A COUPLED LACUSTRINE AND ALLUVIAL FAN RECORD FOR THE HOLOCENE OF THE ARID SOUTHERN CENTRAL ANDES

The arid southern central Andes of Chile, a high relief region on western South America at 30°S, contains an untapped record of Holocene climate in lakes developed on the granitic substrate at elevations from 2000 to 4000 masl. Alluvial fans sourced in these catchments affect fluvial deposition along major trunk valleys downstream. Concurrent development of these features has allowed study of landscape response to climate change during the last 12,000 years at the northern fringe of the mobile band of southern Westerlies. We present here data obtained from sedimentology and luminescence geochronology of alluvial fan and fluvial deposits of the Elqui river catchment and from sedimentological, pollen and magnetic and geochemical stratigraphy analyses in two lake corings at elevations of 2900 (El Cepo) and 3850 masl (Cerritos Blancos). Driest conditions at El Cepo from pollen analysis appear between 8.0 and 6.0 ka BP; other dry periods are recorded at 11.0-9.5 and around 4.0-3.0 ka BP. Charcoal analysis show highest values around 7.0 ka BP, decreasing continuously until the present. By comparison, recorded alluvial fan and fluvial events were discrete and regional in extent and do not correspond in timing (occurring ca. 8 and 4 ka BP) or scope to vegetation changes recorded in the lakes. Terrigenous proxies in the lakes are at highest values at 8.3-7.7 ka, 5.5-4.9 ka, 4.4-2.6 ka, and 2.2 ka to present. The latter three intervals are consistent with periods of northern movement of the Westerlies recorded in central Chile, suggesting that the moisture source for storm events lies in the nearby Pacific. The first interval however is chronologically equivalent with thick (>10 m) alluvial fan deposition partially damming the main valleys at 8.2±0.1 and 7.9±0.3 ka BP, at a time when no meridional movement of the Westerlies is recorded south. We hypothesize that this event has easterly moisture sources, linked to displacement south of tropical convection over the eastern Andes, through a teleconnection involving southward displacement of the intertropical convergence zone linked to an increase in the sea surface temperature gradient across the Equator during the 8.2 ka cold event. Our findings have implications to assess a hypothetical response of Andean landscapes to cold northern Hemisphere events like the Younger Dryas.