Distribution of organic carbon in surface sediments covered during SONNE cruise SO184

Sediments were sampled and oxygen profiles of the water column were determined in the Indian Ocean off west and south Indonesia in order to obtain information on the production, transformation, and accumulation of organic matter (OM). The stable carbon isotope composition (d13Corg) in combination with C/N ratios depicts the almost exclusively marine origin of sedimentary organic matter in the entire study area. Maximum concentrations of organic carbon (Corg) and nitrogen (N) of 3.0% and 0.31%, respectively, were observed in the northern Mentawai Basin and in the Savu and Lombok basins. Minimum d15N values of 3.7 per mil were measured in the northern Mentawai Basin, whereas they varied around 5.4 per mil at stations outside this region. Minimum bottom water oxygen concentrations of 1.1 mL L**1, corresponding to an oxygen saturation of 16.1%, indicate reduced ventilation of bottom water in the northern Mentawai Basin. This low bottom water oxygen reduces organic matter decomposition, which is demonstrated by the almost unaltered isotopic composition of nitrogen during early diagenesis. Maximum Corg accumulation rates (CARs) were measured in the Lombok (10.4 g C m**-2 yr**-1) and northern Mentawai basins (5.2 g C m**-2 yr**-1). Upwelling-induced high productivity is responsible for the high CAR off East Java, Lombok, and Savu Basins, while a better OM preservation caused by reduced ventilation contributes to the high CAR observed in the northern Mentawai Basin. The interplay between primary production, remineralisation, and organic carbon burial determines the regional heterogeneity. CAR in the Indian Ocean upwelling region off Indonesia is lower than in the Peru and Chile upwellings, but in the same order of magnitude as in the Arabian Sea, the Benguela, and Gulf of California upwellings, and corresponds to 0.1-7.1% of the global ocean carbon burial. This demonstrates the relevance of the Indian Ocean margin off Indonesia for the global OM burial.

Compilation of scientific results of the OASIS project

Seamounts are of great interest to science, industry and conservation because of their potential role as 'stirring rods' of the oceans, their enhanced productivity, their high local biodiversity, and the growing exploitation of their natural resources. This is accompanied by rising concern about the threats to seamount ecosystems, e.g. through over-fishing and the impact of trawling. OASIS described the functioning characteristics of seamount ecosystems. OASIS' integrated hydrographic, biogeochemical and biological information. Based on two case studies. The scientific results, condensed in conceptual and mass balanced ecosystem models, were applied to outline a model management plan as well as site-specific management plans for the seamounts investigated. OASIS addressed five main objectives: Objective 1: To identify and describe the physical forcing mechanisms effecting seamount systems Objective 2: To assess the origin, quality and dynamics of particulate organic material within the water column and surface sediment at seamounts. Objective 3: To describe aspects of the biodiversity and the ecology of seamount biota, to assess their dynamics and the maintenance of their production. Objective 4: Modelling the trophic ecology of seamount ecosystems. Objective 5: Application of scientific knowledge to practical conservation.

(Table 1) Chemical composition of plagioclase and amphibole-pyroxene containing samples from pyroxene-amphibole-asbestos rock collected in the Hess Deep

Recent sediments with distinct signs of hydrothermal alteration sampled in the Hess Deep(Galapagos Ridge, East Pacific Rise) contained a piece of ash-gray rock, which differed from other rock fragments by degree of consolidation, conchoidal fracture, and had properties of asbestos. Our studies found that the sample represented mixture of asbestos-like pyroxene of diopside-hedenbergite composition, amphibole of tremolite composition and a new mineral, which basic structure consisted of bands of triple pyroxene chains with the radical [Si6O16]. The latter can be regarded as intermediate between amphiboles and layered silicates. Also in some parts of the sample presence of trioctahedral vermiculite-chlorite was indicated. Genesis of the studied asbestos rock is considered from the standpoint of high-temperature hydrothermal-metasomatic alteration of sediment by post-magmatic mineralized halide solutions.

Ferromagnetic fraction of Cretaceous and Cenozoic sediments at DSDP Holes 72-515B and 72-516F

Investigation of the ferromagnetic fraction of sediments from the Brazil Basin and Rio Grande Rise shows that its main constituents are magnetite and hematite. The magnetite is detrital, but the hematite is both detrital and chemical in origin. Magnetite is the main carrier of the natural remanent magnetization (NRM); therefore, the NRM is detrital remanent magnetization (DRM). In a number of cases, the change of magnetic parameters along the stratigraphic column permits some refinement of the previously defined boundaries of the lithologic units.

Chemical composition of bottom sediments from the Discovery Deep, Red Sea rift zone

Hot brines in depressions of the central Red Sea contain thousands of times more iron, manganese and other metals than . After removal of salts, approximately half of sediments from these depressions consists of iron hydroxides and they are enriched in zinc, copper, lead and molybdenum. Hydrothermal deposits with the same complex of metals, located along the coast of the Red Sea, are correlated with faults and may be due to occurrences of Tertiary volcanism. Brines of similar composition are known in the Cheleken Peninsula. Certain geological and geochemical data indicate that such brines are of relatively deep origin.

Chemical element contents and accumulation rates in metalliferous sediments from DSDP Hole 34-319, Bauer Deep, Southeast Pacific

Marked variations in the chemical and mineralogical composition of sediments at Site 319 have occurred during the 15 My history of sedimentation at this site. The change in composition through time parallels the variability observed in surface sediments from various parts of the Nazca Plate and can be related to variations in the proportion of hydrothermal, hydrogenous, detrital and biogenous phases reaching this site at different times. Metal accumulation rates at Site 319 reach a maximum near the basement for most elements, suggesting a strong hydrothermal contribution during the early history of this site. The hydrothermal contribution decreased rapidly as Site 319 moved away from the spreading center, although a subtle increase in this source is detectable about the time spreading began on the East Pacific Rise. The most recent sedimentation exhibits a strong detritalhydrogenous influence. Post-depositional diagenesis of amorphous phases has converted them to ironrich smectite and well-crystallized goethite without significantly altering the bulk composition of the sediment.

Barium concentration in sediments of the central Equatorial Pacific

High resolution pore-water dissolved Ba concentration-depth profiles were determined at seven sites across an Equatorial Pacific productivity gradient from 12°S to 9°N, at 140°W. These data are important for understanding the physical, chemical, and biological controls on Ba recycling in the ocean, and for evaluating the paleo-oceanographic significance of Ba content in central Equatorial Pacific sediments. Pore-water Ba concentrations at all sites are higher than in the overlying bottom water, leading to a diffusive flux of Ba into the ocean. A pronounced subsurface concentration maximum exceeding barite solubility characterizes the dissolved Ba pore-water profiles, suggesting that the Ba regenerated in the upper few millimeters of sediment is not controlled by barite solubility. A few centimeters down-core Ba concentrations reach a relatively constant value of approximately barite saturation. The benthic Ba flux shows a clear zonal trend, with a maximum between 2°S and 2°N, most probably due to higher productivity at the equatorial divergence zone, and with lowest values at the southern and northern extremes of the transect. The dissolved Ba flux between 2°S and 2°N is ~30 nmol/cm**2 yr and drops to 6 nmol/cm**2 yr at 12°S. Even the lowest fluxes are significantly higher than those previously reported for the open ocean. In the Equatorial Pacific the calculated Ba recycling efficiency is about 70%. Thus, ~30% of the particulate Ba flux to the deep ocean is preserved in the sediments, compared with less than 1% for organic carbon and ~5% for biogenic silica. Mass balance calculation of the oceanic Ba cycle, using a two-box model, implies benthic Ba fluxes similar to those reported here for a steady-state ocean.

Deep thermohaline Circulation in the low-latitude Atlantic during the Last Glacial

Present-day low-latitude eastern and western Atlantic basins are geochemically distinct below the sill depth of the Mid-Atlantic Ridge. While Antarctic Bottom Water (AABW) circulates freely in the western Atlantic, flow into the eastern Atlantic is restricted below 4 km which results in filling the abyssal depths of this basin with water of geochemical similarity to nutrient depleted North Atlantic Deep Water. Using carbon isotopes and Cd/Ca ratios in benthic foraminifera we reconstruct the geochemistry of these basins during the last glacial maximum. Results indicate that deep eastern and western Atlantic basins became geochemically identical during the last glacial. This was achieved by shoaling of the upper surface of AABW above the sill depth of the Mid-Atlantic Ridge, which allowed bottom waters in both basins to be filled with the same water mass. Although AABW became the dominant water mass in the deep eastern Atlantic basin during the glacial, Holocene-glacial delta13C-PO4 shifts in this basin are in Redfield proportions, unlike the disproportionate Holocene-glacial delta13C-PO4 shifts observed in the Southern Ocean. By examining the composition of deep and intermediate waters throughout the Atlantic, we show that this effect was induced by a change in gradient of the delta13C-PO4 deepwater mixing line during glacial times. Evidence from high-latitude planktonic data suggests that the change in gradient of the deepwater mixing line was brought about through a significant reduction in the thermodynamic effect on Southern Ocean surface waters. By using coupled delta13C-PO4 data to constrain the composition of end member water masses in the glacial Atlantic, we conclude that deep waters in the low-latitude glacial Atlantic were composed of a mixture of northern and southern source waters in a ratio of 1:3.