LATE PLEISTOCENE-HOLOCENE PALEO-PRECIPITATION RECORDS FROM THE DRYLANDS OF NORTHERN MEXICO

The drylands of northern Mexico belong to the North American Desert system. This desert system comprises Sonora, Chihuahua, Mojave and Great Basin Deserts and spreads over USA and Mexico. Secular variations in lake sediment geochemistry, pollen, diatoms, ostracodes, packrat midden palynology, speleothem growth rate and stable isotope geochemistry and geomorphological indicators of paleohydrological changes have been focussed mainly on the dynamics of winter precipitation and latitudinal displacement of the westerly storm tracks during the late Quaternary. There are very few records indicating the variation in summer precipitation during the late Pleistocene-Holocene. The sediment record of the Baldwin Lake, located in the southern California, suggests expansion of the North American Monsoon (NAM) to 34 °N during 33 ka and 57 ka, respectively. We present the sedimentary registers from the ephemeral paleolakes San Felipe and Babicora as direct proxies to understand the dynamics of NAM during the late Quaternary in the Sonora and Chihuahua Deserts of Mexico. The sedimentary sequences from San Felipe (31 °N) and Babicora (29 °N) cover last 55 ka and 80 ka, respectively. During the present day, both the basins receive dominant summer precipitation by the NAM. The geomorphological and climatological factors indicate that winter runoff has always been minimal during the Quaternary. The Ti record from San Felipe is used as a proxy for summer precipitation in the western Sonora Desert and compared with geochemistry of watershed rocks and aeolian deposits to understand the provenance of the transported sediments. Influx of abundant aeolian sediment during >50 ka suggest arid conditions. Lower but gradually increasing runoff into the basin till 14 ka is comparable to the registers of paleovegetation and lake salinity obtained from pollen and diatoms. In the last 14 ka, the higher and varying Ti suggests dominant but fluctuating summer precipitation in the region. Comparison of Ti with pollen, diatoms and ostracodes from Babicora suggest a dominant regime of summer precipitation during >58 ka and 30-40 ka in the western Chihuahua Desert. In the last 30 ka, the runoff is characterized by high frequency fluctuations and indicates a variable summer rainfall.