The influence of climate on hillslope development in arid regions : an examination of eolian accumulation and pedogenesis
Brody, Saisha Althea Taylor
May 8, 2019
Department or Program
Geology - Environmental Studies
A pronounced period of alluvial fan aggradation in the Mojave Desert is linked to a period of climatic change associated with the Pleistocene-Holocene transition. Quaternary climatic variations affected erosional and/or depositional modes of operation of streams that deposited or entrenched alluvium in response to changes in their watersheds. One such potential change could be hillslope development; hillslopes are the primary source of sediment production. Currently, the response of hillslopes to climatic changes is not well understood. An east-west oriented drainage basin in the upper elevations of the Mojave Desert provides the opportunity to study the timing and controls on hillslope processes. This study examines the origin of hillslope sediment as a means to better understand responses of hillslopes to climatic changes. Samples collected on the north and south-facing slope allow us to use a space for time substitution to understand hillslope development on slopes of variable wetness. X-ray fluorescence and grain size analysis was performed to determine the relative importance of eolian dust and weathered bedrock in the total amount of colluvium. Our results display a distinct bimodal grain size distribution, one being fine sediment (~40 μm) and the other coarse sediment (~515 μm). These two modes have distinctly different chemical compositions, indicating that one is eolian dust and the other weathered bedrock. The north-facing slope is dominated by the finer grain size fraction, thus, differences in climate effects the ability of hillslopes to effectively entrap eolian materials. North-facing slopes that receive more moisture display denser vegetation communities and as a result, more eolian fines in the soil. Our results imply the necessity of reassessing the response of hillslopes to aridification resulting from global climate change, with implications for ecosystem functions in arid regions of southwestern North America and other equally vulnerable areas of the world.