PRINCIPAL RESULTS

Site 1075 is the deep-water drill site on a depth transect in the Lower Congo Basin. It is located in 2995-m deep water in a complex environment dominated by (1) freshwater input from the Congo River, (2) seasonal coastal upwelling 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 upcoming two sites (1076 and 1077) in the Lower Congo Basin, will allow us to reconstruct the changing influence of Congo River, coastal upwelling, and open-ocean contributions to the dynamics of the region.

Three holes were cored with the advanced hydraulic piston corer (APC) at Site 1075 to a maximum depth of 207.2 meters below seafloor (mbsf), which recovered an apparently continuous interval of Pleistocene to upper Pliocene age. Hole 1075A was cored with the APC to 201.5 mbsf. Twenty-two cores were taken at Hole 1075B with the APC to a depth of 204.5 mbsf, and Hole 1075C was cored to 207.2 mbsf. Sediments from Site 1075 consist of one lithostratigraphic unit composed entirely of greenish gray diatomaceous clay and nannofossil-bearing diatomaceous clay. The sediment is bioturbated and shows a gradual increase in lithification with depth but no structural or lithologic change. The sediments have overall low calcium carbonate contents of generally <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 foraminiferal 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 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. An integrated, high-resolution biostratigraphy was developed 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. 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 mbsf, which is commensurate with increases in alkalinity and ammonium, all of which record the degradation of the abundant 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 hydrates at any depth at Site 1075.

 

The average concentration of total organic carbon (TOC) is 2.6 wt%, 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 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 caused by factors other than those recorded 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 the 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, whereas a strong precessional signal appears in the red/blue ratio in sediment reflectance.