STROMATOLITES SUGGEST TIMING, DURATION, AND CLIMATE DYNAMICS OF THE TERMINAL AFRICAN HUMID PERIOD AT CHOW BAHIR, ETHIOPIA

The most recent African Humid Period, canonically ca. 15-5 ka, demonstrates that influential regional climate processes such as the East African-Asian monsoon and the Congolese Air Mass migration are teleconnected to global insolation with dramatically nonlinear feedbacks; and that equatorial climate dynamics are different from mid- and high-latitude responses across the last deglacial transition. Determining the spatio-temporal character of those feedbacks is critical for understanding extremes of wetness and drought; for realising general circulation model outcomes as specific regional climate and vegetation responses; and for tracking human movements within and out of Africa. Existing records support a debate as to whether, and when, the recent African Humid Period terminated abruptly or diminished gradually. These records often are imported from sites distant from the East African Rift Valley nexus of atmospheric air mass boundaries and H. sapiens cultural development. We report geomorphologic discovery of a palaeo-lake margin in Chow Bahir, the southernmost rift valley of the Ethiopian Dome, and radiocarbon age data on freshwater river oysters and encrusting stromatolites which constrain the timing and duration of African Humid Period termination. These data also suggest earlier, wetter stages, decadal climate fluctuations, and multi-decadal drought cycles during the waning AHP. The stromatolites document semi-annual growth increments, continuously growing by centimeters over millennial timescales and recrystallizing geologically rapidly. This reveals aspects of stromatolite growth fundamental to interpreting palaeo-environments in Deep Time as well.