Paleocene-Eocene age glendonites from the Norwegian Margin &ndash; Indicators of cold snaps in the hothouse?

Vickers, Madeleine L.; Jones, Morgan T.; Longman, Jack; Evans, David; Ullmann, Clemens V.; Stokke, Ella Wulfsberg; Vickers, Martin; Frieling, Joost; Harper, Dustin T.; Clementi, Vincent J.; the IODP Expedition 396 Scientists,

doi:https://doi.org/10.5194/egusphere-2023-1651

Preprints

https://doi.org/10.5194/egusphere-2023-1651

Preprints

03 Aug 2023

| 03 Aug 2023

Paleocene-Eocene age glendonites from the Norwegian Margin – Indicators of cold snaps in the hothouse?

Madeleine L. Vickers, Morgan T. Jones, Jack Longman, David Evans, Clemens V. Ullmann, Ella Wulfsberg Stokke, Martin Vickers, Joost Frieling, Dustin T. Harper, Vincent J. Clementi, and the IODP Expedition 396 Scientists

Abstract. The International Ocean Discovery Program (IODP) Expedition 396 to the mid-Norwegian margin recovered >1300 m of pristinely preserved, volcanic ash-rich sediments deposited during the late Paleocene and early Eocene, from close to the centre of the North Atlantic Igneous Province (NAIP). Remarkably, many of these cores contain glendonites, pseudomorphs after the purported cold-water mineral ikaite, from sediments dated to the late Paleocene, Paleocene – Eocene boundary and early Eocene. These time intervals span some of the hottest climates of the Cenozoic, including the Paleocene-Eocene Thermal Maximum (PETM). Global deep ocean temperatures are not thought to have dropped below 10 °C at any point during this time, making the occurrence of supposedly cold-water (near-freezing temperature) glendonite pseudomorphs seemingly paradoxical. This study presents a detailed sedimentological, geochemical, and microscopic study of the Exp. 396 glendonites, and presents an updated model for the ikaite-to-calcite transformation for these glendonites. Specifically, we show that early diagenesis of basaltic ashes of the NAIP appear to have chemically promoted ikaite growth in the sediments in this region. Together with existing knowledge of late Paleocene and early Eocene glendonites from Svalbard to the north, and early Eocene glendonites from Denmark to the south, these new glendonite finds possibly imply episodic, short-duration, and likely localised cooling in the Nordic Seas region, which may have been directly or indirectly linked to the emplacement of the NAIP.

Received: 18 Jul 2023 – Discussion started: 03 Aug 2023

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Journal article(s) based on this preprint

03 Jan 2024

Paleocene–Eocene age glendonites from the Mid-Norwegian Margin – indicators of cold snaps in the hothouse?

Clim. Past, 20, 1–23, https://doi.org/10.5194/cp-20-1-2024,https://doi.org/10.5194/cp-20-1-2024, 2024

Short summary

Madeleine L. Vickers et al.

Interactive discussion

Status: closed

CC1:
'Comment on egusphere-2023-1651', Niels de Winter, 16 Aug 2023

Dear Prof. Soreghan, dear authors,
After the appearance online of the manuscript titled “Paleocene-Eocene age glendonites from the Norwegian Margin - Indicators of cold snaps in the hothouse?” submitted for review in Climate of the Past by Vickers et al., I read the manuscript with interest and noticed some things which the authors may find useful while revising their text in response to the other review comments.
Overall, I found this to be a well written manuscript which details an important study into the somewhat enigmatic occurrence of glendonites in sediments deposited during warm climates. The authors applied a diverse set of methods used to arrive at a very plausible description of the formation pathway of the glendonites and how they are chemically related to volcanic ashes deposited in the basin. This makes this manuscript a very useful contribution to the field of paleoclimatology and carbonate chemistry, even though I am not fully convinced that the “cold snaps” in Eocene climate put forward by the authors are the only viable mechanism to explain what is seen in the data and observations (see comments below).
Below, I first highlight some general remarks I had while reading through the manuscript. These are followed by some more minor (textual) points and elaboration on the general remarks sorted by line number.
General comments
In my opinion, the observations and data the authors put forward do not conclusively support the hypothesis that the Eocene hothouse was punctuated by geologically brief “cold snaps”. The lack of precise timing of the formation of the glendonites leaves room for other explanations, such as the hypothesis that the fast-growing glendonites form out of isotopic equilibrium (and may record lower temperatures than their growing temperatures) or that they grow seasonally (e.g. during the cold season). Unless the authors can convincingly reject these two alternatives using either evidence from the literature or new observations, they should at least name them as alternative explanations. I think it is well possible that there were indeed geologically short cold periods in the Early Eocene in this locality, but it does not seem like the only possible explanation for the data and observations put forward in this manuscript (see details in my comment on lines 441-462).
The PHREEQC modelling is, in my opinion, the weakest part of the manuscript. The outcomes of this model are hardly described and play only a very small role in the discussion. Perhaps the authors can elaborate a bit more on these results and how they add to the discussion in this manuscript.
On my first read-through, I was quite skeptical about the use of pore water chemistry in Eocene sediments to infer something about paleo-conditions. It seems hard to argue that these preserve a remnant of the original pore waters after such a long time. However, I must acknowledge that I am not an expert in this subject and the authors wrote a nuanced discussion about the observed changes in porewater chemistry (lines 367-397), so I am happy to go along with its qualitative interpretation if more expert reviewers agree that this data can be used to make the inferences the authors put forward in the manuscript.

Minor comments
Line 70: “…the conditions under which this was achieved in the laboratory is unlike any natural setting.” should probably read “…the conditions under which this was achieved in the laboratory are unlike any natural setting.”
Line 76-78: On checking the preprint by Jones et al. (2023), most specifically figures 3 and 6, I did not find any biomarker-based temperature reconstructions that yielded results below 10°C. It seems the only datapoints in this compilation that yield colder temperatures are those originating from Vickers et al. (2020), which represent clumped isotope datapoints on glendonites if I’m not mistaken. Unless I have overlooked any temperature reconstructions in Jones et al. (2023) the authors are referring to here, I think this statement should be rephrased. The authors should acknowledge that the cold temperature reconstructions are only found from analyses on the glendonites themselves, and not from other (independent) proxies and archives. This observation has implications for the “cold snap” hypothesis, as it remains possible that the temperatures reconstructed from the glendonites themselves are underestimations (see comments on lines 441-462).
Line 92-94: The train of thought in this sentence is a bit hard to follow for me. Perhaps the authors could briefly explain how these colder temperature reconstructions from glendonites imply changes in circulation or stratification in this basin. I’m sure this has been discussed in one or more of the papers by the authors cited earlier in the Introduction, but for the sake of clarity I would suggest the authors indulge the reader who has not read these contributions in detail by explaining the line of reasoning here and refer to these previous studies for a more detailed discussion.
Section 2.2-2.6: I commend the authors on their detailed explanation of the geochemical measurement procedures.
Section 3.7: It would probably benefit the manuscript if the results of PHREEQC modelling are described in more detail here. As it stands, this section now contains one sentence referring to a figure and supplement, which leaves the reader searching for the information.
Line 263-269: While I understand the decision to keep the terminology of the different carbonate phases observed within the glendonites consistent with previous literature, I think it would be helpful to include a brief description of the crystal habit (e.g. “botryoidal”, “sparry”, etc.) and texture of each phase (or “type”) here in the Results section.
Line 296: Is “blebs” a scientific term in this context? Perhaps the authors can define it here for clarity.
Line 311: A space is needed between “localized” and “increase”.
Line 313: “due to the rapidity of the reaction” This reasoning requires a bit more explanation, I think. Do the authors mean that the faster mineralization rate causes less discrimination against Mg in the crystal structure compared to phase 1A? If so, it would be helpful if the authors supported this claim about the influence of reaction rate with a reference.
Line 339-344: Here I think the authors should acknowledge that this link between methane seepage and ikaite formation (and the appearance of glendonites in the sedimentary record) has also been recorded in geological history (e.g. Morales et al., 2017)
Line 394-397: I find it hard to see the connection between PHREEQC model results and the discussion in this section, but I think that is mostly because the results of the modelling have not been described in the manuscript. It would be easier for the reader to follow the reasoning here if the outcomes of PHREEQC simulations are first described (in Results) and then discussed earlier in the discussion before they are used to support the discussion here.
Lines 399-440: I think the hypothesis for the formation of ikaite in connection to the deposition of the ashes in this section is very plausible.
Lines 441-462: I’m not sure if I am fully convinced that the data presented in this study and the previous studies cited here are conclusive evidence pointing towards “cold snaps” during the Eocene hothouse. In my opinion, there remain two other possibilities that could reconcile the cool temperatures measured in the glendonites using clumped isotope thermometry with the warmer temperatures inferred from biomarkers.
Firstly, it is possible that the transformation of ikaite to glendonite does not take place in (clumped) isotopic equilibrium. Studies using the comparatively new “dual clumped” method highlight that some carbonates (e.g. those in brachiopod shells or corals) are affected by rate-limiting processes such as the hydration of CO2 in the water and the diffusion of DIC to the mineralization site (Davies et al., 2023). If glendonites indeed grow as fast as hypothesized in the previous section (lines 427-433), these processes may also cause kinetic effects in their isotopic composition. In brachiopods, such effects are demonstrated to cause an offset between the D47-based temperature and the actual environmental temperature under which the carbonate mineralizes of up to 10 degrees, enough to potentially explain a large part of the temperature offset cited here (Bajnai et al., 2020; Davies et al., 2023). While I would not suggest the authors add dual clumped measurements of their glendonites to this study (which might be analytically challenging considering their heterogeneity in terms of carbonate phases), I think this caveat should be recognized as part of the discussion here.
Secondly, I think the authors should consider the possibility that the difference between the temperatures recorded in the glendonites and those in the organic proxies can be explained by a difference in the season in which these materials are formed. If the glendonites formed in the winter and the biomarkers represent a summer signal, it is not inconceivable that the former yields temperatures of 1-9 degrees and the latter 20-30 degrees, especially in the higher latitudes. In fact, similar seasonal temperature contrasts are common in modern marine systems like the North Sea (e.g. Van Aken, 2008) and have been observed in seasonal-scale temperature reconstructions from higher mid-latitudes in the same region during past greenhouse periods (e.g. de Winter et al., 2021). In addition, while the season of growth of recrystallization of the glendonites would be hard to constrain, at least there is some evidence that biomarker-based SST reconstructions may be seasonally biased (Jia et al., 2017; Udoh et al., 2022). It seems plausible to me that the ikaites form in the winter season when temperatures drop enough for the mineralization to start, even if the chemical conditions that seed ikaite formation (made possible by the volcanic ashes) are in place earlier in the year. Therefore, I think this hypothesis cannot be disregarded in this discussion.
Finally, the authors make a link between crystal size and growth rates (lines 427-433). This allows room for the argument that relatively large (cm-scale) crystals can grow in relatively short cold periods. However, even considering the fast mineralization rates observed for ikaite, I wonder how the very large (up to meter-scale) in the Fur formation can form in a setting that is otherwise indicative of typical warm Eocene hothouse conditions (e.g. Schultz et al., 2020). Even assuming growth rates in the order of centimeters per year, growing such crystals would still require decades to centuries of cold periods in this area. While this observation seems to favor the “cold snap” hypothesis rather than the seasonal growth I suggested as an alternative above, it still seems hard to explain such prolonged cool periods of which no evidence is present in the local geological record beyond the glendonites. This makes me think that perhaps the thickness or composition of the ash layers across the basin might be a factor influencing the differences in glendonite size between the locations (with the larger glendonites begin found further south). Perhaps the authors could comment on this in the manuscript.
I hope the comments above will be helpful to the authors in improving their manuscript during the review process, and I welcome any replies to my comments above in case I overlooked anything in my reasoning above.
Kind regards,
Niels de Winter

References
Bajnai, D., Guo, W., Spötl, C., Coplen, T. B., Methner, K., Löffler, N., Krsnik, E., Gischler, E., Hansen, M., Henkel, D., Price, G. D., Raddatz, J., Scholz, D., and Fiebig, J.: Dual clumped isotope thermometry resolves kinetic biases in carbonate formation temperatures, Nat Commun, 11, 4005, https://doi.org/10.1038/s41467-020-17501-0, 2020.
Davies, A. J., Brand, U., Tagliavento, M., Bitner, M. A., Bajnai, D., Staudigel, P., Bernecker, M., and Fiebig, J.: Isotopic disequilibrium in brachiopods disentangled with dual clumped isotope thermometry, Geochimica et Cosmochimica Acta, https://doi.org/10.1016/j.gca.2023.08.005, 2023.
van Aken, H. M.: Variability of the water temperature in the western Wadden Sea on tidal to centennial time scales, Journal of Sea Research, 60, 227–234, https://doi.org/10.1016/j.seares.2008.09.001, 2008.
de Winter, N. J., Müller, I. A., Kocken, I. J., Thibault, N., Ullmann, C. V., Farnsworth, A., Lunt, D. J., Claeys, P., and Ziegler, M.: Absolute seasonal temperature estimates from clumped isotopes in bivalve shells suggest warm and variable greenhouse climate, Commun Earth Environ, 2, 1–8, https://doi.org/10.1038/s43247-021-00193-9, 2021.
Jia, G., Wang, X., Guo, W., and Dong, L.: Seasonal distribution of archaeal lipids in surface water and its constraint on their sources and the TEX86 temperature proxy in sediments of the South China Sea, Journal of Geophysical Research: Biogeosciences, 122, 592–606, https://doi.org/10.1002/2016JG003732, 2017.
Morales, C., Rogov, M., Wierzbowski, H., Ershova, V., Suan, G., Adatte, T., Föllmi, K. B., Tegelaar, E., Reichart, G.-J., and De Lange, G. J.: Glendonites track methane seepage in Mesozoic polar seas, Geology, 45, 503–506, 2017.
Schulz, B. P., Vickers, M. L., Huggett, J., Madsen, H., Heilmann-Clausen, C., Friis, H., and Suess, E.: Palaeogene glendonites from Denmark, Bulletin of the Geological Society of Denmark, 68, 23–35, https://doi.org/10.37570/bgsd-2020-68-03, 2020.
Udoh, E. C., Li, L., Chen, M., Dan, S. F., Chen, L., Zhang, J., Jia, G., and He, J.: Distribution characteristics of terrestrial and marine lipid biomarkers in surface sediment and their implication for the provenance and palaeoceanographic application in the northern South China Sea, Marine Geology, 452, 106899, https://doi.org/10.1016/j.margeo.2022.106899, 2022.

Citation: https://doi.org/10.5194/egusphere-2023-1651-CC1
- AC5: 'Reply on CC1', Madeleine Vickers, 07 Nov 2023
  
  We thank Niels de Winter for his thoughtful community comment, and are happy to address every point raised. We believe that the changes we will make in response make the manuscript a much more thorough and considered discussion on the conditions that may have driven glendonite formation throughout the Paleocene-Eocene-aged sediments from the mid-Norwegian margin (Exp. 396). We detail our responses to each comment, major and minor, in the attached supplementary file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1651-AC5
RC1:
'Comment on egusphere-2023-1651', Mikhail Rogov, 19 Aug 2023

The reviewed MS by Vickers et al. provides an important contribution devoted to the enigmatic glendonite occurrences across the PETM climatic optimum. It is well-written, and interpretation provided by the authors is well-supported and in agreement with previously known data about the glendonite records.

I have a few minor comments / corrections, listed below:
Line 83: “numerous glendonites in volcanic sediments” – rather, in “ash-bearing deposits”
Line 96: “glendonites in the stratigraphy of the Exp. 396 cores” – in my opinion, usage of the word ‘stratigraphy’ in such a meaning if doubtful; I propose to replace it by "glendonites recovered from Exp. 396 cores and their stratigraphic distribution"
Line 108: “The Modgunn locality is a transect of boreholes” – can the transect be considered as a ‘locality’?
Line 116: “and the biostratigraphic marker taxa Apectodinium augustum and Hemiaulus proteus” – rather, “FAD of the the biostratigraphic marker taxa…”. Can you show key biostratigraphic events (FAD and LAD of dinocyst species) on the figures? Intwill be useful for readers
Line 228: “hydrothermal vent infilling sediments”- or “deposits” (here and above in the text)? the term 'sediments' more frequently used for modern unconsolidated ones
Line 242: at least a short review of biostratigraphic data, which are crucial for further discussion, is necessary prior the description of glendonites.
Lines 247-249: “Most show the characteristic shape of stellate or bladed ‘crystals’, although the individual blades are no longer a single crystal but rather a heterogeneous mix of smaller crystals” - as follow from photographs provided in the MS, nearly all recorded glendonite specimens can be ascribed to a single rosette morphotype (following terminology proposed by Frank et al., 2008), except for specimens from figs. S4 and S8-S9, which morphology is unclear.
Lines 267, 300, 320: “Counts et al., in review” it is not necessary to cite such a paper, which still not accepted yet; in all the cases it cited along with other refs
Lines 301-302: “green Type 0 calcite identified in this study has not been observed in other glendonite thin section” - can this newly recorded generation be related with an influence of the nearby ash horizons?
Line 311: “localisedincrease” – please split this word
Unfortunately, an information about the precise coordinates of studied boreholes is missing in the MS; I propose to add a table with these data to the Supplementary.

Citation: https://doi.org/10.5194/egusphere-2023-1651-RC1
- AC2: 'Reply on RC1', Madeleine Vickers, 07 Nov 2023
  
  We are pleased that you find the MS by Vickers et al. an important contribution devoted to the enigmatic glendonite occurrences across the PETM climatic optimum, and that in general you find the manuscript well-written, with well-supported interpretations of the findings. We attach detailed responses to the line by line comments in the supplementary pdf here.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1651-AC2
RC2:
'Comment on egusphere-2023-1651', Anonymous Referee #2, 07 Sep 2023

The subject of the paper is interesting, and very well presented by the authors. A few spelling errors may be removed before final publication.

Citation: https://doi.org/10.5194/egusphere-2023-1651-RC2
- AC1: 'Reply on RC2', Madeleine Vickers, 02 Oct 2023
  
  We are pleased to hear you found our article interesting and well-presented, and will proceed to correct any spelling errors.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1651-AC1
RC3:
'Comment on egusphere-2023-1651', William Rush, 10 Oct 2023

The manuscript by Vickers et al. provides an important insight into the formation of glendonites during the PETM with exciting implications for regional climate and circulation and is worthy of publication pending minor edits. The methods are sound, but I have some concerns related to the interpretation. There is extensive literature on the formation of thermogenic methane in the North Atlantic during the PETM, the anaerobic oxidation of which could have influenced the formation of ikaite, as the authors have noted occurs in modern settings. I feel this should be addressed before final publication. I have listed my concerns below in the order of appearance within the manuscript, which includes minor notes such as typos as well as my major concern as to the interpretation of the observations.
Lines 48-49: specify °C when saying “temperatures approaching zero”
Lines 62-63: explain why the glendonites are a topic of controversy, there should be more discussion on these previous studies that are referenced
Lines 78-80: “Whether these cooling events reflect a localised as well as short-lived cooling is not understood, nor the mechanism by which such cooling could occur, although these have been speculated upon” discuss how these have been speculated upon and what some of the proposed mechanisms have been
Line 244: Somewhat minor, but I believe there could be a better word choice than “mush”, perhaps mixture or amalgam?
Lines 296, 310: Again, I believe there is a better word choice than “blebs”. I will leave that choice to the author, as I’m not exactly certain what is meant by blebs.
Line 311: there is a space missing between “localised” and “increase”
Line 340-341: This is my major concern with the paper. The authors reference anaerobic oxidation of methane as playing a key role in ikaite precipitation, yet do not discuss the extensive literature on the thermogenic methane production in the North Atlantic during the PETM associated with North Atlantic Igneous Province vulcanism. There are many papers on this topic, including Jones et al., 2019 and Frieling et al., 2016 among others. At the very least, there should be discussion as to the possibility that anaerobic oxidation of methane could have played a role in the formation of ikaite as an alternative to the volcanic ash hypothesis, why this methane could not have played a role, or the potential that these mechanisms could have worked in conjunction.
Line 393: there is a missing space between “CaCO₃” and “in”
Line 438: Again, here the influence of methane should be discussed in addition to the rapid diagenesis of ash
Lines 449-450: A recent paper in this journal (Rush et al., 2023) found transient cooling associated with ETM2 in the Mid-Atlantic of the United States. This would not have been an enclosed basin but provides evidence of another region that experienced cooling during an Eocene hyperthermal and may be worth a citation.

Citation: https://doi.org/10.5194/egusphere-2023-1651-RC3
- AC3: 'Reply on RC3', Madeleine Vickers, 07 Nov 2023
  
  We thank William Rush for his considered comments on the manuscript. In addition to some minor comments, RC3 expressed their major concern that that more discussion of anaerobic oxidation of methane was required, as there is extensive literature on the formation of thermogenic methane in the North Atlantic during the PETM, the anaerobic oxidation of which could have influenced the formation of ikaite. This is a valid point, and we will accordingly add a discussion of the possible source and role of methane in ikaite precipitation to the manuscript. Please refer to the attached supplementary file for details of our response to this and all the more minor line-by-line comments.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1651-AC3
EC1:
'Comment on egusphere-2023-1651', Gerilyn (Lynn) Soreghan, 12 Oct 2023

Minor comments:
117— the strata “exhibit” rather than “contain”
246— Section 3.2— it might be helpful to begin here by noting the size (and maybe shape) range, where you note they are variable in “size and appearance,” in addition to the references to the relevant figures.
Fig. 3A illustrates a core that appears to have mud invasion and be a bit disrupted, and indeed the caption notes that these are glendonite fragments in drill mud. Doesn’t that mean that these could be out of place, and thus not reliably placed stratigraphically? In other words, why show this example, given the uncertainty of its stratigraphic position? I see that you mention this in lines 257-260, so that is fine.
315— space needed between words.
367— what is meant by “Pore waters… were taken on board at low resolution…”— really two questions— how were the pore waters sampled, and at what resolution? Did I miss this in the Methods? (I couldn’t find this detailed in the methods).
495— are they found “throughout” or within discrete intervals? It seems more like the latter?
This is a well-written, well-organized paper with an interesting story to relay about ikaite formation and climate perturbations during the late Paleocene-early Eocene.

Citation: https://doi.org/10.5194/egusphere-2023-1651-EC1
- AC4: 'Reply on EC1', Madeleine Vickers, 07 Nov 2023
  
  We thank Gerilyn Soreghan for these helpful comments, and attach detailed responses to the line-by-line comments in the attached supplementary file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1651-AC4

Interactive discussion

Status: closed

CC1:
'Comment on egusphere-2023-1651', Niels de Winter, 16 Aug 2023

Dear Prof. Soreghan, dear authors,
After the appearance online of the manuscript titled “Paleocene-Eocene age glendonites from the Norwegian Margin - Indicators of cold snaps in the hothouse?” submitted for review in Climate of the Past by Vickers et al., I read the manuscript with interest and noticed some things which the authors may find useful while revising their text in response to the other review comments.
Overall, I found this to be a well written manuscript which details an important study into the somewhat enigmatic occurrence of glendonites in sediments deposited during warm climates. The authors applied a diverse set of methods used to arrive at a very plausible description of the formation pathway of the glendonites and how they are chemically related to volcanic ashes deposited in the basin. This makes this manuscript a very useful contribution to the field of paleoclimatology and carbonate chemistry, even though I am not fully convinced that the “cold snaps” in Eocene climate put forward by the authors are the only viable mechanism to explain what is seen in the data and observations (see comments below).
Below, I first highlight some general remarks I had while reading through the manuscript. These are followed by some more minor (textual) points and elaboration on the general remarks sorted by line number.
General comments
In my opinion, the observations and data the authors put forward do not conclusively support the hypothesis that the Eocene hothouse was punctuated by geologically brief “cold snaps”. The lack of precise timing of the formation of the glendonites leaves room for other explanations, such as the hypothesis that the fast-growing glendonites form out of isotopic equilibrium (and may record lower temperatures than their growing temperatures) or that they grow seasonally (e.g. during the cold season). Unless the authors can convincingly reject these two alternatives using either evidence from the literature or new observations, they should at least name them as alternative explanations. I think it is well possible that there were indeed geologically short cold periods in the Early Eocene in this locality, but it does not seem like the only possible explanation for the data and observations put forward in this manuscript (see details in my comment on lines 441-462).
The PHREEQC modelling is, in my opinion, the weakest part of the manuscript. The outcomes of this model are hardly described and play only a very small role in the discussion. Perhaps the authors can elaborate a bit more on these results and how they add to the discussion in this manuscript.
On my first read-through, I was quite skeptical about the use of pore water chemistry in Eocene sediments to infer something about paleo-conditions. It seems hard to argue that these preserve a remnant of the original pore waters after such a long time. However, I must acknowledge that I am not an expert in this subject and the authors wrote a nuanced discussion about the observed changes in porewater chemistry (lines 367-397), so I am happy to go along with its qualitative interpretation if more expert reviewers agree that this data can be used to make the inferences the authors put forward in the manuscript.

Minor comments
Line 70: “…the conditions under which this was achieved in the laboratory is unlike any natural setting.” should probably read “…the conditions under which this was achieved in the laboratory are unlike any natural setting.”
Line 76-78: On checking the preprint by Jones et al. (2023), most specifically figures 3 and 6, I did not find any biomarker-based temperature reconstructions that yielded results below 10°C. It seems the only datapoints in this compilation that yield colder temperatures are those originating from Vickers et al. (2020), which represent clumped isotope datapoints on glendonites if I’m not mistaken. Unless I have overlooked any temperature reconstructions in Jones et al. (2023) the authors are referring to here, I think this statement should be rephrased. The authors should acknowledge that the cold temperature reconstructions are only found from analyses on the glendonites themselves, and not from other (independent) proxies and archives. This observation has implications for the “cold snap” hypothesis, as it remains possible that the temperatures reconstructed from the glendonites themselves are underestimations (see comments on lines 441-462).
Line 92-94: The train of thought in this sentence is a bit hard to follow for me. Perhaps the authors could briefly explain how these colder temperature reconstructions from glendonites imply changes in circulation or stratification in this basin. I’m sure this has been discussed in one or more of the papers by the authors cited earlier in the Introduction, but for the sake of clarity I would suggest the authors indulge the reader who has not read these contributions in detail by explaining the line of reasoning here and refer to these previous studies for a more detailed discussion.
Section 2.2-2.6: I commend the authors on their detailed explanation of the geochemical measurement procedures.
Section 3.7: It would probably benefit the manuscript if the results of PHREEQC modelling are described in more detail here. As it stands, this section now contains one sentence referring to a figure and supplement, which leaves the reader searching for the information.
Line 263-269: While I understand the decision to keep the terminology of the different carbonate phases observed within the glendonites consistent with previous literature, I think it would be helpful to include a brief description of the crystal habit (e.g. “botryoidal”, “sparry”, etc.) and texture of each phase (or “type”) here in the Results section.
Line 296: Is “blebs” a scientific term in this context? Perhaps the authors can define it here for clarity.
Line 311: A space is needed between “localized” and “increase”.
Line 313: “due to the rapidity of the reaction” This reasoning requires a bit more explanation, I think. Do the authors mean that the faster mineralization rate causes less discrimination against Mg in the crystal structure compared to phase 1A? If so, it would be helpful if the authors supported this claim about the influence of reaction rate with a reference.
Line 339-344: Here I think the authors should acknowledge that this link between methane seepage and ikaite formation (and the appearance of glendonites in the sedimentary record) has also been recorded in geological history (e.g. Morales et al., 2017)
Line 394-397: I find it hard to see the connection between PHREEQC model results and the discussion in this section, but I think that is mostly because the results of the modelling have not been described in the manuscript. It would be easier for the reader to follow the reasoning here if the outcomes of PHREEQC simulations are first described (in Results) and then discussed earlier in the discussion before they are used to support the discussion here.
Lines 399-440: I think the hypothesis for the formation of ikaite in connection to the deposition of the ashes in this section is very plausible.
Lines 441-462: I’m not sure if I am fully convinced that the data presented in this study and the previous studies cited here are conclusive evidence pointing towards “cold snaps” during the Eocene hothouse. In my opinion, there remain two other possibilities that could reconcile the cool temperatures measured in the glendonites using clumped isotope thermometry with the warmer temperatures inferred from biomarkers.
Firstly, it is possible that the transformation of ikaite to glendonite does not take place in (clumped) isotopic equilibrium. Studies using the comparatively new “dual clumped” method highlight that some carbonates (e.g. those in brachiopod shells or corals) are affected by rate-limiting processes such as the hydration of CO2 in the water and the diffusion of DIC to the mineralization site (Davies et al., 2023). If glendonites indeed grow as fast as hypothesized in the previous section (lines 427-433), these processes may also cause kinetic effects in their isotopic composition. In brachiopods, such effects are demonstrated to cause an offset between the D47-based temperature and the actual environmental temperature under which the carbonate mineralizes of up to 10 degrees, enough to potentially explain a large part of the temperature offset cited here (Bajnai et al., 2020; Davies et al., 2023). While I would not suggest the authors add dual clumped measurements of their glendonites to this study (which might be analytically challenging considering their heterogeneity in terms of carbonate phases), I think this caveat should be recognized as part of the discussion here.
Secondly, I think the authors should consider the possibility that the difference between the temperatures recorded in the glendonites and those in the organic proxies can be explained by a difference in the season in which these materials are formed. If the glendonites formed in the winter and the biomarkers represent a summer signal, it is not inconceivable that the former yields temperatures of 1-9 degrees and the latter 20-30 degrees, especially in the higher latitudes. In fact, similar seasonal temperature contrasts are common in modern marine systems like the North Sea (e.g. Van Aken, 2008) and have been observed in seasonal-scale temperature reconstructions from higher mid-latitudes in the same region during past greenhouse periods (e.g. de Winter et al., 2021). In addition, while the season of growth of recrystallization of the glendonites would be hard to constrain, at least there is some evidence that biomarker-based SST reconstructions may be seasonally biased (Jia et al., 2017; Udoh et al., 2022). It seems plausible to me that the ikaites form in the winter season when temperatures drop enough for the mineralization to start, even if the chemical conditions that seed ikaite formation (made possible by the volcanic ashes) are in place earlier in the year. Therefore, I think this hypothesis cannot be disregarded in this discussion.
Finally, the authors make a link between crystal size and growth rates (lines 427-433). This allows room for the argument that relatively large (cm-scale) crystals can grow in relatively short cold periods. However, even considering the fast mineralization rates observed for ikaite, I wonder how the very large (up to meter-scale) in the Fur formation can form in a setting that is otherwise indicative of typical warm Eocene hothouse conditions (e.g. Schultz et al., 2020). Even assuming growth rates in the order of centimeters per year, growing such crystals would still require decades to centuries of cold periods in this area. While this observation seems to favor the “cold snap” hypothesis rather than the seasonal growth I suggested as an alternative above, it still seems hard to explain such prolonged cool periods of which no evidence is present in the local geological record beyond the glendonites. This makes me think that perhaps the thickness or composition of the ash layers across the basin might be a factor influencing the differences in glendonite size between the locations (with the larger glendonites begin found further south). Perhaps the authors could comment on this in the manuscript.
I hope the comments above will be helpful to the authors in improving their manuscript during the review process, and I welcome any replies to my comments above in case I overlooked anything in my reasoning above.
Kind regards,
Niels de Winter

References
Bajnai, D., Guo, W., Spötl, C., Coplen, T. B., Methner, K., Löffler, N., Krsnik, E., Gischler, E., Hansen, M., Henkel, D., Price, G. D., Raddatz, J., Scholz, D., and Fiebig, J.: Dual clumped isotope thermometry resolves kinetic biases in carbonate formation temperatures, Nat Commun, 11, 4005, https://doi.org/10.1038/s41467-020-17501-0, 2020.
Davies, A. J., Brand, U., Tagliavento, M., Bitner, M. A., Bajnai, D., Staudigel, P., Bernecker, M., and Fiebig, J.: Isotopic disequilibrium in brachiopods disentangled with dual clumped isotope thermometry, Geochimica et Cosmochimica Acta, https://doi.org/10.1016/j.gca.2023.08.005, 2023.
van Aken, H. M.: Variability of the water temperature in the western Wadden Sea on tidal to centennial time scales, Journal of Sea Research, 60, 227–234, https://doi.org/10.1016/j.seares.2008.09.001, 2008.
de Winter, N. J., Müller, I. A., Kocken, I. J., Thibault, N., Ullmann, C. V., Farnsworth, A., Lunt, D. J., Claeys, P., and Ziegler, M.: Absolute seasonal temperature estimates from clumped isotopes in bivalve shells suggest warm and variable greenhouse climate, Commun Earth Environ, 2, 1–8, https://doi.org/10.1038/s43247-021-00193-9, 2021.
Jia, G., Wang, X., Guo, W., and Dong, L.: Seasonal distribution of archaeal lipids in surface water and its constraint on their sources and the TEX86 temperature proxy in sediments of the South China Sea, Journal of Geophysical Research: Biogeosciences, 122, 592–606, https://doi.org/10.1002/2016JG003732, 2017.
Morales, C., Rogov, M., Wierzbowski, H., Ershova, V., Suan, G., Adatte, T., Föllmi, K. B., Tegelaar, E., Reichart, G.-J., and De Lange, G. J.: Glendonites track methane seepage in Mesozoic polar seas, Geology, 45, 503–506, 2017.
Schulz, B. P., Vickers, M. L., Huggett, J., Madsen, H., Heilmann-Clausen, C., Friis, H., and Suess, E.: Palaeogene glendonites from Denmark, Bulletin of the Geological Society of Denmark, 68, 23–35, https://doi.org/10.37570/bgsd-2020-68-03, 2020.
Udoh, E. C., Li, L., Chen, M., Dan, S. F., Chen, L., Zhang, J., Jia, G., and He, J.: Distribution characteristics of terrestrial and marine lipid biomarkers in surface sediment and their implication for the provenance and palaeoceanographic application in the northern South China Sea, Marine Geology, 452, 106899, https://doi.org/10.1016/j.margeo.2022.106899, 2022.

Citation: https://doi.org/10.5194/egusphere-2023-1651-CC1
- AC5: 'Reply on CC1', Madeleine Vickers, 07 Nov 2023
  
  We thank Niels de Winter for his thoughtful community comment, and are happy to address every point raised. We believe that the changes we will make in response make the manuscript a much more thorough and considered discussion on the conditions that may have driven glendonite formation throughout the Paleocene-Eocene-aged sediments from the mid-Norwegian margin (Exp. 396). We detail our responses to each comment, major and minor, in the attached supplementary file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1651-AC5
RC1:
'Comment on egusphere-2023-1651', Mikhail Rogov, 19 Aug 2023

The reviewed MS by Vickers et al. provides an important contribution devoted to the enigmatic glendonite occurrences across the PETM climatic optimum. It is well-written, and interpretation provided by the authors is well-supported and in agreement with previously known data about the glendonite records.

I have a few minor comments / corrections, listed below:
Line 83: “numerous glendonites in volcanic sediments” – rather, in “ash-bearing deposits”
Line 96: “glendonites in the stratigraphy of the Exp. 396 cores” – in my opinion, usage of the word ‘stratigraphy’ in such a meaning if doubtful; I propose to replace it by "glendonites recovered from Exp. 396 cores and their stratigraphic distribution"
Line 108: “The Modgunn locality is a transect of boreholes” – can the transect be considered as a ‘locality’?
Line 116: “and the biostratigraphic marker taxa Apectodinium augustum and Hemiaulus proteus” – rather, “FAD of the the biostratigraphic marker taxa…”. Can you show key biostratigraphic events (FAD and LAD of dinocyst species) on the figures? Intwill be useful for readers
Line 228: “hydrothermal vent infilling sediments”- or “deposits” (here and above in the text)? the term 'sediments' more frequently used for modern unconsolidated ones
Line 242: at least a short review of biostratigraphic data, which are crucial for further discussion, is necessary prior the description of glendonites.
Lines 247-249: “Most show the characteristic shape of stellate or bladed ‘crystals’, although the individual blades are no longer a single crystal but rather a heterogeneous mix of smaller crystals” - as follow from photographs provided in the MS, nearly all recorded glendonite specimens can be ascribed to a single rosette morphotype (following terminology proposed by Frank et al., 2008), except for specimens from figs. S4 and S8-S9, which morphology is unclear.
Lines 267, 300, 320: “Counts et al., in review” it is not necessary to cite such a paper, which still not accepted yet; in all the cases it cited along with other refs
Lines 301-302: “green Type 0 calcite identified in this study has not been observed in other glendonite thin section” - can this newly recorded generation be related with an influence of the nearby ash horizons?
Line 311: “localisedincrease” – please split this word
Unfortunately, an information about the precise coordinates of studied boreholes is missing in the MS; I propose to add a table with these data to the Supplementary.

Citation: https://doi.org/10.5194/egusphere-2023-1651-RC1
- AC2: 'Reply on RC1', Madeleine Vickers, 07 Nov 2023
  
  We are pleased that you find the MS by Vickers et al. an important contribution devoted to the enigmatic glendonite occurrences across the PETM climatic optimum, and that in general you find the manuscript well-written, with well-supported interpretations of the findings. We attach detailed responses to the line by line comments in the supplementary pdf here.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1651-AC2
RC2:
'Comment on egusphere-2023-1651', Anonymous Referee #2, 07 Sep 2023

The subject of the paper is interesting, and very well presented by the authors. A few spelling errors may be removed before final publication.

Citation: https://doi.org/10.5194/egusphere-2023-1651-RC2
- AC1: 'Reply on RC2', Madeleine Vickers, 02 Oct 2023
  
  We are pleased to hear you found our article interesting and well-presented, and will proceed to correct any spelling errors.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1651-AC1
RC3:
'Comment on egusphere-2023-1651', William Rush, 10 Oct 2023

The manuscript by Vickers et al. provides an important insight into the formation of glendonites during the PETM with exciting implications for regional climate and circulation and is worthy of publication pending minor edits. The methods are sound, but I have some concerns related to the interpretation. There is extensive literature on the formation of thermogenic methane in the North Atlantic during the PETM, the anaerobic oxidation of which could have influenced the formation of ikaite, as the authors have noted occurs in modern settings. I feel this should be addressed before final publication. I have listed my concerns below in the order of appearance within the manuscript, which includes minor notes such as typos as well as my major concern as to the interpretation of the observations.
Lines 48-49: specify °C when saying “temperatures approaching zero”
Lines 62-63: explain why the glendonites are a topic of controversy, there should be more discussion on these previous studies that are referenced
Lines 78-80: “Whether these cooling events reflect a localised as well as short-lived cooling is not understood, nor the mechanism by which such cooling could occur, although these have been speculated upon” discuss how these have been speculated upon and what some of the proposed mechanisms have been
Line 244: Somewhat minor, but I believe there could be a better word choice than “mush”, perhaps mixture or amalgam?
Lines 296, 310: Again, I believe there is a better word choice than “blebs”. I will leave that choice to the author, as I’m not exactly certain what is meant by blebs.
Line 311: there is a space missing between “localised” and “increase”
Line 340-341: This is my major concern with the paper. The authors reference anaerobic oxidation of methane as playing a key role in ikaite precipitation, yet do not discuss the extensive literature on the thermogenic methane production in the North Atlantic during the PETM associated with North Atlantic Igneous Province vulcanism. There are many papers on this topic, including Jones et al., 2019 and Frieling et al., 2016 among others. At the very least, there should be discussion as to the possibility that anaerobic oxidation of methane could have played a role in the formation of ikaite as an alternative to the volcanic ash hypothesis, why this methane could not have played a role, or the potential that these mechanisms could have worked in conjunction.
Line 393: there is a missing space between “CaCO₃” and “in”
Line 438: Again, here the influence of methane should be discussed in addition to the rapid diagenesis of ash
Lines 449-450: A recent paper in this journal (Rush et al., 2023) found transient cooling associated with ETM2 in the Mid-Atlantic of the United States. This would not have been an enclosed basin but provides evidence of another region that experienced cooling during an Eocene hyperthermal and may be worth a citation.

Citation: https://doi.org/10.5194/egusphere-2023-1651-RC3
- AC3: 'Reply on RC3', Madeleine Vickers, 07 Nov 2023
  
  We thank William Rush for his considered comments on the manuscript. In addition to some minor comments, RC3 expressed their major concern that that more discussion of anaerobic oxidation of methane was required, as there is extensive literature on the formation of thermogenic methane in the North Atlantic during the PETM, the anaerobic oxidation of which could have influenced the formation of ikaite. This is a valid point, and we will accordingly add a discussion of the possible source and role of methane in ikaite precipitation to the manuscript. Please refer to the attached supplementary file for details of our response to this and all the more minor line-by-line comments.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1651-AC3
EC1:
'Comment on egusphere-2023-1651', Gerilyn (Lynn) Soreghan, 12 Oct 2023

Minor comments:
117— the strata “exhibit” rather than “contain”
246— Section 3.2— it might be helpful to begin here by noting the size (and maybe shape) range, where you note they are variable in “size and appearance,” in addition to the references to the relevant figures.
Fig. 3A illustrates a core that appears to have mud invasion and be a bit disrupted, and indeed the caption notes that these are glendonite fragments in drill mud. Doesn’t that mean that these could be out of place, and thus not reliably placed stratigraphically? In other words, why show this example, given the uncertainty of its stratigraphic position? I see that you mention this in lines 257-260, so that is fine.
315— space needed between words.
367— what is meant by “Pore waters… were taken on board at low resolution…”— really two questions— how were the pore waters sampled, and at what resolution? Did I miss this in the Methods? (I couldn’t find this detailed in the methods).
495— are they found “throughout” or within discrete intervals? It seems more like the latter?
This is a well-written, well-organized paper with an interesting story to relay about ikaite formation and climate perturbations during the late Paleocene-early Eocene.

Citation: https://doi.org/10.5194/egusphere-2023-1651-EC1
- AC4: 'Reply on EC1', Madeleine Vickers, 07 Nov 2023
  
  We thank Gerilyn Soreghan for these helpful comments, and attach detailed responses to the line-by-line comments in the attached supplementary file.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1651-AC4

Peer review completion

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

ED: Publish subject to technical corrections (08 Nov 2023) by Gerilyn (Lynn) Soreghan

AR by Madeleine Vickers on behalf of the Authors (13 Nov 2023) Author's response Manuscript

Journal article(s) based on this preprint

03 Jan 2024

Paleocene–Eocene age glendonites from the Mid-Norwegian Margin – indicators of cold snaps in the hothouse?

Clim. Past, 20, 1–23, https://doi.org/10.5194/cp-20-1-2024,https://doi.org/10.5194/cp-20-1-2024, 2024

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Madeleine L. Vickers et al.

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396 glendonite and ash data Madeleine L. Vickers, Morgan T. Jones, Jack Longman, David Evans, Clemens V. Ullmann, Ella Wulfsberg Stokke, Martin Vickers, Joost Frieling, Dustin T. Harper, Vincent J. Clementi https://doi.org/10.5281/zenodo.8159662

Madeleine L. Vickers et al.

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The discovery of cold-water glendonite pseudomorphs in sediments deposited during the hottest part of the Cenozoic poses an apparent climate paradox. This study examines their occurrence, association with volcanic sediments, and speculates on the timing and extent of cooling, fitting this with current understanding of global climate during this period. We propose that volcanic activity was key to both physical and chemical conditions that enabled the formation of glendonites in these sediments.


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