HR: 1340h
AN: PP33A-0908    [Abstracts]
TI: Distinguishing Between Provenance Changes and Sorting Effects on the Rb-Sr Systematics in Glacial and Holocene South Atlantic Sediments
AU: * Culkin, S
EM: sculkin1@jhu.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: * Culkin, S
EM: sculkin1@jhu.edu
AF: Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 United States
AU: Franzese, A M
EM: franzese@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AB: The results of recent studies show that the Strontium isotope composition of Holocene sediments in the South Atlantic and western Indian Oceans follow systematic geographic variations consistent with the distribution of modern surface currents. Our conclusion based on these results is that surface and intermediate, rather than deep currents are the first order means of transporting terrigenous sediments in this region, and studies of the terrigenous composition in marine sediment cores should yield insights into past ocean circulation. The two major sediment sources for the eastern South Atlantic can be identified by their Sr isotopic signatures, and appear to be transported by the Agulhas Current and the Antarctic Circumpolar Current (ACC) or South Atlantic Current (SAC). Although the Rb/Sr isotope system is a reliable measure of the provenance of terrigenous sediments, it is also known to be influenced by sorting. The goals of this study are to characterize the grain size compositions of the end-member sediment sources in both Holocene and last glacial maximum (LGM) aged core samples, and to evaluate the relative effects of provenance changes and sorting effects on the Rb/Sr system in this region during both time periods. Additionally, the silt to clay ratios of sediments may be useful in determining deep current strength at the time of deposition, due to their different hydrodynamic behaviors. Deep-sea cores from the Atlantic sector of the Southern Ocean and from the Mozambique Channel were selected to represent sediments deposited by the ACC and Agulhas Current, respectively. Samples were taken depths thought to represent Holocene and LGM. The core samples, as well as one sample taken from the Orange River in South Africa, were sieved to remove the coarse ($>$63 micron) fraction. The carbonate, oxide and opal components were leached from the remaining fine-fraction. Sub-samples of the leached fine-fraction sediments were separated into silt (2-63 micron) and clay ($<$2 micron) fractions according to Stoke's law of settling. Sr isotope ratios and Rb/Sr ratios were measured in the lithogenic $<$63 micron fraction, as well as the in silt and clay fractions of each sample. The silt/clay ratios range from 0.3 to 3.6, and are generally higher in the west and lower in the east. The silt/clay of the Orange River is 4.6, higher than any of the marine sediment samples. There is no clear pattern of change in silt/clay between the Holocene and the LGM. Sr isotope ratios are consistently higher in the clay fractions than in the silt. Sr isotope ratios of both the silt and clay fractions of the Agulhas samples are higher than those of the South Atlantic samples for both Holocene and LGM sediments. Rb/Sr isotope systematics for the slit and clay is consistent with sorting of both Holocene and LGM sediments from the Agulhas region, yielding Rb/Sr apparent ages that are higher for the silts than for the clays, and range from 256 Ma to 1.25 Ga. These results imply that most of the variability in the Rb-Sr systematics in South Atlantic sediments is due to differing provenance, rather than to systematic changes in the degree of sorting of a single sediment source. A more detailed look at the Rb-Sr systematics coupled with a study of Sm-Nd systematics in these samples may help us to distinguishing between provenance changes and sorting effects in South Atlantic Sediments.
DE: 4825 Geochemistry
DE: 4863 Sedimentation
DE: 4267 Paleoceanography
DE: 3022 Marine sediments--processes and transport
DE: 1040 Isotopic composition/chemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting