Northern versus southern gateways control on ocean overturning circulation at the Eocene-Oligocene transition and possible implications to global cooling

There is a growing body of evidence indicating reorganization in the ocean overturning circulation during the transition from the Eocene to the Oligocene, in particular, the initiation of the northern-sourced overturning circulation that comprises the northern component water (NCW) and northward flowing intermediate water. This evidence includes Nd isotopes from the Southern Hemisphere, δ13C from intermediate water in the subtropical North Atlantic, and increasing difference in the benthic δ18O between the Northern and Southern Hemispheres. We compare all these sets of proxies and show that the increasing trends have been conjugated throughout most of the Oligocene indicating enhancement of the northern-sourced overturning circulation that began around the Eocene-Oligocene transition (EOT). This reorganization was attributed mainly to the development of the wind-driven Antarctic circumpolar current (ACC). Based on variety of geological inferences, we suggest that the shift to interhemispheric bipolar overturning circulation was initiated by buoyancy fluxes formed by the development of anti-estuarine circulation between the Nordic Seas and the North Atlantic around the EOT, which significantly intensified the NCW. The Nordic anti-estuarine circulation began at a critical threshold during rapid tectonic subsidence of the Greenland-Scotland Ridge (GSR) triggered by the suppression of the Iceland mantle plume. We show that the development of northern-sourced circulation cell during the Oligocene correlates compellingly with the histories of the GSR subsidence and the activity of the Iceland mantle plume, and to a lesser degree with the ACC history. Accordingly, this circulation was enhanced as long as the GSR subsided rapidly. The onset of the northern-sourced overturning circulation, which is the biologically productive circulation domain, has probably triggered the marked increase in the ocean productivity during the EOT. The relationship between the EOT climatic changes (δ18O, CO2 proxies) and circulation proxies may suggest that growing rates of burial of organic carbon caused by the productivity increase could have triggered an accelerated decrease in atmospheric CO2, which in turn caused the cooling at the first stage of the EOT.