Rapid changes in ocean circulation and atmospheric radiocarbon

A latitude-depth, coupled global ocean-ice-atmosphere model is extended to include a simple biosphere component. A physically reasonable adjustment of runoff into the North Atlantic is invoked to achieve a transient response to glacial meltwater perturbations, which closely resembles the Younger Dryas climate event. We then investigate the evolution of the isotopic ratio of atmospheric radiocarbon, Delta(14)C, due to the rapid changes of deep ocean circulation. When the North Atlantic branch of the conveyor belt circulation is interrupted, the oceanic uptake of radiocarbon is reduced, resulting in an increase of atmospheric Delta(14)C by about 35 parts per thousand. The reduction of ventilation in the North Atlantic is partly compensated by an increase of the C-14 ratios of the biosphere, the Southern Ocean, and the upper ocean above 1000 m depth. A plateau of the C-14 year/calendar year relation can be generated at the time of the rapid reinitiation of deep ocean ventilation which begins coincident with the major temperature increase and lasts for about 60 years. It is hence significantly shorter than that found by analyzing tree rings during the termination of Younger Dryas (longer than 400 years). A sensitivity study reveals that the duration of the plateau depends strongly on the transient evolution of the gas exchange rate and can increase to 150-300 years if changes of pCO(2) or sea ice coverage are taken into account.