HR: 0800h
AN: PP21A-1552    [Abstracts]
TI: Lithogenic Sediments as a Proxy Record of Tropical Aridity and Monsoon Intensity: An Example from Lake Tanganyika, Africa
AU: * Soreghan, M J
EM: msoreg@ou.edu
AF: University of Oklahoma, School of Geology and Geophysics 100 E. Boyd St, Norman, OK 73019 United States
AU: Scholz, C A
EM: casholz@syr.edu
AF: Syracuse University, Department of Earth Sciences 219 Heroy Geology Laboratory, Syracuse, NY 13244-1070 United States
AU: Cohen, A S
EM: acohen@geo.arizona.edu
AF: Univesity of Arizona, Department of Geosciences 1040 E. 4th St, Tucson, AZ 85721 United States
AB: The flux and grain size of wind-blown sediment to Lake Tanganyika allows reconstruction of wind regimes in this tropical continental setting, which is important for understanding temporal changes of the Asian Monsoon. In Lake Tanganyika, Africa, a 6 m core was collected from an isolated bathymetric ridge in 393 m of water. Geomorphic and seismic evidence suggests that clastic sedimentation to this site is primarily suspension derived. A previously published sedimentation model based on 11 calibrated C-14 AMS dates of bulk sediment suggests a slow, linear sedimentation rate. Samples collected on 4-cm spacing were subjected to a multi-stage chemical treatment to remove carbonate, organic, and oxide phases. The remaining lithogenic fraction in the sampled interval ranged between 5 and 74 % (by weight) and mean grain size of the fraction ranged between 5.9 and 101 μm. The temporal trends show significant variation: low lithogenic fraction and mean grain sizes during the Holocene (core top to ~9 kyrs BP), abruptly changed downward to very high lithogenic fraction and maximal grain size at ~11 kyrs BP, which corresponds to Younger Dryas (YD) interval. Immediately preceding the YD, lithogenic fraction and mean grain size were low, but during the Last Glacial Maximum (LGM) both variables were much higher than Holocene values. Prior to ~20 kyrs BP, the lithogenic fraction was generally lower, although broad peaks of higher lithogenic fraction occur at roughly 29, 34, 40, 46 and 53 kyrs BP. The mean grain size data prior to ~20 kyrs BP does not necessarily track the lithogenic fraction, and exhibits more abrupt peaks, particularly prior to ~42 kyrs BP. The dataset as a whole correlate well with previous data from the Indian Ocean that suggests enhanced monsoonal circulation during the Late Glacial Maximum and correspondingly enhanced dust loads. Further, the lithogenic fraction data show an inverse correlation with the ice-core methane record from Vostok, Antarctica. Trace element geochemical analysis of the lithogenic fraction suggests that the source regions for the eolian dust did not vary significantly, except possibly during the Younger Dryas. However, the geochemical analyses were performed on a coarse temporal scale and more work is needed to confirm this conclusion. Finally, the lithogenic fraction shows a strong positive covariance with previously collected carbon isotope data on the bulk organic fraction, such that higher lithogenic fractions correspond to less negative carbon isotopes in the organic fraction.
DE: 3305 Climate change and variability (1616, 1635, 3309, 4215, 4513)
DE: 3344 Paleoclimatology (0473, 4900)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 9305 Africa
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005