Description: Sections from two 'Polarstern' cruises in austral winter 1992 and summer 1992/93 were used to track the course of Circumpolar Deep Water (CDW) in the Weddell Sea. Total inorganic carbon (TCO2) is a valuable tracer for that water mass because it allows to distinguish features that cannot be seen in the distributions of temperature and salinity. Upon entrance into the eastern Weddell Gyre, a shallow maximum in TCO2 at about 200 m (likewise a temperature maximum and oxygen minimum) indicates the depth level to which vertical mixing with Winter Water penetrates the CDW layer in the Weddell Gyre. The lower boundary of this CDW layer, which is not apparent in the temperature and salinity profiles, is a TCO2 maximum at 1000-1500 m (sq27.835), originating through the superposition of the recently advected CDW from the Antarctic Circumpolar Current and the Weddell Sea Deep Water (WSDW) with opposite vertical gradients. A coinciding, weak oxygen minimum is only present on the prime meridian and is probably caused bydifferent biological histories of the CDW and the WSDW underneath. Using this TCO2 maximum, the newly injected CDW can be traced as a well-defined band around the Weddell Sea up to south of the South Orkney plateau. Downstream in the northern limb of the Weddell Gyre at the prime meridian its trace has disappeared.The band of 'new' CDW, as part of the boundary current, envelopes a central area where currents are significantly smaller. In this interior a special modification of CDW, the Central Intermediate Water (CIW), can be distinguished. This water mass is characterised by a secondary TCO2 maximum and oxygen minimum, with no comparable structures in the temperature and salinitiy fields. CIW is enriched in CO2 compared to the CDW that enters the Weddell Gyre and is most pronounced in the western part of the Weddell basin. Data in the west suggest that the CIW is related to the lower part of the 'new'-CDW layer. Thus, the central Weddell basin is replenished from the west side rather than from the east. Within the interior the CDW is further modified by mixing with the WSDW below and by entrainment into the surface layer above. Part of it is also advected out of the Weddell Sea into the bottom layer of the ACC, conveying water that has been biologically enriched in CO2 to the abyssal oceans.

Description: Deep and bottom water from the Enderby basin, which is strongly enriched in silicate, enters the Weddell Sea off Kapp Norvegia parallel to the coast. However, the bottom water in this region originates from the northern Weddell Sea, indicating a southward return flow of bottom water west of the prime meridian. The eastern Weddell Sea margin was identified as the place where a significant silicate enrichment (at least 15 µmol kg-1) and a weak CO2 enrichment of the bottom water takes place, related to a regional recirculation cell. The deep and bottom water continue their course through the Weddell Sea along the base of the continental slope, where further in the west they are underridden by a thin layer of new, silicate-poor bottom water. A silicate maximum and weak TCO2 maximum are formed at the interface between deep and bottom water at approximately 4000 m. This silicate maximum occurs in the central Weddell Sea as well. This indicates an exchange of the deep water between the boundaries and the interior of the Weddell basin; as an important site for this the northwestern Weddell Sea was identified. Bottom layer enrichment by CO2 in the central Weddell Sea (3 µmol kg-1) is comparable to that in the eastern Weddell Sea, but silicate enrichment in the former is much less than in the latter. The extent of bottom layer enrichment suggests that about 2% of the primary produced material reaches the seafloor, supporting the view that the biological pump mechanism in this area is effectively transporting down a significant amount of CO2.

Description: High precision Total CO2 (TCO2) data are presented from the NW Weddell Sea obtained during two cruises which were 3 years apart. A TCO2 increase from 1993 to 1996 was observed in the newly formed bottom water, whereas no TCO2 increase was found in the surrounding water masses. Accompanying this TCO2 increase in the bottom water was an oxygen decrease. Obviously, bottom water with variable characteristics is produced along the margins of the Weddell Sea. Examination of possible causes leads to the conclusion that the bottom water variability is largely due to varying amounts of Warm Deep Water intruding onto the shelves of the Weddell Sea, thus changing the shelf water end-member of bottom water formation. Analysis of the data, using the observed differences of oxygen to perform a correction, is suggesting that some part of the TCO2 increase of the bottom water is due to the increased level of anthropogenic CO2. The TCO2 increase of the bottom water is commensurate to a tentative annual increase of about 1 µmol kg-1 in the surface water source of this bottom water. This would agree fairly well with the increase of the partial pressure of CO2 in the atmosphere.

Description: Carbon cycling in the Weddell Sea was investigated during the ANT X/7 cruise with 'FS Polarstern' Dec. 1992 - Jan. 1993. Samples were taken on a cross section from Kapp Norvegia to Joinville Island, and on a section from the Larsen Ice Shelf to the north-east. The following quantities were measured: total carbon dioxide (TCO2), fluorescence from humic substances and total organic carbon. The distribution of TCO2 was strongly positively correlated to the time elapsed since the various water masses were last ventilated. In general, humic substance fluorescence was positively correlated with TCO2, with the exception of the productive part of the western Weddell Sea, where the correlation was negative in the surface mixed layer. The increased fluorescence at the surface is suggested to be a result of biological production. The distribution of total organic carbon showed less structure, since this quantity includes a particulate component, which is subject to dispersion processes different from those of the dissolved components TCO2 and humic substances. The mean total organic carbon concentration below the surface mixed layer was 50 µmol l1. At some stations, a steep maximum around 2000 m depth was observed. This was interpreted to result from mass sinking of phytoplankton blooms. Total organic carbon had a maximum in surface water and, at some stations also a second subsurface maximum. In the Warm Deep Water, TCO2 and fluorescence had their maximum values, while total organic carbon tended to be low. In low productivity surface water in the eastern part of the Kapp Norvegia - Joinville Island section, the lowest fluorescence was found. Surface water is eventually formed from Warm Deep Water, which had the highest fluorescence values, and therefore it is concluded that humic substances were removed in situ from surface water. In the central area of the Weddell Sea, TCO2 and fluorescence showed their highest Warm Deep Water maxima, while total organic carbon was low. The Warm Deep Water in this area, close to the axis of the Weddell Gyre, is part of the so-called Central Intermediate Water which circulates for a long time within the Gyre. Reduced total organic carbon, which coincides with the most pronounced Central Intermediate Water characteristics, and high TCO2 can thus both be accounted for by continued degradation of organic matter in this water mass. The associated fluorescence maximum implies that humic substances are also produced during mineralisation. Recently formed bottom water, by contrast, could be seen as patches of low TCO2, low fluorescence and high total organic carbon along the western slope of the Weddell Sea.