Revisiting the Middle Eocene Climatic Optimum “Carbon Cycle Conundrum” With New Estimates of Atmospheric pCO 2 From Boron Isotopes

Michael J. Henehan; Kirsty M. Edgar; Gavin L. Foster; Donald E. Penman; Pincelli M. Hull; Rosanna Greenop; Eleni Anagnostou; Paul N. Pearson

doi:10.1029/2019PA003713

Revisiting the Middle Eocene Climatic Optimum “Carbon Cycle Conundrum” With New Estimates of Atmospheric pCO 2 From Boron Isotopes

Michael J. Henehan^*
, Kirsty M. Edgar
, Gavin L. Foster
, Donald E. Penman
, Pincelli M. Hull
, Rosanna Greenop
, Eleni Anagnostou
, Paul N. Pearson

^*Corresponding author for this work

School of Earth Sciences

Research output: Contribution to journal › Article (Academic Journal) › peer-review

76 Citations (Scopus)

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 δ¹³C 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 δ¹³C-pCO₂ proxy. In addition, this rise in pCO₂ was focused around the peak of the 400 kyr warming trend. Before this, considerable global carbonate δ¹⁸O 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.

Original language	English
Article number	e2019PA003713
Journal	Paleoceanography and Paleoclimatology
Volume	35
Issue number	6
Early online date	9 Mar 2020
DOIs	https://doi.org/10.1029/2019PA003713 https://doi.org/10.1029/2019PA003713
Publication status	Published - 5 Jun 2020

Bibliographical note

Funding Information:
This research used samples provided by the International Ocean Discovery Program (IODP). We thank Steve Bohaty for assistance with stratigraphy, provision of sample material from Sites 702 and 1263, and helpful discussion. We thank Paul Wilson for providing access to washed sample residues for Sites 1260. We thank Paige Breen for providing δ13C and δ18O data from IODP Site 1408. We thank Tom Chalk for assistance with trace element analysis and the rest of the B-Team at the University of Southampton for their help and cooperation. We thank Bärbel Hönisch, Christopher Pearce, David Evans, and Mathis Hain for helpful discussion and Ellen Thomas for providing feedback on an earlier draft of the manuscript. We also thank Peter Bijl and one anonymous reviewer for helpful feedback which improved the manuscript. Financial support was provided from via the Yale Peabody Museum to M. J. H., a NERC Postdoctoral Research Fellowship (NE/H016457/1), NERC Standard Grant (NE/P013112/1) and Leverhulme Early Career Fellowship (ECF-2013-608) to K. M. E., a Flint Postdoctoral Fellowship to D. E. P., and NERC grants awarded to P. N. P. (NE/I005870/1) and G. L. F. (NE/I005595/1). We report no conflicts of interests for any coauthors. All data supporting our conclusions are available in the supporting information and is also archived on PANGAEA (https://doi.pangaea.de/10.1594/PANGAEA.909432).

Funding Information:
This research used samples provided by the International Ocean Discovery Program (IODP). We thank Steve Bohaty for assistance with stratigraphy, provision of sample material from Sites 702 and 1263, and helpful discussion. We thank Paul Wilson for providing access to washed sample residues for Sites 1260. We thank Paige Breen for providing δC and δO data from IODP Site 1408. We thank Tom Chalk for assistance with trace element analysis and the rest of the B‐Team at the University of Southampton for their help and cooperation. We thank Bärbel Hönisch, Christopher Pearce, David Evans, and Mathis Hain for helpful discussion and Ellen Thomas for providing feedback on an earlier draft of the manuscript. We also thank Peter Bijl and one anonymous reviewer for helpful feedback which improved the manuscript. Financial support was provided from via the Yale Peabody Museum to M. J. H., a NERC Postdoctoral Research Fellowship (NE/H016457/1), NERC Standard Grant (NE/P013112/1) and Leverhulme Early Career Fellowship (ECF‐2013‐608) to K. M. E., a Flint Postdoctoral Fellowship to D. E. P., and NERC grants awarded to P. N. P. (NE/I005870/1) and G. L. F. (NE/I005595/1). We report no conflicts of interests for any coauthors. All data supporting our conclusions are available in the supporting information and is also archived on PANGAEA ( https://doi.pangaea.de/10.1594/PANGAEA.909432 ). 13 18

Publisher Copyright:
©2020. The Authors.

UN SDGs

This output contributes to the following UN Sustainable Development Goals (SDGs)

SDG 13 Climate Action
SDG 14 Life Below Water

Keywords

boron isotopes
carbon cycle
cryosphere
Middle Eocene Climatic Optimum
paleoclimate
pCO reconstruction

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Henehan, M. J., Edgar, K. M., Foster, G. L., Penman, D. E., Hull, P. M., Greenop, R., Anagnostou, E., & Pearson, P. N. (2020). Revisiting the Middle Eocene Climatic Optimum “Carbon Cycle Conundrum” With New Estimates of Atmospheric pCO 2 From Boron Isotopes. Paleoceanography and Paleoclimatology, 35(6), Article e2019PA003713. https://doi.org/10.1029/2019PA003713, https://doi.org/10.1029/2019PA003713

@article{ea90e75bbd734ccd8e052552e1a0e6fb,

title = "Revisiting the Middle Eocene Climatic Optimum {\textquotedblleft}Carbon Cycle Conundrum{\textquotedblright} With New Estimates of Atmospheric pCO 2 From Boron Isotopes",

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 δ13C 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 {\textquotedblleft}carbon cycle conundrum.{\textquotedblright} 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{\textendash}0.75 doublings, thus requiring a much more modest carbon injection than previously indicated by the alkenone δ13C-pCO2 proxy. In addition, this rise in pCO2 was focused around the peak of the 400 kyr warming trend. Before this, considerable global carbonate δ18O 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.",

keywords = "boron isotopes, carbon cycle, cryosphere, Middle Eocene Climatic Optimum, paleoclimate, pCO reconstruction",

author = "Henehan, \{Michael J.\} and Edgar, \{Kirsty M.\} and Foster, \{Gavin L.\} and Penman, \{Donald E.\} and Hull, \{Pincelli M.\} and Rosanna Greenop and Eleni Anagnostou and Pearson, \{Paul N.\}",

note = "Funding Information: This research used samples provided by the International Ocean Discovery Program (IODP). We thank Steve Bohaty for assistance with stratigraphy, provision of sample material from Sites 702 and 1263, and helpful discussion. We thank Paul Wilson for providing access to washed sample residues for Sites 1260. We thank Paige Breen for providing δ13C and δ18O data from IODP Site 1408. We thank Tom Chalk for assistance with trace element analysis and the rest of the B-Team at the University of Southampton for their help and cooperation. We thank B{\"a}rbel H{\"o}nisch, Christopher Pearce, David Evans, and Mathis Hain for helpful discussion and Ellen Thomas for providing feedback on an earlier draft of the manuscript. We also thank Peter Bijl and one anonymous reviewer for helpful feedback which improved the manuscript. Financial support was provided from via the Yale Peabody Museum to M. J. H., a NERC Postdoctoral Research Fellowship (NE/H016457/1), NERC Standard Grant (NE/P013112/1) and Leverhulme Early Career Fellowship (ECF-2013-608) to K. M. E., a Flint Postdoctoral Fellowship to D. E. P., and NERC grants awarded to P. N. P. (NE/I005870/1) and G. L. F. (NE/I005595/1). We report no conflicts of interests for any coauthors. All data supporting our conclusions are available in the supporting information and is also archived on PANGAEA (https://doi.pangaea.de/10.1594/PANGAEA.909432). Funding Information: This research used samples provided by the International Ocean Discovery Program (IODP). We thank Steve Bohaty for assistance with stratigraphy, provision of sample material from Sites 702 and 1263, and helpful discussion. We thank Paul Wilson for providing access to washed sample residues for Sites 1260. We thank Paige Breen for providing δC and δO data from IODP Site 1408. We thank Tom Chalk for assistance with trace element analysis and the rest of the B‐Team at the University of Southampton for their help and cooperation. We thank B{\"a}rbel H{\"o}nisch, Christopher Pearce, David Evans, and Mathis Hain for helpful discussion and Ellen Thomas for providing feedback on an earlier draft of the manuscript. We also thank Peter Bijl and one anonymous reviewer for helpful feedback which improved the manuscript. Financial support was provided from via the Yale Peabody Museum to M. J. H., a NERC Postdoctoral Research Fellowship (NE/H016457/1), NERC Standard Grant (NE/P013112/1) and Leverhulme Early Career Fellowship (ECF‐2013‐608) to K. M. E., a Flint Postdoctoral Fellowship to D. E. P., and NERC grants awarded to P. N. P. (NE/I005870/1) and G. L. F. (NE/I005595/1). We report no conflicts of interests for any coauthors. All data supporting our conclusions are available in the supporting information and is also archived on PANGAEA ( https://doi.pangaea.de/10.1594/PANGAEA.909432 ). 13 18 Publisher Copyright: {\textcopyright}2020. The Authors.",

year = "2020",

month = jun,

day = "5",

doi = "10.1029/2019PA003713",

language = "English",

volume = "35",

journal = "Paleoceanography and Paleoclimatology",

issn = "2572-4517",

publisher = "John Wiley and Sons Inc",

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Henehan, MJ, Edgar, KM, Foster, GL, Penman, DE, Hull, PM, Greenop, R, Anagnostou, E & Pearson, PN 2020, 'Revisiting the Middle Eocene Climatic Optimum “Carbon Cycle Conundrum” With New Estimates of Atmospheric pCO 2 From Boron Isotopes', Paleoceanography and Paleoclimatology, vol. 35, no. 6, e2019PA003713. https://doi.org/10.1029/2019PA003713, https://doi.org/10.1029/2019PA003713

Revisiting the Middle Eocene Climatic Optimum “Carbon Cycle Conundrum” With New Estimates of Atmospheric pCO 2 From Boron Isotopes. / Henehan, Michael J.; Edgar, Kirsty M.; Foster, Gavin L. et al.
In: Paleoceanography and Paleoclimatology, Vol. 35, No. 6, e2019PA003713, 05.06.2020.

Research output: Contribution to journal › Article (Academic Journal) › peer-review

TY - JOUR

T1 - Revisiting the Middle Eocene Climatic Optimum “Carbon Cycle Conundrum” With New Estimates of Atmospheric pCO 2 From Boron Isotopes

AU - Henehan, Michael J.

AU - Edgar, Kirsty M.

AU - Foster, Gavin L.

AU - Penman, Donald E.

AU - Hull, Pincelli M.

AU - Greenop, Rosanna

AU - Anagnostou, Eleni

AU - Pearson, Paul N.

N1 - Funding Information: This research used samples provided by the International Ocean Discovery Program (IODP). We thank Steve Bohaty for assistance with stratigraphy, provision of sample material from Sites 702 and 1263, and helpful discussion. We thank Paul Wilson for providing access to washed sample residues for Sites 1260. We thank Paige Breen for providing δ13C and δ18O data from IODP Site 1408. We thank Tom Chalk for assistance with trace element analysis and the rest of the B-Team at the University of Southampton for their help and cooperation. We thank Bärbel Hönisch, Christopher Pearce, David Evans, and Mathis Hain for helpful discussion and Ellen Thomas for providing feedback on an earlier draft of the manuscript. We also thank Peter Bijl and one anonymous reviewer for helpful feedback which improved the manuscript. Financial support was provided from via the Yale Peabody Museum to M. J. H., a NERC Postdoctoral Research Fellowship (NE/H016457/1), NERC Standard Grant (NE/P013112/1) and Leverhulme Early Career Fellowship (ECF-2013-608) to K. M. E., a Flint Postdoctoral Fellowship to D. E. P., and NERC grants awarded to P. N. P. (NE/I005870/1) and G. L. F. (NE/I005595/1). We report no conflicts of interests for any coauthors. All data supporting our conclusions are available in the supporting information and is also archived on PANGAEA (https://doi.pangaea.de/10.1594/PANGAEA.909432). Funding Information: This research used samples provided by the International Ocean Discovery Program (IODP). We thank Steve Bohaty for assistance with stratigraphy, provision of sample material from Sites 702 and 1263, and helpful discussion. We thank Paul Wilson for providing access to washed sample residues for Sites 1260. We thank Paige Breen for providing δC and δO data from IODP Site 1408. We thank Tom Chalk for assistance with trace element analysis and the rest of the B‐Team at the University of Southampton for their help and cooperation. We thank Bärbel Hönisch, Christopher Pearce, David Evans, and Mathis Hain for helpful discussion and Ellen Thomas for providing feedback on an earlier draft of the manuscript. We also thank Peter Bijl and one anonymous reviewer for helpful feedback which improved the manuscript. Financial support was provided from via the Yale Peabody Museum to M. J. H., a NERC Postdoctoral Research Fellowship (NE/H016457/1), NERC Standard Grant (NE/P013112/1) and Leverhulme Early Career Fellowship (ECF‐2013‐608) to K. M. E., a Flint Postdoctoral Fellowship to D. E. P., and NERC grants awarded to P. N. P. (NE/I005870/1) and G. L. F. (NE/I005595/1). We report no conflicts of interests for any coauthors. All data supporting our conclusions are available in the supporting information and is also archived on PANGAEA ( https://doi.pangaea.de/10.1594/PANGAEA.909432 ). 13 18 Publisher Copyright: ©2020. The Authors.

PY - 2020/6/5

Y1 - 2020/6/5

N2 - 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 δ13C 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 δ13C-pCO2 proxy. In addition, this rise in pCO2 was focused around the peak of the 400 kyr warming trend. Before this, considerable global carbonate δ18O 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.

AB - 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 δ13C 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 δ13C-pCO2 proxy. In addition, this rise in pCO2 was focused around the peak of the 400 kyr warming trend. Before this, considerable global carbonate δ18O 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.

KW - boron isotopes

KW - carbon cycle

KW - cryosphere

KW - Middle Eocene Climatic Optimum

KW - paleoclimate

KW - pCO reconstruction

UR - https://www.scopus.com/pages/publications/85086874049

U2 - 10.1029/2019PA003713

DO - 10.1029/2019PA003713

M3 - Article (Academic Journal)

SN - 2572-4517

VL - 35

JO - Paleoceanography and Paleoclimatology

JF - Paleoceanography and Paleoclimatology

IS - 6

M1 - e2019PA003713

ER -

Revisiting the Middle Eocene Climatic Optimum “Carbon Cycle Conundrum” With New Estimates of Atmospheric pCO 2 From Boron Isotopes

Abstract

Bibliographical note

UN SDGs

Keywords

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