LATE PLEISTOCENE CLIMATE IN SEDIMENTS OF LAKE CHALCO, BASIN OF MEXICO

Sediment cores (60.90 to 122.36 mblf, representing ~100 to 220 ka) collected from Lake Chalco at the southern limit of the Basin of Mexico were analyzed by scanning XRF to construct a regional late Pleistocene paleoclimate record. Signals of varying aridity, humidity, and volcanic activity were interpreted from XRF measurements of Sr:Ca, Si:Ti, Ca, K:Ti, Fe, and magnetic susceptibility records. Volcanic activity heavily influenced the climate of the region over sporadic periods, often covering the landscape with layers of ash. Fe, magnetic susceptibility, and K:Ti record the frequency of pyroclastic volcanic eruptions within the Trans-Mexican Volcanic Belt. Si:Ti functions as a proxy for diatom abundance and productivity throughout the record. Because Lake Chalco is essentially a closed basin, sedimentary calcite (XRF Ca) records the delivery of dissolved calcium via enhanced precipitation. In contrast, during two episodes a lack of carbonate preservation, coupled with Sr:Ca (high in silicates, low in carbonates), indicates that Lake Chalco briefly switched to open basin conditions, probably draining northward within the Basin of Mexico. The Ca record also appears to respond to precessional forcing, with enhanced rainfall at times of greater insolation. Collectively these data show uses for various geochemical methods and their application to constructing paleoclimate records of lacustrine sediments and the surrounding local regions.