LATE- AND POSTGLACIAL HYDROCLIMATIC RECORDS IN A TRANSITIONAL DESERT: THE SOUTHERN GREAT BASIN

The southern hydrographic Great Basin encompasses a pronounced transition: Its southern-most (<34° N lat.) basins lie below 500 m amsl in hyper-arid settings, while those north of ca. 37° N lat. rise above 1500 m amsl and support steppe. The height and size of encircling mountain ranges, and runoff from them, also increase to the north into the central Great Basin. This gradient in elevation of >1000 m over only 5-6° of latitude affected paleoclimate just as it does current local climate. The southern Great Basin also is located closer to sources of maritime tropical moisture, including both the Southwest monsoon, as well as storms embedded in transitory low-pressure systems moving inland from the Pacific. Finally, watersheds of the southern hydrographic Great Basin do not lie in the Sierra Nevada’s rain shadow.

Quaternary geological investigations of Ivanpah Dry Lake, ca. 60 km south of Las Vegas (ca. 35°30’ N lat., 794 m amsl), suggest that local geomorphic processes were more active around this basin than those farther north. This pluvial lake’s stratigraphic record is preserved primarily beyond the edge of the playa, where prograding alluvial fan toes and sand sheets have buried (and protected) both shoreline features and lacustrine strata. Evidence from these suggest multiple episodes of filling and desiccation over the last 13 ka. Some appear to correlate with other Great Basin hydroclimatic records while others, particularly the middle Holocene paleolake(s) (ca. 7.5 – 4.5 ka), appear to correlate instead with flood records from the monsoonal Southwest. Thus, there are both physiographic and meteorological bases to expect that pluvial lake chronologies like Ivanpah, in the southern Great Basin and on the edge of today’s monsoon, may differ from correlative hydroclimatic records in the central Great Basin. Contrasts, as well as concordances, among paleoclimatic records provide crucial keys to understanding the effects of global warming on precipitation patterns and flooding events in North America’s deserts.