SITE 1075

Site 1075 is the deep-water drill site on a depth transect in the Lower Congo Basin (Fig. 1). It is located in 2995 m deep water in a complex environment dominated by (1) the freshwater input from the Congo River, (2) seasonal coastal upwelling activity and associated filaments and eddies moving offshore, and (3) incursions of open-ocean waters, especially from the South Equatorial Countercurrent. We expect a close tie-in of climatic records from the continent and the ocean in this area. In the fan-margin deposits, the intercalation of pelagic and terrigenous information provides an excellent opportunity for studying cross-correlations of climatic effects on land and at sea. Site 1075, in connection with the two sites (1076 and 1077) in the Lower Congo Basin, allows us to reconstruct the changing influence of Congo River coastal upwelling and open-ocean contributions to the dynamics of the region.

Sediments at Site 1075 consist of one lithostratigraphic unit composed entirely of greenish gray diatomaceous clay and nannofossil-bearing diatomaceous clay (Fig. 2). The section at Site 1075 is apparently continuous and of Pleistocene to upper Pliocene age (Fig. 3). The sediment is bioturbated and displays a gradual increase in lithification with depth, but no structural or lithologic change can be observed. The sediments have overall low calcium carbonate contents of generally less than 2.5 wt%. Biogenic portions of the sediment contain abundant diatoms with variable amounts of nannofossils, rare silicoflagellates, siliceous sponge spicules, phytoliths, and traces of radiolarian and foraminifer fragments. Sedimentation rates for the recovered sequence average 100 m/m.y.

Detailed comparisons between the magnetic susceptibility record generated on the multisensor track (MST) and the high-resolution color reflectance measured with the Minolta spectrophotometer demonstrated complete recovery of the sedimentary sequence down to 234 meters composite depth (mcd).

Calcareous microfossils are poorly preserved, particularly in the lower section. Siliceous microfossils are relatively unaffected by dissolution and are abundant throughout Hole 1075A. We were able to develop an integrated, high-resolution biostratigraphy for the site that is in agreement with paleomagnetic interpretations. No apparent reworking has been identified. Diatoms are represented by marine and freshwater taxa. Clay minerals show varying contributions of kaolinite. Fluctuations of freshwater diatom and phytolith assemblages and kaolinite reflect changing continental climatic conditions.

A complete magnetostratigraphy was determined at Site 1075 after alternating-field (AF) demagnetization at 20 mT (Fig. 3). All chrons from the Brunhes (C1n) to the onset of C2n (Olduvai) at 1.95 Ma could be identified. Magnetic intensity is low and decreases with depth, although no decreasing trend was observed in the magnetic susceptibility. This suggests that the magnetic minerals that carry the remanent magnetization differ from those that dominate the magnetic susceptibility.

Interstitial water profiles record the complete consumption of sulfate at 30 meters below seafloor (mbsf), commensurate with increases in alkalinity and ammonium, all of which record the degradation of the high levels of sedimentary organic matter. The distribution of dissolved strontium, calcium, and magnesium suggests that the uppermost 50 mbsf is a region of calcite dissolution and dolomite precipitation. A sharp 2%-3% increase in the measured values of dissolved chloride through the upper 20 mbsf appears to reflect a stacked and damped diffusional signal of glacial seawater. We found no chemical evidence of methane hydrate at any depth at Site 1075.

The average concentration of total organic carbon (TOC) is 2.6%, which is rather high for ocean margin areas and reflects a history of elevated primary production in this area. The organic matter appears to be mostly marine in origin. Its microbial degradation in the sediments has fueled a sequence of redox processes. One consequence of the degradation has been the production of moderate amounts of biogenic methane and carbon dioxide and additional dissolution of calcareous sediment components within the sediment.

Physical sediment properties were determined both by high-resolution MST core logging and index properties measurements. Magnetic susceptibility (Fig. 3) and gamma-ray attenuation porosity evaluator (GRAPE) signals reveal pronounced cyclicities. The high-resolution multichannel seismic record, which was acquired during the presite survey, reveals a reflection pattern that seems to be overprinted by in situ chemical or physical conditions not reflected in core log data. Clathrates, dissolved gas, or pore-pressure anomalies are potential explanations. High gas concentrations (CO2 and methane) were found mainly in the interval of higher reflectivity beneath 100 mbsf.

Highlights of Site 1075 results include complete recovery of an apparently continuous Quaternary record, with a chance for extensive reconstruction of the response of the regional system to climatic forcing. First indications are that all major Milankovitch cycles are represented within the record, but with different spectral power depending on the type of record. For example, a strong 100-k.y. signal may be present in magnetic susceptibility in the upper Quaternary sediment, whereas a strong precessional signal appears in the red/green ratio in sediment reflectancy.

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