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 (similar to 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 records(4-6), 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 (similar to 280 p.p.m.v.) is much lower than that for Antarctica (similar to 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 time-frame, 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 degrees C and Antarctic ice that is less isotopically depleted (-30 to -35 parts per thousand) than previously suggested(15,16). Proxy CO2 estimates remain above our model's northern- hemispheric glaciation threshold of similar to 280 p.p.m.v. until similar to 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).