Article

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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    ABSTRACT: The Amundsen Sea Embayment of West Antarctica hosts one of the most rapidly changing sectors of the West Antarctic Ice Sheet. With the fastest-flowing ice streams in Antarctica, the region around Pine Island Bay is characterized by rapid ice-sheet thinning and grounding-line retreat. Published surface-exposure data are limited to a few isolated nunataks making it difficult to assess the long-term deglacial history of the area. To address this, we correlate existing records of lateral ice-stream retreat from marine sediment cores with onshore glacial thinning in two key areas of eastern Marie Byrd Land: the Kohler Range and Pine Island Bay. Our 10Be surface-exposure ages are the first from the isolated Kohler Range and show that the nunataks there became ice-free between 8.6 and 12.6 ka. This implies a minimum long-term average thinning rate of 3.3 ± 0.3 cm/yr, which is one order of magnitude lower than recent rates based on satellite data. We also present pre- to early Holocene 10Be surface-exposure ages from two islands located approximately 80 km downstream of the Pine Island Glacier ice-shelf front to constrain the lateral deglacial history in the Pine Island Bay area. This study provides insight into the significance of local ice sheet variations and suggests that the post-LGM history in the Amundsen Sea sector was characterized by glacial thinning as well as lateral retreat in pre- to early Holocene times.
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    ABSTRACT: Recent palaeoglaciological studies on the West Antarctic shelf have mainly focused on the wide embayments of the Ross and Amundsen seas in order to reconstruct the extent and subsequent retreat of the West Antarctic Ice Sheet (WAIS) since the Last Glacial Maximum (LGM). However, the narrower shelf sectors between these two major embayments have remained largely unstudied in previous geological investigations despite them covering extensive areas of the West Antarctic shelf. Here, we present the first systematic marine geological and geophysical survey of a shelf sector offshore from the Hobbs Coast. It is dominated by a large grounding zone wedge (GZW), which fills the base of a palaeo-ice stream trough on the inner shelf and marks a phase of stabilization of the grounding line during general WAIS retreat following the last maximum ice-sheet extent in this particular area (referred to as the Local Last Glacial Maximum, ‘LLGM’). Reliable age determination on calcareous microfossils from the infill of a subglacial meltwater channel eroded into the GZW reveals that grounded ice had retreated landward of the GZW before ∼20.88 cal. ka BP, with deglaciation of the innermost shelf occurring prior to ∼12.97 cal. ka BP. Geophysical sub-bottom information from the inner-, mid- and outer shelf indicates grounded ice extended to the shelf edge prior to the formation of the GZW. Assuming the wedge was deposited during deglaciation, we infer the timing of maximum grounded ice extent occurred before ∼20.88 cal. ka BP. This could suggest that the WAIS retreat from the outer shelf was already underway during or even prior to the global LGM (∼23–19 cal. ka BP). Our new findings give insights into the regional deglacial behaviour of this understudied part of the West Antarctic shelf and at the same time support early deglaciation ages recently presented for adjacent drainage sectors of the WAIS. If correct, these findings contrast with the hypothesis that initial deglaciation of Antarctic Ice Sheets occurred synchronously at ∼19 cal. ka BP.
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    ABSTRACT: Modern Pine Island and Thwaites Glaciers, which both drain into Pine Island Bay, are among the fastest changing portions of the cryosphere and the least stable ice streams in Antarctica. Here we show that the uppermost sediment unit in Pine Island Bay was deposited from a meltwater plume, a plumite, during the late stages of ice sheet retreat ∼7–8.6 k cal yr BP and argue that this deposit records episodes of meltwater intensive sedimentation. The plumite is a hydraulically sorted, glacially sourced, draping deposit that overlies proximal glacimarine sediments and thickens towards the modern grounding line. The uppermost sediment unit is interpreted as a product of non-steady-state processes in which low background sedimentation in large bedrock-carved basins alternates with episodic purging of sediment-laden water from these basins. The inner part of Pine Island Bay contains several basins that are linked by channels with a storage capacity on the order of 70 km3 of stagnant water and significant sediment storage capacity. Purging of these basins is caused by changes in hydraulic potential and glacial reorganization. The sediment mobilized by these processes is found here to total 120 km3. This study demonstrates that episodes of meltwater-intensive sedimentation in Pine Island Bay occurred at least three times in the Holocene. The most recent episode coincides with rapid retreat of the grounding line in historical time and has an order of magnitude greater flux relative to the entire unit. We note that the final phase of ice stream retreat in Marguerite Bay was marked by a similar sedimentary event and suggest that the modern Thwaites Glacier is poised for an analogous meltwater-intensive phase of retreat.
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May 28, 2014

Claus-Dieter Hillenbrand