RAPID CHANGES IN OCEAN CIRCULATION AND ATMOSPHERIC RADIOCARBON

A latitude-depth, coupled global ocean-ice-atmosphere model is extende d to include a simple biosphere component. A physically reasonable adj ustment of runoff into the North Atlantic is invoked to achieve a tran sient response to glacial meltwater perturbations, which closely resem bles the Younger Dryas climate event. We then investigate the evolutio n 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 up take of radiocarbon is reduced, resulting in an increase of atmospheri c Delta(14)C by about 35 parts per thousand. The reduction of ventilat ion 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 v entilation 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 D ryas (longer than 400 years). A sensitivity study reveals that the dur ation of the plateau depends strongly on the transient evolution of th e gas exchange rate and can increase to 150-300 years if changes of pC O(2) or sea ice coverage are taken into account.