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 Southwest Indian Monsoon System, forced by northern hemisphere insolation variations in the precession band and/or glacial boundary conditions, is suggested as the mechanism for the abrupt shift to more arid conditions. Geoarchaeological evidence suggests that agriculture was more widespread and evolved alongside the development of irrigation technologies during a period when rainfall was more plentiful than today. Here we investigate the surficial record of the dynamic fluvial response to the late Quaternary climate shift and reconstruct the geochronology of the geomorphic evolution of a significant portion of the ca. 125 km length of Wadi Sana, a north-flowing tributary to the Wadi Hadramout system. Using differential-corrected GPS-based survey, combined with 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 ivchanging 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 12,000-7,000 years ago and continued aggrading until about 5,000 years ago. Paralleling the climate shift, Wadi Sana began incising and eroding the thick sediment infilling about 4,500 years ago, which has continued to the present time. Field reconnaissance and map analysis reveals structural and lithologic controls on the source and availability of these fluvial sediments for downstream deposition during the late Pleistocene and Holocene. Hydrologic modeling of active present-day channels within Wadi Sana estimates stream velocities at 2.2 m/s and stream discharges of 444 m3/s. We propose that a change in hydrologic regime, driven by the monsoon shift, is the cause of the middle Holocene channel adjustment from an aggradational to incising mode in Wadi Sana.

Tufa and lacustrine deposits are useful paleoclimate archives in reconstructing the early to middle Holocene climate and paleohydrology of southern Yemen's Wadi Idim and Wadi Sana, which are north-flowing tributaries to Wadi Hadramawt. Numerical age estimates and oxygen-isotopes are used to assess the onset and cessation of tufa formation and reconstruct the environment of lacustrine sediment deposition in the region in order to understand the broader early to middle Holocene hydrologic system.Numerical age estimates from the studied wadis show a correspondence between early to mid-Holocene humid-phase sediment deposition and the northward shift of the ITCZ, as documented in paleoclimate records from other East Africa -- Arabia -- India continental and marine sediments. The interval between ca. 10-5 ka B.P. corresponds to a period of greater availability of moisture from the Arabian Sea region. Increases in precipitation allowed for a lake and wetland systems to develop, and increased spring discharge contributed to the formation of the tufa. Within the lacustrine sediments are ostracodes, mollusks, and flora casts that are found in a much wetter climate compared to today's hyper-arid environment. This early to mid-Holocene humid phase corresponds with a more northerly positioned ITCZ, which shifted south to its present day position around 5,000 yr B.P.Oxygen isotope measurements from ostracods show a range of isotope values from ~ -4.0%â?? at approximately 10 ka B.P. to ~ -6.0%â?? at approximately 5 ka B.P. Theses values represent the early to middle Holocene pluvial phase. Changes in the oxygen isotopic signature represent a change in evaporation or a possible change in source.The early to middle Hol ocene humid phase also corresponds with periods of agricultural activity, which are being investigated by the archaeological team of the Roots of Agriculture in Southern Arabia Project (RASA). Research into the effects of climate change on human activities, specifically agricultural processes, is the focus of RASA. Southern Arabia offers not only a convergence of three major agricultural regions, but also preserves a sedimentary record of the climate shift that affected the region during the period of study.