River incision, circulation, and wind regime of Pleistocene Lake Bonneville, USA

a r t i c l e i n f o Pleistocene Lake Bonneville of the western U.S. and its associated alluvial systems present a unique opportunity to understand the relationship between prevailing winds of the time, lake circulation, and river incision. The lake underwent a catastrophic flooding event ∼ 18,300 yr B.P. resulting in the incision of streams entering the lake along its eastern border. Incision patterns of twelve streams and rivers suggest that they were influenced by the prevailing circulation in the lake at the time. In order to match patterns of river incisions, simulations of lake circulation were performed with a state-of-the-art numerical model for the maximum transgressive (Bonneville) lake elevation. Simulations were conducted using the forcing of westerly and easterly prevailing winds. Simulated circulation can be described in terms of simple geostrophic balances in which the currents are generally cyclonic (counter clockwise) for westerly winds and anti-cyclonic (clockwise) for easterly winds. Irregularities in lake shorelines and bathymetry cause localized variation of this general pattern. Comparison of model output with the deflected stream incision patterns suggest that prevailing winds during the Pleistocene in the Great Basin of North America were westerly and that, unlike the interior of North America, the continental ice sheet did not exert significant influence on the climatological wind patterns of the Great Basin. Paleoclimate studies have long benefited from a symbiotic relationship between numerical models and paleoclimate proxies. While this relationship has worked particularly well for understanding climate variables such as precipitation and temperature, field evidence of paleowind direction and magnitude is more difficult. Analysis of dunes and associated sedimentary features is the most obvious and widely employed methodology (e.g., Allen, 1982; Kocurek, 1999). Fallen trees can provide indications of ancient wind directions where they are preserved in the geologic record (Allen, 1998). Ancient pyroclastic deposits sometimes have asymmetric thickness that can be used to infer wind direction (Fisher and Schmincke, 1984; Oviatt and Nash, 1989). More recently, isotopic and geochemical analyses of loess and dune deposits have shown promise in understanding the provenance and hence, wind direction (Aleinik-off et al., 2008; Muhs et al., 2008). The sediments, shorelines and geomorphology of lakes have occasionally been used to estimate wind strength (Krist and Schaetz, 2001; Adams, 2003). Both field and modeling studies suggest that the wind regime during the last glacial maximum (LGM) was considerably different from that of the modern atmosphere. General …