Environmental Characteristics of Lake Tecocomulco, northern basin of Mexico, for the last 50,000 years

Paleoenvironmental studies have documented the late Pleistocene to Holocene evolution of the lakes in the central and southern parts of the basin of M exico (Texcoco and Chalco). No information was available, however, for the lakes in the north-eastern part of this basin. The north-eastern and the ce ntral and southern areas represent, at present, different environmental con ditions: an important gradient exists between the dry north and the moister south. To investigate the late Pleistocene to Holocene characteristics of the north-eastern lakes in the basin of Mexico two parallel cores (TA and T B) were drilled at the SE shore of Lake Tecocomulco. Stratigraphy, magnetic properties, granulometry, diatom and pollen analyses performed on these se diments indicate that the lake experienced a series of changes between ca. > 42,000 yr BP and present. Chronological control is given by five radiocar bon determinations. The base of the record is represented by a thick, rhyol itic air-fall tephra that could be older than ca. 50,000 yr BP. After this Plininan event, and until ca. 42,000 yr BP, Lake Tecocomulco was a moderate ly deep, freshwater lake surrounded by extended pine forests that suggest t he presence of cooler and moister conditions than present. Between ca. 42,0 00 and 37,000 yr BP, the lake became shallower but with important fluctuati ons and pollen suggests slightly warmer conditions. Between ca. 37,000 and 30,000 yr BP the lake experienced two relatively deep phases separated by a dry interval. A second Plinian eruption, represented in the sequence by a dacitic an air-fall tephra layer dated at 31,000 yr BP, occurred in the are a by the end of this dry episode. Between ca. 30,000 and 25,7000 yr BP Teco comulco was a fresh to slightly alkaline lake with a trend towards lower le vel. After ca. 25,700 yr BP very low lake levels are inferred, and after ca . 16,000 yr BP the data indicate the presence of a very dry environment tha t was persistent until the middle Holocene. After 3,500 yr BP lacustrine co nditions were re-established and the vegetation cover shows a change toward s higher percentages of herbaceous taxa.