Andrew S. Hein

A modelling reconstruction of the last glacial maximum ice sheet and its deglaciation in the vicinity of the Northern Patagonian Icefield, South America

ABSTRACT. A time‐dependent model is used to investigate the interaction between climate, extent and fluctuations of Patagonian ice sheet between 45° and 48°S during the last glacial maximum (LGM) and its subsequent deglaciation. The model is applied at 2 km resolution and enables ice thickness, lithospheric response and ice deformation and sliding to interact freely and is perturbed from present day by relative changes in sea level and equilibrium line altitude (ELA). Experiments implemented to identify an LGM configuration compatible with the available empirical record, indicate that a stepped ELA lowering of 750 to 950 m is required over 15000 years to bracket the Fenix I‐V suite of moraines at Lago Buenos Aires. However, 900 m of ELA lowering yields an ice sheet which best matches the Fenix V moraine (c. 23000 a BP) and Caldenius’ reconstructed LGM limit for the entire modelled area. This optimum LGM experiment yields a highly dynamic, low aspect ice sheet, with a mean ice thickness of c. 1130 m drained by numerous large ice streams to the western, seaward margin and two large, fast‐flowing outlet lobes to the east. Forcing this scenario into deglaciation using a re‐scaled Vostok ice core record results in an ice sheet that slowly shrinks by 25% to c. 14500 a bp, after which it experiences a rapid collapse, loosing some 85% of its volume in c. 800 years. Its margins stabilize during the Antarctic Cold Reversal after which it shrinks to near present‐day limits by 11 000 a bp.

Exposure dating outwash gravels to determine the age of the greatest Patagonian glaciations

The relict moraines in Argentine Patagonia archive major expansions of the Patagonian Ice Sheet throughout the Quaternary, and are one of the few terrestrial climate proxies in the middle latitudes of the Southern Hemisphere that extends beyond the last glacial cycle. Determining their individual ages has proved challenging but has important implications for our understanding of terrestrial climate change in southern South America over the duration of the Quaternary. Here, for the first time, we demonstrate that sediment on outwash terraces can be directly dated to determine the timing of early–middle Quaternary glacial advances in southern South America. Cosmogenic 10 Be and 26 Al surface exposure ages were obtained from outwash gravels associated with two of the oldest glacial sequences in the Lago Pueyrredon valley (47.5°S), Argentina. The outermost Gorra de Poivre glacial sequence marks the greatest extent of the Patagonian Ice Sheet. A cobble from this surface gives 10 Be and 26 Al surface exposure ages of ca. 1.2 Ma, consistent with bracketing 40 Ar/ 39 Ar age constraints obtained elsewhere in Patagonia. This is the first time early Pleistocene glacial surfaces have been directly dated in Patagonia. Cobbles on a younger Canadon de Caracoles outwash terrace give exposure ages of ca. 600 ka, while 4 of 5 boulders on an associated moraine give exposure ages that are significantly younger. If the demonstrated stability of outwash terraces in the valley is common throughout the region, it will be possible to extend Patagonian glacial chronologies as far back as the early Pleistocene.

Evidence for the stability of the West Antarctic Ice Sheet divide for 1.4 million years

Past fluctuations of the West Antarctic Ice Sheet (WAIS) are of fundamental interest because of the possibility of WAIS collapse in the future and a consequent rise in global sea level. However, the configuration and stability of the ice sheet during past interglacial periods remains uncertain. Here we present geomorphological evidence and multiple cosmogenic nuclide data from the southern Ellsworth Mountains to suggest that the divide of the WAIS has fluctuated only modestly in location and thickness for at least the last 1.4 million years. Fluctuations during glacial–interglacial cycles appear superimposed on a long-term trajectory of ice-surface lowering relative to the mountains. This implies that as a minimum, a regional ice sheet centred on the Ellsworth-Whitmore uplands may have survived Pleistocene warm periods. If so, it constrains the WAIS contribution to global sea level rise during interglacials to about 3.3 m above present.

Corrigendum to “The million-year evolution of the glacial trimline in the southernmost Ellsworth Mountains, Antarctica” [Earth and Planetary Science Letters 469 (2017) 42–52]

This corrigendum fixes an error in the reporting of 21Ne concentrations, which affected one batch of samples that included the bedrock depth profile from which cosmogenic 10Be, 26Al and 21Ne were modelled to constrain the age and exposure history of the Patriot Hills (Fig. 8 in the manuscript). Re-modelling the cosmogenic nuclide data using the corrected 21Ne data yields an apparent exposure age of 3.5–5.1 Ma. This corrects an age published as 2.1–2.6 Ma in Sugden et al. (2017), and reinforces the conclusion of the original paper that the glacial trimline is pre-Quaternary and that the climatic conditions necessary for its erosion last occurred in the Mid-Miocene. The revised Supplementary Table 1 has been updated with corrected 21Ne concentrations and consistent reporting of 10Be concentrations. The revised Supplementary Table 2 has been updated with 21Ne exposure ages for the affected batch of samples. Below, we describe the revised model results and present a revised Fig. 8. Tables 1 and 2, Fig. 8 and its caption replace those in the original paper. The corrections reinforce the conclusions of the original paper.