Palynological and magnetic susceptibility records of Lake Chalco, central Mexico (original) (raw)

1994, Palaeogeography, Palaeoclimatology, Palaeoecology

Pollen and magnetic susceptibility data from a 11.27-m-long core drilled in the central part of Lake Chalco (99°00'W, 19°15'N), located at the southeastern part of the Basin of Mexico, are interpreted in terms of paleoenvironmental changes. The record of this subtropical highland lake, dated by radiocarbon methods, spans approximately 24,000 yr B.P. and documents changes in climate, vegetation, lake level and volcanic activity. Seven palynozones were established by using the method of cluster analysis for correlation purposes. Zones 7, 6 and 5, between 20,600 and 18,300 yr B.P., indicate reduced forests with extensive grasslands, relatively high input of sediments by water discharge of surface streams and high lake level. The evidences obtained in zone 4, between 18,300 and 17,500 yr B.P., suggest a dry and warm climate with xerophytic vegetation, lower lake level and erosion rates. Zone 3, between 17,500 and 10,000 yr B.P., shows a trend to increasing moisture and cooler temperatures. Volcanic activity during this period is intense, and strongly influences the natural environments. At around 12,000 yr B.P., a clear expansion of forests and fulfillment of the lake are recorded. At the beginning of the Holocene, an oak forest expansion and a lowering of lake levels are initiated, and by the mid-Holocene (Zone 2) subhumid-temperate conditions are detected, although lake level continues to be low. The human impact is recorded in the last 4000 yr.