ALBERT

Description: 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.

Description: Stable isotope measurements on the planktonic foraminifer Globigerinoides ruber (white) have been carried out on a number of selected deep-seas sediment cores from the South Lau and Norlh Fiji Basins. The d18O-curves show good correlation with the inter-ocean oraphic correlation composite d18O-record of the standard reference section (Prell et al. 1986), which, in combination with the chronostratigraphic classifications of Herterich & Sarnthein (1984, modified) and Imbrie et al. 1984), allows a detailed dating of the sedimentary sequences. The deepest layers in core no. 119 (southern Lau Basin) could be assigned to Isotope Stage 24. Measurements made on bulk carbonate in two cores show a much higher glacial-interglacial amplitude, allowing the general identification of the conventional oxygen isotope stages. The d13C-values of the benthic foraminifer Cibicidoides wuellerstorfi show progressively lighter values northwards reflecting an increasing contribution of the isotopically lighter CO2 from the remineralisation of organic matter during the general northward movement of the deep water masses. Cyclicities in the sedimentation rates were observed in core nos. 117 and 119 (both southern Lau Basin) where the interglacials exhibit higher levels than the glacials. Calculated new or export paleoproductivity show that the glacials had higher productivity in the euphotic zone. From the oxygen isotope stratigraphy, the five ash layers in core nos. 117 and 119 could be dated as about 530 ka B.P. in Stage 14, 695 ka B.P. in Stage 18, 775 ka B.P. in Stage 21, 790 ka B.P. and 825 ka B.P. in Stage 22. Carbonate dissolution occurred during stages 5, 8 and 10 to 12.

Description: Oligocene to Quaternary sediments were recovered from the Antarctic continental margin in the eastern Weddell Sea during ODP Leg 113 and Polarstern expedition ANT-VI. Clay mineral composition and grain size distribution patterns are useful for distinguishing sediments that have been transported by ocean currents from those that were ice-rafted. This, in turn, has assisted in providing insights about the changing late Paleogene to Neogene sedimentary environment as the cryosphere developed in Antarctica. During the middle Oligocene, increasing glacial conditions on the continent are indicated by the presence of glauconite sands, that are interpreted to have formed on the shelf and then transported down the continental slope by advancing glaciers or as a result of sea-level lowering. The dominance of illite and a relatively high content of chlorite suggest predominantly physical weathering conditions on the continent. The high content of biogenic opal from the late Miocene to the late Pliocene resulted from increased upwelling processes at the continental margin due to increased wind strength related to global cooling. Partial melting of the ice-sheet occurred during an early Pliocene climate optimum as is shown by an increasing supply of predominantly current-derived sediment with a low mean grain size and peak values of smectite. Primary productivity decreased at ~ 3 Ma due to the development of a permanent sea-ice cover close to the continent. Late Pleistocene sediments are characterized by planktonic foraminifers and biogenic opal, concentrated in distinct horizons reflecting climatic cycles. Isotopic analysis of AT. pachyderma produced a stratigraphy which resulted in a calculated sedimentation rate of 1 cm/k.y. during the Pleistocene. Primary productivity was highest during the last three interglacial maxima and decreased during glacial episodes as a result of increasing sea-ice coverage.

Description: Surface sediment samples taken with a vented box corer from the eastern Weddell Sea on four profiles perpendicular to the continental margin have been investigated for their benthic foraminiferal content. The live fauna was differentiated from empty tests comprising the foraminiferal death assemblage. Based on the dead assemblages, potential fossil assemblages were calculated to facilitate the analogy with late Neogene core material. Five distinct live assemblages inhabit the continental margin today. Six dead assemblages and five potential fossil assemblages, respectively, correspond to these biocoenoses. A predominantly calcareous live fauna dominated byTrifarina angulosa is correlated with strong bottom currents and sandy sediments at the shelf break and on the uppermost continental slope. Below this, on the upper slope down to 2000 m water depth, the predominantly calcareousBulimina aculeata assemblage coincides with the core of warm (〉0°C) Weddell Deep Water and with fine and more organic-rich sediments. These calcareous live assemblages completely change composition during early diagenesis because of calcite dissolution within the uppermost sediment, which depends largely on the grain size distribution of the sediment and the fluxes of organic matter. Therefore, a still calcareousT. angulosa-dominated fossil assemblage indicates the sandy substrates on the shelf break and the upper slope, whereas the deeper slope with hemipelagic calm sedimentation and with high fluxes of organic matter is indicated byMartinottiella nodulosa, the characteristic arenaceous fossil remnant of the former predominantly calcareous liveB. aculeata fauna. On a continental terrace between 2500 and 3500 m water depthCribrostomoides subglobosus dominates the live fauna, but because of rapid disintegration of the empty tests of this agglutinated species a predominantly calcareous fauna characterized byOridorsalis umbonatus andEpistominella exigua comprises the dead assemblage and the potential fossil assemblage, respectively. On the lower continental slope, between the carbonate lysocline (3500 m) and the carbonate compensation depth (4000 m), tests ofNuttallides umbonifer are the characteristic dead and potential fossil remnants of a former predominantly arenaceous live fauna, which is associated with the lower part of the Antarctic Bottom Water (AABW). This corroborates earlier investigations suggesting a relationship between the carbonate-corrosiveness of water masses and the distribution of N. umbonifer. This is important for inferring paleo-routes and estimates of paleo-production rates of AABW during the Neogene.