Organic carbon and opal in deep sea sediments of the southern and eastern Weddell Sea

During the ANT V14 (1986187) and ANT V113 (1987188) cruises of R.V. Polarstern sedirnents from the eastern, southern and central Weddell Sea were sarnpled with a boxcorer andlor a multicorer. The 24 sampling locations are distributed over the whole depth range, from shelf to pelagic environments. Porewater concentrations of aluminium, fluoride, manganese, nitrate, nitrite, oxygen and silicate, the pH and the alkalinity were measured. Of the sediment the opal, calcium carbonate and organic carbon content were quantified. The 210Pb-profile was measured for three sedirnent cores. This investigation deals with the estimation of the amounts of opal and organic carbon (Corg) that are transported into the sediment, the regional distribution of these flux rates and the early diagenetic processes that control the preservation of organic carbon and opal in the sediment. The flux and degradation rates of organic carbon were determined by modelling the rneasured oxygen and nitrate profiles. The highest flux and degradation rates were found in the eastern shelf sediments. Due to the high Corg-flux (>500 mmol C m**-2 a-1) in this area the oxic environment is restricted to the upper 3 cm of the sediment. In contrast to this, the oxic Zone in the pelagic sedirnents of the Weddell Sea has probably an extension of a few meters. The Corg-flux here, computed from the flux of nitrate throug h the sedimentlwater-interface, is less than 50 mmol C m**-2 a**-1. The flux of organic carbon into the sediments of the continental slope area is usually intermediate between the values computed for the shelf and pelagic sediments. Exceptions are the continental slope region north of Halley Bay. In these sediments the measured oxygen and nitrate profiles indicate a relatively high organic carbon flux. This could be a result of the recurrent development of a coastal polynia in this area. The bioturbation rate determined in this region by a 210Pb-profile is 0,019 cm**2 a**-1. In the Weddell Sea the opal content at the sediment surface (0-1 cm depth) varies between 0,1 and 7 %-wt. These opal concentrations are rnuch lower than the opal contents determined for the sediments of the ROSS Sea by Ledford-Hoffmann et al. (1986 doi:10.1016/0016-7037(86)90263-2). Therefore the importance of the Antarctic shelf regions for the global silica cycle as stated by Ledford-Hoffmann et al. (1986) has to be reconsidered. The regional distribution of the opal content and the computed opal flux rates are correlated with the organic carbon flux rates. The processes controlling the preservation of opal are discussed based On the measured aluminium and silicate concentrations in the Pore water and the opal content of the sediment.The depth distribution of the Si- and Al-concentration of the porewater indicates that the reconstitution of clay minerals takes place in the immediate vicinity of the sediment-water nterface. A characterization of these minerals e.g. the estimation of the Si/AI-ratio (Mackin and Aller, 1984 a doi:10.1016/0016-7037(84)90251-5, 1984 b doi:10.1016/0016-7037(84)90252-7) is not possible. With the program WATEQ2 saturation indices are computed to estimate which minerals could reconstitute. In this context the applicability of programs like WATEQ2 for computations of the species distribution and saturation indices in solutions with the ionic strength of sea water is investigated.