Variations in water level for Lake Turkana in the past 8500 years near Mt. Porr, Kenya and the transition from the African Humid Period to Holocene aridity
A stratigraphic and geomorphic study of relict beaches up to 80 m above current water level of Lake Turkana reveal four major lake level fluctuations of >50 m between ca 8.5 and 4.5 ka during the transition from the African Humid Period to Holocene aridity. We hypothesize that high stands of Lake Turkana reflect complex hydrological processes with variability in Atlantic and Indian Ocean-derived moisture into East Africa, ultimately linked to changes in sea surface temperatures (SSTs). Lake level rise was also amplified regionally by increased precipitation causing overflow from the adjacent Suguta and Chew Bahir basins, particularly during high stands at ca >8.5 ka and at 6.4 ka. The timing of transgressive and regressive events is constrained by 14C dating of carefully selected lacustrine mollusks, incorporation of previous shell ages with sufficient elevational control, and OSL dating of quartz grains from littoral and sublittoral deposits by a multiple aliquot regeneration (MAR) approach. There was a high water level up to at least 70 m at >8.5 ka that probably reached the spillover limit ∼100 m. A brief (<500 yr) high stand up to at least 50 m at ca 7 ka appears to be coincident with warm postglacial SSTs in the western Indian Ocean and thus may reflect a strengthened East African Monsoon, though some precipitation was probably derived from Atlantic sources as well. A pronounced high stand inferred at ca 6.4 ka, possibly up to 100 m, is associated with warming across the Indian Ocean and equatorward compression of the ITCZ, thus reflecting a strengthened East African Monsoon. A high stand occurred at ca 5.5-5.0 ka up to at least 80 m, with a precipitous drop in lake level by 4.6 ka. Evidence from relict Porr strand plain indicate that lake level was probably below 20 m since ca 4.5 ka, though there were two noticeable high stands up to 12-20 m at ca 830 years ago and <100 years, the latter age is consistent with the historic record.