Indicators of climate and sediment-source variations at Site 959: implications for the reconstruction of paleoenvironments in the Gulf of Guinea through Pleistocene times

Abstract

Leg 159 Ocean Drilling Project Site 959 (in 2100 m water depth and ~120 km off the west African coast) is located on a small plateau that extends on the northern flank of the Côte d'Ivoire-Ghana Marginal Ridge. Because of its topographic position, this site was largely isolated from mass gravitational or turbidity flows coming from the continental shelf and slope. This very steep transform margin, on the other hand, has probably established interference of bottom-water circulation. The Hole 959C depth corresponds roughly to the interface between North Atlantic Deep Water and South Atlantic Intermediate Water. The very moderate linear sedimentation rates (~1−2 cm/k.y.) are relatively constant throughout the Pleistocene interval. This pronounced low sedimentation rate most likely reflects the persistent activity of deep-water circulation. Based on the oxygen isotopic record, two stratigraphic gaps are reported between 1.6 and 0.99 Ma and between 0.20 and 0.12 Ma, and are attributed to enhanced bottom-water circulation. This water circulation implies that significant changes in winnowing activity occurred in this site during the last 1 m.y. Detailed microstructural analysis indicates a number of laminae 1 mm to 1 cm thick with increased concentration of foraminifers resulting from the recurrent winnowing process (contourite). Many more glauconitic infillings of foraminifers were observed ~1−0.9 Ma, just after the major hiatus. Assuming that the superficial redistribution of the sediment was favorable to longer contact with seawater and to mineralization, the green grain concentration supports the indication of an uneven beginning of sedimentation after the hiatus.The carbonate average concentrations tend to increase from ~20%−30% in the lower part of the section to ~35%−40% in the upper part. Organic carbon increases ~0.7%−0.8% in the lower part to 1%−1.1% in the upper part. However, accumulation rates for carbonate and organic carbon closely follow bulk accumulation rate records that correspond to an average of the bottom-current winnowing intensity. Consequently, cyclic variations in relative abundances of carbonate and noncarbonate components do not appear to correspond clearly or closely to glacial/interglacial cycles. The older section (before 1.5 Ma) shows some evidence of maxima in productivity of calcareous foraminifers during interglacial cycles. Conversely, maxima of organic carbon are connected to glacials and may be enhanced by local upwelling. Both of these indicators have been used in the same way to identify the steps of the last 125 k.y. Such cyclic variations are similar to those of neighboring sites on the slope of the eastern equatorial Atlantic, but from ~1 to ~0.2 Ma, the highest variabilities are recorded and attributed to enhanced sediment redistribution. In the same section, the highest variation ranges of major components prevailed at the time when the climatic change had the maximum effect. It is difficult to assess, therefore, if eustatic and climatic cycles or stronger current in...

Orbital-driven environmental changes recorded at ODP Site 959 (eastern equatorial Atlantic) from the Late Miocene to the Early Pleistocene

“…1). Today, river discharge along the coastline is moderate and most of the mass gravitational flows are canalized into the Deep Ivorian Basin (Giresse et al 1998). Fluvial material thus does not often reach Site 959 (Wagner 1998).…”

A Deep-Water Glauconitization Process on the Ivory Coast—Ghana Marginal Ridge (ODP Site 959): Determination of Fe3+-Rich Montmorillonite in Green Grains

“…The successive maxima of green-grain concentrations are not clearly linked to glacial-interglacial cycles. These concentrations and especially this 11-12 mbsf accumulation correspond to periods of slowing down of the erosional process after a long period of maximum bottom-water activity (Giresse et al, 1998). Where the content of greenish grains is low, the fillings are generally white to pale green, whereas higher contents tend to be medium green to dark green and more or less split by fissures.…”

“…After wet sieving through a 50 txm mesh, green fillings of foraminifers were readily distinguishable in the sand size-fraction with the binocular microscope. Part of this sand fraction was treated with 0.1 N HC1 to remove the carbonate and to determine the percentage of the insoluble sand fraction, which consisted largely of green clay fillings (Giresse et al, 1998). Further analyses were performed on another aliquot of the sand fraction using 1 N acetic acid so as to preserve the original chemical composition of the green pigment.…”

“…With regard to the very steep topography along the Ridge, bottomwater velocities are especially strengthened. According to changes in the intensity of winnowing in response to variations in bottom-current velocities, a greater sand content, corresponding to larger amounts of green grains, indicates stronger bottom-water turbulence (Giresse et al, 1998) as was described in Holocene upper slope sediments from Peru (Suits and Arthur, 2000). The lithology comprises nanofossil oozes and foraminifer oozes as end-member types of sediment, with various admixtures of the two forming interbeds.…”

“…The deposits consist of interbedded varicolored sediments (verdant green, olive brown to olive, greyish green), which generally lack sedimentary structure except for some scour contacts and bioturbation. Stratigraphic control was obtained from the oxygen isotope records, which were established on tests of the epibenthic taxon Cibicides wuellerstorfi (Giresse et al, 1998;Wagner, 1998). This record spans the last 25 oxygen isotope stages and indicates a low accumulation rate of 1-2 cm/ka, which is three to four times lower than that recorded at nearly the same depth on the Gabonese slope (Bonifay aud Giresse, 1992).…”