FLUVIAL RESPONSE AND ITS IMPLICATIONS FOR HUMAN-ENVIRONMENT INTERACTIONS DURING THE MIDDLE TO LATE PLEISTOCENE AND AFRICAN HUMID PERIOD IN ETHIOPIA

The Busidima Formation in the Afar region, Ethiopia, spans the Quaternary and records the cultural evolution of the genus Homo. This paper examines the stratigraphy, geochronology and paleoenvironments of the newly designated Odele Member of the uppermost Busidima Formation (<160 ka), which has received little attention despite being a critical time period on the evolution of early Homo sapiens and its migration out of Africa. The Odele Member ranges from 30 to 40 m thick and spans 160-7 ka, defined at the base by the widespread Waidedo Vitric Tuff (WAVT, ~160 ka). There are two prominent erosional unconformities in the Odele Member, a lower one immediately after the WAVT deposition and > 47 ka; and an upper one involving widespread alluvial fan incision sometime between 25.7 and 12.95 cal. kyr BP. The top of the Odele member is bounded by a black, fine-grained relict soil, the Erole Paleosol, with mollic and vertic properties that has been active since 12 cal. kyr BP. The uppermost Odele Member contains black, organic-rich mats, redox features, reed casts, and semi-aquatic and aquatic gastropods marking wetter conditions during the African Humid Period (AHP) 13-7 ka. Paleo-rainfall estimates suggest that the AHP-aged Erole Paleosol weathered under a climate with mean annual rainfall of 722-780 mm yr^-1, in contrast to today where rainfall is 650 mm yr^-1. A compilation of ¹⁴C ages from aquatic gastropods, carbonized wood and charcoal from the upper Odele Member show wetter and possibly more vegetated conditions during late MIS-3 and the African Humid Period that are tightly coupled with precession-driven summer insolation maxima. Archaeological remains in the Odele Member at Gona appear to be more widespread during these insolation maxima and suggest that human occupation in the region expanded as water became more available in these alluvial fan environments. The Odele Member revises upward the age of the Busidima Formation, showing that it spans into the Holocene and now includes Middle and Late Stone Age archaeological traditions.