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Fourteen organic-rich sedimentary layers in the deposits at Quebrada de las Burros, in coastal southern Peru (Tacna department), lie between two debris-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-grained and low-energy sediments of this deposit during the mid-Holocene is incompatible 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 increased condensation of fog at midaltitudes, These results suggest a lower intensity 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 increased soil moisture episodes on the coast, implying a long-term difference in climate between these two regions, (C) 1999 University of Washington.