Oceanic carbon and water masses during the Mystery Interval: A model‐data comparison study

Abstract

The ‘Mystery Interval’ (17.5–14.5 ka BP) is characterized by a large decline in atmospheric Δ14C synchronous with an increase in atmospheric CO2. The most widely accepted hypothesis to explain these observed shifts involves the existence of an isolated ‘old’ ocean carbon reservoir that was subsequently ventilated. Here we use the UVic Earth System Climate Model to locate a potential carbon rich and Δ14C depleted water mass under 17.5 ka BP boundary conditions. We then investigate two mechanisms for the potential ventilation of such a reservoir, namely the weakening of the North Atlantic Meridional Overturning due to iceberg calving and latitudinal shifts in Southern Hemisphere Westerlies (SHW) due to southern hemispheric warming. We find that simulations derived from an equilibrium state forced with present‐day SHW and moderate North Atlantic Deep Water (NADW) formation are in better agreement with atmospheric and ocean Δ14C reconstructions than simulations derived from an equilibrium state forced with a northward shifted SHW belt resulting in a shut‐down of the Atlantic Meridional Overturning and formation of North Pacific Deep Water. For simulations with present‐day SHW, the oldest water masses are found in the North Pacific, although the Southern Ocean cannot be ruled out as a potential ‘Mystery Reservoir’. According to our simulations, the strength of Atlantic overturning is the dominant mechanism in increasing the ocean‐atmosphere carbon flux, while shifting SHW results in a rearrangement of deep ocean carbon largely between the Atlantic and Pacific basins. In our ‘best case’ scenario, the model can account for 58% of the atmospheric CO2 increase and 48% of the atmospheric Δ14C decline. While the rate of ventilation and the age of ventilated water masses are comparable with observations, the ventilation in the model could not be sustained long enough to account for the full excursion seen in paleodata.