What is an ice stream?
Ice streams are corridors of fast flow within an ice sheet (ca. 800 metres per year). They discharge most of the ice and sediment from these ice sheets, flowing orders of magnitude faster than their surrounding ice. Their behaviour and stability is therefore essentially important to overall ice sheet dynamics and mass balance. The Antarctic Ice Sheet currently discharges 90% of ice and sediment through ice streams. Antarctic Ice Streams are fed by complex tributaries that extend up to 1000 km into the interior of the ice sheet. These can be seen beautifully in the video below, released by NASA:
Ice streams are typically large features (> 20 km in width, >150 km in length), with a convergent onset zone feeding in to a main channel. Modern ice streams are associated with pervasively deformed till and offshore trough-mouth fans, depo-centres for the large volumes of sediment that are transported from the interior of the ice sheet outwards to the continental shelves.
Ice streams can be constrained by topography or by areas of slow moving ice. They are called topographic ice streams or pure ice streams respectively. Both types show variations in behaviour (both through time and space), which indicates potential for instability and are therefore particularly interesting. Their discharge of ice into ocean basins effects thermal and saline ocean circulation. Ice streams have therefore been a focus for research worldwide over the last 30 years.
Ice streams tend to occupy topographic lows, because:
- thicker ice leads to greater driving stress at the bed and faster velocity, because internal deformation of ice is controlled by basal shear stress[1, 4];
- thicker ice is better insulated and has greater basal temperatures, enhancing rates of ice deformation and bed slip from basal melting;
- Meltwater flows towards and accumulates in topographic lows, and melt rate is greater beneath thicker ice, both of which encourage basal sliding.
- This positive feedback system, with enhanced flow increasing temperature and basal lubrication, which in turn increases flow, leads to ice stream development in topographic corridors.
Ice streams can also develop in areas with weaker ice, or with a lubricated bed to aid basal motion. Some ice streams are a combination of topographic and pure, bounded by both ice and topography. There is growing evidence that soft deformable sediments are a pre-requisite for fast ice flow; subglacial geology therefore is essential in determining ice stream location.
Ice streams lower surface topography, with greater ice-sheet drawdown for pure ice streams, which tend to have greater ice flow volumes. Pure ice streams are also likely to be variable through time and space, shifting location and switching on and off.
The flow velocity, thickness and grounding lines of ice streams are variable over decadal timescales, with observations in Antarctica of thinning, acceleration, deceleration, stagnation and lateral migration[6-9]. However, mechanisms controlling this fast and variable flow are complex and poorly understood. There are a number of potential forcings, which include ocean temperatures, sea level changes, air temperatures, ocean tides, subglacial bathymetry, subglacial geomorphology, topographic pinning points, meltwater beneath the ice stream, thermodynamics and the size of the drainage basin.
Ice streams around Antarctica
The velocity map by Eric Rignot, showing ice velocities in 2007-2009, shows how the Antarctic continent today is drained by ice streams, with tributary glaciers reaching hundreds to thousands of kilometres inland. These dendritic drainage systems pass ice from the interior, near the ice divide, and flow into the ocean or ice shelves.
Siple Coast ice streams
The ice streams around Siple Coast in West Antarctica (Ice Streams A to F) discharge 40% of the ice from the entire West Antarctic Ice Sheet. The behaviour of these ice streams is of particular interest, because they may be important to the stability of the West Antarctic Ice Sheet (see Marine Ice Sheet Instability)[1, 13]. These ice streams are the world’s only current pure ice streams (except perhaps in NE Greenland). Other glaciers draining into the Ross Ice Shelf are topographically constrained.
These ice streams are 50 km wide, 300-500 km long, with ice thicknesses ~1 km. Ice velocities are between 0.1 and 0.8 km per year. There are lateral shear zones along the margins of each ice stream. There are many crevasses near the shear zone as a result of intense deformation. In between the ice streams the glacier ice is cold and frozen to the bed. Deformable subglacial sediments seem to be a requirement for ice-stream formation on the Siple Coast, with continuous sedimentary basins below the accumulation areas of Ice Streams C and D. The adjacent non-streaming areas overlie harder bedrock, with thin or no basal sediments.
The velocity of these ice streams is variable. For example, there is evidence of deceleration on Ice Stream B (Whillans). Ice Stream C shut down ~150 years ago[7, 17]. Ice Stream D, which currently flows rapidly, shut down ~450 years ago. This is because these wide, pure ice streams are inherently unstable. The glaciers are currently thinning, which may reduce driving stress, thus explaining some of the deceleration. However, in general, the accumulation areas of these ice streams are thickening. Ice Stream C has a strongly positive mass balance because of its negative outflow, and it is the stoppage of this ice stream that has contributed to the positive mass balances. The positive imbalance is therefore driven by internal ice-stream dynamics. Ice flow in the area that once discharged into Ice Stream C now drains into Ice Stream B (Whillans), following thinning of Ice Stream B. During these rapid changes, the Siple Coast grounding line has remained static, rather than undergoing continuous change. These grounding lines may be prone to rapid, rather than continuous recession – see Marine Ice Sheet Instability.
These ice streams are highly variable over short timescales, which makes it difficult to draw meaningful conclusions for short-term observations. Analysis and ice sheet reconstructions over centennial to millennial timescales are therefore very important in analysing cryospheric response to modern environmental change.
Ice stream structures
The pattern of velocity across the surface of an ice stream is complex, and varies between ice streams. It is captured by surficial structures, such as crevasses and longitudinal flow structures (also known as flow stripes, flow lines, and streak lines). In places, they can be traced for > 100 km. They form on valley glaciers, outlet glaciers and ice streams, all flowing at a variety of velocities. Longitudinal structures are typically developed parallel to the margins of glacier flow units.
Glasser and Gudmundsson (2012) have mapped surface structures on a number of Antarctic glaciers.
There are three key hypotheses for longitudinal surface structure formation:
- They form as a result of lateral compression in topographic situations where glaciers flow from wide accumulation basins into a narrow tongue.;
- They form where two glacier tributaries converge, and are associated with shear margins between flow units;
- They are the surface expression of subglacial bed perturbations created during rapid basal sliding.
Glasser and Gudmundsson 2012 make the following key observations regarding longitudinal surface structures on Antarctic glaciers:
- They are common features on Antarctic glaciers and ice streams, forming at a variety of scales from entire glacier catchments to individual small valley glaciers;
- At confluences, larger glaciers “pinch out” structures where they meet smaller tributary glaciers;
- The structures can be followed from cirque headwalls to glacier snouts and are continuous;
- Longitudinal surface structures sometimes intensify in zones of lateral compression;
- Longitudinal structures are more closely spaced at flow-unit boundaries than away from these boundaries.
- They are most prominent where ice flow is convergent, but can also be maintained where flow diverges;
- They start abruptly, particularly behind bedrock obstacles and nunataks;
- They can turn more than 90° without interruption or increased lateral compression;
- They are sometimes, but not always, associated with surface debris.
Glasser and Gudmundsson suggest that these surface structures can develop in two main situations: within glacier flow units, and where there is convergent flow around nunataks of glacier confluence zones. Development within flow units is relatively well understood and derives from basal perturbations on the ice-stream surface.
Where these structures start abruptly, they form in areas with rapid longitudinal extension, suggesting that extensional flow can explain these structures. The confluence of two glaciers or flow units, for example, results in strong transverse convergence and longitudinal extension.
Ice stream geomorphology
A classic paper by Chris Stokes and Chris Clark from 1999 suggests that the geomorphological record provides diagnostic criteria for identifying palaeo-ice streams. Understanding the locations and dynamics of palaeo-ice streams is important for understanding palaeo-ice sheets. This is because their large ice flux would have effected ice-sheet configurations; investigations on former ice-streams helps understand glacial processes; their interactions with climate help reconstruct past climate change, as well as predicting the response of contemporary ice sheets to future climatic perturbations; their sedimentary flux is comparable with the largest fluvial basins.
Palaeo-ice streams leave characteristic features in the sedimentological and geomorphological record, which are summarised in the table below (after Stokes and Clark, 1999).
|Contemporary ice stream characteristic||Geomorphological signature|
|Characteristic shape and dimensions||
|Sharply delineated shear margin||
|Deformable bed conditions||
The palaeo-landsystem left behind by an ice stream includes mega-scale glacial lineations (MSGLs) and highly attenuated drumlins. Ice-stagnation features may overprint these landforms as an ice stream switches off or recedes. On the sea floor, grounding zone wedges indicate past pauses in ice stream recession, and scours made by icebergs document the travel of icebergs across the shallow continental shelf.
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