Late and postglacial paleoenviroments of Tierra del Fuego: terrestial and marine palynological evidence

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

Santiago, Chile

Congreso; GEOSUR 2007; 2007

 Dpto. de Geología, FCFM, Universidad de Chile; Proyecto ARTG-04, Conecciones Antártica - Patagonia, PBCT-CONICYT; Dto. de Ciencias Ecológicas, Pontifica Universidad Católica de Chile

Late- and Postglacial paleovegetational communities and paleoenvironments in southern Tierra del Fuego (55º S) are interpreted from pollen assemblages, organic walled and palynofacies in 14C-dated peat-bogs, glaciolacustrine sediments and marine deposits. The available evidence of the pollen sequences from the tributary Fuegian valleys close to the Beagle Channel area allows a comparison with the pollen record from the Beagle Channel peat-bog sections. 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 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 the 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 10-11ºC and 400-500 mm annual precipitation resemble modern forest-steppe ecotone of the central part of Tierra del Fuego. The beginning of the transgressive event into the Beagle Channel, ca. 7500 14C yr B.P, flooded the west sector of the channel at Lago Roca - Bahía Lapataia area. The overall palynological assemblages reflect inner estuarine environments related to low and variable salinities and/or turbulence, cool-temperate seawater temperature and abundant dissolved nutrients associated with significant dilution of surface waters from freshwater runoff. The littoral vegetation at the time of the marine incursion in the Beagle Channel was mainly arboreal while a forest-steppe vegetational pattern spread at the 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. Changes in temperature and/or precipitation inferred from different proxy sources (plant macrofossils, pollen, fungal spores, testate amebae and peat humification) during the last ca. 1400 yr records evidence for a period of warming-induced drier conditions from 960-1020 AD. This period may correspond to the Medieval Warm period in Europe (AD 950-1045) suggesting that this was a global warm event, synchronous in both hemispheres.