On the relation between organic and inorganic carbon in the Weddell Sea

Carbon cycling in the Weddell Sea was investigated during the ANT X/7 cruis e with 'FS Polarstern' December 1992-January 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 northeast. The following quantities were measu red: total carbon dioxide (TCO2), fluorescence from humic substances and to tal organic carbon. The distribution of TCO2 was strongly positively correl ated to the time elapsed since the various water masses were last ventilate d. 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 increa sed fluorescence at the surface is suggested to be a result of biological p roduction. The distribution of total organic carbon showed less structure, since this quantity includes a particulate component, which is subject to d ispersion processes different from those of the dissolved components TCO2 a nd humic substances. The mean total organic carbon concentration below the surface mixed layer was 50 mu mol l(-1). At some stations, a steep TOC maxi mum around 2000 m depth was observed. This was interpreted to result from m ass sinking of phytoplankton blooms. Total organic carbon had a maximum in surface water, and at some stations also a second subsurface maximum. In th e Warm Deep Water (WDW), TCO2 and fluorescence had their maximum values, wh ile total organic carbon tended to be low. In low productivity surface wate r in the eastern part of the Kapp Norvegia-Joinville Island section, the lo west flourescence was found. Surface water is eventually formed from Warm D eep Water, which had the highest fluorescence values, and therefore it is c oncluded that humic substances were removed in situ from surface water. In the central area of the Weddell Sea, TCO2 and fluorescence showed the highe st Warm Deep Water maxima, while total organic carbon was low. The Warm Dee p Water in this area is part of the so-called Central Intermediate Water wh ich circulates for a long time within the Weddell Gyre. Reduced total organ ic carbon, which coincides with the most pronounced Central Intermediate Wa ter characteristics, and high TCO2 can thus both be accounted for by contin ued degradation of organic matter in this water mass. The associated fluore scence maximum implies that humic substances are also produced during miner alisation. Recently formed bottom water, by contrast, could be seen as patc hes of low TCO2, low fluorescence and high total organic carbon along the w estern slope of the Weddell Sea.