Investigating the Timing of Carbonate Precipitations and Their Potential Impact on Fossil Preservation in the Hell Creek Formation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript highlights the importance of studying the sediments containing fossils. To me this important point is not highlighted enough in the paper, and if the author wished, this discussion could be expanded to include more details on how/why this type of study has the potential to impact the field of paleontology. An example could be expansion of the discussion and reasoning for your conclusions in lines 360-368 and 391-392. In general, not enough attention is probably given to the sediment surrounding fossils in paleontology, and this manuscript makes a case for changing this.

Also, in line 82 (in situ should be italicized), is this citation from 1952 the most recent definition for the upper boundary of the contact for the Hell Creek Formation? I know that there has been more recent stratigraphic work on this formation (e.g., Hartman et al. 2014, Moore et al. 2014, Fowler 2020). The 1952 definition sounds imprecise.

There are a few minor edits to improve the manuscript:

- In Figure 4, it is very difficult to differentiate the symbols used for the different rock types from one another. Could alternative symbology be used or could color be added to enhance distinction between these symbols?

- Line 232 would read better if it said 'herein', 'this study', or something similar, rather than "thesis".

-In section 4.2.4, it would be helpful to remind the read what taxon the femur and rib belonged to so we don't have to go back to the beginning of the manuscript.

-Line 269 has a typo in the figure caption with two letter B panels and not a letter F panel. Also, the description of letter G is incomplete.

-The first paragraph in section 5.1 (start of the Discussion) does not belong there. It is fascinating but should be moved elsewhere in the manuscript or have its own section.

 

Author Response

Thank you for reviewing the manuscript and constructive feedback.

Comment 1: This manuscript highlights the importance of studying the sediments containing fossils. To me this important point is not highlighted enough in the paper, and if the author wished, this discussion could be expanded to include more details on how/why this type of study has the potential to impact the field of paleontology. An example could be expansion of the discussion and reasoning for your conclusions in lines 360-368 and 391-392. In general, not enough attention is probably given to the sediment surrounding fossils in paleontology, and this manuscript makes a case for changing this.

-I have added sentences to highlight the importance of limited fractionation between bones and carbonate cement/pore-fluid. Some studies cited in my paper (e.g. Kohn et al., 2005; Fricke and Pearson, 2008; Suarez et al., 2013) use structural carbonate of fossil bones and teeth. Limited fractionation between fossils and pore fluid in this study supports the authenticity of the isotope compositions in previous studies.

Comment 2: in line 82 (in situ should be italicized), is this citation from 1952 the most recent definition for the upper boundary of the contact for the Hell Creek Formation?

-Originally, 1952 paper was cited because of the local context (i.e. Glendive area), but description and citation were replaced with Hartman et al. (2014) as their study also include Glendive area.

Comment 3: The first paragraph in section 5.1 (start of the Discussion) does not belong there. It is fascinating but should be moved elsewhere in the manuscript or have its own section.

-This paragraph was moved to the Geologic Settings section.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The author Daigo Yamamura has conducted a detailed and interesting study about the diagenetic history of fossil bones from the Hell Creek Formation, investigating the sedimentological and isotopic features of different sediment and bone samples collected from crevasse splay deposits cropping out at Glendive (Montana).

The paper is a well-organized, interesting, and valuable manuscript and I think it should be published on Minerals after a minor revision. The topic and the analysis are of interests of sedimentologists, paleontologists and petrologists, and whoever is interested in fossil diagenesis.

However, some modifications should be applied before publishing this article. All the suggestions and minor adjustments are reported in the attached PDF and three major points are reported here as follows:

1) Please, improve the quality and the editing of the figures. There are some suggestions about figures for implementing them for publication. Please, see the comments on the attached pdf.

2) Please, add a description of the fossil assemblage in the results. In the geological setting, there is a paragraph but this should include the fossil assemblage of the Hell Creek Fm based on literature. It would be interesting to add also a small paragraph about the analyzed fossil assemblage of the study area.

3) In the discussion and conclusions, there is an important point to discuss. The author addresses that the bones and sediments do not show similarities in the isotope composition, and for this reason, the concretions have a limited impact on fossil preservation (see lines 20-21 and 367). However, I suggest another point of view, so that the concretions have actually a great impact on the bones, protecting them from late diagenetic fluids that affected sediments and concretions but not the bones. This concept is explained by Bosio et al. 2021, where the trace elements and REE uptake of bones in low permeability environments, such as nodules, is limited by the concretion itself that isolated the bones during diagenesis.

Finally, maybe for a future work, it should be very interesting to make some further analyses on the bones and the concretions, for example LA-ICP-MS for REE patterns and microprobe analysis for major elements.

Please, see the attached file for minor changes. I suggest publication after a minor revision.

Comments for author File: Comments.pdf

Author Response

Thank you for reviewing the manuscript and constructive feedback.

Comment 1: Please, improve the quality and the editing of the figures. There are some suggestions about figures for implementing them for publication. Please, see the comments on the attached pdf.

-I have revised figures and attached figures with higher resolution. Conversion to pdf seem to affect the figures, and I will check with the editor as well.

Comment 2: Please, add a description of the fossil assemblage in the results. In the geological setting, there is a paragraph but this should include the fossil assemblage of the Hell Creek Fm based on literature. It would be interesting to add also a small paragraph about the analyzed fossil assemblage of the study area.

-Description of the fossil assemblage was added in the geological settings section, and another short paragraph was added to the results section.  The paragraph in results section explains how bone assemblage (isolated, unsorted and multi-taxa assemblage) coincides with the sedimentological interpretation of depositional environment.

Comment 3: In the discussion and conclusions, there is an important point to discuss. The author addresses that the bones and sediments do not show similarities in the isotope composition, and for this reason, the concretions have a limited impact on fossil preservation (see lines 20-21 and 367). However, I suggest another point of view, so that the concretions have actually a great impact on the bones, protecting them from late diagenetic fluids that affected sediments and concretions but not the bones. This concept is explained by Bosio et al. 2021, where the trace elements and REE uptake of bones in low permeability environments, such as nodules, is limited by the concretion itself that isolated the bones during diagenesis.

-I have revised my interpretation slightly and included "early carbonate precipitation have potential impact on fossil preservation." The concretions in later diagenesis only partially cover the fossils, and uncovered portion of bones does not show significant differences in preservation. I have also added "precipitation of the concretion may have impact in fossil preservation by taking up ions for alteration of fossils" and noted ICP-MS would be a great addition to the future study. Bosio et al. (2021) was cited in these additional interpretations.

 

Reviewer 3 Report

Comments and Suggestions for Authors

The submitted manuscript is one of the not-so-common research works that apply the methods of petrography to investigate the taphonomy of a prime fossil vertebrate assemblage. I think it shall be worth being published in Minerals after suitable moderate revisions.

A more detailed characterization of the studied bone specimens should be provided. An overview of the 'fossil assemblage' was provided in paragraph 2.2, but whether this assemblage is the same as the investigated one is not clearly stated. In addition, a reader like me would enjoy more information on the taxonomic affinities of the studied bones. For example, no information is provided as for the affinities of the studied rib. Dear author, please be as detailed as you can on this very point. 

I tend to disagree with the author's conclusion that concretions have a limited impact on the preservation of the Glendive fossil assemblage because their isotope characteristics are so different from those of the embedded bones. Indeed, concretions did likely shelter the bones from late diagenetic fluids that in turn affected the surrounding nodules and sedimentary matrix. Here, I have to refer to a paper, Bosio et al. (J. S. Am. Earth Sci., Mineralogical and geochemical characterization..., 2021), where the element uptake by bones preserved in low-permeability envelopes such as carbonate concretions was interpreted to have been substantially limited by the presence of the concretions themselves, which in turn would have also limited the decay of organic matter during the early diagenetic phases by slowing the interactions with the oxidants. I think this alternative viewpoint on the role of concretions in vertebrate fossil-diagenesis should at least be discussed in the context of the present work (hence my choice of indicating 'moderate' revisions, though this is not an option available on the MDPI SuSy portal). 

Some figures appear to be too low-resolution, e.g., Figs. 1, 2 and 3. Maybe that's only due to the low resolution of images in the PDF I reviewed, but some figures do really look of screenshot-quality. Conventional symbols/patterns, possibly the same as Fig. 3, should be used for lithology in Fig. 4. 

 

Author Response

Thank you for reviewing the manuscript and constructive feedback.

Comment 1: A more detailed characterization of the studied bone specimens should be provided. An overview of the 'fossil assemblage' was provided in paragraph 2.2, but whether this assemblage is the same as the investigated one is not clearly stated. In addition, a reader like me would enjoy more information on the taxonomic affinities of the studied bones. For example, no information is provided as for the affinities of the studied rib.

-New paragraphs for fossil assemblage were added to the geological setting and results sections. Fossils for petrographic analyses were fragmentary and could not identify beyond "indeterminate dinosaur". New paragraphs describe other fossils found in the assemblage.

Comment 2: I tend to disagree with the author's conclusion that concretions have a limited impact on the preservation of the Glendive fossil assemblage because their isotope characteristics are so different from those of the embedded bones. Indeed, concretions did likely shelter the bones from late diagenetic fluids that in turn affected the surrounding nodules and sedimentary matrix. Here, I have to refer to a paper, Bosio et al. (J. S. Am. Earth Sci., Mineralogical and geochemical characterization..., 2021), where the element uptake by bones preserved in low-permeability envelopes such as carbonate concretions was interpreted to have been substantially limited by the presence of the concretions themselves, which in turn would have also limited the decay of organic matter during the early diagenetic phases by slowing the interactions with the oxidants. I think this alternative viewpoint on the role of concretions in vertebrate fossil-diagenesis should at least be discussed in the context of the present work (hence my choice of indicating 'moderate' revisions, though this is not an option available on the MDPI SuSy portal). 

-The interpretation was modified and included "early carbonate precipitation may have influenced preservation of fossil bones." At the time of discovery, large fossils were only partially covered by concretions, and uncovered portions do not show significant difference in preservation. I agree with the potential use of ICP-MS for more detailed analysis, and mentioned that in conclusion as a future study. Bosio et al. (2021) was cited for modified interpretation.

Comment 3: Some figures appear to be too low-resolution, e.g., Figs. 1, 2 and 3. Maybe that's only due to the low resolution of images in the PDF I reviewed, but some figures do really look of screenshot-quality. Conventional symbols/patterns, possibly the same as Fig. 3, should be used for lithology in Fig. 4. 

-Figures were edited and higher resolution images were added to the revised manuscript. I failed to submit higher resolution images separately due to the file size in initial submission, and will contact editor about this issue.

Author Response File: Author Response.docx

Reviewer 4 Report

Comments and Suggestions for Authors

Thank you for finishing this good work. Please consider the comments within the attached file. 

Best wishes

Comments for author File: Comments.pdf

Author Response

Thank you for reviewing my manuscript and constructive feedback.

Figures were edited and higher resolution images were attached on the revised manuscript. I had issues with uploading high resolutions images separately in initial submission, and I will consult with the editor further with this issue.

The order and labels of figures 5, 6 and 7 were revised.  The missing descriptions were also revised for these figures.

Author Response File: Author Response.docx