THE PLEISTOCENE-HOLOCENE TRANSITION IN SUBTROPICAL NORTH AND SOUTH AMERICA

Paleoindian occupation in the American subtropics happened in dynamic environments adjusting to step changes in climate, hydrology and biota. Changes in hydrology and vegetation were not necessarily in phase within or across regions, and thus may have produced human responses that were fundamentally different. My presentation will compare and contrast two well-studied areas, the southwestern USA and the Atacama Desert, which today respond asymmetrically to Pacific climate and would have responded differently to asymmetric variations in seasonal insolation during the Pleistocene-Holocene (P/H) transition.

The P/H transition has been well studied in the southwestern USA, where cool season precipitation generally increases (decreases) during El Niño (La Niña). Pluvial lake and ground-water levels dropped dramatically at 14 cal kyr BP, but vegetation held steady until ~13 cal kyr B.P. around the time of the major megafaunal extinction. Holocene deserts replaced Pleistocene woodlands, and in return Holocene woodlands replaced Pleistocene forests. If Vance Haynes is correct about the Clovis megadrought, vegetation change entailed massive tree dieoffs, widespread insect outbreaks, and catastrophic wildfires in forests above 2000 m. These disturbances cleared the way for woodlands of ponderosa pine, and must have translated pulses of sediment from the mountains to the lowlands. The resulting packets of charcoal and sediment have yet to be identified, even though they may incorporate Paleoindian remains.

The Atacama Desert harbors numerous Paleoindian/early Archaic sites, and boasts a surprisingly complete paleoclimatic record. Precipitation occurs mostly in the austral summer (DJF). Wet (dry) summers are associated with cooling (warming) of the tropical Pacific, shrinking (expansion) of the tropical troposphere and enhanced (impeded) easterly flow from the Amazon lowlands in response to changes in the meridional pressure gradient between tropical and subtropical latitudes. Lake level histories, plant assemblages from rodent middens, and mapped and dated spring deposits indicate 13.5-9.6 cal kyr BP as the maximum summer wetness, when perennial vegetation actually invaded areas that are now absolute desert and there was an impressive concentration of Paleoindian/early Archaic populations.