23,000 yr of vegetation history of the Upper Lerma, a tropical high-altitude basin in Central Mexico

Pollen analysis on a 9.54-rn sediment core from lake Chignahuapan in the upper Lerma, basin, the highest intermontane basin in Central Mexico (2570 in asl), documents vegetation and limnological changes over the past similar to 23,000 C-14 yr. The core was drilled near the archaeological site of Santa Cruz Atizapan, 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 C-14 yr B.P. and the Tres Cruces Tephra of 8500 C-14 yr B.P., provide the chronological framework for the lacustrine sequence. From ca. 23, 000 C-14 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. 2 1,000 to 16,000 C-14 yr B.P., and at ca. 12,600 C-14 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 C-14 yr B.P. After 10,000 C-14 yr B.P., tree cover in the area increased, and a more variable lake level is documented. Late Holocene (ca. 3100 C-14 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 C-14 yr B.P. (550 A.D.) with the appearance of Zea mays pollen and abundant charcoal particles. (c) 2005 University of Washington. All rights reserved.