El Nino variability in the coastal desert of southern Peru during the mid-Holocene

Fourteen organic-rich sedimentary layers in the deposits at Quebrada de las Burros, in coastal southern Peru (Tacna department), lie between two debri s-flow units, interpreted to result from El Nino events, at 8980 cal yr B.P . and after 3380 cal yr B.P., respectively. The accumulation of the fine-gr ained and low-energy sediments of this deposit during the mid-Holocene is i ncompatible with the occurrence of Fl Niiio events in this region, as these would produce catastrophic flood deposits. The occurrence of organic-rich sediments and evidence of an enhancement of upwelling strength at this time imply the existence of a permanent water supply resulting from an increase d condensation of fog at midaltitudes, These results suggest a lower intens ity and, perhaps, a lower frequency of occurrence of the El Nino phenomenon during the mid-Holocene, It is precisely during this period that the most important human settlements are found at this site, probably indicating the presence of reliable supply of fresh water. The chronologies far wetlands in the central south altiplano are out of phase with those indicating incre ased soil moisture episodes on the coast, implying a long-term difference i n climate between these two regions, (C) 1999 University of Washington.