Climate threshold at the Eocene-Oligocene transition; Antarctic ice sheet influence on ocean circulation

We present an overview of the Eocene-Oligocene transition from a marine perspective and posit that growth of a continent-scale Antarctic ice sheet (25X10 (super 6) km (super 3) ) was a primary cause of a dramatic reorganization of ocean circulation and chemistry. The Eocene-Oligocene transition (EOT) was the culmination of long-term (10 (super 7) yr scale) CO (sub 2) drawdown and related cooling that triggered a 0.5 ppm-0.9 ppm transient precursor benthic foraminiferal delta (super 18) O increase at 33.80 Ma (EOT-1), a 0.8 ppm delta (super 18) O increase at 33.63 Ma (EOT-2), and a 1.0 ppm delta (super 18) O increase at 33.55 Ma (oxygen isotope event Oi-1). We show that a small ( approximately 25 m) sea-level lowering was associated with the precursor EOT-1 increase, suggesting that the delta (super 18) O increase primarily reflected 1-2 degrees C of cooling. Global sea level dropped by 80+ or -25 m at Oi-1 time, implying that the deep-sea foraminiferal delta (super 18) O increase was due to the growth of a continent-sized Antarctic ice sheet and 1-4 degrees C of cooling. The Antarctic ice sheet reached the coastline for the first time at ca. 33.6 Ma and became a driver of Antarctic circulation, which in turn affected global climate, causing increased latitudinal thermal gradients and a "spinning up" of the oceans that resulted in: (1) increased thermohaline circulation and erosional pulses of Northern Component Water and Antarctic Bottom Water; (2) increased deep-basin ventilation, which caused a decrease in oceanic residence time, a decrease in deep-ocean acidity, and a deepening of the calcite compensation depth (CCD); and (3) increased diatom diversity due to intensified upwelling.