FINE SEDIMENT IN ODP SITE 1094 (SOUTH ATLANTIC): EOLIAN VERSUS HEMIPELAGIC DETERMINATION

The record of natural gamma radiation (NGR) from ODP Site 1094 in the southeast Atlantic Ocean displays millennial variability throughout the last glacial period and shows a strong correlation to dust concentration in the Vostok Ice Core. One hypothesis to explain this resemblance is that NGR reflects predominantly fine terrigenous sediments transported to the site by eolian processes, the intensity of which fluctuate in response to changes in sea-ice coverage and associated ocean-land temperature gradients (Petit et al., 1999; Kanfoush et al. 2002). Alternatively, NGR may represent terrigenous material that is hemipelagic in origin, the delivery of which is alternately hindered and enhanced by millennial sea-level fluctuations that result in alternate inundation and exposure of circum-South Atlantic source areas during the glaciation (Diekmann et al., 2000; Latimer and Filippelli, 2001).

To decipher between these two hypotheses, grain size analysis was performed on four hundred samples taken every 5-cm from 0-20 mbsf. The record of accumulation of the less than 63 micron size fraction resembles the NGR record, and throughout much of the last glacial period and the Holocene, the accumulation rate of the clay-sized sediment (less than 3.9 microns) at Site 1094 is two orders of magnitude greater than that of dust in Vostok providing preliminary evidence against an eolian origin for terrigenous fines at Site 1094. If the NGR represents the fine sediment component at Site 1094 and this component is hemipelagic in origin, then the record of NGR may serve as a recorder of millennial fluctuations in bottom current intensity and perhaps larger-scale thermohaline circulation. Additional analyses (SEM and XRD) will be performed, and comparisons of the grain size at Site 1094 with millennial records of sea-level will be made to further test and (in)validate this interpretation.