HOLE 1076D

Position: 5°4.1312'S, 11°6.1150'E

Start hole: 1155 hr, 28 August 1997

End hole: 0215 hr, 29 August 1997

Time on hole: 14.33 hr

Seafloor (drill pipe measurement from rig floor, mbrf): 1412.5

Total depth (drill pipe measurement from rig floor, mbrf): 1526

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

Water depth (drill pipe measurement from sea level, m): 1401

Penetration (mbsf): 113.5

Coring totals:

Type: APC

Number: 12

Cored: 113.5 m

Recovered: 118.41 m (104.33%)

Lithology:

Unit I: organic carbon-rich olive-gray and greenish gray clay

Principal results: Site 1076 is the shallow-water drill site on a depth transect in the Lower Congo Basin. It is located in 1402 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 1076, in connection with Sites 1075 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.

Four holes were cored with the advanced hydraulic piston corer (APC) at Site 1076 to a maximum depth of 204.3 meters below seafloor (mbsf), which recovered an apparently continuous hemipelagic sedimentary section spanning the last 1.5-1.6 m.y of the Pleistocene. Hole 1076A was cored with the APC to 204.3 mbsf. Seven APC cores were taken at Hole 1076B before the hole was abandoned because of lack of recovery. Hole 1076C was cored with the APC to 203.1 mbsf, and Hole 1076D to 113.5 mbsf.

The sediments form one lithostratigraphic unit composed of biotur-bated organic carbon-rich olive-gray clay and greenish gray clay. Small shell fragments are present in many intervals. Above 150 mbsf, the calcium carbonate concentration alternates between 3 and 16 wt% and is limited below 150 mbsf to a maximum of 3.5 wt%. The biogenic portion of the sediment contains rare to abundant diatoms with rare nannofossils, silicoflagellates, siliceous sponge spicules, phytoliths, and traces of radiolarian and foraminifer fragments. Diatoms are abundant in both greenish gray and olive-gray intervals. Authigenic components are dominated by the presence of glauconite, dolomite, and iron sulfides. Rare, friable nodules, possibly phosphatic, are sometimes disseminated throughout certain intervals. Sedimentation rates vary between 250 m/m.y. in the uppermost 80 mbsf, 50 m/m.y. between 80 and 120 mbsf, and 210 m/m.y. between 120 and 200 mbsf.

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 140 meters composite depth (mcd).

Calcareous microfossils show evidence of reworking. Their abundance and preservation deteriorates gradually between 100 and 200 mbsf. Siliceous microfossils are relatively abundant, well preserved, and show no evidence of reworking. The calcareous nannofossil-based biostratigraphy disagrees with the paleomagnetic time frame for the lower half-interval of Hole 1076A. We tentatively explain this discrepancy as a result of both poor preservation and reworking of calcareous nannofossils. Both calcareous nannofossil and benthic foraminiferal assemblages suggest a discontinuity within the sedimentary record at a depth of 120 mbsf. Downcore changes in planktonic foraminiferal and diatom assemblages are used as indices of variable surface and subsurface hydrography, as well as proxies for coastal upwelling and fluvial input.

A magnetostratigraphy was determined after alternating-field (AF) demagnetization at 20 mT. The Matuyama/Brunhes boundary occurs at ~138 mbsf, and the onset and termination of the Jaramillo Subchron (C1r.1n) was identified in the lower part of the section. A short reversal event in the Brunhes Chron (possibly the Blake event) occurs at all four holes.

Interstitial water profiles record the complete consumption of dissolved sulfate within the uppermost 20 mbsf. In this interval, alkalinity and ammonium also increase sharply, recording the degradation of organic matter. The distributions of dissolved strontium, calcium, and magnesium suggest two depth domains of carbonate dissolution and reprecipitation reactions: the first from 0 to 50 mbsf and the deeper from 120 to 200 mbsf.

The average concentration of total organic carbon (TOC) is 2 to 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, which were used for high-quality stratigraphic correlation in conjunction with digital color data.

Through its position within the domain of Congo River sedimentation and its high-resolution continuous record back through much of the Quaternary, Site 1076 will provide the basis for a tie-in of climatic records of west Africa, Congo River activity, coastal upwelling, and eastern tropical ocean dynamics. Of special interest are the competing source effects for land-derived materials, with some (most?) being brought directly by the river and the rest originating from reworked shelf sediments, especially during low sea-level periods.