Thresholds for Cenozoic bipolar glaciation

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 ago¹), followed by the onset of northern-hemispheric glacial cycles in the late Pliocene epoch, about 31 million years later². The pivotal early Oligocene event is characterized by a rapid shift of 1.5 parts per thousand in deep-sea benthic oxygen-isotope values³ (Oi-1) within a few hundred thousand years⁴, reflecting a combination of terrestrial ice growth and deep-sea cooling. The apparent absence of contemporaneous cooling in deep-sea Mg/Ca records^4,5,6, however, has been argued to reflect the growth of more ice than can be accommodated on Antarctica; this, combined with new evidence of continental cooling⁷ 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 forcing¹². 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.