Sensitivity of the ocean–atmosphere carbon cycle to ice-covered and ice-free conditions in the Nordic Seas during the Last Glacial Maximum

A global carbon-cycle box model forced by an ocean-general circulation model (OGCM) is used to investigate how different sea-surface reconstructions for the northern North Atlantic Ocean (Nordic Seas) for the Last Glacial Maximum (LGM) affect the ocean–atmosphere carbon cycle via changes in the large-scale thermohaline circulation. For perennial sea-ice cover in the Nordic Seas [CLIMAP, Chart Ser. (Geol. Soc. Am.) MC-36 (1981)], the model-predicted deep-water formation areas, δ13C distribution and 14C ventilation ages are partly inconsistent with palaeoceanographic data for the glacial Atlantic Ocean. Considering ice-free conditions during LGM summer in the Nordic Seas [Weinelt et al., Palaeoclimatology 1 (1996) 283] brings the model results in better agreement with palaeoceanographic findings, thus supporting this particular sea-surface reconstruction. For both LGM scenarios, the ocean circulation model simulates a reduction in the export of North Atlantic Deep Water (NADW) to the Southern Ocean by 50% compared to today. The effect of changes in the intensity of the thermohaline circulation on atmospheric CO2 content alone is rather small, accounting for approximately 20% of the net CO2 lowering during the LGM.