Evidence from Ice Shelves for Channelized Meltwater Flow Beneath the Antarctic Ice Sheet
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LETTERS PUBLISHED ONLINE: 6 OCTOBER 2013 | DOI: 10.1038/NGEO1977 Evidence from ice shelves for channelized meltwater flow beneath the Antarctic Ice Sheet Anne M. Le Brocq1*, Neil Ross2, Jennifer A. Griggs3, Robert G. Bingham4, Hugh F. J. Corr5, Fausto Ferraccioli5, Adrian Jenkins5, Tom A. Jordan5, Antony J. Payne3, David M. Rippin6 and Martin J. Siegert3 Meltwater generated beneath the Antarctic Ice Sheet exerts a A key unknown, however, is whether a persistent channelized strong influence on the speed of ice flow, in particular for major subglacial hydrological system exists widely beneath the Antarctic ice streams1,2. The subglacial meltwater also influences ocean Ice Sheet, where water input is solely from basal melt, in the absence circulation beneath ice shelves, initiating meltwater plumes of surface melt. As a result, and owing to limits imposed by present that entrain warmer ocean water and cause high rates of ice-sheet-model resolution, existing attempts to incorporate a melting3. However, despite its importance, the nature of the model of subglacial water flow into ice-sheet models have largely hydrological system beneath the grounded ice sheet remains focused on distributed-type water flow10. poorly characterized. Here we present evidence, from satellite Here we present evidence for persistent subglacial channels and airborne remote sensing, for large channels beneath the beneath the grounded ice sheet, derived from satellite imagery of the floating Filchner–Ronne Ice Shelf in West Antarctica, which continent's floating ice shelves. The MODIS (Moderate Resolution we propose provide a means for investigating the hydrological Imaging Spectroradiometer) Mosaic of Antarctica imagery provides system beneath the grounded ice sheet. We observe features a map of surface morphology, enhancing features using a multi- on the surface of the ice shelf from satellite imagery and, image approach11. On some ice shelves around Antarctica, features using radar measurements, show that they correspond with that are distinct from standard flow stripes12 are apparent, of the channels beneath the ice shelf. We also show that the order of 1 km in width, aligned to the flow direction but occasionally sub-ice-shelf channels are aligned with locations where the migrating across the general flow direction of the ice shelf. These outflow of subglacial meltwater has been predicted. This are particularly discernible on part of the Filchner–Ronne Ice Shelf agreement indicates that the sub-ice-shelf channels are formed (FRIS; Fig.1 a) but similar features also appear on the Ross Ice Shelf by meltwater plumes, initiated by subglacial water exiting (RIS) and on smaller fringing ice shelves and ice tongues around the upstream grounded ice sheet in a focused (channelized) Antarctica (Fig.1 b–d; see Supplementary Section S1 for imagery of manner. The existence of a channelized hydrological system all ice shelves). Here, we focus on the most striking features on the has implications for the behaviour and dynamics of ice FRIS, but discuss these in the context of the wider coupled Antarctic sheets and ice shelves near the grounding lines of ice subglacial-hydrological/ocean system. streams in Antarctica. Airborne radar measurements over the Institute Ice Stream (IIS) From extensive research in mountain glacier environments, the and Möller Ice Stream (MIS) system13, feeding into the FRIS, cross subglacial hydrological system has been characterized as having one of the features downstream of the MIS (Fig.2 ). The radar two potential states: distributed or channelized4. In a system where measurements reveal a sub-ice-shelf channel about 250 m in height there is limited meltwater availability, an inefficient, low-volume, and around 300 m wide incised upwards into the ice-shelf base. high-pressure distributed system exists. When sufficiently large The ice-shelf surface feature is, therefore, the surface expression amounts of meltwater are available, the system evolves into of a sub-ice-shelf channel, due to the lower flotation level of the an efficient, higher volume, lower pressure channelized system. thinner ice in the channel. Despite producing a surface expression, Channels may be incised into the ice or into the basal substrate. The corresponding ice surface elevation measurements demonstrate basal velocity of an ice sheet is a function of the subglacial water that the ice is not in hydrostatic equilibrium, with bridging stresses depth and pressure, which is, in turn, a function of the nature of preventing the full relaxation of the ice surface14 (Fig.2 b). the hydrological system. Recent research in Greenland has demon- The ice-shelf features identified correspond with the predicted strated that similar relationships between subglacial water depth exit location of subglacial meltwater beneath the grounded ice and pressure also hold true in ice-sheet settings, where even beneath sheet10 (Fig.1 ). We propose, therefore, that the ice shelf features are multi-kilometre-depth ice, both the nature and evolution of a sub-ice-shelf extension of subglacial meltwater channels present subglacial water systems impose significant effects on ice velocity5,6. beneath the grounded ice sheet. The physical mechanism for the Channelized subglacial water flow beneath former ice sheets creation of a sub-ice-shelf channel has already been described3,15,16 has previously been postulated from geomorphological evidence in as follows: subglacial meltwater reaches the grounding line of the Antarctica7 and North America8. Subglacial lake drainage in the ice sheet and, being fresh relative to the sub-shelf ocean water, centre of East Antarctica has also been assumed to be channelized9. rises underneath the base of the ice shelf, entraining the warmer 1Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UK, 2School of Geography, Politics and Sociology, Newcastle University, Newcastle Upon Tyne NE1 7RU, UK, 3Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK, 4School of Geosciences, University of Edinburgh, Edinburgh EH8 9XP, UK, 5British Antarctic Survey, Cambridge CB3 0ET, UK, 6Environment Department, University of York, York YO10 5DD, UK. *e-mail: [email protected] NATURE GEOSCIENCE j VOL 6 j NOVEMBER 2013 j www.nature.com/naturegeoscience 945 © 2013 Macmillan Publishers Limited. All rights reserved LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1977 bcde c d e a b a Flux (m3 s¬1) 02040 60 80 100+ 0246810+ 45° W 80° S 0 25 50 km 67.5° W Figure 1 j Selected ice shelf features, with calculated meltwater flux beneath the grounded ice sheet overlain. a, FRIS, arrowed features are downstream of (left to right) Institute, Möller, Foundation and Support Force ice streams. b, MacAyeal Ice Stream (RIS). c,d, Smaller East Antarctic ice shelves. e, Lambert Glacier (Amery Ice Shelf). Meltwater flux colour labels in black—parts a,b,e, labels in grey—parts c,d. Orange circles indicate evidence of migration of the exit point of subglacial channels. The green line is the MODIS grounding line11. Dashed lines are airborne radar flight lines, with the yellow section indicating the part shown in Fig. 2, and purple in Supplementary Fig. S4. a Atmosphere along the channel becomes supercooled, as a result of the falling 0 pressure, and freezes, filling in the channel. This can be seen on the Ice shelf satellite imagery with the disappearance of the feature after several ¬500 m hundred kilometres (Fig.1 ). Sub-ice-shelf channels on this scale have been noted ¬1,000 previously17–19. Similar channels on Petermann Glacier in Green- Ocean land were attributed to a purely oceanographic source17, initiated by 0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 irregularities in the ice along the grounding line16. The mechanism m Upper ice surface elevation (m) proposed here to explain the FRIS features does not preclude other b 150 features having a purely oceanographic source. However, results Atmosphere from plume models3,16 demonstrate that the outflow of sub-glacial 0 100 meltwater at an ice shelf grounding line increases overall melt rates. Ice shelf ¬500 The additional buoyancy associated with the sub-glacial outflow 50 leads to a more vigorous plume and enhanced transfer of ocean heat ¬1,000 to the ice shelf base. Subglacial outflows are thus a more effective Ocean 0 means of initiating sub-ice-shelf plumes (and hence of creating 0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 sub-ice-shelf channels) compared with an ocean-driven plume Lower ice surface elevation (m) m alone. The agreement between subglacial water flow routes and sub-ice-shelf channels in Fig.1 indicates that in this circumstance, Figure 2 j Geophysical data for the Möller sub-ice-shelf channel. a, Radar the plume is likely to be driven by a focused subglacial water input. echogram. b, Measured elevations: lower ice surface elevation picked from Elsewhere, sub-ice-shelf channels on the Amery Ice Shelf have been the radar data (solid black line), upper ice surface elevation from radar attributed to suture zones (shear margins) between discrete ice altimeter (solid grey line, note different vertical scale). The dashed line is stream flow units18 that feed the Lambert Glacier and join near the lower ice surface as inferred from the ice surface and a hydrostatic the grounding line. However, in many cases on the FRIS, there assumption (firn correction of 17 m). Ice flow is into the page. are no distinct suture zones, and the subglacial channels are away from the shear margins, largely following the basal topography ocean water as it flows (Fig.3 ). This process induces large but (most notably, MIS; see Supplementary Fig. S4). We can, therefore, localized sub-ice-shelf melt rates beneath the ice shelf. Once a small dismiss suture zones as the cause of the ice shelf features in the FRIS sub-ice-shelf channel is formed, the localized melt rates quickly region (see Supplementary Section S2 for further discussion).