Major vegetation trends during the Cenozoic at Patagonia (Argentina), and their paleoclimatic relationship: an overview

PALAZZESI, L., BARREDA, V. Y PRIETO, A.R

Gramado, Brasil

Otro; 11º Reunión de Paleobotánicos y Palinológos; 2004

UNISINOS

The patterns of the Patagonian vegetation changes suggest a strong relationship between the major thermal characters of the flora and the global paleoclimatic trends during the Cenozoic. This conclusion was reached from the assessment of fossil pollen data from Patagonia throughout the Cenozoic and its comparison to the global deep-sea oxygen and carbon isotope data. Four main time intervals were recognized according to the temporal distribution of selected angiosperm key taxa. The first stage (Paleocene-Early Eocene) is characterized by the megathermal elements presence such as Nypa sp. and Pandanus sp., probably representing mangrove communities in the Patagonian lowlands. On the other hand, isotope records indicate a warm trend during this stage. The second stage (Middle Eocene-Early Oligocene) is defined by the wide spread of a mesothermal flora across Patagonia with the Nothofagus forest development, prevailing the N. brasii-type, Casuarinaceae and Proteaceae. However, a few megathermal elements were still present (e.g. Cupania, Ilex) at the beginning of this stage, while at the end of it the microthermal ones were dominant. This evidence is consistent with the cooling trend proposed by the isotope records. The third stage (Late Oligocene-Middle Miocene) is characterized by the reappearance of many non-seasonal/megathermal components such as Arecaceae and Cupania in the Patagonian lowlands. These changes in vegetation strongly contrast with the presence and in some cases the first appearance of sclerophyllous taxa such as Asteraceae, Chenopodiaceae and Fabaceae, probably representing isolated patches of open vegetation. From the latter part of the Oligocene until the late Middle Miocene a warming trend is suggested by the isotope records. The last stage (Late Miocene-Quaternary) was clearly recognized by the spreading of xerophytic taxa (e.g. Asteraceae, Fabaceae) in the extra Andean Patagonia and the retreat and expansion of forests in the Andes. These changes are related to a gradual cooling trend and to the late Quaternary glacial-interglacial cycles. The current vegetation was reached during this last stage with the forest development under wetter conditions, restricted to the Andean regions, and a spread steppe across the extra Andean Patagonia.