LATE-QUATERNARY HYDROLOGY AND PALEOCLIMATOLOGY OF LAKE TURKANA, KENYA FROM COMPOUND-SPECIFIC HYDROGEN ISOTOPES

Lake Turkana (3.5°N, 36°E) is the largest lake in the world in a desert with annual cumulative rainfall of <200 mm over the lake surface. However, paleoclimate proxies from sediment cores suggest that Lake Turkana’s climate was much more humid in the past. A ~20 ka record of precipitation from the hydrogen isotope ratio of terrigenous leaf waxes (dD_wax) from sediment cores shows significant variation in the amount of isotope enrichment over this interval. Differences in enrichment are dependent upon the amount of rainfall in the arid tropics as well as moisture source and transport history. The dD_wax values range from -69 to -143‰. The most depleted values occur during the period of deglaciation in the northern hemisphere where values range between -90 and -69‰. The record exhibits the most significant enrichment, near -130‰, from 13 to 11 ka during the more humid Bølling-Allerød period and enrichment of up to -143‰ between 11 and 8.3 ka, a time when many tropical African climate records display more humid conditions. At 11.1 ka, depleted dD_wax of -112‰ occurs just after the Younger Dryas. At 8.1 ka, isotope values are more depleted at -93‰ for a short period of time, which occurs just after a cooling event at high latitudes. After 8.1 ka, values return to near -130‰ followed by a slow return to more depleted values in the mid Holocene. After 5 ka, the record is erratic but trend toward more depleted dD_wax values. The general trend of the record mimics average daily summer insolation of the catchment, which is the main driver of the migration of the Intertropical Convergence Zone (ITCZ). The amount of enrichment of the dD_wax values is controlled by the amount of rainfall delivered by the ITCZ. The isotope composition the ITCZ air mass is influenced by ice volume and moisture gradients between the Indian Ocean and Africa. Anomalies in this precipitation record at 11.1 and 8.1 ka, however, show evidence for a northern hemisphere influence during times of increased cooling at high latitudes. While data density is not sufficient enough to identify a definitive lead or lag in these events, it does suggest that northern hemisphere climate forcings are present and identifiable in near-equatorial climate records that are otherwise more closely linked to the ITCZ.