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Using 10Be cosmogenic isotopes to estimate erosion rates and landscape changes during the Plio-Pleistocene in the Cradle of Humankind, South Africa
15 May 2016
Concentrations of cosmogenic听10Be, measured in quartz from chert and river sediment around the Cradle of Humankind (CoH), are used to determine basin-averaged erosion rates and estimate incision rates for local river valleys.
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This study focusses on the catchment area that hosts Malapa cave with听Australopithecus sediba, in order to compare regional versus localized erosion rates, and better constrain the timing of cave formation and fossil entrapment. Basin-averaged erosion rates for six sub-catchments draining the CoH show a narrow range (3.00听卤听0.28 to 4.15听卤听0.37听m/Mega-annum [Ma]; 卤1蟽) regardless of catchment size or underlying geology; e.g. the sub-catchment with Malapa Cave (3听km2) underlain by dolomite erodes at the same rate (3.30听卤听0.30听m/Ma) as the upper Skeerpoort River catchment (87听km2) underlain by shale, chert and conglomerate (3.23听卤听0.30听m/Ma). Likewise, the Skeerpoort River catchment (147听km2) draining the northern CoH erodes at a rate (3.00听卤听0.28听m/Ma) similar to the Bloubank-Crocodile River catchment (627听km2) that drains the southern CoH (at 3.62听卤听0.33 to 4.15听卤听0.37听m/Ma). Dolomite- and siliciclastic-dominated catchments erode at similar rates, consistent with physical weathering as the rate controlling process, and a relatively dry climate in more recent times. Erosion resistant chert dykes along the Grootvleispruit River below Malapa yield an incision rate of 鈭�8听m/Ma at steady-state erosion rates for chert of 0.86听卤听0.54听m/Ma. Results provide better palaeo-depth estimates for Malapa Cave of 7-16听m听at the time of deposition of听A.听sediba. Low basin-averaged erosion rates and concave river profiles indicate that the landscape across the CoH is old, and eroding slowly; i.e. the physical character of the landscape changed little in the last 3-4听Ma, and dolomite was exposed on surface probably well into the Miocene. The apparent absence of early Pliocene- or Miocene-aged cave deposits and fossils in the CoH suggests that caves only started forming from 4听Ma onwards. Therefore, whilst the landscape in the CoH is old, cavities are a relatively young phenomenon, thus controlling the maximum age of fossils that can potentially be preserved in caves in the CoH.
Using听10Be cosmogenic isotopes to estimate erosion rates and landscape changes during the Plio-Pleistocene in the Cradle of Humankind, South Africa
Paul H.G.M. Dirks, Christa J. Placzek, David Fink, Anthony Dosseto, Eric Roberts
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