10BE-DERIVED PALEOEROSION RATES FROM NORTHERN AWASH AND WEST TURKANA HOMININ SITES AND PALEOLAKES DRILLING PROJECT (HSPDP) DRILL CORES

Drill cores collected by the Hominin Sites and Paleolakes Drilling Project (HSPDP) in Ethiopia and Kenya provide an unusual opportunity to evaluate paleoerosion rates and basin-scale landscape dynamics during key intervals in Plio-Pleistocene hominin evolution. We collected four sand samples from the Northern Awash (NA; Afar, Ethiopia) drill cores (11.3152^o, 40.7370^o and 11.3254^o, 40.7649^o), and one sample from the West Turkana (WTK; Kenya) core (4.1097^o, 35.8718^o) for in situ cosmogenic radionuclide ¹⁰Be analyses. The ¹⁰Be concentration in quartz accumulates while the material is actively being eroded and transported within the hillslope and fluvial system (lake watershed) and decays at a fixed rate over time upon burial and shielding. The youngest detrital zircon ages from the NA samples indicate deposition between ~3–2.9 Ma for three of the samples and deposition at ~1.7 Ma for the fourth. ⁴⁰Ar/³⁹Ar dating and detrital zircon ages indicate deposition at ~1.5 Ma for the WTK sample. We used these ages to determine the amount of ¹⁰Be lost to radioactive decay, and we used sedimentation rates derived from age models of the cores to constrain post-depositional nuclide accumulation. Paleoerosion rates from the NA samples at ~3–2.9 Ma ranged from 0.012 to 0.014 mm/yr. Deposition of these samples was relatively contemporaneous, and the narrow range of erosion rates indicates the uniform conditions under which samples were eroded. The NA sample deposited at ~1.7 Ma yielded a lower erosion rate of 0.0097 mm/yr. Despite a significant tectonic reorganization within the Afar Depression at ~2.9 Ma, erosion rates were relatively consistent across the Pliocene to Pleistocene. Interestingly, all detrital zircons from the NA samples are <45 Ma and derived from silicic volcanic activity, thus not suggesting an identifiable quartz-bearing basement source. The NA erosion rates may therefore reflect erosion of vein quartz within the rift-related magmatic and hydrothermal systems. The single WTK sample deposited at ~1.5 Ma yielded an erosion rate of 0.024 mm/yr, higher than the NA erosion rate during a similar interval. This rate is consistent with modern erosion rates measured within the Kenya Rift, which show a relationship with vegetation cover where denser cover yields lower erosion rates for a given hillslope gradient.