SITE 1078

Site 1078 is located outside the Bight of Angola in 438 m deep water (Fig. 1). The site is part of a transect that will provide information on "pelagic background" sedimentation for the latest Neogene, as it is situated between the high-productivity regions to the north and south. Sediments from this region indicate lower primary productivity in overlying waters compared to 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 there is a strong influence of subsurface waters, which originate elsewhere in the system, possibly in the subtropical convergence. Other topics of importance are the control of variation by precession, reflecting the changing dominance of trade wind and monsoonal effects.

A relatively continuous hemipelagic sedimentary section spanning the last 360 k.y. of the Pleistocene was recovered from Site 1078. Sediments at this site form one lithostratigraphic unit composed predominantly of a moderately bioturbated, olive-gray silty clay with varying amounts of nannofossils and foraminifers (Fig. 2). Sediments in the uppermost 3 m contain rare intact gastropod and mollusc shells, pteropods, and abundant shell fragments. Below 80 mbsf all holes contain several sections with 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 MST and high resolution color reflectance measured with the Minolta spectrophotometer demonstrated complete recovery of the sedimentary sequence down to 136 mcd.

Calcareous microfossils are abundant and well preserved in the entire section, except for the laminated intervals, which 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 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% total organic carbon, 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 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; despite high background productivity, the absence of diatoms 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.

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