Holocene hydrological and climatic changes in the southern Bolivian Altiplano according to diatom assemblages in paleowetlands

This paper presents the first Holocene continuous record from the southern Bolivian Altiplano. In this area, the climate is now characterized by weak summer monsoon rains. The record is located north of Salar de Uyuni in a non-glacial valley (Rio Baja). Between ∼11 600 and ∼2210 cal year BP, the rivers accumulated fine deposits, while under the present climatic conditions, the fine particles are carried downstream by strong water floods. These deposits contain a rich diatom flora showing that the valley floor was occupied by paleowetlands. Water input needed to be more or less continuous to explain that the paleowetlands survived over a long period of time. We show that diatoms can be used to reconstruct the relative variations in the water level and the salinity throughout time, despite of the spatial complexity of this type of environment. During the Holocene, the water level was low except during some periods, dated ∼11 600–9800, ∼6330–5300, and ∼3110–2210 cal year BP. Saline and freshwater microhabitats were simultaneously present in the valley floor as indicated by a mixed diatom flora evidenced throughout the record. We propose a paleoclimatic scenario based on the assumption that the NE wet atmospheric flow of the monsoon was replaced by the westerlies of the southern hemisphere at the latitude of the study site.