Holocene precipitation and atmospheric changes inferred from river paleowetlands in the Bolivian Andes

摘要

In the southern tropical Andes, previous studies performed on lacustrine basins display large water-level fluctuations due to changes in the precipitation minus evaporation balance during the Holocene. Significant changes in groudwater levels were also inferred from river paleowetland deposits in northern Chile and the northern Bolivian Altiplano but the discrepancies which appeared in the data from paleolakes and paleowetlands are still not understood. In this paper we present stratigraphy and diatom analyses in new paleowetland records from non-glacial valleys. These data, compared with previous results, show that non-stormy precipitation (recorded by fine and/or organic sedimentation) dominated continuously from ～11200 to ～1500 cal yr BP and that convective rainfall (recorded by strong erosion) occurred only episodically in the northern Altiplano. Convective rainfall was similar to the type of precipitation which nowadays occurs during the rainy season (austral summer) when the tropical easterlies from the Atlantic reach the Bolivian Andes. Non-stormy precipitation is interpreted as a results of an intensification of cold-air incursions from southern polar regions, in good agreement with data from Brazil. The maximum intensity in cold-air incursions occurred during the Early Holocene and coincided with a climatic optimum in Antarctica and a considerably reduced El Nino Southern Oscillation (ENSO). During the Middle and Late Holocene, cold-air incursions decreased and the ENSO frequency increased. We suggest that the interactions between the southern high and low latitudes, by means of cold-air incursions and associated changes in the west wind flow, have been the main mechanisms involved in climatic changes at the latitudes of Bolivia.