QUATERNARY PALEOENVIRONMENTS AND LAKE LEVEL RECORDS FROM CUITZEO LAKE, WEST CENTRAL MEXICO

Cuitzeo lake located at the high plateau (1820 m.o.s.l.) in North Michoacan is a large vulcanotectonic lake. Twenty seven meters coring in the Cuitzeo lake spanning the last ca. 127,000 y. B. P. shows a high resolution lacustrine record involving diatoms, pollen, mineralogy, TOC and TIC analysis. Sediments of this closed basin are mainly composed of clayed silts, silts and diatomite. Tephra deposition observed in three periods (ca.80 ky, ca.50 ky and ca.35 to 25 ky)modifies palaeogeography and paleoenvironments. The record from Cuitzeo lake canbe divided fairly in two episodes: the first one from Late Pleistocene and the second one in Early Holocene. Evidence indicates that in Late Pleistocene dry conditions dominate, Staurosira construens and herbaceous vegetation(Poaceae, Cheno-Am, Ambrosia) and subaquatic communities (Cyperaceae)dominates the pollen diagram. Mineralogically constant low TOC values and the presence of gypsum are a characteristic of low water levels. This trend was punctuated by small humid phases of relatively short duration at ca. 17 ky, 44 ky and 42 ky. The most evident short humid phase is observed in the last glacial maximum ( ca.17 ky B. P .) in which increases up to 80% the percents of Stephanodiscus niagarae and also shows an expansion of open pine-oak forest. However warm weather and arid summer are recorded by Quercus, Ambrosia and Coelastrum and bentonic diatoms indicating also low lacustrine levels. An important aridity interval was recorded from ca. 23 ky to 18 ky and the lake went dry or dropped significantly below modem level. A hiatus from 8 ky B. P. to 17 ky B. P. product of erosion cut the biological and sedimentary record but the trend is toward arid conditions. The second episode from Early to Middle Holocene suggest an improvement in the temperature conditions with an increase in rain precipitation and subhumid temperate forest. For the Middle Holocene the lake changes from freshwater (Staurosira construens) to a shallow saline water lake. Finally a sharp increase of aloctonous material and successive eutrophication are suggested for the Botryococcus disappearance and the Cyclotella meneghiniana increase evidence the establishment of the modem conditions. Clearly more coring sites with higher resolution are needed to understand the palaeoenvironmental evolution in a big lake with a tectonic and volcanic configuration as this one.