Understanding controls on rapid ice‐stream retreat during the last deglaciation of Marguerite Bay, Antarctica, using a numerical model

Abstract

[1] Using a one-dimensional numerical model of ice-stream flow with robust grounding-line dynamics, we explore controls on paleo-ice-stream retreat in Marguerite Bay, Antarctica, during the last deglaciation. Landforms on the continental shelf constrain the numerical model and suggest that retreat was rapid but punctuated by a series of slowdowns. We investigate the sensitivity of ice-stream retreat to changes in subglacial and lateral topography and to forcing processes including sea-level rise, enhanced melting beneath an ice shelf, atmospheric warming, and ice-shelf debuttressing. Our experiments consistently reproduce punctuated retreat on a bed that deepens inland, with retreat-rate slowdowns controlled by narrowings in the topography. Sensitivity experiments indicate that the magnitudes of change required for individual forcing mechanisms to initiate retreat are unrealistically high but that thresholds are reduced when processes act in combination. The ice stream is, however, most sensitive to ocean warming and associated ice-shelf melting, and retreat was most likely in response to external forcing that endured throughout the period of retreat rather than to a single triggering "event." Timescales of retreat are further controlled by the delivery of ice from upstream of the grounding line. Due to the influence of topography, modeled retreat patterns are insensitive to the temporal pattern of forcing evolution. We therefore suggest that despite regionally similar forcing mechanisms, landscape controls significant contrasts in retreat behavior between adjacent but topographically distinct catchments. Patterns of ice-stream retreat in the past, present, and future should therefore be expected to vary significantly.

Palaeo-ice stream pathways and retreat style in the easternmost Amundsen Sea Embayment, West Antarctica, revealed by combined multibeam bathymetric and seismic data

“…Dramatic changes in West Antarctic ice streams -such as major thinning, flow acceleration, and grounding line retreat -have recently been observed, leading to concerns over future changes and their contribution to sea level rise (e.g., Vaughan, 2008;Pritchard et al, 2009Pritchard et al, , 2012Tinto and Bell, 2011;Favier et al, 2014;Joughin et al, 2014;Rignot et al, 2014). The complex and nonlinear behaviour of ice streams suggests that the contemporary observational record spanning only the last two to three decades cannot fully elucidate processes controlling long-term ice stream behaviour (i.e., over centuries to millennia; Jamieson et al, 2014). Therefore, numerical simulations of ice sheet changes over longer timescales (i.e., from the last glacial period throughout the deglaciation until present) are required to (i) better understand ice stream behaviour and (ii) predict future changes more precisely, after reliably simulating past ice sheet configurations known from empirical records.…”

“…While some models reconstructing the ice sheet configuration at the Last Glacial Maximum (LGM;~23-19 cal. ka BP -calibrated kiloyears before present) have already included geological data (e.g., LeBrocq et al, 2011;Golledge et al, 2013Golledge et al, , 2014Jamieson et al, 2014), gaps are still significant in understanding West Antarctic Ice Sheet (WAIS) behaviour (e.g., Larter et al, 2014;The RAISED Consortium, 2014). …”

A review of topographic controls on moraine distribution

“…During retreat from stable locations into wider/deeper sections of troughs and/or away from islands or bathymetric highs, ablation will increase significantly (perhaps enhanced by the incursion of warm water into previously enclosed basins -see Jenkins et al, 2010), and glaciers will undergo rapid retreat until the next topographic pinning point is reached (Warren and Hulton, 1990;Boyce et al, 2007;Nick et al, 2009;Jamieson et al, 2012;Carr et al, 2013). This propensity for periodic stability during retreat is highlighted by Jamieson et al (2012Jamieson et al ( , 2014 who used a numerical model of ice-stream flow, constrained by mapped grounding-zone wedges, to analyse the post-LGM retreat of the Marguerite Bay Ice Stream (western Antarctic Peninsula). These data (Fig.…”

“…This effectively represents a control on glacier dimensions, dynamics, and margin stability (i.e., where and when glacial still-stands occur). The role of topography as a regulator of modern ice-mass stability is a topic of ongoing research interest (e.g., Mercer, 1961;Hughes, 1987;Payne and Sugden, 1990;Kerr, 1993;Tomkin, 2003;Singer et al, 2004;Taylor et al, 2004;Kessler et al, 2006), particularly when considering marine-terminating outlets of the Greenland and Antarctic ice sheets (e.g., Bennett, 2003;Schoof, 2007;Jamieson et al, 2012Jamieson et al, , 2014Carr et al, 2013), but associated implications for the palaeorecord are rarely discussed (c.f. Warren and Hulton, 1990;Warren, 1991;Kaplan et al, 2009;Anderson et al, 2012;Pedersen and Egholm, 2013).…”

“…Topography (fjord/trough geometry) plays a key role in governing these factors and is therefore fundamental to the stability of lake-and marine-terminating glaciers (more so than in the case of land-terminating examples) (Warren and Hulton, 1990;Warren, 1991Warren, , 1992. In particular, topographic pinning points represent potentially stable locations where moraine formation can occur (Warren, 1991;O'Neel et al, 2005;Jamieson et al, 2012Jamieson et al, , 2014Carr et al, 2013). These pinning points often consist of laterally confined shallow basins, subaerial topographic obstructions (i.e., islands) or bathymetric highs (such as the adverse slopes of overdeepenings) (Warren and Hulton, 1990;Warren, 1991Warren, , 1992Cook and Swift, 2012).…”