DEGLACIATION OF THE LATE GLACIAL PATAGONIAN ICESHEET

The North and South Patagonian Ice sheets are presently separated by a 100km long gap. During glacial periods the icesheets expanded and apparently coalesced to form a single ice sheet which stretched from north of the Chilean Lake District to the southern tip of Tierra del Fuego. One impact of such an expanse of ice was the blocking of westward flowing rivers to the Pacific and the re-routing of drainage to the South Atlantic. In Central Patagonia the principal river, the Rio Baker, drains the cross-border lakes of Lago General Carrera (Argentine name: Lago Buenos Aires) and Lago Cochrane (Argentine name: Lago Puerrydon) as well as the eastern outlets of the North Patgonian Icefield, to the Pacific. Field evidence indicates that much of the drainage basin was once occupied by large proglacial lakes which drained via meltwater channels to the east. Mercer (1976) hypothesised that a minimum date for lake drainage could be retrieved by dating the abandonment of these channel. Thus, lake drainage forms a proxy for icesheet separation.

This project relies on new geomorphogical mapping in the area. The initial conclusions are:

· The proglacial lake immediately prior to final drainage covered an area of 6,315km2

· Radiocarbon dating show the lake drained earlier than 13,550 14Cyr BP.

· Modelling suggests that the lake can be dammed by a relatively modest advance of glaciers from the North Patagonian Icefield.

· The North Patagonian Icefield had retreated to within 30km of its present margins by 13,550 14C yr BP and no subsequent advance has gone beyond this point. This indicates a lack of any Younger Dryas aged event or an Antarctic Cold Reversal aged event similar in magnitude to that in the Straits of Magellan.

Here we present a more detailed chronology based on a suite of radiocarbon dates and tephrochronology.