SITE 1077

Site 1077 is the intermediate-water drill site on a depth transect in the Lower Congo Basin (Fig. 1). It is located in 2394 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 1077, in connection with Sites 1075 and 1076 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.

Drilling at Site 1077 recovered a continuous hemipelagic sedimentary section spanning the entire Pleistocene (1.77 to 0 Ma). Sediments form one lithostratigraphic unit composed of intercalated, 40- to 150-cm-thick intervals of greenish gray diatom-rich, diatom-bearing, nannofossil-bearing, and nannofossil-rich clay (Fig. 2). The relative abundances of the biogenic components vary greatly with depth. Most of the sediment apparently is strongly bioturbated. Pteropod shells and small shell fragments are present in many intervals. Rare, friable nodules, possibly phosphatic, are interspersed throughout the sediment. The calcium carbonate contents varies between 0.8 and 13.2 wt%. The biogenic portions of sediments contain rare to frequent diatoms, rare nannofossils, silicoflagellates, siliceous sponge spicules, and phytoliths, and traces of radiolarian and foraminifer fragments. Authigenic components are dominated by the presence of glauconite, dolomite, and iron sulfides. X-ray diffraction analysis shows that the clastic fraction is dominated by smectite, kaolinite/illite, quartz, and minor amounts of albitic feldspar.

Detailed comparisons between the magnetic susceptibility record generated on the MST and high resolution color reflectance measured with the Minolta spectrophotometer demonstrated complete recovery of the sedimentary sequence down to 183 mcd, with gaps in the continuous record at 25 and 125 mcd.

Calcareous microfossil abundance and preservation varies between the different groups and deteriorates with depth. Benthic foraminifers are abundant and well preserved down to 120 mbsf, calcareous nannofossils down to 130 mbsf, and planktonic foraminifers down to 150 mbsf. Siliceous microfossils are abundant and well preserved throughout the entire section. Both planktonic and benthic foraminifer assemblages display a major change at 52 mbsf. This change may represent a change in position of water masses at this location.

A magnetostratigraphy was determined after AF demagnetization at 20 mT. The Matuyama/Brunhes boundary occurs at around 120 mbsf, and the termination and onset of the Jaramillo Subchron (C1r.1n) was identified in the lower part of the section at around 130 and 140 mbsf, respectively.

Sediments average 2.3% TOC, which is rather high for ocean margin areas and reflects a history of elevated primary production in this area. Interstitial water chemistry studies document a sequence of diagenetic processes caused largely by the degradation of organic matter and carbonate dissolution/reprecipitation reactions. Among these are moderately high levels of methane and carbon dioxide generated by in situ microbial activity. These postdepositional processes are strongly similar to those found at nearby Sites 1075 and 1076 on the Congo Margin. A high resolution study of interstitial water and headspace methane was conducted over the depth range at which a prominent seismic reflector exists to test whether this reflector is caused by methane hydrate. None of the profiles of salinity, dissolved chloride, or methane are characteristic of hydrate presence. There is no chemical evidence of the presence of methane hydrate in any portion of the sequence recovered from Site 1077.

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.

Hole 1077A was logged with a limited suite of sensors to test the presence of gas hydrate as a potential cause of pronounced changes in seismic reflectivity between 80 and 120 mbsf to provide data for core-log integration and to obtain a continuous record as a proxy for paleoclimatic changes. The tool string included the spectral gamma-ray, long-spacing sonic, phasor dual induction, and Lamont high-resolution temperature (TLT) sondes. The hole was logged at 400 m/hr from 202 to 74 mbsf. The recorded data are of good quality, although density variations were within the resolution of the sensors and the sonic tool measured very low in situ velocities between 1470 and 1510 m/s. No anomalous data indicating hydrate accumulations were found at the reflector depths. Natural gamma-ray values correlate with core measurements and are valuable indicators of coring-induced deformation. The gamma-ray profile is correlated to the changing clay content of the sediment and clearly follows the glacial/interglacial stages of the oxygen-isotope record. Preliminary spectral analysis of the tuned natural gamma data shows a dominance of both the eccentricity and obliquity orbital cycles with a well-identified precessional signal.

On the whole, results from Site 1077 are similar to those from Site 1075, although with somewhat higher sedimentation rates for Site 1077. The shallower water depth at Site 1077 accounts for better preservation of calcareous fossils, especially in the upper portion of the sequence, where even pteropods (aragonite) were found in places. In combination with the high-resolution continuous 2-m.y. record of Site 1075, Site 1077 should greatly contribute to our understanding of the changing conditions of sedimentation in this area. Depositional patterns will reflect climatically driven changes in the supply of riverine materials (Congo), upwelling export (seasonal coastal upwelling), and open-ocean contributions. The Milankovitch-related cyclicity of the changes is evident already at this preliminary stage of analysis from inspection of a number of records.

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