A late Quaternary paleolimnological history from the Mexican highlands has been obtained by diatom analysis of short cores and stratigraphic sections of lake and marsh sediments from the Cuenca de Me??xico, the large, endorheic, graben basin that holds Mexico City. The records, dated by radiocarbon and tephrochronologic methods, extend back to about 30 ka BP and document the presence of extensive, saline lakes in the basin until 25 ka BP. Thereafter, lake levels fell and marginal sites became shallow and fresh under the influence of surficial drainage and (especially) spring discharge. A shallow, saline lake existed 18 ka BP in Texcoco, the central and lowest basin in the system, reflecting moderately increased effective moisture at that time. By 16 ka BP, Lake Texcoco had become so shallow that diatoms were no longer preserved. However, marginal sites nourished by spring flow recorded changes in the local hydrologic balance resulting from increased infiltration at higher elevations. These changes appear to coincide with glacial advances between about 14 and 10 ka BP on the volcanic mountains surrounding the basin. Dry climates with reduced infiltration characterized the early Holocene, but by 5 ka BP a modest increase in precipitation established the modern climatic regime. These lacustrine records offer important insights for evaluating the paleoenvironmental history of the Cuenca de Me??xico based on other evidence. They confirm glaciological, stratigraphic and palynologic data that suggest dry climates and the absence of large pluvial lakes in the Cuenca de Me??xico during and after the full glacial, but document climates of significantly increased precipitation at least 10 ka prior to 18 ka BP. ?? 1989.
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Late Quaternary lacustrine paleoenvironments in the Cuenca de Me??xico