PALEOENVIRONMENTAL RECONSTRUCTION OF A LARGE WETLAND, A FRESHWATER RESOURCE FOR HOMININS IN THE EARS AT ~1.3 MA

Tropical portions of the East African Rift System (EARS) typically have a negative hydrologic budget, with high evapotranspiration and low precipitation. Yet a rich paleontological record including at least three species of hominins is preserved in Plio-Pleistocene volcaniclastic sequences in a number of semi-arid rift basins within the EARS. Recent geological excavations revealed a laterally persistent siliceous deposit in Upper Bed II, Olduvai Gorge, Tanzania, which provides evidence for a large paleo wetland (1.5 km wide) in an otherwise arid to hyper-arid rift basin. The deposit is in a lake-margin stratigraphic sequence that is sandwiched between incised fluvial channels suggesting episodic tectonic or climatic fluctuations. Geologic mapping and facies analyses indicate that the wetland likely formed at the margin of a small lake or pond at the distal end of a fluvial-deltaic system. The system was fed by surface water and groundwater from a 3-4,000 m high volcano (Lemagurut) that lay to the southeast. Dated to ~1.3 Ma by an underlying tuff (Tuff IID), the deposit consists of siliceous claystone overlain by a carbonate (tufa), each deposit ranging from 30-70 cm in thickness. Evidence of freshwater is provided by preserved siliceous rhizoliths, sponge spicules, diatoms, and phytoliths in the siliceous clay, and by ostracods and a freshwater stable isotope signature of the carbonate (δ¹³C -2 to -1‰; δ¹⁸O -4 to -1‰). The rhizoliths are abundant and are typically up to 5 cm in length and 5 mm wide and are composed mostly of opal-A, although some are calcareous. Rhizolith facies abut laterally with silty volcaniclastic facies and vertisols, which potentially represent local waterways and interfluves. Hominin fossils (Homo erectus and Australopithecus spp.) and tools (Developed Oldowan B) have been found in association with the deposit, suggesting it was an important freshwater resource for hominins in this arid to hyper-arid landscape.