Abstract

Spanning both summer and winter rainfall belts, precipitation in the Atacama Desert of northern Chile shifts from 90% summer at 22°S to 90% winter at 26°S and offers an ideal opportunity for reconstructing past climate fluctuations and interactions between these two major features of the climate system. Over the last several years, we have been documenting the timing, seasonality, extent and intensity of past precipitation changes along an 1100 km N-S transect in the hyperarid Atacama Desert, using plant macrofossils and pollen preserved in rodent middens: accumulations of feces, bones, plant remains and insects encased in a crystallized mass of rodent urine (amberat). To date, over 200 14C-dated middens constitute a unique record of vegetation and climate change spanning the past 50,000 yrs. Past precipitation changes are documented by repeated invasions of plant species typical of higher altitude belts into the present hyperarid ‘absolute desert' (lacking vegetation). Our results indicate that a prominent and widespread summer pluvial occurred between 17.5 – 16 and 13.8 – 9.5 kyr BP (thousands of calibrated years before present). Lesser increases in precipitation also occurred during the middle (7.6 – 6.0 kyr BP) and late Holocene (3.5 – 2.3 kyr BP). Displacements in the lower limits of Andean steppe and Puna species indicate that mean annual rainfall may have been more than twice the present values at 17.5 - 16.3 kyr BP. Evidence for increased winter precipitation during the LGM was, however, restricted to our southern sites at 25°30'S ocurring between 25 – 15 kyr BP. Emerging trends from the cumulative midden record across the region indicates that paleoclimate changes were also spatially heterogeneous. For example, abrupt desiccation during the early Holocene, cited as the major cause for the paucity of archaeological sites in the region, occurred progressively earlier farther south. Increased summer rainfall in the Atacama at 17.5 kyr BP is synchronous with onset of ENSO variability in Peru, infilling of paleolake Tauca and Heinrich Event 1. This implies common forcing of past precipitation changes in the region, most likely through changes in equatorial Pacific sea surface temperature gradients at a time of major southern hemisphere deglaciation.