Australia: The Land Where Time Began
Antarctica – Larsen C Ice Shelf – Basal Crevasses and Implications for Meltwater Ponding and Hydrofracture
According to McGrath et al. meltwater-driven crevasse propagation was the key mechanism leading to the rapid collapse of both the Larsen A and Larsen B Ice Shelves. Basal crevasses, which are large-scale structural features within ice shelves, may have contributed to this mechanism in 3 important ways:
i) Deformation of the surface of the shelf as a result of modified buoyancy and gravitation forces above the basal crevasse, which formed a more than 10 m deep compressional surfaces depressions where meltwater is able to collect,
ii) Surface crevassing is driven by bending stresses from the modified shape, with crevasses reaching up to 40 m in width, on the flanks of the basal crevasses-induced trough and
iii) The propagation distance before a full-thickness rift forms as it is minimised by the thickness of the ice.
In this study McGrath et al. examined a basal crevasse in the Cabinet Inlet sector of the Larsen C Ice Shelf, that was 4.5 km long and about 230 m high, as well as the corresponding surface features, by a combination of high resolution (0.5 m) satellite imagery, kinematic GPS and in situ ground penetrating radar. They discuss the mechanism by why which basal crevasses may have contributed to the breakup of the Larsen B Ice Shelf by controlling directly the location of meltwater ponding and highlight the presence of similar features on the Amery and Getz Ice Shelves with high resolution imagery.
According to McGrath et al. meltwater-driven crevasse propagation is a key mechanism for the rapid and catastrophic collapse of the Larsen A and Larsen B Ice Shelves (Rott et al., 1996; Scambos et al., 2000, 2003, 2009). It is contended by this mechanism that when sufficient meltwater has drained into a surface crevasse, the crevasse will propagate through the entire thickness of the ice shelf, due to differences between the density of the water and that of the ice, which fractured the ice shelf into many elongated icebergs (van der Veen, 1998, 2007; Scambos et al., 2003, 2009; Weertman, 1973). These icebergs are distinguished from tabular icebergs by their narrow along-flow width and their elongated across-flow length, which likely facilitates a a positive feedback during the process of disintegration as elongate icebergs overturn and initiate further ice shelf calving (MacAyeal et al., 2003; Gutenberg et al., 2011; Burton et al., 2012).
Over the past 5 decades dramatic atmospheric warming has increased the production of meltwater along the Antarctic Peninsula (AP) (Vaughan et al., 2003; van den Broeke, 2005; Vaughan, 2006). The Antarctic Peninsula is sensitive to even modest warming, which differs from the interior of Antarctica, as the air temperature of large portions of the Antarctic Peninsula in the austral summer hovers near 0oC (Vaughan, 2006). The final disintegration of the Larsen A and Larsen B Ice Shelves has been attributed to crevasse propagation that was driven by meltwater, but there are many processes that pre-condition an ice shelf for rapid collapse (Doake et al., 1998; Vieli et al., 2007; Khazendar et al., 2007; Glasser & Scambos, 2008). Surface meltwater ponds are allowed to form on the surface of the ice shelf by the densification of firn, which can take multiple melt seasons to accomplish (Scambos et al., 2000, 2003). Concurrently, an increase in basal submarine melting or a reduction of marine ice accretion can thin an ice shelf and can lead to reduced cohesion between parallel flow bands and/or shear margins (Glasser & Scambos, 2008; Jansen et al., 2010). Acceleration of ice flow can result from this, with increased crevassing and rifting that result from increased rates of strain, as was observed on the Larsen B Ice Shelf prior to its collapse (Rignot et al., 2004). Also a clear harbinger of ice shelf disintegration is increased calving and subsequent frontal retreat, particularly if the retreat of the ice front progresses past a critical compressive arch in the strain field, at which point substantial retreat will occur before a new stable configuration is reached (Doake et al., 1998).
The Larsen C Ice Shelf, at more than 50,000 km2 of floating ice, is the largest remaining ice shelf on the Antarctic Peninsula, is fed by 12 major outlet glaciers (Glasser et al., 2009; Cook & Vaughan, 2010). The extent of the Larsen C Ice Shelf has remained relatively stable over the last 5 decades, apart from losing about 7,700 km2 in 1986 and about 1,500 km2 in 2004/2005 due to calving events (Glasser et al., 2009; Cook & Vaughan, 2010). The elevation of the Larsen C Ice Shelf has lowered at a rate of 0.06-0.09 m per annum over the period 1978-2008, the greatest lowering occurring in the northern sector (Fricker & Padman, 2012; Shepard et al., 2003). McGrath et al. suggest firn densification, that has been driven by warmer temperatures and an increase in production of meltwater/freezing, has dominated the lowering of the surface (Holland et al., 2011; Fricker & Padman, 2012) and not an increase in basal melting that is driven by ocean forcing (Shepard et al., 2003). It is suggested by oceanographic observations that the primary water mass in the Larsen C cavity is Modified Weddell Deep Water, which has been cooled to the surface freezing point, and therefore it is not likely to drive high rates of basal melting (Nicholls et al., 2004). It was found (Khazendar et al., 2011) that the northern sector of the ice shelf accelerated by 80 m per year, or 15 %, between 2000 and 2006, and between 2006-2008 a further 6-8 % in the vicinity of Cabinet Inlet, possibly resulting from a reduction of backstress from the Borden Ice Rise and/or the erosion of marine ice that previously has sutured the parallel flow bands together.
Large hyperbolic radar returns were identified by airborne radar surveys that began in the 1970s which were interpreted as basal crevasses within the Ross Ice Shelf (Jezek et al., 1979; Shabtaie & Bentley, 1982, the Larsen Ice Shelf (Swithinbank, 1977), and the Riiser-Larsen Ice Shelf (Orheim, 1982). These features have received relatively little attention, especially in light of a number of recent disintegrations of ice shelves, in spite of their magnitude and abundance. Many basal crevasses, as well as their corresponding surface expressions, on the Pine Island Glacier, Fimbul Ice Shelf and the Larsen C Ice Shelf, have been identified by recent work (Bindschadler et al, 2011; Humbert & Steinhage, 2011; Luckman et al., 2012; McGrath et al., 2012). Basal crevasses penetrate between 69 and 217 m into the overlying ice shelf, which represents about 24% and about 66% of the thickness of the ice, and are likely to have basal opening widths that range from 10s to 100s of metres have been found in 2 different regions of the Larsen C Ice Shelf (Luckman et al., 2012; McGrath et al., 2012). Basal crevasses also regulate the mass and energy exchange between the ice shelf and the ocean by increasing the area of interface between the ocean and the ice (Luckman et al., 2012) and the roughness of the basal surface, as well as representing structural weaknesses in the ice shelf. The presence of basal crevasses make it difficult to speculate on the amount of net basal melting or accretion, as these processes are dependent of ocean properties and circulation that are not known in close proximity to basal crevasses.
Hydrofracture, that is driven by meltwater, the process by which water filled crevasses fracture downwards, has been suggested to be a mechanism that is important in the final breakup of several ice shelves (Weertman, 1973; van der Veen, 1998, 2007; Scambos et al., 2000, 2003). McGrath et al. have shown that as well as introducing ice shelf weaknesses, on a large scale, basal crevasses can form both surface depressions and surface crevasses. The implication of meltwater ponding in the surface depression that is most apparent is that if the meltwater intercepted a flanking crevasse, and established a channel subsequently which could drain the pond it would thereby provide the necessary volume of water for the fracture to continue. Possibly less obvious, however, the increased load in the trough will increase extensional stresses along the flanks in the vicinity of the apex of the basal crevasse, which could potentially lead to further propagation and there is the possibility that a shear fracture could connect these features (Bassis & Walker, 2012). If hydrofracture originates from the base of the surface trough, where hydrostatic pressure is at its greatest, and where incipient surface cracks/fractures are still likely to be present, the structural weakness could be exploited still further, in spite of the large-scale compressional environment (Fountain et al., 2005). The ice thickness is reduced by the presence of the basal crevasse in the vicinity, which thereby minimises the distance these small fractures need to propagate prior to leading to a rift that is full thickness. This latter case highlights the possibility that if the presence of the basal crevasse is more important for the stability of an ice shelf, though the ice shelf is certainly weakened by surface crevasse. Basal crevasses have been found to have a width and depth that are an order of magnitude larger than the surface crevasse they cause to form, and the location of a fracture, and therefore the disintegration of an ice shelf, can be controlled by basal crevasses by concentrating the ponding of meltwater directly above them.
As well as the observations of the drainage of melt ponds on the Larsen B Ice Shelf and sediment cores that were retrieved from beneath the Larsen A Ice Shelf and the Prince Gustav Ice Shelf record sediment pulses, that are spatially discrete, have been interpreted as the drainage of supraglacial lakes and/or crevasses prior to the ice shelf disintegration event (Gilbert & Domack, 2003). Together, clear evidence is provided by these observations that fractures do propagate through ice shelves, though it is not clear where the hydrofracture originated (i.e. whether it was a proximal surface crevasse or incipient beneath the pond.) According to McGrath et al. a corollary can be drawn to supraglacial lake drainage on the Greenland Ice Sheet, where within the lake boundary fractures, and later moulins, develop (Das et al., 2008). Therefore, the presence of the basal crevasse should make a full-thickness rift exceedingly efficient, if a hydrofracture does indeed originate within the pond boundary.
It has been concluded by previous studies that the Larsen C Ice Shelf is largely stable, so not likely to undergo a catastrophic collapse in the short term, in spite of observations of thinning and flow acceleration in the northerly sector (Jansen et al., 2010; Khazendar et al., 2011). It’s suggested by McGrath et al. that basal crevasses have probably existed on the ice shelf for at least about 400 years (McGrath et al., 2012), and therefore are probably not a reflection of recent changes in the thickness of the ice or the speed, and don’t suggest the Larsen C is becoming unstable (Khazendar et al., 2011). According to McGrath et al. for the stability of the Larsen C Ice Shelf to be affected, both meltwater production and meltwater ponding would need to increase significantly. There are only a limited number of melt ponds from each summer at present, mostly near the Cabinet Inlet grounding line, which is probably a response to fδhn air flow over the peninsula (van den Broeke, 2005). McGrath et al. suggest it is probably the case that firn densification has contributed significantly to the lowering of the surface over the past 3 decades, though melt ponds are limited spatially at present (Holland et al., 2011; Fricker & Padman, 2012). It is likely there will be an increase of meltwater production and firn densification if the long-term temperature trends on the Antarctic Peninsula continue (Vaughan et al., 2003), and if this occurs basal crevasses and their surface expressions, which include both crevasses and depressions, could have a significant role in future ice shelf disintegration events.
McGrath, D., K. Steffen, H. Rajaram, T. Scambos, W. Abdalati and E. Rignot (2012). "Basal crevasses on the Larsen C Ice Shelf, Antarctica: Implications for meltwater ponding and hydrofracture." Geophysical Research Letters 39(16): L16504.
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