The early Eocene Climatic Optimum (EECO; ~ 53.3 to 49.1 Ma) was a period of the warmest sustained temperatures of the Cenozoic caused by perturbations to the global carbon cycle. Deep sea sediment cores and the microfossils preserved within them are the primary sources of information for these changes in climate and global carbon cycling but are subject to diagenetic alteration after deposition. One of the great challenges in paleoclimate research is determining how to accurately interpreting the proxy record by identifying the amount of chemical alteration of the isotopic and elemental compositions locked within microfossils such as foraminifera. The planktic foraminifera record has been biased by digenesis, provoking questions about the strength of the latitudinal temperature gradient throughout the EECO, specifically with respect to mismatches between proxy data and climate model simulations that remain unresolved. To investigate this question, we selected three deep sea sites that span across the Pacific Ocean, ODP Sites 865, 1209 and DSDP Site 207. From these sediments we extracted carefully screened planktic foraminifera and conducted analysis by two independent approaches on splits of the same individual foraminiferal shells. We measured the δ18O composition by conventional analysis (gas source mass spectrometry), and Mg/Ca ratios on fragments of the same shells by LA-ICP-MS that allows for a careful diagenetic screening. We then independently estimate sea surface temperatures and compare records to quantify the extent of bias in the planktonic foraminifera record. This approach helps to reassess the latitudinal temperature gradients across the EECO.
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The latitudinal temperature gradient and its climate dependence as inferred from foraminiferal δ 18 O over the past 95 million years
The latitudinal temperature gradient is a fundamental state parameter of the climate system tied to the dynamics of heat transport and radiative transfer. Thus, it is a primary target for temperature proxy reconstructions and global climate models. However, reconstructing the latitudinal temperature gradient in past climates remains challenging due to the scarcity of appropriate proxy records and large proxy–model disagreements. Here, we develop methods leveraging an extensive compilation of planktonic foraminifera δ 18 O to reconstruct a continuous record of the latitudinal sea-surface temperature (SST) gradient over the last 95 million years (My). We find that latitudinal SST gradients ranged from 26.5 to 15.3 °C over a mean global SST range of 15.3 to 32.5 °C, with the highest gradients during the coldest intervals of time. From this relationship, we calculate a polar amplification factor (PAF; the ratio of change in >60° S SST to change in global mean SST) of 1.44 ± 0.15. Our results are closer to model predictions than previous proxy-based estimates, primarily because δ 18 O-based high-latitude SST estimates more closely track benthic temperatures, yielding higher gradients. The consistent covariance of δ 18 O values in low- and high-latitude planktonic foraminifera and in benthic foraminifera, across numerous climate states, suggests a fundamental constraint on multiple aspects of the climate system, linking deep-sea temperatures, the latitudinal SST gradient, and global mean SSTs across large changes in atmospheric CO 2 , continental configuration, oceanic gateways, and the extent of continental ice sheets. This implies an important underlying, internally driven predictability of the climate system in vastly different background states.
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- PAR ID:
- 10333429
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 119
- Issue:
- 11
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1073/pnas.2111332119
