Andreas Wisotzki

Winter-summer differences of carbon dioxide and oxygen in the Weddell Sea surface layer

Abstract Mid-winter total inorganic carbon (TCO 2 ) and oxygen measurements are presented for the central fully ice-covered Weddell Sea. Lateral variations of these properties in the surface layer of the central Weddell Sea were small but significant. These variations were caused by vertical transport of Warm Deep Water into the surface layer and air-sea exchange before the ice cover. Oxygen saturation in the surface layer of the central Weddell Sea was near 82%, whereas in the eastern shelf area this was 89%. Surprisingly, p CO 2 , as calculated under the assumption of (reported) conservativeness of alkalinity, was also found to be below saturation (86–93%). This was not expected since ongoing Warm Deep Water entrainment into the surface layer tends to increase the p CO 2 . Rapid cooling and subsequent ice formation during the previous autumn, however, might have brought about a sufficiently low undersaturation of CO 2 , that as to the point of sampling had not yet been replenished through Warm Deep Water entrainment. In the ensuing early summer the measurements were repeated. In the shelf area and the central Weddell Sea, where the ice-cover had almost disappeared, photosynthesis had caused a decrease of p CO 2 and an increase of oxygen compared to the previous winter. In between these two regions there was an area with significant ice-cover where essentially winter conditions prevailed. Based on the summer-winter difference a (late-winter) entrainment rate of 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 summer cruises CO 2 and oxygen had both been absorbed from the atmosphere. The TCO 2 increase due to entrainment of Warm Deep Water was partly countered by (autumn) cooling, and partly through biological drawdown. Part of the CO 2 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 CO 2 . However, whether the Weddell Sea as a whole is a sink for CO 2 depends on the ratio of two counteracting processes i.e. entrainment, which increases CO 2 in the surface and the biological pump, which decreases 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.