Ice-stream stability on a reverse bed slope

Abstract

Citation for published item: t miesonD F F F nd ieliD eF nd vivingstoneD FtF nd ¡ y gof ighD gF nd tokesD gF F nd rillen r ndD gEhF nd howdeswellD tFeF @PHIPA 9s eEstre m st ility on reverse ed slopeF9D x ture geos ien eFD S @IIAF ppF UWWEVHPF Further information on publisher's website: httpXGGdxFdoiForgGIHFIHQVGngeoITHH Publisher's copyright statement:Additional information:Read Durham University's press release about this article, available at:http://www.dur.ac.uk/news/newsitem/?itemno=15572 Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Ice streams are fast-flowing arteries of ice sheets that dominate ice discharge into the oceans, impacting directly upon sea level. Many ice streams possess beds that are below sea level and typically deepen inland on a reverse-slope 9 . Theory suggests that ice discharge increases rapidly with water depth 3 , and in the absence of lateral-drag induced buttressing from a floating ice shelf, grounding lines (marking the transition from grounded to floating ice) on reverse-bed slopes may be unstable 1-2 . Bed topography is therefore cited as a strong control on ice-stream retreat rate 3,10 and modern satellite observations of rapid ice-stream thinning and recession appear consistent with this theory 4-6 . However, with just two decades of data, these records are too short to identify the longerterm centennial to millennial-scale trends crucial for constraining future sea-level projections.Major uncertainties in predictions of ice-sheet vulnerability 11 relate to limitations in understanding processes controlling grounding-line motion and, importantly, to deficiencies in grounding-line treatment in ice-sheet models 12 . In recent years, significant advances in model development have been made 3,12-17 , but tests have only been applied to simplified bed geometries or to steady-state conditions and lack validation against data over timescales longer than a few decades. We aim to understand the long-term controls and stability of marine ice streams and, for the first time, 'pinning points' (e.g. at 500 km; Fig. 2). However, in the reverse-sloping portion of the bed and, significantly, in the absence of topographic highs, several slow-downs also occur. Moreover, the simulated stabilisations are broadly consistent with previously mapped GZW positions (Fig. 2).These short-term changes in retreat-rate occur adjacent to GZWs 1, 2, 3, 6, 7 and 8 and prolonged grounding-line stability is modelled between GZWs 4 and 5 where the reverse slope steepens into the overdeepened portion of Marguerite Trough...

Future sea-level rise from Greenland’s main outlet glaciers in a warming climate

“…S8) can temporarily slow down the terminus on an upward bed slope (Fig. 2a+b), similar to modelled paleo ice-stream behaviour 25 . Our results show that over the full range of parameters used for each glacier, and despite episodic and short-lived peaks in discharge, century-averaged icedischarge does not exceed 1.7 times the pre-acceleration values of the late-1990s (Table 1).…”

On the reconstruction of palaeo-ice sheets: Recent advances and future challenges

“…A major development in ice sheet reconstructions has been the increased use of geophysical techniques (seismic, sidescan sonar, swath bathymetry) to investigate the marginal areas of palaeo-ice sheet beds that are now submerged beneath sea level (Ó Cofaigh, 2012). Imagery from continental slopes has revealed sedimentary depocentres (trough mouth fans : Vorren and Laberg, 1997;Batchelor and Dowdeswell, 2014), the architecture of which often indicates rapid, episodic sedimentation by ice streams (Dowdeswell et al, 1996;Dowdeswell and Elverhoi, 2002;Nygård et al, 2007).…”

“…There is, therefore, a requirement for additional chronological constraints on palaeo-ice stream activity, including from marine IRD records (e.g. which has much potential to test numerical models and refine ice sheet reconstructions (Mackintosh et al, 2011;Jamieson et al, 2012).…”

A review of topographic controls on moraine distribution

“…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).…”