A 15,000-yr paleo-environmental record from Lake Chapala, west-central Mexico

A 27.15-m-long core from Lake Chapala, west-central Mexico (1520 masl) possesses a continuous 15,000-year record of sediment accumulation with abundant diatoms, pollen, charcoal particles, and organic and inorganic carbon. Analyses of multiple sediment variables revealed the lake was deep, dominated by Stephanodiscus niagarae, and was surrounded by temperate forests of Pinus, Quercus, and Alnus in the late Pleistocene (14,700-12,760 cal yr BP). During the Pleistocene-Holocene transition (12,760-9660 cal yr BP), the lake level dropped and a temperate forest of Pinus and Quercus occupied the watershed and was persistently affected by fire. During the earliest Holocene, Quercus nearly disappeared and Pinus, relatively abundant, declined abruptly. Pollen of dry forests became more common, indicating the onset of drier conditions in the region. During the interval from 7500 to 6500 cal yr BP, more humid climate conditions prevailed and abundant Surirella spp. diatom valves were well preserved. Sediments of the time also contain Stephanodiscus niagarae and Ulnaria acus, suggesting deep-water conditions. Between 6500 and 2000 cal yr BP, the lake experienced higher conductivity and longer water residence times. The Pinus and Quercus forests began to be replaced by open vegetation, with Asteraceae and Poaceae dominating the pollen spectrum. Lake level fluctuated, but overall declined. Between the interval of ca. 2 ky to 0.5 cal yr BP basin conditions remained relatively wet. During the last similar to 0.5 kyr BP to present day, the lake level declined and ionic concentration increased, as inferred from representation of planktonic and benthic saline diatom taxa and TIC increases. Numerous micro-tephras are common in this interval; additionally, frequent fires favored the dominance of Poaceae. Paleolimnological records suggest that after the Last Glacial Maximum, Chapala and other lakes of west-central Mexico maintained relatively high water levels. Some records from the region showed evidence of increased humidity beginning ca. 8200 cal yr BP, a consequence of strength intensification in North American Monsoon, associated with ENSO variability and/or tropical cyclones. The record from the last 0.5 cal yr BP in Lake Chapala is similar to records from other lakes in Central Mexico, as it displays a trend toward drier conditions with a more variable precipitation regime.