Water Level History of Lake Turkana, Kenya and Hydroclimate Variability during the African Humid Period
thesisposted on 28.10.2014 by Christopher Bloszies
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The present chronology of East African paleoclimate suggests the transitions associated with the African Humid Period (AHP) at ca. 15 and 5 ka were binary. Previous studies have indicated water levels for Lake Turkana as relatively unchanging at ~88 to 98 m above present level for the AHP, and infer outflow into the White Nile Basin. This interpretation is reinforced by many proxy-inferences for East African hydroclimate, which suggest ubiquitous wet conditions between ca. 11.5 and 6 cal. ka. Also, previous studies suggest that peak water levels for Lake Turkana resulted from precipitation delivered by a strengthened African monsoon and were amplified by overflow from adjacent lake basins. This study presents an intensive sedimentologic investigation of the western strand plain of Lake Turkana. New AMS 14C ages on seventeen freshwater gastropod and bivalve shells taken from littoral deposits provide chronologic control. This reconstruction of water level for Lake Turkana is based on these results, which are combined with previous ages that have been ranked systematically. Water level of Lake Turkana may have varied by ± 60 m at least ten times, potentially reaching the outlet elevation at ca.11.3, 10.3, 9.0, 6.3 and 5.1 cal. ka. Inferences on the source of moisture to sustain these many high stands are based on isotopic data on leaf wax (δDwax), various metrics of runoff and associated sea surface temperature (SST) records from the Indian and the Atlantic oceans. Multiple water level oscillations, possibly to the outlet sill elevation, between 14.5 and 8.5 cal. ka are associated with intervals of elevated Atlantic-derived moisture flux to East Africa, and a zonally variable CAB. After ca. 7.5 cal. ka, water level oscillations may be linked to rising SSTs in the Western Indian Ocean, and moisture delivered by the East African Monsoon. In turn, evidence from relict beach landforms supports low lake levels associated with Heinrich Event 1, the Younger Dryas, the 8.2 ka meltwater event and mid-Holocene aridity for East Africa post 4.5 ka. These regressions are attributed to intervals of suppressed SSTs for the Eastern Atlantic Ocean, and limited westerly moisture flux to East Africa.