Geomorphology under ice streams: Moving from form to process

Abstract

Ice streams are integral components of an ice sheet's mass balance and directly impact on sea level. Their flow is governed by processes at the ice-bed interface which create landforms that, in turn, modulate ice stream dynamics through their influence on bed topography and basal shear stresses. Thus, ice stream geomorphology is critical to understanding and modelling ice streams and ice sheet dynamics. This paper reviews developments in our understanding of ice stream geomorphology from a historical perspective, with a focus on the extent to which studies of modern and palaeo-ice streams have converged to take us from a position of near-complete ignorance to a detailed understanding of their bed morphology. During the 1970s and 1980s, our knowledge was limited and largely gleaned from geophysical investigations of modern ice stream beds in Antarctica. Very few palaeo-ice streams had been identified with any confidence. During the 1990s, however, glacial geomorphologists began to recognise their distinctive geomorphology, which included distinct patterns of highly elongated mega-scale glacial lineations, ice stream shear margin moraines, and major sedimentary depocentres. However, studying relict features could say little about the time-scales over which this geomorphology evolved and under what glaciological conditions. This began to be addressed in the early 2000s, through continued efforts to scrutinise modern ice stream beds at higher resolution, but our current understanding of how landforms relate to processes remains subject to large uncertainties, particularly in relation to the mechanisms and time-scales of sediment erosion, transport and deposition, and how these lead to the growth and decay of subglacial bedforms. This represents the next key challenge and will require even closer cooperation between glaciology, glacial geomorphology, sedimentology, and numerical modelling, together with more sophisticated methods to quantify and analyse the anticipated growth of geomorphological data from beneath active ice streams.

Topographic Steering of Enhanced Ice Flow at the Bottleneck Between East and West Antarctica

“…While the pattern of this topography is presumably tectonic in origin (Rose, 1979;Studinger et al, 2001;Jordan, Ferraccioli, Armadillo, & Bozzo, 2013), the current over-deepened geometry of the troughs is also attributed here to fluvial and glacial modification during more restricted paleo ice sheet configurations (e.g., in Oligocene and Miocene times; Young et al, 2011). As deforming sediments at the base of ice sheets can alter frictional stress and modify ice flow speeds (Bell et al, 1998;Peters et al, 2006;Stokes, 2018) it is important to assess the possibility of marine sediments at the base of Patuxent Trough, Foundation Trough, and the Offset Rift Basin. Three subglacial troughs provide a series of lowelevation conduits that channelize ice flow from interior Antarctica to Foundation Ice Stream and ice streams of the Siple Coast.…”

“…In these deep troughs RES returns are often weak or intermittent, so we explore the potential for marine sediments in our study area with the use of an isostatically rebounded elevation map ( Figure S2), calculated using a simple Airy isostatic compensation of the current ice load (assuming an ice density of 915 kg/m 3 and mantle density of 3.33 kg/m 3 ; Jordan, Ferraccioli, Ross, et al, 2013). Any terrestrial sediment accumulations could therefore extend the inland extent of subglacial sediments, recorded by Studinger et al (2001), and impact ice flow dynamics (Bell et al, 1998;Peters et al, 2006;Stokes, 2018). While we recognize that erosion, flexural isostatic effects, and faulting will generate different paleotopographies compared to the simple predictions of an Airy isostatic model (Paxman et al, 2016(Paxman et al, , 2017, major recent erosion and faulting is not known to have occurred in this region.…”

“…As deforming sediments at the base of ice sheets can alter frictional stress and modify ice flow speeds (Bell et al, 1998;Peters et al, 2006;Stokes, 2018) it is important to assess the possibility of marine sediments at the base of Patuxent Trough, Foundation Trough, and the Offset Rift Basin. In these deep troughs RES returns are often weak or intermittent, so we explore the potential for marine sediments in our study area with the use of an isostatically rebounded elevation map ( Figure S2), calculated using a simple Airy isostatic compensation of the current ice load (assuming an ice density of 915 kg/m 3 and mantle density of 3.33 kg/m 3 ; Jordan, Ferraccioli, .…”

“…However, our analysis does not exclude the possibility of terrestrial sedimentation by geological, glaciological, and fluvial processes. Any terrestrial sediment accumulations could therefore extend the inland extent of subglacial sediments, recorded by Studinger et al (2001), and impact ice flow dynamics (Bell et al, 1998;Peters et al, 2006;Stokes, 2018).…”

Reply to the comment by H.A.J. Russell on “Converging ice streams: a new paradigm for reconstructions of the Laurentide Ice Sheet in southern Ontario and deposition of the Oak Ridges Moraine”

“…A wealth of work at the margins of modern ice sheets in Antarctica, Greenland, and Spitsbergen has shown that MSGLs are the geomorphic fingerprint of dynamic fast-flowing ice streams (King et al 2009;Stokes 2011;Livingstone et al 2012;Dowdeswell et al 2016;Fernandez et al 2018). They are now widely identified on the beds of Pleistocene and pre-Pleistocene ice sheets, indicating that they too were similarly structured (Le Heron 2017; Putkinen et al 2017;Stokes et al 2016;Margold et al 2014Margold et al , 2015Margold et al , 2018Stokes 2018). Today, almost 200 ice streams are currently recognized in the last Cordilleran and Laurentide ice sheets (Clark 1993;Patterson 1997;Dyke 2008;Kleman 2005, 2007;Evans et al 2006Evans et al , 2008Evans et al , 2014Ross et al 2006Ross et al , 2009Ross et al , 2011Ó Cofaigh et al 2010aÓ Cofaigh et al , 2013MacLean et al 2010MacLean et al , 2015MacLean et al , 2017Occhietti et al 2011;Paulen et al 2017;McMartin 2017;McMartin et al 2018;Veillette et al 2017;Shaw and Longva 2017;Margold et al 2018;Eyles et al 2018a).…”