Beatriz Ortega-Guerrero

Late Quaternary environmental changes of the central part of the Basin of Mexico; correlation between Texcoco and Chalco basins

Abstract A 10.8 m core was collected from Lake Texcoco to investigate paleoenvironmental changes for the late Quaternary in the central Basin of Mexico. Results from pollen, magnetic susceptibility and loss-on-ignition analyses reveal important fluctuations. A time scale has been established from five AMS 14 C dates and tephra correlation. A humid period is recorded from at least 34,000 to ca. 23,000 yr B.P. where mesophytic and wetland taxa are important. A trend of dry and cold conditions began during the late glacial maximum, reflected by an expansion of the pine forests and lowering of pollen diversity and concentration. This trend continued during the late glacial when grasslands developed, and culminated at ca. 14,000 yr B.P. Volcanic activity was particularly noteworthy during this time in association with this environmental change. A major hiatus marks the culmination of the dry period. Wetter conditions during the early (?) Holocene-mid-Holocene allowed Texcoco to be a persistent saline lake. Correlation between Chalco and Texcoco sub-basins based on 14 C dated pollen assemblages suggests that the major environmental changes affecting both sites are of climatic origin. However, volcanic eruptions also have played an important role in modeling the landscape as well as the evolution of vegetation and the lacustrine systems within the basin, and some of the limnological changes in the lakes could be explained by volcanism rather than by climatic causes.

Palynological and magnetic susceptibility records of Lake Chalco, central Mexico

Abstract 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 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.