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

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Bakker, D. C. E. , Hoppema, M. , Schröder, M. , Geibert, W. and Baar, H. J. W. d. (2007): Rapid transition from ice covered CO2-rich waters to a biologically mediated sink in the eastern Weddell Gyre , IP CarboOcean annual meeting, 4-7 December 2007, Bremen, Germany. .
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Upwelling and ice cover exert an important control on dissolved inorganic carbon (DIC) and the fugacity of carbon dioxide (fCO2) in Weddell Sea surface waters in early spring. Ultimately these processes drive CO2 air-sea fluxes. Data were collected during cruise ANT XX/2 on RV Polarstern from December 2002 to January 2003. Deep CTD sections were made along 0°W, a northwest-southeast cross-section and along 20°E. Warm Circumpolar Deep Water (CDW, later WDW) enters the Weddell Gyre on the southeastern side of the gyre, roughly at 20-30°E. The upward movement of this water (upwelling) creates a source for CO2 in the Weddell Gyre. The effects of upwelling and entrainment on surface water characteristics were notably large in the southern Weddell Gyre, both at 0°W and 20°E, confirming observations by Gordon and Huber (1990). Entrainment during the winter months had increased the fCO2 difference across the sea surface, dfCO2(w-a), to 20-40 µatm and had preconditioned a CO2 source upon disappearance of the ice cover. Surface water fCO2 was close to the atmospheric value in areas with less upwelling. Once the ice had gone, biological activity locally reduced dfCO2(w-a) to -50 µatm, thus creating CO2 sinks. Despite the tendency of upwelling to cause CO2 oversaturation, the Weddell Gyre may thus still be a CO2 sink on an annual basis. It is probable that the CO2 source originating from upwelling of old, pre-industrial CDW is declining as atmospheric CO2 levels continue to increase (Hoppema, 2004). The relatively small residence time of surface waters in the gyre (2.5 years on average, considerably less in the southern part of the gyre Gordon and Huber, 1990) and prolonged ice cover (up to 8 months per year) partly explain the low anthropogenic CO2 content of bottom waters originating in the Weddell Gyre (Hoppema et al., 2001).

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