Abstract
AbstractFew proxies exist to identify aridity in the depositional record, although drylands cover ca 30% of the modern continental surface. New exposures in a siliciclastic and carbonate sequence in an arid to hyperarid basin at Olduvai Gorge, Tanzania provide a unique multi‐proxy record of a 1·85 Ma landscape that was exploited by early humans. The 2 m thick sequence of clastics and carbonates that are exposed along a 450 m outcrop records climate change over a single precession (dry‐wet‐dry) cycle. Siliciclastic data (sedimentary structures, grain size, mineralogy) and biological data are combined with data for a 10 to 35 cm thick limestone (stable isotopes, elemental geochemistry, petrography) to generate a depositional facies model for a site DK (Douglass Korongo) on this dry rift basin landscape. This site was situated on a low gradient, distal portion of a volcaniclastic alluvial fan. The clastics are intercalated distal alluvial fan sandy silts and lake clays that accumulated in a low energy environment. Groundwater discharge and the alkaline springs and seeps during wet‐to‐dry change in climate made a freshwater carbonate‐rich environment. Bedded lithofacies (a lime mudstone with fossils) were deposited in shallow standing (spring‐fed) pools, while nodular lithofacies with calcite spherulites indicate permanently saturated ground (seeps). Both environments experienced similar diagenesis, that is, the precipitation of authigenic barite from supersaturated groundwater, desiccation and pedogenesis, and late‐stage calcite precipitation. Compositional and isotopic data suggest that a fresh groundwater‐fed system was available to early humans even during dry intervals of the precession cycle.
Highlights
Drylands account for ca 30% of the modern Earth’s surface
The sedimentary infill of the Olduvai Basin is composed of intercalated pyroclastics, minor lava flows, and weathered volcaniclastic detritus transported from the basin margin by ephemeral rivers and authigenic Mg-rich clays produced in the lake (Hay, 1976; Hay & Kyser, 2001; Hover & Ashley, 2003; Deocampo et al, 2009)
The DK carbonates are divided into two lithofacies endmembers; (i) bedded limestone consisting of lime mudstone or wackestone and (ii) nodular limestone consisting of calcite spherulites in a micrite-smectite-rich matrix
Summary
Drylands account for ca 30% of the modern Earth’s surface. They occur where moisture-bearing weather systems are prevented from reaching an area, such as the interior of a continent, the distal sides of a topographic high (rain shadow), and within high-pressure climate zones (global circulation easterlies). Interpreting the rock record of arid lands requires the use of proxies to estimate ‘dryness’. Aridity is the net result of a limited moisture supply (precipitation, P) and moisture losses (evaporation, E, and transpiration from plants, T). Speleothems provide robust records of aridity from evidence of variable carbonate precipitation rate and changes in both d13C and d18O (Burns et al, 1998; McDermott, 2004)
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