Pollen analysis on a 9.54-m sediment core from lake Chignahuapan in the upper Lerma basin, the highest intermontane basin in Central Mexico (2570 m asl), documents vegetation and limnological changes over the past
23,000
14C yr. The core was drilled near the archaeological site of Santa Cruz Atizapán, a site with a long history of human occupation, abandoned at the end of the Epiclassic period (ca. 900 AD). Six radiocarbon AMS dates and two well-dated volcanic events, the Upper Toluca Pumice with an age of 11,600
14C yr B.P. and the Tres Cruces Tephra of 8500
14C yr B.P., provide the chronological framework for the lacustrine sequence. From ca. 23,000
14C yr B.P. to ca. 11,600
14C yr B.P. the plant communities were woodlands and grasslands based on the pollen data. The glacial advances MII-1 and MII-2 correlate with abundant non-arboreal pollen, mainly grasses, from ca. 21,000 to 16,000
14C yr B.P., and at ca. 12,600
14C yr B.P. During the late Pleistocene, lake Chignahuapan was a shallow freshwater lake with a phase of lower level between 19,000 and 16,000
14C yr B.P. After 10,000
14C yr B.P., tree cover in the area increased, and a more variable lake level is documented. Late Holocene (ca. 3100
14C yr B.P.) deforestation was concurrent with human population expansion at the beginning of the Formative period (1500 B.C.). Agriculture and manipulation of the lacustrine environment by human lakeshore populations appear at 1200
14C yr B.P. (550 A.D.) with the appearance of
Zea mays pollen and abundant charcoal particles.