Hydrologic Controls on Individual Ostracode Stable Isotopes in a Desert Lake: A Modern Baseline for Lake Turkana

Stable carbon (d(13)C) and oxygen (d(18)O) isotope measurements in lacustrine ostracodes are widely used as paleolimnological indicators of past climate change. Previous work has used individual ostracode valves to resolve seasonal and subdecadal paleohydrological signals, yet replicated assessments of co-occurring specimens from modern samples are rare. Here we focus on ostracodes from sediments collected in 1979 from Lake Turkana, an alkaline desert lake in tropical eastern Africa. Sourced from the Ethiopian highlands, the Omo River is the dominant inflow into this lake. We present individual ostracode valve analyses (IOVA) of delta C-13 and delta O-18 measurements (n = 329) of the extant benthic ostracode Sclerocypris clavularis from 17 sites spanning the entire lake (n(avg) = similar to 19 specimens/site). We demonstrate that the pooled statistics of individual valve measurements at each site are shaped by lake hydrology, superimposed over inter-individual isotopic variations. Within-site variance in IOVA-delta C-13 is larger (similar to 60%) than that of IOVA-delta O-18. Yet, site-based averages exhibit a systematic pattern, with higher d values toward the southern part of the lake, away from Omo River inflow. We suggest that the latitudinal delta C-13 gradient may arise from low riverine delta C-13 and low organic matter delta C-13 as a productivity response to nutrient-rich Omo River inflow toward the north. The delta O-18 pattern may be explained by the diminishing influence of Omo River inflows and more evaporation driving higher IOVA-delta O-18 values toward the windier, southern basin. We conclude that pooled IOVA statistics in Omo-Turkana sediments can aid interpretations of past regional paleohydrology and its variability in this basin. Plain Language Summary Ostracodes are microscopic, aquatic crustaceans that deposit shells ("valves") made of calcium carbonate. Retrieved from sediment cores, these valves are widely used to estimate past water conditions using their shell chemistry. Here we focus on stable oxygen and carbon isotopes in numerous individual valves in modern sediments taken from multiple sites across Lake Turkana, Kenya. This lake is located in a hot and arid region in tropical eastern Africa. The Omo River, which is sourced from rainfall in the Ethiopian Highlands region, is the major river that flows into Lake Turkana. We find that ostracode stable isotopes are useful recorders of lake hydrology in this basin and reveal a strong fingerprint of Omo River inflow as well as evaporation toward the windier southern part of Lake Turkana. Analyzing numerous individual valves at each site can help produce better estimates of average changes in past lakewater conditions versus changes arising from seasonal variations. Our results provide a modern baseline of ostracode stable isotopes at Lake Turkana. We suggest that both the average of, and variations within individual-valve measurements of ostracode geochemistry can help characterize past hydrological variability and lake water conditions.