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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.
Implications
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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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |