Ocean circulation and carbon cycle changes around the Eocene-Oligocene boundary

Abstract

At the Eocene-Oligocene boundary (34 Myr ago) a sudden cooling of climate happened and the Antarctic Ice sheet developed, lowering sea level by over 100 m. This ice sheet exists up to the present date. Other changes at the time were a faster thermohaline circulation and a 1 km deepening of the carbon compensation depth. The cause of this event is unclear; there are several hypotheses. One of them points out a change in the ocean circulation pattern - possibly by gateway changes - as the cause. In this thesis the hypothesis is tested using a coupled ocean-atmosphere-ice box model with a biochemical module. The focus lies on two circulation patterns: south polar sinking (SPP state) and south and north polar sinking (TH state). The ?first pattern existed in the Eocene, the latter is the current MOC state. The in uence of the MOC state on the atmospheric CO2 concentration through the biological pump and solubility pump mechanisms is examined. It is found that a change from SPP to TH state lowers the atmospheric concentrations, cooling the deep ocean. The TH pattern has a faster circulation, this increases biomass; in this way atmospheric CO2 is drawn dawn by the biological pump. When starting from a CO2 level close enough to the ice growth threshold, the MOC state change also starts the growth of an Antarctic ice sheet, together with the temperature drop this mimics the two-step evolution in the d18O records. An envisioned expansion of the present model with the carbonate pump mechanism can be used to test whether the proposed model can also reproduce the observed carbon compensation depth change.