Coupled tectonic-ocean-atmosphere-weathering-pCO2 feedback as a trigger for Eocene-Oligocene Antarctic glaciation

English Abstract:

Rapid emplacement of the Antarctic ice sheet spanning the Eocene-Oligocene Transition (EOT) was initially attributed to thermal isolation of Antarctica due to tectonic widening of Southern Ocean gateways, but in recent years it has been more frequently attributed to atmospheric pCO₂ drawdown. A combination of geochemical evidence and modeling results suggest the possibility that a series of Earth system feedbacks actually linked these two mechanisms together by reducing atmospheric pCO₂ in response to an initial tectonic change and thereby triggering Antarctic glaciation. Large vertical gradients in benthic foraminiferal δ¹³C in the Southern Ocean, evident from ODP Sites 689 [Diester-Haass and Zahn, 1996] and 1090 [Pusz et al., 2011], suggest that strong stratification of the Southern Ocean developed during an ~2 million year interval of the latest Eocene. We suggest that during this time, the deepening of Southern Ocean gateways prevented southward geostrophic transport of saline subtropical water to high southern latitudes, causing the North Atlantic to salinify in compensation, and thereby strengthening the Atlantic Meridional Overturning Circulation. The consequent effects of changes in ocean heat transport, amplified by radiative cloud feedbacks, would have resulted in a warming of the northern hemisphere relative to the southern hemisphere [Yang et al., 2013], consistent with temperature reconstructions [Plancq et al., 2014; Zanazzi et al., 2007]. Because the northern hemisphere contains more land area, the interhemispheric temperature shift would have accelerated the global rate of silicate weathering, providing a mechanism for the drawdown of atmospheric pCO₂ and global cooling during the latest Eocene. The decline of atmospheric pCO₂ below a certain threshold would have enabled rapid growth of the Antarctic ice sheet across the EOT, with the subsequent formation of cold, dense waters ventilating the Southern Ocean and subsequently eliminating the vertical benthic foraminiferal δ¹³C gradient. This series of Earth system feedbacks – initiated by the tectonic deepening of Southern Ocean gateways and culminating in atmospheric pCO₂ drawdown driven by silicate weathering – reconcile competing hypotheses for the mechanisms responsible for onset of Antarctic glaciation during the EOT.