HOLE 1077C

Position: 5°10.7995'S, 10°26.1687'E

Start hole: 0455 hr, 31 August 1997

End hole: 1206 hr, 31 August 1977

Time on hole: 7.18 hr

Seafloor (drill pipe measurement from rig floor, mbrf): 2396.7

Total depth (drill pipe measurement from rig floor, mbrf): 2419.5

Distance between rig floor and sea level (m): 11.5

Water depth (drill pipe measurement from sea level, m): 2385.2

Penetration (mbsf): 22.8

Coring totals:

Type: APC

Number: 3

Cored: 22.8 m

Recovered: 16.46 m (72.19%)

Lithology:

Unit I: diatom-rich, diatom-bearing, nannofossil-bearing, and nanno-fossil-rich clay

Principal results: Site 1077 is the intermediate-water drill site on a depth transect in the Lower Congo Basin. It is located in 2382-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 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.

Three holes were cored with the advanced hydraulic piston corer (APC) at Site 1077 to a maximum depth of 205.1 meters below seafloor (mbsf), which recovered an apparently continuous hemipelagic sedimentary section spanning the entire Pleistocene (1.77-0 Ma). Drilling at Hole 1077A recovered 22 APC cores to a total depth of 204.5 mbsf, Hole 1077B was cored with the APC to 205.1 mbsf, and three APC cores were taken at Hole 1077C to a total depth of 22.8 mbsf.

Sediments form one lithostratigraphic unit comprised of intercalated, 40- to 150-cm-thick intervals of greenish gray diatom-rich, diatom-bearing, nannofossil-bearing, and nannofossil-rich clay. 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 content varies between 0.8 and 13.2 wt%. The biogenic fraction contains rare to frequent diatoms, rare nannofossils, silicoflagellates, siliceous sponge spicules, phytoliths, and traces of radiolarian and foraminifer fragments. Authigenic components are dominated by the presence of glauconite, dolomite, and iron sulfides. X-ray diffraction (XRD) 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 multisensor track (MST) and high-resolution color reflectance measured with the Minolta spectrophotometer demonstrated com-plete recovery of the sedimentary sequence down to 183 meters composite depth (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 foraminiferal assemblages display a major change at 52 mbsf, which may represent a change in position of water masses at this location.

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

Sediments average 2.3 wt% total organic carbon (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 gamma-ray attenuation porosity evaluator (GRAPE) signals reveal pronounced cyclicities, which 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 for 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 natural gamma spectroscopy tool (NGT), long-spaced sonic (LSS), phasor dual-induction-spherically focused resistivity (DITE-SFL), and Lamont-Doherty high-resolution temperature (LDEO-TLT) sondes. The hole was logged 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. The natural gamma-ray profile correlates with core measurements and is a valuable indicator 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-ray data shows a dominance of both the eccentricity and obliquity orbital cycles with a well-identified precessional signal.

On the whole, results for Site 1077 are similar to those for Site 1075, although sedimentation rates are higher. 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 from inspection of a number of records, even at this preliminary stage of analysis.