43. INDICATORS OF CLIMATE AND SEDIMENT-SOURCE VARIATIONS AT SITE 959: IMPLICATIONS FOR THE RECONSTRUCTION OF PALEOENVIRONMENTS IN THE GULF OF GUINEA THROUGH PLEISTOCENE TIMES1P. Giresse,2 F. Gadel,2 L. Serve,3 and J.P. Barusseau2 |
ABSTRACTLeg 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 ~1after the major hiatus. Assuming that the superficial redistribution of -0.9 Ma, just 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 intensity would be the primary cause of the observed variations in lithologic cycles. After 1 Ma, the enlarged land areas during lower sea-level stands may have resulted in more source terranes available for erosion. The Mesozoic and Cenozoic deposits of the emerged shelf may have resulted in a significant detrital input contaminating reworked micrite and clinoptilolite. Before ~1 Ma, and particularly during the 1.5-1.8 Ma interval, significant occurrences of the ligneous phenols and, sometimes, of the cyannyl phenols, were observed. Such organic material likely originated from the grass savanna, indicating a maximum fragmentation of the forest block during this phase. Despite the major 1.5- to 1-Ma hiatus, the recorded events of Hole 959C may be integrated into the postulated ocean-circulation history of the eastern Atlantic Ocean formulated by previous authors: 1.3 Ma would have been the time of the major deterioration of the climate, and ~0.9 Ma the intensification of the cold episode took place. It is proposed that major changes in deep-water circulation patterns had an influence on the time- and space-varying nature of the sediment equilibrium near the top of the margin crest of Hole 959C. |
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