In this manuscript, Tomašových et al. assess the preservation, pyritization and age and depth distribution of valves of the hypoxia-tolerant bivalve Varicorbula gibba in Adriatic Sea sediments. The authors compare these V. gibba taphonomic data to sedimentary and radiometric proxies for mixed layer depth and sedimentation rate, as well as sedimentary biogeochemical data. They conclude that sedimentation rate likely played a strong role in enhancing long-term pyritization of V. gibba valves (i.e., by limiting extents of oxidation in the uppermost sediment pile, and by shuttling valves below the mixed layer on relatively rapid time scales). The authors also observe that V. gibba valves with pyrite linings appear to be more prevalent in the portions of cores corresponding to the late 20^th century, correlative with increases in seasonal hypoxic events and episodes of eutrophication. They therefore conclude that these hypoxic episodes, by deleteriously impacting the local infaunal community (particularly bioirrigators), directly resulted in valve-associated reducing microniches and decreased potential for reoxidation of valve pyrite linings, allowing burial of pyritized valves to outpace bioirrigator-mediated reoxidation in sites characterized by high sedimentation rates. With this work, the authors lend new insights into the role played by bioirrigation (and feedbacks between sedimentation and bioirrigation) and eutrophification in pyritization, with important implications for how pyritized fossils in the stratigraphic record can be used to reconstruct not only the taphonomy of body fossil assemblages but also changes in the extent and timescales of bioturbation.

This manuscript represents an impressive body of work with potentially important implications for not only reconstructing environmental-ecological-taphonomy feedbacks on historic time scales but also in the deep-time stratigraphic record. I am therefore supportive of publication, and below I highlight a few relatively minor aspects that I have confidence the authors should be able to readily address in a revised version of the manuscript:

I would have liked to have seen more extensive discussion, up front (e.g., in the Introduction, the Methods or a new section of its own) of the ecology of Varicorbula gibba—for instance, whether it is infaunal, semi-infaunal or epifaunal; feeding ecology; seasonal variation in abundance; and ecological relationships to other local taxa. We are not told until near the end of the manuscript (l. 580-581) that V. gibba is among the assemblage of shallowly burrowing detritivores and deposit-feeders that have been previously documented at Adriatic prodelta sites such as Po and Isonzo. However, the ecology of V. gibba seems very relevant to the manuscript’s consideration of impacts of hypoxia and eutrophification on local benthic communities. For instance, is it an opportunistic taxon? Does its relative abundance in benthic communities actually increase under conditions of hypoxia—or in spite of being hypoxia-tolerant, are impacts of hypoxia on V. gibba, on the whole, deleterious (as they are inferred to be for the bioirrigating community)? This in turn could impact some of the authors’ assumptions regarding rates of V. gibba input to the sediment pile. The authors explore various models for the distribution of V. gibba valves at depth (and from that exercise conclude that a model of uniform loss from the mixed layer for pyritized and non-pyritized valves is most parsimonious), but this appears to be premised upon an assumption of invariant input rates. If V. gibba abundance (or relative abundance) varies with bottom-water redox state and degree of nutrient loading this may not be a valid assumption, however. In addition to providing further detail on V. gibba ecology earlier in the manuscript, the authors should discuss these assumptions and the extent to which they can be constrained or justified.

Similarly, in the manuscript’s discussion of rates of “loss” of valves from the mixed layer (e.g., l. 201-204 and elsewhere), I suggest replacing use of the term “loss” (which is interpretive and connotates a null model that other processes—for instance, additive processes that may variable impact pyritized vs. non-pyritized valves, or those of different age ‘cohorts’—do not lead to differences in abundance or distribution) and instead phrasing this in terms of relative abundance or distribution—at least prior to the Discussion section of the manuscript.

I would also have liked to have seen additional information on sedimentation rates and mixed layer depths, specifically how these were constrained—given the importance of each of these to the authors’ conclusions. For instance, it would be good to include the ²¹⁰Pb data, which do not appear to currently be part of any of the figures or tables. The authors state that sedimentation rates, although variable between the different sites and cores, appear to have been largely invariant throughout the deposition of individual cores (e.g., l. 266). It would be good to see that data upon which that assessment is based, as well as further discussion by the authors of whether this is surprising or expected for the prodeltaic sediments of their study sites over the hundred- to thousand-year time scales recorded by these cores.

There is also some ambiguity in the authors’ discussion of the role of organic matter in fostering precipitation of pyrite linings on V. gibba valves. Pyritization is, as the authors acknowledge, typically limited by the supply of organic matter (as well a requiring a redox interface between iron and sulfate reduction at the localized supply of organic matter). However, something the authors do not directly discuss (though they perhaps allude to this in l. 529-532) but which is, in contrast, discussed by some of the studies they cite (e.g., Raiswell et al., 1993, Marine Geology; Farrell et al., 2009, Geology; as well as Raiswell et al., 2008, AJS) is that the presence of abundant disseminated organic matter in the sedimentary matrix tends to be detrimental to extensive pyritization of macroorganism carcasses. So although the hypoxic conditions fostered by eutrophification may, in the case of their Adriatic sediment samples, have played an important role in the development of a shallow redoxcline and thus pyrite precipitation on V. gibba, high rates of organic matter delivery to the seafloor are unlikely to foster extensive and exceptional fossilization of macroorganism carcasses via, for instance, pyrite templating or replacement in geologic analogues. In other words, early diagenetic precipitation of pyrite framboids on V. gibba valves under these conditions does not necessarily equate to exceptional pyritization—particularly given the abundance of sedimentary organic matter noted by the authors. The authors should therefore temper their discussion of how their findings bear upon understanding of pathways of exceptional fossilization via pyritization, and incorporate discussion of these caveats. Similarly, obrution in the typical sense need not involve deep burial, but rather rapid burial (and the associated ‘smothering’ of benthic communities). Particularly if the redoxcline (due to hypoxic conditions) is located in the uppermost centimeters of the sediment pile, an “obrution scenario” and a “hypoxia-mediated reduced bioirrigation scenario” are entirely compatible, and should not be discussed as diametrically opposed alternative models (e.g., as in l. 475-484, l. 622-627). On a more minor note, previous studies of pyritization have suggested that relatively more recalcitrant organics may preferentially undergo pyritization (or more rapid pyritization) (e.g., Briggs et al., 1991, Geology; Raiswell et al., 1993, Marine Geology); the authors should therefore take care to not oversimplify fossil pyritization as targeting solely the most labile tissues.

l. 196: Although D/L amino acid ratios are of course broadly used, given that this paper may attract a broad audience (including those who do not commonly employ organic geochemical methods), I suggest providing further detail here.

l. 351-352: Please state here how sites were partitioned into “high-” and “low-” sedimentation rates (e.g., what range of values were used for this categorization).

Figure 9: the burial rates calculated for A) and D) (without and with pyrite linings, respectively for Po 3) seem substantially different (by a factor of 2). It would be good to see additional interrogation of the grounds on which it was determined that these are essentially indistinguishable.

Technical Corrections:

l. 97: “…may not be surprising…”

l. 171-172: This sentence contains two separate notations of the mixed layer depth at Brijuni—perhaps a typo?

l. 388: increments

l. 469: reach

For Figure 11 in particular (and, to a lesser extent, some of the other figures), the plots are so closely packed together that it is a little challenging to read the axis labels and attribute these to the appropriate plots. Could the panel components be spaced slightly further apart?

Figure 15: For the A) label, is this supposed to be > (not <)?

Sincerely,

Lidya Tarhan

This is an original manuscript on a highly relevant topic. I am sure that we will soon witness an explosion in this style of papers. This study is definitely timely. My comments are all minor and aimed towards improving the clarity of the argument, pointing in places to additional background literature.

The authors highlight the role of deltaic systems on sedimentary dynamics, noting the higher sedimentation rates in the prodelta in comparison with areas in the northwest Adriatic Sea. Some cores are coming from prodelta settings (Po and Isonzo) and others from areas off strandplains. Deltaic settings are characterized by a complex array of stressors. It is clear that sedimentation rate is a first-rate controlling factor in this area. However, what about other potential factors, such as freshwater discharge, hypoxia, or substrate consolidation in connection with a deltaic source? Some of these (e.g. hypoxia) are assessed through the text, but a better articulation with the deltaic context would be advisable. A discussion on other influences on pyritization would be useful as well. For example, the higher abundance of pyritized shells is present in nearshore areas where restricted circulation may have been associated with lower oxygen content. Also, these are areas with higher amounts of organic carbon and iron in the fine-grained sediment. In particular, bioturbation is strongly affected by the interplay of these parameters. There are various papers published on this topic during the last fifteen years or so. I suggest, for example, to check MacEachern, J. A., Bann, K. L., Bhattacharya, J. P., 2005. Ichnology of deltas: Organisms’ responses to the dynamic interplay of rivers, waves, storms and tides. In: Giosan, L., Bhattacharya, J. P. (Eds.), River Deltas: Concepts, Models, and Examples. SEPM Special Publication, 83, 49–85. Also of relevance is: Bhattacharya, J.P., Howell, C.D., MacEachern, J.A. and Walsh, J.P., 2020. Bioturbation, sedimentation rates, and preservation of flood events in deltas. Palaeogeography, Palaeoclimatology, Palaeoecology, 560, p.110049. In short, the proposed interpretation relies heavily on sedimentation rates, but bringing other parameters to the discussion would be important to reflect more adequately the complex dynamics of deltaic systems.

In line 61, the classic paper in this regard is: Bromley, R.G. and Ekdale, A.A., 1986. Composite ichnofabrics and tiering of burrows. Geological magazine, 123(1), pp.59-65.

Review of manuscript by Tomašových et al.

This paper presents an exceptionally well documented case study of a phenomenon rarely seen in modern environments but common in the deep time fossil record: pyrite formation within shells. The authors present a wealth data on the relative frequency and association of pyrite with other taphonomic indicators of residence time and also provide data on the actual ages of the shells based on amino-acid racemization. The results provide a powerful case for the development and preservation of pyrite in closed spaces of dead organisms and shows that pyrite linings are most frequent in areas of higher sedimentation rates and slow mixing or bioirrigation. The authors even present data that indicate an upward increase in pyrite formation within shells of the later 20^th century that parallels evidence for increased eutrophication owing to anthropogenic activity. In fact, the presence of pyrite may be a more sensitive indicator of sluggish rates of bioirrigation than ichnofabrics.

The paper is extremely well written, well organized and thoroughly referenced. And supported by a large data set and thorough statistical analysis.  I noted only a few minor errors in the text and references, which are marked on the pdf.  I also have the following queries, mainly out of interest in the subject; I refer to relevant line numbers. The authors may wish to comment on them.

Line 75. Here and elsewhere, throughout, the term “microniches” is not quite right; perhaps “microenvironments” is preferable.

183, see also 613.: this paragraph well explains the tight closure of Varicorbula valves. But is there a reason that Varicorbula shells remain closed after death instead of splaying open at the hinge ligaments as do most bivalves?  Does the ligament groove serve as a sort of locking devise? This may explain why they are most frequently pyrite lined. Perhaps a brief discussion of comparative taphonomy of these clams would be useful here.

180s and 480s: Alternatively, do these clams frequently perish within their burrows as opposed to many that seem to rise to the surface during mortality. Under such conditions, burial within sediment may not require obrution, but I do not think this applies to most other articulated and closed bivalves let alone completely articulated multi-element skeletons.

319-320: how is it that pyrite formation does not set in until after 10 years in the sediments? If, as assumed the development of sulfides is associated or at least initiated with decay of organic matter contained within the enclosed spaces of shell cavities. But one would guess that such OM should be largely gone after just a year or so. Is the issue that the initial sulfides are monosulfide gels that only later recrystallize to recognizable pyrite?

349: it seems odd that shells with periostracum preservation should be negatively correlated with pyrite linings as one would suspect both might indicate higher rates of burial and reduced decay.

385: this result: increased borings in the HST vs. TST seems paradoxical as sedimentation rates are normally predicted to increase into the later HST. Any explanation?

 Is it also possible that anthropogenic activity is related to increased rates of runoff and therefore, faster burial of organisms?

Lines 443-445. This is a very interesting finding in line with observations of iron sulfide blackened shells in ancient assemblages in which there is a strong positive correlation between other taphonomic indicators of long residence time and darkening:  Kolbe, S., Zambito, IV, J.J., Brett, C.E., Wise, J.L., Wilson, R.D., 2011. Brachiopod shell discoloration as an indicator of taphonomic alteration in the deep-time fossil record, Palaios 26: 682-692.  

480-onward. The result that shells of the same age may or may not show pyritization does not seem necessarily to support the contention that the pyritization occurred gradually during normal burial rather than during burial events. It is certainly possible that pulses of burial could entomb not only live or recently dead individuals, but many others that had died and decayed prior to the event. Pyrite would be localized in those that were buried with soft tissues intact whereas the “dead shells” would show little tendency toward sulfate reduction or pyrite formation.

559. this observation (of loss of Fe from sediments under persistent anoxia) is very important, as it may help explain the absence of pyritized fossils in truly black, laminated sediments that we have documented repeatedly (see Brett, C.E., Dick, V.B. and Baird, G.C.,1991. Comparative taphonomy and paleoecology of Middle Devonian dark gray and black shale facies from western New York. In Landing, E. and Brett, C.E., eds., Dynamic Stratigraphy and Depositional Environments of the Hamilton Group in New York Pt. II. State Museum Bulletin 469, p. 5‑36.)

One implication is that, while eutrophication and increased organic matter may enhance pyrite formation up to a point (because of lower benthic O₂ and reduced bioirrigation); too much OM may shut down the process.

570: forgive my ignorance, but I do not understand your use of hysteresis here. Perhaps explain  a bit.

577: it is not intuitive that large size would be associated with lowered oxygen levels (resulting from eutrophication); in many cases low oxygen has been attributed to stunting and diminutive organisms. Apparently in Varicorbula the stunting affect is offset by increased growth. Presumably this is well documented; a reference might be useful.

613: but why do these bivalves not open shortly after death? If they are not buried rapidly (episodically) then how do they come to remain tightly closed?

Line 635: this also contrasts with nanopyrite infillings that seem to increase together with other signs of degradation with residence time.

Illustrations are fine and require no modification.

References I have gone through and highlighted all that appear to be correctly cited; most are fine but there are a number of minor problems:

The following are slightly out of alphabetical order:

Briggs et al. 1991., Boyle, Bjereskov, Degobbis. Meysman, Schieber 2012 (should come before Schieber and Baird)

Wrong date

 Brush et al. 2020 (2021 intext)

Kralje et al. 2019 (2020 in text)

Slagter et al. 2021 (2020 intext)

No date (in ref list) Hunda et al. [2006]

In reference list but apparently not referenced in text:

Aller, 1982

Arcon et al. 1999

Clark ad Lutz, 1980

Eliott et al. 2007

Faganeli et al 1985

Palinkas et al. 2007

Powell and Stanton 1985

Cited in text but not in reference list:

Alvizi et al. 2016

Stanton and Powell, 1985

Wignall et al., 2010

Need to designate a and b references in text:

Tomašových 2019 a vs. b

Overall: this is an excellent paper, which needs only minor revision for publication.  The paper provides much new data and reaches important conclusions regarding the little known phenomenon of early diagenetic pyritization; it has important implications for sedimentation rates and rates of bioirrigation, for the taphonomy of exceptional fossils, and potentially for conservation paleobiology and evidence for anthropogenic effects. The results will be of interest to geochemists, sedimentologists, taphonomists, paleobiologists and perhaps conservation paleobiologists. I strongly recommend publication with slight correction.