Perennial stream discharge in the hyperarid Atacama Desert of northern Chile during the latest Pleistocene.

A large fraction of the vital groundwater in the Atacama Desert of northern Chile is likely composed of "fossil" or "ancient" reserves that receive little or no recharge in today's hyperarid climate. Here, we present evidence for latest Pleistocene perennial streamflow in canyons from the hyperarid core of the Atacama Desert in northern Chile. Fluvial terraces in the Pampa del Tamarugal (PdT) basin (21 degrees S) contain widespread fossil wood, in situ roots, and well preserved leaf litter deposits indicative of perennial surface flow currently absent in these channels. Nineteen radiocarbon dates on these deposits from four separate drainages within this endorheic basin indicate ages from 16,380 to 13,740 cal yr BP, synchronous with paleolake Tauca on the Bolivian Altiplano and other regional evidence for wetter conditions during the latest Pleistocene. Groundwater-fed riparian ecosystems and associated fluvial deposits abound today in the absence of direct rainfall in northern Atacama canyons with perennial discharge. Our relict riparian ecosystems from the PdT basin are indicative of conditions similar to these northern canyons. Given that discharge was higher than present during this time, we propose that these deposits represent the most important groundwater recharge events of the last 18,000 years. A lesser recharge event occurred during the Holocene, when phreatophytic trees also grew in these drainages between 1,070 and 700 cal yr BP, during the Medieval Climatic Anomaly. Taken together, our evidence lends further support for gradient changes in the equatorial Pacific as a major driver of hydrologic change in the Atacama on both centennial and millennial time scales.