A rapid transition from ice covered CO2-rich waters to a biologically mediated CO2 sink in the eastern Weddell Gyre

Circumpolar Deep Water (CDW), locally called Warm Deep Water (WDW), enters the Weddell Gyre in the southeast, roughly at 25 degrees E to 30 degrees E. In December 2002 and January 2003 we studied the effect of entrainment of WDW on the fugacity of carbon dioxide (fCO(2)) and dissolved inorganic carbon (DIC) in Weddell Sea surface waters. Ultimately the fCO(2) difference across the sea surface drives airsea fluxes of CO2. Deep CTD sections and surface transects of fCO(2) were made along the Prime Meridian, a northwest-southeast section, and along 17 degrees E to 23 degrees E during cruise ANT XX/2 on FS Polarstern. Upward movement and entrainment of WDW into the winter mixed layer had significantly increased DIC and fCO(2) below the sea ice along 0 degrees W and 17 degrees E to 23 degrees E, notably in the southern Weddell Gyre. Nonetheless, the ice cover largely prevented outgassing of CO2 to the atmosphere. During and upon melting of the ice, biological activity rapidly reduced surface water fCO(2) by up to 100 mu atm, thus creating a sink for atmospheric CO2. Despite the tendency of the surfacing WDW to cause CO2 supersaturation, the Weddell Gyre may well be a CO2 sink on an annual basis due to this effective mechanism involving ice cover and ensuing biological fCO(2) reduction. Dissolution of calcium carbonate (CaCO3) in melting sea ice may play a minor role in this rapid reduction of surface water fCO(2).