Abstract
The Amundsen Sea sector of the Antarctic ice sheet presently dominates the contribution from Antarctica to sea level rise. Several large ice streams that currently drain the sector have experienced rapid flow acceleration, grounding line retreat and thinning during the past few decades. However, little is known of their longer-term – millennial-scale – retreat history, despite the reliance of several ice sheet and glacial-isostatic adjustment models on such data for improving sea level prediction from this critical region. This study investigates the timing and extent of surface lowering of one of those ice streams, Pope Glacier, since the Last Glacial Maximum (LGM), using glacial-geological evidence for former ice cover. We present a new deglacial chronology for the glacier, derived from surface exposure dating of glacially-deposited cobbles and ice-scoured bedrock from Mount Murphy and its surrounding peaks. Cosmogenic 10Be exposure ages from 44 erratic cobbles and 5 bedrock samples, and in situ 14C exposure ages from one erratic and 8 bedrock samples are predominantly in the range 5.5-16 ka. Although 10Be inheritance from prior exposure is prevalent in some erratics and probably all bedrock samples, none of the ages pre-date the LGM. From these results we infer that the surface of Pope Glacier lowered by 560 m during the early- to mid-Holocene (9-6 ka), at an average rate of 0.13 ± 0.09/0.04 m yr−1. The lowering coincided with a period of enhanced upwelling of warm Circumpolar Deep Water onto the continental shelf in the region. A reduction in buttressing − facilitated by such upwelling − by an ice shelf that is thought to have spanned the embayment until 10.6 cal kyr BP could have triggered simultaneous early Holocene thinning of Pope Glacier and glaciers elsewhere in the Amundsen Sea Embayment.
Highlights
The West Antarctic Ice Sheet (WAIS), and its Amundsen Sea sector, is key to sea level prediction because it rests on bedrock that is, in places, more than 1000 m below sea level and deepens towards the centre of the ice sheet
Three possible explanations for absence of erratics at higher elevations are: i) the Last Glacial Maximum (LGM) ice sheet did not thicken sufficiently to deposit erratics there; ii) the overriding ice sheet was not transporting enough material to deposit erratics there; and iii) erratics deposited during a previous ice-free period were removed by the LGM ice sheet as it flowed over the sites
This study investigates the timing and extent of surface lowering of Pope Glacier since the LGM
Summary
The West Antarctic Ice Sheet (WAIS), and its Amundsen Sea sector, is key to sea level prediction because it rests on bedrock that is, in places, more than 1000 m below sea level and deepens towards the centre of the ice sheet. Subsequent glacial-geological studies have provided evidence that: i) grounding line retreat in the eastern ASE occurred with similar style and timing as in the central ASE, with retreat onto the inner continental shelf by the early Holocene (10.6 cal kyr BP; Smith et al, 2014); ii) extremely rapid thinning of Pine Island Glacier occurred a few thousand years later, in the early. The largest peak in the ASE that is not entirely ice-covered at present is Mt Murphy, a large volcanic edifice This is bounded by Crosson Ice Shelf to the north and Pope Glacier – a tributary of Smith Glacier – to the west (Fig. 1a). A similar “dipstick” approach has been used in several other studies of the WAIS (e.g. Stone et al, 2003; Ackert et al, 2011; Johnson et al, 2014)
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