the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Serpulid-microbialitic bioconstructions from the upper Sarmatian (Middle Miocene) of the Central Paratethys Sea (NW Hungary) – witnesses of a microbial sea
Mathias Harzhauser
Oleg Mandic
Werner E. Piller
Abstract. We present so far unknown stacked bowl-shaped bioherms reaching a size of 45 cm in diameter and 40 cm in height from weakly solidified peloidal sand from the upper Sarmatian of the Paratethys Sea. The bioherms were mostly embedded in sediment and the ‘stages’ reflect a reaction on sediment accretion and sinking into the soft sediment. The bioherms are spirorbid-microclots-acicular cement boundstones with densely packed Janua tubes surrounded by microclots and acicular cement solidifying the bioherm. The surrounding sediment is a thrombolite made of peloids and polylobate particles (mesoclots) which are solidified synsedimentarily by micrite cement and dog-tooth cement in a later stage. The shape of the bioherms reflects a series of growth stages with an initial stage (‘start-up’ phase) followed by a more massive ‘keep-up stage’ which grades into a structure with a collar-like outer rim and a central protrusion and finally by a termination of growth (‘give-up stage’). The setting was a shallow subtidal environment with normal marine or hypersaline, oligotrophic conditions with an elevated alkalinity. The stacked bowl-shaped microbialites are a unique feature so far undescribed. Modern and Neogene microbialite occurrences are no direct analogues to the described structures but the marine examples, like at the Bahamas, Shark Bay and the Persian Gulf offer insight into the microbial composition and environmental parameters.
The microbialites and the surrounding sediment document a predominance of microbial activity in the shallow marine environments of the Paratethys Sea during the late Middle Miocene, which was characterized by a warm, arid climate.
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Mathias Harzhauser et al.
Status: final response (author comments only)
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RC1: 'Comment on bg-2023-105', Anonymous Referee #1, 04 Sep 2023
The paper deals with new metazoan/microbial build-ups formed by Janua tubes and autochthonous peloidal micrite, from the upper Sarmatian of the Paratethys Sea. The Authors describe these unusual bioconstructions and interpreted them in term of paleoenvironmental evolution of the Paratethys Sea during the late Middle Miocene.
The peculiarity of the described bioherms, their comparison with other microbial/metazoan bioconstructions spanning in time and space and their paleoecological and paleoenvironmental implication make the research novel and suitable for the Biogeosciences journal.
The paper is interesting, well-structured and clearly written, the title reflects the contents of the paper and the abstract provide a complete summary. The main concern is the interpretation of the microbialite component with the only observation at mesoscale and thin sections. In my opinion the paper can benefit of more detailed micromorphological, mineralogical and biogeochemical analyses to prove irrefutably the biotic nature of the micritic sediments and their interpretation as microbial carbonates.
Specific comment and suggestions:
Introduction
The introduction on the paleoecological evolution and bioconstructions of the Central Paratethys Sea is well addressed and introduces the readers to the topic, but the paragraph lacks the aim of the present research, for example:
- why these biconstructions were studied?
- what new aspects want to be evaluated in this research compared to those described in previous papers?
- what the research wants to evaluate about the role of the various skeletal and non-skeletal organisms in the formation of these build-ups and what palaeoenvironmental indications can be deduced from them?
- how these new data can be interpreted in terms of the palaeoenvironmental evolution of the Parathethys Sea?
All these aspects were addressed in the following paragraphs but they were not introduced as aims of the research in the introduction.
Results
In the paragraph “4.1 Bioherm morphology” were described the four stage of growth of the bioherm but this is an interpretation. Here it would be more appropriate to describe the morphological and compositional characteristics at the mesoscale of the bioherms. The interpretation must be moved after the result paragraph.
In general, the microfacies are well described and illustrated but the microbial or organic induced nature of the micrite is based only on morphological observations at mesoscale and thin section observations, so the interpretation as microbialite seems speculative. To confirm their model, the authors should supplement the data with SEM/EDS, Raman spectroscopy, UV-fluorescence or other techniques aimed to put in evidence: 1) details at higher magnification of the micro and nano-structures of the putative microbialite fraction; 2) the mineral composition of the presumed autochthonous micrite; 3) the organic nature of the microbialites.
Discussion
The discussion section is well structured and the comparison between the studied metazoan/microbial bioconstructions and those reported in literature, which spam in time and space, is well addressed. The only doubt is about the support of the discussion with the acquired data, mainly concerning the interpretation of the peloidal and clotted peloidal micrite as microbially mediated. The morphologies seem attributable to microbial activities but more micro- nano-morphological, mineralogical and biogeochemical investigations could prove definitively their origin.
I think that the topic proposed in the paper is interesting and opens to a new discussion on not-usual build-ups, but the interpretation and discussion must be supported by more convincing micro-nanomorphological and biogeochemical data to prove mainly the induced or influenced biomineralization of the putative microbialite fractions.
Citation: https://doi.org/10.5194/bg-2023-105-RC1 -
RC2: 'Comment on bg-2023-105', Anonymous Referee #2, 14 Sep 2023
The manuscript under discussion is a well written and well illustrated documentation of a new serpulid-microbialite bioherm occurrence in the Sarmatian Paratethys. The introduction and geological background as well as the lithological and microfacies descriptions are solid. The microbialite origin appears evident from the microstructure illustrated by thin section micrographs. Interpretations with respect to depositional setting and growth dynamics are sound, too. Consequently, I suggest accepting this manuscript with only minor modifications.
Improvements are possible with respect to the comparison with similar Miocene to present-day microbialites. Although this discussion chapter is well elaborated, it remained without broader implications (except that there are always some minor differences to the investigated serpulid-microbialites from Piuspuszta). In my view, a short discussion on the significance of these (and similar) microbialites in the Phanerozoic record would be desirable, in particular, why calcified cyanobacteria do not occur in the shallow-water serpulid-microbialites described, although microbial precipitates and acicular aragonites are formed. In this context, it would also have been good to measure stable carbon and oxygen isotopes of material of the studied section. Currently, the paper relies on data on gastropods from the adjacent section "Altes Zollhaus", with no data from the microbialites themselves. In my view, it would have been worth trying to obtain separate phases for this purpose. Also a series of mixed samples from the thrombolites would have been quite informative.
Specific comments and suggestions:
Title: What is the meaning of "microbial sea"? I am not sure if this wording is appropriate: The Sarmatian sea was certainly in the same way populated by microbial communities as present day or other fossil marine settings. Please consider rewording.Introduction:
- Please specify here or in the methods chapter to which salinity scale your refer (Venice system? Reference?). This is important especially for the term "hypersaline".
- Lines 103 f.: Caliche commonly forms in "semi-arid" rather than in real "arid" environments. What is the indication to infer "arid" conditions in the present case?
- Lines 105 f.: oligotrophic conditions: If I understand correctly, this statement is solely based on the low amount of siliciclastic input? I could imagine that the conditions were indeed rather oligotrophic, but the line of argumentation is not compelling. There are examples of low-siliciclastic peloidal limestones within highly eutrophic settings (e.g. Ries crater lake). Is the absence of organisms indication eutrophication the major argument? Please add a short clarification in the text.Results
- Line 169 and throughout the text: Why not calling the polylobate grains "aggregate grains"?Discussion
- Please provide the argument(s) of an increased alkalinity at the time of bioherm development.
- Salinity: Since no geochemical/isotope data are available, please expand on the associated fossils. In lines 166 f. you mention "ostracods, foraminifera (miliolids, rotalids), a few molluscs and fish teeth/otoliths". Is there anything known on the identity of the ostracods (which might be very useful for salinity interpretations)? If miliolid foraminifera are abundant, you may simply state that the setting deviated from open marine conditions and was affected by salinity fluctuations. Besides of Potamides disjunctus, which mollusc species are present in the bed of the microbialites? Currently I do not see real evidence for "hypersaline" conditions (in the sense of >40 permill salinity). Are there any echinoderm fragments in the thin sections?Minor comments and suggestions:
- Terminology: [Not that important...] Please use the term "bioherm" consistently throughout the manuscript. "Bioconstruction" is, as far as I remember various discussions, an ambiguous term as you may consider e.g. metazoan skeletons bioconstruction, too.
- Line 66: replace "Glibert-type" by "Gilbert-type"
- Line 85: Please put "Sarmatimactra vitaliana" in italics.
- Line 173: In my understanding, these are grain- to rudstones. A floatstone is characterized by large components floating in a fine-grained (micritic or mud) matrix.
- Line 315: Consider replacing "happens by" by "resulted from".
- Line 356: On basis of the current manuscript, the statement that "the entire shallow water area in the late Sarmatian of the Vienna Basin (and eventually in all the Paratethys) was dominated by microbial sediments" cannot be derived. Major parts of the investigated section are composed of peloidal sands, not microbial sediments. The statement above may therefore be made in the other paper in review (corresponding evidence provided), but could be removed from the present manuscript.
- Figure 2: Please increase size and clarity of the symbols for gastropods and bivalves. Consider placing some of the symbols right to the sediment column
(e.g. sedimentary structures, label for caliche, fossils) so that they become more prominent and recognizable to the reader. You may also consider indicating shallowing-upwards sequences and discontinuities where evident. Likewise, the stratigraphic stages Sarmatian, Pannonian, and the corresponding boundary (at basis of unit 10) can be shown right or left to the column. Currently, the term "Pannonian" appears isolated at the level of unit 10.Citation: https://doi.org/10.5194/bg-2023-105-RC2
Mathias Harzhauser et al.
Mathias Harzhauser et al.
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