MIDDLE PALEOLITHIC HOMININ LAKE ENVIRONMENTS IN SAHARAN NORTH AFRICA

One key process in the transition from archaic to modern Homo sapiens (200-130 ka) involved their movement out of East Africa, coeval with episodic drying of the eastern Sahara. Some hypothesize that Pleistocene lakes in the Egyptian Western Desert reflect northward migration of the ITCZ, increasing Atlantic-sourced, African monsoon precipitation during the summer. To evaluate this hypothesis, we determined precipitation sources and seasonality at the Middle Paleolithic Bir Tarfawi site, southern Egypt, using oxygen stable isotope analysis of the climate proxy, Melanoides tuberculata. This gastropod secretes its shell in equilibrium with surrounding water, providing a robust proxy for water compositions. All Melanoides were collected from a coquina horizon in Pleistocene White Lake sediments, and were unaltered diagenetically. Shell oxygen isotopes varied from -7.7‰ to 0.8‰, and intrashell isotope zoning showed variations of 3 to 6 ‰, with no evidence for seasonally low values. The shell oxygen isotope values are higher than previously published perennial lake sediment carbonates (-8.3‰ to -5.7‰), and imply water compositions of -6.6‰ to 2‰ at 25°C. Higher oxygen isotope shell values suggest that the coquina may have formed in evaporative waters separated from the main lake perhaps the result of a regression. High oxygen isotope values for lake sediment and Melanoides shells contradict predicted northward ITCZ movement, suggesting that additional precipitation sources, e.g. from the Indian Ocean, contributed to the perennial White Lake.