WINTER-SUMMER DIFFERENCES OF CARBON-DIOXIDE AND OXYGEN IN THE WEDDELLSEA-SURFACE LAYER

Mid-winter total inorganic carbon (TCO2) and oxygen measurements are p resented for the central fully ice-covered Weddell Sea, Lateral variat ions of these properties in the surface layer of the central Weddell S ea were small but significant. These variations were caused by vertica l transport of Warm Deep Water into the surface layer and air-sea exch ange before the ice cover. Oxygen saturation in the surface layer of t he central Weddell Sea was near 82%, whereas in the eastern shelf area this was 89%, Surprisingly, pCO(2), as calculated under the assumptio n of (reported) conservativeness of alkalinity, was also found to be b elow saturation (86-93%). This was not expected since ongoing Warm Dee p Water entrainment into the surface layer tends to increase the pCO(2 ). Rapid cooling and subsequent ice formation during the previous autu mn, however, might have brought about a sufficiently low undersaturati on of CO2, that as to the point of sampling had not yet been replenish ed through Warm Deep Water entrainment. In the ensuing early summer th e measurements were repeated. In the shelf area and the central Weddel l Sea, where the ice-cover had almost disappeared, photosynthesis had caused a decrease of pCO(2) and an increase of oxygen compared to the previous winter, In between these two regions there was an area with s ignificant ice-cover where essentially winter conditions prevailed, Ba sed on the summer-winter difference a (late-winter) entrainment rate o f Warm Deep Water into the surface layer of 4-5 m/month was calculated . A complete surface water balance, including entrainment, biological activity and air-sea exchange, showed that between the winter and summ er cruises CO2 and oxygen had both been absorbed from the atmosphere. The TCO2 increase due to entrainment of Warm Deep Water was partly cou ntered by (autumn) cooling, and partly through biological drawdown. Pa rt of the CO2 removed through biological activity sinks down the water column as organic material and is remineralised at depth. It is well- known that bottom water formation constitutes a sink for atmospheric C O2. However, whether the Weddell Sea as a whole is a sink for CO2 depe nds on the ratio of two counteracting processes, i.e. entrainment, whi ch increases CO2 in the surface and the biological pump, which decreas es it. As deep water is not only entrained into the surface, but also conveyed out of the Weddell Sea, the relative importances of these (CO 2-enriched) deep water transports are important as well.