GEOMORPHIC RESPONSE TO MIDDLE HOLOCENE CLIMATE CHANGE IN HIGHLAND SOUTHERN YEMEN

Middle Holocene climate change forced significant environmental response and influenced human activities throughout southern Arabia. Climate models and proxy data indicate that climate along the southern Arabian peninsula changed from a moist phase, spanning the early to middle Holocene, to an arid phase, which persisted for the last ca. 5,000 years. A weakening and southward shift of the East African-Indian monsoon system, forced by insolation variations, is suggested as the mechanism for the abrupt shift to more arid conditions. Abundant archaeological evidence suggests that agriculture was much more widespread and evolved alongside the development of irrigation technologies during a period when rainfall was more plentiful than today.

Through geoarchaeological survey we have investigated the surficial record of the dynamic fluvial response to the mid-Holocene climate shift and documented a rich archaeological record of neolithic artifacts, human settlement, and water management structures through the ca. 80km length of Wadi Sana, a significant north-flowing tributary to the Wadi Hadramawt system. Using differential-corrected GPS-based survey, combined with field and lab analysis of the sedimentary record, the RASA (Roots of Agriculture in Southern Arabia) Project has created a paleohydrologic reconstruction of Wadi Sana in order to provide a context for understanding how fluvial landscapes, hydrologic regime, and human activity reacted to changing middle Holocene climate. Radiocarbon and luminescence dating of remnant silt terraces suggests that fine-grained sediment began accumulating on an older (late-Pleistocene?) coarse cobble surface between 14,000-8,000 years ago and continued aggrading until about 5,000 years ago. Paralleling the climate shift, Wadi Sana began incising and eroding thick sediment infilling about 4,500 years ago, which has continued to the present time. Our primary hypothesis is that a change in hydrologic regime, driven by the monsoon shift, is the cause of channel adjustment from an aggradational to incising mode in Wadi Sana. Additional paleohydrologic modeling will quantify paleoflow, paleodischarge, and provide estimates of sediment flux in the Wadi Sana system.