Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 5 5 2008 10.5194/bg-5-1373-2008 http://www.biogeosciences.net/5/1373/2008/ http://www.biogeosciences.net/5/1373/2008/bg-5-1373-2008.html http://www.biogeosciences.net/5/1373/2008/bg-5-1373-2008.pdf 1373 1386 2008-09-30 A rapid transition from ice covered CO₂–rich waters to a biologically mediated CO₂ sink in the eastern Weddell Gyre D. C. E. Bakker d.bakker@uea.ac.uk M. Hoppema M. Schröder W. Geibert H. J. W. de Baar School of Environmental Sciences, University of East Anglia, Norwich, UK Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany Royal Netherlands Institute for Sea Research, Texel, The Netherlands now at: Earth Science, School of Geosciences, University of Edinburgh, Edinburgh, UK Circumpolar Deep Water (CDW), locally called Warm Deep Water (WDW), enters the Weddell Gyre in the southeast, roughly at 25&deg; E to 30&deg; E. In December 2002 and January 2003 we studied the effect of entrainment of WDW on the fugacity of carbon dioxide (fCO₂) and dissolved inorganic carbon (DIC) in Weddell Sea surface waters. Ultimately the fCO₂ difference across the sea surface drives air-sea fluxes of CO₂. Deep CTD sections and surface transects of fCO₂ were made along the Prime Meridian, a northwest-southeast section, and along 17&deg; E to 23&deg; E during cruise ANT XX/2 on FS <i>Polarstern</i>. Upward movement and entrainment of WDW into the winter mixed layer had significantly increased DIC and fCO₂ below the sea ice along 0&deg; W and 17&deg; E to 23&deg; 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 fCO₂ by up to 100 μatm, thus creating a sink for atmospheric CO₂. 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