DEGLACIAL PALEOCLIMATE OF THE AMERICAN SECTOR OF THE SOUTHERN-OCEAN -LATE GLACIAL-HOLOCENE RECORDS FROM THE LATITUDE OF CANAL BEAGLE (55-DEGREES-S), ARGENTINE TIERRA-DEL-FUEGO

Paleoclimate at the time of deglaciation (14,000-10,000 C-14 yr B.P.) in different sectors of the Southern Ocean has followed a continuous w arming trend, but also warming broken by a transitory episode, the You nger Dryas stadial event, during which on the order of a millennium, c older conditions prevailed. Evidence is from Antarctic ice cores, deep -sea cores, glacier fluctuations, and pollen and beetle records. Reaso ns for the apparent nonuniformity include differential sensitivity of each sector to climatic change, uneven dating control, and the variabl e forcing-response nature of biota used as climatic indicators. Establ ishing the Late Glacial paleoclimate of the Southern Ocean is of signi ficance, as it bears on comprehending the character of deglaciation in a global context. With the object of investigating further the sequen ce of deglacial climatic events, pollen stratigraphic studies at Canal Beagle at the southern extremity of South America were extended westw ard to the vicinity of Ushuaia and Lapataia, supplementing work done p reviously at Caleta Robalo, Puerto Harberton, and Bahia Moat. At Ushua ia, pollen frequency and influx records of the Late Glacial, as well a s the Holocene, chronologically controlled by 25 C-14-age determinatio ns, are from four Sphagnum-Empetrum mires. Results obtained at sites d ating from 12,500 to 12,000 yr B.P. show Late Glacial climatic variabi lity. Most indicative among tundra communities of grass and dwarf shru b heath is a drop both in frequency and influx of the singular arborea l component Nothofagus at the time of the Younger Dryas, when estimate d summer (January) temperature was greater than or equal to 3 degrees C lower than at present at Ushuaia. In general terms, the Late Glacial pattern is not unlike the northwestern European sequence (sensu Walke r, 1995), which exhibits a Late Glacial interstadial (13,000-11,000 yr B.P.) followed by a Younger Dryas stadial (11,000-10,000 yr B.P.). Wi thin limits of the chronology in the American sector, there is no indi cation, as shown by marine cores from the Southern Ocean and by Antarc tic ice cores, that deglacial climatic events at higher latitudes of t he Southern Hemisphere, exemplified by the Younger Dryas, lead similar events in the Northern Hemisphere. Uninterrupted warming evident in t he marine and ice cores occurred after 11,600-11,500 yr B.P; at Canal Beagle, in contrast, the trend toward steadily milder climate, indicat ed by continuous expansion of Nothofagus frequency, followed in the ea rly Holocene after 10,000 yr B.P. Early Holocene Nothofagus woodland c ontaining grass and composites replaced tundra under a warmer and drie r set of conditions coupled with burning. Estimated summer temperature averaged similar to 1 degrees C higher than today and annual precipit ation similar to 100 mm less. Pollen influx remained generally low unt il about 5000 yr B.P., when the rate of Nothofagus greatly increased t ogether with increase in the rate of Empetrum, a characteristic mire c omponent. The implication is of late Holocene spread of both closed fo rest and mire, the result of greater precipitation and lower temperatu res, while burning abated. Summer temperatures evidently varied within about 0.5 degrees C and precipitation by similar to 100 mm compared w ith the present. Renewed, albeit fluctuating, glacial activity is reco rded after 5000 yr B.P. under a variable, cooler and more humid, late Holocene climatic regime. (C) 1998 Elsevier Science B.V. All rights re served.