DESERT WETLANDS AND THE YOUNGER DRYAS COLD EVENT IN THE MOJAVE DESERT AND SOUTHERN GREAT BASIN

The Younger Dryas (YD) cold event has long been recognized as one of the preeminent abrupt climate change events of the recent geologic past. It began at 12.9 ka with the warm-to-cold onset occurring over just a few decades and the cold-to-warm termination at 11.7 ka taking place over decades to years. We have discovered and studied paleowetland deposits at sites throughout the Mojave Desert and southern Great Basin that firmly date to the YD based on radiocarbon dating of charcoal, organic-rich sediment, and Succineidae shells. The YD cooling is expressed in various ways within the deposits: (1) organic-rich silts/clays informally referred to as "black mats" (Piute Valley, CA), (2) a classic sequence of green silts/clays overlain by black mats and topped with tan silt (Las Vegas Valley, NV; Corn Creek Springs, NV; Cactus Springs, NV; Valley Wells, CA), (3) multiple black mats interbedded with alluvial, colluvial, and/or eolian sediments (Dove Spring, CA; Little Dixie Wash, CA; Mesquite Springs, CA), and (4) thick green silts/clays overlain by multiple black mats interbedded with alluvial sediments (Death Valley, CA). Each of these sequences represents climatic conditions that were wetter/cooler than what prevailed both before and after, with the lithologic differences determined by their geologic settings, hydrologic characteristics, and especially sedimentation rates. The spectacular and newly discovered set of YD-age deposits of the Rogers Beds in the northern part of Death Valley is especially noteworthy as the sediments contain evidence of marshes that persisted for several centuries in the lower part of the section and are overlain by alternating black mats and alluvial sediments that represent climatic fluctuations that occurred on century to decadal timescales. This sequence is similar and corresponds temporally to that observed in YD marine and lacustrine sediments in the North Atlantic in which stable, cold/wet conditions in the early part of the cold event gave way to unstable, oscillating conditions in the latter part. In addition to being an important source of climatic and hydrologic information, the paleowetland deposits at these sites often contain Pleistocene megafaunal remains, which affords researchers the opportunity to study these faunas prior to the terminal extinction.