16. ISOTOPIC STRATIGRAPHY OF AMAZON FAN SEDIMENTS1William J. Showers,2 Ralph Schneider,3 Naja Mikkelson,4 and Mark Maslin5 |
Delta18O stratigraphies of planktonic foraminifers were completed for 17 sites drilled on the Amazon Fan during Leg 155 (March-May 1994). These sites penetrated sediments of the last glacial and, for the first time, sediments of the previous interglacial on the Amazon Fan. Downslope, sediment depositional processes and hiatuses dominate the record in sites from the lower portion of the fan making the construction of age models from isotopic data alone problematic. Sites from the central axis of the fan have extremely high sedimentation rates. Age models could only be constructed with isotopic data at these sites into isotopic Stages 2–3, because sediment reworking processes dominate the record in sediments older than Stage 3 (>25 k.y.). High resolution isotopic records were obtained in the hemipelagic portions of the mud-rich upper fan (Sites 937 through 939) and from a site located to the west of the central axis of the fan (Site 942). Age models constructed from these isotopic stratigraphies show that sediments accumulated at rates up to 30 m/k.y. on the central fan during the last glacial lowstand. On the far western portion of the fan at Site 942, sedimentation rates are higher in the interglacials than during the last glacial period. Continental margin drilling can yield high resolution isotopic records if sites are carefully chosen to avoid areas where depositional processes are dominated by sediment gravity flows and reworking. |
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