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

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

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

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

Controls on rapid supraglacial lake drainage in West Greenland: An Exploratory Data Analysis approach

ABSTRACTThe controls on rapid surface lake drainage on the Greenland ice sheet (GrIS) remain uncertain, making it challenging to incorporate lake drainage into models of GrIS hydrology, and so to determine the ice-dynamic impact of meltwater reaching the ice-sheet bed. Here, we first use a lake area and volume tracking algorithm to identify rapidly draining lakes within West Greenland during summer 2014. Second, we derive hydrological, morphological, glaciological and surface-mass-balance data for various factors that may influence rapid lake drainage. Third, these factors are used within Exploratory Data Analysis to examine existing hypotheses for rapid lake drainage. This involves testing for statistical differences between the rapidly and non-rapidly draining lake types, as well as examining associations between lake size and the potential controlling factors. This study shows that the two lake types are statistically indistinguishable for almost all factors investigated, except lake area. Thus, we are unable to recommend an empirically supported, deterministic alternative to the fracture area threshold parameter for modelling rapid lake drainage within existing surface-hydrology models of the GrIS. However, if improved remotely sensed datasets (e.g. ice-velocity maps, climate model outputs) were included in future research, it may be possible to detect the causes of rapid drainage.</jats:p

Modeling the response of subglacial drainage at Paakitsoq, west Greenland, to 21^st century climate change

Although the Greenland Ice Sheet (GrIS) is losing mass at an accelerating rate, much uncertainty remains about how surface runoff interacts with the subglacial drainage system and affects water pressures and ice velocities, both currently, and into the future. Here, we apply a physically-based, subglacial hydrological model to the Paakitsoq region, west Greenland, and run it into the future to calculate patterns of daily subglacial water pressure fluctuations in response to climatic warming. The model is driven with moulin input hydrographs calculated by a surface routing model, forced with distributed runoff. Surface runoff and routing are simulated for a baseline year (2000), before the model is forced with future climate scenarios for the years 2025, 2050 and 2095, based on the IPCC’s Representative Concentration Pathways (RCPs). Our results show that as runoff increases throughout the 21st century, and/or as RCP scenarios become more extreme, the subglacial drainage system makes an earlier transition from a less efficient network operating at high water pressures, to a more efficient network with lower pressures. This will likely cause an overall decrease in ice velocities for marginal areas of the GrIS. However, short-term variations in runoff, and therefore subglacial pressure, can still cause localized speedups, even after the system has become more efficient. If these short-term pressure fluctuations become more pronounced as future runoff increases, the associated late-season speedups may help to compensate for the drop in overall summer velocities, associated with earlier transitioning from a high to a low pressure system.This work was funded by a Derek Brewer MPhil Studentship (Emmanuel College, Cambridge) awarded to J.R.M, a UK Natural Environment Research Council Doctoral Training Grant to A.F.B. (LCAG/133) (CASE Studentship with the Geological Survey of Denmark and Greenland (GEUS)), and a Bowring Junior Research Fellowship (St Catharine’s College, Cambridge), also to A.F.B.This is the accepted manuscript. An edited version of this paper was published by AGU. Copyright 2015 American Geophysical Union

Antarctic surface hydrology and impacts on ice-sheet mass balance

Melting is pervasive along the ice surrounding Antarctica. On the surface of the grounded ice sheet and floating ice shelves, extensive networks of lakes, streams and rivers both store and transport water. As melting increases with a warming climate, the surface hydrology of Antarctica in some regions could resemble Greenland’s present-day ablation and percolation zones. Drawing on observations of widespread Antarctica surface water and decades of study in Greenland, we consider three modes by which meltwater could impact Antarctic mass balance: increased runoff, meltwater injection to the bed, and meltwater-induced ice-shelf fracture, all of which may contribute to future ice sheet mass loss from Antarctica

Dual-satellite (Sentinel-2 and Landsat 8) remote sensing of supraglacial lakes in Greenland

© 2018 All rights reserved. Remote sensing is commonly used to monitor supraglacial lakes on the Greenland Ice Sheet (GrIS); however, most satellite records must trade off higher spatial resolution for higher temporal resolution (e.g. MODIS) or vice versa (e.g. Landsat). Here, we overcome this issue by developing and applying a dual-sensor method that can monitor changes to lake areas and volumes at high spatial resolution (10-30&thinsp;m) with a frequent revisit time ( ~ 3 days). We achieve this by mosaicking imagery from the Landsat 8 Operational Land Imager (OLI) with imagery from the recently launched Sentinel-2 Multispectral Instrument (MSI) for a ~ 12 000 km2area of West Greenland in the 2016 melt season. First, we validate a physically based method for calculating lake depths with Sentinel-2 by comparing measurements against those derived from the available contemporaneous Landsat 8 imagery; we find close correspondence between the two sets of values (R2Combining double low line 0.841; RMSE Combining double low line 0.555 m). This provides us with the methodological basis for automatically calculating lake areas, depths, and volumes from all available Landsat 8 and Sentinel-2 images. These automatic methods are incorporated into an algorithm for Fully Automated Supraglacial lake Tracking at Enhanced Resolution (FASTER). The FASTER algorithm produces time series showing lake evolution during the 2016 melt season, including automated rapid ( ≤ 4 day) lake-drainage identification. With the dual Sentinel-2-Landsat 8 record, we identify 184 rapidly draining lakes, many more than identified with either imagery collection alone (93 with Sentinel-2; 66 with Landsat 8), due to their inferior temporal resolution, or would be possible with MODIS, due to its omission of small lakes &lt; 0.125 km2. Finally, we estimate the water volumes drained into the GrIS during rapid-lake-drainage events and, by analysing downscaled regional climate-model (RACMO2.3p2) run-off data, the water quantity that enters the GrIS via the moulins opened by such events. We find that during the lake-drainage events alone, the water drained by small lakes ( &lt; 0.125 km2) is only 5.1 % of the total water volume drained by all lakes. However, considering the total water volume entering the GrIS after lake drainage, the moulins opened by small lakes deliver 61.5 % of the total water volume delivered via the moulins opened by large and small lakes; this is because there are more small lakes, allowing more moulins to open, and because small lakes are found at lower elevations than large lakes, where run-off is higher. These findings suggest that small lakes should be included in future remote-sensing and modelling work.NER

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

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 ${\bf \gt} \bf 75\,{\bf Hz}$ and flexural-gravity waves at frequency $\bf {\bf \lt}25\,{\bf Hz}$. 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