Assessing Environmental Change Associated with Early Eocene Hyperthermals in the Atlantic Coastal Plain, USA

Rush, William Davis; Self-Trail, Jean; Zhang, Yang; Sluijs, Appy; Brinkhuis, Henk; Zachos, James; Ogg, James; Robinson, Marci

doi:https://doi.org/10.5194/egusphere-2023-58

Preprints

https://doi.org/10.5194/egusphere-2023-58

Preprints

27 Jan 2023

| 27 Jan 2023

Assessing Environmental Change Associated with Early Eocene Hyperthermals in the Atlantic Coastal Plain, USA

William Davis Rush, Jean Self-Trail, Yang Zhang, Appy Sluijs, Henk Brinkhuis, James Zachos, James Ogg, and Marci Robinson

Abstract. Eocene transient global warming events (hyperthermals) can provide insight into a future, warmer world. While much research has focused on the Paleocene-Eocene Thermal Maximum (PETM), hyperthermals of smaller magnitude can be used to characterize climatic responses over different magnitudes of forcing. This study identifies two events, Eocene Thermal Maximum 2 (ETM2 and H2) for the first time in a shallow marine setting along the United States Atlantic margin in the Salisbury Embayment of Maryland, based on magnetostratigraphy, calcareous nannofossil and dinocyst biostratigraphy, and recognition of negative stable carbon isotope excursions (CIEs) in biogenic calcite. We assess local environmental change in the Salisbury Embayment utilizing clay mineralogy, marine palynology, δ¹⁸O of biogenic calcite, and biomarker paleothermometry (TEX₈₆). Paleo-temperature proxies show broad agreement between surface water and bottom water temperature changes. However, the timing of warming does not correspond to the CIE of ETM2 as expected from other records, and the highest values are observed during H2, suggesting factors other than pCO₂ forcing influenced temperature changes in the region. The ETM2 interval exhibits a shift in clay mineralogy from smectite-dominated facies to illite-rich facies, suggesting hydroclimatic changes but with a rather dampened weathering response relative to that of the PETM in the same region. Organic walled dinoflagellate cyst assemblages show large fluctuations throughout the studied section, none of which seem systematically related to ETM2. These observations are contrary to the typical tight correspondence between climate change and assemblages across the PETM, regionally and globally, and ETM2 in the Arctic Ocean. The data do indicate very warm and (seasonally) stratified conditions, likely salinity-driven, across H2. The absence of evidence for strong perturbations in local hydrology and nutrient supply during ETM2 and H2, compared to the PETM, is consistent with the less extreme forcing and the warmer pre-event baseline, as well as the non-linear response in hydroclimates to greenhouse forcing.

Received: 16 Jan 2023 – Discussion started: 27 Jan 2023

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Journal article(s) based on this preprint

17 Aug 2023

Assessing environmental change associated with early Eocene hyperthermals in the Atlantic Coastal Plain, USA

William Rush, Jean Self-Trail, Yang Zhang, Appy Sluijs, Henk Brinkhuis, James Zachos, James G. Ogg, and Marci Robinson

Clim. Past, 19, 1677–1698, https://doi.org/10.5194/cp-19-1677-2023,https://doi.org/10.5194/cp-19-1677-2023, 2023

Short summary

William Davis Rush et al.

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-58', Anonymous Referee #1, 13 Mar 2023

The manuscript provides new data of the ETM2 and H2 early Eocene Hyperthermal events in the Salisbury Embayment (Mid Atlantic coastal plain). This area has been studied in some sequences in the last years with particular interest on the PETM and the hyperthermal events of the lower Eocene , however the data presented by the authors are interesting as they show short - term changes and long - term trends which are often not consistent with data from pelagic, hemipelagic and coastal successions.
The first part of the manuscript is very robust and characterizes the stratigraphic framework of the succession which allows to correlate the investigated succession with the hyperthermal events ETM2 and H2.
I suggest the authors not to use informal terms in their biostratigraphic schemes and in any case to present them in detail in the methods. Specifically, the use of lower/upper before the biozone cannot be added in the biostratigraphy column of the figures. The authors can instead add events in the figure in order to highlight the possible informal subdivision of the biozone. However, these events should be detailed in the text (more details in the text).
The second part is dedicated to the changes that occurred in this depositional setting and to their substantial inconsistency with the available data both in terms of temporal relationships and actual modifications of the environment during these perturbations.
In the pdf file, I have included many comments and suggestions that the authors can use to implement their manuscript. In particular the authors must make sure not to mix results and discussion of them throughout the ms (details are provided in the edited file).
As for the discussion, there are some points, especially in the part that offers scenarios useful to explain the peculiar results of this study, which are either superficially presented or are inconsistent with the available literature (e.g. , the AMOC hypothesis, more details on specific points are provided in the edited file).
Another point that could potentially be improved is the comparison with all the available data and in any case with those of the Spanish sections which, in addition to having played a important role in the development of fundamental hypotheses related to mechanisms active during the hyperthermals (e.g., weathering and clay mineralogy)also represent a suitable continuous hemipelagic-continental transect ideal for a comparison with the record of the Mid Atlantic coastal plain.
Finally, I provide there are some suggestions to improve the figures, These are related to both the lack of units and the use of informal nomenclature ( more details in the text)

Citation: https://doi.org/10.5194/egusphere-2023-58-RC1
- AC1: 'Reply on RC1', William Rush, 02 Jun 2023
  
  The authors thank both referees for their constructive comments. We attempt to address each of their comments point by point below. For referee 1:
  I suggest the authors not to use informal terms in their biostratigraphic schemes and in any case to present them in detail in the methods. Specifically, the use of lower/upper before the biozone cannot be added in the biostratigraphy column of the figures. The authors can instead add events in the figure in order to highlight the possible informal subdivision of the biozone. However, these events should be detailed in the text (more details in the text).
  We have updated the biostratigraphic schemes to refer only to formally defined zones and removed the references to the upper and lower zones. More background information has been added about the biozones.
  In the pdf file, I have included many comments and suggestions that the authors can use to implement their manuscript. In particular the authors must make sure not to mix results and discussion of them throughout the ms (details are provided in the edited file).
  Comments from the pdf have been acknowledged and incorporated, particularly as it pertains to separation of results and discussion.
  As for the discussion, there are some points, especially in the part that offers scenarios useful to explain the peculiar results of this study, which are either superficially presented or are inconsistent with the available literature (e.g. , the AMOC hypothesis, more details on specific points are provided in the edited file).
  Reference to AMOC has been removed, alternate hypotheses have been put forward (e.g. sampling bias).
  Another point that could potentially be improved is the comparison with all the available data and in any case with those of the Spanish sections which, in addition to having played a important role in the development of fundamental hypotheses related to mechanisms active during the hyperthermals (e.g., weathering and clay mineralogy)also represent a suitable continuous hemipelagic-continental transect ideal for a comparison with the record of the Mid Atlantic coastal plain.
  We have attempted to edit this section for clarity.
  Finally, I provide there are some suggestions to improve the figures, These are related to both the lack of units and the use of informal nomenclature (more details in the text)
  Notes on the figures have been taken into account and the figures have been updated accordingly.
  
  Citation: https://doi.org/10.5194/egusphere-2023-58-AC1
RC2:
'Comment on Rush et al. manuscript', Anonymous Referee #2, 11 Apr 2023

General Comments:
This study presents an interesting and exciting body of work which focuses on the second largest hyperthermal of the early Eocene, ETM2, and its orbitally-paced partner event H2 in an under-studied depositional environment. It is interesting in part due to the curious environmental response (in terms of the timing and patterns of temperature and ecologic change relative to the CIE) which appears to be unique to the region. Except for the handling of uncertainty, the proxy-based environmental reconstructions are robust. Temperature responses are reproduced using two separate proxy systems, perhaps providing confirmation of the temperature trends.
Overall, the records are well discussed and interpreted. The paper is well-written and organized. The topic is certainly of interest to the Cenozoic paleoclimate community and following some minor revisions the manuscript is suitable for publication.
In my opinion, the primary finding of the study is the counterintuitive response of temperature in the region to CO₂ release for ETM2. ETM2 cooling like this has not been observed elsewhere, thus a detailed discussion of changes in the depositional environment, hydroclimate and ocean circulation is warranted. Therefore, I think the manuscript would benefit from a more detailed sedimentological interpretation of the changes in the depositional setting (i.e., physical sedimentology), in addition to the clay mineralogy work presented here.
Unfortunately, there is little to no consideration of proxy uncertainty in the manuscript. Temperature reconstruction figures provide no graphical estimation of uncertainty. Given the current state of the science, this is somewhat misleading and should be addressed prior to publication. Specifically, the authors should note 2sd or 0.95 quantile uncertainty. The paired mean δ¹⁸O and mean TEX₈₆ warming responses add to the validity of the interpreted cooling during ETM2, but a more detailed discussion of potential mechanisms influencing these proxy systems besides surface cooling would improve the manuscript.
In my opinion the manuscript would benefit from a more organized approach to interpreting changes in sedimentation, salinity, bathymetry, ocean and atmospheric circulation, surface productivity, and carbonate chemistry. The narrative of the discussion could be focused somewhat or organized in a way that considers how all the above-mentioned influence sedimentation, perhaps with a schematic figure. As written, the discussion tends to be harder to follow than necessary.
Additionally, some specific aspects of the discussion could be improved. I provide more detailed comments below where I think the authors could bolster their arguments by providing a more thorough discussion. The authors may find that quantitative salinity, seawater δ¹⁸O, reconstructions using δ¹⁸O and TEX₈₆-based temperatures would aid certain arguments in the discussion section.

Specific Comments:
Line 27: I recommend avoiding statements of novelty. If the authors wish to include these statements, please be more specific. As written, it is difficult to decipher if it is the first time both ETM2 and H2 have been identified in a shallow marine setting, or one event, or if it is the first time they’ve been identified along the Atlantic margin, or in the Salisbury embayment.
Line 30: The terminology here is a bit confusing. Was it the Salisbury Embayment in the early Eocene?
Line 33: Perhaps “in addition to pCO₂ forcing” is more appropriate phrasing?
Line 37: This is a bit vague. Systematically related to what ETM2 environmental response? The local CIE? The warming? The lithologic change? I think these fluctuations may still be related to ETM2 and/or H2 in the most general sense. Consider changing to “…related to CIE warming.”
Line 48: Relatively well constrained compared to what? Is it well constrained? The studies the authors cite have C release estimates that range from 4500 to 12000 GtC. Consider removing “well” and “relatively”.
Line 107: This is somewhat confusing. Consider rephrasing the following sentence: “Samples… have no control on their declination orientation.”
Line 139: A single mass, but two weights? I suggest rephrasing – confusing as written.
Line 214: Admittedly nit-picky, but doesn’t the first ETM2 CIE look closer to -2 to -2.5‰?
Line 263: It may be worth calculating paleo-salinity with δ¹⁸O temps and TEX₈₆-temps, to show that the effect is minimal, or to glean any subtle changes in freshwater input as this would be relevant to the present study.
Line 282: “Rare” or lacking entirely? The abstract indicated that this was the first shallow marine ETM2, but perhaps that needs to be clarified as suggested above.
Line 302: Unless baseline pH is very low, chemical erosion in shallow marine environments requires a strong pH decrease since the lysocline is typically at depths much deeper than the depositional environment discussed here. If acidification is pointed to as a previously hypothesized mechanism driving lower carbonate in this setting, this section could be improved from a discussion of acidification in this type of environment. Perhaps this could be done by providing what is known about lysocline fluctuations from deep marine sites, with a consideration of any regional amplifiers of acidification in the coastal environment. This would help prop-up the author’s statement that changes in siliciclastic flux is driving changes in carbonate content.
Additionally, how would shifts in nutrient availability influence these records? How is this linked to shifts in regional hydrology and circulation in the embayment (if it was still an embayment in the early Eocene)? Though these topics are briefly mentioned w.r.t. palynological assemblage changes, it seems to not be considered currently in the discussion of carbonate accumulation. As the authors note below this section, sedimentation rates (Fig 6) show little variation. Increased siliciclastic flux would tend to increase sedimentation rates if biogenic carbonate deposition is constant. Therefore, given the interpretation, I assume shifts in biogenic carbonate are required, and this could be estimated from sed rate and %CaCO₃. Considering the smaller pH anomaly of ETM2 compared with the PETM, and the shallow paleo-depth of the study site, this may not be due to acidification as the authors point out, but then what would be causing this drop in carbonate if sed rates are uniform and siliciclastic flux is changing? Perhaps shifts in surface productivity? The manuscript would benefit from a discussion of this here. The sedimentation rates provide a primary constraint and could be used quantitively, but this is not done here. Doing this may improve the discussion.
Line 309: More moderate increases in sedimentation rates for ETM2 and H2 or the PETM? At this locality specifically? I see that the authors expand this discussion on sed rates in the following paragraph, but this sentence is somewhat unclear as written.
Line 360: There is much discussion regarding palynologic assemblages pointing to salinity changes. Can this be identified with quantitative salinity reconstructions? If this cannot be done, please explain why in the manuscript.
Line 391: If possible, add a reference detailing the response of hydrologic cycle to various magnitude C release (i.e., [non]linearity of hydro response). This is needed for this line and for the following discussion in this paragraph.
Line 410: I don’t follow this argument. If the broad δ¹⁸O warming of ETM2 with onset of ~20kyr is missing, then why would you preserve the CIE in the same foram samples? Please expand on this topic or remove the argument altogether.

Citation: https://doi.org/10.5194/egusphere-2023-58-RC2
- AC2: 'Reply on RC2', William Rush, 02 Jun 2023
  
  The authors thank both referees for their constructive comments. We attempt to address each of their comments point by point below. For referee 2:
  In my opinion, the primary finding of the study is the counterintuitive response of temperature in the region to CO₂ release for ETM2. ETM2 cooling like this has not been observed elsewhere, thus a detailed discussion of changes in the depositional environment, hydroclimate and ocean circulation is warranted. Therefore, I think the manuscript would benefit from a more detailed sedimentological interpretation of the changes in the depositional setting (i.e., physical sedimentology), in addition to the clay mineralogy work presented here.
  Regarding the physical sedimentology, we believe we have provided a detailed characterization of the event in terms of weathering, sedimentation, and environmental changes, and have provided context to both the PETM and ETM2 at other sites. Questions of large-scale ocean circulation would likely require additional sites and go beyond the scope of this study.
  Unfortunately, there is little to no consideration of proxy uncertainty in the manuscript. Temperature reconstruction figures provide no graphical estimation of uncertainty. Given the current state of the science, this is somewhat misleading and should be addressed prior to publication. Specifically, the authors should note 2sd or 0.95 quantile uncertainty. The paired mean δ18O and mean TEX86 warming responses add to the validity of the interpreted cooling during ETM2, but a more detailed discussion of potential mechanisms influencing these proxy systems besides surface cooling would improve the manuscript.
  Figures have been updated to reflect uncertainty. However, in interpretation of δ¹⁸O, the largest uncertainty is not due to instrument precision or proxy calibration, but due to uncertainty in bottom-water salinity, as there are no estimates of δ¹⁸O_swfor this time in this area. Therefore, this figure has been updated to reflect the uncertainty related to a +/- 1 psu change relative to our estimates. Error associated with TEX₈₆ measurements is on the order of 0.2 °C and has been incorporated into the manuscript and figures.
  In my opinion the manuscript would benefit from a more organized approach to interpreting changes in sedimentation, salinity, bathymetry, ocean and atmospheric circulation, surface productivity, and carbonate chemistry. The narrative of the discussion could be focused somewhat or organized in a way that considers how all the above-mentioned influence sedimentation, perhaps with a schematic figure. As written, the discussion tends to be harder to follow than necessary.
  The section on the CIE magnitude and low carbonate intervals has been reworked and expanded.
  Additionally, some specific aspects of the discussion could be improved. I provide more detailed comments below where I think the authors could bolster their arguments by providing a more thorough discussion. The authors may find that quantitative salinity, seawater δ¹⁸O, reconstructions using δ¹⁸O and TEX₈₆-based temperatures would aid certain arguments in the discussion section.
  While we agree that quantitative salinity estimates would enhance the manuscript, we were unable to extract planktonic foraminifera in large enough quantity in order to obtain surface δ¹⁸O values. While the TEX₈₆ values record surface temperatures and could be used for salinity reconstructions when paired to surface δ¹⁸O values, at this time only bottom water δ¹⁸O values are available.
  Line 27: Statement of novelty removed.
  Line 30: We have clarified the terminology. “This study identifies two events, Eocene Thermal Maximum 2 (ETM2 and H2) in shallow marine sediments of the Eocene-aged Salisbury Embayment of Maryland, based on magnetostratigraphy, calcareous nannofossil and dinocyst biostratigraphy, and recognition of negative stable carbon isotope excursions (CIEs) in biogenic calcite.”
  Line 33: Done
  Line 37: Changed to “related to CIE warming”
  Line 48: Removed “well” and “relatively”
  Line 107: Clarified
  Line 139: Rephrased to clarify that we were measuring a mass between these values
  Line 214: Sure
  Line 263: As noted above, this is not possible with δ¹⁸O_Benthic
  Line 282: They are rare. This is the first time it has been identified in this region. Our earlier statement of novelty has been removed for clarification.
  Line 302: This argument has been thoroughly explored in Bralower et al., 2018 for this region during the PETM. Further exploration of the changing lysocline during ETM2 could be a manuscript on its own and goes beyond the scope of this study. With respect to productivity, our results show little correlation between sedimentation rates/palynological markers for productivity and the timing of either event. We have expanded upon this in the discussion.
  Line 309: We have clarified and expanded upon the sedimentation rate changes.
  Line 360: Again, our dataset in unable to quantifiably reconstruct salinity
  Line 391: Removed reference to linearity
  Line 410: The entirety of the CIE is not preserved in foraminifera samples and is partially reconstructed from bulk carbonate. This has been clarified in the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2023-58-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-58', Anonymous Referee #1, 13 Mar 2023

The manuscript provides new data of the ETM2 and H2 early Eocene Hyperthermal events in the Salisbury Embayment (Mid Atlantic coastal plain). This area has been studied in some sequences in the last years with particular interest on the PETM and the hyperthermal events of the lower Eocene , however the data presented by the authors are interesting as they show short - term changes and long - term trends which are often not consistent with data from pelagic, hemipelagic and coastal successions.
The first part of the manuscript is very robust and characterizes the stratigraphic framework of the succession which allows to correlate the investigated succession with the hyperthermal events ETM2 and H2.
I suggest the authors not to use informal terms in their biostratigraphic schemes and in any case to present them in detail in the methods. Specifically, the use of lower/upper before the biozone cannot be added in the biostratigraphy column of the figures. The authors can instead add events in the figure in order to highlight the possible informal subdivision of the biozone. However, these events should be detailed in the text (more details in the text).
The second part is dedicated to the changes that occurred in this depositional setting and to their substantial inconsistency with the available data both in terms of temporal relationships and actual modifications of the environment during these perturbations.
In the pdf file, I have included many comments and suggestions that the authors can use to implement their manuscript. In particular the authors must make sure not to mix results and discussion of them throughout the ms (details are provided in the edited file).
As for the discussion, there are some points, especially in the part that offers scenarios useful to explain the peculiar results of this study, which are either superficially presented or are inconsistent with the available literature (e.g. , the AMOC hypothesis, more details on specific points are provided in the edited file).
Another point that could potentially be improved is the comparison with all the available data and in any case with those of the Spanish sections which, in addition to having played a important role in the development of fundamental hypotheses related to mechanisms active during the hyperthermals (e.g., weathering and clay mineralogy)also represent a suitable continuous hemipelagic-continental transect ideal for a comparison with the record of the Mid Atlantic coastal plain.
Finally, I provide there are some suggestions to improve the figures, These are related to both the lack of units and the use of informal nomenclature ( more details in the text)

Citation: https://doi.org/10.5194/egusphere-2023-58-RC1
- AC1: 'Reply on RC1', William Rush, 02 Jun 2023
  
  The authors thank both referees for their constructive comments. We attempt to address each of their comments point by point below. For referee 1:
  I suggest the authors not to use informal terms in their biostratigraphic schemes and in any case to present them in detail in the methods. Specifically, the use of lower/upper before the biozone cannot be added in the biostratigraphy column of the figures. The authors can instead add events in the figure in order to highlight the possible informal subdivision of the biozone. However, these events should be detailed in the text (more details in the text).
  We have updated the biostratigraphic schemes to refer only to formally defined zones and removed the references to the upper and lower zones. More background information has been added about the biozones.
  In the pdf file, I have included many comments and suggestions that the authors can use to implement their manuscript. In particular the authors must make sure not to mix results and discussion of them throughout the ms (details are provided in the edited file).
  Comments from the pdf have been acknowledged and incorporated, particularly as it pertains to separation of results and discussion.
  As for the discussion, there are some points, especially in the part that offers scenarios useful to explain the peculiar results of this study, which are either superficially presented or are inconsistent with the available literature (e.g. , the AMOC hypothesis, more details on specific points are provided in the edited file).
  Reference to AMOC has been removed, alternate hypotheses have been put forward (e.g. sampling bias).
  Another point that could potentially be improved is the comparison with all the available data and in any case with those of the Spanish sections which, in addition to having played a important role in the development of fundamental hypotheses related to mechanisms active during the hyperthermals (e.g., weathering and clay mineralogy)also represent a suitable continuous hemipelagic-continental transect ideal for a comparison with the record of the Mid Atlantic coastal plain.
  We have attempted to edit this section for clarity.
  Finally, I provide there are some suggestions to improve the figures, These are related to both the lack of units and the use of informal nomenclature (more details in the text)
  Notes on the figures have been taken into account and the figures have been updated accordingly.
  
  Citation: https://doi.org/10.5194/egusphere-2023-58-AC1
RC2:
'Comment on Rush et al. manuscript', Anonymous Referee #2, 11 Apr 2023

General Comments:
This study presents an interesting and exciting body of work which focuses on the second largest hyperthermal of the early Eocene, ETM2, and its orbitally-paced partner event H2 in an under-studied depositional environment. It is interesting in part due to the curious environmental response (in terms of the timing and patterns of temperature and ecologic change relative to the CIE) which appears to be unique to the region. Except for the handling of uncertainty, the proxy-based environmental reconstructions are robust. Temperature responses are reproduced using two separate proxy systems, perhaps providing confirmation of the temperature trends.
Overall, the records are well discussed and interpreted. The paper is well-written and organized. The topic is certainly of interest to the Cenozoic paleoclimate community and following some minor revisions the manuscript is suitable for publication.
In my opinion, the primary finding of the study is the counterintuitive response of temperature in the region to CO₂ release for ETM2. ETM2 cooling like this has not been observed elsewhere, thus a detailed discussion of changes in the depositional environment, hydroclimate and ocean circulation is warranted. Therefore, I think the manuscript would benefit from a more detailed sedimentological interpretation of the changes in the depositional setting (i.e., physical sedimentology), in addition to the clay mineralogy work presented here.
Unfortunately, there is little to no consideration of proxy uncertainty in the manuscript. Temperature reconstruction figures provide no graphical estimation of uncertainty. Given the current state of the science, this is somewhat misleading and should be addressed prior to publication. Specifically, the authors should note 2sd or 0.95 quantile uncertainty. The paired mean δ¹⁸O and mean TEX₈₆ warming responses add to the validity of the interpreted cooling during ETM2, but a more detailed discussion of potential mechanisms influencing these proxy systems besides surface cooling would improve the manuscript.
In my opinion the manuscript would benefit from a more organized approach to interpreting changes in sedimentation, salinity, bathymetry, ocean and atmospheric circulation, surface productivity, and carbonate chemistry. The narrative of the discussion could be focused somewhat or organized in a way that considers how all the above-mentioned influence sedimentation, perhaps with a schematic figure. As written, the discussion tends to be harder to follow than necessary.
Additionally, some specific aspects of the discussion could be improved. I provide more detailed comments below where I think the authors could bolster their arguments by providing a more thorough discussion. The authors may find that quantitative salinity, seawater δ¹⁸O, reconstructions using δ¹⁸O and TEX₈₆-based temperatures would aid certain arguments in the discussion section.

Specific Comments:
Line 27: I recommend avoiding statements of novelty. If the authors wish to include these statements, please be more specific. As written, it is difficult to decipher if it is the first time both ETM2 and H2 have been identified in a shallow marine setting, or one event, or if it is the first time they’ve been identified along the Atlantic margin, or in the Salisbury embayment.
Line 30: The terminology here is a bit confusing. Was it the Salisbury Embayment in the early Eocene?
Line 33: Perhaps “in addition to pCO₂ forcing” is more appropriate phrasing?
Line 37: This is a bit vague. Systematically related to what ETM2 environmental response? The local CIE? The warming? The lithologic change? I think these fluctuations may still be related to ETM2 and/or H2 in the most general sense. Consider changing to “…related to CIE warming.”
Line 48: Relatively well constrained compared to what? Is it well constrained? The studies the authors cite have C release estimates that range from 4500 to 12000 GtC. Consider removing “well” and “relatively”.
Line 107: This is somewhat confusing. Consider rephrasing the following sentence: “Samples… have no control on their declination orientation.”
Line 139: A single mass, but two weights? I suggest rephrasing – confusing as written.
Line 214: Admittedly nit-picky, but doesn’t the first ETM2 CIE look closer to -2 to -2.5‰?
Line 263: It may be worth calculating paleo-salinity with δ¹⁸O temps and TEX₈₆-temps, to show that the effect is minimal, or to glean any subtle changes in freshwater input as this would be relevant to the present study.
Line 282: “Rare” or lacking entirely? The abstract indicated that this was the first shallow marine ETM2, but perhaps that needs to be clarified as suggested above.
Line 302: Unless baseline pH is very low, chemical erosion in shallow marine environments requires a strong pH decrease since the lysocline is typically at depths much deeper than the depositional environment discussed here. If acidification is pointed to as a previously hypothesized mechanism driving lower carbonate in this setting, this section could be improved from a discussion of acidification in this type of environment. Perhaps this could be done by providing what is known about lysocline fluctuations from deep marine sites, with a consideration of any regional amplifiers of acidification in the coastal environment. This would help prop-up the author’s statement that changes in siliciclastic flux is driving changes in carbonate content.
Additionally, how would shifts in nutrient availability influence these records? How is this linked to shifts in regional hydrology and circulation in the embayment (if it was still an embayment in the early Eocene)? Though these topics are briefly mentioned w.r.t. palynological assemblage changes, it seems to not be considered currently in the discussion of carbonate accumulation. As the authors note below this section, sedimentation rates (Fig 6) show little variation. Increased siliciclastic flux would tend to increase sedimentation rates if biogenic carbonate deposition is constant. Therefore, given the interpretation, I assume shifts in biogenic carbonate are required, and this could be estimated from sed rate and %CaCO₃. Considering the smaller pH anomaly of ETM2 compared with the PETM, and the shallow paleo-depth of the study site, this may not be due to acidification as the authors point out, but then what would be causing this drop in carbonate if sed rates are uniform and siliciclastic flux is changing? Perhaps shifts in surface productivity? The manuscript would benefit from a discussion of this here. The sedimentation rates provide a primary constraint and could be used quantitively, but this is not done here. Doing this may improve the discussion.
Line 309: More moderate increases in sedimentation rates for ETM2 and H2 or the PETM? At this locality specifically? I see that the authors expand this discussion on sed rates in the following paragraph, but this sentence is somewhat unclear as written.
Line 360: There is much discussion regarding palynologic assemblages pointing to salinity changes. Can this be identified with quantitative salinity reconstructions? If this cannot be done, please explain why in the manuscript.
Line 391: If possible, add a reference detailing the response of hydrologic cycle to various magnitude C release (i.e., [non]linearity of hydro response). This is needed for this line and for the following discussion in this paragraph.
Line 410: I don’t follow this argument. If the broad δ¹⁸O warming of ETM2 with onset of ~20kyr is missing, then why would you preserve the CIE in the same foram samples? Please expand on this topic or remove the argument altogether.

Citation: https://doi.org/10.5194/egusphere-2023-58-RC2
- AC2: 'Reply on RC2', William Rush, 02 Jun 2023
  
  The authors thank both referees for their constructive comments. We attempt to address each of their comments point by point below. For referee 2:
  In my opinion, the primary finding of the study is the counterintuitive response of temperature in the region to CO₂ release for ETM2. ETM2 cooling like this has not been observed elsewhere, thus a detailed discussion of changes in the depositional environment, hydroclimate and ocean circulation is warranted. Therefore, I think the manuscript would benefit from a more detailed sedimentological interpretation of the changes in the depositional setting (i.e., physical sedimentology), in addition to the clay mineralogy work presented here.
  Regarding the physical sedimentology, we believe we have provided a detailed characterization of the event in terms of weathering, sedimentation, and environmental changes, and have provided context to both the PETM and ETM2 at other sites. Questions of large-scale ocean circulation would likely require additional sites and go beyond the scope of this study.
  Unfortunately, there is little to no consideration of proxy uncertainty in the manuscript. Temperature reconstruction figures provide no graphical estimation of uncertainty. Given the current state of the science, this is somewhat misleading and should be addressed prior to publication. Specifically, the authors should note 2sd or 0.95 quantile uncertainty. The paired mean δ18O and mean TEX86 warming responses add to the validity of the interpreted cooling during ETM2, but a more detailed discussion of potential mechanisms influencing these proxy systems besides surface cooling would improve the manuscript.
  Figures have been updated to reflect uncertainty. However, in interpretation of δ¹⁸O, the largest uncertainty is not due to instrument precision or proxy calibration, but due to uncertainty in bottom-water salinity, as there are no estimates of δ¹⁸O_swfor this time in this area. Therefore, this figure has been updated to reflect the uncertainty related to a +/- 1 psu change relative to our estimates. Error associated with TEX₈₆ measurements is on the order of 0.2 °C and has been incorporated into the manuscript and figures.
  In my opinion the manuscript would benefit from a more organized approach to interpreting changes in sedimentation, salinity, bathymetry, ocean and atmospheric circulation, surface productivity, and carbonate chemistry. The narrative of the discussion could be focused somewhat or organized in a way that considers how all the above-mentioned influence sedimentation, perhaps with a schematic figure. As written, the discussion tends to be harder to follow than necessary.
  The section on the CIE magnitude and low carbonate intervals has been reworked and expanded.
  Additionally, some specific aspects of the discussion could be improved. I provide more detailed comments below where I think the authors could bolster their arguments by providing a more thorough discussion. The authors may find that quantitative salinity, seawater δ¹⁸O, reconstructions using δ¹⁸O and TEX₈₆-based temperatures would aid certain arguments in the discussion section.
  While we agree that quantitative salinity estimates would enhance the manuscript, we were unable to extract planktonic foraminifera in large enough quantity in order to obtain surface δ¹⁸O values. While the TEX₈₆ values record surface temperatures and could be used for salinity reconstructions when paired to surface δ¹⁸O values, at this time only bottom water δ¹⁸O values are available.
  Line 27: Statement of novelty removed.
  Line 30: We have clarified the terminology. “This study identifies two events, Eocene Thermal Maximum 2 (ETM2 and H2) in shallow marine sediments of the Eocene-aged Salisbury Embayment of Maryland, based on magnetostratigraphy, calcareous nannofossil and dinocyst biostratigraphy, and recognition of negative stable carbon isotope excursions (CIEs) in biogenic calcite.”
  Line 33: Done
  Line 37: Changed to “related to CIE warming”
  Line 48: Removed “well” and “relatively”
  Line 107: Clarified
  Line 139: Rephrased to clarify that we were measuring a mass between these values
  Line 214: Sure
  Line 263: As noted above, this is not possible with δ¹⁸O_Benthic
  Line 282: They are rare. This is the first time it has been identified in this region. Our earlier statement of novelty has been removed for clarification.
  Line 302: This argument has been thoroughly explored in Bralower et al., 2018 for this region during the PETM. Further exploration of the changing lysocline during ETM2 could be a manuscript on its own and goes beyond the scope of this study. With respect to productivity, our results show little correlation between sedimentation rates/palynological markers for productivity and the timing of either event. We have expanded upon this in the discussion.
  Line 309: We have clarified and expanded upon the sedimentation rate changes.
  Line 360: Again, our dataset in unable to quantifiably reconstruct salinity
  Line 391: Removed reference to linearity
  Line 410: The entirety of the CIE is not preserved in foraminifera samples and is partially reconstructed from bulk carbonate. This has been clarified in the manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2023-58-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

ED: Publish subject to minor revisions (review by editor) (21 Jun 2023) by Luc Beaufort

AR by William Rush on behalf of the Authors (21 Jun 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (05 Jul 2023) by Luc Beaufort

AR by William Rush on behalf of the Authors (13 Jul 2023) Manuscript

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17 Aug 2023

Assessing environmental change associated with early Eocene hyperthermals in the Atlantic Coastal Plain, USA

William Rush, Jean Self-Trail, Yang Zhang, Appy Sluijs, Henk Brinkhuis, James Zachos, James G. Ogg, and Marci Robinson

Clim. Past, 19, 1677–1698, https://doi.org/10.5194/cp-19-1677-2023,https://doi.org/10.5194/cp-19-1677-2023, 2023

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William Davis Rush et al.

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The Eocene contains several brief warming periods referred to as “hyperthermals”. Studying these events and how they varied between locations can help provide insight into our future, warmer world. This study provides the first identification of 2 of these events in the Mid-Atlantic region of the United States. The records of climate we measured demonstrate significant changes during this time period, but the type and timing of these changes highlight the complexity of climatic changes.


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Assessing Environmental Change Associated with Early Eocene Hyperthermals in the Atlantic Coastal Plain, USA

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