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Revisiting the Middle Eocene climatic optimum ‘Carbon Cycle Conundrum’ with new estimates of atmospheric pCO2 from boron isotopes

Revisiting the Middle Eocene climatic optimum ‘Carbon Cycle Conundrum’ with new estimates of atmospheric pCO2 from boron isotopes
Revisiting the Middle Eocene climatic optimum ‘Carbon Cycle Conundrum’ with new estimates of atmospheric pCO2 from boron isotopes
The Middle Eocene Climatic Optimum (MECO) was a gradual warming event and carbon cycle perturbation that occurred between 40.5 and 40.1 Ma. A number of characteristics, including greater-than-expected deep-sea carbonate dissolution, a lack of globally-coherent negative δ 13 40 C excursion in marine carbonates, a duration longer than the characteristic timescale of carbon-cycle recovery, and the absence of a clear trigger mechanism, challenge our current understanding of the Earth system and its regulatory feedbacks. This makes the MECO one of the most enigmatic events in the Cenozoic, dubbed a middle Eocene ‘carbon cycle conundrum’. Here we use boron isotopes in planktic foraminifera to better constrain pCO2 changes over the event. Over the MECO itself, we find that pCO2 rose by only 0.55-0.75 doublings, thus requiring a much more modest carbon injection than previously indicated by the alkenone δ 13 C-pCO2 proxy. In addition, this rise in pCO2 was focused around the peak of the 400 kyr warming trend. Before this, considerable global carbonate δ 18 O change was asynchronous with any coherent ocean pH (and hence pCO2) excursion. This finding suggests that middle Eocene climate (and perhaps a nascent cryosphere) was highly sensitive to small changes in radiative forcing.
2572-4525
Henehan, Michael J.
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Edgar, Kirsty M.
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Foster, Gavin
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Penman, Donald E.
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Hull, Pincelli M.
ec74bbf9-239b-439f-a158-cfd68b575b92
Greenop, Rosanna
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Anagnostou, Eleni
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Pearson, Paul N.
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Henehan, Michael J.
1dae087e-6389-4f29-b966-26929951881d
Edgar, Kirsty M.
c180d215-e9ee-4a79-8274-a1e8a300c07c
Foster, Gavin
fbaa7255-7267-4443-a55e-e2a791213022
Penman, Donald E.
4356d201-8ca9-4edb-8e71-cc7ce08e39dd
Hull, Pincelli M.
ec74bbf9-239b-439f-a158-cfd68b575b92
Greenop, Rosanna
8d7defb5-8a8e-4e86-8734-15dffcfc81f0
Anagnostou, Eleni
4527c274-f765-44ce-89ab-0e437aa3d870
Pearson, Paul N.
76269a23-3411-45a1-bc81-b3a668ef1d13

Henehan, Michael J., Edgar, Kirsty M., Foster, Gavin, Penman, Donald E., Hull, Pincelli M., Greenop, Rosanna, Anagnostou, Eleni and Pearson, Paul N. (2020) Revisiting the Middle Eocene climatic optimum ‘Carbon Cycle Conundrum’ with new estimates of atmospheric pCO2 from boron isotopes. Paleoceanography and Paleoclimatology. (In Press)

Record type: Article

Abstract

The Middle Eocene Climatic Optimum (MECO) was a gradual warming event and carbon cycle perturbation that occurred between 40.5 and 40.1 Ma. A number of characteristics, including greater-than-expected deep-sea carbonate dissolution, a lack of globally-coherent negative δ 13 40 C excursion in marine carbonates, a duration longer than the characteristic timescale of carbon-cycle recovery, and the absence of a clear trigger mechanism, challenge our current understanding of the Earth system and its regulatory feedbacks. This makes the MECO one of the most enigmatic events in the Cenozoic, dubbed a middle Eocene ‘carbon cycle conundrum’. Here we use boron isotopes in planktic foraminifera to better constrain pCO2 changes over the event. Over the MECO itself, we find that pCO2 rose by only 0.55-0.75 doublings, thus requiring a much more modest carbon injection than previously indicated by the alkenone δ 13 C-pCO2 proxy. In addition, this rise in pCO2 was focused around the peak of the 400 kyr warming trend. Before this, considerable global carbonate δ 18 O change was asynchronous with any coherent ocean pH (and hence pCO2) excursion. This finding suggests that middle Eocene climate (and perhaps a nascent cryosphere) was highly sensitive to small changes in radiative forcing.

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Accepted/In Press date: 2020

Identifiers

Local EPrints ID: 438394
URI: http://eprints.soton.ac.uk/id/eprint/438394
ISSN: 2572-4525
PURE UUID: cd266155-829a-4f48-9127-8159571e9052
ORCID for Gavin Foster: ORCID iD orcid.org/0000-0003-3688-9668

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Date deposited: 09 Mar 2020 17:31
Last modified: 18 Feb 2021 17:14

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Contributors

Author: Michael J. Henehan
Author: Kirsty M. Edgar
Author: Gavin Foster ORCID iD
Author: Donald E. Penman
Author: Pincelli M. Hull
Author: Rosanna Greenop
Author: Eleni Anagnostou
Author: Paul N. Pearson

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