81 research outputs found
Ice-shelf fracture due to viscoelastic flexure stress induced by fill/drain cycles of supraglacial lakes
AbstractUsing a previously derived treatment of viscoelastic flexure of floating ice shelves, we simulated multiple years of evolution of a single, axisymmetric supraglacial lake when it is subjected to annual fill/drain cycles. Our viscoelastic treatment follows the assumptions of the well-known thin-beam and thin-plate analysis but, crucially, also covers power-law creep rheology. As the ice-shelf surface does not completely return to its un-flexed position after a 1-year fill/drain cycle, the lake basin deepens with each successive cycle. This deepening process is significantly amplified when lake-bottom ablation is taken into account. We evaluate the timescale over which a typical lake reaches a sufficient depth such that ice-shelf fracture can occur well beyond the lake itself in response to lake filling/drainage. We show that, although this is unlikely during one fill/drain cycle, fracture is possible after multiple years assuming surface meltwater availability is unlimited. This extended zone of potential fracture implies that flexural stresses in response to a single lake filling/drainage event can cause neighbouring lakes to drain, which, in turn, can cause lakes farther afield to drain. Such self-stimulating behaviour may have accounted for the sudden, widespread appearance of a fracture system that drove the Larsen B Ice Shelf to break-up in 2002.Alison Banwell acknowledges the support of an Antarctic Science International Bursary from Antarctic Science Ltd. and a Bowring Junior Research Fellowship from St Catharine’s College, Cambridge.This is the author accepted manuscript. The final version is available from Cambridge University Press via http://dx.doi.org/10.1017/S095410201500029
A model of viscoelastic ice-shelf flexure
AbstractWe develop a formal thin-plate treatment of the viscoelastic flexure of floating ice shelves as an initial step in treating various problems relevant to ice-shelf response to sudden changes of surface loads and applied bending moments (e.g. draining supraglacial lakes, iceberg calving, surface and basal crevassing). Our analysis is based on the assumption that total deformation is the sum of elastic and viscous (or power-law creep) deformations (i.e. akin to a Maxwell model of viscoelasticity, having a spring and dashpot in series). The treatment follows the assumptions of well-known thin-plate approximation, but is presented in a manner familiar to glaciologists and with Glen’s flow law. We present an analysis of the viscoelastic evolution of an ice shelf subject to a filling and draining supraglacial lake. This demonstration is motivated by the proposition that flexure in response to the filling/drainage of meltwater features on the Larsen B ice shelf, Antarctica, contributed to the fragmentation process that accompanied its collapse in 2002.Olga Sergienko acknowledges the support of National Oceanic and Atmospheric Administration of the US (NOAA) grant NA13OAR431009. Alison Banwell acknowledges the support of a Bowring Junior Research Fellowship from St Catharine’s College, Cambridge, and a bursary from Antarctic Science Ltd.This is the final version of the article. It first appeared from the International Glaciological Society via http://dx.doi.org/10.3189/2015JoG14J16
Breakup of the Larsen B Ice Shelf triggered by chain reaction drainage of supraglacial lakes
The explosive disintegration of the Larsen B Ice Shelf poses two unresolved
questions: What process (1) set a horizontal fracture spacing sufficiently small to
pre-dispose the subsequent ice-shelf fragments to capsize, and (2) synchronized the
widespread drainage of >2750 supraglacial meltwater lakes observed in the days
prior to break-up? We answer both questions through analysis of the ice shelf’s
elastic-flexure response to the supraglacial lakes on the ice shelf prior to break-up.
By expanding the previously articulated role of lakes beyond mere water-reservoirs
supporting hydrofracture, we show that lake-induced flexural stresses produce a
fracture network with appropriate horizontal spacing to induce capsize-driven
break-up. The analysis of flexural stresses suggests that drainage of a single lake can
cause neighboring lakes to drain, which, in turn, cause farther removed lakes to
drain. Such self-stimulating behavior can account for the sudden, widespread
appearance of a fracture system capable of driving explosive break-up.This research is supported by the U.S. National Science Foundation under grant
GEOP/ANT – 0944248 awarded to D.R.M and grants ANT-0838811 and ARC-0934534
awarded to O.V.S.This is the accepted manuscript. An edited version of this paper was published by AGU. Copyright 2014 American Geophysical Union
Seasonal evolution of supraglacial lakes on a floating ice tongue, Petermann Glacier, Greenland
ABSTRACTSupraglacial lakes are known to trigger Antarctic ice-shelf instability and break-up. However, to date, no study has focused on lakes on Greenland's floating termini. Here, we apply lake boundary/area and depth algorithms to Landsat 8 imagery to analyse the inter- and intraseasonal evolution of supraglacial lakes across Petermann Glacier's (81°N) floating tongue from 2014 to 2016, while also comparing these lakes to those on the grounded ice. Lakes start to fill in June and quickly peak in total number, volume and area in late June/early July in response to increases in air temperatures. However, through July and August, total lake number, volume and area all decline, despite sustained high temperatures. These observations may be explained by the transportation of meltwater into the ocean by a river, and by lake drainage events on the floating tongue. Further, as mean lake depth remains relatively constant during this time, we suggest that a large proportion of the lakes that drain, do so completely, likely by rapid hydrofracture. The mean areas of lakes on the tongue are only ~20% of those on the grounded ice and exhibit lower variability in maximum and mean depth, differences likely attributable to the contrasting formation processes of lakes in each environment.</jats:p
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Formation of pedestalled, relict lakes on the McMurdo Ice Shelf, Antarctica
ABSTRACTSurface debris covers much of the western portion of the McMurdo Ice Shelf and has a strong influence on the local surface albedo and energy balance. Differential ablation between debris-covered and debris-free areas creates an unusual heterogeneous surface of topographically low, high-ablation, and topographically raised (‘pedestalled’), low-ablation areas. Analysis of Landsat and MODIS satellite imagery from 1999 to 2018, alongside field observations from the 2016/2017 austral summer, shows that pedestalled relict lakes (‘pedestals’) form when an active surface meltwater lake that develops in the summer, freezes-over in winter, resulting in the lake-bottom debris being masked by a high-albedo, superimposed, ice surface. If this ice surface fails to melt during a subsequent melt season, it experiences reduced surface ablation relative to the surrounding debris-covered areas of the ice shelf. We propose that this differential ablation, and resultant hydrostatic and flexural readjustments of the ice shelf, causes the former supraglacial lake surface to become increasingly pedestalled above the lower topography of the surrounding ice shelf. Consequently, meltwater streams cannot flow onto these pedestalled features, and instead divert around them. We suggest that the development of pedestals has a significant influence on the surface-energy balance, hydrology and flexure of the ice shelf.Ia
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Diurnal seismicity cycle linked to subsurface melting on an ice shelf
ABSTRACTSeismograms acquired on the McMurdo Ice Shelf, Antarctica, during an Austral summer melt season (November 2016–January 2017) reveal a diurnal cycle of seismicity, consisting of hundreds of thousands of small ice quakes limited to a 6–12 hour period during the evening, in an area where there is substantial subsurface melting. This cycle is explained by thermally induced bending and fracture of a frozen surface superimposed on a subsurface slush/water layer that is supported by solar radiation penetration and absorption. A simple, one-dimensional model of heat transfer driven by observed surface air temperature and shortwave absorption reproduces the presence and absence (as daily weather dictated) of the observed diurnal seismicity cycle. Seismic event statistics comparing event occurrence with amplitude suggest that the events are generated in a fractured medium featuring relatively low stresses, as is consistent with a frozen surface superimposed on subsurface slush. Waveforms of the icequakes are consistent with hydroacoustic phases at frequency and flexural-gravity waves at frequency . Our results suggest that seismic observation may prove useful in monitoring subsurface melting in a manner that complements other ground-based methods as well as remote sensing.</jats:p
Calving and rifting on the McMurdo Ice Shelf, Antarctica
ABSTRACTOn 2 March 2016, several small en échelon tabular icebergs calved from the seaward front of the McMurdo Ice Shelf, and a previously inactive rift widened and propagated by ~3 km, ~25% of its previous length, setting the stage for the future calving of a ~14 km2 iceberg. Within 24 h of these events, all remaining land-fast sea ice that had been stabilizing the ice shelf broke-up. The events were witnessed by time-lapse cameras at nearby Scott Base, and put into context using nearby seismic and automatic weather station data, satellite imagery and subsequent ground observation. Although the exact trigger of calving and rifting cannot be identified definitively, seismic records reveal superimposed sets of both long-period (>10 s) sea swell propagating into McMurdo Sound from storm sources beyond Antarctica, and high-energy, locally-sourced, short-period (<10 s) sea swell, in the 4 days before the fast ice break-up and associated ice-shelf calving and rifting. This suggests that sea swell should be studied further as a proximal cause of ice-shelf calving and rifting; if proven, it suggests that ice-shelf stability is tele-connected with far-field storm conditions at lower latitudes, adding a global dimension to the physics of ice-shelf break-up.</jats:p
Ocean tides and Heinrich events
Climate varied enormously over the
most recent ice age1 — for example,
large pulses of ice-rafted debris2, originating
mainly from the Labrador Sea3, were
deposited into the North Atlantic at roughly
7,000-year intervals, with global climatic
implications3. Here we show that ocean
tides within the Labrador Sea were exceptionally
large over the period spanning
these huge, abrupt ice movements, which
are known as Heinrich events. We propose
that tides played a catalytic role in liberating
iceberg armadas during that time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/84375/1/nature_tidesheinrich.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/84375/2/432460a-s1.do
Meltwater produced by wind–albedo interaction stored in an East Antarctic ice shelf
Surface melt and subsequent firn air depletion can ultimately
lead to disintegration of Antarctic ice shelves1,2 causing
grounded glaciers to accelerate3 and sea level to rise. In
the Antarctic Peninsula, foehn winds enhance melting near
the grounding line4, which in the recent past has led to the
disintegration of the most northerly ice shelves5,6. Here, we
provide observational and model evidence that this process
also occurs over an East Antarctic ice shelf, where meltwaterinduced
firn air depletion is found in the grounding zone.
Unlike the Antarctic Peninsula, where foehn events originate
from episodic interaction of the circumpolar westerlies with
the topography, in coastal East Antarctica high temperatures
are caused by persistent katabatic winds originating from the
ice sheet’s interior. Katabatic winds warm and mix the air
as it flows downward and cause widespread snow erosion,
explaining >3 K higher near-surface temperatures in summer
and surface melt doubling in the grounding zone compared with
its surroundings. Additionally, these winds expose blue ice and
firn with lower surface albedo, further enhancing melt. The
in situ observation of supraglacial flow and englacial storage
of meltwater suggests that ice-shelf grounding zones in East
Antarctica, like their Antarctic Peninsula counterparts, are
vulnerable to hydrofracturing7
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