Evidence from the Rio Bayo valley on the extent of the North Patagonian Icefield during the Late Pleistocene-Holocene transition

This paper presents data on the extent of the North Patagonian Icefield during the Late Pleistocene-Holocene transition using cosinogenic nuclide exposure age and optically stimulated luminescence dating. We describe geomorphological and geochronological evidence for glacier extent in one of the major valleys surrounding the North Patagonian Icefield, the Rio Bayo valley. Geomorphological mapping provides evidence for the existence of two types of former ice masses in this area: (i) a large outlet glacier of the North Patagonian Icefield, which occupied the main Rio Bayo valley, and (ii) a number of small glaciers that developed in cirques on the slopes of the mountains surrounding the valley. Cosmogenic nuclide exposure age dating of two erratic boulders on the floor of the Rio Bayo valley indicate that the outlet glacier of the icefield withdrew from the Rio Bayo valley after 10,900 +/- 1000 yr (the mean of two boulders dated to 11,400 +/- 900 yr and 10,500 800 yr). Single-grain optically stimulated luminescence (OSL) dating of an ice-contact landform constructed against this glacier indicates that this ice mass remained in the valley until at least 9700 700 yr. The agreement between the two independent dating techniques (OSL and cosmogenic nuclide exposure-age dating) increases our confidence in these age estimates. A date obtained from a boulder on a cirque moraine above the main valley indicates that glaciers advanced in cirques surrounding the icefield some time around 12,500 900 yr. This evidence for an expanded North Patagonian Icefield between 10,900 +/- 1000 yr and 9700 +/- 700 yr implies cold climatic conditions dominated at this time. (c) 2005 University of Washington. All rights reserved.