New boundary conditions for the West Antarctic ice sheet: Subglacial topography beneath Pine Island Glacier

Abstract

Predictions about future changes in the Amundsen Sea sector of the West Antarctic ice sheet (WAIS) have been hampered by poorly known subglacial topography. Extensive airborne survey has allowed us to derive improved subglacial topography for the Pine Island Glacier basin. The trunk of this glacier lies in a narrow, 250‐km long, 500‐m deep sub‐glacial trough, suggesting a long‐lived and constrained ice stream. Two tributaries lie in similar troughs, others lie in less defined, shallower troughs. The lower basin of the glacier is surrounded by bedrock, which, after deglaciation and isostatic rebound, could rise above sea level. This feature would impede ice‐sheet collapse initiated near the grounding line of this glacier, and prevent its progress into the deepest portions of WAIS. The inland‐slope of the bed beneath the trunk of the glacier, however, confirms potential instability of the lower basin, containing sufficient ice to raise global sea by ∼24 cm.

Sustained retreat of the Pine Island Glacier

“…Satellite observations of relative tidal motion show that the PIG hinge line retreated by up to 25 km between 1992 and 2009 [Rignot, 1998b;Joughin et al, 2010]. This retreat corresponds to a reduction in ice thickness of around 90 m at the glacier terminus considering the recent geometry [Vaughan et al, 2006], a value that is consistent with direct observations of thinning acquired by satellite altimetry during the same period [Wingham et al, 2009].…”

“…The progressive retreat of the PIG hinge line has occurred presumably as a result of changes in both the rate of ocean melting and the glacier surface slope, the latter occurring as a dynamical response to reduced grounding [Joughin et al, 2003;Payne et al, 2004]. Although a substantial bedrock peak occurs around 25 km inland of the present hinge-line position (Figure 3d), this feature does not straddle the entire glacier width [Vaughan et al, 2006], and relief to the south side of the glacier, in particular, slopes gently. However, although the hinge line has retreated since 2009, analysis of glacier velocities reveals that the glacier speed has yet to increase (I. Joughin, personal communication), which suggests that further dynamical imbalance has yet to occur.…”

“…Wingham et al, 2009] suggest that the surface slope in the vicinity of the PIG hinge line has altered considerably over time. To account for this variation, we estimate a averaged between the locations of successive hinge-line positions using a sequence of glacier surface slopes generated from satellite and airborne data [Bamber and Bindschadler, 1997;DiMarzio et al, 2007;Krabill, 2011], and we estimate b using a model of the bedrock elevation [Vaughan et al, 2006]. In the absence of direct observations, we use a model of Antarctic ocean tide (an updated version of the regional inverse model described by Padman et al [2002]) to assess the fluctuation in grounding line position due to tidal displacement.…”

Megagrooves and streamlined bedrock in NW Scotland: The role of ice streams in landscape evolution

“…They are readily distinguishable in Antarctica by synthetic aperture radar measurements over time (Bamber et al, 2000). Several studies have shown that onset zones are conicident with a thermal boundary within the ice sheet, where cold slow-flowing ice becomes warmer, wet-based, and, hence, faster flowing (Engelhardt and Kamb, 1998;Bamber et al, 2000;Vaughan et al, 2006). Relatively few studies, to date, have identified palaeo-ice-stream onset zones in the landscape record.…”

Retreat of Pine Island Glacier controlled by marine ice-sheet instability

“…This has reduced the buttressing exerted by the ice shelf, leading to the acceleration of the ice stream and the ongoing retreat of the grounding line (GL) along the glacier's trunk observed since 1992 [17]. Today the GL lies over bedrock that has a steep retrograde slope [29] (Figure 1c) raising the possibility that PIG may already be engaged in an irrevocable retreat. ProvidedAssuming that ice flow is dominated * durand@lgge.obs.ujf-grenoble.fr 2 by basal sliding and lateral variation can be ignored, grounding lines located on retrograde slopes are always unstable [24,3], but in realistic, three-dimensional geometries lateral drag and buttressing in the ice shelf can act to prevent unstable retreat [9].…”