Revised alkenone &delta;<sup>13</sup>C based CO<sub>2</sub> estimates during the Plio-Pleistocene

Seki, Osamu; Bendle, James

doi:https://doi.org/10.5194/cp-2021-62

Preprints

https://doi.org/10.5194/cp-2021-62

Preprints

17 Jun 2021

| 17 Jun 2021

Status: this discussion paper is a preprint. It has been under review for the journal Climate of the Past (CP). The manuscript was not accepted for further review after discussion.

Revised alkenone δ¹³C based CO₂ estimates during the Plio-Pleistocene

Osamu Seki and James Bendle

Abstract. Here we revisit reported alkenone δ¹³C (δ¹³C_C37) based CO₂ records during the Plio-Pleistocene and apply a refined approach to better constrain the dynamic range of CO₂ during the time. Specifically, we consider ways to correct for regional differences in physical oceanographic factors. As a result of our relatively simple approach we find that offsets of ~150 ppm between reported δ¹³C_C37 CO₂ records from different sites can be significantly reduced. This confirms that better constraints on environmental variables, including physical oceanographic controls on depth and season of production are key aspects for improving δ¹³C_C37 based CO₂ estimates. The revised δ¹³C_C37 CO₂ datasets suggest that Plio-Pleistocene CO₂ levels are 180–350 ppm, which is consistent with the most of reported CO₂ reconstructions, though their upper end of Pliocene CO₂ levels are lower than that of some CO₂ estimates.

Received: 04 Jun 2021 – Discussion started: 17 Jun 2021

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.

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Osamu Seki and James Bendle

Status: closed

RC1: 'Comment on cp-2021-62', Anonymous Referee #1, 22 Jul 2021

Overview

This manuscript covers precisely as its title suggests, “Revised alkenone δ13C based CO2 estimates during the Plio-Pleistocene”. In the manuscript, the authors apply the traditional method for the alkenone-based CO2 proxy, in which the proxy model assumes passive diffusion of CO2 into the cell. Their revisions to the alkenone-based proxy are based on corrections for regional differences in physical oceanographic factors, including depth, season of production, and air-ocean disequilibrium. Correcting for these physical oceanographic factors can result in offsets of up to 150 ppm and now set the alkenone-based CO2 estimates in the range of other reported CO2 reconstructions.

Overall, the paper addresses a relevant scientific question within the scope of CP: testing the influence of regional differences in physical oceanographic factors on the alkenone-based CO2 proxy. However, I have some questions regarding the aim of the study and some of the assumptions made, and there are unfortunately many presentation issues, which I outline below.

Major Comments

My main comments are related to the aim/perspective and some of the assumptions in the approach.

The authors state in the introduction (Line 44-46) that “the aim is to not provide newly definitive CO2 estimates but to demonstrate the importance of physical oceanography”. However, the authors cite that Pagani et al. (2010; 2011) has already identified that physical oceanography affect values up to 150 ppm (Line 47, Pagani et al., 2010). This calls the question the novelty of this study.

I would also like some clarifications on the assumptions used to make these revised calculations and whether these revisions could be replicated in future studies, e.g. at different sites or during different time periods.

For example, there needs to be a bit more care around [PO43-]. It seems like mixed messages to state on one hand that this must be corrected for, and then on the other, state (Line 103-105) that modern values can be used under the assumption that it has not differed in these open marine settings. In other words, the study emphasizes the importance of changes in region but entirely neglects time. Have the authors considered estimating [PO43-] from δ15N values?

Another example of needed clarification: in some sections, the authors refer to these sites at open marine settings, while in other sections, the authors discuss upwelling and sea-air disequilibrium. Are these open or upwelling sites?

The authors state that they control for active upwelling (Line 154). However, the studies that the authors cited for this upwelling data (Takahashi et al., 2007) demonstrate that there are large changes within the year, large changes from year-to-year, and that these dynamics have been influenced by anthropogenic CO2 rise. Based on this, these upwelling intensities would presumably change over these million year timescales. Again, this appears to be correcting everything to modern day conditions. I think this section needs more explanation.

Another example is with the regional b-term regressions. Three of the six sites use global regressions and three use regional regressions. I think the lack of consistent treatment among sites needs more care.

All raw data needs to be included, either in the manuscript or as a supplement, and all related calculations. This is crucial for replicability and transparency. In addition to those reasons, I would also like to see the raw Uk’37 abundances and ratios, given that some tropical Uk’37 ratios have been known to approach saturation during this time.

Along with the raw data, all estimates of uncertainty should be included. For example, the authors only include the analytical error on the Uk’37-based proxy, but do not include the error on the proxy itself which is much greater (closer to 4 degrees). These uncertainties should be included in the CO2 estimates shown throughout the manuscript.

In the Introduction, the authors should include a couple sentences to explain what alkenones are (producer, organic compound, etc.) and how the alkenone-based proxy works (e.g. isotopic fractionation), given that this entire manuscript revolves the improvement of this proxy.

Specific Comments

Is Site 1000 used? It is occasionally mentioned (e.g. Fig 2, Line 168) but then is not included in other places (e.g. Fig. 1 map of the sites or discussions).

There are numerous mistakes in equations, grammar, spelling, and formatting throughout the manuscript, which have made this manuscript difficult to read and understand. This is not an exhaustive list, but some examples include:

Eq. 2: Fix the equation to read: epsilon_p = epsilon_f – b / [CO2]. In its current form, there is no epsilon_f and there is a missing bracket on the CO2(aq).

Line 16-21: Multiple grammatical issues (“though”, “understating”)

Line 48: Fix year, current cited as Pagani et al., 1010

Line 57: Delete “an”

Line 85-88: Why are these, for example, δCaCO3 instead of δ13C of CaCO3

There are two sections labeled 3.7.

Fig. 2: Add a title to explain what this figure is, add consistency in the panel labeling, and correct the numerous reference formatting issues.

Fig. 6: Change the scale to show all data and uncertainty (the error bars appear to extend beyond the figure). Change the y-axis label to “Reconstructed CO2” or similar (currently, the y-axis is labeled as boron-based CO2, but the figure shows both boron-based and alkenone-based CO2).

Define terms the first time they appear (e.g. δ13C37, δCaCO3).

Citation: https://doi.org/10.5194/cp-2021-62-RC1
RC2:
'Comment on cp-2021-62', Anonymous Referee #2, 26 Jul 2021
The manuscript considers the potential CO2 implications of published determinations of carbon isotopic fraction in phytoplankton biomarkers (ep shorthand for epsilon p throughout) over the last 5 million years. The manuscript does not present any new data nor propose a new framework for calculation of pCO2 from ep. Rather it suggests two minor adjustments in the original calculation of pCO2. First it proposes application of a constant adjustment factor of pCO2 ranging from -36 ppm to +30 ppm to the results of a given site. Secondly it suggests two small adjustments in the "b" coefficient used to convert ep to CO2 at a given site, which in some models have been estimated from PO4 concentrations - the adjustment to use average mixed layer rather than surface ocean PO4, and for two sites the use of a specific regional rather than global correlation.

Overall, I do not find this contribution suitable for further consideration for publication in Climate of the Past. The contribution does not provide a reasonable discussion of the current and recent advances of the framework for interpreting ep and its relationship with CO2, and neglects key references. The contribution is not well organized to clearly set up the rationale for its approach and why this approach is advantageous in light of current understanding. Errors in key equations (eg Eq 2) further give the impression of a hastily prepared contribution. FInally the scope of the contribution - with no new data, no new interpretative framework - is really insufficient for an article in a journal of the standing of Climate of the Past. It jiggles with a few details but does not provide much helpful for improving the big picture or useful to future studies.

Missing a meaningful assimilation of previous work and its relevance to the proposed recalculations

Recent evaluation of available ep data for periods in which CO2 is independently known (water column, core top, and late Quaternary glacial cycles) has highlighted a number of challenges with the original model for pCO2 estimation used by the authors here (assuming a CO2 slope following a diffusive regulation of carbon uptake and the inference of an empirical relationship of the form intended to be presented in equation 2)

ep=ef- (b/CO2aq)

The introduction and discussion of Seki and Bendle give only cursory citation to the state of the art, without addressing the challenges recognized in previous studies and the implications they have for the exercise presented here.

(a) Zhang et al 2019 have documented that the non-CO2 (physicological) factor b is highest during the interglacial and lowest during the glacial, suggesting that present day photic zone correlations between b and PO2 are not broadly representative of the past. They highlight a greater ep sensitivity to glacial interglacial variations in sites of higher CO2 than lower CO2, opposite to predictions of diffusive behavior.

(b) Stoll et al (2019) document that over glacial cycles, the difference between observed ep and that predicted by diffusive models is correlated to CO2 aq, suggesting that diffusive models overestimate the ep sensitivity to CO2 aq, a finding consistent with additional data presented by Badger et al (2019).

Both results a) and b) imply that application of a constant "b" derived from interglacial (core top or water column) regressions to PO4 to ep measurements downcore can lead to errors in estimating the magnitude of past changes in CO2,

(c) Phelps et al (2021) have suggested that integrated irradiance, rather than growth rate, is the main physiological influence on ep

d) Wilkes and Pearson (2019) have highlighted that ef in alkenone producers was observed to be -11 rather than -25 as generally implemented in Eq 2, implicating a significantly different sensitivity of ep to CO2 and a large role for CCM-related processes in producing high ep values

e) Hernandez-Almeida et al (2020) have presented several proxies for estimation of mixed layer PO4 concentrations in the past (should be acknowledged line 104-105); and also document that growth rate is not the key driver of b (contrast with line 176).

These results have additional implications for the processes responsible for the observed covariation of b with PO4 and its potential to generate robust estimates of pCO2 in the past.

This paper therefore needs to acknowledge these previous findings and account for them and justify why it is useful to neglect all of these advances to return to previous paradigms despite the large errors that have been shown to result from application of such paradigms.

In the introduction the paper gives as a motivation improving the estimates of absolute values of CO2 by reducing the "up to 150 ppm errors" from not accounting for "physical oceanography" . (lines 47-48.) yet in lines 185-187 appears to suggest that the neglect of CCM in this approach will at least generate robust trends, implying that the absolute values might not be accurate but this is oK for the goal of the study. This reads as an inconsistency in the goal of the study.

The idea to use mixed layer PO4 concentrations to estimate b is not novel here, and the previous papers describing the idea has not been cited nor contrasted with the current approach.

Andersen et al (1999) were using PO4 concentrations averaged over the 0-50m depth interval to evaluate b vs PO4 relationships in the South Atlantic. Pagani et al (2002) interpreted core top data in the Pacific using consideration of alkenone production depth in each setting (ranging from 0 to 160 m) and considering production seasonality, and estimated PO4 for the relevant production depths, a more sophisticated approach than that taken here.

Even the publication in which much of the ep data recalcuated here were originally presented describes " We use alkenone unsaturation indices (UK′37) to determine SST and assume haptophyte production depths between 0 and 75 m at each site, bounding a range of [PO43−] determined from modern mean-annual-phosphate depth profiles (Supplementary Information) "

Thus line 111 which states " standard approach assumes alkenone production depths at the surface of the mixed layer" is not an accurate representation of the more sophisticated approaches that do exist in previous studies.

Section 3.4 on variations in cell geometry is not well-reasoned

This section cites studies without discussing the fundamental issues. First of all Zhang et al 2020 contrast with previous approaches and suggest that cell size (coccolith size) is a proxy for growth rate, and the net effect of decreasing cell size on pCO2 estimates is opposite to the effect considered in previous treatments evaluating the surface area to volume process or empirical culture data (eg Henderiks et al 2007; Phelps et al 2021, Stoll et al 2019). This fundamental difference should be acknowledged, not lumped into a single statement that all suggest ep is affected by cell geometry. Additionally, the authors claim that an influence of cell geometry based on changes in coccolith size should not be considered, because alkenone accumulation rate in one site does not covary with the accumulation rate of coccoliths inferred to produce the alkenones. This interpretation ignores the important influence that contrasting preservation factors may have on accumulation rate of organic components (sensitive to stabilizing mineral and deep sea O2) and calcium carbonate components (sensitive to bottom water carbonate undersaturation). The authors furthermore present no alternative producer of alkenones other than the lineage responsible for modern alkenone production.

The presentation of the paper:

Error in the key equation 2: the second ep should be ef

In methods section, it is unclear why all of the steps in ep and SST calculation are described. Was ep recalculated from the individual data, and if so why? ONly because the authors have adopted alternate SST equations? Also it is unclear why the authors have adopted these particular Uk'37 calibrations rather than Tierney et al Bayspline; additionally it is misleading to report only analytical error component on SST and not the calibration error which is substantial in the high temperature range. The first few lines of the methods should clarify what was taken "as published" and what was recalculated and why.

Really the introduction should have framed the question clearly and indicated exactly what aspects of "physical circulation" were leading to such gross errors in pCO2 as to justify a new paper.

Why is the post-industrial Takahashi et al (2009) used as the reference for pre-industrial (through Pliocene) air-sea disequilibrium corrections. This is not a coherent approach, the pre-industrial air sea disequilibrium must be used at each site.

Table 1 I would expect to list the source of the input ep data for each site.

References cited:

Andersen, N., P. J. Müller, G. Kirst, and R. R. Schneider. "Alkenone δ 13 C as a Proxy for PastPCO 2 in Surface Waters: Results from the Late Quaternary Angola Current." In Use of Proxies in Paleoceanography, pp. 469-488. Springer, Berlin, Heidelberg, 1999.

HernándezâAlmeida, Iván, Kristen M. Krumhardt, Hongrui Zhang, and Heather M. Stoll. "Estimation of physiological factors controlling carbon isotope fractionation in coccolithophores in photic zone and coreâtop samples." Geochemistry, Geophysics, Geosystems 21, no. 11 (2020): e2020GC009272.

Pagani, Mark, Katherine H. Freeman, Nao Ohkouchi, and Ken Caldeira. "Comparison of water column [CO2aq] with sedimentary alkenoneâbased estimates: A test of the alkenoneâCO2 proxy." Paleoceanography 17, no. 4 (2002): 21-1.

Wilkes, Elise B., and Ann Pearson. "A general model for carbon isotopes in red-lineage phytoplankton: Interplay between unidirectional processes and fractionation by RubisCO." Geochimica et Cosmochimica Acta 265 (2019): 163-181.

Zhang, Yi Ge, Ann Pearson, Albert Benthien, Liang Dong, Peter Huybers, Xiaoqing Liu, and Mark Pagani. "Refining the alkenone-pCO2 method I: Lessons from the Quaternary glacial cycles." Geochimica et Cosmochimica Acta 260 (2019): 177-191.
Citation: https://doi.org/10.5194/cp-2021-62-RC2

Status: closed

RC1: 'Comment on cp-2021-62', Anonymous Referee #1, 22 Jul 2021

Overview

This manuscript covers precisely as its title suggests, “Revised alkenone δ13C based CO2 estimates during the Plio-Pleistocene”. In the manuscript, the authors apply the traditional method for the alkenone-based CO2 proxy, in which the proxy model assumes passive diffusion of CO2 into the cell. Their revisions to the alkenone-based proxy are based on corrections for regional differences in physical oceanographic factors, including depth, season of production, and air-ocean disequilibrium. Correcting for these physical oceanographic factors can result in offsets of up to 150 ppm and now set the alkenone-based CO2 estimates in the range of other reported CO2 reconstructions.

Overall, the paper addresses a relevant scientific question within the scope of CP: testing the influence of regional differences in physical oceanographic factors on the alkenone-based CO2 proxy. However, I have some questions regarding the aim of the study and some of the assumptions made, and there are unfortunately many presentation issues, which I outline below.

Major Comments

My main comments are related to the aim/perspective and some of the assumptions in the approach.

The authors state in the introduction (Line 44-46) that “the aim is to not provide newly definitive CO2 estimates but to demonstrate the importance of physical oceanography”. However, the authors cite that Pagani et al. (2010; 2011) has already identified that physical oceanography affect values up to 150 ppm (Line 47, Pagani et al., 2010). This calls the question the novelty of this study.

I would also like some clarifications on the assumptions used to make these revised calculations and whether these revisions could be replicated in future studies, e.g. at different sites or during different time periods.

For example, there needs to be a bit more care around [PO43-]. It seems like mixed messages to state on one hand that this must be corrected for, and then on the other, state (Line 103-105) that modern values can be used under the assumption that it has not differed in these open marine settings. In other words, the study emphasizes the importance of changes in region but entirely neglects time. Have the authors considered estimating [PO43-] from δ15N values?

Another example of needed clarification: in some sections, the authors refer to these sites at open marine settings, while in other sections, the authors discuss upwelling and sea-air disequilibrium. Are these open or upwelling sites?

The authors state that they control for active upwelling (Line 154). However, the studies that the authors cited for this upwelling data (Takahashi et al., 2007) demonstrate that there are large changes within the year, large changes from year-to-year, and that these dynamics have been influenced by anthropogenic CO2 rise. Based on this, these upwelling intensities would presumably change over these million year timescales. Again, this appears to be correcting everything to modern day conditions. I think this section needs more explanation.

Another example is with the regional b-term regressions. Three of the six sites use global regressions and three use regional regressions. I think the lack of consistent treatment among sites needs more care.

All raw data needs to be included, either in the manuscript or as a supplement, and all related calculations. This is crucial for replicability and transparency. In addition to those reasons, I would also like to see the raw Uk’37 abundances and ratios, given that some tropical Uk’37 ratios have been known to approach saturation during this time.

Along with the raw data, all estimates of uncertainty should be included. For example, the authors only include the analytical error on the Uk’37-based proxy, but do not include the error on the proxy itself which is much greater (closer to 4 degrees). These uncertainties should be included in the CO2 estimates shown throughout the manuscript.

In the Introduction, the authors should include a couple sentences to explain what alkenones are (producer, organic compound, etc.) and how the alkenone-based proxy works (e.g. isotopic fractionation), given that this entire manuscript revolves the improvement of this proxy.

Specific Comments

Is Site 1000 used? It is occasionally mentioned (e.g. Fig 2, Line 168) but then is not included in other places (e.g. Fig. 1 map of the sites or discussions).

There are numerous mistakes in equations, grammar, spelling, and formatting throughout the manuscript, which have made this manuscript difficult to read and understand. This is not an exhaustive list, but some examples include:

Eq. 2: Fix the equation to read: epsilon_p = epsilon_f – b / [CO2]. In its current form, there is no epsilon_f and there is a missing bracket on the CO2(aq).

Line 16-21: Multiple grammatical issues (“though”, “understating”)

Line 48: Fix year, current cited as Pagani et al., 1010

Line 57: Delete “an”

Line 85-88: Why are these, for example, δCaCO3 instead of δ13C of CaCO3

There are two sections labeled 3.7.

Fig. 2: Add a title to explain what this figure is, add consistency in the panel labeling, and correct the numerous reference formatting issues.

Fig. 6: Change the scale to show all data and uncertainty (the error bars appear to extend beyond the figure). Change the y-axis label to “Reconstructed CO2” or similar (currently, the y-axis is labeled as boron-based CO2, but the figure shows both boron-based and alkenone-based CO2).

Define terms the first time they appear (e.g. δ13C37, δCaCO3).

Citation: https://doi.org/10.5194/cp-2021-62-RC1
RC2:
'Comment on cp-2021-62', Anonymous Referee #2, 26 Jul 2021
The manuscript considers the potential CO2 implications of published determinations of carbon isotopic fraction in phytoplankton biomarkers (ep shorthand for epsilon p throughout) over the last 5 million years. The manuscript does not present any new data nor propose a new framework for calculation of pCO2 from ep. Rather it suggests two minor adjustments in the original calculation of pCO2. First it proposes application of a constant adjustment factor of pCO2 ranging from -36 ppm to +30 ppm to the results of a given site. Secondly it suggests two small adjustments in the "b" coefficient used to convert ep to CO2 at a given site, which in some models have been estimated from PO4 concentrations - the adjustment to use average mixed layer rather than surface ocean PO4, and for two sites the use of a specific regional rather than global correlation.

Overall, I do not find this contribution suitable for further consideration for publication in Climate of the Past. The contribution does not provide a reasonable discussion of the current and recent advances of the framework for interpreting ep and its relationship with CO2, and neglects key references. The contribution is not well organized to clearly set up the rationale for its approach and why this approach is advantageous in light of current understanding. Errors in key equations (eg Eq 2) further give the impression of a hastily prepared contribution. FInally the scope of the contribution - with no new data, no new interpretative framework - is really insufficient for an article in a journal of the standing of Climate of the Past. It jiggles with a few details but does not provide much helpful for improving the big picture or useful to future studies.

Missing a meaningful assimilation of previous work and its relevance to the proposed recalculations

Recent evaluation of available ep data for periods in which CO2 is independently known (water column, core top, and late Quaternary glacial cycles) has highlighted a number of challenges with the original model for pCO2 estimation used by the authors here (assuming a CO2 slope following a diffusive regulation of carbon uptake and the inference of an empirical relationship of the form intended to be presented in equation 2)

ep=ef- (b/CO2aq)

The introduction and discussion of Seki and Bendle give only cursory citation to the state of the art, without addressing the challenges recognized in previous studies and the implications they have for the exercise presented here.

(a) Zhang et al 2019 have documented that the non-CO2 (physicological) factor b is highest during the interglacial and lowest during the glacial, suggesting that present day photic zone correlations between b and PO2 are not broadly representative of the past. They highlight a greater ep sensitivity to glacial interglacial variations in sites of higher CO2 than lower CO2, opposite to predictions of diffusive behavior.

(b) Stoll et al (2019) document that over glacial cycles, the difference between observed ep and that predicted by diffusive models is correlated to CO2 aq, suggesting that diffusive models overestimate the ep sensitivity to CO2 aq, a finding consistent with additional data presented by Badger et al (2019).

Both results a) and b) imply that application of a constant "b" derived from interglacial (core top or water column) regressions to PO4 to ep measurements downcore can lead to errors in estimating the magnitude of past changes in CO2,

(c) Phelps et al (2021) have suggested that integrated irradiance, rather than growth rate, is the main physiological influence on ep

d) Wilkes and Pearson (2019) have highlighted that ef in alkenone producers was observed to be -11 rather than -25 as generally implemented in Eq 2, implicating a significantly different sensitivity of ep to CO2 and a large role for CCM-related processes in producing high ep values

e) Hernandez-Almeida et al (2020) have presented several proxies for estimation of mixed layer PO4 concentrations in the past (should be acknowledged line 104-105); and also document that growth rate is not the key driver of b (contrast with line 176).

These results have additional implications for the processes responsible for the observed covariation of b with PO4 and its potential to generate robust estimates of pCO2 in the past.

This paper therefore needs to acknowledge these previous findings and account for them and justify why it is useful to neglect all of these advances to return to previous paradigms despite the large errors that have been shown to result from application of such paradigms.

In the introduction the paper gives as a motivation improving the estimates of absolute values of CO2 by reducing the "up to 150 ppm errors" from not accounting for "physical oceanography" . (lines 47-48.) yet in lines 185-187 appears to suggest that the neglect of CCM in this approach will at least generate robust trends, implying that the absolute values might not be accurate but this is oK for the goal of the study. This reads as an inconsistency in the goal of the study.

The idea to use mixed layer PO4 concentrations to estimate b is not novel here, and the previous papers describing the idea has not been cited nor contrasted with the current approach.

Andersen et al (1999) were using PO4 concentrations averaged over the 0-50m depth interval to evaluate b vs PO4 relationships in the South Atlantic. Pagani et al (2002) interpreted core top data in the Pacific using consideration of alkenone production depth in each setting (ranging from 0 to 160 m) and considering production seasonality, and estimated PO4 for the relevant production depths, a more sophisticated approach than that taken here.

Even the publication in which much of the ep data recalcuated here were originally presented describes " We use alkenone unsaturation indices (UK′37) to determine SST and assume haptophyte production depths between 0 and 75 m at each site, bounding a range of [PO43−] determined from modern mean-annual-phosphate depth profiles (Supplementary Information) "

Thus line 111 which states " standard approach assumes alkenone production depths at the surface of the mixed layer" is not an accurate representation of the more sophisticated approaches that do exist in previous studies.

Section 3.4 on variations in cell geometry is not well-reasoned

This section cites studies without discussing the fundamental issues. First of all Zhang et al 2020 contrast with previous approaches and suggest that cell size (coccolith size) is a proxy for growth rate, and the net effect of decreasing cell size on pCO2 estimates is opposite to the effect considered in previous treatments evaluating the surface area to volume process or empirical culture data (eg Henderiks et al 2007; Phelps et al 2021, Stoll et al 2019). This fundamental difference should be acknowledged, not lumped into a single statement that all suggest ep is affected by cell geometry. Additionally, the authors claim that an influence of cell geometry based on changes in coccolith size should not be considered, because alkenone accumulation rate in one site does not covary with the accumulation rate of coccoliths inferred to produce the alkenones. This interpretation ignores the important influence that contrasting preservation factors may have on accumulation rate of organic components (sensitive to stabilizing mineral and deep sea O2) and calcium carbonate components (sensitive to bottom water carbonate undersaturation). The authors furthermore present no alternative producer of alkenones other than the lineage responsible for modern alkenone production.

The presentation of the paper:

Error in the key equation 2: the second ep should be ef

In methods section, it is unclear why all of the steps in ep and SST calculation are described. Was ep recalculated from the individual data, and if so why? ONly because the authors have adopted alternate SST equations? Also it is unclear why the authors have adopted these particular Uk'37 calibrations rather than Tierney et al Bayspline; additionally it is misleading to report only analytical error component on SST and not the calibration error which is substantial in the high temperature range. The first few lines of the methods should clarify what was taken "as published" and what was recalculated and why.

Really the introduction should have framed the question clearly and indicated exactly what aspects of "physical circulation" were leading to such gross errors in pCO2 as to justify a new paper.

Why is the post-industrial Takahashi et al (2009) used as the reference for pre-industrial (through Pliocene) air-sea disequilibrium corrections. This is not a coherent approach, the pre-industrial air sea disequilibrium must be used at each site.

Table 1 I would expect to list the source of the input ep data for each site.

References cited:

Andersen, N., P. J. Müller, G. Kirst, and R. R. Schneider. "Alkenone δ 13 C as a Proxy for PastPCO 2 in Surface Waters: Results from the Late Quaternary Angola Current." In Use of Proxies in Paleoceanography, pp. 469-488. Springer, Berlin, Heidelberg, 1999.

HernándezâAlmeida, Iván, Kristen M. Krumhardt, Hongrui Zhang, and Heather M. Stoll. "Estimation of physiological factors controlling carbon isotope fractionation in coccolithophores in photic zone and coreâtop samples." Geochemistry, Geophysics, Geosystems 21, no. 11 (2020): e2020GC009272.

Pagani, Mark, Katherine H. Freeman, Nao Ohkouchi, and Ken Caldeira. "Comparison of water column [CO2aq] with sedimentary alkenoneâbased estimates: A test of the alkenoneâCO2 proxy." Paleoceanography 17, no. 4 (2002): 21-1.

Wilkes, Elise B., and Ann Pearson. "A general model for carbon isotopes in red-lineage phytoplankton: Interplay between unidirectional processes and fractionation by RubisCO." Geochimica et Cosmochimica Acta 265 (2019): 163-181.

Zhang, Yi Ge, Ann Pearson, Albert Benthien, Liang Dong, Peter Huybers, Xiaoqing Liu, and Mark Pagani. "Refining the alkenone-pCO2 method I: Lessons from the Quaternary glacial cycles." Geochimica et Cosmochimica Acta 260 (2019): 177-191.
Citation: https://doi.org/10.5194/cp-2021-62-RC2

Osamu Seki and James Bendle

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Jul 2024	13	3	3	19
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Short summary

The reconstruction of CO₂ levels in the past is a crucial objective in palaeoclimate research. However, estimates of CO₂ level markedly differ among the data. We revised reported alkenone δ¹³C based CO₂ records from the Pliocene to Pleistocene based on a refined approach. Our approach significantly reduced the large offsets between reported alkenone δ¹³C CO₂ records, confirming that better constraints on environmental variables are key aspects for improving alkenone δ¹³C based CO₂ estimates.


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Revised alkenone δ13C based CO2 estimates during the Plio-Pleistocene

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Revised alkenone δ¹³C based CO₂ estimates during the Plio-Pleistocene