Graduation Year

2007

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Eric A. Oches, Ph.D.

Committee Member

Peter Harries, Ph.D.

Committee Member

Mark Rains, Ph.D.

Keywords

Yemen, RASA, Wadi Hadramawt, paleoclimate, geochronology, paleohydrology

Abstract

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.

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