HOLE 1078D

Coring totals:

Type: APC

Number: 14

Cored: 126.8 m

Recovered: 116.71 m (92.04%)

Lithology:

Unit I: olive-gray and dark olive-gray silty clay with varying amounts of nannofossils and foraminifers

Principal results: Site 1078 is located outside the Bight of Angola in 427 m-deep water. Situated between the high-productivity regions off the Congo River to the north and off Namibia to the south, the site is part of a transect that will provide information on "pelagic background" sedimentation for the latest Neogene. Sediments from this region indicate lower primary productivity in overlying waters compared with the adjacent upwelling areas. Thus, the influence of the open ocean is more pronounced and will provide a tie-in of coastal ocean history to the record of the pelagic environment. This will allow us to study the cross-correlations of climate-driven ocean dynamics across these two regimes. One of the intriguing aspects of this record is the low opal content associated with high organic matter accumulation. This paradox indicates a strong influence of the quality of subsurface waters, which is set elsewhere in the system, possibly in the Subtropical Convergence Zone. Other topics of importance are the control of variation by precession, reflecting the changing dominance of trade-wind and monsoonal effects.

Four holes were cored with the advanced hydraulic piston corer/ extended core barrel (APC/XCB) at Site 1078 to a maximum depth of 165.2 meters below seafloor (mbsf), which recovered an apparently continuous hemipelagic sedimentary section spanning the last 360 k.y. of the Pleistocene. Hole 1078A was cored with the APC to 69.5 mbsf and deepened with the XCB to 77.1 mbsf. Fourteen APC cores were taken at Hole 1078B to 130.1 mbsf. Hole 1078C was cored with the APC to 128.8 mbsf and deepened with the XCB to 165.2 mbsf. Hole 1078D was cored with the APC to 126.8 mbsf.

Sediments from this site form one lithostratigraphic unit composed predominantly of a moderately bioturbated, olive-gray silty clay with varying amounts of nannofossils and foraminifers. Sediments in the uppermost 3 m contain rare intact gastropod and mollusc shells, pteropods, and abundant shell fragments. Below 80 mbsf, several sections contain whitish gray nodules, 1-2 mm in diameter. Diagenetic dolomite concretions, between 3 and 7 cm thick, and laminated intervals were found at various depths. Calcium carbonate varies between 1 and 25 wt%. The clastic fraction is dominated by smectite, kaolinite and/or illite, quartz, feldspar, and muscovite. The biogenic component is represented by frequent foraminifer fragments and nannofossils. Diatoms are abundant only in laminated sequences. Laminated packages also show abundant diatom resting spores. Trace amounts of plant remains and amorphous organic matter were observed occasionally.

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 136 meters composite depth (mcd).

Calcareous microfossils are abundant and well preserved in the entire section, except for the laminated intervals that are barren of nannofossils. Both calcareous nannofossils and planktonic foraminifers show evidence of reworking within the middle part of Hole 1078B. With the exception of the laminated intervals, diatoms, silicoflagellates, and radiolarians are absent at this site. The nannofossil-based biostratigraphy suggests that Site 1078 terminated within the upper half of Zone NN20, between 0.26 and 0.36 Ma.

Assuming a linear sedimentation rate between the two available datum events, sediments accumulated at a rate close to 600 m/m.y. for the interval from 0.09 to 0.26 Ma (Zone NN21a).

Magnetic inclinations and declinations after alternating-field (AF) demagnetization at 20 mT from all four holes indicate that only the Brunhes (C1n) normal polarity Chron is recorded. Short reversal events in the Brunhes Chron were not found, despite the high sedimentation rates.

Sediments average 2.5 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 Sites 1075, 1076, and 1077 on the Congo Margin. Profiles of salinity, dissolved chloride, and methane do not indicate the presence of gas hydrate in Site 1078 sediments.

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.

The site provides a good high-resolution record for the reconstruction of the oceanography of the Eastern Angola Basin. Of special interest are the changing position of the Angola-Benguela Front and the supply of nutrients from the subsurface waters rising within the Angola Dome. The possibility that silicate content varies through time within that water must be considered; the absence of diatoms, despite high background productivity, is intriguing in this context. Much diatom dissolution takes place during early diagenesis within a zone of high alkalinity generated by sulfate reduction. Occasional development of short laminated sequences (in one case cemented by dolomite) indicates sporadic oxygen deficiency in bottom waters. Possibly, such events are tied to change in the quality of upwelled waters, as suggested by the high abundance of diatoms within laminated sediments. Dolomite layers are present at some depths; several were found near 100 mbsf. Such layers may be important in determining the seismic reflectancy of sediments.