Article

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

Geology (Impact Factor: 4.88). 10/2012; 41(1). DOI: 10.1130/G33469.1

ABSTRACT

Ice loss from the marine-based, potentially unstable West Antarctic 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 Embayment (ASE) have shown accelerated ice flow, 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 similar to 100 km of its modern position before ca. 10,000 calibrated (cal.) yr B.P. This early deglaciation 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 exceptional during the Holocene and may originate in recent changes in regional climate, ocean circulation, or ice-sheet dynamics.

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    • "19,000 yr B.P., but retreat to the inner shelf was largely complete by the start of the Holocene ca. 10,000 yr B.P. (e.g., Lowe and Anderson, 2002; Jakobsson et al., 2011; Hillenbrand et al., 2013; Nitsche et al., 2013). Cosmogenic-isotope exposure dating in the Hudson Mountains near PIG shows a rapid ice thinning of more than 100 m at ∼8000 yr B.P. to near its modern level, with no evidence of additional large or rapid changes until recently (Johnson et al., 2014). "
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    ABSTRACT: Pine Island Glacier (PIG) in the Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS) is losing mass and contributing to global sea-level rise at an accelerating rate. Although recent observations and modeling have identified the incursion of relatively warm Circumpolar Deep Water (CDW) beneath the PIG ice shelf (PIGIS) as the main driver of this ice-mass loss, the lack of precise bathymetry limits furthering our understanding of the ice–ocean interactions and improving the accuracy of modeling. Here we present updated bathymetry and sediment distribution beneath the PIGIS, modeled by the inversion of aerogravity data with constraints from active-source seismic data, observations from an autonomous underwater vehicle, and the regional gravity-anomaly field derived from satellite gravity observations. Modeled bathymetry shows a submarine ridge beneath the middle of PIGIS that rises ∼350 to 400 m above the surrounding sea floor, with a minimum water-column thickness of ∼200 m above it. This submarine ridge continues across the whole width of the 45-km wide ice shelf, with no deep troughs crossing it, confirming the general features of the previously predicted sub-ice-shelf ocean circulation. However, the relatively low resolution of the aerogravity data and limitations in our inversion method leave a possibility that there is an undetected, few-kilometers-wide or narrower trough that may alter the predicted sub-ice-shelf ocean circulation. Modeled sediment distribution indicates a sedimentary basin of up to ∼800 m thick near the current grounding zone of the main PIG trunk and extending farther inland, and a region seaward of the submarine ridge where sediments are thin or absent with exposed crystalline basement that extends seaward into Pine Island Bay. Therefore, the submarine ridge marks the transition from a thick sedimentary basin providing a smooth interface over which ice could flow easily by sliding or sediment deformation, to a region with no to little sediments and instead a rough interface over which ice flows mainly by deformation. We hypothesize that the post-Last Glacial Maximum retreat of PIG stabilized at this location because of the spatial transition in basal conditions. This in turn supports the hypothesis that the recent retreat of PIG was strongly forced, probably by changes in ocean circulation, rather than occurring because of ongoing response to the end of the ice age or other changes inland of or beneath PIG.
    No preview · Article · Jan 2016 · Earth and Planetary Science Letters
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    • "unit 1A and Unit 1 . Unit 1 is observed as blanketing all of PIB . Based on this style of sedimentation , the estimated volume of Unit 1 is approximately 120 km 3 . The flux rate for Unit 1 is 0 . 018 km 3 / yr based on a 7000 year onset age ( Kirshner et al . , 2012 ) to 0 . 014 km 3 / yr based on a 8660 year onset age for a similar silt facies ( Hillenbrand et al . , 2013a , b ) . We calculate the volume of sediments that comprise subunit 1A as 23 . 8 km 3 . Flux rates for subunit 1A may have been as high as 0 . 60 km 3 / yr to as low as 0 . 04 km 3 / yr , based on end member onsets of subunit 1A deposition ( 40 years before present from Core KC04 to 560 years before present from Core TC49 ) . A more like"
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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.
    Full-text · Article · Feb 2014 · Quaternary Science Reviews
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    • "unit 1A and Unit 1 . Unit 1 is observed as blanketing all of PIB . Based on this style of sedimentation , the estimated volume of Unit 1 is approximately 120 km 3 . The flux rate for Unit 1 is 0 . 018 km 3 / yr based on a 7000 year onset age ( Kirshner et al . , 2012 ) to 0 . 014 km 3 / yr based on a 8660 year onset age for a similar silt facies ( Hillenbrand et al . , 2013a , b ) . We calculate the volume of sediments that comprise subunit 1A as 23 . 8 km 3 . Flux rates for subunit 1A may have been as high as 0 . 60 km 3 / yr to as low as 0 . 04 km 3 / yr , based on end member onsets of subunit 1A deposition ( 40 years before present from Core KC04 to 560 years before present from Core TC49 ) . A more like"

    Full-text · Conference Paper · Sep 2013
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