Determining seasonal variability in the Omo-Turkana Basin from the Pleistocene to modern using ostracod stable isotope analysis

Located in the eastern branch of the East African Rift System, the Turkana Basin preserves an exceptional archive of eastern African climate and primate (especially hominin) evolution over the last 20 Ma. As a desert lake, Lake Turkana experiences intense evaporative loss that results in 18O enrichment relative to its tributaries. During the wet season, the discharge from the Omo River increases causing the entire lake to become less enriched in 18O, especially near the river’s mouth. Ostracods, a class of sand-sized crustaceans that produce molts every few months, record the δ18O and temperature fluctuations in their shells. By analyzing δ18O from a large number of individual ostracod valves, we determined the spread in δ18O from an assemblage to reconstruct past seasonal intensity. We attribute the spread in δ18O to seasonal runoff changes rather than evaporation as water temperatures remain relatively constant throughout the year. 

Previous studies indicate that increased tropical solar insolation intensifies the eastern African monsoon and boosts overall humidity. The mechanisms through which this increase in rainfall manifests over seasonal timescales remain poorly understood. If high insolation is correlated with a large δ18O spread in the lake, it would suggest a stronger seasonal signal with concentrated rainfall in the wet season (low δ18O) and intense evaporation (leading to 18O enrichment) in the dry season. We compared the δ18O spread among Limnocythere from the Galana Boi (Southwest Turkana 13-11 kyrs) and Kibish (Omo River-Kibish Member II, 194-104 kyrs) Formations. The dominance of Limnocythere in both the Kibish (100% of ostracods) and Galana Boi (87% of ostracods) assemblages suggests that waters were alkaline with high concentrations of sodium carbonate. The Limnocythere from Kibish (n = 25) showed a larger standard deviation in δ18O (2.3‰ vs. 0.9‰, p < 0.001) and a lower mean δ18O (-5.1‰ vs. 0.9‰, p < 0.001) compared to those from Galana Boi (n = 53). The difference in mean δ18O is due to the Limnocythere in Kibish living more proximal to the Omo River. We interpret the larger standard deviation in the Kibish assemblage as evidence of heightened seasonal variability in Omo discharge.