Abstract :
Paleoclimate at the time of deglaciation (14,000–10,000 14C yr B.P.) in different sectors of the Southern Ocean has followed a continuous warming trend, but also warming broken by a transitory episode, the Younger Dryas stadial event, during which on the order of a millennium, colder conditions prevailed. Evidence is from Antarctic ice cores, deep-sea cores, glacier fluctuations, and pollen and beetle records. Reasons for the apparent nonuniformity include differential sensitivity of each sector to climatic change, uneven dating control, and the variable forcing-response nature of biota used as climatic indicators. Establishing the Late Glacial paleoclimate of the Southern Ocean is of significance, as it bears on comprehending the character of deglaciation in a global context. With the object of investigating further the sequence of deglacial climatic events, pollen stratigraphic studies at Canal Beagle at the southern extremity of South America were extended westward to the vicinity of Ushuaia and Lapataia, supplementing work done previously at Caleta Róbalo, Puerto Harberton, and Bahı́a Moat. At Ushuaia, pollen frequency and influx records of the Late Glacial, as well as the Holocene, chronologically controlled by 25 14C-age determinations, are from four Sphagnum–Empetrum mires. Results obtained at sites dating from 12,500 to 12,000 yr B.P. show Late Glacial climatic variability. Most indicative among tundra communities of grass and dwarf shrub heath is a drop both in frequency and influx of the singular arboreal component Nothofagus at the time of the Younger Dryas, when estimated summer (January) temperature was ≥3°C lower than at present at Ushuaia. In general terms, the Late Glacial pattern is not unlike the northwestern European sequence (sensu Walker, 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.). Within limits of the chronology in the American sector, there is no indication, as shown by marine cores from the Southern Ocean and by Antarctic ice cores, that deglacial climatic events at higher latitudes of the Southern Hemisphere, exemplified by the Younger Dryas, lead similar events in the Northern Hemisphere. Uninterrupted warming evident in the marine and ice cores occurred after 11,600–11,500 yr B.P.; at Canal Beagle, in contrast, the trend toward steadily milder climate, indicated by continuous expansion of Nothofagus frequency, followed in the early Holocene after 10,000 yr B.P. Early Holocene Nothofagus woodland containing grass and composites replaced tundra under a warmer and drier set of conditions coupled with burning. Estimated summer temperature averaged ∼1°C higher than today and annual precipitation ∼100 mm less. Pollen influx remained generally low until about 5000 yr B.P., when the rate of Nothofagus greatly increased together with increase in the rate of Empetrum, a characteristic mire component. The implication is of late Holocene spread of both closed forest and mire, the result of greater precipitation and lower temperatures, while burning abated. Summer temperatures evidently varied within about 0.5°C and precipitation by ∼100 mm compared with the present. Renewed, albeit fluctuating, glacial activity is recorded after 5000 yr B.P. under a variable, cooler and more humid, late Holocene climatic regime.
