A Progressively Wetter Climate in Southern East Africa Over the Past 1.3 Million Years

Abstract:

We present a 1.3 million year record of hydroclimate in the basin of Lake Malawi, the second deepest lake in Africa, located at ~10 - 15ºS latitude in the East African Rift Valley. The lake is ~550 km long, has a maximum depth of 706 m, and is presently anoxic below ~200 m. While the lake is an open basin today with outflow through the Shire River at its southern end, the surface of Lake Malawi has dropped well below the elevation of its outlet on several occasions in its past. We examined a 380 m sediment sequence taken from a water depth of 590 m, from Cores MAL05-1B and MAL05-1C of the Lake Malawi Drilling Project. Sediment samples were analyzed for the carbon isotopic composition of the C₂₉ – C₃₃ n-alkanes derived from fossil leaf waxes, which primarily reflect the relative abundance of C₃ (mostly trees and shrubs) and C₄ (mostly grass) vegetation, i.e., relatively humid or arid conditions, respectively, in the lake basin. The δ¹³C_wax  record portrays a transition from a highly variable and predominantly arid climate prior to 900 ka to a progressively more humid environment after the Mid-Pleistocene Transition, dominated by 100 ky cycles consisting of warm, wet interglacial periods alternating with relatively cool, dry glacial periods. This shift towards more humid conditions in the Lake Malawi basin contrasts with the well-documented progression towards a more arid environment in North Africa over the same period, as reflected in the carbon isotopic record of soil carbonates and in dust records from marine sediment recovered from the Atlantic Ocean and Gulf of Aden. Aridification in the Horn of Africa has been attributed to a cooling of the Indian Ocean. Model results suggest that this would be accompanied by a weakening of a localized Walker circulation over the Indian Ocean, less ascending air over the western Indian Ocean and coastal Africa, and more precipitation in the Rift Valley.