Water level history for Lake Turkana, Kenya in the past 15,000 years and a variable transition from the African Humid Period to Holocene aridity

Abstract

Relict beaches adjacent to Lake Turkana, Kenya provide a record of water level variability for the Late Quaternary. This study focused on deciphering the geomorphology, sedimentology, stratigraphy and 14C chronology of strand plain sequences in the Kalokol and Lothagam areas, at the western margin of the Lake. Nine >30m oscillations in water level were documented between ca. 15 and 4ka. The earliest lake level oscillation between ca. 14.5 and 13ka is not well constrained with water level to at least 70m above the present surface and subsequently fell to at least 50m. Lake level increased to at least 90m between ca. 11.2 and 10.4ka, post Younger Dryas cooling. Water level fell by >30m by 10.2ka, with another potential rise at ca. 8.5ka to >70m above current level. Lake level regressed by >40m at 8.2ka coincident with cooling in the equatorial eastern Atlantic Ocean. Two major >70m lake level oscillations centered at 6.6 and 5.2ka may reflect enhanced convection with warmer sea surface temperatures in the western Indian Ocean. The termination of the African Humid Period occurred from ca. 8.0 to 4.5ka and is characterized by highly variable lake level (±>40m), rather than one monotonic fall in water level. This lake level variability reflects a complex response to variations in the extent and intensity of the East and West African Monsoons near geographic and topographic limits within the catchment of Lake Turkana. Also, for this closed lake basin excesses and deficits in water input are amplified with a cascading lake effect in the East African Rift Valley and through the Chew Bahir Basin. The final regression from a high stand of >90m above the present lake began at 5.2ka and water level was below 20m by 4.5ka; and for the remainder of the Holocene. This sustained low stand is associated with weakening of the West African Monsoon, a shift of the mean position of Congo Air Boundary west of the Lake Turkana catchment and with meter-scale variability in lake level linked to Walker circulation across the Indian Ocean.