the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Feb 2025
Clumped isotope temperature and salinity constrains for the Maastrichtian Chalk Sea based on planktonic and benthic foraminifera from Poland
Abstract. The Maastrichtian (~72–66 Ma), the final stage of the Cretaceous, experienced long-term cooling with high atmospheric CO₂ and weak latitudinal temperature gradients. Tectonic movements and variations in climate lead to sea-level changes and dynamic ocean conditions. This background probably affected the seawater circulation regime of the shallow epeiric Chalk Sea that covered a large portion of the Northern European continent. The connections to the evolving Northern Atlantic, the Arctic Basin and the tropical Thetis Ocean in the South and their impact on the seawater circulation and stratification in the open Chalk Sea is still not well understood. This study applies carbonate clumped-isotope thermometry (Δ47) to well preserved planktonic and benthic foraminifera from the Polanówka UW-1 core (Poland) to reconstruct the local surface and bottom water conditions prevailing during the Maastrichtian in the Chalk Sea.
The results from planktonic foraminifera reveal dynamic surface water conditions of alternating warmer and more saline with colder and less saline surface waters compared to stable, warm, and saline bottom waters from the benthic foraminifera. Comparisons with previous studies indicate the new planktonic Δ47-SST reconstructions align more with oxygen isotope-based SST than the SST based on organic proxies such as TEX₈₆-SST, with differences attributed to calibration, seasonality, and habitat depth. These findings suggest a stratified water column where the surface water is influenced by sporadic water entrainment with strong depleted ẟ18O that could be associated to freshwater runoff. The observed more stable and warm bottom water conditions and the range of ẟ18O may be associated with 2 scenarios: (1) a warm, saline bottom water periodically influenced by incursions of colder and fresher North Atlantic waters, or (2) bottom water conditions influenced by increased water input from the Tethys during periods of sea-level rise. Our reconstructions on the central European Chalk Sea conditions provide new insights into the thermal structure and water circulation in the Chalk Sea during the Mid-Maastrichtian Event and highlight the need for further research to refine the understanding of hydrology dynamics during this mild greenhouse climate interval.
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CC1: 'Comment on egusphere-2025-502', Thamizharasan Sakthivel, 14 Apr 2025
This manuscript presents a thoughtful interpretation of clumped and stable oxygen isotope data from the Maastrichtian Chalk Sea. While the study provides valuable contributions to our understanding of past marine conditions, there are a few aspects that would benefit from further clarification and discussion to strengthen the scientific interpretation. The main points are summarized below:
1) The authors apply the Δ47–temperature calibration from Daëron & Gray (2023) to both planktonic and benthic foraminifera. However, it is important to note that this calibration was developed specifically for planktonic species and has been shown to be poorly suited for benthic taxa. Applying it to benthic foraminifera may introduce systematic bias in bottom-water temperature reconstructions, which could affect interpretations related to thermal stratification and deep-water circulation. It would be more appropriate to apply a benthic-specific calibration and assess whether the results align with those from the Daëron & Gray (2023) calibration within the stated uncertainties.
2) At several stratigraphic levels (86, 75, and 46 m), surface water temperatures are reported as being lower than those of bottom waters. This inversion contrasts with expectations for greenhouse climates, where surface waters are typically warmer. The authors suggest freshwater input as a potential cause, but mixing freshwater with seawater generally increases specific heat capacity, which would likely buffer against cooling rather than promote it—unless the input was from meltwater.
Additionally, the explanation involving upwelling is debatable. While upwelling can lower surface temperatures, it typically brings saline, deeper water to the surface, which would be expected to have a heavier, not lighter, δ¹⁸O signature. Clarifying these mechanisms with supporting evidence would strengthen the argument.
3) The manuscript compares Δ47-based sea surface temperatures from the study site with mean δ¹⁸O-derived SST estimates from other locations. While this offers useful context, a more direct and meaningful comparison would involve estimating SST from δ¹⁸O values measured on the same foraminiferal specimens used for clumped isotope analysis. This within-sample comparison would allow for a clearer evaluation of proxy agreement and would strengthen the argument that Δ47-SST aligns more closely with δ18O-based temperatures than with TEX86 reconstructions.
4) Understanding the depth habitat of the studied foraminifera is critical when reconstructing vertical water column profiles. While the manuscript assigns species as either surface or bottom dwellers, no ecological justification or calcification depth estimates are provided. Including at least approximate depth ranges based on literature or modern analogs would improve confidence in the stratification interpretations.
5) The authors propose mechanisms such as freshwater runoff and upwelling to explain the observed isotopic patterns. However, potentially relevant supporting data—such as TOC/TN ratios, δ¹⁵N and δ¹³C of organics, or foraminiferal assemblage changes—are not presented. These datasets, which are available from the same core (https://doi.org/10.1016/j.gr.2023.11.010), could provide important context and would help substantiate the proposed scenarios.
6) The study notes that each sample was measured for clumped isotopes between 6 and 16 times, but the rationale behind this level of replication is not explained. It is unclear whether this was intended to improve counting statistics. A brief explanation of the replication strategy and the method used to assess uncertainties would help clarify the robustness of the reported Δ47 values and temperature estimates.
7) Dubicka et al. (2024) have conducted detailed SEM and elemental analyses to assess diagenetic alteration in foraminifera from the same core used in the present study. It would strengthen the manuscript to briefly mention this elemental evidence, highlighting the absence of diagenetic alteration to support the reliability of the geochemical data.
Citation: https://doi.org/10.5194/egusphere-2025-502-CC1 -
AC4: 'Reply on CC1', Marion Peral, 04 Aug 2025
Comment by Thamizharasan Sakthivel - reply:
This manuscript presents a thoughtful interpretation of clumped and stable oxygen isotope data from the Maastrichtian Chalk Sea. While the study provides valuable contributions to our understanding of past marine conditions, there are a few aspects that would benefit from further clarification and discussion to strengthen the scientific interpretation. The main points are summarized below:
1) The authors apply the Δ47–temperature calibration from Daëron & Gray (2023) to both planktonic and benthic foraminifera. However, it is important to note that this calibration was developed specifically for planktonic species and has been shown to be poorly suited for benthic taxa. Applying it to benthic foraminifera may introduce systematic bias in bottom-water temperature reconstructions, which could affect interpretations related to thermal stratification and deep-water circulation. It would be more appropriate to apply a benthic-specific calibration and assess whether the results align with those from the Daëron & Gray (2023) calibration within the stated uncertainties.
Daëron and Gray (2023) mentioned a potential issue with the Piasecki et al. (2019) dataset that could be due to the measurements themselves: “After N. Meckler, who was one of the authors of the Piasecki et al. (2019) study and who reviewed the present work, suggested that this particular data set may not be as robust as it would be following today's best practices, we reviewed the corresponding raw data that she kindly shared and we concur that the level of replication of unknown samples and the temporal distribution of unknown versus standard replicates in that study was not ideal, making final average Δ47 values potentially susceptible to substantial standardization errors. This hypothesis would also be supported, albeit circumstantially, by the much-improved agreement between benthic δ18Oc-derived and Δ47-derived Cenozoic temperatures when using our new planktic calibration.” Also, it is important to note that the dataset of Peral et al. (2018) includes two benthic foraminiferal species, showing a good agreement with the planktonic D47. Therefore, the calibration used here is a good choice given the current state of the art, as new Δ47 measurements of benthic foraminifera from well-constrained core tops would be needed to verify the need to potentially use a benthic-specific calibration. The use of the calibration of Meinecke et al. (2021 – only planktonic) or Peral et al. (2022 – mainly planktonic + 2 benthic) will just change the absolute values, shifting to slightly warmer temperatures of maximum 2 deg.C. This shift will not change the observation made in the comparison with the range of temperature from other proxies.
2) At several stratigraphic levels (86, 75, and 46 m), surface water temperatures are reported as being lower than those of bottom waters. This inversion contrasts with expectations for greenhouse climates, where surface waters are typically warmer. The authors suggest freshwater input as a potential cause, but mixing freshwater with seawater generally increases specific heat capacity, which would likely buffer against cooling rather than promote it—unless the input was from meltwater.
Modern planktonic foraminifera have their growth season often related to enhanced nutrient input in spring or early summer and they grow for a limited amount of time, roughly few weeks. In case we deal at the studied site with enhanced winter or spring rainfall that brings in colder freshwater it will create a so-called freshwater lid that can be colder if the resulting surface water mix is only few PSU units less saline than the main seawater component. This enhanced freshwater input season can create a temporary stable stratification and at the same time enhanced freshwater input from nearby river inlets will bring in nutrients that could stimulate planktonic foraminifera growth recording this temporary signal. Also keeping in mind that the Maastrichtian is thought to record mild greenhouse climate not much different than the modern one where rainfall, formed in the atmosphere is cold. There is no requirement for glaciation and meltwater to form colder freshwater during the winter/spring season with temperatures between 10 to 15 °C. We tried to formulate this clearer in the revised version.
Additionally, the explanation involving upwelling is debatable. While upwelling can lower surface temperatures, it typically brings saline, deeper water to the surface, which would be expected to have a heavier, not lighter, δ¹⁸O signature. Clarifying these mechanisms with supporting evidence would strengthen the argument.
True, we modified our paragraph and support hypotheses with statistic.
3) The manuscript compares Δ47-based sea surface temperatures from the study site with mean δ¹⁸O-derived SST estimates from other locations. While this offers useful context, a more direct and meaningful comparison would involve estimating SST from δ¹⁸O values measured on the same foraminiferal specimens used for clumped isotope analysis. This within-sample comparison would allow for a clearer evaluation of proxy agreement and would strengthen the argument that Δ47-SST aligns more closely with δ18O-based temperatures than with TEX86 reconstructions.
We are not convinced that using d18O as thermometer is the best way to do, however, to show the importance better constraint on d18Osw, we estimate the sub-surface temperature using the same calibration and correction than previous articles. However, it is important to note that despite the dependance on d18Osw, other limitations have to be taken into account when working with d18O as thermometers, such as the so-called vital effect, living depth and seasonality of the foraminifera… leading to the need of specific calibrations. These d18O-temperatures have therefore to be used carefully.
4) Understanding the depth habitat of the studied foraminifera is critical when reconstructing vertical water column profiles. While the manuscript assigns species as either surface or bottom dwellers, no ecological justification or calcification depth estimates are provided. Including at least approximate depth ranges based on literature or modern analogs would improve confidence in the stratification interpretations.
We added the only reference we found in the material and method section to describe the ecology of the planktonic species we worked on. Applying modern analogs is almost impossible on most of the Cretaceous species, as this genius disappeared at the end of the Cretaceous.
5) The authors propose mechanisms such as freshwater runoff and upwelling to explain the observed isotopic patterns. However, potentially relevant supporting data—such as TOC/TN ratios, δ¹⁵N and δ¹³C of organics, or foraminiferal assemblage changes—are not presented. These datasets, which are available from the same core (https://doi.org/10.1016/j.gr.2023.11.010), could provide important context and would help substantiate the proposed scenarios.
No significant changes are observed with these other proxies, this is why we did not mention them. Now we added a paragraph with a summary of these proxies variations, to explain which one are interested to compare with our data.
6) The study notes that each sample was measured for clumped isotopes between 6 and 16 times, but the rationale behind this level of replication is not explained. It is unclear whether this was intended to improve counting statistics. A brief explanation of the replication strategy and the method used to assess uncertainties would help clarify the robustness of the reported Δ47 values and temperature estimates.
Done
7) Dubicka et al. (2024) have conducted detailed SEM and elemental analyses to assess diagenetic alteration in foraminifera from the same core used in the present study. It would strengthen the manuscript to briefly mention this elemental evidence, highlighting the absence of diagenetic alteration to support the reliability of the geochemical data.
These data/images were presented in Fig. 2 (now Fig. 3) and mentioned in the material and method section. We added a section about previous work in the new section 2 (geological, stratigraphical and environmental context) and discus preservation there.
Citation: https://doi.org/10.5194/egusphere-2025-502-AC4
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AC4: 'Reply on CC1', Marion Peral, 04 Aug 2025
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RC1: 'Comment on egusphere-2025-502', Anonymous Referee #1, 06 May 2025
REVIEW
Dear Editors and Authors,
Thank you for the opportunity to review the manuscript entitled “Clumped isotope temperature and salinity constraints for the Maastrichtian Chalk Sea based on planktonic and benthic foraminifera from Poland.” I sincerely appreciate the authors’ effort in compiling new geochemical data and applying state-of-the-art clumped isotope methods to an important paleoclimate question.
The manuscript presents novel Δ₄₇-based temperature reconstructions using both planktonic and benthic foraminifera from the Polanówka core, offering valuable insights into surface and bottom water conditions of the Maastrichtian Chalk Sea. The dual-species approach in an open marine context is innovative and helps fill a gap in the existing literature, which has often focused on coastal or single-depth records. The study addresses important scientific questions related to water column structure, salinity variation, and possible links to climate change and sea-level fluctuations during the Late Cretaceous.
While the dataset is promising and the scientific questions relevant, I believe the manuscript would benefit from several substantial revisions to enhance clarity, contextual grounding, and robustness of interpretation. My comments below are intended to be constructive and supportive, with the goal of strengthening the manuscript for publication.
Geological and Stratigraphic Context
At present, the manuscript does not include a dedicated description of the geological setting or stratigraphic context of your study sections. Given the importance of timing and depositional environment in interpreting geochemical signals, I suggest the authors consider adding:
A concise summary of the regional geology and depositional environment of the Chalk Sea;
Stratigraphic logs indicating lithology, sampling levels, and potential sequence boundaries;
Relevant sedimentological or facies observations that might support environmental shifts;
Photographs or thin-section images to illustrate sample preservation and context.
This information would help readers assess the temporal and environmental significance of the presented data and strengthen the broader interpretations.
Sample Resolution and Seasonal Bias
The study is based on a limited number of samples (five planktonic and five benthic), which understandably reflects preservation and analytical challenges. However, the small sample size does restrict the temporal resolution of interpretations. Furthermore, as noted by the authors, planktonic foraminifera are sensitive to seasonal variations, which could introduce growth season bias in the surface temperature reconstructions.
To strengthen this aspect, I encourage the authors to:
More explicitly address the potential influence of seasonal signals in the Discussion section;
Consider and discuss alternative interpretations of the observed surface–bottom temperature gradients (e.g., ecological or seasonal overprints);
Indicate whether additional supporting proxies (such as δ¹⁸O profiles, faunal changes, or sedimentological features) are available to contextualize the geochemical data.
Data Quality and Visualization
The Δ₄₇ methods are clearly explained and follow established protocols. However, the current presentation of the data could be enhanced to increase clarity and accessibility, particularly for readers less familiar with the technique. I recommend:
Including the number of replicates and standard deviations for each sample in relevant figures;
Displaying the reproducibility of carbonate standards (e.g., ETH-4) visually, perhaps in supplementary material;
Expanding Figure 3 to incorporate sea-level curves, glacial indicators, or broader climatic markers, to allow comparison between geochemical trends and regional/global environmental signals.
These additions would help convey the robustness of the results and strengthen the link between data and interpretation.
Hypotheses
The manuscript proposes two plausible mechanisms to explain bottom water signals: incursions of cold North Atlantic water and inputs of warmer Tethyan water during transgressive phases. While both are scientifically reasonable, the current data do not allow these hypotheses to be clearly distinguished or independently tested.
To improve this section, I suggest presenting these interpretations as working hypotheses and clearly outlining the supporting and limiting evidence. If additional constraints—such as εNd provenance interpretation, faunal changes, or stratigraphic markers—are available or could be incorporated in future work, this would enhance the credibility of the proposed scenarios.
Writing, Terminology, and Title
A few writing and formatting issues could be addressed to improve clarity and consistency:
The title should be revised to correct “salinity constrains” to “salinity constraints.” Fossil names (e.g., Planohedbergella praeihilensis, Cibicidoides voltzianus) should be italicized throughout. In the Abstract and Conclusions, I suggest more clearly linking your findings to broader climate and oceanographic frameworks, while maintaining a balanced tone regarding uncertainties. A brief explanation in the Methods section about the need for repeated standard measurements would also be helpful for non-specialist readers.
Overall Assessment
This manuscript explores a timely and scientifically important topic with the use of advanced geochemical tools and well-preserved microfossils. The data are promising, and the approach is appropriate. However, the study would benefit from enhanced geological context, more cautious interpretation of the geochemical signals (especially given the limited sample size), and improved visual presentation of data and uncertainties. With these revisions, the paper would be well-positioned to make a valuable contribution to the study of Late Cretaceous paleoceanography and shallow marine systems.
Recommendation: Encouraged Resubmission
I look forward to seeing a revised version and thank the authors again for their efforts on this important topic.
Warm regards.
Citation: https://doi.org/10.5194/egusphere-2025-502-RC1 -
AC2: 'Reply on RC1', Marion Peral, 04 Aug 2025
Referee #1 reply:
Dear Editors and Authors,
Thank you for the opportunity to review the manuscript entitled “Clumped isotope temperature and salinity constraints for the Maastrichtian Chalk Sea based on planktonic and benthic foraminifera from Poland.” I sincerely appreciate the authors’ effort in compiling new geochemical data and applying state-of-the-art clumped isotope methods to an important paleoclimate question.
The manuscript presents novel Δ₄₇-based temperature reconstructions using both planktonic and benthic foraminifera from the Polanówka core, offering valuable insights into surface and bottom water conditions of the Maastrichtian Chalk Sea. The dual-species approach in an open marine context is innovative and helps fill a gap in the existing literature, which has often focused on coastal or single-depth records. The study addresses important scientific questions related to water column structure, salinity variation, and possible links to climate change and sea-level fluctuations during the Late Cretaceous.
While the dataset is promising and the scientific questions relevant, I believe the manuscript would benefit from several substantial revisions to enhance clarity, contextual grounding, and robustness of interpretation. My comments below are intended to be constructive and supportive, with the goal of strengthening the manuscript for publication.
Thank you for your valuable suggestions. We have taken them all into account and hope that the manuscript has been significantly improved.
Geological and Stratigraphic Context
At present, the manuscript does not include a dedicated description of the geological setting or stratigraphic context of your study sections. Given the importance of timing and depositional environment in interpreting geochemical signals, I suggest the authors consider adding:
A concise summary of the regional geology and depositional environment of the Chalk Sea;
Stratigraphic logs indicating lithology, sampling levels, and potential sequence boundaries;
Relevant sedimentological or facies observations that might support environmental shifts;
Photographs or thin-section images to illustrate sample preservation and context.
This information would help readers assess the temporal and environmental significance of the presented data and strengthen the broader interpretations.
Thank you for the comment. We agree and added a new section named Geological, stratigraphical and environmental context in section 2. Unfortunately, we do not have thin section images, but we have SEM pictures of the foraminifera that we present in the context. We added a new figure with the stratigraphic context as presented in Dubicka et al. (2024) – Figure 2.
Sample Resolution and Seasonal Bias
The study is based on a limited number of samples (five planktonic and five benthic), which understandably reflects preservation and analytical challenges. However, the small sample size does restrict the temporal resolution of interpretations. Furthermore, as noted by the authors, planktonic foraminifera are sensitive to seasonal variations, which could introduce growth season bias in the surface temperature reconstructions.
To strengthen this aspect, I encourage the authors to:
More explicitly address the potential influence of seasonal signals in the Discussion section;
Consider and discuss alternative interpretations of the observed surface–bottom temperature gradients (e.g., ecological or seasonal overprints);
Indicate whether additional supporting proxies (such as δ¹⁸O profiles, faunal changes, or sedimentological features) are available to contextualize the geochemical data.
Thank you for the recommendation. We added more detail in discussion section about potential seasonality effect. We tried to link better our results with already published dataset at our site, but little changes are found in the lithology, in the foraminiferal assemblages and in geochemical proxies. We also want to point out that this dataset is probably considering as limited in the number of samples, but to get this dataset, a total of 112 measurements (excluding the standards) at 0.5 mg of foraminifera per analyses, which is quite significative.
Data Quality and Visualization
The Δ₄₇ methods are clearly explained and follow established protocols. However, the current presentation of the data could be enhanced to increase clarity and accessibility, particularly for readers less familiar with the technique. I recommend:
Including the number of replicates and standard deviations for each sample in relevant figures;
Displaying the reproducibility of carbonate standards (e.g., ETH-4) visually, perhaps in supplementary material;
Expanding Figure 3 to incorporate sea-level curves, glacial indicators, or broader climatic markers, to allow comparison between geochemical trends and regional/global environmental signals.
These additions would help convey the robustness of the results and strengthen the link between data and interpretation.
Yes, we definitely agree. We modified the methodology section. We added details in the table with the summary of the results and an active link in a repository. The external reproducibility is mentioned and we moved the quality control values (ETH 4) in the method section rather than in the result section. Replicate number and uncertainties are presented in the Table and the figure are plotted with 2SE uncertainties.
We modified the Figure 3, by dividing it into 2 parts, one with a focus on the section (our data and other already published data) and one with the comparison with other temperature data.
Hypotheses
The manuscript proposes two plausible mechanisms to explain bottom water signals: incursions of cold North Atlantic water and inputs of warmer Tethyan water during transgressive phases. While both are scientifically reasonable, the current data do not allow these hypotheses to be clearly distinguished or independently tested.
To improve this section, I suggest presenting these interpretations as working hypotheses and clearly outlining the supporting and limiting evidence. If additional constraints—such as εNd provenance interpretation, faunal changes, or stratigraphic markers—are available or could be incorporated in future work, this would enhance the credibility of the proposed scenarios.
We tried to present the hypotheses better as hypotheses. We discuss the evidence. However, no significant changes are observed in other proxies available at our site. But we include a section to put into perspective these previous results.
Writing, Terminology, and Title
A few writing and formatting issues could be addressed to improve clarity and consistency:
The title should be revised to correct “salinity constrains” to “salinity constraints.”
Done
Fossil names (e.g., Planohedbergella praeihilensis, Cibicidoides voltzianus) should be italicized throughout.
Done, it was due a formatting issue, sorry for that.
In the Abstract and Conclusions, I suggest more clearly linking your findings to broader climate and oceanographic frameworks, while maintaining a balanced tone regarding uncertainties.
Done
A brief explanation in the Methods section about the need for repeated standard measurements would also be helpful for non-specialist readers.
Done
Overall Assessment
This manuscript explores a timely and scientifically important topic with the use of advanced geochemical tools and well-preserved microfossils. The data are promising, and the approach is appropriate. However, the study would benefit from enhanced geological context, more cautious interpretation of the geochemical signals (especially given the limited sample size), and improved visual presentation of data and uncertainties. With these revisions, the paper would be well-positioned to make a valuable contribution to the study of Late Cretaceous paleoceanography and shallow marine systems.
Thank you for your valuable comments. The manuscript has been significantly improved.
Recommendation: Encouraged Resubmission
I look forward to seeing a revised version and thank the authors again for their efforts on this important topic.
Warm regards.
Citation: https://doi.org/10.5194/egusphere-2025-502-AC2 -
AC3: 'Reply on RC1', Marion Peral, 04 Aug 2025
Referee #1 reply:
Dear Editors and Authors,
Thank you for the opportunity to review the manuscript entitled “Clumped isotope temperature and salinity constraints for the Maastrichtian Chalk Sea based on planktonic and benthic foraminifera from Poland.” I sincerely appreciate the authors’ effort in compiling new geochemical data and applying state-of-the-art clumped isotope methods to an important paleoclimate question.
The manuscript presents novel Δ₄₇-based temperature reconstructions using both planktonic and benthic foraminifera from the Polanówka core, offering valuable insights into surface and bottom water conditions of the Maastrichtian Chalk Sea. The dual-species approach in an open marine context is innovative and helps fill a gap in the existing literature, which has often focused on coastal or single-depth records. The study addresses important scientific questions related to water column structure, salinity variation, and possible links to climate change and sea-level fluctuations during the Late Cretaceous.
While the dataset is promising and the scientific questions relevant, I believe the manuscript would benefit from several substantial revisions to enhance clarity, contextual grounding, and robustness of interpretation. My comments below are intended to be constructive and supportive, with the goal of strengthening the manuscript for publication.
Thank you for your valuable suggestions. We have taken them all into account and hope that the manuscript has been significantly improved.
Geological and Stratigraphic Context
At present, the manuscript does not include a dedicated description of the geological setting or stratigraphic context of your study sections. Given the importance of timing and depositional environment in interpreting geochemical signals, I suggest the authors consider adding:
A concise summary of the regional geology and depositional environment of the Chalk Sea;
Stratigraphic logs indicating lithology, sampling levels, and potential sequence boundaries;
Relevant sedimentological or facies observations that might support environmental shifts;
Photographs or thin-section images to illustrate sample preservation and context.
This information would help readers assess the temporal and environmental significance of the presented data and strengthen the broader interpretations.
Thank you for the comment. We agree and added a new section named Geological, stratigraphical and environmental context in section 2. Unfortunately, we do not have thin section images, but we have SEM pictures of the foraminifera that we present in the context. We added a new figure with the stratigraphic context as presented in Dubicka et al. (2024) – Figure 2.
Sample Resolution and Seasonal Bias
The study is based on a limited number of samples (five planktonic and five benthic), which understandably reflects preservation and analytical challenges. However, the small sample size does restrict the temporal resolution of interpretations. Furthermore, as noted by the authors, planktonic foraminifera are sensitive to seasonal variations, which could introduce growth season bias in the surface temperature reconstructions.
To strengthen this aspect, I encourage the authors to:
More explicitly address the potential influence of seasonal signals in the Discussion section;
Consider and discuss alternative interpretations of the observed surface–bottom temperature gradients (e.g., ecological or seasonal overprints);
Indicate whether additional supporting proxies (such as δ¹⁸O profiles, faunal changes, or sedimentological features) are available to contextualize the geochemical data.
Thank you for the recommendation. We added more detail in discussion section about potential seasonality effect. We tried to link better our results with already published dataset at our site, but little changes are found in the lithology, in the foraminiferal assemblages and in geochemical proxies. We also want to point out that this dataset is probably considering as limited in the number of samples, but to get this dataset, a total of 112 measurements (excluding the standards) at 0.5 mg of foraminifera per analyses, which is quite significative.
Data Quality and Visualization
The Δ₄₇ methods are clearly explained and follow established protocols. However, the current presentation of the data could be enhanced to increase clarity and accessibility, particularly for readers less familiar with the technique. I recommend:
Including the number of replicates and standard deviations for each sample in relevant figures;
Displaying the reproducibility of carbonate standards (e.g., ETH-4) visually, perhaps in supplementary material;
Expanding Figure 3 to incorporate sea-level curves, glacial indicators, or broader climatic markers, to allow comparison between geochemical trends and regional/global environmental signals.
These additions would help convey the robustness of the results and strengthen the link between data and interpretation.
Yes, we definitely agree. We modified the methodology section. We added details in the table with the summary of the results and an active link in a repository. The external reproducibility is mentioned and we moved the quality control values (ETH 4) in the method section rather than in the result section. Replicate number and uncertainties are presented in the Table and the figure are plotted with 2SE uncertainties.
We modified the Figure 3, by dividing it into 2 parts, one with a focus on the section (our data and other already published data) and one with the comparison with other temperature data.
Hypotheses
The manuscript proposes two plausible mechanisms to explain bottom water signals: incursions of cold North Atlantic water and inputs of warmer Tethyan water during transgressive phases. While both are scientifically reasonable, the current data do not allow these hypotheses to be clearly distinguished or independently tested.
To improve this section, I suggest presenting these interpretations as working hypotheses and clearly outlining the supporting and limiting evidence. If additional constraints—such as εNd provenance interpretation, faunal changes, or stratigraphic markers—are available or could be incorporated in future work, this would enhance the credibility of the proposed scenarios.
We tried to present the hypotheses better as hypotheses. We discuss the evidence. However, no significant changes are observed in other proxies available at our site. But we include a section to put into perspective these previous results.
Writing, Terminology, and Title
A few writing and formatting issues could be addressed to improve clarity and consistency:
The title should be revised to correct “salinity constrains” to “salinity constraints.”
Done
Fossil names (e.g., Planohedbergella praeihilensis, Cibicidoides voltzianus) should be italicized throughout.
Done, it was due a formatting issue, sorry for that.
In the Abstract and Conclusions, I suggest more clearly linking your findings to broader climate and oceanographic frameworks, while maintaining a balanced tone regarding uncertainties.
Done
A brief explanation in the Methods section about the need for repeated standard measurements would also be helpful for non-specialist readers.
Done
Overall Assessment
This manuscript explores a timely and scientifically important topic with the use of advanced geochemical tools and well-preserved microfossils. The data are promising, and the approach is appropriate. However, the study would benefit from enhanced geological context, more cautious interpretation of the geochemical signals (especially given the limited sample size), and improved visual presentation of data and uncertainties. With these revisions, the paper would be well-positioned to make a valuable contribution to the study of Late Cretaceous paleoceanography and shallow marine systems.
Thank you for your valuable comments. The manuscript has been significantly improved.
Recommendation: Encouraged Resubmission
I look forward to seeing a revised version and thank the authors again for their efforts on this important topic.
Warm regards.
Citation: https://doi.org/10.5194/egusphere-2025-502-AC3
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AC2: 'Reply on RC1', Marion Peral, 04 Aug 2025
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RC2: 'Comment on egusphere-2025-502', Anonymous Referee #2, 19 May 2025
Dear Dr. Donnadieu,
The study by Peral et al. presents a dataset of clumped isotope paleotemperature and calculated water isotope values from the Maastrichtian Chalk Sea of Europe. What is interesting about the dataset is that it presents measurements of the geochemistry of both planktic and benthic foraminifera, using recent clumped isotope methodological advancements in measurements of small samples. This potentially provides a new insight into the depth profile of the water column in the Maastrichtian, an important understanding for the oceanography of warm climates. Based on this alone I think the study can make for a solid contribution to Climate of the Past. I do have several concerns though about the rigor of hypothesis testing in the discussion and need for more description of samples, which I consider to be crucial to address before publication to ensure the quality of the ultimate article. I have left a detailed review since I think the study has good potential although it still needs some attention to detail and there are some additional considerations to address.
General comments:
1) Small sample set and potential for overinterpretation of the data. Although the data are certainly novel, I am concerned about the potential for overinterpretation in some aspects of the current version of the manuscript. For example, the abstract and conclusion of the manuscript highlight findings on the mid-Maastrichtian Event (MME), however, it seems this may be based on only 4 or fewer samples. The MME is not described or placed in the strat column, so its correlation is unclear. It is also unclear whether differences between pairs of benthic and planktic temperature measurements are statistically significant. It seems like temperatures overlap in figure 3 for the pairs, but d18Osw is more likely to be statistically different. In general the data are insightful and the authors should not feel pressure to fully reconstruct Maastrichtian oceanographic circulation locally based on 10 data points. First order hypothesis testing will suffice and temporal changes are likely beyond the resolution of the sampling here. Also the statistical question about whether paired samples are resolvabely different should be addressed in the text.
2) First-order testing of interpretations of water column structure. Ultimately the scenarios for water column structure based on the clumped isotope data depend on a density contrast (less dense surface waters vs more dense bottom water) to be valid. Given temperature and inferred salinity ranges from the clumped data, can ranges for the water density contrast between shallow/deep waters in the different scenarios help to contextualize the validity of either or both scenarios? Salinity is referenced in the title and discussion but can a quantitative range on salinity be inferred from the d18Osw values if a reasonable range of d18O is assumed for local precipitation?
3) More information needs to be provided on the samples and site. For example, where in eastern Poland is the UW-1 core located (lat./long., nearby municipality). How many foraminifera were picked for each analysis and from each sampled horizon? How many horizons in the core were sampled previously and why were certain horizons chosen for clumped isotope analysis? Is there past sedimentological research of the core and what are the lithologies? Have events like the MME and K/Pg been correlated to the core in other studies, and if so where do they occur? How is the foraminifera test preservation determined? Descriptors range from “good” to “well” to “exceptionally well” preserved. Can this terminology be tightened?
4) Some improvements to figures would benefit the visualization of the data. Figure 1 is pixelated, and it would be helpful to have an additional regional map (either inset or separate panel) since this study is focused on the relatively regional paleoceanography of the chalk sea (see Fig 1 from Tagliavento et al. 2019). At current global scale, it is difficult to visualize the local basins and degree of connection vs restriction. Figure 3 is somewhat confusing, particularly the left plot. Please add subscripts to the d18O axis title label in left subplot and add VPDB or VSMOW for units after per mil symbols. Please add “Δ47 temperature (°C)” or similar as the lower left x-axis label. Consider removing some of the shaded polygons on the left panel since these are distracting from viewing of this study’s data. If comparison to other temperature estimates from other proxies/studies is important, consider adding a new figure to meet that objective. I was also a bit concerned by the co-plotting of Δ47 temperature with δ18Ocarb in the same panel. Is the δ18O upper x-axis scale aligned with the bottom temperature scale somehow? Since δ18O data are sometimes interpreted as temperature data by assuming a δ18Ow value, the δ18Ocarb data might be better plotted in a separate panel. Given all the information in this figure, consider creating an additional figure to compare statistically summarized results from this study with other studies/proxies (the shaded boxes).
5) The writing is generally of good quality but there are instances where grammar or punction need to be corrected to avoid confusion. I have noted a few examples below but I encourage a close look before final publication.
Geochemical nomenclature recommendation:
Ln 123. Add subscript “sw” in d18Osw throughout. Also the discussion of d18Ow and d18Ocarb throughout the manuscript has the potential to be a point of confusion. I recommend using the following nomenclature δ18Ocarb and δ18Osw, or something along those lines. For differentiating results from benthic vs planktonic forams, some type of shorthand such as δ18Osw,p orδ18Osw,b could be used to reduce ambiguity in reporting of results in section 3 and discussion. Additionally, when reporting d18O permil values, I recommend adding “VPDB” or “VSMOW” after the per mil symbol. In general, be sure to define all geochemical abbreviations as they are introduced throughout the text.
Specific comments
Title: should read “constraints” I think
Ln 14. The phasing “…experienced long-term cooling with high atmospheric CO2…” was a bit confusing. Please rephrase.
Ln 18. Spelled “Tethys Ocean” elsewhere
Ln 27. Change to “strongly depleted” and “associated with”
Ln 32. Consider phrasing “…hydroclimate dynamics…”
Ln 42. “aquifer-eustasy”
Ln 52-53. Is there some type of classic citation on this concept that could be added to support the sentence?
Ln. 56-57. Remove Huber et al. 2002 citation – no TEX86 measured in that study
Ln 69. Rephrase to “…independence from…”
Ln 73. Results from this study are compared with data from O’Hora et al 2022 in the discussion. Perhaps a note of caution is needed in the discussion unless the data are considered relatively comparable.
Ln 85. Not much context is provided for the neodymium isotopes. The abbreviation should be defined at a minimum but consider adding a short phrase with context for that type of dataset and how it is useful to compare with clumped isotope data.
Ln 105-106. It might be helpful to note which of these specimens are benthonic vs planktonic for non-specialists and because the comparison between planktic v benthic data are a unique attribute of this study.
Ln 108. Be sure to italicize species names here and throughout
Ln 126. Are the eNd data from the same stratigraphic horizons sampled for this foram-based study?
Table 1. Consider adding δ18Ocarb and δ13Ccarb, as well as units for depth and temperature.
Ln 187-188. This is a good to include comparisons with existing Maastrichtian datasets. Note that Maastrichtian D47 temperature data from Meyer et al. 2018 from the Prairie Bluff Formation were reprocessed using updated 17-O parameters by Jones et al 2022 Geology https://doi.org/10.1130/G49998.1 (appendix tables). These recalibrated temperatures may allow for a more direct comparison with this study.
Ln 198. Were data discussed also collected in deeper sea settings compared to this site?
Ln 200. Are these findings statistically significant given analytical and calibration uncertainties on clumped isotope measurements? Please include brief note on this if so.
Lns 201-205. The comparison with the modern Baltic Sea is helpful and interesting context. These Baltic Sea surface d18Osw values though are much lower than what is reported from the Maastrichtian foraminifera. Can water body density contrasts be explored to more rigorously test the hypotheses? (see major comment above)
Ln 266. "coastal"
Ln 286-288. Do foram samples analyzed in this study come from different lithologies or only chalk? Unless foraminifera are sampled from these different lithologies, foraminifera-derived data cannot be used to explain lithological cycles.
Ln 298. "milder"
Conclusions style recommendation: Keep as one paragraph as opposed to having paragraphs with 1-2 sentences.
Citation: https://doi.org/10.5194/egusphere-2025-502-RC2 -
AC1: 'Reply on RC2', Marion Peral, 04 Aug 2025
Referee #2 Reply:
Dear Dr. Donnadieu,
The study by Peral et al. presents a dataset of clumped isotope paleotemperature and calculated water isotope values from the Maastrichtian Chalk Sea of Europe. What is interesting about the dataset is that it presents measurements of the geochemistry of both planktic and benthic foraminifera, using recent clumped isotope methodological advancements in measurements of small samples. This potentially provides a new insight into the depth profile of the water column in the Maastrichtian, an important understanding for the oceanography of warm climates. Based on this alone I think the study can make for a solid contribution to Climate of the Past. I do have several concerns though about the rigor of hypothesis testing in the discussion and need for more description of samples, which I consider to be crucial to address before publication to ensure the quality of the ultimate article. I have left a detailed review since I think the study has good potential although it still needs some attention to detail and there are some additional considerations to address.
Thank you for your comment.
General comments:
- Small sample set and potential for overinterpretation of the data. Although the data are certainly novel, I am concerned about the potential for overinterpretation in some aspects of the current version of the manuscript. For example, the abstract and conclusion of the manuscript highlight findings on the mid-Maastrichtian Event (MME), however, it seems this may be based on only 4 or fewer samples.
It is true that the resolution is low, due to preservation and analytical challenges, however, we provide 5 datapoints on planktonic and 5 datapoints on the benthic foraminifera, representing a total of 112 analyses (without the standards), which is not a small dataset in total.
The MME is not described or placed in the strat column, so its correlation is unclear.
We added this stratigraphic information in the figure.
It is also unclear whether differences between pairs of benthic and planktic temperature measurements are statistically significant. It seems like temperatures overlap in figure 3 for the pairs, but d18Osw is more likely to be statistically different. In general the data are insightful and the authors should not feel pressure to fully reconstruct Maastrichtian oceanographic circulation locally based on 10 data points. First order hypothesis testing will suffice and temporal changes are likely beyond the resolution of the sampling here. Also the statistical question about whether paired samples are resolvabely different should be addressed in the text.
We added statistical analysis to better interpretate our hypothesis. We hope it is better nuanced. Our objective was just to list some potential hypothesis to explain these unexpected data between the sub-surface and the bottom water. We added a sentence at the beginning of the discussion to highlight the fact that the discussion is based on hypothesis and conclude that additional works are needed to test these hypotheses.
2) First-order testing of interpretations of water column structure. Ultimately the scenarios for water column structure based on the clumped isotope data depend on a density contrast (less dense surface waters vs more dense bottom water) to be valid. Given temperature and inferred salinity ranges from the clumped data, can ranges for the water density contrast between shallow/deep waters in the different scenarios help to contextualize the validity of either or both scenarios? Salinity is referenced in the title and discussion but can a quantitative range on salinity be inferred from the d18Osw values if a reasonable range of d18O is assumed for local precipitation?
Thank you for the comment. We added statistical analysis to better discriminate surface and bottom conditions, and changed the term salinity to d18Osw, as d18Osw is not only salinity. Therefore, the interpretation is now focusing on d18Osw changes rather than salinity. Salinity is already very difficult to estimate over Quaternary, so over the Maastrichtian, it is almost impossible with this approach, as the uncertainties will be too large due to all the assumptions needed here.
3) More information needs to be provided on the samples and site. For example, where in eastern Poland is the UW-1 core located (lat./long., nearby municipality). How many foraminifera were picked for each analysis and from each sampled horizon? How many horizons in the core were sampled previously and why were certain horizons chosen for clumped isotope analysis? Is there past sedimentological research of the core and what are the lithologies? Have events like the MME and K/Pg been correlated to the core in other studies, and if so where do they occur? How is the foraminifera test preservation determined? Descriptors range from “good” to “well” to “exceptionally well” preserved. Can this terminology be tightened?
Done, we added all this information in the new section (2. Context) and in the material and method. However, it is difficult to range the preservation as we do not have comparison point. Additional SEM images are included in this revised version that show good preservation. Previous works on the section also show absence of enhance Fe, Mn concentrations that are typical for diagenetic alteration phases. We now cite properly the references for the readers to refer to.
4) Some improvements to figures would benefit the visualization of the data. Figure 1 is pixelated, and it would be helpful to have an additional regional map (either inset or separate panel) since this study is focused on the relatively regional paleoceanography of the chalk sea (see Fig 1 from Tagliavento et al. 2019). At current global scale, it is difficult to visualize the local basins and degree of connection vs restriction. Figure 3 is somewhat confusing, particularly the left plot. Please add subscripts to the d18O axis title label in left subplot and add VPDB or VSMOW for units after per mil symbols. Please add “Δ47 temperature (°C)” or similar as the lower left x-axis label. Consider removing some of the shaded polygons on the left panel since these are distracting from viewing of this study’s data. If comparison to other temperature estimates from other proxies/studies is important, consider adding a new figure to meet that objective. I was also a bit concerned by the co-plotting of Δ47 temperature with δ18Ocarb in the same panel. Is the δ18O upper x-axis scale aligned with the bottom temperature scale somehow? Since δ18O data are sometimes interpreted as temperature data by assuming a δ18Ow value, the δ18Ocarb data might be better plotted in a separate panel. Given all the information in this figure, consider creating an additional figure to compare statistically summarized results from this study with other studies/proxies (the shaded boxes).
We added the additional map. We modified the figure 3 (now figure 4) and add another figure (5) to focus only on temperature comparison.
5) The writing is generally of good quality but there are instances where grammar or punction need to be corrected to avoid confusion. I have noted a few examples below but I encourage a close look before final publication.
Thank you, we have checked this.
Geochemical nomenclature recommendation:
Ln 123. Add subscript “sw” in d18Osw throughout. Also the discussion of d18Ow and d18Ocarb throughout the manuscript has the potential to be a point of confusion. I recommend using the following nomenclature δ18Ocarb and δ18Osw, or something along those lines. For differentiating results from benthic vs planktonic forams, some type of shorthand such as δ18Osw,p orδ18Osw,b could be used to reduce ambiguity in reporting of results in section 3 and discussion. Additionally, when reporting d18O permil values, I recommend adding “VPDB” or “VSMOW” after the per mil symbol. In general, be sure to define all geochemical abbreviations as they are introduced throughout the text.
Thank you, we modified all of them following your recommendations.
Specific comments
Title: should read “constraints” I think
Done
Ln 14. The phasing “…experienced long-term cooling with high atmospheric CO2…” was a bit confusing. Please rephrase.
Done
Ln 18. Spelled “Tethys Ocean” elsewhere
Done
Ln 27. Change to “strongly depleted” and “associated with”
Done
Ln 32. Consider phrasing “…hydroclimate dynamics…”
Done
Ln 42. “aquifer-eustasy”
Done
Ln 52-53. Is there some type of classic citation on this concept that could be added to support the sentence?
Done
Ln. 56-57. Remove Huber et al. 2002 citation – no TEX86 measured in that study
Done
Ln 69. Rephrase to “…independence from…”
Done
Ln 73. Results from this study are compared with data from O’Hora et al 2022 in the discussion. Perhaps a note of caution is needed in the discussion unless the data are considered relatively comparable.
This is true, we added a caution in the discussion.
Ln 85. Not much context is provided for the neodymium isotopes. The abbreviation should be defined at a minimum but consider adding a short phrase with context for that type of dataset and how it is useful to compare with clumped isotope data.
This is true, we introduced it in the new section 2.3 environmental context at the studied site.
Ln 105-106. It might be helpful to note which of these specimens are benthonic vs planktonic for non-specialists and because the comparison between planktic v benthic data are a unique attribute of this study.
Done
Ln 108. Be sure to italicize species names here and throughout
Done, formatting mistake to match climate of the past template.
Ln 126. Are the eNd data from the same stratigraphic horizons sampled for this foram-based study?
Most of them, or from narrow stratigraphic layers (see Fig. 3).
Table 1. Consider adding δ18Ocarb and δ13Ccarb, as well as units for depth and temperature.
Done
Ln 187-188. This is a good to include comparisons with existing Maastrichtian datasets. Note that Maastrichtian D47 temperature data from Meyer et al. 2018 from the Prairie Bluff Formation were reprocessed using updated 17-O parameters by Jones et al 2022 Geology https://doi.org/10.1130/G49998.1 (appendix tables). These recalibrated temperatures may allow for a more direct comparison with this study.
Interesting, thank you. However, I am scare that reprocessing using 17-O parameters of Brand won’t be enough to ensure a good comparison, as the same standards need to be measured for an accurate comparison. But we will discuss the reprocessed values in the discussion.
Ln 198. Were data discussed also collected in deeper sea settings compared to this site?
It is difficult to answer this question. It is likely yes, as our study site is located in the Chalk sea, that is expected to be quite shallow.
Ln 200. Are these findings statistically significant given analytical and calibration uncertainties on clumped isotope measurements? Please include brief note on this if so.
We added statistical analysis.
Lns 201-205. The comparison with the modern Baltic Sea is helpful and interesting context. These Baltic Sea surface d18Osw values though are much lower than what is reported from the Maastrichtian foraminifera. Can water body density contrasts be explored to more rigorously test the hypotheses? (see major comment above)
Answer in the major comment
Ln 266. "coastal"
Done
Ln 286-288. Do foram samples analyzed in this study come from different lithologies or only chalk? Unless foraminifera are sampled from these different lithologies, foraminifera-derived data cannot be used to explain lithological cycles.
Lithology is constant throughout the studied section.
Ln 298. "milder"
Done
Conclusions style recommendation: Keep as one paragraph as opposed to having paragraphs with 1-2 sentences.
Done
Citation: https://doi.org/10.5194/egusphere-2025-502-AC1
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AC1: 'Reply on RC2', Marion Peral, 04 Aug 2025
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