Добірка наукової літератури з теми "Basal crevasses"

Abstract. Basal crevasses extend upwards from the base of ice bodies and can penetrate more than halfway through the ice column under conditions found commonly on ice shelves. As a result, they may locally modify the exchange of mass and energy between ice shelf and ocean, and by altering the shelf's mechanical properties could play a fundamental role in ice shelf stability. Although early studies revealed that such features may be abundant on Antarctic ice shelves, their geometrical properties and spatial distribution has gained little attention. We investigate basal crevasses in Larsen C Ice Shelf using field radar survey, remote sensing and numerical modelling. We demonstrate that a group of features visible in MODIS imagery are the surface expressions of basal crevasses in the form of surface troughs, and find that basal crevasses can be generated as a result of stresses well downstream of the grounding line. We show that linear elastic fracture mechanics modelling is a good predictor of basal crevasse penetration height where stresses are predominantly tensile, and that measured surface trough depth does not always reflect this height, probably because of snow accumulation in the trough, marine ice accretion in the crevasse, or stress bridging from the surrounding ice. We conclude that all features visible in MODIS imagery of ice shelves and previously labelled simply as "crevasses", where they are not full thickness rifts, must be basal crevasse troughs, highlighting a fundamental structural property of many ice shelves that may have been previously overlooked.

Abstract. Basal crevasses extend upwards from the base of ice bodies and can penetrate more than halfway through the ice column under conditions found commonly on ice shelves. As a result, they may locally modify the exchange of mass and energy between ice shelf and ocean, and by altering the shelf's mechanical properties could play a fundamental role in ice shelf stability. Although early studies revealed that such features may be abundant on Antarctic ice shelves, their geometrical properties and spatial distribution has gained little attention. We investigate basal crevasses in Larsen C Ice Shelf using field radar survey, remote sensing and numerical modelling. We demonstrate that a group of features visible in MODIS imagery are the surface expressions of basal crevasses in the form of surface troughs, and find that basal crevasses can be generated as a result of stresses well downstream of the grounding line. We show that linear elastic fracture mechanics modelling is a good predictor of basal crevasse penetration height where stresses are predominantly tensile, and that measured surface trough depth does not always reflect this height, probably because of snow accumulation in the trough, marine ice accretion in the crevasse, or stress bridging from the surrounding ice. We conclude that all features visible in MODIS imagery of ice shelves and previously labelled simply as "crevasses", where they are not full thickness rifts, must be basal crevasse troughs, highlighting a fundamental structural property of many ice shelves that may have been previously overlooked.

AbstractWe identify a series of basal crevasses along a 31 km transect across the northern sector of the Larsen C ice shelf, Antarctica, using in situ ground-penetrating radar. The basal crevasses propagate from a region of multiple, shallow basal fractures to form widely spaced (0.5–2.0 km) but deeply incised (70–134 m) features. Surface troughs, observed in visible imagery, exist above the basal crevasses as the ice vertically shears to reach hydrostatic equilibrium, while widespread surface crevassing occurs along the crests and on the flanks of the undulations, primarily aligned with the topography. We suggest, based on the location of the surface crevasses and the along-flow evolution of the basal crevasses, that the former are induced by a bending stress created by gradients in hydrostatic forces. Using a linear elastic fracture mechanics model, we investigate the sensitivity of basal crevasse propagation to observed trends of ice-shelf thinning and acceleration. Basal crevasses are large-scale structural weaknesses that can both control meltwater ponding and induce surface crevassing. Together, these features may represent an important mechanism in both past and future ice-shelf disintegration events on the Antarctic Peninsula.

ABSTRACTThe presence of water-filled crevasses is known to increase the penetration depth of crevasses and this has been hypothesized to play an important role controlling iceberg calving rate. Here, we develop a continuum-damage-based poro-mechanics formulation that enables the simulation of water-filled basal and surface crevasse propagation. The formulation incorporates a scalar isotropic damage variable into a Maxwell-type viscoelastic constitutive model for glacial ice, and the effect of the water pressure on fracture propagation using the concept of effective solid stress. We illustrate the model by simulating quasi-static hydrofracture in idealized rectangular slabs of ice in contact with the ocean. Our results indicate that water-filled basal crevasses only propagate when the water pressure is sufficiently large, and that the interaction between simultaneously propagating water-filled surface and basal crevasses can have a mutually positive influence leading to deeper crevasse propagation, which can critically affect glacial stability. Therefore, this study supports the hypothesis that hydraulic fracture is a plausible mechanism for the accelerated breakdown of glaciers.

AbstractBasal crevasses may play an important precursory role in determining both the location and propagation of rifts and iceberg dimensions. For example, icebergs calved recently from Thwaites Glacier, Antarctica, have the same width as surface undulations, strengthening the connection between basal crevasses, rifting and calving. We explore a novel method for estimating the heights of basal crevasses formed at the grounding lines of ice shelves and ice streams. We employ a thin-elastic beam (TEB) formulation and tensional yielding criterion to capture the physics of flexed ice at grounding lines. Observations of basal crevasse heights compare well with model predictions in the Siple Coast region of the Ross Ice Shelf. We find that the TEB method is most accurate in areas of low strain rate. We also test the method in other areas of Antarctica to produce order-of-magnitude maps of grounding-line basal crevasses and find general agreement with reported observations assuming basal crevasses develop in spatio-temporal sequence and are advected downstream. This method is computationally cheap and could be relatively easy to implement into damage-oriented large-scale ice models which aim at physically simulating calving and fracture processes.

AbstractThe transition from limited-slip conditions at the base of grounded ice to free-slip conditions beneath floating ice occurs across the few-kilometers-wide grounding zone. This region involves either an elastic flexural transition from bedrock to hydrostatically supported elevations (often tidally influenced), a transition from thicker to thinner ice over a flat bed, or some combination of these two processes. In either case, ice must flow across a changing stress field, often resulting in brittle deformation, manifested as basal crevassing. Thus the position and morphology of basal crevasses reveal important information about the stress state across this transition. Our gridded ground-based radar surveys on Whillans Ice Stream, West Antarctica, indicate a complex pattern of basal crevasses, but most are associated with regions where the surface elevation gradient is steepest. Due to the high reflectivity of sea water, we image many off-nadir crevasses from a corner-reflector geometry involving reflections from the ice/sea-water interface and then from the crevasse, producing echoes with an inverted phase that could be misinterpreted as subglacial returns. Our results indicate that basal crevasses offer a rich dataset for diagnosing stress state and salient processes across grounding zones, and that special care is needed when interpreting subglacial returns in radar data.

AbstractSequential satellite imagery and modeling are used to investigate crevasse patterns at the head of Ice Stream B tributary B1b. The crevasses, informally called the “chromosomes”, form at the upstream limit to B1b’s northern shear margin and chaotic crevasse zone. We find that the onset to crevasse formation, and by inference the onset to streaming flow, has migrated upstream over time at a mean rate of 230(16) m a−1. A possible cause for that migration is changes in net basal friction due to changes in basal water production rate and storage.

Abstract. The Totten Ice Shelf (IS) has a large drainage basin, much of which is grounded below sea level, leaving the glacier vulnerable to retreat through the marine ice sheet instability mechanism. The ice shelf has also been shown to be sensitive to changes in calving rate, as a very small retreat of the calving front from its current position is predicted to cause a change in flow at the grounding line. Therefore understanding the processes behind calving on the Totten IS is key to predicting its future sea level rise contribution. Here we use the Helsinki Discrete Element Model (HiDEM) to show that not all of the fractures visible at the front of the Totten IS are produced locally, but that the across-flow basal crevasses, which are part of the distinctive cross-cutting fracture pattern, are advected into the calving front area from upstream. A separate simulation of the grounding line shows that re-grounding points may be key areas of basal crevasse production, and can produce basal crevasses in both an along- and across-flow orientation. The along-flow basal crevasses at the grounding line may be a possible precursor to basal channels, while we suggest the across-flow grounding-line fractures are the source of the across-flow features observed at the calving front. We use two additional models to simulate the evolution of basal fractures as they advect downstream, demonstrating that both strain and ocean melt have the potential to deform narrow fractures into the broad basal features observed near the calving front. The wide range of factors which influence fracture patterns and calving on this glacier will be a challenge for predicting its future mass loss.

Abstract. In cold and arid climates, small glaciers with cold accumulation zones are often thought to be entirely cold based. However, scattering in ground-penetrating radar (GPR) measurements on the Rikha Samba Glacier in the Nepal Himalayas suggests a large amount of temperate ice that seems to be influenced by the presence of crevassed areas. We used a coupled thermo-mechanical model forced by a firn model accounting for firn heating to interpret the observed thermal regime. Using a simple energy conservation approach, we show that the addition of water percolation and refreezing in crevassed areas explains these observations. Model experiments show that both steady and transient thermal regimes are significantly affected by latent heat release in crevassed areas. This makes half of the glacier base temperate, resulting in an ice dynamic mainly controlled by basal friction instead of ice deformation. The timescale of thermal regime change, in response to atmospheric warming, is also greatly diminished, with a potential switch from cold to temperate basal ice in 50–60 years in the upper part of the glacier instead of the 100–150 years that it would take without the effect of the crevasses. This study highlights the crucial role of water percolation through the crevasses on the thermal regime of glaciers and validates a simple method to account for it in glacier thermo-mechanical models.

Abstract. The marine-terminating outlet in Basin 3, Austfonna ice cap, has been accelerating since the mid-1990s. Stepwise multi-annual acceleration associated with seasonal summer speed-up events was observed before the outlet entered the basin-wide surge in autumn 2012. We used multiple numerical models to explore hydrologic activation mechanisms for the surge behaviour. A continuum ice dynamic model was used to invert basal friction coefficient distributions using the control method and observed surface velocity data between April 2012 and July 2014. This has provided input to a discrete element model capable of simulating individual crevasses, with the aim of finding locations where meltwater entered the glacier during the summer and reached the bed. The possible flow paths of surface meltwater reaching the glacier bed as well as those of meltwater produced at the bed were calculated according to the gradient of the hydraulic potential. The inverted friction coefficients show the “unplugging” of the stagnant ice front and expansion of low-friction regions before the surge reached its peak velocity in January 2013. Crevasse distribution reflects the basal friction pattern to a high degree. The meltwater reaches the bed through the crevasses located above the margins of the subglacial valley and the basal melt that is generated mainly by frictional heating flows either to the fast-flowing units or potentially accumulates in an overdeepened region. Based on these results, the mechanisms facilitated by basal meltwater production, crevasse opening and the routing of meltwater to the bed are discussed for the surge in Basin 3.

Majoriteten av geomorfologin i Sveriges landskap är produkten av den senaste nedisningen som avslutades för ca 8000 år sedan. De Geer-morän är en typ av moränavlagring som härstammar från denna nedisning. De Geer-moränavlagringarna har ett omdebatterat bildningssätt samt är landformer med en omtvistad innebörd i landskapen världen över. Vad är det som moränavlagringarna återspeglar? Är det avlagringar från svämmande glaciärer, avlagringar formade vid grundstötningslinjen för en ryckvis avsmältande glaciär eller kanske avlagringar bildade av sammanpressat material i basala sprickor hos en glaciär? Diskussionen om De Geer-moränen har varit långvarig och teorierna är många. Här är forskningen inte överens. Det förefaller också att studerad De Geer-morän på olika platser världen över inte nödvändigtvis behöver ha haft samma bildningsätt. I Sverige förekommer De Geer-morän främst i två områden, längs med kusten i Norrbotten och Västerbotten, samt vid den mellansvenska israndzonen. I dessa områden förekommer moränavlagringarna som små ryggar i svärmar med några meters höjd. De studier som gjorts i Sverige om De Geer-morän har förklarligt nog tenderat att fokusera på dessa två större områden. Men, utöver dessa två större områden finns det också ett mycket mindre område beläget längs med Sveriges västkust, ett område med samma typ av moränavlagringar. De Geer-moränen här, benämnda Hallands kustmorän, är både äldre och färre, dessutom är senaste nedisningens avsmältning i detta område mer ovisst. I denna uppsats gjordes ett försök till att övergripligt redogöra för några av de presenterade teorierna som föreligger De Geer-moränens bildningssätt. Uppsatsen syftar till att studera och kartera De Geer-moränen belägen längs med Sveriges västkust med hjälp av höjddata och sedan jämföra dessa avlagringar med den teori som presenteras i bakgrunden för denna uppsats. De Geer-moränen kommer även att jämföras med en isavsmältningsrekonstruktion gjord i programmet ICESHEET 1.0, med lokala karterade isräfflor i området samt även analyseras utifrån beskrivningarna till SGUs utgivna jordartskartor. Förhoppningen var att denna uppsatsen skulle kunna bringa mer klarhet om omfattningen och utbredningen av De Geer-moränen längs med Sveriges västkust. Resultatet som erhölls presenterades bland annat visuellt med framtagna kartbilder. Några av de slutsatser som kunde dras utifrån denna studie var att De Geer-moränen längs med Sveriges västkust är starkt påverkade av svallning och överlagring av yngre sediment vilket gjorde dem svåra att upptäcka i landskapet. Högst troligt är det att förekomsten av De Geer-morän längs med Sveriges västkust är större än vad som syns vid dagens marknivå på grund av överlagring, dessutom tyder mycket på att avlagringarna fortsätter ut i Kungsbackafjorden och Kattegatt. Det gick för det studerade området att se att det fanns ett samband mellan jorddjup och förekomsten av De Geer-morän. Ryggarna var främst avlagrade ovanpå drumliner och förekom främst i områden med ett jorddjup på ≥ 5m. Det gick inte att dra några slutsatser om De Geer-moränens bildningssätt i området.
Most of the geomorphology in the Swedish landscape is a product of the last deglaciation that ended about 8000 years ago. De Geer moraine is moraine deposits that has debated origins as well as debated implications worldwide. What message does the deposits send? What kind of conditions do they reflect? Are they perhaps the deposits of surging glaciers, or deposits formed at the grounding lines of melting glaciers or perhaps are they squeezed ridges formed in basal crevasses of glaciers? The discussion in research worldwide about the origin of these features have been going on for a long time and there is a lot of different theories about how the deposits are formed. The De Geer moraines in Sweden are mainly located in two larger areas, one of them being along the east coast in Norrbotten and Västerbotten, and the other one being a large belt of deposits that are located parallel to the big Swedish lake Vänern. The majority of the published articles about De Geer moraine in Sweden are therefore understandably about these two larger areas, however there are also a much smaller area of De Geer moraine along Sweden’s west coast. These deposits are called Halland’s coastal moraines and are not studied to the same extent as the other two areas. This bachelor thesis aims to, in a general way, describe some of the different theories there are about the formation of these ridges. The purpose of this thesis is to study and map the De Geer moraine along the Swedish west coast and compare them to the theory featured in the background of this paper. The ridges will then also be studied on the basis of a glacier melting reconstruction in the computer software ICESHEET 1.0, of ice striations in the area and lastly on the basis of the descriptions to the quaternary maps published by Geological Survey of Sweden. Hopefully, this paper will bring some clarity about the De Geer moraine ridges along the Swedish west coast. The results from this thesis were presented visually in different types of map sheets. Some of the conclusions that could be drawn from this bachelor thesis was that ridges along the Swedish west coast were both wave-washed and superimposed by younger sediments which therefore made them hard to both map and distinguish. There is a high probability that the occurrence of De Geer moraine is much larger in the area than what could be seen at today’s ground level. Also, the distribution of the ridges suggested that they continued out in Kattegatt and the Kungsbackafjord. In general, the ridges were superimposed on drumlins and there was shown to be a connection between soil depth and the occurrence of De Geer moraine, the ridges mainly occurred where the soil depth were ≥5 metres. Despite of this, no conclusion could be drawn about the formation of the features.