the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Oct 2024
Oligocene-early Miocene paradox of pCO2 inferred from alkenone carbon isotopic fractionation and sea surface temperature trends
Abstract. Atmospheric carbon dioxide decline is hypothesized to drive the progressive cooling over the Cenozoic. However, at multimillion-year timescales during the Oligocene to Miocene time interval the existing reconstructions, most based on the phytoplankton carbon isotopic fractionation (εp) proxy, differ from what is expected to drive the climate observations.
Here, we produce two new long-term records of εp over the Oligocene to early Miocene time interval from widely separated locations at IODP Site 1406 and ODP 1168 and increase the resolution of determinations at the equatorial Atlantic ODP 925. These new results confirm a global footprint of εp shift occurring during this interval. Abrupt 3 ‰ declines are found from 27 to 24.5 Ma and 24 to 22.5 Ma, and minimum εp is attained at 19 Ma. Between 28.7 and 29.7 Ma at IODP 1406, a higher resolution sampling reveals orbital scale 100 kyr cyclicity in εp. Making use of alkenone-based sea surface temperature (SST) estimates and benthic δ18O estimated extracted from the same samples, we perform a direct comparison with εp to evaluate the relationship with climate dynamics. We observe that across the long Oligocene to early Miocene interval the two sites’ relationships contrast with what is expected if CO2 was the main driver of εp and average earth surface temperature evolution was registered at the local surface SST and global benthic δ18O. Moreover, at orbital timescale, εp and benthic δ18O appear to follow an inverse relationship, although located within the multimillion-year period with the strongest direct correlation between these variables (>25.5 Ma). To evaluate the physiological, non- CO2 influences on εp, we use modern cultures to evaluate the impact of changing cell size and growth rate on the trends in εp. Although at specific time intervals, those drivers seem to explain part of the εp divergence with SST or benthic δ18O, most periods remain largely divergent, particularly the late Oligocene warming. We infer that a common CO2 forcing is likely the dominant control on the coherent temporal trends in εp at widely separated sites, which experienced contrasting temperature evolution and likely experienced different variations in nutrient availability. While CO2 changes likely caused significant changes in radiative forcing, SST variation at the examined sites may have been conditioned by regional heat transport, and the relationship between benthic δ18O and εp could reflect variable phasing between ice growth and global temperature.
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CC1: 'Comment on cp-2024-65', Peter Bijl, 11 Nov 2024
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I read the manuscript with great interest, it represents an interesting set of new data spanning the Oligocene-Miocene boundary. I am in general happy with the manuscript, but do feel that a lot of published information on Site 1168 is omitted, while these actually shed light on the interpretation of the data. First of all, the dinoflagellate cyst-based oceanographic reconstructions (Hoem et al., 2021; Hou et al., 2023b) shed light on the prevailing ocean conditions at the site: potential changes in upwelling, the ocean zone overlying the site, proximity of the subtropical front, presence of Leeuwin Current Influence, etc. all of which are crucial to at least qualitatively assess growth conditions and thus the ep and pCO2 reconstructions from this site. The papers referenced above would show (by the rather constant dinoflagellate cyst assemblage composition) little latitudinal migration of fronts and the high abundance of Spiniferites evidence for a persistent influence of the Leeuwin Current at the site. Secondly, the high-resolution TEX86-based SST data is also available for Site 1168 (Hou et al., 2023a). Although it makes sense to infer SST from alkenones to infer CO2 and ep, for arguments made in the paper, I think the TEX86 record still has value in the presentation of the oceanography at the site. I understand that this work was the result of a PhD project and that this part of the thesis was finished before the publication of these papers, and in that light it it understandable that the said studies were omitted. I suggest the authors do incorporate this information in a more comprehensive picture of the oceanographic development at ODP Site 1168 so that the whole study becomes more complete.
Also, the ACC development illustration in Figure 8 is somewhat outdated by recent insights that suggest that the modern-strength ACC did not start until the late Miocene (Evangelinos et al., 2022; 2024). Before that time, the ACC remained arguably weak (see, e.g., Sauermilch et al., 2021 for a recent modelling study).
Regards, Peter Bijl
References used in this comment
Evangelinos, D., et al. (2022). "Absence of a strong, deep-reaching Antarctic Circumpolar Current zonal flow across the Tasmanian gateway during the Oligocene to early Miocene." Global and Planetary Change 208.
Evangelinos, D., et al. (2024). "Late Miocene onset of the modern Antarctic Circumpolar Current." Nature Geoscience 17(2): 165-170.
Hoem, F. S., et al. (2021). "Late Eocene-early Miocene evolution of the southern Australian subtropical front: a marine palynological approach." Journal of Micropalaeontology 40(2): 175-193.
Hou, S., et al. (2022). "Lipid biomarker-based sea (sub)surface temperature record offshore Tasmania over the last 23 million years." Clim. Past Discuss. 2022: 1-33.
Hou, S., et al. (2023). "Equatorward subtropical front migration and strong dee-sea cooling in the Neogene." Nature Communications 14: 7230.
Sauermilch, I., et al. (2021). "Gateway-driven weakening of ocean gyres leads to Southern Ocean cooling." Nature Communications 12(1).
Citation: https://doi.org/10.5194/cp-2024-65-CC1 -
CC2: 'Comment on cp-2024-65', Xiaoqing Liu, 27 Nov 2024
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Guitián et al. built two new records of εp from the Oligocene to early Miocene based on IODP Site 1406 and ODP Site 1168, along with the new εp record from Site 925 to supplement previous published low-resolution record. As εp, the carbon isotopic fractionation during the photosynthesis of phytoplankton, is determined by both aqueous CO₂ levels and physiological parameters of phytoplankton. To extract variations in atmospheric CO₂ levels based on changes in εp from the Oligocene to the early Miocene, the authors evaluated the influence of varying physiological parameters, including cell size and growth rate, on εp evolution. The influence of changing cell size was assessed using measured coccolith sizes and a statistical multilinear regression model developed by Stoll et al. (2019), which shows that εp is a function of aqueous CO₂ concentration, light, growth rate, and cell radius. For growth rate, they assumed it is controlled by temperature and used the sensitivity of εp to temperature, as derived from the culture experiments by Torres Romero et al. (2024), to represent the sensitivity of εp to growth rate. They used changes in biogenic silica (bioSi) concentrations in sediments to indicate variations in nutrient concentrations; higher bioSi concentrations suggest increased nutrients and growth rates. They conclude that size and temperature effects have a negligible impact on the long-term declining trend of εp, and that a global CO₂ decline is the most likely cause of the decrease in εp.
Overall, the manuscript reads well, but a lot of phrases/sentences are confusing and difficult to understand. Please look at the specific comments below. In addition, I see several major shortcomings of the analysis and presentation of the results.
First, I do not think it is appropriate to use a single value for temperature-εp sensitivity (0.48‰ decrease in εp per 1°C warming) to represent the sensitivity during the Oligocene to early Miocene. The 0.48‰ decrease in εp per 1°C warming is based on the linear regression of temperature and εp data (22 samples) from culture studies by Torres Romero et al. (2024), conducted at 22 different combinations of temperature, CO₂(aq), and light. Torres Romero et al. (2024) demonstrates that the sensitivity of εp to temperature varies significantly across different CO₂(aq) ranges: 0.37‰ decrease in εp per 1°C warming when CO₂(aq) ranges from 4 to 22.5μmol/kg, and 0.95‰ decrease in εp per 1°C warming when CO₂(aq) ranges from 22.5 to 41 μmol/kg. Given that CO₂(aq) likely fluctuated across these ranges during the Oligocene to early Miocene, the temperature-εp sensitivity may also have varied. Therefore, relying on a single value derived from the culture experiment is not reliable for representing the entire period.
Second, the sensitivity of εp to temperature derived from culture experiments does not necessarily represent the sensitivity of εp to growth rate in real geological environments. Growth rate is influenced by a combination of factors, including light, temperature, nutrient availability, CO₂ levels, cell size, and other variables, all of which varied significantly over the geologic past. Although Torres Romero et al. (2024) demonstrates that the temperature-sensitive εp variation can be fully explained by the temperature sensitivity of growth rate, this does not imply that the relationship is directly applicable to real-world conditions of the geologic past.
Third, more evidence is needed to justify using changes in biogenic silica (bioSi) content in sediments to represent variations in surface ocean nutrient concentrations, particularly nitrate and phosphate, which are critical for coccolithophore growth. Sedimentary bioSi is primarily linked to the Si biogeochemical cycle, which likely differs from N and P cycles. Additionally, sedimentary bioSi is influenced by factors such as dissolution and preservation, limiting its reliability as a proxy for ocean nutrient concentrations, especially for nitrate and phosphate. Furthermore, the interpretation of bioSi results is inconsistent, with the authors at times referring to bioSi content as the delivery rate (Line 305) and at other times as the burial rate (Line 316).
Fourth, the linear relationship between εp and SST (or benthic δ¹⁸O) shown for several time slices in Figure 6 is not statistically meaningful, as the sample sizes for most of these time slices are fewer than 10. Therefore, the conclusions drawn from Figure 6 are unreliable.
Lastly, the relationship between εp and benthic δ¹⁸O at orbital scales (Figures 7b and 7c) does not yield a clear conclusion, as variations in benthic δ¹⁸O are influenced by both deep-water temperature and ice volume. A more meaningful comparison would be between εp and estimated global mean SST (Gaskell et al., 2022; https://doi.org/10.1073/pnas.2111332119) or surface temperature (Evans et al., 2024; https://doi.org/10.1029/2023PA004788).
Specific comments
Line 12-13: The statement “most based on the phytoplankton carbon isotopic fractionation (εp) proxy” is not accurate. Between 25 and 16 Ma, most of CO2 estimates are based on boron isotopes, not alkenone carbon isotopic fractionation.
Line 17: Full name of “Ma” is needed here
Line 17-18: “a higher resolution sampling” —higher than what?
Line 20: Please specify “the two sites”.
Line 20: climate dynamics is a broad concept. Please clarify it.
Line 21-22: This sentence is confusing, especially the phrase 'average earth surface temperature evolution.' Are the authors referring to the global mean surface temperature?
Line 22-23: what does the inverse relationship between εp and benthic δ18O indicate?
Line 25: what do “specific time intervals” represent?
Line 26: this sentence is incomprehensible.
Line 25-27: Confusing. How does the changing cell size and growth rate explain the divergence between εp and benthic δ18O?
Line 29: “While CO2 changes likely caused significant changes in radiative forcing” is not connected to the following sentence “SST variation at the examined sites may have been conditioned by regional heat transport”.
Line 31: How does “the relationship between benthic δ18O and εp” reflect the phasing between ice growth and global temperature?
Line 34: Please specify “long-term trends”. What trend?
Line 44: please specify what time interval shows “multimillion year warming” and what time intervals shows “cooling trends”
Line 54: delete “globally”?
Line 56: References are needed.
Line 60: Full name of “m. y.” is needed
Line 60: Please specify “two sites on the south American margin”.
Line 61: I would add the name of the Site for the additional North Atlantic record
Line 56-63: please reorganize these sentences. The current sentences are not in logic order. I would put the sentence “In this study, we produce a new long-term record of εp over the Oligocene to Miocene time interval at two new, widely separated locations” right after “One approach to evaluate the relative contribution of physiological factors vs CO2 is to produce εp records from sites of widely contrasting oceanographic setting…”. The difference of environmental factors (important for physiological factors of coccolithophores) between Site 1406 and Site 1168 should also be clarified in order to make it connected to the previous sentence.
Line 68-69: “an indicator of high-latitude temperature and Antarctic ice sheet extent
and/or volume” is not accurate. Variations in Benthic δ18O are controlled by changes in both deep-water temperature and ice volume.
Line 69: what do “These long-term relationships” indicate? “higher resolution” —higher than what?
Line 70: climate dynamics is a broad concept. Please clarify it.
Line 74: Please add the full name of CO2[aq].
Line 75: what do “These” refer to?
Line 80-81: Please add the equation εp = εf – b/CO2[aq], which makes it easy to read.Line 83-87: Some statements are incorrect. Zhang et al. (2013) also applied modern relationships between b and phosphate. Bolton et al. (2016) and Henderiks and Pagani (2007) do not estimate the difference between the modern b value at the site and the paleo-setting b value. Bolton et al. (2016) uses previous formulations of the relationship between cell size and b, which is derived from Henderiks and Pagani (2007).
Line 87-88: Please add references.
Line 88: The sentence “b term is not well predicted by growth rate, light or cell size alone in a diffusive model” is confusing.
Line 93-94: Could the author provide a brief implication of lower Rubisco fractionation?
Line 96: what does “This approach” refer to? The previous sentence does not mention any approach.
Line 96-97: “the observed slope of εp dependence on CO2” is difficult to understand.
Line 103: Change “growth rate” to “growth rate μi”
Line 107: Please add references after “While cell size can be estimated from coccolith length”.
Line 116-117: Please clarify how 0.5 ‰ decrease in εp per 1°C warming is indistinguishable from the prediction of growth rate effect on εp? Krumhardt et al. (2017) only demonstrates the increases in sea surface temperature lead to faster coccolithophore growth rates.
Line 134: Please specify “higher resolution”.
Line 139-140: Replace “The ODP Site 1168 age model” to “The age model of ODP Site 1168”. Similar issues occur throughout this manuscript. Please revise accordingly.
Line 146: what ages do “the two ODP 1168 samples deeper than the Sr isotope measurements” correspond to?
Figure 1: what is ODSN?
Line 159, 170, and 189: Change the bold text to normal formatting. Similar issues are present throughout the manuscript. Please revise them accordingly.
Line 171: please specify the age of “the young set of samples”
Line 173-176: the sentence flow is not clear. Please reorganize these sentences.
Line 207-208: References are needed.
Line 209-210: Guitián et al. (2020) does not demonstrate that the bulk carbonate is dominated by Reticulofenestra coccoliths.
Line 212-213: Here the authors assume that δ13C of bulk carbonates is equivalent to the δ13C of coccolith. However, they do not provide any evidence to support this assumption.
Line 213: The citation should be the original paper, McClelland et al. (2017), rather than Stoll et al. (2019).
Line 215: Guitián et al. (2019) describes the method for measuring stable isotopes of benthic foraminifera, not bulk carbonate. Before the section “Estimation of aqueous carbon dioxide δ¹³C”, a section describing the method for measuring stable isotopes of bulk carbonate is needed, including details on sample preprocessing.
Line 237-238: Do the authors use the value in equation (1) or the linear relationship between εp and cell radius? The slope of the linear relationship between εp and cell radius, derived from a compilation of culture experiments, is certainly different from the value in equation (1). Please specify the sensitivity of εp to cell radius used here and provide justification for its selection.
Line 247: Replace “26 ma” to “26 Ma”.
Line 249: Replace “from 28.8 to 29.6 Ma” to “from 29.6 to 28.8 Ma”. Similar issues occur throughout this manuscript. Please revise accordingly.
Line 250: “Several ̴ 100 ky orbital scale variations of 0.75 ‰ benthic δ18O and bulk δ18O” is incomprehensible.
Figure 2: Did the authors measure δ¹³C of the bulk carbonate for all three sites? The methods section does not clarify which sites were analyzed for bulk carbonate carbon isotopes.
Line 255: Instead of using solid and transparent lines, I recommend using different colors for the lines.
Line 258: White symbols are not visible on this figure; consider using a more visible color.
Lines 259–260: To maintain consistency, I suggest using either 1σ or 2σ for all the error bars.
Line 267: Change “an overall low and stable early Miocene” to “overall low and stable values in the early Miocene”?
Line 272: Is Curry et al. (1995) the correct citation? Curry et al. (1995) is the Initial Report for Leg 154, covering ODP Site 925 alone. It does not include DSDP 516 or ODP 608.
Line 272: The phrase “As seen in sites 1168 and 1406” is confusing, as this paper does not present the Sr isotopic stratigraphy of Sites 1168 and 1406.
Line 282 and 287: “within age uncertainty of the decrease” and “within the age model uncertainty of the minimum” are difficult to understand.
Line 284: “5‰ peak” is confusing. Do the authors mean “5‰ increase”?
Line 295: the title is not accurate.
Line 296: The authors have not discussed the effect of CO2 on εp yet.
Line 296: replace “cell surface area to volume ratio” with “cell size”
Line 304: Confusing. How does the deeper mixing cause the lower mean light levels?
Line 316-322: The main point of this paragraph is not clear.
Line 318 and 323: Misra and Froelich (2012) do not suggest an increase in erosion and weathering rates from the Oligocene to the early Miocene. In fact, their δ⁷LiSW data show little change from the middle Oligocene to the early Miocene.
Line 399: Is “1 ‰ range” typo?
Line 403: what do “these variables” indicate?
Figure 7: The numeric labels on the x- and y-axes (e.g., "2,2" and similar) are difficult to read. Please adjust them to a clearer format, such as "2.2." Similar issues occur in other figures. Please revise accordingly.
Figure 7: please add a, b, and c to each panel of this figure.
Line 409: r2=-0.34 is not possible. R-squared is always a positive value.
Figure 8: Please add the full name of MMCO, Mi-1, LOW, and MOGI.
Line 443: delete very. what do “a different set of feedbacks” mean? Different from what?
Line 446: Please clarify “a substantially different relationship between ice expansion and CO2.”
Line 451-452: Please delete “and decline in radiative forcing from the greenhouse effect.”
Line 453: there is no evidence to support the claim “the ODP Site 1168 temperature trend reflects global temperature”. SST change of Site 1168 is likely a regional signal.
Line 462: The term "late Oligocene divergence" is not easy to understand. Please consider replacing "divergence" with a clearer term throughout the manuscript to improve clarity.
Line 465: The conclusion section merely repeats the results presented in earlier sections. In addition to summarizing the findings, the conclusion should discuss the broader implications of the results.
Figures and supplementary figures: The current color scheme, particularly the use of red and green in the same figure, is not color-blind-friendly. Please adjust the colors to enhance accessibility and readability for all readers.
Citation: https://doi.org/10.5194/cp-2024-65-CC2 -
RC1: 'Comment on cp-2024-65', Anonymous Referee #1, 02 Dec 2024
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Hello,
Thank you for the opportunity to review this paper. I found that it was an interesting read with useful data, and with some points addressed will fit well into Climate of the Past. I've attached my comments in the PDF below.
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