Book Description

10Be terrestrial cosmogenic nuclide dating of alluvial surfaces in the Coachella Valley, southern California, shows two alluvial fan surfaces with ages of 124.2{plusmin}6 ka (KOFo) and 80.1{plusmin}3.5 ka (KOFi), with two additional probable depositional episodes at 60 ka (KOFy2) and 32 ka (KOFy1). When combined with additional fan dates from the Mojave and Sonoran Desert climatic regimes, statistically relevant episodes of fan production in southern California and Baja California were identified by mean standard weighted deviance analysis. Three major episodes have been identified at 73.27{plusmin}2.02 ka (episode 1), 42.4{plusmin}1.1 ka (episode 2) and 36.5{plusmin}0.8 and a possible episode (1b) at 60 ka. These correlate well with the first 2 ka of glacial advance with the Sierra Nevada as established by dating of moraines and rock flour analysis of lake cores from Owens Lake. These data suggest that regional climate rather than global climate or local tectonics influences the production, transport, and deposition of sediment onto fan surfaces in the Sonoran and Colorado Deserts. Regional data on the ages of fan surfaces highlights a temporal link between the initiation of glacial advance and fan production. Global climate changes initiate Sierra Nevada glaciations which in turn produce a response in sedimentary processes over regional-scale areas. This suggests changes in regional climate initiate fan production when sediment is mobilized due to increased rainfall, before vegetation anchors sediment. Furthermore, the model focused upon transitions into more humid climate appropriate for the Sonoran and Colorado Deserts may be applicable to areas of similar arid climate where precipitation levels clearly control the abundance of vegetation but not globally, and so a unifying global model for fan formation may not be possible. The relationship between humidity and the presence or absence of vegetation is the key control in the Sonoran and Colorado Deserts. It is possible that during some humid periods the amount of precipitation could override the anchoring ability of vegetation bringing additional material into valleys from mountain slopes.