Sensitivity of Ocean Circulation to Warming During the Early Eocene Greenhouse

Authors

Sandra Kirtland Turner, University of California, RiversideFollow
Andy Ridgwell, University of California, Riverside
Allison L. Keller, University of California
Maximilian Vahlenkamp, University of Bremen
Adam K. Aleksinski, University of California, Riverside
Philip F. Sexton, The Open University
Donald E. Penman, Utah State UniversityFollow
Pincelli M. Hull, Yale University
Richard D. Norris, University of California - San Diego

Document Type

Article

Author ORCID Identifier

Sandra Kirtland Turner https://orcid.org/0000-0002-3606-5071

Andy Ridgwell https://orcid.org/0000-0003-2333-0128

Philip F. Sexton https://orcid.org/0000-0003-2873-3989

Donald E. Penman https://orcid.org/0000-0003-4191-0505

Pincelli M. Hull https://orcid.org/0000-0001-8607-4817

Richard D. Norris https://orcid.org/0000-0001-5288-1733

Journal/Book Title/Conference

Proceedings of the National Academy of Sciences of the United States of America

Volume

121

Issue

24

Publisher

National Academy of Sciences

Publication Date

6-3-2024

Journal Article Version

Version of Record

First Page

1

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 License.

Last Page

8

Abstract

Multiple abrupt warming events ("hyperthermals") punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ¹³C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation's sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ¹³C and δ¹⁸O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ¹³C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ¹³C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ¹³C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ¹³C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future.

Recommended Citation

S. Kirtland Turner, A. Ridgwell, A.L. Keller, M. Vahlenkamp, A.K. Aleksinski, P.F. Sexton, D.E. Penman, P.M. Hull, R.D. Norris, Sensitivity of ocean circulation to warming during the Early Eocene greenhouse, Proc. Natl. Acad. Sci. U.S.A. 121 (24) e2311980121, https://doi.org/10.1073/pnas.2311980121 (2024).