Grounding-line retreat of the West Antarctic Ice Sheet from inner Pine Island Bay

Geology (Impact Factor: 4.64). 10/2012; DOI: 10.1130/G33469.1

ABSTRACT Ice loss from the marine-based, potentially unstable West Ant-arctic Ice Sheet (WAIS) contributes to current sea-level rise and may raise sea level by ≤3.3 m or even ≤5 m in the future. Over the past few decades, glaciers draining the WAIS into the Amundsen Sea Embay-ment (ASE) have shown accelerated ice fl ow, rapid thinning, and fast retreat of the grounding line (GL). However, the long-term context of this ice loss is poorly constrained, limiting our ability to accurately predict future WAIS behavior. Here we present a new chronology for WAIS retreat from the inner continental shelf of the eastern ASE, based on radiocarbon dates from three marine sediment cores. The ages document a retreat of the GL to within ~100 km of its modern position before ca. 10,000 calibrated (cal.) yr B.P. This early deglacia-tion is consistent with ages for GL retreat from the western ASE. Our new data demonstrate that, in contrast to the Ross Sea, WAIS retreat from the ASE shelf was largely complete by the start of the Holocene. Our results further suggest either slow GL retreat from the inner ASE shelf throughout the Holocene, or that any episodes of fast GL retreat must have been short-lived. Thus, today's rapid retreat may be excep-tional during the Holocene and may originate in recent changes in regional climate, ocean circulation, or ice-sheet dynamics. INTRODUCTION Pine Island Glacier, Thwaites Glacier, and Smith Glacier drain the West Antarctic Ice Sheet (WAIS) into Pine Island Bay in the eastern Amundsen Sea Embayment (ASE) (Fig. 1). Ice loss from this sector of the WAIS is currently raising global sea level at a rate of ~0.15–0.30 mm/yr, making it Antarctica's main contributor to present sea-level rise (Joughin and Alley, 2011, and references therein). Continued WAIS melting in the ASE sector has the potential to raise global sea level by ≤1.5 m, and thus to dominate sea-level change over coming centuries (Vaughan, 2008; Wing-ham et al., 2009). The current negative mass balance, which is mainly attributed to signifi cant sub–ice shelf melting by upwelling of relatively warm Circumpolar Deep Water (e.g., Rignot and Jacobs, 2002; Pritchard et al., 2012), is characterized by fast grounding line (GL) retreat (Pine Island Glacier, ~25 km from 1992 to 2009; Joughin et al., 2010; Thwaites Glacier, ≤14.5 km from 1992 to 2009; Tinto and Bell, 2011), accelerated ice dis-charge (Rignot, 2008; Joughin et al., 2010), and rapid thinning of grounded ice and ice shelves draining into the ASE (e.g., Joughin and Alley, 2011). However, it is unknown if the contemporary dynamic changes are simply part of long-term WAIS retreat since the Last Glacial Maximum (LGM, ca. 23,000–19,000 cal. yr B.P.), or solely recent phenomena. The deglacial history in the ASE sector since the LGM is poorly con-strained. Subglacial bedforms mapped by multibeam bathymetry and infor-mation from marine sediment cores revealed that Pine Island, Thwaites, and Smith Glaciers coalesced on the inner shelf during the LGM to form a major ice stream that advanced through a bathymetric trough to the outer shelf (Lowe and Anderson, 2002; Graham et al., 2010; Jakobsson et al., 2012). Radiocarbon ages from the sediment cores constrain deglaciation of the middle shelf at ~73°S to before 12,503 cal. yr B.P., while just a single 14 C date from core NBP99–02 PC41 (Fig. 1) constrains the timing of grounded ice retreat within ~250 km of the modern GL of Pine Island Glacier to before 10,256 cal. yr B.P. (Lowe and Anderson, 2002; Kirshner et al., 2012). Our study investigates whether rapid GL retreat similar to the modern GL retreat has typifi ed WAIS retreat from the ASE shelf during the Holocene. This knowledge will improve our understanding of current mass loss of West Antarctic glaciers and model-based predictions of future changes.

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May 28, 2014

Claus-Dieter Hillenbrand