the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Maastrichtian–Rupelian paleoclimates in the southwest Pacific – a critical re-evaluation of biomarker paleothermometry and dinoflagellate cyst paleoecology at Ocean Drilling Program Site 1172
Joost Frieling
Margot J. Cramwinckel
Christine Boschman
Appy Sluijs
Francien Peterse
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- Final revised paper (published on 25 Nov 2021)
- Supplement to the final revised paper
- Preprint (discussion started on 11 Mar 2021)
- Supplement to the preprint
Interactive discussion
Status: closed
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RC1: 'Comment on cp-2021-18', Chris Hollis, 16 Apr 2021
This is a very comprehensive review and update of the Late Cretaceous-Paleogene climate record of a key high-latitude locality. The authors are to be commended on bring such a wealth of data together in such a thoroughly integrated and informative way. The combination new ways of seeing established proxies and several new proxies gives us important insights into how this part of the planet functioned under a greenhouse climate. Especially interesting is the potential to compare pollen-based and dinocyst-based proxies with a new suite of GDGT and GDMT proxies. And the work to identify runoff indicators in this record (Fig. 16) is really important for understanding how the hydrological cycle is affected by greenhouse climate events.
There are however a few shortcomings with the paper that need to be addressed. Firstly, the authors need to appreciate that this is not a SW Pacific record but a Tasman Sea record. The degree to which it is representative of the wider SW Pacific is still a matter of debate with so few other records for comparison. However, there is one record that warrants more direct comparison, namely the New Zealand mid-Waipara section, which really is on the western margin of the SW Pacific Ocean. The similarities in the GDGT records from the two sites has been demonstrated in numerous publications (most recently Hollis et al., 2019). The trends in both SST and MAAT for the two sites are within the margin of error for the two GDGT proxies. This in my view calls into question the ocean circulation model perpetuated in this paper and recently reproduced in Huurdeman et al. (2020, Fig. 1). It is simply impossible to see how the same SST values can be generated from the cool southern-sourced “Tasman Current” at site 1172 and from the warm proto-East Australian Current that bathed the eastern margin of Zealandia. This applies for both the super-warm SSTs of the PETM and EECO as it does to the cool SSTs of the mid-Paleocene. A much simpler explanation is that the same currents bathed both sites, with the Tasman Current dominant in the Paleocene and later Eocene and the EAC dominant in the early Eocene (e.g. as suggested in Hollis et al., 2012).
Moreover, the Tasman Current is described as “western boundary current” (p. 12). These currents are the deep currents that drive deep-water circulation. This is quite distinct from the surface-water of the Ross gyre. Also worth noting that abbreviations for compass directions are always in upper case, hence SW not sw.
The effort the authors have made to compare the geochemical results with other approaches to temperature reconstruction are commendable. This should be standard practice in studies of this sort where so much uncertainty surrounds the absolute values generated by GDGT-based proxies. However, I find the introductory section on p. 4 poorly organised and a little misleading. It jumps from marine calcitic proxies to terrestrial pollen-based proxies and then back to marine TEX86. The problems that affect calcitic proxies are raised as a source of significant uncertainty but the much greater (in my view) uncertainties associated with applying a modern analogue approach to Eocene pollen grains is not mentioned at all. And no mention at all of brGDGT-based terrestrial approaches. I understand that this is covered in detail below, but some reorganisation is needed in these introductory paragraphs to set the scene for what follows.
Along these lines, a greater issue arises when comparing the dinocyst ecogroups with SST estimates. No mention is made of the fact that two of the ecogroups have several taxa in common – open ocean and thermophilic (Table 3). So, more consideration needs to be given to the argument that an increase in open ocean taxa signals sea-level rise during warm events, notably the PETM. Also, I am surprised that there is no comment of the mismatch between the abundance of thermophilic taxa and SST. Fig. 15 shows there is a general correlation, but a very weak increase in thermophile during the EECO that is quite at odds with the major SST increase. The authors will know that a similar muted response is recorded at mid-Waipara, for both dinocysts and nannofossils (Crouch et al., 2019). This begs the oft-posed question, are the fossils recording an annual signal and TEX86 a summer one. This warrants some comment together with reference to the NZ record. It is also worth noting that the thermophilic and opern ocean dinocysts decrease rather abruptly at 50 Ma, whereas TEX86 decreases more gradually. The major rise in endemics directly above the peak in SST, to me suggests a greater influence of the Ross-gyre from ~50 Ma.
In general, the text would benefit from a thorough edit to simplify sentence structures. Some of the references are cited out of context and others have been superseded by later work (Huber and Cabellero 200 vs Lunt et al. 2021; Huber et al. 2004 vs Sijp et al., 2016).
The figures are very informative but suffer from being too small with a too limited colour range and lacking guidelines to help match the text descriptions to the records. In some cases more explanation of methods and legends for symbols are needed (e.g. Figs. 15 and 16).
Numerous additional comments and edits are provided in the annotated MS
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AC1: 'Reply on RC1', Peter Bijl, 14 Jul 2021
Reply to Reviewer 1 (Chris Hollis)
The effort the authors have made to compare the geochemical results with other approaches to temperature reconstruction are commendable. This should be standard practice in studies of this sort where so much uncertainty surrounds the absolute values generated by GDGT-based proxies.
Response to reviewer: We thank the review for his positive overall assessment of the paper
However, I find the introductory section on p. 4 poorly organised and a little misleading. It jumps from marine calcitic proxies to terrestrial pollen-based proxies and then back to marine TEX86. The problems that affect calcitic proxies are raised as a source of significant uncertainty but the much greater (in my view) uncertainties associated with applying a modern analogue approach to Eocene pollen grains is not mentioned at all. And no mention at all of brGDGT-based terrestrial approaches. I understand that this is covered in detail below, but some reorganisation is needed in these introductory paragraphs to set the scene for what follows.
Response to reviewer: We understand the point raised about the mismatch between introduction and discussion when it comes to fully appreciating the uncertainties of the absolute temperature reconstructions from existing proxy records.
Proposed adjustments to the paper: We will carefully review the introduction section on existing proxy records (Lines 86-102) to better reflect the uncertainties and improve the order of the proxies mentioned.
Along these lines, a greater issue arises when comparing the dinocyst ecogroups with SST estimates. No mention is made of the fact that two of the ecogroups have several taxa in common – open ocean and thermophilic (Table 3). So, more consideration needs to be given to the argument that an increase in open ocean taxa signals sea-level rise during warm events, notably the PETM.
Response to reviewer: This is a good point, and we will mention this in the discussion. Generally, we see no problem in this co-variance, but we do acknowledge that the uncertainty in distinguishing between open ocean and warm conditions is not immediately apparent to the reader and provide further clarification:
The most important open-ocean dinocysts (e.g., Impagidinium spp., Operculodinium spp., Nematosphaeropsis spp.) are very low in abundance, which supports our view that the depositional setting was on the continental shelf. In the absence of the few genera that can be considered indicative of open ocean conditions (Zonneveld et al., 2013; Prebble et al., 2013), the remaining dinocysts are largely those that are also commonly associated to warmer conditions (according to Frieling and Sluijs, 2018). This overlap shows that within the range of genera and data studied by Frieling & Sluijs (2018) ‘warm’ and ‘open marine’ conditions cannot be easily distinguished based on dinocyst assemblages. However, this is perhaps not unexpected, as a general co-variance of higher sea level and temperature on continental shelfs in the ice-free Eocene is deemed likely. The risk of circular reasoning is countered by the extra evidence for deeper marine conditions in the middle Eocene, from the lithology (e.g., more CaCO3; Rohl et al., 2004).
Proposed adjustments to the paper: We will add the discussion above to the discussion section 5.3.1
Also, I am surprised that there is no comment of the mismatch between the abundance of thermophilic taxa and SST. Fig. 15 shows there is a general correlation, but a very weak increase in thermophile during the EECO that is quite at odds with the major SST increase. The authors will know that a similar muted response is recorded at mid-Waipara, for both dinocysts and nannofossils (Crouch et al., 2019). This begs the oft-posed question, are the fossils recording an annual signal and TEX86 a summer one. This warrants some comment together with reference to the NZ record.
Response to reviewer: We agree with the reviewer that there is no perfect correlation between TEX86 and thermophilic dinocyst assemblages, and that this requires better explanation and embedding into existing data from other microfossil groups (e.g. ,the recent paper by Shepherd et al., 2021). We however argue that a multitude of explanations, seasonality being just one of them, could explain this mismatch. Another explanation could be a reduced response of dinocyst assemblages at these high temperatures, and as such a non-linear behaviour to the temperature forcing. There are simply no cold-water species left to respond under such warm conditions. This has been observed in other microfossil groups as well, e.g., during the EECO on Maud Rise (Site 690; Thomas and Shackleton, 1996). What remains unexplained, however, is that the onset of proliferation of endemism occurs at the end of the EECO, when SSTs are still very warm (see also Bijl et al., 2011). We further note that dinocyst assemblages are sensitive to more environmental factors than just temperature: salinity, nutrients, thermocline depth, and seasonality, many which tend to have strong gradients on an inshore-to-offshore transect.
Proposed adjustments to the paper: We will add the above discussion to section 5.3.1.
It is also worth noting that the thermophilic and open ocean dinocysts decrease rather abruptly at 50 Ma, whereas TEX86 decreases more gradually. The major rise in endemics directly above the peak in SST, to me suggests a greater influence of the Ross-gyre from ~50 Ma.
Response to reviewer: We agree this is an interesting observation.
Proposed adjustments to the paper: we will add some notes on the termination of the EECO, and the dinocyst response to the paper, in section 5.3.1 (thereby citing Bijl et al., 2011; see also a note on this above).
In general, the text would benefit from a thorough edit to simplify sentence structures. Some of the references are cited out of context and others have been superseded by later work (Huber and Cabellero 200 vs Lunt et al. 2021; Huber et al. 2004 vs Sijp et al., 2016).
Response to reviewer: We disagree that because these papers are superseding one another, that they should therefore be replaced by the most recent one. Each of these papers use a different model, with different boundary conditions and functional feedbacks (e.g., ocean only versus fully coupled). More importantly, these simulations are quite sensitive to input parameters, and should therefore be considered as experiments, rather than full representation of reality, with the most recent one being the most accurate. Consistency in the outcome of these experiments strengthens the inferences from them, just as more proxy records strengthen inferences. This justifies citing all of them instead of just the most recent one.
Proposed adjustments to the paper: We will carefully reconsider the text to simplify sentence structures and reconsider referencing to properly reflect the current state of the literature, by including both older and newer references.
The figures are very informative but suffer from being too small with a too limited colour range and lacking guidelines to help match the text descriptions to the records. In some cases more explanation of methods and legends for symbols are needed (e.g. Figs. 15 and 16).
Response to reviewer: we will make sure figures adhere to the guidelines of the journal, and expand the captions for clarity.
Numerous additional comments and edits are provided in the annotated MS. Please also note the supplement to this comment: https://cp.copernicus.org/preprints/cp-2021-18/cp-2021-18-RC1-supplement.pdf
Response to reviewer: Please find responses to the comments in the annotated text. We thank the reviewer for his thorough assessment, which allows us to improve our manuscript.
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AC1: 'Reply on RC1', Peter Bijl, 14 Jul 2021
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RC2: 'R2 Comment on cp-2021-18', Anonymous Referee #2, 15 Jun 2021
General comment
The manuscript presents new paleoclimatic data from high southern latitudes that is consistent with previous interpretations for the region. A strength of the manuscript is that it also evaluates the strengths and weaknesses of proxies for sea surface temperature (SST) including isoGDGTs and mean annual air temperature (MAAT) including soil-derived branched GDGTs. The authors conclude that MAAT is consistently lower than SST during the early Eocene, independent of the calibration chosen and moreover, that the proxies fail to document a rise in MAAT during the PETM and MECO. The factors contributing to this discrepancy (i.e., a change in GDGT source) are discussed, however the incorporation of mixing models may help demonstrate this now that new data (see Lauretano et al. 2021, Nature Geoscience, accepted) is available for the peat/coal of interest.
Specific comments
The authors discuss the potential contribution of terrestrial material from Australia throughout the manuscript. As such, reference pollen-based vegetation reconstructions from southeastern Australia should be included in lines 94-96.
In lines 172-175 the authors detail the incorporation of “substantial terrestrial input”. Could you please clarify whether the source of the terrestrial input is deemed contemporaneous or reworked or both?
Could the authors please elaborate on why smaller Eocene hypothermal events do not stand out clearly at Site 1172. Is it for the same reasons as the PETM and MECO or other factors?
Can you please elaborate on possible mechanisms facilitating the warm bias for TEX86-based SSTs in the sw Pacific?
The inability to document a MAAT rise during the PETM and MECO is attributed to a switch in brGDGT sources, namely from soils and peaty lakes, that dampened the proxy-response. Here you cite Holdgate et al. 2009 and say the source could be peats in SE Australia. However, earlier in the manuscript you mention that “rivers flowing from southeast Australia drained into the Gippsland and Bass Basins, and that terrigenous material is unlikely to have reached the ETP.” Can you please clarify whether or not you think material from SE Australia could have reached site 1172? In addition, have you considered incorporating new brGDGT data (see Lauretano et al. 2021, Eocene to Oligocene terrestrial Southern Hemisphere cooling caused by declining pCO2, Nature Geoscience) derived from co-eval peats and deriving a mixing model (see Baczynsk et al. 2016 or Lyons et al. 2020 for mixing model examples)? This way you could test whether shifting sources of brGDGTs could be contributing to the absence of MAAT responses to the PETM and MECO.
You mention that diversity and TEX86 have a modest correlation for long-term trends and short-term trends (PETM), but not the MECO. Can you please elaborate on why this trend doesn’t hold true for the MECO?
You regularly refer to “Australian hinterland” and “hinterland catchment”. Can this please be illustrated on one of your maps?
How do the authors know the site drifted out of the zone of intense precipitation? Is there fossil or geochemical evidence for this? Latitudinal zones have shifted through time so there is no guarantee the northward movement of the Australian plate would have shifted the site into a new latitudinal/precipitation zone.
Technical corrections
Line 186 – please add ‘in’ after brGMGTs
Line 942 – please change ‘bothe’ to ‘both’
Line 1249 – please change ‘prodcution” to “production”
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AC2: 'Reply on RC2', Peter Bijl, 14 Jul 2021
Reviewer 2 (anonymous)
General comment
The manuscript presents new paleoclimatic data from high southern latitudes that is consistent with previous interpretations for the region. A strength of the manuscript is that it also evaluates the strengths and weaknesses of proxies for sea surface temperature (SST) including isoGDGTs and mean annual air temperature (MAAT) including soil-derived branched GDGTs. The authors conclude that MAAT is consistently lower than SST during the early Eocene, independent of the calibration chosen and moreover, that the proxies fail to document a rise in MAAT during the PETM and MECO. The factors contributing to this discrepancy (i.e., a change in GDGT source) are discussed, however the incorporation of mixing models may help demonstrate this now that new data (see Lauretano et al. 2021, Nature Geoscience, accepted) is available for the peat/coal of interest.
Response to reviewer: We thank the reviewer for this suggestion. As of today (Jul 14, 2021) the cited paper is not yet available, therefore we cannot use the information as yet but we invite the reviewer to contact us so that we can incorporate this suggestion.
Specific comments
The authors discuss the potential contribution of terrestrial material from Australia throughout the manuscript. As such, reference pollen-based vegetation reconstructions from southeastern Australia should be included in lines 94-96.
Response to reviewer: This is a good suggestion.
Proposed changes to the ms: we will incorporate an overview of pollen-based vegetation in the suggested section.
In lines 172-175 the authors detail the incorporation of “substantial terrestrial input”. Could you please clarify whether the source of the terrestrial input is deemed contemporaneous or reworked or both?
Response to reviewer: The studies done on the Arctic Paleogene material (e.g., Willard et al., 2019; Sluijs et al., 2020) do not show any indication that the terrestrial OM is abnormally out of stratigraphic context to the marine sediments it was found in.
Proposed changes to the ms: we will add this to the section. We thank the reviewer for his/her review.
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EC1: 'Editor Reply on AC2', Erin McClymont, 23 Jul 2021
Editor reply: Reviewer 2 has raised several questions or suggestions but only the reply to the general comment and some of the specific comments are outlined here. 9e.g. there is no response to the comment about hyperthemals, Australian hinterland being flagged....). Can the authors please upload their remaining responses to the Reviewer: do you disagree or plan to include these suggestions from Reviewer 2?
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AC3: 'Reply on AC2', Peter Bijl, 24 Jul 2021
Apologies, something must have gone wrong in the process. Please find the complete review to Reviewer 2 below.
Reviewer 2 (anonymous)
General comment
The manuscript presents new paleoclimatic data from high southern latitudes that is consistent with previous interpretations for the region. A strength of the manuscript is that it also evaluates the strengths and weaknesses of proxies for sea surface temperature (SST) including isoGDGTs and mean annual air temperature (MAAT) including soil-derived branched GDGTs. The authors conclude that MAAT is consistently lower than SST during the early Eocene, independent of the calibration chosen and moreover, that the proxies fail to document a rise in MAAT during the PETM and MECO. The factors contributing to this discrepancy (i.e., a change in GDGT source) are discussed, however the incorporation of mixing models may help demonstrate this now that new data (see Lauretano et al. 2021, Nature Geoscience, accepted) is available for the peat/coal of interest.
Response to reviewer: We thank the reviewer for this suggestion. As of today (Jul 6, 2021) the cited paper is not yet available, therefore we cannot use the information as yet.
Specific comments
The authors discuss the potential contribution of terrestrial material from Australia throughout the manuscript. As such, reference pollen-based vegetation reconstructions from southeastern Australia should be included in lines 94-96.
Response to reviewer: This is a good suggestion.
Proposed changes to the ms: we will incorporate an overview of pollen-based vegetation in the suggested section.
In lines 172-175 the authors detail the incorporation of “substantial terrestrial input”. Could you please clarify whether the source of the terrestrial input is deemed contemporaneous or reworked or both?
Response to reviewer: The research done on the record (e.g., Willard et al., 2019; Sluijs et al., 2020) do seem to suggest that that terrestrial OM is quasi-contemporaneous to the marine sediments it was found in.
Proposed changes to the ms: we will add this to the section.
Could the authors please elaborate on why smaller Eocene hypothermal events do not stand out clearly at Site 1172. Is it for the same reasons as the PETM and MECO or other factors?
Response to reviewer: Continental shelf records in the Southern Ocean have a general tendency not to show much climate change associated to the early Eocene post-PETM hyperthermals (e.g., Bijl et al., 2013; PNAS), e.g., in TEX86-based SST and bulk organic carbon isotopes (P.K. Bijl, unpublished data). The reason for this is unexplained.
Proposed changes to the ms: no changes made.
Can you please elaborate on possible mechanisms facilitating the warm bias for TEX86-based SSTs in the sw Pacific?
Response to Reviewer: We are unsure where the reviewer wants us to elaborate on this, in the introduction or in the discussion? It either represents an overestimation of temperature by the SST proxies, or an underestimation of regional SSTs by the coarse-resolution fully coupled climate models, or a combination of both.
Proposed changes to the ms: We will make sure that this is adequately presented in the paper, e.g., at lines 79-83.
The inability to document a MAAT rise during the PETM and MECO is attributed to a switch in brGDGT sources, namely from soils and peaty lakes, that dampened the proxy response. Here you cite Holdgate et al. 2009 and say the source could be peats in SE Australia. However, earlier in the manuscript you mention that “rivers flowing from southeast Australia drained into the Gippsland and Bass Basins, and that terrigenous
material is unlikely to have reached the ETP.” Can you please clarify whether or not you think material from SE Australia could have reached site 1172?
Response to Reviewer: We thank the reviewer for pointing out this contradiction, and we understand the confusion. We feel it is unlikely that all terrigenous matieral found at ETP came from Australian hinterland: a source from Tasmania is more likely. There might however have been a minor contribution of clay, and clay-bound organic matter, from further sources, like the Australian hinterland.
Proposed changes to the ms: We will make the uncertainties in the sources of the terrigenous material clearer in Section 2.4, which leaves plenty of room for further interpreting our results in the discussion.
In addition, have you considered incorporating new brGDGT data (see Lauretano et al. 2021, Eocene to Oligocene terrestrial Southern Hemisphere cooling caused by declining pCO2, Nature Geoscience) derived from co-eval peats and deriving a mixing model (see Baczynsk et al. 2016 or Lyons et al. 2020 for mixing model examples)? This way you could test whether shifting sources of brGDGTs could be contributing to the absence of MAAT responses to the PETM and MECO.
Response to Reviewer: As much as we are looking forward to seeing the paper by Lauretano in Nature Geoscience, the paper has not yet been published, unless we overlooked.
Proposed changes to the ms: If it becomes available in time, we will incorporate that work into our paper, and consider the approach of a mixing model, provided that we think that would be appropriate for our investigation.
You mention that diversity and TEX86 have a modest correlation for long-term trends and short-term trends (PETM), but not the MECO. Can you please elaborate on why this trend doesn’t hold true for the MECO?
Response to Reviewer: The why of this was left intentionally open, because we fail to have an explanation. Possible causes might be that the climate shift of the MECO is of critically slower time scales, leading to a less-dramatic ecological disruption. It is then strange, however, that longer-term climate changes, such as tat leading into the EECO, is represented in the diversity.
Proposed changes to the ms: We will add this discussion the text, to at least highlight the paradox..
You regularly refer to “Australian hinterland” and “hinterland catchment”. Can this please be illustrated on one of your maps?
Response to Reviewer: I am afraid we can put no bounds or limits to the hinterland catchment, because of the uncertainties described above, and because of the fact that the mountain ranges in se Australia are younger than our record (HOLDGATE G. R., WALLACE M. W., GALLAGHER S. J., WAGSTAFF B. E. & MOORE D. 2008. No mountains to snow on: major post-Eocene uplift of the East Victoria Highlands; evidence from Cenozoic
deposits. Australian Journal of Earth Sciences 55, 211–334.).
Proposed changes to the ms: However, we agree that that is a fair point to make, and so we will describe the uncertainties of the hinterland better in the site description.
How do the authors know the site drifted out of the zone of intense precipitation? Is there fossil or geochemical evidence for this? Latitudinal zones have shifted through time so there is no guarantee the northward movement of the Australian plate would have shifted the site into a new latitudinal/precipitation zone.
Response to Reviewer: Our inference of a drying of the hinterland comes purely from the northward drift of Australia, and prevailing climate conditions at those latitudes today. The coincidental evidence for a more seasonal concentration of precipitation in the middle-late Eocene we derive from our records confirms a climate shift that is not directly related to temperature (the Paleocene was cool and wet, while the middle-late Eocene was cool and drier).
Proposed changes to the ms: We will make that more explicit in the discussion.
Technical corrections
Line 186 – please add ‘in’ after brGMGTs
Line 942 – please change ‘bothe’ to ‘both’
Line 1249 – please change ‘prodcution” to “production”
We thank the reviewer, and will adapt these technical errors.
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EC1: 'Editor Reply on AC2', Erin McClymont, 23 Jul 2021
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AC2: 'Reply on RC2', Peter Bijl, 14 Jul 2021
Peer review completion

