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Letter. Thresholds for Cenozoic bipolar glaciation

Letter. Thresholds for Cenozoic bipolar glaciation
Letter. Thresholds for Cenozoic bipolar glaciation
The long-standing view of Earth's Cenozoic glacial history calls for the first continental-scale glaciation of Antarctica in the earliest Oligocene epoch (33.6 million years ago), followed by the onset of northern-hemispheric glacial cycles in the late Pliocene epoch, about 31 million years later. The pivotal early Oligocene event is characterized by a rapid shift of 1.5 parts per thousand in deep-sea benthic oxygen-isotope values (Oi-1) within a few hundred thousand years, reflecting a combination of terrestrial ice growth and deep-sea cooling. The apparent absence of contemporaneous cooling in deep-sea Mg/Ca records, however, has been argued to reflect the growth of more ice than can be accommodated on Antarctica; this, combined with new evidence of continental cooling7 and ice-rafted debris in the Northern Hemisphere during this period, raises the possibility that Oi-1 represents a precursory bipolar glaciation. Here we test this hypothesis using an isotope-capable global climate/ice-sheet model that accommodates both the long-term decline of Cenozoic atmospheric CO2 levels and the effects of orbital forcing. We show that the CO2 threshold below which glaciation occurs in the Northern Hemisphere (280 p.p.m.v.) is much lower than that for Antarctica (750 p.p.m.v.). Therefore, the growth of ice sheets in the Northern Hemisphere immediately following Antarctic glaciation would have required rapid CO2 drawdown within the Oi-1 timeframe, to levels lower than those estimated by geochemical proxies and carbon-cycle models. Instead of bipolar glaciation, we find that Oi-1 is best explained by Antarctic glaciation alone, combined with deep-sea cooling of up to 4 °C and Antarctic ice that is less isotopically depleted (-30 to -35) than previously suggested. Proxy CO2 estimates remain above our model's northern-hemispheric glaciation threshold of 280 p.p.m.v. until 25 Myr ago, but have been near or below that level ever since. This implies that episodic northern-hemispheric ice sheets have been possible some 20 million years earlier than currently assumed (although still much later than Oi-1) and could explain some of the variability in Miocene sea-level records.

0028-0836
652-656
DeConto, Robert M.
b920aac8-fc77-487c-a85a-95820d007871
Pollard, David
2ecf15f2-499c-4f91-ba64-da03f9f8a789
Wilson, Paul A.
f940a9f0-fa5a-4a64-9061-f0794bfbf7c6
Pälike, Heiko
b9bf7798-ad8c-479b-8487-dd9a30a61fa5
Lear, Caroline H.
c0caf74d-ed6c-4b04-b5c8-de8fc794943c
Pagani, Mark
fea192f2-422c-4380-a5be-f6e2e75398e6
DeConto, Robert M.
b920aac8-fc77-487c-a85a-95820d007871
Pollard, David
2ecf15f2-499c-4f91-ba64-da03f9f8a789
Wilson, Paul A.
f940a9f0-fa5a-4a64-9061-f0794bfbf7c6
Pälike, Heiko
b9bf7798-ad8c-479b-8487-dd9a30a61fa5
Lear, Caroline H.
c0caf74d-ed6c-4b04-b5c8-de8fc794943c
Pagani, Mark
fea192f2-422c-4380-a5be-f6e2e75398e6

DeConto, Robert M., Pollard, David, Wilson, Paul A., Pälike, Heiko, Lear, Caroline H. and Pagani, Mark (2008) Letter. Thresholds for Cenozoic bipolar glaciation. Nature, 455 (7213), 652-656. (doi:10.1038/nature07337).

Record type: Article

Abstract

The long-standing view of Earth's Cenozoic glacial history calls for the first continental-scale glaciation of Antarctica in the earliest Oligocene epoch (33.6 million years ago), followed by the onset of northern-hemispheric glacial cycles in the late Pliocene epoch, about 31 million years later. The pivotal early Oligocene event is characterized by a rapid shift of 1.5 parts per thousand in deep-sea benthic oxygen-isotope values (Oi-1) within a few hundred thousand years, reflecting a combination of terrestrial ice growth and deep-sea cooling. The apparent absence of contemporaneous cooling in deep-sea Mg/Ca records, however, has been argued to reflect the growth of more ice than can be accommodated on Antarctica; this, combined with new evidence of continental cooling7 and ice-rafted debris in the Northern Hemisphere during this period, raises the possibility that Oi-1 represents a precursory bipolar glaciation. Here we test this hypothesis using an isotope-capable global climate/ice-sheet model that accommodates both the long-term decline of Cenozoic atmospheric CO2 levels and the effects of orbital forcing. We show that the CO2 threshold below which glaciation occurs in the Northern Hemisphere (280 p.p.m.v.) is much lower than that for Antarctica (750 p.p.m.v.). Therefore, the growth of ice sheets in the Northern Hemisphere immediately following Antarctic glaciation would have required rapid CO2 drawdown within the Oi-1 timeframe, to levels lower than those estimated by geochemical proxies and carbon-cycle models. Instead of bipolar glaciation, we find that Oi-1 is best explained by Antarctic glaciation alone, combined with deep-sea cooling of up to 4 °C and Antarctic ice that is less isotopically depleted (-30 to -35) than previously suggested. Proxy CO2 estimates remain above our model's northern-hemispheric glaciation threshold of 280 p.p.m.v. until 25 Myr ago, but have been near or below that level ever since. This implies that episodic northern-hemispheric ice sheets have been possible some 20 million years earlier than currently assumed (although still much later than Oi-1) and could explain some of the variability in Miocene sea-level records.

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Published date: 2 October 2008
Organisations: Ocean and Earth Science

Identifiers

Local EPrints ID: 55276
URI: http://eprints.soton.ac.uk/id/eprint/55276
ISSN: 0028-0836
PURE UUID: 00e20caa-3c32-43ef-b04f-4677290b21f3
ORCID for Paul A. Wilson: ORCID iD orcid.org/0000-0001-6425-8906

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Date deposited: 28 Jul 2008
Last modified: 16 Mar 2024 03:13

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Contributors

Author: Robert M. DeConto
Author: David Pollard
Author: Paul A. Wilson ORCID iD
Author: Heiko Pälike
Author: Caroline H. Lear
Author: Mark Pagani

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