Palinologia y paleoambientes pleistocénicos de la Cuenca de México

Abstract

This palynological study is based on an 80 m core at Texcoco Lake. The aim of this work is to establish the palynoestratigraphy by mean s of the paleoecologic and paleoclimatic analysis. The changes observed through the palynological analysis are supported by the geochemical changes recorded in the lacustrine sediments. The palynoestratigraphic sequen ce is explained in temlS of the climatic variations as well as a result of the volcanic activity that caused alterations of soil and vegetation. Based on the lithostratigraphic correlation and taking into account the sedimentation rate, the lacustrine clays of the core are encompassed between Middle and Upper Pleistocene. Three zones are established, according to the palynological content and the geochemical characteristics of the sediments. Zone A (Upper Middle Pleistocene) dominated by pine and oak pollen and cloud forest pollen is also presento This assemblage seems to indicate that the climate was humid-temperate. The paleolimnological evidence exhibit the existenee of a fresh-water lake during this periodo Zone B (Upper Middle-Pleistocene) presents important variations in the palynological content, with prevalen ce of pollen from herbaceous communities; there are signals of an important cooling correlated with the Tomicoxco glaciation. There is a great contribution of allochthonous detritic material and the values of Na and K rise. Thereis also a considerable in crease of the magnetic susceptibility and an important lowering of the lake leve. Towards  the end of this zone the lake leve! in creases. the woodlands are restablished and the herbaceous communities retract. Zone C (Upper Pleistocene) is characterized by the prevalen ce of arboreal pollen (Pinus, Quercus, Alnus). Paleoclimatic fluctuations are observed. In the wet stages, pollen of cloud forest is registered and in less humidstages woodlands of pine and oak prevail. In general, there is a moderate but progressive tendency towards aridity. The lake becomes progressively brackish, possibly due to a combination of hydrothermalism and a dryer climate.