POTENTIAL AND PITFALLS OF STABLE ISOTOPE ANALYSES OF STROMATOLITES IN PALEO-CRITICAL ZONE RECONSTRUCTIONS FOR THE TURKANA BASIN, KENYA

Fossilized organisms are important proxies for reconstructing paleoenvironments through the analysis of their chemical composition. One such fossil, stromatolites, can provide important paleoenvironmental information about marine, fluvial, and lacustrine settings in the paleo-Critical Zone. Stromatolites are the remnants of photosynthetic bacteria, commonly cyanobacteria, that precipitate calcium carbonate around a mucilaginous nucleus. High Ca⁺²: Mg⁺² ratios in source waters favor precipitation of aragonite over calcite in stromatolites. Aragonite is the meta-stable form of calcium carbonate and can be isotopically analyzed for δ¹³C and δ¹⁸O which can provide information about the paleoenvironment and paleohydrology in which these organism formed. However, because aragonite is not stable at surface, or near-surface, conditions and readily transforms into calcite at these conditions, post-depositional diagenesis of the stromatolites can alter this original material into calcite and complicate the interpretation of isotopic data. The stromatolites for this study are from the KBS Member of the Koobi Foora Formation of the Turkana Basin, Kenya and were formed in a fluvial-deltaic setting in the Pleistocene at ~1.64 Ma. The active tectonics and volcanism of this basin within the East African Rift System, resulted in Ca⁺² and Mg⁺² rich water, generating aragonite-based stromatolites. Preliminary investigation of five stromatolites from the KBS Member for bulk geochemistry (EDS on a scanning electron microscope) and mineral phase (cathodoluminescence on a nuclide luminoscope) revealed two of the stromatolites contain original aragonite material, while the other three are dominated by secondary calcite. The comparison of the stable isotopic data of the diagenically altered stromatolites to the unaltered samples can provide important insight into the influence of diagenesis on isotopic concentrations, which can have further implications for paleoenvironmental reconstructions that use aragonite-based fossils as proxies. Additional isotopic analysis of the unaltered samples can also provide data on the paleoenvironmental conditions of the basin during the Pleistocene.