Patagonian vegetation turnovers during the Paleogene-Early Neogene: origin of arid-adapted floras

BARREDA, V.; PALAZZESI, L.

La Plata

Congreso; 16º Congreso Geológico Argentino; 2006

Universidad Nacional de La Plata

The structure of the living Patagonian flora, dominated by arid-adapted communities, is a direct consequence of past climatic and tectonic events. Paleoclimatic evidence provided by global deep-sea oxygen isotope data suggests a general global cooling trend throughout the Cenozoic. However, some warm intervals were also identified during Early Eocene, Late Oligocene and Middle Miocene times. The progressive Cenozoic Andes uplift caused a major paleoenvironmental change, mainly related to a marked west to east rainfall gradient. Arid-adapted communities were widespread during Late Neogene, but their origin in Patagonia may be traced back to the Paleogene. This assumption was achieved considering the analysis of the available paleobotanical and palynological data from Patagonia. Paleocene and Early Eocene floras were rainforest-dominated, including many angiosperms with warm temperate affinities. Evidence from both pollen and macrofossil remains indicates that diversity was particularly high. The presence of mangrove palms (Nypa type), Juglandaceae (Plicatopollis spp.), cycads, araucarian conifers, diverse podocarps, and casuarinacean cones (Gymnostoma type), along with the absence of Nothofagaceae, provide evidence of a warm climate, with abundant rainfall. However, mainly in the Early Eocene, some patches influenced by warm but drier conditions are suggested by the occurrence of Anacardiaceae, Amaranthaceae and Casuarinaceae (Casuarina type). These patches of an arid-adapted flora would have risen in Patagonian inland areas, in a generally wet continent. Volcanism, which began to spread from the Eocene, could have played a major forcing factor on both, diversity and vegetation distribution. Ash soils would have been drier niches to be occupied by xerophytic taxa. The Middle-Late Eocene was distinct by the irruption of Nothofagus forests. Progressive replacements of megathermal communities by meso and microthermal rainforest-dominated were documented. Nothofagus forest expansion suggests a marked cooling trend at this time, although some megathermal elements (Aquifoliaceae (Ilex type), Tiliaceae-Bombacaceae, Sapindaceae) were still present. Megathermal Juglandaceae and Aquifoliaceae became extinct at these high latitudes during the Eocene, and they were not recorded later in other warm episodes. Latest Eocene-Early Oligocene fossil floras indicate the development of temperate forests along with a reduction of floristic diversity. Communities were dominated by Nothofagaceae and Podocarpaceae without megathermal elements. During the Late Oligocene termophillous angiosperms were again recorded in Patagonia in accordance with the increase in global temperatures. Late Oligocene floras are represented by forests of Nothofagaceae, Podocarpaceae and Araucariaceae, with abundant ferns, palms and megathermal angiosperms of the Rubiaceae, Combretaceae, Sapindaceae, Chlorantaceae on lowland areas. During the Late Oligocene, and particularly in the Early Miocene, the occurrence of shrubby-herbaceous elements belonging to Asteraceae, Chenopodiaceae, Mimosaceae and Poaceae began to give a modern appearance to plant communities. However, forests were still dominant in the Early and Middle Miocene. The presence of some megathermal species of the Sapindaceae (Cupania type) and Euphorbiaceae (Alchornea type) in the earliest Middle Miocene supports a new temperature increase at this time. Xerophitic formations would have occupied coastal saltmarshes and pockets in inland areas. A contrast between coastal and inland environments may have been developed. Late Miocene records show an increasing diversity and abundance of xerophyticadapted taxa, including Asteraceae, Chenopodiaceae and Convolvulaceae (Cressa/Wilsonia type). Expansion of these xerophytic adapted taxa, coupled with extinctions of megathermal/no seasonal elements, would have been associated with both, tectonic and climatic forcing factors. They would have led to the development of aridity and extreme seasonality. These arid-adapted Late Miocene floras are closely related to modern communities, with the steppe widespread across the extra Andean Patagonia and forest restricted to western, humid, uplands regions.