Age determination of sediment cores from the Shelf Edge off Mauritania, Northwest Africa

The impact of late glacial changes on the sedimentary record was investigated in two long vibracores, collected from the shelf edge off Mauritania, northwest Africa. Lithology and radiocarbon dates indicate that the sedimentary sequences were mainly controlled by sea-level changes on the shelf. The upper Pleistocene sequence is characterized by deposition in coastal environments, while the Holocene sequence represents deposition in shelf environments. During low sea level, much sediment was supplied to the present outer shelf, and the data imply an average accumulation rate of up to 43.0 cm/1000 yrs during the late Pleistocene, which is substantially higher than the Holocene rate. Shelf sediments were continuously reworked and redistributed on a regional scale during falling and rising sea level. The presence of reworked material results in radiocarbon ages which are too old. The mollusc. Venus striatula, which presently is found north of, but not along, the Mauritanian coast, occurs in the upper Pleistocene sequence, suggesting cooler water conditions in the shelf during late glacial times. This species probably migrated to the south during late glacial times, following the southward extension of the cold Canary Current. Radiocarbon dates of the shells broadly coincide with a lowstand of sea level over this part of the continental shelf.

Radiocarbon ages of bottom sediments and sedimentation rates in depressions of the Red Sea rift zone

Sedimentation rates (SR) for metalliferous and ore sediments containing ore material in 15 depressions of the Red Sea rift zone are discussed. SR for normal sediments was ca. 3.2 cm/ka in the second half of Holocene, 14.3 cm/ka yrs in the first half of Holocene, and 21.3 cm/ka in Late Würmian. Accumulation of metalliferous and ore sediments requires considerable accumulation of hydrothermal matter. Ore sediments have been found primarily in the Atlantis II and Chain Deeps; average sedimentation rate in these depressions is 90.0 cm/ka. In other depressions geothermal activity during considered time intervals was lower, and ore material occurs as admixture or in layers.

Carbonate accumulation rates of the Ninetyeast Ridge, Indian Ocean

More than 95% of the carbon lost from the "blue-ocean" reservoir to the sedimentary sink appears to be transferred as skeletal CaCO3, produced in the surface waters. This skeletal CaCO3 carries a productivity signal which is much better preserved in the underlying pelagic carbonate sediments than that of the refractory organic carbon accompanying it. Here, we develop a new method to quantify this signal in terms of organic carbon paleoproductivity, using the sedimentary mass accumulation rates of pelagic carbonate. These are converted into carbonate transit-paleofluxes, which are then translated into the corresponding transit-fluxes of organic carbon, via the carbonate to organic carbon ratios reported from deep-moored sediment trap experiments in modern blue-ocean environments. Paleoproductivity can then be estimated quantitatively by using published algorithms describing the relationship between the export production of particulate organic carbon at depth and primary productivity in the euphotic zone. Although our approach seems rather straightforward, it contains several pitfalls, the effects of which are highlighted by an example comprising three Paleocene/Oligocene to Recent pelagic carbonate sequences drilled during ODP Leg 121 in the eastern Indian Ocean. Although some extreme values are likely due to errors, such as poorly constrained datum levels and dissolution peaks, the results for the Quaternary and Neogene correlate well from site to site and are within the productivity range of present-day low to medium latitude open oceans. Our method may provide an opportunity to actually quantify blue-ocean primary productivity in sedimentary carbonate environments, but requires validation by other, more established ones.

Sedimentary records of ODP Site 198-1209 on Shatsky Rise

The accumulation of wind blown (eolian) dust in deep-sea sediments reflects the aridity/humidity conditions of the continental region supplying the dust, as well as the "gustiness" of the climate system. Detailed studies of Pleistocene glacial-interglacial dust fluxes suggest changes in accumulation rates corresponding to orbital variations in solar insolation (Milankovitch cycles). While the orbital cycles found in sedimentary archives of the Pleistocene are intricately related to glacial growth and decay, similar global orbital signals recognized in deep-sea sediments of early Paleogene age, the last major greenhouse interval ~65-45 million years ago, could not have been linked to the waxing and waning of large ice sheets. Thus orbital signals recorded in early Paleogene sediments must reflect some other climate response to changes in solar insolation. To explore the potential connection between orbital forcing and the climate processes that control dust accumulation, we generated a high-resolution dust record for ~58 Myr old sediments from Shatsky Rise (ODP Site 1209, paleolatitude ~15°N-20°N). The dust accumulation data provide the first evidence of a correlation between dust flux to the deep sea and orbital cyclicity during the early Paleogene, indicating dust supply responded to insolation forcing during the last major interval of greenhouse climate. Furthermore, the relative amplitude of the dust flux response during the early Paleogene greenhouse was comparable to that during icehouse climates. Thus, subtle variations in solar insolation driven by changes in Earth's orbit about the Sun may have had a similar impact on climate during intervals of overall warmth as they did during glacial-interglacial states.

Rare earth element composition and mass accumulation rates at DSDP Hole 92-598

Site 598 sediments were analyzed to determine the factors controlling the rare earth element (REE) geochemistry of the hydrothermal component. Site 598 provides an ideal sample suite for this purpose. Samples are lithologically "simple," primarily consisting of a hydrothermal component and biogenous carbonates. Also, the composition of the hydrothermal component appears unchanged through time or space, and the site appears to have undergone minimal diagenetic alteration. The shale-normalized REE patterns are similar to the pattern of seawater, varying only in absolute REE content. The REE content increases with distance from the paleorise crest and exhibits a pronounced increase in sediments deposited below the paleolysocline. Results presented are consistent with the following model: the source mechanism for the REE content of hydrothermal sediments is scavenging by Fe oxyhydroxides from seawater. With prolonged exposure to seawater resulting from transport far from the injection point and/or long residence at the seawatersediment interface, the absolute REE content of hydrothermal sediments increases and becomes more like seawater.