Modelling the concentration of atmospheric CO2 during the Younger Dryas climate event

The Younger Dryas (YD, dated between 12.7-11.6 ky BP in the GRIP ice core, Central Greenland) is a distinct cold period in the North Atlantic region during the last deglaciation. A popular, but controversial hypothesis to explain the cooling is a reduction of the Atlantic thermohaline circulation (THC) and associated northward heat flux as triggered by glacial meltwater. Recently, a CH4-based synchronization of GRIP delta(18)O and Byrd CO2 records (West Antarctica) indicated that the concentration of atmospheric CO2 (CO2atm) rose steadily during the YD, suggesting a minor influence of the THC on CO2atm at that time. Here we show that the CO2atm change in a zonally averaged, circulation-biogeochemistry ocean model when THC is collapsed by freshwater flux anomaly is consistent with the Byrd record. Cooling in the North Atlantic has a small effect on CO2atm in this model, because it is spatially limited and compensated by far-field changes such as a warming in the Southern Ocean. The modelled Southern Ocean warming is in agreement with the anti-phase evolution of isotopic temperature records from GRIP (Northern Hemisphere) and from Byrd and Vostok (East Antarctica) during the YD. delta(13)C depletion and PO4 enrichment are predicted at depth in the North Atlantic, but not in the Southern Ocean. This could explain a part of the controversy about the intensity of the THC during the YD. Potential weaknesses in our interpretation of the Byrd CO2 record in terms of THC changes are discussed.