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

Abstract

Abstract 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 14 C 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 14 C yr B.P. and the Tres Cruces Tephra of 8500 14 C yr B.P., provide the chronological framework for the lacustrine sequence. From ca. 23,000 14 C yr B.P. to ca. 11,600 14 C 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 14 C yr B.P., and at ca. 12,600 14 C 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 14 C yr B.P. After 10,000 14 C yr B.P., tree cover in the area increased, and a more variable lake level is documented. Late Holocene (ca. 3100 14 C 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 14 C yr B.P. (550 A.D.) with the appearance of Zea mays pollen and abundant charcoal particles.