CLIMATE-CHANGE AT THE MONSOON WESTERLY BOUNDARY IN NORTHERN MEXICO/

Northern Mexico lies close to the present day boundary between mid-lat itude (Westerly) and tropical (monsoonal) sources of moisture. Studies from the adjacent southwest USA have shown major changes in lake leve ls and vegetation distributions over the late Quaternary which have be en interpreted in terms of significant variations in the relative stre ngths and positions of these climate systems. Palaeoclimatic data from this area have, however, left a number of unresolved questions which can only be answered by extending work into northern Mexico, closer to the major source of summer (monsoonal) rain, the Gulf of Mexico. Stud ies of palaeolake sediments from a series of hydrologically closed lak e basins across a range of altitudes (1280 to 2200 m a.s.l.) in northe rn Chihuahua are in progress using geochemical, mineral magnetic, diat om and plant microfossil analyses. Preliminary results are presented f rom the Alta Babicora and Encinillas basins. The sites provide records of lacustrine deposition between >11000 and about 2500 yr BP. The dia tom record from Babicora provides clear evidence for a deep water lake in this basin in the late glacial which persisted into the early Holo cene. A dry episode coinciding with the timing of the Younger Dryas is recorded in Alta Babicora. Conditions wetter than present are indicat ed up to at least 7000 yr BP.