the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Jun 2025
Living on the edge: Response of rudist bivalves (Hippuritida) to hot and highly seasonal climate in the low-latitude Saiwan site, Oman
Abstract. Earth’s climate history serves as a natural laboratory for testing the effect of warm climates on the biosphere. The Cretaceous period featured a prolonged greenhouse climate characterized by higher-than-modern atmospheric CO2 concentrations and mostly ice-free poles. In such a climate, shallow seas in low latitudes probably became very hot, especially during the summers. At the same time, life seems to have thrived there in reef-like ecosystems built by rudists, an extinct group of bivalve molluscs. To test the seasonal temperature variability in this greenhouse period, and whether temperature extremes exceed the maximum tolerable temperatures of modern marine molluscs, we discuss a detailed sclerochronological (incrementally sampled) dataset of seasonal scale variability in shell chemistry from fossil rudist (Torreites sanchezi and Vaccinites vesiculosus) and oyster (Oscillopha figari) shells from the late Campanian (75-million-year-old) low latitude (3° S paleolatitude) Saiwan site in present-day Oman. We combine trace element data and microscopy to screen fossil shells for diagenesis, before sampling well-preserved sections of a Torreites sanchezi rudist specimen for clumped isotope analysis. Based on this specimen alone, we identify a strong seasonal variability in temperature of 19.2 ± 3.8 °C to 44.2 ± 4.0 °C in the seawater at the Saiwan site. The oxygen isotopic composition of the seawater (δ18Osw) varied from -4.62 ± 0.86 ‰ VSMOW in winter to +0.86 ± 1.6 ‰ VSMOW in summer.
We use this information in combination with age modelling to infer temperature seasonality from incrementally sampled oxygen isotope profiles sourced from the literature, sampling multiple shells and species in the assemblage. We find that, on average, the Saiwan seawater experienced strong seasonal fluctuations in monthly temperature (18.7 ± 3.8 to 42.6 ± 4.0 °C seasonal range) and water isotopic composition (-4.33 ± 0.86 to 0.59 ± 1.03 ‰ VSMOW). The latter would strongly bias the interpretation of stable oxygen isotopes in shell carbonate without independent control on either temperature or seawater composition.
Combining our seasonal temperature estimates with shell chronologies based on seasonal cyclicity in stable isotope records and daily variability in trace element data, we show that T. sanchezi rudists record temperatures during the hottest periods of the year as well as during the winters, which were characterized by cooler temperatures and enhanced influx of freshwater. Both O. figari and V. vesiculosus plausibly stopped growing during these seasonal extremes. This study aims to demonstrate how high-resolution geochemical records through fossil mollusc shells can shed light on the variability in past warm ecosystems and open the discussion about the limits of life in the shallow marine realm during greenhouse climates. Future work should apply the clumped isotope paleothermometer on incrementally sampled bio-archives to explore the upper-temperature limits experienced by calcifiers in different environments throughout geological history.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-2308', Anonymous Referee #1, 16 Aug 2025
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AC2: 'Reply to comments of reviewer 1', Niels de Winter, 26 Sep 2025
Dear Antje Voelkner, dear reviewers,
We appreciate the supportive and helpful comments by both reviewers on our manuscript. The feedback by both reviewers is minor and can be addressed quite easily in our opinion. Below, we detail how we address the points raised by Reviewer 1 in point-by-point fashion. Line numbers refer to the originally submitted version of the manuscript (as in the reviewer’s report). In addition, we are happy to submit an annotated version of our manuscript on resubmission in which we track all changes made in reply to these comments. We trust that the suggested changes will address the concerns raised by the reviewer on the current manuscript version and make our contribution suitable for publication in Climate of the Past.
Reviewer 1:
The manuscript would be an excellent addition to this journal upon some moderate revisions. The manuscript exhibits excellent scientific significance, as it explores seasonal climate extremes during Cretaceous greenhouse climates. Relating this information to modern bivalves and their maximum thermal tolerance provides excellent context for why this study is important and the requirement for additional research. The quality of the work is sound. The implication of the results is appropriate and within reason, given the data. The presentation quality is good, but could be improved. Figures and figure captions need to be revised to be clearer. Sections of the manuscript require reorganization to enhance clarity. More detailed revisions are provided below. It is in my opinion that this manuscript is accepted subject to minor revisions.
We thank the reviewer for their positive reception of our manuscript and constructive comments on the presentation of our data and figures. We hope that our suggested changes make our manuscript more accessible to the reader.
Moderate Comments:
Figures 2, 3, 4, 5, 8, and 9 all have red and green color elements that need to be changed.
We appreciate that our current color scheme may not be accessible for colorblind readers and thank the reviewer for bringing that to our attention. We propose different solutions to this issue for different figures to preserve a consistent color scheme throughout the manuscript. To choose an accessible and inclusive color scheme, we used the Colorbrewer tool (https://colorbrewer2.org/), and we cite the unique HEX codes of the colors we plan to use in our responses below.
- Figure 2: All red text, lines and boxes in panels A and B will be changed to grey. Green and red colors used to indicate calcite and silica mineralogies will be changed to light green (HEX: #66c2a5) and orange (HEX: #fc8d62), respectively (Using “Set2” in the Colorbrewer tool).
- Figures 3, 4, 5, 8 and 9: The green, blue and red color scheme used for the three species ( sanchezi, V. vesiculosus and O. figari, respectively), will be changed according to scheme “Dark2” of the Colorbrewer tool, using green (HEX: #1b9e77), purple (HEX: #7570b3) and orange (HEX: #d95f02) for these three species, respectively. We will retain the red and blue colors marking carbon and oxygen isotope values in Figure 4 and 6, and the blue-yellow-red diverging color scheme (“RdYlBu” in the Colorbrewer tool) used for clumped isotope temperatures in Figure 4 and 7, as well as the green shading in Figure 8B, D and F (“Greens” color scheme in Colorbrewer tool).
Figure 4. The caption says 9 profiles, but I count 7 unique samples with 11 time series.
We realize that this caption is slightly confusing, but there are indeed 9 profiles through T. sanchezi shells and one profile to both V. vesiculosus and O. figari. To clarify that this is what we mean, we will rephrase the first sentence of the caption as follows:
“Figure 4: Overview of 11 incrementally sampled stable oxygen (δ18Oc; blue) and carbon isotope (δ13C; red) profiles: 9 profiles through 5 T. sanchezi specimens, of which 5 parallel profiles through specimen H579 (A, B, C, D & F; green frame), one profile through V. vesiculosus specimen B6 (E; purple frame) and one profile through O. figari specimen B11 (G; orange frame).”
Figures 4, 5, and 8. The X- and Y-axis labels need to be made larger for improved legibility.
We will increase the font size of the axes of these figures to improve legibility
Figure 8. δ18O is listed as d18O in the x-axis of plots C and D. This should be changed to the correct format.
The vertical axis label in panels C and D of this figure will be updated to reference the correct delta symbol notation.
Font size for Table 4 needs to be increased.
We will attempt to increase the font size of the text in this table as much as possible while still allowing the table to fit on the page. If the font size needs to be increased further, we provide the raw data of the table and suggest to resolve this issue with the type editor.
Line 775. Figure 10 is mentioned but not found in the manuscript.
Thank you for pointing this out. The figure reference was left over from a previous manuscript version and should instead refer to Figure 9. This will be corrected.
Minor Comments
Lines 74-78. Break into two sentences.
This sentence will be divided into two sentences as follows:
“For example, some of these past environments, most notably shallow marine ecosystems, are thought to have reached temperatures exceeding the temperature range of modern equivalent ecosystems (de Winter et al., 2020; Huang et al., 2017; Jones et al., 2022). These temperatures probably exceeded the maximum temperature tolerance at which modern shallow marine species can complete their life cycle, which is typically estimated in the order of 38-42°C (Compton et al., 2007).”
Lines 573-577. Break into two sentences.
This sentence will be divided into two sentences as follows:
“Instead, a positive correlation between δ18Oc and δ13C values is often observed in modern (non-diagenetically altered) photosymbiotic species, such as tridacnids (Elliot et al., 2009; Killam et al., 2020). Such a correlation has been proposed to be caused by seasonal changes in the isotopic composition of the dissolved inorganic carbon pool due to variability in the activity of photosymbionts in phase with the seasonal effect of temperature on the oxygen isotope composition of the shell (Elliot et al., 2009; McConnaughey and Gillikin, 2008).”
Line 670. Change classical to classic
This will be changed accordingly
Lines 851-856. Break into two sentences.
This sentence will be divided into two sentences as follows:
“T. sanchezi apparently thrived under these conditions while V. vesiculosus and O. figari stopped producing their shell at temperatures close to those that limit modern shallow marine bivalves (~34°C; Clarke (2014); Compton et al. (2007)) and perhaps also during the high-precipitation and lower-salinity phases of the winter season. This suggests that T. sanchezi may have been particularly well-adapted to the high seasonality in temperature and precipitation and hot summers in its environment.”
Citation: https://doi.org/10.5194/egusphere-2025-2308-AC2
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AC2: 'Reply to comments of reviewer 1', Niels de Winter, 26 Sep 2025
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RC2: 'Comment on egusphere-2025-2308', Werner Piller, 13 Sep 2025
The manuscript presents an excellent study on rudist and ostreid bivalves from the late Campanian. Since the data are dealing with a warmhouse climate the results and conclusions are of wide relevance. The methods applied are adequate and state-of-the-art. The explanation of the methods is excellent and can also be followed by non-specialists. The interpretation of the results are sound and of great importance since they not only provide information on temperature but also on (paleo)biological aspects in respect to the high temperatures which cause shut off of growth of some taxa and continuous growth in others. The paper is therefore not only a geochemical approach but combines geochemical results with critical (paleo)biological parameters.
I made several comments directly in the pdf file which is attached. I suggest to carry out minor changes before the manuscript should be accepted.
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AC1: 'Reply to comments of reviewer 2', Niels de Winter, 26 Sep 2025
Dear Antje Voelkner, dear reviewers,
We appreciate the supportive and helpful comments by both reviewers on our manuscript. The feedback by both reviewers is minor and can be addressed quite easily in our opinion. Below, we detail how we address the points raised by Reviewer 2 in his annotated PDF in point-by-point fashion. Line numbers refer to the originally submitted version of the manuscript (as in the reviewer’s report). In addition, we are happy to submit an annotated version of our manuscript on resubmission in which we track all changes made in reply to these comments. We trust that the suggested changes will address the concerns raised by the reviewer on the current manuscript version and make our contribution suitable for publication in Climate of the Past.
Reviewer 2 (Werner Piller):
The manuscript presents an excellent study on rudist and ostreid bivalves from the late Campanian. Since the data are dealing with a warmhouse climate the results and conclusions are of wide relevance. The methods applied are adequate and state-of-the-art. The explanation of the methods is excellent and can also be followed by non-specialists. The interpretation of the results are sound and of great importance since they not only provide information on temperature but also on (paleo)biological aspects in respect to the high temperatures which cause shut off of growth of some taxa and continuous growth in others. The paper is therefore not only a geochemical approach but combines geochemical results with critical (paleo)biological parameters.
We appreciate the thoughtful endorsement of our study by the reviewer and their constructive comments on our text in the annotated PDF, which we plan implement to improve our manuscript.
I made several comments directly in the pdf file which is attached. I suggest to carry out minor changes before the manuscript should be accepted.
We directly implemented all comments pertaining to grammar, spelling and interpunction and briefly reply below to the written comments by the reviewer:
Line 127: We will mention specifically in the caption of Figure 1 that the marly layer referred to here is the layer in between the two biostromes indicated in green in Figure 1B:
“The marly layer containing O. figari referred to in the text is the brown layer in between the two green members in the red box.”
Line 139: We will rephrase this to “longitudinal cross section” for clarity. This was originally implied with the statement “through the axis of maximum growth” later in the sentence, but we appreciate that just “cross” section may cause confusion. Throughout the revised manuscript, “cross section” will be written without the hyphen.
Line 525: The first use of “sample” will be removed here to prevent repetition of the word in this sentence and promote clarity.
Line 549: We appreciate the suggestion of citing a textbook here for the diagenetic alteration discussion, but we retain the current citations since these are landmark studies of calcite shell preservation and deal specifically with rudist shells, making them, in our opinion, more directly relevant to this study’s specimens.
Citation: https://doi.org/10.5194/egusphere-2025-2308-AC1
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AC1: 'Reply to comments of reviewer 2', Niels de Winter, 26 Sep 2025
Data sets
Supplementary Information to: "Living on the edge: Response of rudist bivalves (Hippuritida) to hot and highly seasonal climate in the low-latitude Saiwan site, Oman" Niels J. de Winter et al. https://doi.org/10.5281/zenodo.12567712
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The manuscript would be an excellent addition to this journal upon some moderate revisions. The manuscript exhibits excellent scientific significance, as it explores seasonal climate extremes during Cretaceous greenhouse climates. Relating this information to modern bivalves and their maximum thermal tolerance provides excellent context for why this study is important and the requirement for additional research. The quality of the work is sound. The implication of the results is appropriate and within reason, given the data. The presentation quality is good, but could be improved. Figures and figure captions need to be revised to be clearer. Sections of the manuscript require reorganization to enhance clarity. More detailed revisions are provided below. It is in my opinion that this manuscript is accepted subject to minor revisions.
Moderate Comments:
Figures 2, 3, 4, 5, 8, and 9 all have red and green color elements that need to be changed.
Figure 4. The caption says 9 profiles, but I count 7 unique samples with 11 time series.
Figures 4, 5, and 8. The X- and Y-axis labels need to be made larger for improved legibility.
Figure 8. δ18O is listed as d18O in the x-axis of plots C and D. This should be changed to the correct format.
Font size for Table 4 needs to be increased.
Line 775. Figure 10 is mentioned but not found in the manuscript.
Section
Minor Comments
Lines 74-78. Break into two sentences.
Lines 573-577. Break into two sentences.
Line 670. Change classical to classic
Lines 851-856. Break into two sentences.