Carbonates from Olduvai Gorge, Tanzania: Palaeohydrology and geochronology

Carbonates are abundant in the Pleistocene sedimentary sequence at Olduvai Gorge, Tanzania. This study reports their potential for investigating palaeoenvironments and for radiometric dating using U-Pb geochronology. Using their textural characteristics the, commonly nodular, terrestrial carbonates have been placed in one of five groups. By using multiple textural and geochemical analytical techniques, the palaeohydrological origin of each group has been proposed. When referenced to the geographical and stratigraphic framework at the eastern lake margin, the carbonates have been used to identify the palaeohydrological conditions beneath specific land surfaces and how it changed through time. The results identify the onset of synsedimentary faulting below Tuff IB, the palaeohydrological significance of fault control in landscape development, and the persistence of water in discrete settings. This helps to explain why hominin activity is located in certain areas in a fault compartment. The study has proved that detailed investigation of carbonates offers an effective method for understanding the wider palaeohydrology at exposure surfaces and the factors influencing hominin exploitation at particular locations and has the potential to provide a predictive tool for future archaeological investigations. Two types of dolomite are found at different stratigraphic levels, identifying episodes of high Mg/Ca ratios in the lake, and dolomite precipitation occurring in both a basinal and a lake marginal setting. Sand-sized calcite crystals formed in the shallow sub-surface sediments on the lake floor and lake margins under anoxic to sub-oxic conditions. 238U - 206Pb dating of these zoned calcite crystals using Laser Ablation MC-ICP-MS and has produced dates only a little older than those using 40Ar/39Ar on tuffs in the same stratigraphic intervals. 234U/238U activity ratios of the Pleistocene crystals indicate that different levels are more affected by open system behaviour than others. Early-diagenetic, authigenic calcite crystals show exciting promise for directly dating saline, alkaline lake sediments which may be useful in similar hominin sites where geochronology is less well constrained