Atmospheric circulation patterns during late Pleistocene climate changes at Lake Malawi, Africa

The climate of tropical Africa transitioned from an interval of pronounced, orbitally-paced megadroughts to more humid and stable conditions approximately 70,000years ago (Scholz et al., 2007). The regional atmospheric circulation patterns that accompanied these climatic changes, however, are unclear due to a paucity of continental paleoclimate records from tropical Africa extending into the last interglacial. We present a new 140-kyr record of the deuterium/hydrogen isotopic ratio of terrestrial leaf waxes (δD_wax) from drill cores from Lake Malawi, southeast Africa, that spans this important climatic transition. δD_wax shifts from highly variable and relatively D-depleted to more stable and D-enriched around 56ka, contemporary with the onset of more humid conditions in the region. Moisture source and transport history dominate the δD_wax signal at Lake Malawi, with local rainfall amount playing a secondary role for much of the paleorecord. Analysis of modern moisture sources for Lake Malawi suggests that D-depletion of waxes during the megadroughts may have been caused by an enhanced contribution of the drier, D-depleted air mass currently located in central southern Africa to the Lake Malawi catchment. This D-depleted air mass is associated with the descending limb of the Hadley cell, which implies significant changes in the Hadley circulation during the megadroughts and related changes in the position of the Intertropical Convergence Zone over Africa. These findings demonstrate the ability of δD_wax to serve as an atmospheric tracer when used in conjunction with additional proxy records for moisture balance, and elucidate potential mechanisms for pronounced hydrological change in southeast Africa during the late Pleistocene.