Olivier Marchal, Thomas Stocker, Fortunat Joos, Andreas Inderühle, Thomas Blunier, Jürg Tschumi, Climate and Environmental Physics, Physics Insitute, University of Bern, Bern, Switzerland
The Younger Dryas (YD, dated between 12.7-11.6 kyr 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 slow-down of the Atlantic thermohaline circulation (THC) and associated northw ard heat flux as triggered by glacial meltwater. Recently, a CH4-based synchronization of GRIP d18O and Byrd CO2 records (West Antarctica) indicated that the concentration of atmospheric CO2, CO2(atm), rose steadily during the YD, suggesting a minor influence of the THC on CO2(atm) at that time. Here we show that the CO2(atm) change in a zonally averaged, circulation-biogeochemical 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 CO2(atm) in this model, because it is spatially limited and compensated by far-field changes such as a warming in the Southern Ocean. The modeled Southern Ocean warming is in agreement with the anti-phase evolution of isotopic temperature records from GRIP, Byrd, and Vostok (East Antarctica) during the YD. d13C 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.