T165 African Lakes and Paleolakes: Processes, Paleoenvironments, and Paleoclimate

Tuesday, 2 November 2010
8:00 AM - 12:00 PM
Mile High Ballroom 1AB

SEDIMENTARY GEOCHEMISTRY OF DIATOMACEOUS RIFT-VALLEY FILL: EXAMPLES FROM THE PLEISTOCENE OLORGESAILIE FORMATION, KENYA

Paper number: T165-4
Program: Technical Sessions
Time: 8:35 AM – 8:50 AM

Authors

Abstract

Recent advances in micropaleontology, palynology, isotope geochemistry, clay mineralogy, and related fields provide a powerful, multi-proxy tool kit for paleolimnology and allow detailed reconstructions of paleoenvironments and paleoclimatic conditions in East African Rift basins. This study presents the results of whole-rock geochemical analyses of altered lacustrine sediment in the Pleistocene Olorgesailie Formation, Kenya, within paleosols of Member 1 (UM1p) and Member 7 (UM7p). Based on Ar/Ar dates these are constrained to ~0.99-0.97 and ~0.90-0.78 Ma in age, respectively. Several major oxides and ratios provide useful indicators of paleoenvironmental processes. For example, SiO2 in these units ranges from 62-83 weight%, and TiO2 ranges from 0.4-1.7 weight%. These two indicators are strongly inversely related (e.g. UM1p: r2=0.84, p<0.0001), reflecting the competing geochemical signals of diatom productivity and preservation versus detrital input. Samples of UM1p laterally distributed over ~3km of the basin have lowest SiO2/TiO2 ratios to the west, suggesting a detrital source in that direction. In contrast, UM7p SiO2/TiO2 ratios are less variable across the field area, consistent with observations of more uniform primary lithology in the unit.

Elemental ratios such as Ba/Sr and (Fe2O3+MnO)/TiO2 provide data on variability in post-depositional paleo-hydrolytic or paleo-oxidative conditions, respectively. Significant relationships are also found between these indicators of diagenesis (e.g. UM1p: r2=0.45, p<0.001). Both UM1p and UM7p have higher Ba/Sr and (Fe2O3+MnO)/TiO2 to the east, suggesting greater weathering in that direction at times of subaerial exposure. Weathering indicators are overall more severe in UM7p, consistent with its interpreted longer period of exposure and pedogenesis. These diagenetic and pedogenic conditions are of particular interest for paleoenvironmental studies focused on terrestrial flora, fauna, or hominin paleoecology, as they reflect conditions during low lake level and subaerial exposure.

Lateral variability of primary and diagenetic geochemical signatures is important to consider in the interpretation of data from sediment cores, serving to remind us of the importance of sedimentary facies models to contextualize geochemical data.