Challenges in constraining pluvial events and hominin activity: Examples of ESR dating molluscs from the Western Desert, Egypt

Receiving <0.1 mm/y of precipitation, Egypt’s hyperarid Western Desert, today lacks naturally occurring surface water. Artesian spring deposits, tufa deposited by springs and carbonate-rich silty lacustrine sediment attest that oases in the Western Desert had surface water during the Pleistocene. Paleolithic artefacts, fossil ungulate teeth, and snails occurring within the Pleistocene deposits and dotting the surface record times when higher rainfall and/or groundwater tables during pluvial events allowed surface water to exist in wetlands, small ponds and lakes, enabling hominin habitation. Archaeological finds ranging from Early to Later Stone Age (ESA–LSA) occur in gravel lags, within sedimentary deposits, and on the older geomorphic surfaces. Near Kharga, large tufa deposits ranging from a few hectares to more than 10 km2 in area, such as Matana and Medauwara, dot the edge of the Libyan Plateau. Molluscs were dated using standard ESR protocols. To test for reworked fossils, multiple samples from a single sample were dated independently. In some units at Medauwara, multiple gastropod populations from different times have been preserved, while others appear to only preserve a single population. To see the effects of the cosmic dose rate on ESR ages, ages were calculated using zero cosmic dose rate, the full modern cosmic dose rate, and time-averaged cosmic and sedimentary dose rates. For gastropods from Matana, no significant difference in ESR ages resulted from different cosmic dose rate assumptions. Therefore, at Matana 2, the shells dated at 27.7 ± 1.9 assuming time-averaged external dose rates, while at Matana 3, they averaged 65.1 ± 4.1 ka, suggesting that water was present for hominin use at times during OIS 2 and 4.

► Matana 2 lacustrine silts: 27.7 ± 1.9 ka.
► Matana 3 lacustrine silts: 65.1 ± 4.1 ka.
► water present: OIS 2 & 4.
► hominins also likely present: OIS 2 & 4.