Isotopic ratios of rainfall in eastern Africa: insights into reconstructing past climate from terrestrial archives

The timing and intensity of rainfall dominate climate variability in eastern Africa on seasonal, interannual, and precessional timescales. Today, rainfall in eastern Africa is coupled to the position of the Intertropical Convergence Zone and the Congo Air Boundary; major wet and dry intervals during the late Pleistocene and Holocene are viewed with respect to the movement of these convergence zones. Oxygen and hydrogen isotopic ratios of rainfall in eastern Africa today reflect rainfall amount, moisture source and position relative to convergence zones, such that rainfall sourced in the Indian Ocean yields δ18O and δD values that are lower than δ18O and δD values of rainfall from interior sources (e.g., Congo Basin, Sud). An interior, recycled moisture source is likely responsible for the high δ18O and δD values of meteoric waters in Ethiopia today relative to other regions in East Africa. Here we propose that the connections between isotopic composition and moisture source in rainfall today can be used as a template for identifying shifts in moisture source and the position of convergence zones in the past. The isotopic composition of meteoric water is recorded in a variety of terrestrial materials such as soil and spring carbonates, bioapatites, mollusks and plant waxes, which have the potential to document the seasonality, intensity and source of rainfall. Soil carbonates and bioapatite from Pliocene and Pleistocene rift sediments in Kenya and Ethiopia indicate a >4‰ increase in δ18O values of rainfall since 2.0 Ma. In the Turkana Basin of northern Kenya, this record indicates more intense rainfall from the southeasterly monsoon prior to 2.0 Ma. In the Awash Basin of northeastern Ethiopia, low δ18O values in Plio-Pleistocene carbonates and bioapatite likely reflect the input of Indian Ocean moisture, which does not contribute substantial amounts of rainfall to the Awash Basin today. A northwestward shift of the Congo Air Boundary and an intensified southeasterly monsoon would explain the δ18O record from both the Awash and Turkana Basins. Considering the isotopic data from these older sediments, we suggest that migrations in the Congo Air Boundary and intensification of the southeasterly monsoon during the late Pleistocene and Holocene should be accompanied by a predictable decrease (>4‰) in the oxygen isotopic composition of terrestrial materials from these regions.