Lake-Level Chronology and Climate Change in the Southern Bolivian Altiplano

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ABSTRACTThere is perhaps no more visible a record of low-latitude shifts in climate than the hundreds of miles of tufa-encrusted shorelines and dissected diatomites laid down by an enormous (about the size of paleo-Lake Bonneville) paleolake that once occupiedUyuni/Coipasa/Poopo basin in southern Bolivia. Appropriately these deposits have been the focus of decades of research, including several major coring efforts by both American and French teams. These lake records are fuel for a heated debate over the relative role of regional tropical insolation versus global temperature in controlling moisture balance in lakes and glaciers in the past.For all the research effort expended on the lake deposits, the geochronology is still in disarray. In particular, dating of long cores from the Uyuni basin show poor agreement for the critical period 30-10 ka, with one chronology supporting the notion of regional summer insolation control on lake level, and the other flatly contradicting it.A key challenge in relating the lake-core records to such parameters as regionalinsolation and global temperature is that there is no direct physical evidence for lake depth. Such evidence, however, is available in the superb natural exposures of dissected lake deposits, which we surveyed for the first time two field seasons ago. The exposures are so impressive that we are confident that they will provide the key to reconstructing lake depths and lake chronology over the last ~120 ka. Our results so far show that the lake reached high elevations only twice in the last 120 ka, from 17 (?) to 14 ka, and from 108 to 94 ka. We request funds to continue our reconstruction of lake-level variations in the Uyuni/Coipasa/Poopo basin in southern Bolivia. Our intent is to establish a coherentstratigraphy and chronology of lake-level fluctuations using the natural exposures of lake sediments and shoreline tufas, and by both U-Th and 14 C dating of lacustrine tufas and mollusks. We also propose to employ 87Sr/86Sr and 234U/238U ratios in lake carbonates as tools for tracking changes in the proportions of runoff to ground-water inputs to the lake, and to infer lake depths from cores. Our revised lake-level chronology will in turn constitute an important test of the potential linkages between lake-level stages and variations in global temperature and low-latitude seasonal insolation over land and sea.