Description: Using contemporary CO2 data from the subsurface Weddell Sea, the source/sink function of this region against the changing atmospheric CO2 level has been investigated. As in the central Weddell Sea, surface water is supplied by upwelling of subsurface water, the CO2 content is also forced by it. TCO2 data of four cruises were used to determine a robust value for the subsurface Warm Deep Water (WDW). After accounting for biological activity in the surface layer and salinity differences between the subsurface and surface waters, the forcing CO2 partial pressure (pCO2) was calculated from the TCO2 of the WDW and the conservative alkalinity as taken from the literature. As the WDW contains negligible anthropogenic CO2, the pCO2 forcing by the WDW has been prevalent both in the pre-industrial and modern Weddell Sea. The calculated pCO2 forcing amounts to 300–310 μatm at a minimum in late winter/early spring and possibly 30 μatm more during spring and summer. This figure does not represent the actual pCO2, but rather the value before air–sea exchange gets effective. Hence, in pre-industrial times when the atmospheric pCO2 was about 280 μatm, the Weddell Sea must have been a relatively strong source of atmospheric CO2. Because of the steadily rising atmospheric CO2 levels to more than the pCO2 forcing by the WDW, the Weddell Sea turned into a CO2 sink in recent times. The storage of anthropogenic CO2 in the Weddell Sea surface layer is estimated to be 4.1 mol C m−2. Applying the WDW forcing method to O2, a steady state O2 uptake from the atmosphere of 3.6 mol O2 m−2 year−1 is computed.

Description: Upon one decade of research it is a well established fact that iron limits photosynthetic CO2 fixation and phytoplankton growth in the Southern Ocean - intense blooms are scarse. However, the input of iron to the Southern Ocean is considerable. An important factor for diminished phytoplankton production refers to the meridional circulation of the Southern Ocean. Intense upwelling of Upper Circumpolar Deep Water (UCDW) causes a large iron flux into the surface layer. However, the main entrainment of upwelled UCDW into the surface layer occurs in autumn and winter which strongly restricts the usefullness of iron supply for phytoplankton due to unfavorable light conditions. Moreover, the meridional transport within the Ekman layer is intense enough to export at least 25% of the iron input away form the Antarctic Zone before it can be used by phytoplankton. This also depresses the potential phytoplankton primary production by at least 25%. Most iron that crosses the Polar Front unused leaves the surface ocean north of the Polar Front because the surface water participates in Antarctic Intermediate Water/Mode Water formation.

Description: A simple model, using concentrations of nitrate and phosphate in austral winter 1992, reveals that the Antarctic Surface Water (AASW) of the southernmost Antarctic Circumpolar Current (ACC) between the Southern ACC Front and the Weddell Front is made up of about 90% Upper Circumpolar Deep Water (UCDW) and 10% northward-flowing AASW from the Weddell Gyre. With a typical time scale of about 1 year, the upwelling velocity was calculated to be as high as 60-100 m y-1. Knowing the composition of the surface water with respect to its sources, changes due to several processes in the surface layer were deduced for carbon dioxide, oxygen and silicate. As the time scale of changes in the surface layer of the southern ACC is about 1 year, this allows us to calculate changes on an annual basis without interference of short-term variations. Balancing the contributions by upwelling, biological activity and air-sea exchange to the concentrations in the surface layer, the area was found to be a large sink for atmospheric oxygen of 6.0 mol m-2 y-1 (53 µmol kg-1) and a small sink for atmospheric carbon dioxide of 1.0 mol m-2 y-1 (9 µmol kg-1). The most important cause for the oxygen sink is the upwelling of oxygen-poor UCDW, which surpasses the oxygen-elevating effect of primary productivity. This large oxygen sink, in between areas to the north and south which are only a small sink or even a source, conforms with the latitudinal distribution of atmospheric oxygen. The small CO2 sink is largely brought about by biological activity. The annual carbon utilization amounts to 76 ± 22 g C m-2 y-1, which is relatively high for an open ocean region in the Antarctic. However, it supports recent estimates of primary production of the Antarctic Ocean that are higher than early published values. The annual silicate consumption was calculated to be 126 ± 19 g Si m-2 y-1. This is considerably higher than the Southern Ocean mean in current estimates. Although the southernmost ACC may be atypical for the Southern Ocean, the current estimate for Southern Ocean silica production may well be an underestimation. The silicate to carbon utilization ratio derived here is 0.53 which aligns with investigations on Antarctic phytoplankton and thus underscores the consistency of our results.

Description: Using Weddell Sea data collected during a cruise with "FS Polarstern" in austral summer 1992/1993, depletions of nutrients and TCO2 in the summer surface layer were calculated. Also the analogous depletion-like properties for temperature (Heat Storage) and salinity were computed. The latter properties are useful to describe the physical conditions over the time period pertinent to the depletions. For different areas a strong correlation exists of the Heat Storage and the nutrient/TCO2 depletions, which is caused by a common factor, the period of light availability. Offshore of the Larsen shelf, an area usually inaccessible due to perennial ice cover, high nutrients/TCO2 depletions are achieved in a short period of time, pointing to a rapidly producing biological system. Primary productivity, calculated from the TCO2 depletion, amounts to about 100 mg C m-2 d-1 for the central Weddell Sea, but 570-1140 mg C m-2 d-1 for the offshore Larsen region. These values agree fairly well with the open ocean Antarctic and other coastal areas, respectively.

Description: The region influenced by the Polar Front in the Southern Ocean is characterized by relatively high productivity, which is mirrored instrong depletions of 234Th in the surface water, a good tracer of export production, and by high accumulation rates on the underlyingseabed. Farther south, the Weddell Sea is generally considered a low productivity region with very low export fluxes. This finding is basedon satellite observations, sediment accumulation rates, trap deployments, and phytoplankton distribution. If this would be true, 234Thshould be close to equilibrium with its parent. However, in a series of high-resolution transects of 234Th/238U across the AntarcticCircumpolar Current (ACC), 234Th was found to be depleted by 10-15% throughout the clear Weddell Gyre, only to reach equilibrium insea-ice covered regions of the coastal zone. Vertical profiles showed that the depletion was limited to the upper mixed layer and wasbalanced by an enrichment of similar magnitude at 100-250m depth. This implies that the export of particles below 250 m is negligible.Such shallow remineralization is in line with the discrepancies between biogenic silica production rates and sediment trap data observedin the Weddell and Ross Seas. These observations in the Weddell Sea are fully consistent with our inverse modeling results for bothorganic carbon and opal, and they are not inconsistent with TCO2 and oxygen sections that show a TCO2 enriched, oxygen reducedshallow subsurface layer. This blue ocean, characterized by upwelling of CO2-enriched deep waters, supports sufficient productivity tobe a net sink for CO2 to abyssal depths [Hoppema et al., 1999]. No record of this productivity and export is stored in the underlyingsediment, which has important palaeoceanographic consequences.