Taphonomy of modern and ancient vertebrate traces in the marginal sediments of saline, alkaline and freshwater lakes, Baringo-Bogoria basin, Kenya Rift Valley

Abstract

Actualistic, sedimentological, and experimental approaches to the study of vertebrate trace taphonomy in the Kenya Rift Valley have permitted the characterization and even quantification of factors that influence trace morphology and preservation potential in semi-arid lake margins. Several important taphonomic factors were identified from the modern lake-marginal sediments of saline, alkaline Lake Bogoria and freshwater Lake Baringo in the Baringo-Bogoria basin (1º N and 36º E). The investigation of these factors, considering vertebrate ecology and sedimentology of the deposits, together with the study of early and later diagenetic processes that cement substrates, provided a framework for the paleoecological interpretation of three Pleistocene fossil footprint localities, also in the Baringo-Bogoria basin. The most important taphonomic factors appear to be related to the semi-arid climate (e.g., high evaporation:precipitation ratio), frequent lake level changes, the closed nature of the lake basins, bedrock geology (mainly volcaniclastic) within the catchment, and the chemical composition of lake and pore waters. Notable factors that cause the alteration, destruction, and stabilization of traces include efflorescent salt crystallization, which may temporarily cement substrates or destroy traces during crystal growth in the capillary fringe; substrate wetting and drying, which can induce soil-crusting and the shrinking and swelling of smectitic clays; and the presence of benthic microbial mats and biofilms, which may temporarily stabilize substrates or contribute to their early cementation by mediating carbonate precipitation. Experiments to quantitatively and statistically test the effects of salt efflorescence, the rate and temperature of substrate drying (e.g., sun-baking), and swelling and non-swelling clays supported field observations. Preservational processes interpreted from Pleistocene footprint-bearing sediments include the early cementation of substrates by carbonates (e.g., calcite), and during prolonged, stable dry phases, the precipitation of zeolitic cements and Mn- and Fe-oxide minerals.

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