SITE 1081

At Site 1081, a relatively continuous hemipelagic sedimentary section spanning the Holocene to upper Miocene (0-9 Ma) was recovered. Sediments at Site 1081 consist of two lithostratigraphic units (Fig. 2). Unit I is composed of olive-gray to black clays that contain varying amounts of diatoms, nannofossils, foraminifers, and radiolarians. Three subunits are defined based on the various abundances and types of microfossils in the sediments: nannofossil- and foraminifer-rich clay (0-77 mbsf), diatom-rich clay (77-230 mbsf), and nannofossil-rich clay (230-390 mbsf). Lithostratigraphic Unit II (390-452 mbsf) is composed of olive-gray clayey nannofossil ooze. The detrital component of the sediments is clay with rare silt-sized, angular and subangular, mono- and polycrystalline quartz grains, feldspar, and mica. Muscovite and biotite also are present in trace amounts. The biogenic component is represented by varying abundances of foraminifers, nannofossils, and diatoms. Varying amounts of particulate organic matter were observed. Authigenic minerals, such as glauconite, rare framboidal pyrite, and dolomite, were observed. Quartz, microcline, and albite, which are probably supplied by the wind, and opal show parallel low-amplitude variations in the Miocene and lower Pliocene sediments. In the upper Pliocene and Pleistocene sediments, the amplitudes are larger, and the above eolian components are partly or completely decoupled from each other. These features suggest that the eolian supply of terrigenous material was dominated mainly by the strength of the trade winds during the late Miocene and early Pliocene. The later variations may be more readily caused by changes in the position of the atmospheric circulation system. Sedimentation rates are fairly constant within the upper Miocene and lower Pliocene sequences (40 m/m.y.). Sedimentation rates within the upper Pliocene sequences are the highest recorded at this site (90-150 m/m.y.) but are reduced at the Pliocene/Pleistocene boundary (70 m/m.y.).

Detailed comparisons between the magnetic susceptibility and the GRAPE density records generated on the MST demonstrated complete recovery of the sedimentary sequence down to 215 mcd.

Fine biostratigraphic resolution was achieved by integrating datums from all microfossil groups. Except for planktonic foraminifers, all microfossil groups show marked coeval fluctuations in abundance that are reflected in the lithology. Planktonic foraminifers are barren, rare, or replaced by pyrite from 105 mbsf to the bottom of the hole. Calcareous nannofossils are abundant and well preserved within the upper (0-77 mbsf) and lower parts of the section (147-452 mbsf). Samples between 77 and 147 mbsf are commonly barren or poor, and the calcareous nannofossils are heavily etched, although some nannofossil-rich sediments are found occasionally within short intervals. The biostratigraphy of the Neogene is poorly constrained because of the scarcity of index species. Diatom abundance increases substantially in the upper Pliocene and lower Pleistocene sediments and reaches a maximum in the upper Pliocene sediment, whereas overall abundance levels remain low in the upper Miocene and lower Pliocene sediments. This pattern resembles that of DSDP Site 532. The diatom assemblage consists of a mixture of upwelling-related and oceanic species. Upwelling-related species dominate the diatom assemblage during highest abundance times in the upper Pliocene sediments. They are not common in lower Pliocene and Miocene sediments where oceanic species tend to dominate. The diatom content in the sediments possibly reflects a varying nutrient supply that could be related to upwelling of nutrient-rich deeper waters and high biological productivity over the Walvis Ridge, especially in the late Pliocene.

A complete magnetostratigraphy was determined in the uppermost 120 mbsf of the APC section at Site 1081 after AF demagnetization at 20 mT. All chrons could be identified from the Brunhes Chron (C1n) to the termination of C2n (Olduvai Chron) at 1.77 Ma.

Well-developed nannofossil ooze/clay cycles, in which concentrations of CaCO3 and organic carbon vary between 1% and 53% and 1.4% to 8.2%, respectively, reflect fluctuations in the elevated marine production associated with the Benguela Current. Higher concentrations of organic carbon from 0 to 200 mbsf record higher productivities during the last 2 m.y. than those recorded earlier in the history of this upwelling system. The interstitial water chemical profiles at this deeply drilled site record a relatively shallow (0 to 80 mbsf) region affected by the diagenetic degradation of organic matter that results in sulfate consumption, as well as increases in alkalinity, ammonium, and phosphate. These changes are accompanied by calcite dissolution and dolomite precipitation, which are recorded by increases in dissolved strontium and decreases in dissolved magnesium and calcium. Additionally, the diatomaceous sequence in lithostratigraphic Subunit IB causes an increase in dissolved silica and a second peak in dissolved phosphate deeper in the sediments.

Physical sediment properties were determined both by high-resolution MST core logging and index properties measurements. Magnetic susceptibility and GRAPE signals reveal pronounced cyclicities that were used for high-quality stratigraphic correlation in conjunction with digital color data.

The geophysical downhole logs show well-identified levels of high velocity, resistivity, and density, attributed mainly to dolomitic layers (Fig. 4). The Formation MicroScanner (FMS) shows dolomitic layers and numerous conductive horizons that might be related to high-porosity assemblages of diatomaceous microfossils. Porosity, density, and natural gamma-radiation logs show high-frequency cyclical patterns that will be used as valuable indicators of paleoclimatic history and will provide an age estimate of the sediment until 2 Ma.

Site 1081, in 790 m deep water on the Walvis Ridge (Fig. 1), is the shallow-water drill site of the Walvis Ridge/Walvis Basin transect. The other anchors are DSDP Sites 532 and 362, in 1300 m water depth, and Sites 1082 and 1083. The DSDP sites are seaward of the upwelling center but contain an upwelling signal that has been transported by the Benguela Current. Site 1081 will give a better record of the upwelling itself. The transect, located above the carbonate compensation depth (CCD) in a passive-margin area with high sedimentation rates, will provide high-resolution records of these important processes and add important new dimensions to the records now available. This transect, situated as it is on the only topographic high over which the Benguela Current passes, is central to the reconstruction of the history of the current.

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