The million-year evolution of the glacial trimline in the southernmost Ellsworth Mountains, Antarctica

David E Sugden,Andrew S Hein,John Woodward,Shasta M Marrero,Ángel Rodés,Stuart A Dunning,Finlay M Stuart,Stewart P.H.T Freeman,Kate Winter,Matthew J Westoby

doi:10.1016/j.epsl.2017.04.006

Abstract

An elevated erosional trimline in the heart of West Antarctica in the Ellsworth Mountains tells of thicker ice in the past and represents an important yet ambiguous stage in the evolution of the Antarctic Ice Sheet. Here we analyse the geomorphology of massifs in the southernmost Heritage Range where the surfaces associated with the trimline are overlain by surficial deposits that have the potential to be dated through cosmogenic nuclide analysis. Analysis of 100 rock samples reveals that some clasts have been exposed on glacially moulded surfaces for 1.4 Ma and perhaps more than 3.5 Ma, while others reflect fluctuations in thickness during Quaternary glacial cycles. Modelling the age of the glacially moulded bedrock surface based on cosmogenic 10Be, 26Al and 21Ne concentrations from a depth-profile indicates a minimum exposure age of 2.1–2.6 Ma. We conclude that the glacially eroded surfaces adjacent to the trimline predate the Last Glacial Maximum and indeed the Quaternary. Since erosion was by warm-based ice near an ice-sheet upper margin, we suggest it first occurred during the early glaciations of Antarctica before the stepped cooling of the mid-Miocene at ∼14 Ma. This was a time when the interior Antarctic continent had summers warm enough for tundra vegetation to grow and for mountain glaciers to consist of ice at the pressure melting point. During these milder conditions, and subsequently, erosion of glacial troughs is likely to have lowered the ice-sheet surface in relation to the mountains. This means that the range of orbitally induced cyclic fluctuations in ice thickness have progressively been confined to lower elevations.

Highlights

The Ellsworth Mountains, discovered by Lincoln Ellsworth in 1935, form a rugged range 350 km long and 80 km wide on the landward boundary of the Filchner–Ronne Ice Shelf in the Weddell Sea embayment (Fig. 1)
The topography is dominated by a blend of longitudinal mountain ridges separated by glacier-filled basins with ice overspilling from the main ice sheet dome to the west
This paper has examined three massifs in the southernmost Heritage Range in order to help date the Ellsworth glacial trimline, a feature representing a significant thickening and expansion of the West Antarctic Ice Sheet

Summary

Introduction

The Ellsworth Mountains, discovered by Lincoln Ellsworth in 1935, form a rugged range 350 km long and 80 km wide on the landward boundary of the Filchner–Ronne Ice Shelf in the Weddell Sea embayment (Fig. 1). The range runs NNW–SSE and the eastward flowing Minnesota Glacier separates the Sentinel Range to the north from the Heritage Range in the south. The Sentinel Range rises abruptly above the ice sheet and on its western side is an escarpment leading to a succession of peaks exceeding 3500 m and including the highest mountain in Antarctica, Mount Vinson at 4897 m. The Heritage Range is lower with peaks below 2500 m. The topography is dominated by a blend of longitudinal mountain ridges separated by glacier-filled basins with ice overspilling from the main ice sheet dome to the west.

Methods

Discussion

Conclusion