Live benthic foraminifera in surface sediments of the Indo-Pakistan continental margin

The Indo-Pakistan Continental Margin represents an extreme habitat for benthic foraminifera since (1) high fluxes of organic matter offer a high food supply, (2) an intensified oxygen minimum Zone (OMZ) develops from the base of the euphotic Zone to water depths over 1000 m and (3) the monsoon causes seasonal oscillations within the biogeochemical cycle. At three stations from the uppermost (233 m), the central (658 m) and the deeper part (902 m) of the OMZ, living benthic foraminiferal assemblages were analyzed within the uppermost 10 cm of the sediment column. The ecologic structure of foraminiferal faunas is characterized by high abundances at the sediment surface and a rapid decrease within the uppermost 2 cm of the sediment column. Despite dysoxic to suboxic bottom-water conditions, stained benthic foraminifera occurred in all cores down to the base of the sampled interval. High surface abundances, a high dominance by few endobenthic calcareous taxa and a low diversity, which may result from specific physiological adaptations to almost anoxic conditions and the absence of predators, are recognized in the central part of the OMZ. The upper and lower margins of the OMZ are characterized by higher diversities and lower abundances. The shallowest part of the OMZ is dominated by calcareous foraminifera, whereas agglutinated species are the most common taxa in the deeper part. Comparisons with previous studies show that benthic foraminiferal assemblages, that are influenced by seasonal oscillations controlling food supply and/or the availability of oxygen, show variations in faunal density and species composition. Since there is strong evidence that oxygen is not a limiting factor for some taxa, it seems more likely that the distribution pattern of benthic foraminifera is preferentially controlled by trophic conditions.

Species abundance and water and sediment characteristics for three ANDEEP stations on Maud Rise and the Weddell Sea

The benthic fauna was investigated during the expedition ANT-XXIV/2 (2007/08) in relation to oceanographic features, biogeochemical properties and sediment characteristics, as well as the benthic, pelagic and air-breathing fauna. The results document that Maud Rise (MR) differs distinctly from surrounding deep-sea basins investigated during previous Southern Ocean expeditions (ANDEEP 2002, 2005). Considering all taxa, the overall similarity between MR and adjacent stations was low (~20% Bray-Curtis-Similarity), and analyses of single taxa show obvious differences in species composition, abundances and densities. The composition and diversity of bivalves of MR are characterised by extremely high abundances of three species, especially the small sized Vesicomya spp. Exceptionally high gastropod abundance at MR is due to the single species Onoba subantarctica wilkesiana, a small brooder that may prey upon abundant benthic foraminiferas. The abundance and diversity of isopods also show that one family, Haplomunnidae, occurs with a surprisingly high number of individuals at MR while this family was not found at any of the 40 bathyal and abyssal ANDEEP stations. Similarly, polychaetes, especially the tube-dwelling, suspension-feeder fraction, are represented by species not found at the comparison stations. Sponges comprise almost exclusively small specimens in relatively high numbers, especially a few species of Polymastiidae. Water-column sampling from the surface to the seafloor, including observations of top predators, indicate the existence of a prospering pelagic food web. Local concentrations of top predators and zooplankton are associated with a rich ice-edge bloom located over the northern slope of MR. There the sea ice melts, which is probably accelerated by the advection of warm water at intermediate depth. Over the southern slope, high concentrations of Antarctic krill (Euphausia superba) occur under dense sea ice and attract Antarctic Minke Whales (Balaenoptera bonaerensis) and several seabird species. These findings suggest that biological prosperity over MR is related to both oceanographic and sea-ice processes. Downward transport of the organic matter produced in the pelagic realm may be more constant than elsewhere due to low lateral drift over MR.

Quaternary benthic foraminifers from DSDP Sites 95-612 and 95-613

The Quaternary benthic foraminifers from Leg 95 Sites 612 and 613 were examined with respect to paleoceanographic trends. Data from the two sites indicate the presence of markedly different bottom-water masses, during both glacial and interglacial periods. The dominant interglacial species at Site 612 is Uvigerinct peregrina, which is barely present in corresponding intervals at Site 613. Dominant glacial species are Elphidium excavatum and Cassidulina reniforme at Site 612 and Epistominella takayanagii at Site 613.