Boletín de la Sociedad Geológica Mexicana
versão On-line ISSN 1405-3322
ISRADE ALCANTARA, Isabel et al. Paleolimnologic evolution of the Cuitzeo lake, Michoacán, during the Pleistocene-Holocene. Bol. Soc. Geol. Mex [online]. 2010, vol.62, n.3, pp. 345-357. ISSN 1405-3322.
Lake Cuitzeo, located in a volcanic region in northern Michoacan called the Tarascan Plateau, was formed in a tectonic basin and is the second largest lake in Mexico. This study is based on a 27 m long core, drilled in the depocenter of the lake, and four trenches in the proximal areas recently exposed by the retreat of the lake. The predominant materials in the core are clays, silts and diatomaceous muds, while in the trenches fluvial facies are interlayered with deposits of reworked volcaniclastics, ash, pyroclastic surges and air-fall tuffs that were deposited in water or in paleosoils. Chronological control is based on radiocarbon dates for the first part of the core down to 11 m (42 ky BP), and an age of ca. 120 ky BP is inferred for the base of the core. Results from diatom, pollen, mineralogy, geochemistry and macro-charcoal particle analyses from the trenches show that Lake Cuitzeo experienced several environmental changes. Sediment chemistry and diatom fluctuations suggest that before 50 ky BP it was a shallow freshwater lake. Five periods of increasing lake level or more diluted conditions were caused by freshwater entry to the lake (ca. 100 ky BP, ca. 50 ky BP, from 44 ky BP to 42 ky BP, from 32 ky BP to 31 ky BP and from 20 ky BP to 14 ky BP), with Stephanodiscus spp., Aulacoseira spp. and species from the Fragilaria group as dominant taxa, which coincided with high values of Cyperaceae and Typha. From 45 ky BP the lake has shown episodes of aridity, becoming shallow, saline and alkaline, with peaks at ca. 45 ky to 43 ky BP, 38 ky to 35 ky BP, 31 ky to 25 ky BP, and 12 ky to 2 ky BP. Tephra deposits are observed at ca. 80 ky BP, ca. 50 ky BP, ca. 35 ky BP and ca. 25 ky BP. During the middle Holocene (ca. 5 ky BP), for a short period of time the flora indicates more diluted environments, however from 2 ky BP to the present diatom flora is dominated by Discostella meneghiniana, which is indicative of shallow, turbid and saline water. Macro-charcoal particle analysis indicates a strong relation between fire events and the volcanic activity in the area, which decreased during the late Holocene. An interruption in the sedimentation is observed between ca. 8 ky and ca. 12 ky BP. This hiatus also is represented in a core from the northern shore of Zirahuén Lake.
Palavras-chave : Cuitzeo Lake; Mexico; diatoms; pollen; lake levels; Quaternary.