the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Deciphering the origin of dubiofossils from the Pennsylvanian of the Paraná Basin, Brazil
João Pedro Saldanha Saldanha
Joice Cagliari
Rodrigo Sacalise Horodyski
Lucas Del Mouro
Mírian Liza Alves Forancelli Pacheco
Abstract. Minerals are the fundamental record of abiotic processes over time, while biominerals, are one of the most common records of life due to their easy preservation and abundance. However, distinguishing between biominerals and abiotic minerals is challenging because the record is defined by the superimposition and repetition of geologic processes and the interference of ubiquitous and diverse life on Earth's surface and crust. Mineral dubiofossils, located on the threshold between abiotic and biotic, can help resolve this issue. The aim of this contribution is to decipher the origin and history of branched mineralized structures that were previously considered mineral dubiofossils. While this material has different forms and refers to biological aspects, it is difficult to associate it with any known Fossil Group due to the overlapping geological processes that occur in the Rio do Sul Formation (Pennsylvanian of the Paraná Basin), very close to the contact from a sill of the Serra Geral Group – Lower Cretaceous with a proven thermal effect. The samples were described using a protocol that evaluated: 1) morphology, texture, and structure; 2) relationship with the matrix; 3) composition and 4) context, assessing indigeneity and syngenicity, and comparing them with abiotic and biotic products. Despite conducting an extensive comparison with abiotic minerals, as well as controlled, induced, and influenced biominerals, no more probable hypothesis was found, excluding the possibility of it being a controlled biomineral due to its patternless diversity of forms and the purely thermometamorphic origin due to the branched elongated form. The occurrence of these structures suggests a complex history: a syndepositional or eodiagenetic origin of some carbonate or sulfate (gypsum, ikaite, dolomite, calcite, siderite), which may be linked to the presence of microbial mats, could have served as a template for mineralization and possibly mediated mineral growth. Mesodiagenesis could have also modified the occurrence, but the main agent responsible for its formation was the Cretaceous intrusion, which dissolved and replaced the initial mineral and precipitated calcite, resulting in the dubiofossil. Throughout these steps, physical-chemical and biological reactions, aided by the intrinsic characteristics of the matrix, amount of organic matter, and distance from contact with the intrusive body, may have increased the morphological complexity. This material illustrates how dubiofossils can be the result of a complex history and overlapping geological processes. It also highlights the difficulty in differentiating biominerals from abiotic minerals due to the scarcity of biogenicity arguments.
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João Pedro Saldanha Saldanha et al.
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RC1: 'Comment on bg-2023-56', Gregory Retallack, 23 May 2023
These are interesting structures, well described and characterized. My main misgiving is that the paper does not come to a conclusion, other than these structures are not organic skeletal hardparts. Even though they are calcite now, they do not show typical calcite crystal form and are presumably pseudomorphous after other minerals. Resolving their crystallography is needed to make a selection and advance a reasonable explanation.
Also the introduction is more long-winded than necessary (10% reduction would not hurt), and does not address the difference between biologically induced and organic matrix mediated biotic structures. Biologically induced seems more plausible, but the idea that these are actually biological structures seems to be what the text implies. This is explained later on, but be sure to clarify this distinction from the outset.
There are at least 3 alternatives, additional to those discussed, for these small crystal structures which should be addressed somewhere.
- Ice crystal casts are particularly relevant because of independent evidence for glaciation in the rocks in question, and these do have a fossil record. Retallack, G.J., 2021. Ediacaran periglacial sedimentary structures. Journal of Palaeosciences, 70(1-2)), pp.5-30.
- Lichen pruina of oxalate minerals such as weddellite, whewellite or glushinskite, also now have a fossil record Retallack, G.J., 2022. Early Ediacaran lichen from Death Valley, California, USA. Journal of Palaeosciences, 71(2), pp.187-218.
- Acicular evaporite minerals such as mirabilite, also have a fossil record. Retallack, G.J., Gose, B.N. and Osterhout, J.T., 2015. Periglacial paleosols and Cryogenian paleoclimate near Adelaide, South Australia. Precambrian Research, 263, pp.1-18. Retallack, G.J., Chen, Z.Q., Huang, Y. and Fang, Y., 2021. Oxidizing atmosphere and life on land during the late Paleoproterozoic outset of the “boring billion”. Precambrian Research, 364, p.106361.
Kinneyia (l.303) is a misnomer for wrinkle marks. Stimson, Matt, Andrew MacRae, Randall Miller, Steve Hinds, Nic Minter, Zabrina Prescott, Brad Redden, Margaret MacNeil, Sarah Cron, and Stephanie Foster. "Rugalichnus mclaughlinensis: a new ichnospecies of microbially induced sedimentary structure from the nonmarine Early Carboniferous of New Brunswick, Canada." In Fossil Record 6, pp. 281-297. New Mexico Museum of Natural History & Science, 2018. Stimson, M.R., Miller, R.F., MacRae, R.A. and Hinds, S.J., 2017. An ichnotaxonomic approach to wrinkled microbially induced sedimentary structures. Ichnos, 24(4), pp.291-316.
l.15 Fossil Group does not need to be capitalized
l.38 what does this mean? “Ascertaining the biological origin and establishing solid evidence for biogenicity is preponderant”
l.103. why all this Latin here?
l.184 et seq. “sticks” is not the right word here. These appear to be acicular crystals, so “needles” would also work..
l.201 Punctually? Wrong word.
l.273 I have never seen heterolith as a noun, generally heterolithic bed
Figure 11 correct spelling is interstitial
Citation: https://doi.org/10.5194/bg-2023-56-RC1 -
AC1: 'Reply on RC1', João Pedro Saldanha, 07 Jun 2023
We sincerely appreciate the valuable comments and suggestions provided, as they have significantly enhanced the quality of our manuscript and sparked meaningful discussions. Regarding the first point, we acknowledge that the absence of more definitive conclusions may be perceived as a limitation. However, it is important to note that the nature of the material and the inferred geological history impose constraints on achieving a more precise definition. Nevertheless, we have made a concerted effort to address this concern by incorporating two additional paragraphs in the conclusion section (Sect. 4). These paragraphs aim to clarify and enhance the presentation of our results and conclusions. Both are positioned after the description of the 5 steps of the inferred geologic history. The added paragraphs are provided below:
“At each stage, variations in the environmental and physical-chemical characteristics of the substrate play a significant role in shaping the resulting products. Factors such as water content, organic matter, mineral composition, and specific properties of silt and clay layers contribute to the unique conditions for reactions that form and modify the needle-like structures. As a result, distinct processes occur with varying intensities in each silty and muddy layer within this contact section of the turbidites and the sill. These processes include nucleation and agglutination reactions during syndeposition/eodiagenesis, agglutination, stabilization, and mineral growth in mesodiagenesis, dissolution, reprecipitation, replacement, and growth during contact metamorphism, as well as cementation, weathering, and subsequent processes. It is through the interplay of these processes that the diverse forms of dubiofossils emerge.
The precise definition of the original material remains a subject of debate due to two primary factors. Firstly, the morphological diversity observed can be attributed to a succession of processes that have occurred throughout the complex history of the specimen. This has resulted in the presence of diagnostic forms that support a particular hypothesis, as well as other forms that do not refute it. Secondly, the final composition has been influenced by thermometamorphic alteration, which has led to the replacement and modification of the original composition of the recovered calcite needles. This alteration has obscured the initial mineralogy, making it challenging to determine conclusively. As a result, both the hypotheses of biotic and abiotic sulfates and carbonates remain plausible explanations.”
The characteristics of the delicate material and over an undulating surface make it impossible to manufacture thin sections in better quality. Even so, we redescribed all the petrographic slides, carefully evaluating the crystallographic properties and suggesting which minerals are more likely, some characteristics refer to carbonates and others to clay minerals, which are discussed in the composition section: allied to other chemical techniques, they allow inferring the main composition of calcite needle. We hope that now the petrographic description is more complete, as this paragraph has been replaced as follows:
“Despite the different external shapes found in a hand sample, common elements were described in a petrographic thin section that allow inferring that they are the same product (Fig. 5). The needles have a distinct crystallinity from the matrix, generally with well-defined edges, euhedral to subhedral shape, low relief, and nanocrystalline texture. The needles do not show color, twinning or cleavage (Fig. 5A-C). It may have a black opaque central axis in the elongation direction and, less commonly, an opaque brownish outer edge (Fig. 5A-C). Incomplete extinction is oblique with mottled, undulating appearance in larger needles. Birefringence variable depending on needle size, usually low 1st order, but 2nd order present in larger features (Fig. 5C and D; G-H). In general, the needles are organized in layers and may have a central axis that is always opaque, a surrounding layer of greater crystallinity (up to microcrystalline) and high birefringence, and a second layer of 1st order birefringence, other layers may also occur externally as brown linings or faded edge that appears as an irregular gray texture never extinct.
The order and amount of these layers is different between classes. Class A has only the opaque interior and the 1st order layer (Fig. 5A and C); Class B has a more extinct interior, with the two layers of distinct birefringence (2nd order and 1st order; Fig. 5B), sometimes delimited by the opaque lining, lateral branches present a generally extinct central region or the opaque axis (Fig. 5D); Class C presents two opaque axes (different from the others) delimiting an internal portion of 2nd order (Fig. 5E-F); and Class D presents, in radial forms, a less centralized axis and a predominance of 2nd order nanocrystalline material (Fig. 5G-H).
In general, the needles show irregular layers in the direction of the central axis of microporous texture intercalated with smooth or microgranular texture (Fig. 5C), sometimes aligned subspherical blocks (Fig. 5F). The texture and the crystallographic and birefringence variations between the needles make mineral inference difficult. Certainly, the lining is an iron oxide/hydroxide film present in the matrix and that surrounds the needles, the interior, with the most extinguished region or the entirely opaque axis seems to be linked to impurities inside the needle. Mottled extinction refers to a possible clay or phyllosilicate (white mica?), however birefringence resembles a possible carbonate (calcite?), see composition discussion in Sect 3.1.3.”
The introduction was reduced as recommended, before it appeared with about 70 lines now it appears with 66 lines including the differentiation of induced and influenced biominerals as suggested. Now in lines 78-84 is included the sentences:
"Essentially, controlled biominerals are minerals that are directly produced and regulated by living organisms that exercise a high level of control over their formation and composition. Induced biominerals are indirectly formed by living organisms, these play an active role in triggering or influencing their formation, producing certain organic compounds or creating specific environmental conditions. Often an indirect result of the metabolic action. In influenced biominerals, there is a passive role in mineral formation or modification caused by the presence of living or dead organisms (see Dupraz et al. 2009 for a broader review), by exclusion abiotic minerals are the result of physical-chemical reactions, without any biological interference.”
Due to the inappropriate name of Kinneyia discussed in the references indicated by the reviewer, it was decided to exclude the name Kinneyia, keeping it only as “wrinkle structures” (in l.310 and in the caption of the Fig. 9), without major effects for the text or discussion. Thanks a lot for the tip.
The sentence “fossil group" at l.15 was corrected
The sentence " “Ascertaining the biological origin and establishing solid evidence for biogenicity is preponderant” at l.38 was changed to “Acquiring substantial evidence to establish biogenicity is crucial not only for determining the biological origin but also for comprehending the intricate biosphere-lithosphere interface” which makes the sentence clearer.
At l.103 we are sorry because it mistakenly has an automatic Latin text coming from the journal's template, the excerpt has now been removed completely.
The term stick or stick-shape (at l.184 and seq) was chosen due to the length-width ratio not being so intense, even varying between the morphotypes described, in addition to the fact that sometimes the thickness is not fixed along the specimen or does not have tapered ends that would characterize a needle -shape sensu stricto. The use of the term stick in the article by Baucon et al. (2020) cited and compared with the samples also contributed to this choice. However, reviewing the definitions in crystalography, many times both terms are used as synonyms, therefore, whenever possible, it was chosen to use the terms needle and needle-shape as suggested.
at the l. 201 the word “Puctually” was changed to “Rarely”.
at l.273 we corrected the word heterolith to “heterolithic bed”
at the figure 11 the word is now correct spelled, thank you for the thank you for the detailed conference.
The three other alternatives, suggested by the reviewer, were investigated, and added as new topics in the discussion of Section “3.4.1 Syndepositional or diagenetic product”. We are very grateful for the suggestions; they greatly enriched the discussion of the possible original material of the dubiofossils. Like the other alternatives presented in the manuscript previously, there are similar and different characteristics that prevent the classification of the material. The new topics have been appended as shown below:
“Ice casts – freezing minerals: Certain characteristics of dubiofossils suggest that they could be interpreted as ice molds or ice casts. These elongated features are formed when water freezes within silt-dominated mudstones and fine-grained sandstones, in fluvio-lacustrine, marginal marine, or aeolian environments (Dionne, 1985; Pfeifer et al., 2021; Voigt et al., 2021), making their occurrence plausible in a periglacial setting. Moreover, these features are often randomly distributed within the bedding plane without disrupting the layers, as the epistratal ice typically grows horizontally (Voigt et al., 2021). The various shapes observed in these elongated features resemble the different ways in which ice forms under varying temperature conditions, including needle-shaped, branched forms, stubby rods, fanned needles, rosette, and stellate structures (Mason et al., 1963; Pfeifer et al., 2021; Voigt et al., 2021). Additionally, the predominance of a specific morphotype within each slab corresponds to the monotypic pattern observed in ice molds (Voigt et al., 2021). The branching features are explained by cycles of freezing and thawing of water-saturated mud events, possibly occurring on a daily basis, which result in branches forming at acute angles without crossing the principal elongation (Voigt et al., 2021). Particular aspects of dubiofossils, such as crossing branches, distinct from ice casts, can be attributed to diagenetic or thermal modifications. Although similar features have been found in the fossil record, such as those reported by Bandel and Shinaq (2003) and Retallack (2021) in the Precambrian, Pfeifer et al. (2021) and Voigt et al. (2021) in the Permian, linked to LPIA, the corroboration of this hypothesis is hindered by the fact that these structures are typically preserved as epirelief or hyporelief molds formed through the melting of ice crystals and subsequent sedimentary deposition within the resulting cavities (Voigt et al., 2021). It is challenging to explain the syn- or post-depositional preservation of other materials, such as calcite, within these spaces.
Other evaporitic minerals: Other sulphates, such as thenardite, mirabilite, bloedite, loeweite, and glauberite, also exhibit a needle-like morphology in similar geological contexts (Warren, 1999; 2016; Hamdi-Aissa et al., 2004; Benison and Bowen, 2013). The Bemara environment, due to its local temperature conditions, may have provided favorable settings for the formation of these sulfate needles. Once, modern evaporites demonstrate that the nocturnal temperature reduction during winter, reaching close to 0º C, creates the necessary thermodynamic equilibrium for the crystallization of mirabilite and other evaporitic minerals (Hamdi-Aissa et al., 2004; Espinosa-Marzal and Scherer, 2010; Jassim, 2019). It's worth noting that glauberite has also been found in the Karoo basin, in a similar context of LPIA deglacial sequences, although it occurs in concretions (McLachlan and Anderson, 1973). These less common sulphates require a high concentration of specific cations for deposition (Hamdi-Aissa et al., 2004; Warren, 2016) and follow the Usiglio precipitation sequence, necessitating prior carbonate precipitation before their crystallization (see Babel, 2004; Warren, 2010). Consequently, the hypothesis of dubiofossils as evaporitic products becomes less plausible.
Oxalate minerals: oxalates such as whewellite, weddellite, glushinskite, known as organic minerals, occur mainly as biominerals in plants, fungi, algae and in diagenetic, and hydrothermal occurrences (Baran, 2014; Hofmann and Bernasconi, 1998). The hydrothermal origin of the needles is completely discarded, as they do not present features that cut the layers like veins and that should be found as late-stage phase hydrothermal products, in the form of whewellite (Baran, 2014; Hofmann and Bernasconi, 1998) after crystallization of calcite, inversely to what is found in the Bemara outcrop. The shapes and crystallinity also attest against the diagenetic origin of the needles, which although the occurrence of diagenetic whewellite is generally a result of low migration in rocks rich in organic matter, the products are druses, vugs and fissures within septarian concretions, normally larger than 1 cm (see Hofmann and Bernasconi, 1998; for a review). The needles cannot be skeletal parts of the plants and algae, due to their generally elongated format, regular or in small globules (Franceschi and Horner, 1980; Hofmann and Bernasconi, 1998; Francheschi and Nakata, 2007; Baran, 2014), or in the rare occurrences of oxalate crosses included in algae (Pueschel, 2001), much smaller than Bemara needles. However, the origin of dubiofossils as products of mineralization induced/influenced by fungi or lichens is still plausible, mainly the result of the microenvironmental modification of the substrate by the action of hyphae, which leads to the mineralization of whewellite, weddellite (Gadd, 2007; Gadd et al., 2012; 2016; Baran, 2014) or glushinskite for some lichens (Wilson et al., 1980; Baran, 2014). These minerals have varied forms, some branched or ornamented with lateral spines (Whitney, 1989; Dutton and Evans, 1996), similar to dubiofossils. Fungi have a fossil record since the Proterozoic (e.g., Retallack, 2022) and are important degraders of rock and sediments on the surface (Chen et al., 2000; Gadd, 2007; Gadd et al., 2012; 2014), in addition, oxalates are easily modified to calcite under increasing temperature (Baran, 2014). One of the points that argue against this origin is that the needle features are not penetrative in the subsurface (generally subvertical to vertical, Friedmann et al., 1987; Chen et al., 2000; Gadd et al., 2014; Retallack, 2022) and do not they present an expansive distribution from one or more centers, which would be the starting point of mycorrhizae, hyphae or lichens (see Gadd et al., 2014).”
Once again, we thank you for spending your time and pointing out issues that are so relevant to our work. We hope that the modifications meet your expectations and improve the manuscript.
The new references are provided below:
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Bandel, K., Shinaq, R.: Sediments of the Precambrian Wadi Abu Barqa Formation influenced by life and their relation to the Cambrian sandstones in southern Jordan. Freib. Forschungshefte C 499, 78–91, 2003.
Baran, E.J.: Review: Natural oxalates and their analogous synthetic complexes. J. Coord. Chem. 67, 3734–3768. https://doi.org/10.1080/00958972.2014.937340, 2014.
Baucon, A., De Carvalho, C.N., Felletti, F., Cabella, R.: Ichnofossils, cracks or crystals? A test for biogenicity of stick-like structures from vera rubin ridge, mars. Geosci. 10. https://doi.org/10.3390/geosciences10020039, 2020.
Benison, K.C., Bowen, B.B.: Extreme sulfur-cycling in acid brine lake environments of Western Australia. Chem. Geol. 351, 154–167. https://doi.org/10.1016/j.chemgeo.2013.05.018, 2013.
Chen, J., Blume, H.-P., Beyer, L.: Weathering of rocks induced by lichen colonization — a review. CATENA 39, 121–146. https://doi.org/10.1016/S0341-8162(99)00085-5, 2000.
Dionne, J.C.: Formes, figures et faciès sédimentaires glaciels des estrans vaseux des régions froides. Palaeogeogr. Palaeoclimatol. Palaeoecol. 51, 415–451. https://doi.org/10.1016/0031-0182(85)90097-5, 1985.
Dutton, M. V., Evans, C.S.: Oxalate production by fungi: its role in pathogenicity and ecology in the soil environment. Can. J. Microbiol. 42, 881–895. https://doi.org/10.1139/m96-114, 1996.
Espinosa-Marzal, R.M., Scherer, G.W.: Advances in understanding damage by salt crystallization. Acc. Chem. Res. 43, 897–905. https://doi.org/10.1021/ar9002224, 2010.
Franceschi, V.R., Horner, H.T.: Calcium oxalate crystals in plants. Bot. Rev. 46, 361–427. https://doi.org/10.1007/BF02860532, 1980.
Franceschi, V.R., Nakata, P.A.: Calcium oxalate in plants: Formation and function. Annu. Rev. Plant Biol. 56, 41–71. https://doi.org/10.1146/annurev.arplant.56.032604.144106, 2005.
Friedmann, E.J., Weed, R., Land, V.: Abiotic Weathering in the Antarctic Cold Desert. Science (80-. ). 236, 703–705. 1987.
Gadd, G.M., Bahri-Esfahani, J., Li, Q., Rhee, Y.J., Wei, Z., Fomina, M., Liang, X.:Oxalate production by fungi: significance in geomycology, biodeterioration and bioremediation. Fungal Biol. Rev. 28, 36–55. https://doi.org/10.1016/j.fbr.2014.05.001, 2014.
Gadd, G.M., Rhee, Y.J., Stephenson, K., Wei, Z.: Geomycology: Metals, actinides and biominerals. Environ. Microbiol. Rep. 4, 270–296. https://doi.org/10.1111/j.1758-2229.2011.00283.x, 2012.
Gadd, G.M.: Geomycology: biogeochemical transformations of rocks, minerals, metals and radionuclides by fungi, bioweathering and bioremediation. Mycol. Res. 111, 3–49. https://doi.org/10.1016/j.mycres.2006.12.001, 2007
Hamdi-Aissa, B., Valles, V., Aventurier, A., Ribolzi, O.: Soils and Brine Geochemistry and Mineralogy of Hyperarid Desert Playa, Ouargla Basin, Algerian Sahara. Arid L. Res. Manag. 18, 103–126. https://doi.org/10.1080/15324804902796562004, 2004.
Hofmann, B.A., Bernasconi, S.M.: Review of occurrences and carbon isotope geochemistry of oxalate minerals: implications for the origin and fate of oxalate in diagenetic and hydrothermal fluids. Chem. Geol. 149, 127–146. https://doi.org/10.1016/S0009-2541(98)00043-6, 1998.
Jassim, R.Z.: MINERAL RESOURCES OF SODIUM SULFATE IN IRAQ : 289–310. 2019
Mason, B.J., Bryant, G.W., Van den Heuvel, A.P.: The growth habits and surface structure of ice crystals. Philos. Mag. 8, 505–526. https://doi.org/10.1080/14786436308211150, 1963.
McLachlan, I.R., Anderson, A.: A review of the evidence for marine conditions in Southern Africa during Dwyka times, 1973.
Pfeifer, L.S., Birkett, B.A., Driessche, J. Van Den, Pochat, S., Soreghan, G.S.: Ice-crystal traces imply ephemeral freezing in early Permian equatorial Pangea. Geology 49, 1397–1401. https://doi.org/10.1130/G49011.1, 2021.
Pueschel, C.M.: Calcium oxalate crystals in the green alga Spirogyra hatillensis (Zygnematales, Chlorophyta). Int. J. Plant Sci. 162, 1337–1345. https://doi.org/10.1086/322943, 2001.
Retallack, G. J.: Ediacaran periglacial sedimentary structures. J. Palaeosciences 70, 5–30. https://doi.org/10.54991/jop.2021.8, 2021.
Retallack, G.J.: Early Ediacaran lichen from Death Valley, California, USA. J. Palaeosciences 71, 187–218. https://doi.org/10.54991/jop.2022.1841, 2022.
Voigt, S., Oliver, K., Small, B.J.: Potential Ice Crystal Marks From Pennsylvanian–Permian Equatorial Red-Beds of Northwest Colorado, U.S.a. Palaios 36, 377–392. https://doi.org/10.2110/PALO.2021.024, 2021
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Citation: https://doi.org/10.5194/bg-2023-56-AC1
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RC2: 'Comment on bg-2023-56', Flavia Callefo, 12 Jun 2023
Biogeosciences - Preprint bg-2023-56
Manuscript (preprint): Deciphering the origin of dubiofossils from the Pennsylvanian of the Paraná Basin, Brazil
Reviewer: Flavia Callefo
General considerations: The article brings an extremely necessary and pertinent discussion to the current state of the art of research in Geobiology, Paleobiology and related areas, which is about biogenicity assessment using chemical biosignatures, minerals and reconstructions of biogeochemical or purely mineralogical processes.
Geobiology, which has been constantly benefiting from the application of different analytical techniques for the assessment of biogenicity, and any in-depth discussions based on analytical data contribute significantly to different paleoenvironmental interpretations, revisiting dubiofossils and expanding knowledge about modifications of these signatures in different contexts (environmental, chemical, physical, etc).
In particular, the investigation and discussion about the biogenicity of dubiofossils is of paramount importance, since the biological origin makes a complete difference in any paleoenvironmental interpretations from micro to macro scale.
Introduction:
Line 58-59: “As they are on the threshold of knowledge between abiotic and biotic, dubiofossils, fossil-like structures formerly related to life with an ambiguous origin (Hofmann, 1972), hold promising potential for enhancing biosignatures.” – Actually, the dubiofossils hold the potential to be or not to be considered a biosignature, once its biogenicity is proven or not. I suggest rephrasing this sentence.
Line 81 to 86: I believe it is important to emphasize here also the limitations in separating minerals purely products of inorganic (abiotic) processes from a wide range of biologically induced minerals. This difficulty is recognized in the literature, since this category of biominerals does not present intrinsic characteristics (such as crystallographic properties) different from biotic ones. This is one of the main challenges inherent to this issue of mineral biogenicity.
Line 101-103: “Suspendisse a elit ut leo pharetra cursus sed quis diam. Nullam dapibus, ante vitae congue egestas, sem ex semper orci, vel sodales sapien nibh sed lectus. Etiam vehicula lectus quis orci ultricies dapibus. In sit amet lorem egestas, pretium sem sed, tempus lorem.” Is there a purpose for this sentence or was it a mistake?
Practical issue: in order to better present the problem to the readers, I recommend to increase the reasons of why the presented structures are considered dubiofossils in the Introduction; in the line 94 it is saying that they were previously interpreted as sponge spicules. Is there some reference for this? Why? It`s because of the morphology, paleoenvronmental context? I suggest to explore more and encourage readers to start questioning right here.
Line 97-99: “We endeavor to unravel the intricate history of unique mineral occurrence, which has been shaped by the overlapping effects of geologic processes and the omnipresence of life.”. But in the end (in the conclusions) this not happen. I suggest rewrite here.
Geological settings
Figure 2: There are some dark grey colored areas in this figure, it is quite difficult to read what is inside. I suggest to increase the contrast of the background colors and the letters inside he figure.
Materials and methods
Line 168: “One horizontal thin-section…”. Remove “horizontal”.
Results and discussions
Figure 4: very good figure, showing the structures in high magnification in details.
Figure 7: legend. Replace “Raman specters” by “Raman spectra”.
In all discussion text: maybe replace “sticks” by needles, or acicular structures.
A possibility not discussed: aragonite needles are formed in environmental contexts suggested in the text when discussing the possibilities of formation of calcite or dolomite. What happens is that aragonite, a much less stable polymorph, is easily replaced by calcite, which is more stable. This can preserve the original shape of the needles, as if they were replaced relics.
I particularly see no problem with not reaching a more closed conclusion, that is, leaving the discussion open. I believe that the manuscript presented the most in-depth discussions possible about the possibilities, and from my own experience, I believe that it is of great value for approaching the topic of biogenicity of dubiofossils. What I see here is a good example of how to handle complex possibilities and exercise caution in any assertions.
Citation: https://doi.org/10.5194/bg-2023-56-RC2 -
AC2: 'Reply on RC2', João Pedro Saldanha, 20 Jun 2023
We sincerely appreciate the thorough review of our manuscript. Your valuable suggestions and the attention given to specific details have significantly enhanced the quality of this contribution. We highly value your insights regarding the significance of this investigation and the ensuing discussion, despite the remaining inconclusive nature of the issue. Your comments have been thoughtfully taken into account and the necessary modifications have been made in the manuscript as indicated below:
The paragraph with lines 58-59 has been reworded to clarify the explanation of the importance of dubiofossils as shown below:
Dubiofossils, fossil like structures formerly related to life with and ambiguous origin (Hofmanm, 1972), play a crucial role in enhance biosignatures. Through testing and refinement, the biological nature of a dubiofossil can be established, leading to its classification as a genuine fossil; alternatively, if its origin is determined to be the result of abiotic processes, it is categorized as a pseudofossil (Hofmann, 1972; Monroe and Dietrich, 1990; McMahon et al., 2021). Once the biological origin is confirmed, these dubiofossils can be regarded as potential biosignatures or contain distinctive characteristics indicative of past life (McMahon et al., 2021).
To complement the introduction, emphasizing the difficulty of differentiating abiotic minerals from biotic ones as indicated in lines 81 to 86, we chose to include the sentence below, which we hope resolves the issue:
“In practice, it is challenging to differentiate each of these products in the geological record due to the lack of diagnostic characteristics, such as specific shapes or crystallographic properties and compositional signatures that resist modifications over time (see Weiner and Dove, 2003; Dupraz et al., 2009).”
At lines 101-103 we are sorry because it mistakenly has an automatic Latin text coming from the journal's template, the excerpt has now been removed completely.
As suggested, we added a paragraph to explain de first association with sponge spicules and why the needles cannot be sponge. And it is positioned at the end of the penultimate paragraph as follows:
“This material was previously proposed as sponge spicules from the Paraná Basin (Mouro and Saldanha, 2022), since some formats resemble spicules, the distribution of structures could delimit circular and ellipsoidal features such as flattened bodies, in moreover, close to the outcrop, an earlier stratigraphic unit of similar context contains well-preserved fossil sponges in abundance (see Mouro and Saldanha, 2022). However, the diversity of formats and the absence of spicular net prevented the classification of this material as porifera, remaining as a mineral dubiofossil.”
We understood your recomendation and decided to delete the last part of the sentence in lines 97-99, in fact the discussion of the ubiquitous presence of life was less explored in our results and therefore we chose to leave the sentence as below:
“We endeavor to unravel the intricate history of unique mineral occurrence, which has been shaped by the overlapping effects of abiotic and biotic geologic processes.”.
At the Figure 2 we improved the contrast and put a white buffer around the captions, thank you to improve the visualization of this image.
At line 168: the word “horizontal” has been removed.
Thanks for the compliments on Figure 4, the plate was selected to make the morphological differences clearer.
At the caption of the Figure 7, we changed the terms “Raman specters” by “Raman spectra”, thank you to appoint this detail.
The term "stick" or "stick-shape" was selected to describe these structures due to their moderate length-to-width ratio, which may vary among the described morphotypes. Furthermore, these structures may not always exhibit a consistent thickness throughout the specimen or possess tapered ends that would strictly characterize a needle shape. The decision to use the term "stick" in the article by Baucon et al. (2020), which was cited and compared with the samples, also influenced this choice. However, upon reviewing definitions in crystallography, it became evident that both terms are often used interchangeably. Therefore, whenever possible, the terms "acicular structures," "needle," and "needle-shape" were preferred to ensure clarity and accuracy.
In the first version of the manuscript, we ended up not discussing this hypothesis much due to the need for higher temperature linked to aragonite precipitation. After your suggestion, we reassessed the literature in view of our results and found other factors that favored the explanation of the needles as a possible original aragonite that fit our scenario. For this contribution we thank you very much for deepening our discussion and improving our interpretation. In the manuscript we added a paragraph on aragonite in the discussion, right after discussing biotic calcite as follow:
“Aragonite – biotic and abiotic mineral: aragonite is commonly found in mollusk shells, nacre, and as a high-pressure metamorphic mineral (Lipmann, 1973; Ramakrishna et al., 2017; Toffolo, 2021). While it is typically metastable compared to calcite, there are other occurrences where it is found. Needle-shaped aragonite, normally ranging from 5 to 100 μm, can be secreted by algae or deposited in various environments such as caves, hot springs, shallow seas, and lakes (Lowenstam and Epstein, 1957; Lipmann, 1973; Frisia et al., 2002; Jones, 2017; Ramakrishna et al., 2017). The factors contributing to the precipitation of aragonite needles instead of calcite are the influence of temperature, usually above 25°C (Lipmann, 1972; Jones, 2017; Ramakrishna et al., 2017), a high concentration of Mg+ or a high Mg/Ca ratio (Kitano and Hood, 1962; Hu et al., 2009; Jones, 2017; Ramakrishna et al., 2017), and environments with high CO2 degassing rates (Frisia et al., 2002; Sanchez-Moral et al., 2003; Jones, 2017). In the case of dubiofossils at Bemara, the larger needle size cannot be attributed to higher temperatures, as the paleoenvironmental conditions do not support this explanation. However, the presence of a central axis rich in Mg in the needles may indicate remnants of the high concentration necessary for aragonite deposition, in addition to the possible high rate of degassing due to the mediation/degradation of the mats (see Sanchez-Moral et al., 2003). Despite that aragonite can easily be replaced by calcite, the large needle size, exceeding what is commonly reported in the literature, poses challenges in classifying the material as acicular aragonite.”
Once again, we thank you immensely for the careful evaluation of our manuscript. The suggested details and issues have enriched our work and we hope it will meet your expectations.
The new references are provided below:
Baucon, A., De Carvalho, C.N., Felletti, F., Cabella, R.: Ichnofossils, cracks or crystals? A test for biogenicity of stick-like structures from vera rubin ridge, mars. Geosci. 10. https://doi.org/10.3390/geosciences10020039, 2020.
Del Mouro, L., Saldanha, J.P.: sponge fossil of brazil: review and perspectives. Paleontol. EM DESTAQUE - Bol. Inf. da Soc. Bras. Paleontol. 36, 46–61. https://doi.org/10.4072/paleodest.2021.36.75.03, 2021.
Dupraz, C., Reid, R.P., Braissant, O., Decho, A.W., Norman, R.S., Visscher, P.T.: Processes of carbonate precipitation in modern microbial mats. Earth-Science Rev. 96, 141–162. https://doi.org/10.1016/j.earscirev.2008.10.005, 2009.
Frisia, S., Borsato, A., Fairchild, I.J., McDermott, F., Selmo, E.M.: Aragonite-Calcite Relationships in Speleothems (Grotte De Clamouse, France): Environment, Fabrics, and Carbonate Geochemistry. J. Sediment. Res. 72, 687–699. https://doi.org/10.1306/020702720687, 2002
Hofmann, H. J.: Precambrian remains in Canada: fossils, dubiofossils, and pseudofossils. In: Proceedings of the 24th International Geological Congress, Section. 20-30, 1972.
Hu, Z., Shao, M., Li, H., Cai, Q., Zhong, C., Xianming, Z., Deng, Y.: Synthesis of needle-like aragonite crystals in the presence of magnesium chloride and their application in papermaking. Adv. Compos. Mater. 18, 315–326. https://doi.org/10.1163/156855109X434720, 2009.
Jones, B.: Review of aragonite and calcite crystal morphogenesis in thermal spring systems. Sedimentary Geology, 354, 9-23., 2017.
Kitano, Y., Hood, D.W.:Calcium Carbonate Crystal Forms Formed from Sea Water by Inorganic Processes. J. Oceanogr. Soc. Japan 18, 141–145. https://doi.org/10.5928/kaiyou1942.18.141, 1962.
Lippmann, F.: Sedimentary Carbonate Minerals. Springer Berlin Heidelberg, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-65474-9, 1973.
Lowenstam, H.A., Epstein, S.: On the Origin of Sedimentary Aragonite Needles of the Great Bahama Bank. J. Geol. 65, 364–375. https://doi.org/10.1086/626439, 1957
McMahon, S., Ivarsson, M., Wacey, D., Saunders, M., Belivanova, V., Muirhead, D., Knoll, P., Steinbock, O., Frost, D.A.: Dubiofossils from a Mars‐analogue subsurface palaeoenvironment: The limits of biogenicity criteria. Geobiology 19, 473–488. https://doi.org/10.1111/gbi.12445, 2021.
Monroe, J.S., Dietrich, R. V.: Pseudofossils. Rocks Miner. 65, 150–158. https://doi.org/10.1080/00357529.1990.11761667, 1990.
Ramakrishna, C., Thenepalli, T., Ahn, J.W.: A brief review of aragonite precipitated calcium carbonate (PCC) synthesis methods and its applications-. Korean Chem. Eng. Res. 55, 443–455. https://doi.org/10.9713/kcer.2017.55.4.443, 2017.
Sanchez-Moral, S., Canaveras, J.C., Laiz, L., Saiz-Jimenez, C., Bedoya, J., Luque, L.: Biomediated Precipitation of Calcium Carbonate Metastable Phases in Hypogean Environments: A Short Review. Geomicrobiol. J. 20, 491–500. https://doi.org/10.1080/713851131, 2003.
Toffolo, M.B.: The significance of aragonite in the interpretation of the microscopic archaeological record. Geoarchaeology 36, 149–169. https://doi.org/10.1002/gea.21816, 2021.
Weiner, S., Dove, P.: An Overview of Biomineralization Processes and the Problem of the Vital Effect. Rev. Mineral. Geochemistry 54, 1–29. https://doi.org/10.2113/0540001, 2003.
Citation: https://doi.org/10.5194/bg-2023-56-AC2 -
RC3: 'Reply on AC2', Flavia Callefo, 22 Jun 2023
Dear authors
I appreciate the modifications made by the authors, and the accepted suggestions. In my opinion, the manuscript has reached the enough quality for publication. No further modifications are required from me.
Sincerely
Citation: https://doi.org/10.5194/bg-2023-56-RC3 -
AC4: 'Reply on RC3', João Pedro Saldanha, 03 Jul 2023
We are delighted to learn that the manuscript has met your expectations and is prepared for publication. We sincerely appreciate the effort you put into reviewing our manuscript and evaluating the modifications we implemented. Once again, we extend our gratitude for your valuable contributions and suggestions, which have significantly enriched our work.
Citation: https://doi.org/10.5194/bg-2023-56-AC4
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AC4: 'Reply on RC3', João Pedro Saldanha, 03 Jul 2023
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RC3: 'Reply on AC2', Flavia Callefo, 22 Jun 2023
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AC2: 'Reply on RC2', João Pedro Saldanha, 20 Jun 2023
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RC4: 'Comment on bg-2023-56', Luana Morais, 27 Jun 2023
The manuscript by Saldanha et al. entitled "Deciphering the origin of dubiofossils from the Pennsylvanian of the Paraná Basin, Brazil" discusses stick-shaped (needle-like shape) structures, branched or not, and its possible relationships with fossils. For this, the authors used several techniques to highlight the composition, mineralogy, morphology and distribution of these structures and created an analysis protocol to test the biogenicity and indigeneity of the investigated material, which can be certainly useful for taphonomic studies and for interpretation of the structures assigned as biologic in origin.
The work is well written, figures are well presented and the results obtained through the techniques used may help to answer the questions raised.
My main concern is regarding two points:
- The authors' conclusion is not clear. Are these structures dubiofossils or pseudofossils? May this conclusion impossible to reach? Why? These arguments should be clear in the abstract.
- The reasons that led the authors to believe that the structures could be fossils is not clear either. Due to the lack of comparison with recognized fossils throughout the manuscript (until the topic 3.3), may suggest that these structures are diagenetic features typical of the formation of inorganic minerals. The reasons that led the authors to questioning this possibility are unclear. This needs to be clear throughout the abstract and introduction, not only in the discussion.
Below, I present my suggestions and doubts.
Abstract
Line 13: The meaning of the threshold between abiotic and biotic minerals sounds vague without some examples. I suggest drawing attention to at least one example of a mineral dubiofossil and the implication of its interpretation.
Lines 13-14: Although this manuscript may contribute to the investigation of general mineral dubiofossils described in the literature, I suggest to include the geological unit and age, following the sentence “ The aim of this contribution is to decipher the origin and history of branched mineralized structures that were previously considered mineral dubiofossils.” In this way, the text makes it clear that the aim of this manuscript is to understand the taphonomic patchways in this specific case (Rio do Sul Formation, Pennsylvanian of the Paraná Basin).
Lines 14-15: Although it may be hard to attribute the dubiofossils reported here with any known biological affinity, I believe that the authors should compare these structures with possible fossil groups, even if it is to rule out the hypothesis. In this case, I suggest briefly mentioning what are the possibilities here, including what features could be interpreted as belonging to any fossil group and why these possibilities are ruled out.
Lines 19-22: This sentence is too long and confusing. I ́m not sure if I got the author's point. If I did, I suggest: "An extensive comparison with abiotic minerals and controlled, induced, and influenced biominerals excluded the possibility of the samples investigated here being a controlled biomineral, due to its patternless diversity of forms and the purely thermometamorphic origin due to the branched elongated form.”
Lines 24-25: “Mesodiagenesis could have also modified the occurrence …”. Ok. May the authors explain, in a few words, how these modifications could have occurred during mesodiagenesis?
Line 29: Please, include “a” before “result”
Lines 45-46: What is the conclusion of this manuscript? Are the dubiofossils now interpreted as pseudofossils? Or do they still fit in the “dubiofossils” category? Please, clarify it.
Introduction
Lines 41-45: The relation of these sentences is confusing. The dot between them suggests that new information it's coming. However, the second sentence appears to be a continuation (without any link) of the first sentence. Please, clarify it.
Lines 55-58: This “narrow threshold between biotic and abiotic environments” is still vague. Could the authors explain better what they understand as a narrow threshold? Could it be information obtained from compositional signals? Crystallographic characters, in the case of biominerals? Are the authors referring to possible changes during superimposition of diagenetic processes during geologic history? What changes and what are the possible implications for the investigation of biominerals ? Please, clarify it.
Line 64: Is the biogenecity criteria really an emerging area of science? This has been an extensively debated topic since (at least) Buick (1984). Reference: Carbonaceous filaments from North Pole, Western Australia: Are they fossil bacteria in Archean stromatolites?
Line 72: I suggest to change “can be essential for refining…” for “are essential for refining …”.
Line 79: Is important to explain why well-established biogenicity criteria can rarely be applied to minerals. Because of that, I suggest including this argument in the following paragraph, where the authors give the reasons for it.
Lines 95-96: This information was already mentioned in lines 66-67. I suggest removing this sentence (from here, or from lines 66-67) to avoid repetition of information.
Lines 100-101: I suggest including this protocol proposal in the abstract.
Lines 101-103: “Suspendisse a elit ut leo pharetra cursus sed quis diam. Nullam dapibus, ante vitae congue egestas, sem ex semper orci, vel sodales sapien nibh sed lectus. Etiam vehicula lectus quis orci ultricies dapibus. In sit amet lorem egestas, pretium sem sed, tempus lorem.” ????
Geological settings
Lines 123-125: I suggest including the last sentence of this topic (“The detailed outcrop description with sedimentary structures and biotic elements is presented at the Sect. 3.1.4”), following this sentence. Additionally, could it be possible to include the position where the dubiofossils occur in Figure 8?
Material and methods
Line 144: I do not think that the information about the ichnofossil occurrence fits in this topic.
Line 147: How many morphologies were described? Additionally, here is an appropriate place to include the number collection of these thin sections.
Line 147: Does this sample have a number collection? Same question for lines 157, 161 and 168.
Line 168: Why have the authors chosen to make a horizontal thin-section? Is it relevant to say “horizontal!”?
Results and discussion
Line 180: What does simple and complex mean in this case? What characters define the geometry as simple or complex?
Lines 187-189: I suggest to include the approximated total number of specimens per category (at least the measured specimens). Do all categories occur at the same stratigraphic level?
Line 273: I suggest replacing “14 m of heterolith (Fig. 8A) defined by tabular centimetric layers…” by “14 m of centimetric heterolithic layers, measuring 0.2 to 4 cm (Fig. 8A)…”.
Line 279: Could the authors include MISS in the schematic section from Figure 8A?
Lines 281-282: Shouldn't the references follow a chronological order?
Line 302: Same comment for Line 279.
Line 310: I suggest mentioning Figure 5I-J to discuss microfabric.
Lines 327-348:I did not understand the relevance of this discussion for this manuscript. What is the implication of this discussion for the dubiofossils and their origin, once they occur “sometimes” (Line 350) associated with trace fossils and miss?
Line 349: Problematic features and dubiofossils mean the same thing? If so, I suggest replacing “Problematic features” by “These problematic structures…”.
Lines 395-399: This information should be certainly included in the abstract and briefly commented in the introduction.
Line 451: If the controlled biomineral hypotheses have been eliminated, this information should also be included in the abstract.
Figures
Figure 3: Replace the second “C” by “D”. Include “yellow” before “curved contours”. Additionally, It is hard to visualize ramifications in B, F and H. Maybe an arrow could help.
Figure 4: It is hard to visualize ramifications in C-G. Maybe an arrow could help.
Figure 5: "Figure 5” in the figure caption is duplicated. Please, remove one of them. Figure 5A (right) is too dark. Could the authors improve this image?
Figure 8: I suggest replacing “Local geologic features” by "Geological setting and sample features of the Bemara Quarry”. I also suggest replacing the comma between “Serra Geral Group” and “ a schematic section” by “...Serra Geral Group, followed by a schematic section of…”. Figure 8B: Where is the dubiofossils occurrence and the sill, according to the dashed line?
Citation: https://doi.org/10.5194/bg-2023-56-RC4 -
AC3: 'Reply on RC4', João Pedro Saldanha, 28 Jun 2023
We sincerely appreciate your dedication and the meticulous review of our manuscript. We agree with all the notes provided and have made every effort to address each question comprehensively. Regarding the two main points, we have implemented a series of modifications throughout the text to enhance clarity and engage the reader from the outset. We are extremely pleased with these changes and firmly believe that the manuscript has been significantly improved.
Regarding the first main point, we sought to emphasize in the abstract, introduction, and conclusion the inherent challenge of corroborating the origin of the material. We have clearly described the complex history and highlighted how extensive metamorphism has altered a significant portion of the original evidence.
For example, in the abstract, we have included phrases that explicitly convey these aspects: “corroborate the origin of the material becomes even more challenging. Consequently, both the hypotheses pertaining to the formation of biotic and abiotic sulfates and carbonates remain plausible explanations, hence sustaining the classification of the material as a dubiofossil. This material illustrates how dubiofossils can be a result of a complex history and overlapping geological processes.”.
At the conclusion we added and improved the text arguing: “Itararé’s dubiofossils are products of nature, exhibiting a wide range of morphologies that distinguish them from any known minerals. The absence of a consistent pattern in their diverse forms helps dismiss hypotheses suggesting controlled biomineralization. However, it remains uncertain whether the material could have originated as an abiotic mineral or as an induced or influenced biomineral. The complexity of their geological history and the multitude of contributing factors have resulted in this distinctiveness. Consequently, we propose that dubiofossils are likely the outcome of a combination of processes and a complex geological history.” and after: “The precise definition of the original material remains a subject of debate due to two primary factors. Firstly, the morphological diversity observed can be attributed to a succession of processes that have occurred throughout the complex history of the specimen. This has resulted in the presence of diagnostic forms that support a particular hypothesis, as well as other forms that do not refute it. Secondly, the final composition has been influenced by thermometamorphic alteration, which has led to the replacement and modification of the original composition of the recovered calcite needles. This alteration has obscured the initial mineralogy, making it challenging to determine conclusively. As a result, both the hypotheses of biotic and abiotic sulfates and carbonates remain plausible explanations and the material remains as a dubiofossil.”
As for the second point, we have taken steps to elucidate the preliminary proposals regarding the material's origin and the motivations that led us to classify it as a dubiofossil. In the introduction, we have provided a comprehensive argument, stating that the material's diverse forms and absence of a distinct pattern hinder its initial classification as either a sponge or a metamorphic pseudofossil. These sentences have been incorporated into the abstract and introduction, respectively:
“Given the absence of attributes essential for supporting the initial hypotheses proposing the material as a potential set of sponge spicules or a result of contact metamorphism in Pennsylvanian turbidites, the objects are now investigated as mineral dubiofossils.”
“This material was previously proposed as sponge spicules from the Paraná Basin (Mouro and Saldanha, 2021), since some formats resemble spicules, the distribution of structures could delimit circular and ellipsoidal features such as flattened bodies, in moreover, close to the outcrop, an earlier stratigraphic unit of similar context contains well-preserved fossil sponges in abundance (see Mouro and Saldanha, 2021). However, the diversity of formats and the absence of spicular net prevented the classification of this material as porifera, on the other hand, the diversity of formats demonstrate dissimilarities with diagenetic/metamorphic products in a preliminary comparison, remaining as a mineral dubiofossil.”
We have also made minor adjustments to guide the reader towards these key insights throughout the text and facilitate their understanding of our discussions and conclusions. With these updates, we believe that the questions raised have been effectively addressed, and reading the manuscript will meet your expectations.
We express our gratitude for your valuable time and efforts in reviewing and enhancing the abstract. In the initial version of the manuscript, we acknowledged the absence of several crucial details. Therefore, we have thoroughly reworked the abstract, incorporating all the recommended revisions. We believe that the revised version represents a significant improvement, addressing your expectations and providing comprehensive answers to your inquiries.
At line 13, we rewrite the sentence to exclude the vagueness of the “threshold between abiotic and biotic”, replacing by: “Mineral dubiofossils, being potential outcomes of both abiotic and biotic environments, emerge as valuable entities that can contribute significantly to the understanding of this issue, facilitating the testing and refinement of biogenicity criteria.”
As recommended, at lines 13-14, we added the age and unit to specify our object as: “mineral dubiofossils from Pennsylvanian of the Paraná Basin, Brazil”.
As suggested at lines 14-15, we have emphasized the previous proposals biotic and abiotic for the dubiofossils as: “Given the absence of attributes essential for supporting the initial hypotheses proposing the material as a potential set of sponge spicules or a result of contact metamorphism in Pennsylvanian turbidites, the objects are now investigated as mineral dubiofossils” and arguing below why it cannot be one of these hypotheses. “The comparative analysis did allow us to exclude the possibility of the samples being controlled biominerals due to their patternless diversity of morphologies, as well as purely thermometamorphic in origin due to their branched elongated forms.”
The suggestion at Lines 100-101 to put the protocol in the abstract was accepted, as show: To address this challenge, we have developed a descriptive protocol for dubiofossils, building upon prior research in the field. This protocol evaluates the following aspects: 1) morphology, texture, and structure; 2) relationship with the matrix; 3) composition; and 4) context. By assessing indigeneity, syngenicity, and comparing the specimens with abiotic and biotic products.”.
We accepted the suggestion at lines 19-22 and improved the sentence as: “Extensive comparisons were made between the studied samples and a broad spectrum of abiotic minerals, as well as controlled, induced, and influenced biominerals from similar contexts. These comparative analyses encompassed sponge spicules, sea urchin and algae skeletons, minerals induced or influenced by fungi, bacteria, and microbial mats, as well as inorganic pre- and synsedimentary/eodiagenetic minerals like evaporites, springs, and other precipitates, and mesodiagenetic/metamorphic crystals. Despite this comprehensive analysis, no hypothesis emerged as significantly more likely than others.”.
We added the processes of modification of the dubiofossils during the mesodiagenesis to complete the phrase at lines 24-25, as follow: “Mesodiagenesis could have further modified the occurrence through processes such as mineral stabilization, agglutination, aging, and growth”.
The suggestion at Lines 45-46 was accepted and we emphasized at the abstract what is our conclusion, indicating as: “corroborate the origin of the material becomes even more challenging. Consequently, both the hypotheses pertaining to the formation of biotic and abiotic sulfates and carbonates remain plausible explanations, hence sustaining the classification of the material as a dubiofossil.”. That our material remain as dubiofossil.
We changed the sentence at line 29 for “dubiofossils can be a result of a complex history”, thank you for the improvement.
Once again we are grateful for contributions to our abstract. All suggestions were included and improved our text. Below is the complete abstract to check, since we modified a large part of it:
“Minerals are the fundamental record of abiotic processes over time, while biominerals, are one of the most common records of life due to their easy preservation and abundance. However, distinguishing between biominerals and abiotic minerals is challenging due to the superimposition and repetition of geologic processes and the interference of ubiquitous and diverse life on Earth's surface and crust. Mineral dubiofossils, being potential outcomes of both abiotic and biotic environments, emerge as valuable entities that can contribute significantly to the understanding of this issue, facilitating the testing and refinement of biogenicity criteria. The aim of this contribution is to decipher the origin and history of branched mineralized structures that were previously considered mineral dubiofossils from Pennsylvanian of the Paraná Basin, Brazil. While this material has different forms and refers to biological aspects, yet it is challenging to associate it with any known fossil group due to the overlapping geological processes occurring in a transitional deposit of Rio do Sul Formation (Itararé Group of the Paraná Basin), particularly in close proximity to a sill from the Serra Geral Group, which has undergone thermal effects. Given the absence of attributes essential for supporting the initial hypotheses proposing the material as a potential set of sponge spicules or a result of contact metamorphism in Pennsylvanian turbidites, the objects are now investigated as mineral dubiofossils. To address this challenge, we have developed a descriptive protocol for dubiofossils, building upon prior research in the field. This protocol evaluates the following aspects: 1) morphology, texture, and structure; 2) relationship with the matrix; 3) composition; and 4) context. By assessing indigeneity, syngenicity, and comparing the specimens with abiotic and biotic products. Applying this protocol to our samples revealed a wide range of morphologies with internal organization, predominantly composed of calcite with impurities such as iron, magnesium, aluminum, and oxygen. The inferred indigeneity suggests the presence of these minerals can be concurrently or prior to the intrusion of the sill. Extensive comparisons were made between the studied samples and a broad spectrum of abiotic minerals, as well as controlled, induced, and influenced biominerals from similar contexts. These comparative analyses encompassed sponge spicules, sea urchin and algae skeletons, minerals induced or influenced by fungi, bacteria, and microbial mats, as well as inorganic pre- and synsedimentary/eodiagenetic minerals like evaporites, springs, and other precipitates, and mesodiagenetic/metamorphic crystals. Despite this comprehensive analysis, no hypothesis emerged as significantly more likely than others. The comparative analysis did allow us to exclude the possibility of the samples being controlled biominerals due to their patternless diversity of morphologies, as well as purely thermometamorphic in origin due to their branched elongated forms. The occurrence of these structures suggests a complex history: a syndepositional or eodiagenetic origin of some carbonate or sulfate (gypsum, ikaite, dolomite, calcite, aragonite, siderite), potentially associated with the presence of microbial mats, which may have served as templates for mineralization and mediated mineral growth. Mesodiagenesis could have further modified the occurrence through processes such as mineral stabilization, agglutination, aging, and growth. However, the primary agent responsible for the formation of the dubiofossil was the Cretaceous intrusion, which dissolved and replaced the initial minerals, resulting in the precipitation of calcite. Throughout these steps, a combination of physical-chemical and biological reactions, influenced by intrinsic matrix characteristics, organic matter content, and distance from the intrusive body, may have contributed to the heightened morphological complexity observed, thus, corroborate the origin of the material becomes even more challenging. Consequently, both the hypotheses pertaining to the formation of biotic and abiotic sulfates and carbonates remain plausible explanations, hence sustaining the classification of the material as a dubiofossil. This material illustrates how dubiofossils can be a result of a complex history and overlapping geological processes. It also highlights the difficulty in differentiating biominerals from abiotic minerals due to the scarcity of biogenicity arguments.”
For the rest of the manuscript:
We reformulated the second paragraph of the introduction (lines 41-45) in order to clarify the sentences and link the information, as follows:
“Thus, in addition to the conventional perspective that organisms are delimited and conditioned to the environment, there is growing evidence of the significant influence of life on natural processes and events (Knoll, 2013; Davies et al., 2020). As a result, it has become increasingly challenging to recognize large-scale physical and chemical cycles on Earth that are unaffected by biosphere activity (Gargaud et al., 2015). Furthermore, accurately measuring the impact of organisms, which are ubiquitous, on erosion, sedimentation, diagenesis, and mineralization has also become a complex task”.
The introductory paragraph (lines 55-58) raises questions regarding the challenging task of identifying biosignatures within the geological record, given the complex overlap of natural cycles, whether biotic or abiotic. The relevance of biominerals to the issue is addressed later in the introduction (after line 75). Even so, as highlighted the sentence “narrow threshold between biotic and abiotic environments” is vague and has been withdrawn, the paragraph has been rephrased to better align with your expectations. The revised text is presented as follows:
“These biological and geological processes are integral to the Earth system's natural cycles, exhibiting repetition and overlap across multiple scales (Zhang et al., 2017). The Earth's crust and surface are host to a dynamic interaction of physical, chemical, and biological reactions that shape the geological record (Jacobson et al., 2000; Worden and Burley, 2009; Zhang et al., 2017). Consequently, any geological object, whether abiotic or biotic, must be understood in terms of its formation and original conditions, as well as the subsequent processes that contribute to its maintenance, modification, or destruction. Due to the complex interplay of these processes and the ongoing changes throughout geological history, it becomes essential to discern specific life signatures”.
We agree that the debates on biogenicity criteria are not recent dating back to the works of Buick and Hoffman and others cited in the text, in paragraph line 64, the term "emerging" was used to emphasize the expansion in the amount of papers published in the last two decades. To avoid misinterpretation, we agree with the reviewer and replace the sentence on line 64 with: “As an area of science that has received significant attention and prominence in recent years, …”
In Line 72 we change the sentence according to the recommendation, as follows: “are essential for refining …”.
Thank you for suggesting an explanation regarding why the biogenicity criteria cannot be applied to minerals. However, based on the recommendation of another referee, we have removed the paragraph summarizing the establishment of biogenicity criteria for different types of objects, such as microfossil-like and ichnofossil-like artifacts, which emphasized the criteria for minerals. While the information is relevant, it would require further explanations and would expand the introduction excessively. We decided to exclude the sentence. On the other hand, we have made improvements to our introduction following the recommendations of other referees. In the subsequent paragraph, we now explain the theoretical distinctions between biominerals and abiotic minerals and highlight the challenges in differentiating them practically, as presented below. This modification addresses part of the question raised in line 79 and aims to make the text more concise and focused on the objectives of this contribution. We hope this revision aligns with your expectations and those of the other reviewers:
“it lacks arguments to differ purely abiotic minerals from controlled, induced, and influenced biominerals (Dupraz et al., 2009). Essentially, controlled biominerals are minerals that are directly produced and regulated by living organisms that exercise a high level of control over their formation and composition. Induced biominerals are indirectly formed by living organisms, these play an active role in triggering or influencing their formation, producing certain organic compounds or creating specific environmental conditions. Often an indirect result of the metabolic action. In influenced biominerals, there is a passive role in mineral formation or modification caused by the presence of living or dead organisms (see Dupraz et al. 2009 for a broader review), by exclusion abiotic minerals are the result of physical-chemical reactions, without any biological interference. In practice, it is challenging to differentiate each of these products in the geological record due to the lack of diagnostic characteristics, such as specific shapes or crystallographic properties and compositional signatures that resist modifications over time (see Weiner and Dove, 2003; Dupraz et al., 2009). To improve the biogenicity evidence for crystals is essential investigate mineral dubiofossils.”
Thanks for noting the redundancy, we decided to remove the repeated text from the last paragraph of the introduction as recommended (lines 95-96), replacing it with: “these elongated tubes will be examined across the four classes of biogenicity criteria, (1 to 4) explained above”.
At lines 101-103, we apologize for erroneously having automatic Latin text coming from the journal's template, the excerpt has now been removed completely.
We thank you for the detailed review and approve the suggestion to change the placement of the last sentence of Geological settings, including it just after the sentence on line 125, in the middle of the paragraph. We emphasize that the position of the dubiofossils in Figure 8 is indicated in the lateral geological section of Figure 8A, marked by the circular icon that indicates our material and a box with its vertical coverage area close to the contact with the sill.
We agreed that the information on trace fossils (line 144) was isolated in the materials and methods paragraph, and we decided to include it in the text as follows: “(5) indigeneity and syngenicity, we used field-collected data, including some ichnofossils, which were also collected and observed under a stereomicroscope in the laboratory.”
We rewrite the sentence of different morphologies (line 147) as follows: “The morphologies were described, and length, width, and relative angles of the branches were measured.”.
As for the collection numbers, the samples are included in the collection of the Laboratory of Paleontology at UFSC as indicated, their collection number was included (after line 137) as follows: "under the numbers: UFSCLP 395-418, 877-971, 993-1029 totaling 153 samples, the other 100 samples are not included in the collection to avoid redundancy. UFSCLP numbers 1023a and b, 1024-1029 have petrographic slides, stored in the same collection under the number of the respective hand sample." the following topics explain which samples went through the other analyses, some of which were destroyed for Micro CT and XRD.
At line 168: the word “horizontal” has been removed.
As the description of the various morphologies comes right after the sentence on line 180, we chose to exclude the phrase “Thus, geometry ranges from simple to complex.” and we hope that Thank you for suggesting the inclusion of sample amounts by classes, we have inserted a visual estimate just after the general description of the samples (lines 187-189), as follows: "Of the nearly 250 samples, by visual estimation, approximately 40% belong to class A, 35% to class B, 15% to class C and 10% to class D."
As for the issue of samples and morphologies coming from the same stratigraphic level, we have included a new section and thank you for pointing out that this characteristic of the samples was not explicit. We hope that it is now clear and fulfills your expectations. This new paragraph was inserted right after the description of the classes (lines 190-215) and before table 1, as shown:
“Despite the limited stratigraphic control of the collections, the grouping of classes based on different colors/matrix compositions suggests that the morphologies are not consistently present at the same stratigraphic level. It is possible that these forms may occur at various levels with similar compositions. For instance, Class A may or may not be found in the lighter siltstone layers, while Class C may or may not be present in the darker claystone layers. Additionally, it is important to note that there is a possibility of variation within classes occurring at the same stratigraphic level, particularly in the case of Class D. This class exhibits a transition from small needles, similar to the morphotypes of Class A, to dots.”.
We accept the suggestion for line 173 and modify it to “14 m of centimetric heterolithic layers, measuring 0.2 to 4 cm (Fig. 8A)”.
The MISS are included in the geological section of Fig. 8A, the icon referring to ichnofossils also represents the microbial mats found, it was just not indicated in the legend. Thanks for the nomination and the MISS have been added. We hope this resolves the comments for lines 297 and 302.
The Journal's rules are flexible regarding the order of citations in the text, we chose to keep them in chronological order, separating publications by the same author many years apart, keeping them all in chronological order. Therefore, on lines 281-282 it is reordered as: “Balistieri et al., 2002, 2003; Buatois et al., 2006; Gandini et al., 2007; Netto et al., 2009; Lima et al., 2015, 2017; Noll and Netto, 2018; Callefo et al., 2019b; Balistieri et al., 2021; De Barros et al., 2021 Netto et al., 2021”. Other similar list citations were readjusted to this pattern.
Thank to point this detail, we cited the Fig. 5I-L at line 310 to help to discuss the microfabric.
Thanks for the comment about the discussion on lines 327-348, we agree that it was long and with few direct implications for our object of study. We had already modified this passage due to a discussion between the authors. We still consider relevant part of the discussion on trace fossils and MISS for two points: 1) both reveal a paleoenvironmental interpretation of the site distinct from lithostratigraphic data and 2) the occurrence, even if infrequent, of biotic traces with dubiofossils is relevant in Sections 3.4. 1 and 3.5.1. On the other hand, we agree that the discussion about which paleoenvironmental interpretation is more correct is less relevant to the origin of the material. We chose, therefore, to modify the text, shortening this part and making it clearer as shown below:
“The stratigraphic data corroborate the regional interpretation of large turbiditic systems related to melt discharges. The dominance of clayey and silty layers and deformations favor the interpretation of distal turbidites (regional interpretation by Weinschütz and Castro, 2006; Aquino et al., 2016; Schemiko et al., 2019; Vesely et al., 2021). On the other hand, the extensive MISS and Mermia-Scoyenia ichnofacies are interpreted as shallow freshwater lakes, in near marginal marine settings, tidally influenced (lower supratidal) intensively colonized by microbial mats and trace fossil producers, these environments quickly dried up or reduced the water column, evidenced by the dominance of myriapods (other locations interpreted by Balistieri et al., 2002, 2003; Netto et al., 2009; Lima et al., 2015; Noll and Netto, 2018; Callefo et al., 2019b and Balistieri et al., 2021). Both interpretations are postulated for other outcrops of the Itararé group, and further work must be carried out to resolve the issue. As a more detailed description of the outcrop was not carried out and the paleoenvironmental interpretation is not the main objective of this work, both interpretations were considered in the discussion. Even so, the distribution of sand layers and the number of ichnofossils decreasing towards the top may signify a shallowing pattern in any of the interpretations.”
Observation: as no new references were added, we chose not to list the references cited in the excerpts, if necessary, we ask you to check the list of references in the manuscript. Thank you.
At line 349, the problematic features were used as a synonym to dubiofossils. As suggested, we changed to: “These problematic structures…”.
Thank you to review the Figure 3, we change the letter C to D and included the “yellow curved contours” in the caption. We also added more arrows in the plate of Figure 3 and 4 to help to visualize the ramifications.
The second caption was removed and the dark image Figure 5A (right) was improved to facilitate the visualization of the needle structures (attached to check if it's better).
We accepted the suggestions at the Figure 8, replacing the caption for “Geological setting and sample features of the Bemara Quarry”, and the subsequent text by “Serra Geral Group, followed by a schematic section…”. We appreciate these appointments. We also indicate at the Figure 8B where the dubiofossils and the sill are located.
We sincerely apologize for the lengthy responses. Our intention was to thoroughly address each detail and suggestion to ensure the optimal adaptation of the text. We want to emphasize that your questions have been carefully considered, and they have undoubtedly contributed to the improvement of our manuscript. We are genuinely grateful for your valuable input, and we hope that the revised version now meets all the necessary requirements.
Status: closed
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RC1: 'Comment on bg-2023-56', Gregory Retallack, 23 May 2023
These are interesting structures, well described and characterized. My main misgiving is that the paper does not come to a conclusion, other than these structures are not organic skeletal hardparts. Even though they are calcite now, they do not show typical calcite crystal form and are presumably pseudomorphous after other minerals. Resolving their crystallography is needed to make a selection and advance a reasonable explanation.
Also the introduction is more long-winded than necessary (10% reduction would not hurt), and does not address the difference between biologically induced and organic matrix mediated biotic structures. Biologically induced seems more plausible, but the idea that these are actually biological structures seems to be what the text implies. This is explained later on, but be sure to clarify this distinction from the outset.
There are at least 3 alternatives, additional to those discussed, for these small crystal structures which should be addressed somewhere.
- Ice crystal casts are particularly relevant because of independent evidence for glaciation in the rocks in question, and these do have a fossil record. Retallack, G.J., 2021. Ediacaran periglacial sedimentary structures. Journal of Palaeosciences, 70(1-2)), pp.5-30.
- Lichen pruina of oxalate minerals such as weddellite, whewellite or glushinskite, also now have a fossil record Retallack, G.J., 2022. Early Ediacaran lichen from Death Valley, California, USA. Journal of Palaeosciences, 71(2), pp.187-218.
- Acicular evaporite minerals such as mirabilite, also have a fossil record. Retallack, G.J., Gose, B.N. and Osterhout, J.T., 2015. Periglacial paleosols and Cryogenian paleoclimate near Adelaide, South Australia. Precambrian Research, 263, pp.1-18. Retallack, G.J., Chen, Z.Q., Huang, Y. and Fang, Y., 2021. Oxidizing atmosphere and life on land during the late Paleoproterozoic outset of the “boring billion”. Precambrian Research, 364, p.106361.
Kinneyia (l.303) is a misnomer for wrinkle marks. Stimson, Matt, Andrew MacRae, Randall Miller, Steve Hinds, Nic Minter, Zabrina Prescott, Brad Redden, Margaret MacNeil, Sarah Cron, and Stephanie Foster. "Rugalichnus mclaughlinensis: a new ichnospecies of microbially induced sedimentary structure from the nonmarine Early Carboniferous of New Brunswick, Canada." In Fossil Record 6, pp. 281-297. New Mexico Museum of Natural History & Science, 2018. Stimson, M.R., Miller, R.F., MacRae, R.A. and Hinds, S.J., 2017. An ichnotaxonomic approach to wrinkled microbially induced sedimentary structures. Ichnos, 24(4), pp.291-316.
l.15 Fossil Group does not need to be capitalized
l.38 what does this mean? “Ascertaining the biological origin and establishing solid evidence for biogenicity is preponderant”
l.103. why all this Latin here?
l.184 et seq. “sticks” is not the right word here. These appear to be acicular crystals, so “needles” would also work..
l.201 Punctually? Wrong word.
l.273 I have never seen heterolith as a noun, generally heterolithic bed
Figure 11 correct spelling is interstitial
Citation: https://doi.org/10.5194/bg-2023-56-RC1 -
AC1: 'Reply on RC1', João Pedro Saldanha, 07 Jun 2023
We sincerely appreciate the valuable comments and suggestions provided, as they have significantly enhanced the quality of our manuscript and sparked meaningful discussions. Regarding the first point, we acknowledge that the absence of more definitive conclusions may be perceived as a limitation. However, it is important to note that the nature of the material and the inferred geological history impose constraints on achieving a more precise definition. Nevertheless, we have made a concerted effort to address this concern by incorporating two additional paragraphs in the conclusion section (Sect. 4). These paragraphs aim to clarify and enhance the presentation of our results and conclusions. Both are positioned after the description of the 5 steps of the inferred geologic history. The added paragraphs are provided below:
“At each stage, variations in the environmental and physical-chemical characteristics of the substrate play a significant role in shaping the resulting products. Factors such as water content, organic matter, mineral composition, and specific properties of silt and clay layers contribute to the unique conditions for reactions that form and modify the needle-like structures. As a result, distinct processes occur with varying intensities in each silty and muddy layer within this contact section of the turbidites and the sill. These processes include nucleation and agglutination reactions during syndeposition/eodiagenesis, agglutination, stabilization, and mineral growth in mesodiagenesis, dissolution, reprecipitation, replacement, and growth during contact metamorphism, as well as cementation, weathering, and subsequent processes. It is through the interplay of these processes that the diverse forms of dubiofossils emerge.
The precise definition of the original material remains a subject of debate due to two primary factors. Firstly, the morphological diversity observed can be attributed to a succession of processes that have occurred throughout the complex history of the specimen. This has resulted in the presence of diagnostic forms that support a particular hypothesis, as well as other forms that do not refute it. Secondly, the final composition has been influenced by thermometamorphic alteration, which has led to the replacement and modification of the original composition of the recovered calcite needles. This alteration has obscured the initial mineralogy, making it challenging to determine conclusively. As a result, both the hypotheses of biotic and abiotic sulfates and carbonates remain plausible explanations.”
The characteristics of the delicate material and over an undulating surface make it impossible to manufacture thin sections in better quality. Even so, we redescribed all the petrographic slides, carefully evaluating the crystallographic properties and suggesting which minerals are more likely, some characteristics refer to carbonates and others to clay minerals, which are discussed in the composition section: allied to other chemical techniques, they allow inferring the main composition of calcite needle. We hope that now the petrographic description is more complete, as this paragraph has been replaced as follows:
“Despite the different external shapes found in a hand sample, common elements were described in a petrographic thin section that allow inferring that they are the same product (Fig. 5). The needles have a distinct crystallinity from the matrix, generally with well-defined edges, euhedral to subhedral shape, low relief, and nanocrystalline texture. The needles do not show color, twinning or cleavage (Fig. 5A-C). It may have a black opaque central axis in the elongation direction and, less commonly, an opaque brownish outer edge (Fig. 5A-C). Incomplete extinction is oblique with mottled, undulating appearance in larger needles. Birefringence variable depending on needle size, usually low 1st order, but 2nd order present in larger features (Fig. 5C and D; G-H). In general, the needles are organized in layers and may have a central axis that is always opaque, a surrounding layer of greater crystallinity (up to microcrystalline) and high birefringence, and a second layer of 1st order birefringence, other layers may also occur externally as brown linings or faded edge that appears as an irregular gray texture never extinct.
The order and amount of these layers is different between classes. Class A has only the opaque interior and the 1st order layer (Fig. 5A and C); Class B has a more extinct interior, with the two layers of distinct birefringence (2nd order and 1st order; Fig. 5B), sometimes delimited by the opaque lining, lateral branches present a generally extinct central region or the opaque axis (Fig. 5D); Class C presents two opaque axes (different from the others) delimiting an internal portion of 2nd order (Fig. 5E-F); and Class D presents, in radial forms, a less centralized axis and a predominance of 2nd order nanocrystalline material (Fig. 5G-H).
In general, the needles show irregular layers in the direction of the central axis of microporous texture intercalated with smooth or microgranular texture (Fig. 5C), sometimes aligned subspherical blocks (Fig. 5F). The texture and the crystallographic and birefringence variations between the needles make mineral inference difficult. Certainly, the lining is an iron oxide/hydroxide film present in the matrix and that surrounds the needles, the interior, with the most extinguished region or the entirely opaque axis seems to be linked to impurities inside the needle. Mottled extinction refers to a possible clay or phyllosilicate (white mica?), however birefringence resembles a possible carbonate (calcite?), see composition discussion in Sect 3.1.3.”
The introduction was reduced as recommended, before it appeared with about 70 lines now it appears with 66 lines including the differentiation of induced and influenced biominerals as suggested. Now in lines 78-84 is included the sentences:
"Essentially, controlled biominerals are minerals that are directly produced and regulated by living organisms that exercise a high level of control over their formation and composition. Induced biominerals are indirectly formed by living organisms, these play an active role in triggering or influencing their formation, producing certain organic compounds or creating specific environmental conditions. Often an indirect result of the metabolic action. In influenced biominerals, there is a passive role in mineral formation or modification caused by the presence of living or dead organisms (see Dupraz et al. 2009 for a broader review), by exclusion abiotic minerals are the result of physical-chemical reactions, without any biological interference.”
Due to the inappropriate name of Kinneyia discussed in the references indicated by the reviewer, it was decided to exclude the name Kinneyia, keeping it only as “wrinkle structures” (in l.310 and in the caption of the Fig. 9), without major effects for the text or discussion. Thanks a lot for the tip.
The sentence “fossil group" at l.15 was corrected
The sentence " “Ascertaining the biological origin and establishing solid evidence for biogenicity is preponderant” at l.38 was changed to “Acquiring substantial evidence to establish biogenicity is crucial not only for determining the biological origin but also for comprehending the intricate biosphere-lithosphere interface” which makes the sentence clearer.
At l.103 we are sorry because it mistakenly has an automatic Latin text coming from the journal's template, the excerpt has now been removed completely.
The term stick or stick-shape (at l.184 and seq) was chosen due to the length-width ratio not being so intense, even varying between the morphotypes described, in addition to the fact that sometimes the thickness is not fixed along the specimen or does not have tapered ends that would characterize a needle -shape sensu stricto. The use of the term stick in the article by Baucon et al. (2020) cited and compared with the samples also contributed to this choice. However, reviewing the definitions in crystalography, many times both terms are used as synonyms, therefore, whenever possible, it was chosen to use the terms needle and needle-shape as suggested.
at the l. 201 the word “Puctually” was changed to “Rarely”.
at l.273 we corrected the word heterolith to “heterolithic bed”
at the figure 11 the word is now correct spelled, thank you for the thank you for the detailed conference.
The three other alternatives, suggested by the reviewer, were investigated, and added as new topics in the discussion of Section “3.4.1 Syndepositional or diagenetic product”. We are very grateful for the suggestions; they greatly enriched the discussion of the possible original material of the dubiofossils. Like the other alternatives presented in the manuscript previously, there are similar and different characteristics that prevent the classification of the material. The new topics have been appended as shown below:
“Ice casts – freezing minerals: Certain characteristics of dubiofossils suggest that they could be interpreted as ice molds or ice casts. These elongated features are formed when water freezes within silt-dominated mudstones and fine-grained sandstones, in fluvio-lacustrine, marginal marine, or aeolian environments (Dionne, 1985; Pfeifer et al., 2021; Voigt et al., 2021), making their occurrence plausible in a periglacial setting. Moreover, these features are often randomly distributed within the bedding plane without disrupting the layers, as the epistratal ice typically grows horizontally (Voigt et al., 2021). The various shapes observed in these elongated features resemble the different ways in which ice forms under varying temperature conditions, including needle-shaped, branched forms, stubby rods, fanned needles, rosette, and stellate structures (Mason et al., 1963; Pfeifer et al., 2021; Voigt et al., 2021). Additionally, the predominance of a specific morphotype within each slab corresponds to the monotypic pattern observed in ice molds (Voigt et al., 2021). The branching features are explained by cycles of freezing and thawing of water-saturated mud events, possibly occurring on a daily basis, which result in branches forming at acute angles without crossing the principal elongation (Voigt et al., 2021). Particular aspects of dubiofossils, such as crossing branches, distinct from ice casts, can be attributed to diagenetic or thermal modifications. Although similar features have been found in the fossil record, such as those reported by Bandel and Shinaq (2003) and Retallack (2021) in the Precambrian, Pfeifer et al. (2021) and Voigt et al. (2021) in the Permian, linked to LPIA, the corroboration of this hypothesis is hindered by the fact that these structures are typically preserved as epirelief or hyporelief molds formed through the melting of ice crystals and subsequent sedimentary deposition within the resulting cavities (Voigt et al., 2021). It is challenging to explain the syn- or post-depositional preservation of other materials, such as calcite, within these spaces.
Other evaporitic minerals: Other sulphates, such as thenardite, mirabilite, bloedite, loeweite, and glauberite, also exhibit a needle-like morphology in similar geological contexts (Warren, 1999; 2016; Hamdi-Aissa et al., 2004; Benison and Bowen, 2013). The Bemara environment, due to its local temperature conditions, may have provided favorable settings for the formation of these sulfate needles. Once, modern evaporites demonstrate that the nocturnal temperature reduction during winter, reaching close to 0º C, creates the necessary thermodynamic equilibrium for the crystallization of mirabilite and other evaporitic minerals (Hamdi-Aissa et al., 2004; Espinosa-Marzal and Scherer, 2010; Jassim, 2019). It's worth noting that glauberite has also been found in the Karoo basin, in a similar context of LPIA deglacial sequences, although it occurs in concretions (McLachlan and Anderson, 1973). These less common sulphates require a high concentration of specific cations for deposition (Hamdi-Aissa et al., 2004; Warren, 2016) and follow the Usiglio precipitation sequence, necessitating prior carbonate precipitation before their crystallization (see Babel, 2004; Warren, 2010). Consequently, the hypothesis of dubiofossils as evaporitic products becomes less plausible.
Oxalate minerals: oxalates such as whewellite, weddellite, glushinskite, known as organic minerals, occur mainly as biominerals in plants, fungi, algae and in diagenetic, and hydrothermal occurrences (Baran, 2014; Hofmann and Bernasconi, 1998). The hydrothermal origin of the needles is completely discarded, as they do not present features that cut the layers like veins and that should be found as late-stage phase hydrothermal products, in the form of whewellite (Baran, 2014; Hofmann and Bernasconi, 1998) after crystallization of calcite, inversely to what is found in the Bemara outcrop. The shapes and crystallinity also attest against the diagenetic origin of the needles, which although the occurrence of diagenetic whewellite is generally a result of low migration in rocks rich in organic matter, the products are druses, vugs and fissures within septarian concretions, normally larger than 1 cm (see Hofmann and Bernasconi, 1998; for a review). The needles cannot be skeletal parts of the plants and algae, due to their generally elongated format, regular or in small globules (Franceschi and Horner, 1980; Hofmann and Bernasconi, 1998; Francheschi and Nakata, 2007; Baran, 2014), or in the rare occurrences of oxalate crosses included in algae (Pueschel, 2001), much smaller than Bemara needles. However, the origin of dubiofossils as products of mineralization induced/influenced by fungi or lichens is still plausible, mainly the result of the microenvironmental modification of the substrate by the action of hyphae, which leads to the mineralization of whewellite, weddellite (Gadd, 2007; Gadd et al., 2012; 2016; Baran, 2014) or glushinskite for some lichens (Wilson et al., 1980; Baran, 2014). These minerals have varied forms, some branched or ornamented with lateral spines (Whitney, 1989; Dutton and Evans, 1996), similar to dubiofossils. Fungi have a fossil record since the Proterozoic (e.g., Retallack, 2022) and are important degraders of rock and sediments on the surface (Chen et al., 2000; Gadd, 2007; Gadd et al., 2012; 2014), in addition, oxalates are easily modified to calcite under increasing temperature (Baran, 2014). One of the points that argue against this origin is that the needle features are not penetrative in the subsurface (generally subvertical to vertical, Friedmann et al., 1987; Chen et al., 2000; Gadd et al., 2014; Retallack, 2022) and do not they present an expansive distribution from one or more centers, which would be the starting point of mycorrhizae, hyphae or lichens (see Gadd et al., 2014).”
Once again, we thank you for spending your time and pointing out issues that are so relevant to our work. We hope that the modifications meet your expectations and improve the manuscript.
The new references are provided below:
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Bandel, K., Shinaq, R.: Sediments of the Precambrian Wadi Abu Barqa Formation influenced by life and their relation to the Cambrian sandstones in southern Jordan. Freib. Forschungshefte C 499, 78–91, 2003.
Baran, E.J.: Review: Natural oxalates and their analogous synthetic complexes. J. Coord. Chem. 67, 3734–3768. https://doi.org/10.1080/00958972.2014.937340, 2014.
Baucon, A., De Carvalho, C.N., Felletti, F., Cabella, R.: Ichnofossils, cracks or crystals? A test for biogenicity of stick-like structures from vera rubin ridge, mars. Geosci. 10. https://doi.org/10.3390/geosciences10020039, 2020.
Benison, K.C., Bowen, B.B.: Extreme sulfur-cycling in acid brine lake environments of Western Australia. Chem. Geol. 351, 154–167. https://doi.org/10.1016/j.chemgeo.2013.05.018, 2013.
Chen, J., Blume, H.-P., Beyer, L.: Weathering of rocks induced by lichen colonization — a review. CATENA 39, 121–146. https://doi.org/10.1016/S0341-8162(99)00085-5, 2000.
Dionne, J.C.: Formes, figures et faciès sédimentaires glaciels des estrans vaseux des régions froides. Palaeogeogr. Palaeoclimatol. Palaeoecol. 51, 415–451. https://doi.org/10.1016/0031-0182(85)90097-5, 1985.
Dutton, M. V., Evans, C.S.: Oxalate production by fungi: its role in pathogenicity and ecology in the soil environment. Can. J. Microbiol. 42, 881–895. https://doi.org/10.1139/m96-114, 1996.
Espinosa-Marzal, R.M., Scherer, G.W.: Advances in understanding damage by salt crystallization. Acc. Chem. Res. 43, 897–905. https://doi.org/10.1021/ar9002224, 2010.
Franceschi, V.R., Horner, H.T.: Calcium oxalate crystals in plants. Bot. Rev. 46, 361–427. https://doi.org/10.1007/BF02860532, 1980.
Franceschi, V.R., Nakata, P.A.: Calcium oxalate in plants: Formation and function. Annu. Rev. Plant Biol. 56, 41–71. https://doi.org/10.1146/annurev.arplant.56.032604.144106, 2005.
Friedmann, E.J., Weed, R., Land, V.: Abiotic Weathering in the Antarctic Cold Desert. Science (80-. ). 236, 703–705. 1987.
Gadd, G.M., Bahri-Esfahani, J., Li, Q., Rhee, Y.J., Wei, Z., Fomina, M., Liang, X.:Oxalate production by fungi: significance in geomycology, biodeterioration and bioremediation. Fungal Biol. Rev. 28, 36–55. https://doi.org/10.1016/j.fbr.2014.05.001, 2014.
Gadd, G.M., Rhee, Y.J., Stephenson, K., Wei, Z.: Geomycology: Metals, actinides and biominerals. Environ. Microbiol. Rep. 4, 270–296. https://doi.org/10.1111/j.1758-2229.2011.00283.x, 2012.
Gadd, G.M.: Geomycology: biogeochemical transformations of rocks, minerals, metals and radionuclides by fungi, bioweathering and bioremediation. Mycol. Res. 111, 3–49. https://doi.org/10.1016/j.mycres.2006.12.001, 2007
Hamdi-Aissa, B., Valles, V., Aventurier, A., Ribolzi, O.: Soils and Brine Geochemistry and Mineralogy of Hyperarid Desert Playa, Ouargla Basin, Algerian Sahara. Arid L. Res. Manag. 18, 103–126. https://doi.org/10.1080/15324804902796562004, 2004.
Hofmann, B.A., Bernasconi, S.M.: Review of occurrences and carbon isotope geochemistry of oxalate minerals: implications for the origin and fate of oxalate in diagenetic and hydrothermal fluids. Chem. Geol. 149, 127–146. https://doi.org/10.1016/S0009-2541(98)00043-6, 1998.
Jassim, R.Z.: MINERAL RESOURCES OF SODIUM SULFATE IN IRAQ : 289–310. 2019
Mason, B.J., Bryant, G.W., Van den Heuvel, A.P.: The growth habits and surface structure of ice crystals. Philos. Mag. 8, 505–526. https://doi.org/10.1080/14786436308211150, 1963.
McLachlan, I.R., Anderson, A.: A review of the evidence for marine conditions in Southern Africa during Dwyka times, 1973.
Pfeifer, L.S., Birkett, B.A., Driessche, J. Van Den, Pochat, S., Soreghan, G.S.: Ice-crystal traces imply ephemeral freezing in early Permian equatorial Pangea. Geology 49, 1397–1401. https://doi.org/10.1130/G49011.1, 2021.
Pueschel, C.M.: Calcium oxalate crystals in the green alga Spirogyra hatillensis (Zygnematales, Chlorophyta). Int. J. Plant Sci. 162, 1337–1345. https://doi.org/10.1086/322943, 2001.
Retallack, G. J.: Ediacaran periglacial sedimentary structures. J. Palaeosciences 70, 5–30. https://doi.org/10.54991/jop.2021.8, 2021.
Retallack, G.J.: Early Ediacaran lichen from Death Valley, California, USA. J. Palaeosciences 71, 187–218. https://doi.org/10.54991/jop.2022.1841, 2022.
Voigt, S., Oliver, K., Small, B.J.: Potential Ice Crystal Marks From Pennsylvanian–Permian Equatorial Red-Beds of Northwest Colorado, U.S.a. Palaios 36, 377–392. https://doi.org/10.2110/PALO.2021.024, 2021
Warren, J.K.: Evaporites, brines and base metals: What is an evaporite? Defining the rock matrix. Aust. J. Earth Sci. 43, 115–132. https://doi.org/10.1080/08120099608728241, 1996
Warren, J.K.: Evaporites. Springer International Publishing, Cham. https://doi.org/10.1007/978-3-319-13512-0, 2016.
Whitney, K.D.: Systems of Biomineralization in the Fungi, in: Origin, Evolution, and Modern Aspects of Biomineralization in Plants and Animals. Springer US, Boston, MA, pp. 433–441. https://doi.org/10.1007/978-1-4757-6114-6_34, 1989.
Wilson, M.J., Jones, D., Russell, J.D.: Glushinskite, a naturally occurring magnesium oxalate. Mineral. Mag. 43, 837–840. https://doi.org/10.1180/minmag.1980.043.331.02, 1980.
Citation: https://doi.org/10.5194/bg-2023-56-AC1
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RC2: 'Comment on bg-2023-56', Flavia Callefo, 12 Jun 2023
Biogeosciences - Preprint bg-2023-56
Manuscript (preprint): Deciphering the origin of dubiofossils from the Pennsylvanian of the Paraná Basin, Brazil
Reviewer: Flavia Callefo
General considerations: The article brings an extremely necessary and pertinent discussion to the current state of the art of research in Geobiology, Paleobiology and related areas, which is about biogenicity assessment using chemical biosignatures, minerals and reconstructions of biogeochemical or purely mineralogical processes.
Geobiology, which has been constantly benefiting from the application of different analytical techniques for the assessment of biogenicity, and any in-depth discussions based on analytical data contribute significantly to different paleoenvironmental interpretations, revisiting dubiofossils and expanding knowledge about modifications of these signatures in different contexts (environmental, chemical, physical, etc).
In particular, the investigation and discussion about the biogenicity of dubiofossils is of paramount importance, since the biological origin makes a complete difference in any paleoenvironmental interpretations from micro to macro scale.
Introduction:
Line 58-59: “As they are on the threshold of knowledge between abiotic and biotic, dubiofossils, fossil-like structures formerly related to life with an ambiguous origin (Hofmann, 1972), hold promising potential for enhancing biosignatures.” – Actually, the dubiofossils hold the potential to be or not to be considered a biosignature, once its biogenicity is proven or not. I suggest rephrasing this sentence.
Line 81 to 86: I believe it is important to emphasize here also the limitations in separating minerals purely products of inorganic (abiotic) processes from a wide range of biologically induced minerals. This difficulty is recognized in the literature, since this category of biominerals does not present intrinsic characteristics (such as crystallographic properties) different from biotic ones. This is one of the main challenges inherent to this issue of mineral biogenicity.
Line 101-103: “Suspendisse a elit ut leo pharetra cursus sed quis diam. Nullam dapibus, ante vitae congue egestas, sem ex semper orci, vel sodales sapien nibh sed lectus. Etiam vehicula lectus quis orci ultricies dapibus. In sit amet lorem egestas, pretium sem sed, tempus lorem.” Is there a purpose for this sentence or was it a mistake?
Practical issue: in order to better present the problem to the readers, I recommend to increase the reasons of why the presented structures are considered dubiofossils in the Introduction; in the line 94 it is saying that they were previously interpreted as sponge spicules. Is there some reference for this? Why? It`s because of the morphology, paleoenvronmental context? I suggest to explore more and encourage readers to start questioning right here.
Line 97-99: “We endeavor to unravel the intricate history of unique mineral occurrence, which has been shaped by the overlapping effects of geologic processes and the omnipresence of life.”. But in the end (in the conclusions) this not happen. I suggest rewrite here.
Geological settings
Figure 2: There are some dark grey colored areas in this figure, it is quite difficult to read what is inside. I suggest to increase the contrast of the background colors and the letters inside he figure.
Materials and methods
Line 168: “One horizontal thin-section…”. Remove “horizontal”.
Results and discussions
Figure 4: very good figure, showing the structures in high magnification in details.
Figure 7: legend. Replace “Raman specters” by “Raman spectra”.
In all discussion text: maybe replace “sticks” by needles, or acicular structures.
A possibility not discussed: aragonite needles are formed in environmental contexts suggested in the text when discussing the possibilities of formation of calcite or dolomite. What happens is that aragonite, a much less stable polymorph, is easily replaced by calcite, which is more stable. This can preserve the original shape of the needles, as if they were replaced relics.
I particularly see no problem with not reaching a more closed conclusion, that is, leaving the discussion open. I believe that the manuscript presented the most in-depth discussions possible about the possibilities, and from my own experience, I believe that it is of great value for approaching the topic of biogenicity of dubiofossils. What I see here is a good example of how to handle complex possibilities and exercise caution in any assertions.
Citation: https://doi.org/10.5194/bg-2023-56-RC2 -
AC2: 'Reply on RC2', João Pedro Saldanha, 20 Jun 2023
We sincerely appreciate the thorough review of our manuscript. Your valuable suggestions and the attention given to specific details have significantly enhanced the quality of this contribution. We highly value your insights regarding the significance of this investigation and the ensuing discussion, despite the remaining inconclusive nature of the issue. Your comments have been thoughtfully taken into account and the necessary modifications have been made in the manuscript as indicated below:
The paragraph with lines 58-59 has been reworded to clarify the explanation of the importance of dubiofossils as shown below:
Dubiofossils, fossil like structures formerly related to life with and ambiguous origin (Hofmanm, 1972), play a crucial role in enhance biosignatures. Through testing and refinement, the biological nature of a dubiofossil can be established, leading to its classification as a genuine fossil; alternatively, if its origin is determined to be the result of abiotic processes, it is categorized as a pseudofossil (Hofmann, 1972; Monroe and Dietrich, 1990; McMahon et al., 2021). Once the biological origin is confirmed, these dubiofossils can be regarded as potential biosignatures or contain distinctive characteristics indicative of past life (McMahon et al., 2021).
To complement the introduction, emphasizing the difficulty of differentiating abiotic minerals from biotic ones as indicated in lines 81 to 86, we chose to include the sentence below, which we hope resolves the issue:
“In practice, it is challenging to differentiate each of these products in the geological record due to the lack of diagnostic characteristics, such as specific shapes or crystallographic properties and compositional signatures that resist modifications over time (see Weiner and Dove, 2003; Dupraz et al., 2009).”
At lines 101-103 we are sorry because it mistakenly has an automatic Latin text coming from the journal's template, the excerpt has now been removed completely.
As suggested, we added a paragraph to explain de first association with sponge spicules and why the needles cannot be sponge. And it is positioned at the end of the penultimate paragraph as follows:
“This material was previously proposed as sponge spicules from the Paraná Basin (Mouro and Saldanha, 2022), since some formats resemble spicules, the distribution of structures could delimit circular and ellipsoidal features such as flattened bodies, in moreover, close to the outcrop, an earlier stratigraphic unit of similar context contains well-preserved fossil sponges in abundance (see Mouro and Saldanha, 2022). However, the diversity of formats and the absence of spicular net prevented the classification of this material as porifera, remaining as a mineral dubiofossil.”
We understood your recomendation and decided to delete the last part of the sentence in lines 97-99, in fact the discussion of the ubiquitous presence of life was less explored in our results and therefore we chose to leave the sentence as below:
“We endeavor to unravel the intricate history of unique mineral occurrence, which has been shaped by the overlapping effects of abiotic and biotic geologic processes.”.
At the Figure 2 we improved the contrast and put a white buffer around the captions, thank you to improve the visualization of this image.
At line 168: the word “horizontal” has been removed.
Thanks for the compliments on Figure 4, the plate was selected to make the morphological differences clearer.
At the caption of the Figure 7, we changed the terms “Raman specters” by “Raman spectra”, thank you to appoint this detail.
The term "stick" or "stick-shape" was selected to describe these structures due to their moderate length-to-width ratio, which may vary among the described morphotypes. Furthermore, these structures may not always exhibit a consistent thickness throughout the specimen or possess tapered ends that would strictly characterize a needle shape. The decision to use the term "stick" in the article by Baucon et al. (2020), which was cited and compared with the samples, also influenced this choice. However, upon reviewing definitions in crystallography, it became evident that both terms are often used interchangeably. Therefore, whenever possible, the terms "acicular structures," "needle," and "needle-shape" were preferred to ensure clarity and accuracy.
In the first version of the manuscript, we ended up not discussing this hypothesis much due to the need for higher temperature linked to aragonite precipitation. After your suggestion, we reassessed the literature in view of our results and found other factors that favored the explanation of the needles as a possible original aragonite that fit our scenario. For this contribution we thank you very much for deepening our discussion and improving our interpretation. In the manuscript we added a paragraph on aragonite in the discussion, right after discussing biotic calcite as follow:
“Aragonite – biotic and abiotic mineral: aragonite is commonly found in mollusk shells, nacre, and as a high-pressure metamorphic mineral (Lipmann, 1973; Ramakrishna et al., 2017; Toffolo, 2021). While it is typically metastable compared to calcite, there are other occurrences where it is found. Needle-shaped aragonite, normally ranging from 5 to 100 μm, can be secreted by algae or deposited in various environments such as caves, hot springs, shallow seas, and lakes (Lowenstam and Epstein, 1957; Lipmann, 1973; Frisia et al., 2002; Jones, 2017; Ramakrishna et al., 2017). The factors contributing to the precipitation of aragonite needles instead of calcite are the influence of temperature, usually above 25°C (Lipmann, 1972; Jones, 2017; Ramakrishna et al., 2017), a high concentration of Mg+ or a high Mg/Ca ratio (Kitano and Hood, 1962; Hu et al., 2009; Jones, 2017; Ramakrishna et al., 2017), and environments with high CO2 degassing rates (Frisia et al., 2002; Sanchez-Moral et al., 2003; Jones, 2017). In the case of dubiofossils at Bemara, the larger needle size cannot be attributed to higher temperatures, as the paleoenvironmental conditions do not support this explanation. However, the presence of a central axis rich in Mg in the needles may indicate remnants of the high concentration necessary for aragonite deposition, in addition to the possible high rate of degassing due to the mediation/degradation of the mats (see Sanchez-Moral et al., 2003). Despite that aragonite can easily be replaced by calcite, the large needle size, exceeding what is commonly reported in the literature, poses challenges in classifying the material as acicular aragonite.”
Once again, we thank you immensely for the careful evaluation of our manuscript. The suggested details and issues have enriched our work and we hope it will meet your expectations.
The new references are provided below:
Baucon, A., De Carvalho, C.N., Felletti, F., Cabella, R.: Ichnofossils, cracks or crystals? A test for biogenicity of stick-like structures from vera rubin ridge, mars. Geosci. 10. https://doi.org/10.3390/geosciences10020039, 2020.
Del Mouro, L., Saldanha, J.P.: sponge fossil of brazil: review and perspectives. Paleontol. EM DESTAQUE - Bol. Inf. da Soc. Bras. Paleontol. 36, 46–61. https://doi.org/10.4072/paleodest.2021.36.75.03, 2021.
Dupraz, C., Reid, R.P., Braissant, O., Decho, A.W., Norman, R.S., Visscher, P.T.: Processes of carbonate precipitation in modern microbial mats. Earth-Science Rev. 96, 141–162. https://doi.org/10.1016/j.earscirev.2008.10.005, 2009.
Frisia, S., Borsato, A., Fairchild, I.J., McDermott, F., Selmo, E.M.: Aragonite-Calcite Relationships in Speleothems (Grotte De Clamouse, France): Environment, Fabrics, and Carbonate Geochemistry. J. Sediment. Res. 72, 687–699. https://doi.org/10.1306/020702720687, 2002
Hofmann, H. J.: Precambrian remains in Canada: fossils, dubiofossils, and pseudofossils. In: Proceedings of the 24th International Geological Congress, Section. 20-30, 1972.
Hu, Z., Shao, M., Li, H., Cai, Q., Zhong, C., Xianming, Z., Deng, Y.: Synthesis of needle-like aragonite crystals in the presence of magnesium chloride and their application in papermaking. Adv. Compos. Mater. 18, 315–326. https://doi.org/10.1163/156855109X434720, 2009.
Jones, B.: Review of aragonite and calcite crystal morphogenesis in thermal spring systems. Sedimentary Geology, 354, 9-23., 2017.
Kitano, Y., Hood, D.W.:Calcium Carbonate Crystal Forms Formed from Sea Water by Inorganic Processes. J. Oceanogr. Soc. Japan 18, 141–145. https://doi.org/10.5928/kaiyou1942.18.141, 1962.
Lippmann, F.: Sedimentary Carbonate Minerals. Springer Berlin Heidelberg, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-65474-9, 1973.
Lowenstam, H.A., Epstein, S.: On the Origin of Sedimentary Aragonite Needles of the Great Bahama Bank. J. Geol. 65, 364–375. https://doi.org/10.1086/626439, 1957
McMahon, S., Ivarsson, M., Wacey, D., Saunders, M., Belivanova, V., Muirhead, D., Knoll, P., Steinbock, O., Frost, D.A.: Dubiofossils from a Mars‐analogue subsurface palaeoenvironment: The limits of biogenicity criteria. Geobiology 19, 473–488. https://doi.org/10.1111/gbi.12445, 2021.
Monroe, J.S., Dietrich, R. V.: Pseudofossils. Rocks Miner. 65, 150–158. https://doi.org/10.1080/00357529.1990.11761667, 1990.
Ramakrishna, C., Thenepalli, T., Ahn, J.W.: A brief review of aragonite precipitated calcium carbonate (PCC) synthesis methods and its applications-. Korean Chem. Eng. Res. 55, 443–455. https://doi.org/10.9713/kcer.2017.55.4.443, 2017.
Sanchez-Moral, S., Canaveras, J.C., Laiz, L., Saiz-Jimenez, C., Bedoya, J., Luque, L.: Biomediated Precipitation of Calcium Carbonate Metastable Phases in Hypogean Environments: A Short Review. Geomicrobiol. J. 20, 491–500. https://doi.org/10.1080/713851131, 2003.
Toffolo, M.B.: The significance of aragonite in the interpretation of the microscopic archaeological record. Geoarchaeology 36, 149–169. https://doi.org/10.1002/gea.21816, 2021.
Weiner, S., Dove, P.: An Overview of Biomineralization Processes and the Problem of the Vital Effect. Rev. Mineral. Geochemistry 54, 1–29. https://doi.org/10.2113/0540001, 2003.
Citation: https://doi.org/10.5194/bg-2023-56-AC2 -
RC3: 'Reply on AC2', Flavia Callefo, 22 Jun 2023
Dear authors
I appreciate the modifications made by the authors, and the accepted suggestions. In my opinion, the manuscript has reached the enough quality for publication. No further modifications are required from me.
Sincerely
Citation: https://doi.org/10.5194/bg-2023-56-RC3 -
AC4: 'Reply on RC3', João Pedro Saldanha, 03 Jul 2023
We are delighted to learn that the manuscript has met your expectations and is prepared for publication. We sincerely appreciate the effort you put into reviewing our manuscript and evaluating the modifications we implemented. Once again, we extend our gratitude for your valuable contributions and suggestions, which have significantly enriched our work.
Citation: https://doi.org/10.5194/bg-2023-56-AC4
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AC4: 'Reply on RC3', João Pedro Saldanha, 03 Jul 2023
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RC3: 'Reply on AC2', Flavia Callefo, 22 Jun 2023
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AC2: 'Reply on RC2', João Pedro Saldanha, 20 Jun 2023
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RC4: 'Comment on bg-2023-56', Luana Morais, 27 Jun 2023
The manuscript by Saldanha et al. entitled "Deciphering the origin of dubiofossils from the Pennsylvanian of the Paraná Basin, Brazil" discusses stick-shaped (needle-like shape) structures, branched or not, and its possible relationships with fossils. For this, the authors used several techniques to highlight the composition, mineralogy, morphology and distribution of these structures and created an analysis protocol to test the biogenicity and indigeneity of the investigated material, which can be certainly useful for taphonomic studies and for interpretation of the structures assigned as biologic in origin.
The work is well written, figures are well presented and the results obtained through the techniques used may help to answer the questions raised.
My main concern is regarding two points:
- The authors' conclusion is not clear. Are these structures dubiofossils or pseudofossils? May this conclusion impossible to reach? Why? These arguments should be clear in the abstract.
- The reasons that led the authors to believe that the structures could be fossils is not clear either. Due to the lack of comparison with recognized fossils throughout the manuscript (until the topic 3.3), may suggest that these structures are diagenetic features typical of the formation of inorganic minerals. The reasons that led the authors to questioning this possibility are unclear. This needs to be clear throughout the abstract and introduction, not only in the discussion.
Below, I present my suggestions and doubts.
Abstract
Line 13: The meaning of the threshold between abiotic and biotic minerals sounds vague without some examples. I suggest drawing attention to at least one example of a mineral dubiofossil and the implication of its interpretation.
Lines 13-14: Although this manuscript may contribute to the investigation of general mineral dubiofossils described in the literature, I suggest to include the geological unit and age, following the sentence “ The aim of this contribution is to decipher the origin and history of branched mineralized structures that were previously considered mineral dubiofossils.” In this way, the text makes it clear that the aim of this manuscript is to understand the taphonomic patchways in this specific case (Rio do Sul Formation, Pennsylvanian of the Paraná Basin).
Lines 14-15: Although it may be hard to attribute the dubiofossils reported here with any known biological affinity, I believe that the authors should compare these structures with possible fossil groups, even if it is to rule out the hypothesis. In this case, I suggest briefly mentioning what are the possibilities here, including what features could be interpreted as belonging to any fossil group and why these possibilities are ruled out.
Lines 19-22: This sentence is too long and confusing. I ́m not sure if I got the author's point. If I did, I suggest: "An extensive comparison with abiotic minerals and controlled, induced, and influenced biominerals excluded the possibility of the samples investigated here being a controlled biomineral, due to its patternless diversity of forms and the purely thermometamorphic origin due to the branched elongated form.”
Lines 24-25: “Mesodiagenesis could have also modified the occurrence …”. Ok. May the authors explain, in a few words, how these modifications could have occurred during mesodiagenesis?
Line 29: Please, include “a” before “result”
Lines 45-46: What is the conclusion of this manuscript? Are the dubiofossils now interpreted as pseudofossils? Or do they still fit in the “dubiofossils” category? Please, clarify it.
Introduction
Lines 41-45: The relation of these sentences is confusing. The dot between them suggests that new information it's coming. However, the second sentence appears to be a continuation (without any link) of the first sentence. Please, clarify it.
Lines 55-58: This “narrow threshold between biotic and abiotic environments” is still vague. Could the authors explain better what they understand as a narrow threshold? Could it be information obtained from compositional signals? Crystallographic characters, in the case of biominerals? Are the authors referring to possible changes during superimposition of diagenetic processes during geologic history? What changes and what are the possible implications for the investigation of biominerals ? Please, clarify it.
Line 64: Is the biogenecity criteria really an emerging area of science? This has been an extensively debated topic since (at least) Buick (1984). Reference: Carbonaceous filaments from North Pole, Western Australia: Are they fossil bacteria in Archean stromatolites?
Line 72: I suggest to change “can be essential for refining…” for “are essential for refining …”.
Line 79: Is important to explain why well-established biogenicity criteria can rarely be applied to minerals. Because of that, I suggest including this argument in the following paragraph, where the authors give the reasons for it.
Lines 95-96: This information was already mentioned in lines 66-67. I suggest removing this sentence (from here, or from lines 66-67) to avoid repetition of information.
Lines 100-101: I suggest including this protocol proposal in the abstract.
Lines 101-103: “Suspendisse a elit ut leo pharetra cursus sed quis diam. Nullam dapibus, ante vitae congue egestas, sem ex semper orci, vel sodales sapien nibh sed lectus. Etiam vehicula lectus quis orci ultricies dapibus. In sit amet lorem egestas, pretium sem sed, tempus lorem.” ????
Geological settings
Lines 123-125: I suggest including the last sentence of this topic (“The detailed outcrop description with sedimentary structures and biotic elements is presented at the Sect. 3.1.4”), following this sentence. Additionally, could it be possible to include the position where the dubiofossils occur in Figure 8?
Material and methods
Line 144: I do not think that the information about the ichnofossil occurrence fits in this topic.
Line 147: How many morphologies were described? Additionally, here is an appropriate place to include the number collection of these thin sections.
Line 147: Does this sample have a number collection? Same question for lines 157, 161 and 168.
Line 168: Why have the authors chosen to make a horizontal thin-section? Is it relevant to say “horizontal!”?
Results and discussion
Line 180: What does simple and complex mean in this case? What characters define the geometry as simple or complex?
Lines 187-189: I suggest to include the approximated total number of specimens per category (at least the measured specimens). Do all categories occur at the same stratigraphic level?
Line 273: I suggest replacing “14 m of heterolith (Fig. 8A) defined by tabular centimetric layers…” by “14 m of centimetric heterolithic layers, measuring 0.2 to 4 cm (Fig. 8A)…”.
Line 279: Could the authors include MISS in the schematic section from Figure 8A?
Lines 281-282: Shouldn't the references follow a chronological order?
Line 302: Same comment for Line 279.
Line 310: I suggest mentioning Figure 5I-J to discuss microfabric.
Lines 327-348:I did not understand the relevance of this discussion for this manuscript. What is the implication of this discussion for the dubiofossils and their origin, once they occur “sometimes” (Line 350) associated with trace fossils and miss?
Line 349: Problematic features and dubiofossils mean the same thing? If so, I suggest replacing “Problematic features” by “These problematic structures…”.
Lines 395-399: This information should be certainly included in the abstract and briefly commented in the introduction.
Line 451: If the controlled biomineral hypotheses have been eliminated, this information should also be included in the abstract.
Figures
Figure 3: Replace the second “C” by “D”. Include “yellow” before “curved contours”. Additionally, It is hard to visualize ramifications in B, F and H. Maybe an arrow could help.
Figure 4: It is hard to visualize ramifications in C-G. Maybe an arrow could help.
Figure 5: "Figure 5” in the figure caption is duplicated. Please, remove one of them. Figure 5A (right) is too dark. Could the authors improve this image?
Figure 8: I suggest replacing “Local geologic features” by "Geological setting and sample features of the Bemara Quarry”. I also suggest replacing the comma between “Serra Geral Group” and “ a schematic section” by “...Serra Geral Group, followed by a schematic section of…”. Figure 8B: Where is the dubiofossils occurrence and the sill, according to the dashed line?
Citation: https://doi.org/10.5194/bg-2023-56-RC4 -
AC3: 'Reply on RC4', João Pedro Saldanha, 28 Jun 2023
We sincerely appreciate your dedication and the meticulous review of our manuscript. We agree with all the notes provided and have made every effort to address each question comprehensively. Regarding the two main points, we have implemented a series of modifications throughout the text to enhance clarity and engage the reader from the outset. We are extremely pleased with these changes and firmly believe that the manuscript has been significantly improved.
Regarding the first main point, we sought to emphasize in the abstract, introduction, and conclusion the inherent challenge of corroborating the origin of the material. We have clearly described the complex history and highlighted how extensive metamorphism has altered a significant portion of the original evidence.
For example, in the abstract, we have included phrases that explicitly convey these aspects: “corroborate the origin of the material becomes even more challenging. Consequently, both the hypotheses pertaining to the formation of biotic and abiotic sulfates and carbonates remain plausible explanations, hence sustaining the classification of the material as a dubiofossil. This material illustrates how dubiofossils can be a result of a complex history and overlapping geological processes.”.
At the conclusion we added and improved the text arguing: “Itararé’s dubiofossils are products of nature, exhibiting a wide range of morphologies that distinguish them from any known minerals. The absence of a consistent pattern in their diverse forms helps dismiss hypotheses suggesting controlled biomineralization. However, it remains uncertain whether the material could have originated as an abiotic mineral or as an induced or influenced biomineral. The complexity of their geological history and the multitude of contributing factors have resulted in this distinctiveness. Consequently, we propose that dubiofossils are likely the outcome of a combination of processes and a complex geological history.” and after: “The precise definition of the original material remains a subject of debate due to two primary factors. Firstly, the morphological diversity observed can be attributed to a succession of processes that have occurred throughout the complex history of the specimen. This has resulted in the presence of diagnostic forms that support a particular hypothesis, as well as other forms that do not refute it. Secondly, the final composition has been influenced by thermometamorphic alteration, which has led to the replacement and modification of the original composition of the recovered calcite needles. This alteration has obscured the initial mineralogy, making it challenging to determine conclusively. As a result, both the hypotheses of biotic and abiotic sulfates and carbonates remain plausible explanations and the material remains as a dubiofossil.”
As for the second point, we have taken steps to elucidate the preliminary proposals regarding the material's origin and the motivations that led us to classify it as a dubiofossil. In the introduction, we have provided a comprehensive argument, stating that the material's diverse forms and absence of a distinct pattern hinder its initial classification as either a sponge or a metamorphic pseudofossil. These sentences have been incorporated into the abstract and introduction, respectively:
“Given the absence of attributes essential for supporting the initial hypotheses proposing the material as a potential set of sponge spicules or a result of contact metamorphism in Pennsylvanian turbidites, the objects are now investigated as mineral dubiofossils.”
“This material was previously proposed as sponge spicules from the Paraná Basin (Mouro and Saldanha, 2021), since some formats resemble spicules, the distribution of structures could delimit circular and ellipsoidal features such as flattened bodies, in moreover, close to the outcrop, an earlier stratigraphic unit of similar context contains well-preserved fossil sponges in abundance (see Mouro and Saldanha, 2021). However, the diversity of formats and the absence of spicular net prevented the classification of this material as porifera, on the other hand, the diversity of formats demonstrate dissimilarities with diagenetic/metamorphic products in a preliminary comparison, remaining as a mineral dubiofossil.”
We have also made minor adjustments to guide the reader towards these key insights throughout the text and facilitate their understanding of our discussions and conclusions. With these updates, we believe that the questions raised have been effectively addressed, and reading the manuscript will meet your expectations.
We express our gratitude for your valuable time and efforts in reviewing and enhancing the abstract. In the initial version of the manuscript, we acknowledged the absence of several crucial details. Therefore, we have thoroughly reworked the abstract, incorporating all the recommended revisions. We believe that the revised version represents a significant improvement, addressing your expectations and providing comprehensive answers to your inquiries.
At line 13, we rewrite the sentence to exclude the vagueness of the “threshold between abiotic and biotic”, replacing by: “Mineral dubiofossils, being potential outcomes of both abiotic and biotic environments, emerge as valuable entities that can contribute significantly to the understanding of this issue, facilitating the testing and refinement of biogenicity criteria.”
As recommended, at lines 13-14, we added the age and unit to specify our object as: “mineral dubiofossils from Pennsylvanian of the Paraná Basin, Brazil”.
As suggested at lines 14-15, we have emphasized the previous proposals biotic and abiotic for the dubiofossils as: “Given the absence of attributes essential for supporting the initial hypotheses proposing the material as a potential set of sponge spicules or a result of contact metamorphism in Pennsylvanian turbidites, the objects are now investigated as mineral dubiofossils” and arguing below why it cannot be one of these hypotheses. “The comparative analysis did allow us to exclude the possibility of the samples being controlled biominerals due to their patternless diversity of morphologies, as well as purely thermometamorphic in origin due to their branched elongated forms.”
The suggestion at Lines 100-101 to put the protocol in the abstract was accepted, as show: To address this challenge, we have developed a descriptive protocol for dubiofossils, building upon prior research in the field. This protocol evaluates the following aspects: 1) morphology, texture, and structure; 2) relationship with the matrix; 3) composition; and 4) context. By assessing indigeneity, syngenicity, and comparing the specimens with abiotic and biotic products.”.
We accepted the suggestion at lines 19-22 and improved the sentence as: “Extensive comparisons were made between the studied samples and a broad spectrum of abiotic minerals, as well as controlled, induced, and influenced biominerals from similar contexts. These comparative analyses encompassed sponge spicules, sea urchin and algae skeletons, minerals induced or influenced by fungi, bacteria, and microbial mats, as well as inorganic pre- and synsedimentary/eodiagenetic minerals like evaporites, springs, and other precipitates, and mesodiagenetic/metamorphic crystals. Despite this comprehensive analysis, no hypothesis emerged as significantly more likely than others.”.
We added the processes of modification of the dubiofossils during the mesodiagenesis to complete the phrase at lines 24-25, as follow: “Mesodiagenesis could have further modified the occurrence through processes such as mineral stabilization, agglutination, aging, and growth”.
The suggestion at Lines 45-46 was accepted and we emphasized at the abstract what is our conclusion, indicating as: “corroborate the origin of the material becomes even more challenging. Consequently, both the hypotheses pertaining to the formation of biotic and abiotic sulfates and carbonates remain plausible explanations, hence sustaining the classification of the material as a dubiofossil.”. That our material remain as dubiofossil.
We changed the sentence at line 29 for “dubiofossils can be a result of a complex history”, thank you for the improvement.
Once again we are grateful for contributions to our abstract. All suggestions were included and improved our text. Below is the complete abstract to check, since we modified a large part of it:
“Minerals are the fundamental record of abiotic processes over time, while biominerals, are one of the most common records of life due to their easy preservation and abundance. However, distinguishing between biominerals and abiotic minerals is challenging due to the superimposition and repetition of geologic processes and the interference of ubiquitous and diverse life on Earth's surface and crust. Mineral dubiofossils, being potential outcomes of both abiotic and biotic environments, emerge as valuable entities that can contribute significantly to the understanding of this issue, facilitating the testing and refinement of biogenicity criteria. The aim of this contribution is to decipher the origin and history of branched mineralized structures that were previously considered mineral dubiofossils from Pennsylvanian of the Paraná Basin, Brazil. While this material has different forms and refers to biological aspects, yet it is challenging to associate it with any known fossil group due to the overlapping geological processes occurring in a transitional deposit of Rio do Sul Formation (Itararé Group of the Paraná Basin), particularly in close proximity to a sill from the Serra Geral Group, which has undergone thermal effects. Given the absence of attributes essential for supporting the initial hypotheses proposing the material as a potential set of sponge spicules or a result of contact metamorphism in Pennsylvanian turbidites, the objects are now investigated as mineral dubiofossils. To address this challenge, we have developed a descriptive protocol for dubiofossils, building upon prior research in the field. This protocol evaluates the following aspects: 1) morphology, texture, and structure; 2) relationship with the matrix; 3) composition; and 4) context. By assessing indigeneity, syngenicity, and comparing the specimens with abiotic and biotic products. Applying this protocol to our samples revealed a wide range of morphologies with internal organization, predominantly composed of calcite with impurities such as iron, magnesium, aluminum, and oxygen. The inferred indigeneity suggests the presence of these minerals can be concurrently or prior to the intrusion of the sill. Extensive comparisons were made between the studied samples and a broad spectrum of abiotic minerals, as well as controlled, induced, and influenced biominerals from similar contexts. These comparative analyses encompassed sponge spicules, sea urchin and algae skeletons, minerals induced or influenced by fungi, bacteria, and microbial mats, as well as inorganic pre- and synsedimentary/eodiagenetic minerals like evaporites, springs, and other precipitates, and mesodiagenetic/metamorphic crystals. Despite this comprehensive analysis, no hypothesis emerged as significantly more likely than others. The comparative analysis did allow us to exclude the possibility of the samples being controlled biominerals due to their patternless diversity of morphologies, as well as purely thermometamorphic in origin due to their branched elongated forms. The occurrence of these structures suggests a complex history: a syndepositional or eodiagenetic origin of some carbonate or sulfate (gypsum, ikaite, dolomite, calcite, aragonite, siderite), potentially associated with the presence of microbial mats, which may have served as templates for mineralization and mediated mineral growth. Mesodiagenesis could have further modified the occurrence through processes such as mineral stabilization, agglutination, aging, and growth. However, the primary agent responsible for the formation of the dubiofossil was the Cretaceous intrusion, which dissolved and replaced the initial minerals, resulting in the precipitation of calcite. Throughout these steps, a combination of physical-chemical and biological reactions, influenced by intrinsic matrix characteristics, organic matter content, and distance from the intrusive body, may have contributed to the heightened morphological complexity observed, thus, corroborate the origin of the material becomes even more challenging. Consequently, both the hypotheses pertaining to the formation of biotic and abiotic sulfates and carbonates remain plausible explanations, hence sustaining the classification of the material as a dubiofossil. This material illustrates how dubiofossils can be a result of a complex history and overlapping geological processes. It also highlights the difficulty in differentiating biominerals from abiotic minerals due to the scarcity of biogenicity arguments.”
For the rest of the manuscript:
We reformulated the second paragraph of the introduction (lines 41-45) in order to clarify the sentences and link the information, as follows:
“Thus, in addition to the conventional perspective that organisms are delimited and conditioned to the environment, there is growing evidence of the significant influence of life on natural processes and events (Knoll, 2013; Davies et al., 2020). As a result, it has become increasingly challenging to recognize large-scale physical and chemical cycles on Earth that are unaffected by biosphere activity (Gargaud et al., 2015). Furthermore, accurately measuring the impact of organisms, which are ubiquitous, on erosion, sedimentation, diagenesis, and mineralization has also become a complex task”.
The introductory paragraph (lines 55-58) raises questions regarding the challenging task of identifying biosignatures within the geological record, given the complex overlap of natural cycles, whether biotic or abiotic. The relevance of biominerals to the issue is addressed later in the introduction (after line 75). Even so, as highlighted the sentence “narrow threshold between biotic and abiotic environments” is vague and has been withdrawn, the paragraph has been rephrased to better align with your expectations. The revised text is presented as follows:
“These biological and geological processes are integral to the Earth system's natural cycles, exhibiting repetition and overlap across multiple scales (Zhang et al., 2017). The Earth's crust and surface are host to a dynamic interaction of physical, chemical, and biological reactions that shape the geological record (Jacobson et al., 2000; Worden and Burley, 2009; Zhang et al., 2017). Consequently, any geological object, whether abiotic or biotic, must be understood in terms of its formation and original conditions, as well as the subsequent processes that contribute to its maintenance, modification, or destruction. Due to the complex interplay of these processes and the ongoing changes throughout geological history, it becomes essential to discern specific life signatures”.
We agree that the debates on biogenicity criteria are not recent dating back to the works of Buick and Hoffman and others cited in the text, in paragraph line 64, the term "emerging" was used to emphasize the expansion in the amount of papers published in the last two decades. To avoid misinterpretation, we agree with the reviewer and replace the sentence on line 64 with: “As an area of science that has received significant attention and prominence in recent years, …”
In Line 72 we change the sentence according to the recommendation, as follows: “are essential for refining …”.
Thank you for suggesting an explanation regarding why the biogenicity criteria cannot be applied to minerals. However, based on the recommendation of another referee, we have removed the paragraph summarizing the establishment of biogenicity criteria for different types of objects, such as microfossil-like and ichnofossil-like artifacts, which emphasized the criteria for minerals. While the information is relevant, it would require further explanations and would expand the introduction excessively. We decided to exclude the sentence. On the other hand, we have made improvements to our introduction following the recommendations of other referees. In the subsequent paragraph, we now explain the theoretical distinctions between biominerals and abiotic minerals and highlight the challenges in differentiating them practically, as presented below. This modification addresses part of the question raised in line 79 and aims to make the text more concise and focused on the objectives of this contribution. We hope this revision aligns with your expectations and those of the other reviewers:
“it lacks arguments to differ purely abiotic minerals from controlled, induced, and influenced biominerals (Dupraz et al., 2009). Essentially, controlled biominerals are minerals that are directly produced and regulated by living organisms that exercise a high level of control over their formation and composition. Induced biominerals are indirectly formed by living organisms, these play an active role in triggering or influencing their formation, producing certain organic compounds or creating specific environmental conditions. Often an indirect result of the metabolic action. In influenced biominerals, there is a passive role in mineral formation or modification caused by the presence of living or dead organisms (see Dupraz et al. 2009 for a broader review), by exclusion abiotic minerals are the result of physical-chemical reactions, without any biological interference. In practice, it is challenging to differentiate each of these products in the geological record due to the lack of diagnostic characteristics, such as specific shapes or crystallographic properties and compositional signatures that resist modifications over time (see Weiner and Dove, 2003; Dupraz et al., 2009). To improve the biogenicity evidence for crystals is essential investigate mineral dubiofossils.”
Thanks for noting the redundancy, we decided to remove the repeated text from the last paragraph of the introduction as recommended (lines 95-96), replacing it with: “these elongated tubes will be examined across the four classes of biogenicity criteria, (1 to 4) explained above”.
At lines 101-103, we apologize for erroneously having automatic Latin text coming from the journal's template, the excerpt has now been removed completely.
We thank you for the detailed review and approve the suggestion to change the placement of the last sentence of Geological settings, including it just after the sentence on line 125, in the middle of the paragraph. We emphasize that the position of the dubiofossils in Figure 8 is indicated in the lateral geological section of Figure 8A, marked by the circular icon that indicates our material and a box with its vertical coverage area close to the contact with the sill.
We agreed that the information on trace fossils (line 144) was isolated in the materials and methods paragraph, and we decided to include it in the text as follows: “(5) indigeneity and syngenicity, we used field-collected data, including some ichnofossils, which were also collected and observed under a stereomicroscope in the laboratory.”
We rewrite the sentence of different morphologies (line 147) as follows: “The morphologies were described, and length, width, and relative angles of the branches were measured.”.
As for the collection numbers, the samples are included in the collection of the Laboratory of Paleontology at UFSC as indicated, their collection number was included (after line 137) as follows: "under the numbers: UFSCLP 395-418, 877-971, 993-1029 totaling 153 samples, the other 100 samples are not included in the collection to avoid redundancy. UFSCLP numbers 1023a and b, 1024-1029 have petrographic slides, stored in the same collection under the number of the respective hand sample." the following topics explain which samples went through the other analyses, some of which were destroyed for Micro CT and XRD.
At line 168: the word “horizontal” has been removed.
As the description of the various morphologies comes right after the sentence on line 180, we chose to exclude the phrase “Thus, geometry ranges from simple to complex.” and we hope that Thank you for suggesting the inclusion of sample amounts by classes, we have inserted a visual estimate just after the general description of the samples (lines 187-189), as follows: "Of the nearly 250 samples, by visual estimation, approximately 40% belong to class A, 35% to class B, 15% to class C and 10% to class D."
As for the issue of samples and morphologies coming from the same stratigraphic level, we have included a new section and thank you for pointing out that this characteristic of the samples was not explicit. We hope that it is now clear and fulfills your expectations. This new paragraph was inserted right after the description of the classes (lines 190-215) and before table 1, as shown:
“Despite the limited stratigraphic control of the collections, the grouping of classes based on different colors/matrix compositions suggests that the morphologies are not consistently present at the same stratigraphic level. It is possible that these forms may occur at various levels with similar compositions. For instance, Class A may or may not be found in the lighter siltstone layers, while Class C may or may not be present in the darker claystone layers. Additionally, it is important to note that there is a possibility of variation within classes occurring at the same stratigraphic level, particularly in the case of Class D. This class exhibits a transition from small needles, similar to the morphotypes of Class A, to dots.”.
We accept the suggestion for line 173 and modify it to “14 m of centimetric heterolithic layers, measuring 0.2 to 4 cm (Fig. 8A)”.
The MISS are included in the geological section of Fig. 8A, the icon referring to ichnofossils also represents the microbial mats found, it was just not indicated in the legend. Thanks for the nomination and the MISS have been added. We hope this resolves the comments for lines 297 and 302.
The Journal's rules are flexible regarding the order of citations in the text, we chose to keep them in chronological order, separating publications by the same author many years apart, keeping them all in chronological order. Therefore, on lines 281-282 it is reordered as: “Balistieri et al., 2002, 2003; Buatois et al., 2006; Gandini et al., 2007; Netto et al., 2009; Lima et al., 2015, 2017; Noll and Netto, 2018; Callefo et al., 2019b; Balistieri et al., 2021; De Barros et al., 2021 Netto et al., 2021”. Other similar list citations were readjusted to this pattern.
Thank to point this detail, we cited the Fig. 5I-L at line 310 to help to discuss the microfabric.
Thanks for the comment about the discussion on lines 327-348, we agree that it was long and with few direct implications for our object of study. We had already modified this passage due to a discussion between the authors. We still consider relevant part of the discussion on trace fossils and MISS for two points: 1) both reveal a paleoenvironmental interpretation of the site distinct from lithostratigraphic data and 2) the occurrence, even if infrequent, of biotic traces with dubiofossils is relevant in Sections 3.4. 1 and 3.5.1. On the other hand, we agree that the discussion about which paleoenvironmental interpretation is more correct is less relevant to the origin of the material. We chose, therefore, to modify the text, shortening this part and making it clearer as shown below:
“The stratigraphic data corroborate the regional interpretation of large turbiditic systems related to melt discharges. The dominance of clayey and silty layers and deformations favor the interpretation of distal turbidites (regional interpretation by Weinschütz and Castro, 2006; Aquino et al., 2016; Schemiko et al., 2019; Vesely et al., 2021). On the other hand, the extensive MISS and Mermia-Scoyenia ichnofacies are interpreted as shallow freshwater lakes, in near marginal marine settings, tidally influenced (lower supratidal) intensively colonized by microbial mats and trace fossil producers, these environments quickly dried up or reduced the water column, evidenced by the dominance of myriapods (other locations interpreted by Balistieri et al., 2002, 2003; Netto et al., 2009; Lima et al., 2015; Noll and Netto, 2018; Callefo et al., 2019b and Balistieri et al., 2021). Both interpretations are postulated for other outcrops of the Itararé group, and further work must be carried out to resolve the issue. As a more detailed description of the outcrop was not carried out and the paleoenvironmental interpretation is not the main objective of this work, both interpretations were considered in the discussion. Even so, the distribution of sand layers and the number of ichnofossils decreasing towards the top may signify a shallowing pattern in any of the interpretations.”
Observation: as no new references were added, we chose not to list the references cited in the excerpts, if necessary, we ask you to check the list of references in the manuscript. Thank you.
At line 349, the problematic features were used as a synonym to dubiofossils. As suggested, we changed to: “These problematic structures…”.
Thank you to review the Figure 3, we change the letter C to D and included the “yellow curved contours” in the caption. We also added more arrows in the plate of Figure 3 and 4 to help to visualize the ramifications.
The second caption was removed and the dark image Figure 5A (right) was improved to facilitate the visualization of the needle structures (attached to check if it's better).
We accepted the suggestions at the Figure 8, replacing the caption for “Geological setting and sample features of the Bemara Quarry”, and the subsequent text by “Serra Geral Group, followed by a schematic section…”. We appreciate these appointments. We also indicate at the Figure 8B where the dubiofossils and the sill are located.
We sincerely apologize for the lengthy responses. Our intention was to thoroughly address each detail and suggestion to ensure the optimal adaptation of the text. We want to emphasize that your questions have been carefully considered, and they have undoubtedly contributed to the improvement of our manuscript. We are genuinely grateful for your valuable input, and we hope that the revised version now meets all the necessary requirements.
João Pedro Saldanha Saldanha et al.
João Pedro Saldanha Saldanha et al.
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