TRACKING PRECIPITATION SOURCES USING ∂18O ISOTOPES, NORTH BASIN, LAKE TURKANA, KENYA

Lake Turkana (LT) in Northern Kenya is the world’s largest alkaline lake with an area of 6,405 km². The Omo River that flows from the Ethiopian Highlands is the principal source of water to LT. The extent of its influence can be observed in flood stage plumes with high suspended sediment on MODIS satellite images. Local precipitation in Kenya contributes to LT both directly and via flashy discharge streams. LT’s position is unique as the Congo Air Boundary (CAB) falls between the Omo’s source and Kenya. Precipitation in Ethiopian is derived from Atlantic Ocean air masses, whereas precipitation in Kenya is sourced from the Indian Ocean. This should result in unique ∂¹⁸O isotopes in precipitation from Ethiopia vs. Kenya.

Twenty-two 60mL water samples were collected for ∂¹⁸O isotope analysis in the lower Omo River and LT’s North Basin in summer 2010. The oxygen isotope signatures are expected to differ due to the position of the CAB. Sampling sites created a lateral distribution running both N-S and E-W from the Omo Delta, tracking plume dilution across the North Basin. Water was taken from 5m depth at the sites. ∂¹⁸O analyses are being conducted to test whether the water more closely resembled the rainfall in Kenya or the rainfall in Ethiopian Highlands.

New ∂¹⁸O values for the North Basin will also have implications for the assumptions made during isotope analysis of fossils from the Koobi Fora region. Paleotemperature calculations for the Holocene based on ∂¹⁸O of fossils, diatoms, and stromatolites should be re-evaluated to reflect Ethiopian rainfall ∂¹⁸O. This research has broader impacts because the Gibe III Dam and corresponding hydroelectric power plant are currently under construction on the Omo River in Ethiopia. The filling of the Gibe III reservoir will significantly impact the water budget of LT in the future and increases the urgency of understanding the current delicate balance of this system.