LATITUDINAL HYDROLOGIC VARIABILITY ALONG THE EAST AFRICAN RIFT, OVER THE PAST 200 KYR

Abstract:

Within the deep sediments of the large lakes of Africa’s Great Rift Valley are continuous environmental records of remarkable antiquity and fidelity. Not only do stratigraphic sections from these basins extend back millions of years, many of the intervals represented contain high-resolution material of decadal resolution or better. East African lake basins remain sparsely sampled however, with only a few long and continuous records available. Our ability to image the lakes using seismic reflection methods greatly exceeds our opportunities for coring and drilling however; assessing stratal relationships observed in the geophysical data permits powerful inferences about past hydrologic changes. With intensive hydrocarbon exploration work underway in East Africa, industry well data can also help constrain and ground truth basin histories.

Substantial spatio-temporal hydrologic variability is observed in East African basins over the past 200 kyr. Paleohydrological changes in the late Pleistocene and early Holocene are now well constrained in the northern hemisphere East African topics, with widespread aridity and in some cases lake desiccation observed during Heinrich Event 1. A climate recovery followed in the northern hemisphere East African tropics, with the early Holocene African Humid Period a time of positive water balance across most of the rift valley. The paleohydrology of southern hemisphere tropical East Africa is more equivocal, for instance with negligible draw-down of Lake Malawi at HE1.

Whereas these late Pleistocene events represent substantial climate reorganizations, severe droughts during the middle-late Pleistocene (150-65 kyr BP) were far more intense, and produced much more severe drawdowns of Lakes Malawi and Tanganyika. Scientific drill cores, kullenberg cores, and extensive seismic reflection data sets from Lakes Malawi and Tanganyika provide indisputable evidence for lowstands of -500m and -600 m respectively. Climate changes that lowered the levels of the large lakes several hundred meters, or desiccated them completely, were regional in extent and would have undoubtedly impacted our early ancestors and early modern humans, as well as drowning and burying ancient sites of early modern human occupation.