Drivers of the Middle Pleistocene Transition: the Role of the Southern Ocean in Modulating Changes in CO2 and the Transition to the 100ky World

The causes of Mid Pleistocene Transition (MPT) are still a matter of debate. Even its exact length is still being discussed. Thus the onset of the transition from 41 to 100kyrs in the pacing of the glacial-interglacial (G-IG) cycles is placed at different intervals between 1.5, 1.2 and 0.9 Ma. It has been argued that a decrease in CO2 could have led to the crossing of a climatic threshold that through different mechanisms would have changed the pacing of the G-IG (Berger et al., 1999; Paillard, 1998; Raymo, 1997). However, the causes of such a decrease in CO2, and its occurrence are uncertain. Here we show that the Southern Ocean could have played a key role in driving such a hypothetical decrease in atmospheric CO2. We have analyzed a suite of records in the Atlantic and Pacific Oceans and noted that at 1.15Ma a cooling of the surface ocean took place synchronously, which is also recorded as changes in continental conditions. Notably this change is seen as an expansion in polar waters in both hemispheres. In the Southern Ocean, these changes could have had great implications in driving CO2 variations. Thus, in agreement with other studies (Keeling and Stephens, 2001), a cooling of the surface ocean and expansion of sea ice could have caused a change in the ventilation of the deep ocean, peaked at 0.9Ma, through changes in water column stratification and atmosphere-ocean CO2 exchange. This is a model which is also applied to explain in part the G-IG CO2 variability (Sigman and Boyle, 2000; Stephens and Keeling, 2000). Moreover, we have also noted that these changes in surface ocean conditions and deep water ventilation are coincident with an increase in glacial export productivity, observed in the Southern Ocean, but also in the other records in the Atlantic and Pacific. This relative increase in export productivity after 1.15Ma can be caused by shifts of frontal zones and/or changes in atmospheric circulation which could have modified iron fluxes to the ocean and/or trade wind intensity. We argue that in the transition from the 41kyr to the 100kyr world, changes in surface ocean conditions led those in the marine carbon cycle, and in global ice volume. This conceptual model also suggests that significant changes in the marine carbon cycle did occur, which through modifications in physical and biological processes would have contributed in leading the climate towards cooler mean conditions. Berger, A., Li, X.S., Loutre, M.F., (1999) Modelling northern hemisphere ice volume over the last 3 Ma. Quaternary Science Reviews, 18(1), 1-11. Keeling, R.F., Stephens, B.B., (2001) Antarctic sea ice and the control of Pleistocene climate instability. Paleoceanography, 16(1), 112-131. Paillard, D., (1998) The timing of Pleistocene glaciations from a simple multiple-state climate model. Nature, 391(6665), 378. Raymo, M.E., (1997) The timing of major climate terminations. Paleoceanography, 12(4), 577-585. Sigman, D.M., Boyle, E.A., (2000) Glacial/interglacial variations in atmospheric carbon dioxide. Nature, 407(6806), 859-869. Stephens, B.B., Keeling, R.F., (2000) The influence of Antarctic sea ice on glacial-interglacial CO2 variations. Nature, 404(6774), 171-174.