Nature 455, 652-656 (2 October 2008) | doi:10.1038/nature07337; Received 3 April 2008; Accepted 12 August 2008
Thresholds for Cenozoic bipolar glaciation
Robert M. DeConto
1, David Pollard
2, Paul A. Wilson
3, Heiko Pälike
3, Caroline H. Lear
4 & Mark Pagani
5
- Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA
- Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA
- National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK
- School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3YE, UK
- Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA
Correspondence to: Robert M. DeConto
1 Correspondence and requests for materials should be addressed to R.M.D. (Email:
deconto@geo.umass.edu).
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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
1), followed by the onset of northern-hemispheric glacial cycles in the late Pliocene epoch, about 31 million years later
2. The pivotal early Oligocene event is characterized by a rapid shift of 1.5 parts per thousand in deep-sea benthic oxygen-isotope values
3 (Oi-1) within a few hundred thousand years
4, reflecting a combination of terrestrial ice growth and deep-sea cooling.
The apparent absence of contemporaneous cooling in deep-sea Mg/Ca records4, 5, 6, however, has been argued to reflect the growth of more ice than can be accommodated on Antarctica; this, combined with new evidence of continental cooling
7 and ice-rafted debris
8, 9 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
10, 11 and the effects of orbital forcing
12. 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
10, 11 and carbon-cycle models
13, 14. 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
15, 16. 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
10, 11. 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
17, 18.
- Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA
- Earth and Environmental Systems Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA
- National Oceanography Centre, University of Southampton, Southampton SO14 3ZH, UK
- School of Earth and Ocean Sciences, Cardiff University, Cardiff CF10 3YE, UK
- Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA
Correspondence to: Robert M. DeConto
1 Correspondence and requests for materials should be addressed to R.M.D. (Email:
deconto@geo.umass.edu).
