Abstract

Plant macrofossils, grass abundance and plant cuticle analyses from forty-nine radiocarbon dated fossil rodent middens record past changes of local vegetation and precipitation for the last 45 kyr B.P. on the highlands of Chile's central Atacama Desert (22-24 degrees S). Midden sites are located on the upper margin of an absolute desert (2400-3100 m; no vascular plants), which gives way to a sparsely vegetated zone dominated by xeric annuals and halophytic shrubs at higher elevations. Low species richness and few annuals from five middens indicate dry, and possibly cooler, conditions between 45-35 14C kyr B.P. and at 22 cal kyr B. P. A prominent descent (600-1000 m) of steppe grasses, along with abundant C4 grasses/summer annuals, and high species richness characterize middens between 16.2-10.5 cal kyr B.P., suggesting a strong increase in summer rainfall. Drier conditions followed, as middens between 10.5-7 cal kyr B.P. lack summer annuals and exhibit low species richness. Slightly wetter conditions between 7-3 cal kyr B.P. are indicated by increased grass abundance, species richness and presence of extra-local taxa. Present conditions were established after 3 cal kyr B.P. Present-day variability of summer precipitation in the central Atacama Desert reflects the strength of the South American Summer Monsoon (SASM), which is driven by the intensity and position of the Bolivian High and convection over the Amazon Basin. Summer insolation was at its minimum over the central Andes (20 degrees S) during the late Pleistocene-early Holocene when our record indicates an intense pluvial phase, and thus cannot solely account for intensified SASM activity. We conclude that the late Pleistocene-early Holocene pluvial occurred at a time of intensified SASM, in response to extra-regional forcing (increased convection over tropical East Asia) and enhanced Walker circulation with persistent La Nina-like conditions.