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Foraminiferal Mg/Ca evidence for Southern Ocean cooling across the Eocene–Oligocene transition

Foraminiferal Mg/Ca evidence for Southern Ocean cooling across the Eocene–Oligocene transition
Foraminiferal Mg/Ca evidence for Southern Ocean cooling across the Eocene–Oligocene transition
Constraining the magnitude of high-latitude temperature change across the Eocene–Oligocene transition (EOT) is essential for quantifying the magnitude of Antarctic ice-sheet expansion and understanding regional climate response to this event. To this end, we constructed high-resolution stable oxygen isotope (?18O) and magnesium/calcium (Mg/Ca) records from planktic and benthic foraminifera at four Ocean Drilling Program (ODP) sites in the Southern Ocean. Planktic foraminiferal Mg/Ca records from the Kerguelen Plateau (ODP Sites 738, 744, and 748) show a consistent pattern of temperature change, indicating 2–3 °C cooling in direct conjunction with the first step of a two-step increase in benthic and planktic foraminiferal ?18O values across the EOT. In contrast, benthic Mg/Ca records from Maud Rise (ODP Site 689) and the Kerguelen Plateau (ODP Site 748) do not exhibit significant temperature change. The contrasting temperature histories derived from the planktic and benthic Mg/Ca records are not reconcilable, since vertical ?18O gradients remained nearly constant at all sites between 35.0 and 32.5 Ma. Based on the coherency of the planktic Mg/Ca records from the Kerguelen Plateau sites and complications with benthic Mg/Ca paleothermometry at low temperatures, the planktic Mg/Ca records are deemed the most reliable measure of Southern Ocean temperature change. We therefore interpret a uniform cooling of 2–3 °C in both deep surface (thermocline) waters and intermediate deep waters of the Southern Ocean across the EOT. Cooling of Southern Ocean surface waters across the EOT was likely propagated to the deep ocean, since deep waters were primarily sourced on the Antarctic margin throughout this time interval. Removal of the temperature component from the observed foraminiferal ?18O shift indicates that seawater ?18O values increased by 0.6 ± 0.15‰ across the EOT interval, corresponding to an increase in global ice volume to a level equivalent with 60–130% modern East Antarctic ice sheet volume.
Foraminifera, Eocene, Oligocene, Southern Ocean, Stable oxygen isotopes, Mg/Ca, Ocean Drilling Program
0012-821X
251-261
Bohaty, Steven M.
af9dbe78-8b9f-44f2-ba1d-20795837d2d1
Zachos, James C.
c262d59f-aadc-4e09-b844-098db9a0e3c5
Delaney, Margaret L.
05ab5f22-e8f9-4823-840f-71e78bd34f1f
Bohaty, Steven M.
af9dbe78-8b9f-44f2-ba1d-20795837d2d1
Zachos, James C.
c262d59f-aadc-4e09-b844-098db9a0e3c5
Delaney, Margaret L.
05ab5f22-e8f9-4823-840f-71e78bd34f1f

Bohaty, Steven M., Zachos, James C. and Delaney, Margaret L. (2012) Foraminiferal Mg/Ca evidence for Southern Ocean cooling across the Eocene–Oligocene transition. Earth and Planetary Science Letters, 317-318, 251-261. (doi:10.1016/j.epsl.2011.11.037).

Record type: Article

Abstract

Constraining the magnitude of high-latitude temperature change across the Eocene–Oligocene transition (EOT) is essential for quantifying the magnitude of Antarctic ice-sheet expansion and understanding regional climate response to this event. To this end, we constructed high-resolution stable oxygen isotope (?18O) and magnesium/calcium (Mg/Ca) records from planktic and benthic foraminifera at four Ocean Drilling Program (ODP) sites in the Southern Ocean. Planktic foraminiferal Mg/Ca records from the Kerguelen Plateau (ODP Sites 738, 744, and 748) show a consistent pattern of temperature change, indicating 2–3 °C cooling in direct conjunction with the first step of a two-step increase in benthic and planktic foraminiferal ?18O values across the EOT. In contrast, benthic Mg/Ca records from Maud Rise (ODP Site 689) and the Kerguelen Plateau (ODP Site 748) do not exhibit significant temperature change. The contrasting temperature histories derived from the planktic and benthic Mg/Ca records are not reconcilable, since vertical ?18O gradients remained nearly constant at all sites between 35.0 and 32.5 Ma. Based on the coherency of the planktic Mg/Ca records from the Kerguelen Plateau sites and complications with benthic Mg/Ca paleothermometry at low temperatures, the planktic Mg/Ca records are deemed the most reliable measure of Southern Ocean temperature change. We therefore interpret a uniform cooling of 2–3 °C in both deep surface (thermocline) waters and intermediate deep waters of the Southern Ocean across the EOT. Cooling of Southern Ocean surface waters across the EOT was likely propagated to the deep ocean, since deep waters were primarily sourced on the Antarctic margin throughout this time interval. Removal of the temperature component from the observed foraminiferal ?18O shift indicates that seawater ?18O values increased by 0.6 ± 0.15‰ across the EOT interval, corresponding to an increase in global ice volume to a level equivalent with 60–130% modern East Antarctic ice sheet volume.

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More information

Published date: 1 February 2012
Keywords: Foraminifera, Eocene, Oligocene, Southern Ocean, Stable oxygen isotopes, Mg/Ca, Ocean Drilling Program
Organisations: Paleooceanography & Palaeoclimate

Identifiers

Local EPrints ID: 335888
URI: https://eprints.soton.ac.uk/id/eprint/335888
ISSN: 0012-821X
PURE UUID: 767d0552-5bef-426c-8489-9941b5c03b9d
ORCID for Steven M. Bohaty: ORCID iD orcid.org/0000-0002-1193-7398

Catalogue record

Date deposited: 13 Mar 2012 16:32
Last modified: 19 Nov 2019 01:44

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