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SPRI Review 2007: Polar Physical Science

Polar Physical Science

Iceberg calving flux and mass balance of Austfonna, Nordaustlandet, Svalbard

Satellite radar interferometry, 60 MHz airborne ice-penetrating radar data and visible-band satellite imagery were used to calculate the velocity structure, ice thicknes, and changing ice-marginal extent of Austfonna, the largest ice cap in the Eurasian Arctic at 8,120 km2. Most drainage basins of Austfonna have undergone ice-marginal retreat over the past few decades at an average of a few tens of metres per year. Integrating margin change around the whole ice cap gives a total area loss of about 10 km2 yr-1. When mass loss by iceberg production is taken into account, the total mass balance of Austfonna is negative, by between about 2.5 and 4.5 km3 yr-1. The iceberg flux represents about one third of total annual mass loss from Austfonna, with the remainder through surface ablation. Iceberg flux should be included in calculations of the total mass balance of the many large Arctic ice caps, including those located in the Russian and Canadian Arctic that end in tidewater. The neglect of this term has led to underestimates of mass loss from these ice caps and, thus, to underestimates of the contribution of Arctic ice caps to global sea-level rise.

Julian Dowdeswell and Toby Benham

The Isis Remotely Operated Vehicle being deployed in Antarctic waters
Image as described adjacent

Geophysical investigations of a large subglacial palaeolake in the Canadian Arctic

Subglacial lakes identified beneath the Antarctic Ice Sheet are one of Earth's remaining unexplored environments. A key to understanding the origin and longevity of subglacial lakes is contained in their sedimentary record. We have explored the nature of a sedimentary succession in a deep tectonic trough identified as the likely site of a former subglacial lake. The trough is the 100-km-long and 620-m-deep Christie Bay, located in the eastern arm of the Great Slave Lake, Canada. High-resolution seismic reflection data and short sediment cores collected in the deep trough show a 150-m-thick sequence of fine-grained material separating ice-contact deposits from draped Holocene lake sediments. This sequence has been interpreted to comprise sediments that accumulated in a subglacial lake that covered an area larger than 130 km2. The inferred presence of a subglacial water body is supported by results from hydrological modelling of drainage pathways beneath the North American Ice Sheet during the last glacial maximum. The geophysical record from Christie Bay points towards the existence of a dynamic subglacial lake environment where sediments were delivered by discharge of meltwater from a subglacial water system. The project is a collaboration with University of California, Santa Cruz, and the Large Lakes Observatory, University of Minnesota, Duluth.

Poul Christoffersen

SPRI staff making field measurements on an Icelandic glacier
Image as described adjacent

The tundra-taiga ecotone

SPRI Review 2006 described the international research collaboration 'PPS Arctic', a core activity of the International Polar Year intended to develop an improved understanding of the transition zone between the boreal forest and the Arctic tundra. PPS Arctic is jointly coordinated by the Norwegian Institute for Nature Research (NINA) and by SPRI. During 2007, coordinating meetings were held in Tromsø and Cambridge, and an internationally agreed set of measurement protocols was developed. Research activity took place across the circumarctic region from Alaska eastwards to European Russia. In Europe, the main focus of field-based researchwas northern Norway and the Kola Peninsula (Russia). As part of this activity, SPRI contributed to the running of a pilot summer school held in Kirovsk, in the Russian Arctic, to teach field-based and remote sensing methods of ecosystem assessment. Laboratory-based research included the development of techniques for analysing airborne multispectral imagery and laser profiler data from the forest edge, vegetation mapping at global scale using multitemporal satellite imagery and close-range photogrammetric methods for measuring forest structural parameters.

Gareth Rees

Understanding contemporary changes in the West Antarctic Ice Sheet

Recent satellite observations of the Antarctic Ice Sheet show dramatic changes over the last two decades. The first occurs near the coast of the Amundsen Sea where Pine Island Glacier appears to be thinning at a rate of several metres per year. A second change is found along the Siple Coast where one ice stream is thickening due to stagnation while others are slowing down. Contemporary change near the Amundsen Sea may be caused by warm coastal waters entering glacial troughs, leading to increased basal melting and thinning through buoyancy. The contrasting behaviour of the Siple Coast ice streams is not caused by changes in the oceans, as these glaciers are protected by the Ross Ice Shelf, which covers an area of approximately 400,000 km2. Contemporary change in this region is more likely linked to internal variability related to the basal thermal regime and its influence on the mechanical properties of subglacial sediments. The aim of this project is to determine the cause and consequences of contemporary change in different parts of the Antarctic Ice Sheet. We are developing a numerical ice-flow model for the West Antarctic Ice Sheet and its ice streams. The model will feature accurate predictive ability for simulation of the 21st century when coupled to an Earth-system model. The study will make use of a wide range of observations including satellite remote imaging, airborne surveys and ground-based field campaigns. It links expertise in subglacial processes and ice-stream dynamics (Scott Polar Research Institute), large-scale numerical modelling (University of Bristol) and data assimilation techniques (University of Durham). The project is funded by grants from the Natural Environmental Research Council.

Marion Bougamont and Poul Christoffersen

Geometry and mass balance of Langjökull, Iceland

Currently, about 11% (11 200km2) of Iceland is covered by ice. Most is contained within extensive plateau ice caps. The maritime climate means these ice caps receive up to 4 m yr-1 of water-equivalent of snowfall in their accumulation zones and lose about 10 m yr-1 of ice in their smaller ablation areas. This, together with the fact they are temperate, means they are dynamically responsive to small climatically induced mass-balance changes. Langjökull (925 km2) is the second largest ice cap in Iceland and has surface mass balance measurements extending back to 1996. To complement these mass balance measurements, we are currently analysing patterns of surface elevation change across the ice cap. We are using three main data sets: lines of GPS point measurements and a 100 m resolution DEM derived from them for the entire ice cap for 1995; airborne photogrammetrically-derived point measurements and a 25m resolution DEM derived from them for the southern outlet glacier, Hagafellsjökull Vestari; and airborne LiDAR-derived point measurements and a 25 m resolution DEM derived from them for the entire ice cap for 2007. Preliminary results suggest that there are marked spatial variations in elevation changes across the ice cap, reflecting spatial variations in surface mass balance, moderated by changes in ice dynamics. In particular, much of the elevation change observed in the south of the ice cap is associated with the surge of Hagafellsjökull Eystri in 1998/9 and, to a lesser extent, the Hagafellsjökull Vestari surge of 1979/80. Our work is being done in collaboration with Richard Hodgkins (Loughborough University), Adrian Fox (British Antarctic Survey), and Helgi Björnsson and Sverrir Guðmundsson (University of Iceland).

Ian Willis, Neil Arnold and Gareth Rees

Modelling the mass balance of Svalbard glaciers

The Arctic climate is currently warming at a faster rate than observed elsewhere on Earth and future projections suggest this trend will continue well into the 21st century. With glaciers and ice caps covering ~36,600 km2, Svalbard is one of the largest glaciated areas in the Arctic. Future climate change will significantly alter the mass balance of glaciers and ice caps across the archipelago with important consequences for sea level. We are currently developing a numerical mass balance model, which will be used to calculate the spatial and temporal variations in mass balance of the archipelago's ice masses. It is currently being trained and validated for the glacier Midre Lovénbreen, NW Spitsbergen where we have been working for the last few years. The model uses air temperature and precipitation to calculate patterns of accumulation, and an energy balance approach to determine surface melt variations. Recent improvements to the model include a subsurface routine to deal with the processes of conduction and melt water refreezing within the snowpack. We have run the model for the year 2000 using statistically downscaled ERA-40 reanalysis data. The downscaled reanalysis data match measured surface meteorological data very well, and the complete mass balance model reproduces the measured surface mass balance patterns exceptionally well. The work is being carried out together with SPRI PhD student, Cameron Rye.

Ian Willis and Neil Arnold

Cryosat

SPRI staff continued to participate in international campaigns to validate data collected by a new radar altimeter (SIRAL) to be carried by the Cryosat satellite. In Spring 2007, a two-person team from SPRI joined Norwegian and Swedish glaciologists on the Austfonna Ice Cap, Nordaustlandet in eastern Svalbard. Measurements of snow density profiles were made using an automated neutron profiling system and compared with radar returns from a very high bandwidth (VHB) ground–based radar and the airborne ASIRAS radar altimeter, both of which operate at the same band-width as SIRAL. These measurements have extended the Cryosat pre-launch validation studies to an area of high melt rates and formation of thick ice layers within the firn. The snow density profiles were also used to help interpretation of the results of an 800 MHz Ground Penetrating Radar survey and map recent fluctuations in the firn area of Austfonna. During the year, preparations also began for a traverse of the North Greenland Ice Sheet to extend pre-launch validation activities to a region of the dry snow zone where the correlation coefficient of height change versus power change in the ENVISAT altimeter data is high. Density data collected along the traverse will assist in the analysis of this phenomenon. This will be the last of the pre-launch activities, as Cryosat is due to be launched in 2009.

Liz Morris

A major submarine fan in the Bellingshausen Sea, West Antarctica

A 330-km length of the little known continental shelf edge and slope of the Bellingshausen Sea, West Antarctica, was investigated using multibeam swath-bathymetric and sub-bottom profiler evidence. When full-glacial ice advanced across the Antarctic continental shelf to reach the shelf edge, it was partitioned into fast-and slow-flowing elements, with an ice stream filling the trough. This had important consequences for the nature and rate of sediment delivery to the adjacent continental slope. Off Belgica Trough, the upper continental slope has convex-outward contours indicating a major sedimentary fan. Acoustic profiles and cores from the area show that the fan is built from a series of glacigenic debris flows, whose sediment was delivered from a former ice stream that filled the trough under full-glacial conditions. Other morphological features on the Bellingshausen Sea slope are gully systems and channels. The channels provide pathways for sediment by-passing of the upper slope and transfer to the deep ocean. Belgica Fan is about 20,000 km2 in area and about 60,000 km3 in volume. It is the largest submarine fan on the continental margin of the West Antarctic Ice Sheet, fed by an interior ice-sheet basin of approximately 200,000 km2. This work was undertaken in collaboration with staff at the British Antarctic Survey and Durham University.

Julian Dowdeswell, Riko Noormets and Jeff Evans

Assessing the role of subglacial hydrology in the flow of West Antarctic ice streams: a numerical modelling approach

The West Antarctic ice sheet is a marine ice sheet with a bed that lies largely below sea level. It is potentially unstable, and there is a possibility that ice from the interior of the ice sheet may suddenly discharge to the ocean leading to rises in global sea level. Most of the present ice discharge occurs in fast moving ice streams. An understanding of the dynamics of these ice streams is crucial if we are to predict the future evolution of the ice sheet and its effect on global sea level. The most studied ice streams in West Antarctica are those located along the Siple Coast, comprising six major ice streams separated by a series of ice ridges. These ice streams exhibit a century-scale variability in ice discharge and flow direction, that has a strong control on the mass balance of the West Antarctic ice sheet. Our modelling initiative is motivated by new findings showing rapid movement of water stored in lakes beneath the Siple Coast ice streams. To ascertain the role that changes in subglacial hydrology and water storage play on the flow of Antarctic ice streams, we intend to couple a subglacial hydrological model with an existing ice stream model. The coupled model will be applied to the stagnant Ice Stream C and the fast flowing Ice Stream B. Observations show that the subglacial hydrological system beneath these glaciers may exert an important control on their motion. Water is observed to accumulate behind the trunk of Ice Stream C, which stopped approximately 150 years ago, causing an ice bulge to advance progressively downglacier. Subglacial water beneath Ice Stream B, which is currently slowing down, is discharging into the Ross Sea in characteristic discharge events. The temporal character of the subglacial hydrological system, which appear to control ice stream shutdown and reactivation, may thus be a crucial factor when assessing the future contribution to sea level rise from the West Antarctic Ice Sheet. This work is being carried out together with SPRI Ph.D. student, Narelle Baker.

Neil Arnold and Poul Christoffersen