LATE PLEISTOCENE ORGANIC REMAINS IN PAMPA DEL TAMARUGAL, NORTHERN CHILE (21^OS): IMPLICATIONS FOR PAST PRECIPITATION CHANGES IN THE WESTERN PRECORDILLERA

The Pampa del Tamarugal, an endorheic basin located in the Atacama Desert of northern Chile, is today one of the driest regions of the world. Our study represents the first evidence of late Pleistocene climate fluctuation observed in the hyperarid core of the Atacama Desert. Sediments become unconfined at the distal reaches of dry stream channels at elevations <1500 m where several time correlative strath terraces have been identified. These most likely represent regional fluctuations in stream discharge from higher elevations of the Precordillera to the east during the late Quaternary. The strath deposits have aggraded up to 10m of sediment before abandonment and subsequent incision. They contain abundant organic remains such as leaf litter and logs >50 cm in diameter. A radiocarbon date from Escallonia angustifolia (Saxifragaceae) leaves yielded an age of 12,244 ± 96 ¹⁴C yr B.P. Evidence for in situ growth upstream of this deposit includes root casts and rhizoliths. These fluvial deposits are in contrast to both older and younger deposits which are dominated by matrix-supported sheetwash facies, representative of sporadic flood events. Lack of alteration of older surfaces immediately adjacent to the organic-rich deposits argues against a local increase in precipitation. Rather, this temporary shift to semi-permanent running water in the canyons represents a period of increased precipitation at higher elevations to the east and consequent increase of local groundwater tables. Multiple straths, both with and without preserved macrofossils, suggest multiple stages of enhanced precipitation with variable amplitude fluctuations. These deposits are found in streams with no headwater connections to the high elevation of the Altiplano or volcanic chain of the Western Cordillera. Consequently, this would imply either direct precipitation increases over the Sierra Moreno (Precordillera) or alternatively, increased groundwater flow from a more distant Andean source further to the east. Our preliminary results imply that increased precipitation and associated rise in groundwater tables and/or direct runoff may have had considerable impact in the lower reaches of the Pampa del Tamarugal through the establishment of riparian and phreatophytic vegetation in areas that today are absolute desert.