Extensive MIS 3 glaciation in southernmost Patagonia revealed by cosmogenic nuclide dating of outwash sediments

Christopher M Darvill,Michael J Bentley,Chris R Stokes,Andrew S Hein,Ángel Rodés

doi:10.1016/j.epsl.2015.07.030

Abstract

The timing and extent of former glacial advances can demonstrate leads and lags during periods of climatic change and their forcing, but this requires robust glacial chronologies. In parts of southernmost Patagonia, dating pre-global Last Glacial Maximum (gLGM) ice limits has proven difficult due to post-deposition processes affecting the build-up of cosmogenic nuclides in moraine boulders. Here we provide ages for the Río Cullen and San Sebastián glacial limits of the former Bahía Inútil–San Sebastián (BI-SSb) ice lobe on Tierra del Fuego (53–54°S), previously hypothesised to represent advances during Marine Isotope Stages (MIS) 12 and 10, respectively. Our approach uses cosmogenic 10Be and 26Al exposure dating, but targets glacial outwash associated with these limits and uses depth-profiles and surface cobble samples, thereby accounting for surface deflation and inheritance. The data reveal that the limits formed more recently than previously thought, giving ages of 45.6 ka (+139.9−14.3) for the Río Cullen, and 30.1 ka (+45.6−23.1) for the San Sebastián limits. These dates indicate extensive glaciation in southern Patagonia during MIS 3, prior to the well-constrained, but much less extensive MIS 2 (gLGM) limit. This suggests the pattern of ice advances in the region was different to northern Patagonia, with the terrestrial limits relating to the last glacial cycle, rather than progressively less extensive glaciations over hundreds of thousands of years. However, the dates are consistent with MIS 3 glaciation elsewhere in the southern mid-latitudes, and the combination of cooler summers and warmer winters with increased precipitation, may have caused extensive glaciation prior to the gLGM.

Highlights

The terrestrial record of former southern hemisphere ice masses has been used to assess inter-hemispheric synchroneity of glacial advance and retreat (Sugden et al, 2005) and how climatic forcing, such as changes in the Southern Westerly Winds (Figure 1), triggered ice growth or decay through time
We argue that the ages of the Río Cullen and San Sebastián limits remain unclear, and an alternative approach to cosmogenic nuclide exposure dating of boulders in the region is required
The depth profile and surface sample ages for the outwash associated with the Río Cullen and San Sebastián glacial limits suggest that these surfaces are substantially younger than previously thought

Summary

Introduction

The terrestrial record of former southern hemisphere ice masses has been used to assess inter-hemispheric synchroneity of glacial advance and retreat (Sugden et al, 2005) and how climatic forcing, such as changes in the Southern Westerly Winds (Figure 1), triggered ice growth or decay through time. We argue that the ages of the Río Cullen and San Sebastián limits remain unclear, and an alternative approach to cosmogenic nuclide exposure dating of boulders in the region is required.

Results

Conclusion