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

D. C. E. Bakker (1) , M. Hoppema (2) , M. Schröder (2) , W. Geibert (2) , H. J. W. de Baar (3)

1 School of Environmental Sciences [Norwich]
2 Department of Bentho-pelagic processes
3 NIOZ - Royal Netherlands Institute for Sea Research

Circumpolar Deep Water (CDW), locally called Warm Deep Water (WDW), enters the Weddell Gyre in the southeast, roughly at 25° E to 30° 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 CO₂ air-sea fluxes. Deep CTD sections and surface transects of fCO₂ were made along the Prime Meridian, a northwest-southeast section, and along 17° E to 23° 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 fCO₂ below the sea ice along 0° W and 17° E to 23° 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 ?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 fCO₂ reduction. Dissolution of calcium carbonate (CaCO₃) in melting sea ice may also play a role in this rapid reduction of surface water fCO₂. The CO₂ source tendency deriving from the upward movement of "pre-industrial" CDW is declining, as atmospheric CO₂ levels continue to increase, and thus the CO₂ sink of the Weddell Gyre will continue to increase as well (provided the upward movement of WDW does not change significantly).

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]; Interfaces continentales, environnement; Océan, Atmosphère; Sciences de la Terre

D. C. E. Bakker, M. Hoppema, M. Schröder, W. Geibert, H. J. W. de Baar. A rapid transition from ice covered CO₂-rich waters to a biologically mediated CO₂ sink in the eastern Weddell Gyre. Biogeosciences Discussions, 2008, 5 (2), pp.1205-1235. ⟨hal-00297986⟩