Paleovegetational and paleoclimatic changes during the late Plistocene-Holocene in Tierra del Fuego, Southern South America

BORROMEI, A. M.; QUATTROCCHIO, M.; GRILL, S.; CANDEL, S.; PONCE, J. F.

Gramado, RS, Brasil

Otro; XI Meeting of Paleobotanists and Palynologists; 2004

UFRGS y UNISINOS

Late Pleistocene-Holocene paleoenvironment and paleoclimate in the southern part of Tierra del Fuego (55º S) is characterized from pollen assemblages, microplankton and palynofacies by analysis in 14C-dated peat-bogs, glaciolacustrine sediments and marine deposits. The pollen sequences from tributary Fuegian valleys (Valle de Andorra, Valle Carbajal and National Route 3 sites) parallel pollen sequences recorded along the Canal Beagle. The pollen-stratigraphic record of a colder period, associated with the Younger Dryas stadial event (11,000-10,000 14C yr B.P.) in the National Route 3 section, reinforce the evidence of the Late Glacial cooling episode previously recorded in the Canal Beagle area. This transitory episode interrupted the previous expansion of the singular arboreal component Nothofagus under moderating climatic conditions that followed the deglaciation (14,000-10,000 14C yr B.P.). At times of Late Glacial cooling the paleoenvironments were virtually treeless dominated by grass, low scrub and dwarf shrub heath communities spreading into the low and middle slopes, and the temperatures apparently were > 3°C lower than at present at Ushuaia. The interrupted trend towards steadily milder climate, indicated by continuous expansion of Nothofagus forest followed in the early Holocene after 10,000 14C yr B.P. Vegetation communities of Nothofagus woodland containing grass and composites replaced treeless environments under warmer and drier conditions in conjuntion with repeated fires. Estimated summer temperatures of 10-11º C and 400-500 mm annual precipitation resemble modern forest-steppe ecotone of the central part of Tierra del Fuego. The climatic amelioration of the early Holocene coincide with a transgressive event into the Canal Beagle by about 8000 14C yr B.P. The dinoflagellate and acritarch assemblages suggest nearshore environments. Two higher relative sea level were identified, one, between 8240 ± 60 14C yr B.P. and 7260 ± 70 14C yr B.P. and the other, at ca. 6000 14C yr B.P. The vegetation in the littoral at the time of the marine incursion in the Canal Beagle was mainly arboreal, as can be seen in the pollen records by significant increase in Nothofagus frequencies, while a forest-steppe vegetational pattern spread at regional level. About 5000 14C yr B.P. onwards, a relative low sea level is documented until reaching present conditions. The paleoenvironmental conditions became more rigourous, the temperature decreased and the precipitation increased, and as consequence, the Nothofagus woodland started to expand developing a closed forest together with spreading of Empetrum mires, while burning abated. This paleoclimatic conditions resemble those of the modern forest with annual precipitation varying between 500 and 800 mm, and summer temperature averaging 8-9º C.