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SPRI Review 2009: Polar Physical Science

Polar Physical Science

Ice-covered mountain tops on the Antarctic Peninsula
Ice-covered mountain tops on the Antarctic Peninsula

Rates of sediment delivery from the Fennoscandian Ice Sheet through an Ice Age

Sediment delivery rates through an entire Ice Age were investigated using seismic records of glacial erosion products from the Fennoscandian Ice Sheet deposited offshore of mid-Norway in the 2.7 million year old Naust Formation. The mean sedimentation rate over the Ice Age is about 0.25 m per thousand years, with bedrock lowering of about 520 m in the ice-sheet catchment. The mean sediment delivery is 2 to 3 times higher for the most recent 600 thousand years than for earlier Naust sequences. The hypothesis that glacial erosion is most rapid early in an Ice Age, when there was presumably much weathered bedrock and preglacial sediment available, is not accepted for our study area. More important are changing ice-sheet dimensions and dynamics, varying intensity of individual glacial cycles, and complex ice-sheet dynamics in single deglaciations. The mean rate of sediment delivery from ice sheets is an order of magnitude higher than from fluvial activity in the huge Amazon and Mississippi systems, implying that ice is a very effective agent of long-term denudation during cold periods of Earth history. This work is collaborative with Dag Ottesen and Leif Rise of the Norwegian Geological Survey.

Julian Dowdeswell

Circum-Arctic treeline research

Activity related to the international research consortium PPS Arctic continued with a period of summer fieldwork. Studies in northern Sweden have focussed on the birch forest line. Extensive descriptions of vegetation for the period 1905–1912 have been compared with data from the late 20th century and from satellite images to 2009. Results show an altitudinal advance of the birch forest line by up to 100 m. Over the last three decades the advance has been 20-30 m at most. Only part of this advance is attributed to the measured warming of about 1.5 ˚C; other significant factors are reduced grazing pressure and reduced harvest of firewood, both arising from a change in the reindeer herding system in the early 20th century. A similar analysis has been carried out for Finnmark county, Norway, with historical data from as early as 1914 and satellite imagery from 2007. The birch forest line has advanced by around 12 km in the eastern and western parts of the county, but up to 22 km in the middle. These changes are again attributed to a combination of climate warming and a change in the grazing regime. Three key sites on the Russian Kola Peninsula have also been investigated: a very sheltered site in the Khibiny mountains, an industrially exposed site in the Monchetundra range, and a more oceanic site near lake Kanentiavr in the northern part of the peninsula. Treelines at the Khibiny site have moved upslope by about 20-30 m in the last 50 years; those in the industrially exposed areas have moved downslope by hundreds of metres, while in the north the treelines have scarcely moved over the same period. Collectively, these sites have illustrated the wide diversity of factors controlling the position of the tree and forest lines in the north. The European part of the consortium is principally supported through the 'Benefits' project, funded by the Research Council of Norway.

Gareth Rees and Olga Tutubalina

Investigating the dynamic response of the Greenland Ice Sheet to climatic forcing

A growing body of scientific evidence shows that the Greenland Ice Sheet is losing mass in response to climate change. The mass loss of the 1990s is estimated to be around 50 gigatons per year, but the imbalance has grown to almost 200 gigatons per year since 2000. Worldwide concern is associated with this trend because it equates to sea-level rise of 0.5 mm per year. The mass loss is also important because surface melt from Greenland affects sensitive sites of North Atlantic deep-water formation. Surface meltwater drains seasonally to the bed below the ablation zone of the Greenland Ice Sheet and this process is particularly significant on the land-terminating parts of the ice sheet. The mechanism is precarious because accelerated ice flow causes thinning, which in turn leads to an increase in surface melt since a larger part of the ice sheet moves into lower, and therefore warmer, elevations. In a new project funded by the Natural Environment Research Council, members of the Scott Polar Research Institute will observe surface meltwater and ice sheet dynamics using ground-based geophysics, satellite remote-sensing and numerical ice flow models. The integration of observational data into numerical ice-flow models is important because existing models used to project sea-level and freshwater fluxes do not feature atmospherically-coupled hydrological mechanisms.

Poul Christoffersen

Submarine landforms and a geomorphic model based on marine-geophysical data from Svalbard

Well-preserved submarine landforms from the continental shelf and fjords of north-west Svalbard, surveyed using multibeam echo-sounding, provide an example of ice-sheet deposition in an inter-ice stream setting. At the shelf edge, a distinctive and continuous belt of hummocky topography represents the grounding-zone of a slow-moving ice sheet present about 20,000 years ago. Low amplitude transverse moraines cross-cut the lineations, suggesting ice retreat across the outer shelf with brief still-stands. On the middle and inner shelf, large moraine ridges indicate multiple still-stands during deglaciation. Clusters of smaller transverse ridges indicate slow retreat of grounded ice through the fjords. Holocene sedimentation is by rainout from sediment-rich meltwater, producing smooth basin fill. Small slides from the fjord walls are common. Little Ice Age glacier-readvance produced another set of terminal moraines and smaller retreat moraines in the innermost fjords. A schematic model of this inter-ice stream glacial landform assemblage summarizes the geomorphic record. It is compared with a model derived from several Svalbard cross-shelf troughs occupied by fast-flowing full-glacial ice streams. In general, the sea-floor morphology of continental margins affected by ice streams is dominated by streamlined, subglacially produced landforms oriented in the former ice-flow direction, interrupted by major grounding-zone wedges formed during temporary halts in ice retreat. By contrast, between ice streams, shelf and fjord morphology records submarine landforms of various dimensions oriented mainly transverse to ice flow, produced at slowly retreating grounded ice-sheet margins. This work was undertaken in collaboration with Dag Ottesen of the Norwegian Geological Survey

Julian Dowdeswell

CryoSat-2

Liz Morris continued to participate in an international programme to validate data collected by a new radar altimeter (SIRAL) to be carried by the CryoSat-2 satellite, due for launch in February 2010. Analysis of data collected over a series of field seasons continued and a paper on the spatial variability of snow density in Greenland was presented at the Autumn Meeting of the American Geophysical Union. For the first time, it has been possible to determine the separate contributions to ice-sheet elevation change arising from short-term fluctuations in accumulation and compaction.

The analysis showed that these two contributions offset each other, and that it should therefore be possible to distinguish the long-term trend in ice-sheet elevation after a relatively short period of about three years of satellite altimetry data. Preparations began for a repeat traverse along the EGIG line in Greenland in 2010. As in 2004 and 2006, measurements of snow density profiles will be made using an automated neutron profiling system. This will be the first of the post-launch field activities.

Liz Morris

Snow mechanics

Adrian McCallum continued his study of the mechanical properties of snow. With support from Lankelma and the British Antarctic Survey (BAS), he has begun field studies of the strength of snow on the Brunt Ice Shelf in Antarctica. Using a commercial cone penetrometer system, adapted for use in the Antarctic, he is profiling the fracture strength of the natural snow cover and of snow roads and runways prepared using heavy machinery. This research will assist the BAS construction team in planning the transport of a new research station to its final position on the ice shelf. Early results show that cone penetration tests can reveal strong and weak layers in natural snow and the effects of various techniques for forming hard pavements by reworking and compressing the surface snow.

Liz Morris

Effects of digital elevation model resolution on solar radiation calculations on a High Arctic Glacier

Solar radiation is a fundamental control on glacier mass balance, often accounting for 75% or more of the energy available for melt at a glacier surface. To calculate solar radiation receipts, an accurate Digital Elevation Model (DEM) of the glacier and its surrounding mountainous topography is required. Shading by the surrounding terrain, as well as slope and aspect variations over the glacier surface itself, affect the amount of direct solar radiation received; restrictions to the glacier's 'sky-view' by surrounding high relief will reduce the amount of diffuse solar radiation and incoming long-wave radiation to the glacier surface. We have developed a numerical model to calculate solar radiation receipts over a glacier surface, which includes the effects of shading, slope, aspect and sky-view, and we have used this model to investigate the impact of DEM spatial resolution on calculated solar radiation receipts. Our DEMs are based on very high resolution data from Midre Lovénbreen, Svalbard, obtained by SPRI in 2003 and 2005. Degrading DEM resolution has a large impact on calculated radiation totals, as shading by surrounding high ground is progressively under-estimated as DEM resolution coarsens. Changing the resolution from 5 m to 50 m leads to a 5% overestimate in the solar radiation totals; from 50 m to 2000 m there is a further overestimate by over 20% in calculated radiation. Because of the dominance of solar radiation on melt, these results highlight the need for accurate, high-resolution topographic data as one of the key inputs to glacier mass balance models.

Neil Arnold and Gareth Rees

Geometry and mass balance of Langjökull, Iceland

An airborne laser survey of Langjökull, Iceland's second largest ice cap (950 km2), has generated an accurate and detailed Digital Elevation Model (DEM) of the ice cap, although one which was significantly incomplete. We have developed a new technique to fill in the gaps in this fragmented DEM using photoclinometry ('shape-from-shading') applied to the infrared channel of a Landsat satellite image. The estimated accuracy of the interpolated parts of the DEM is around 3 m. Comparison of this new DEM with one generated in 1997 shows that Langjökull has a specific annual mass balance of close to –1 metre per year water equivalent. If this rate is maintained, it would cause the complete disappearance of the ice cap within approximately 200 years. The new DEM is now being used to investigate the dynamics of the ice cap, particularly the possibility of a future surge of the outlet Hagafellsjökull Vestari, and possible discrepancies between in situ measurements of mass balance and those derived from LiDAR. Our work is being done in collaboration with Richard Hodgkins (Loughborough University), Adrian Fox (British Antarctic Survey), and Helgi Björnsson and Sverrir Gudmundsson (University of Iceland).

Gareth Rees, Ian Willis, Neil Arnold, Allen Pope

Modelling the mass balance of Svalbard glaciers

The Arctic climate is currently warming at a faster rate than 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 have developed 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. 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. This model has been trained and validated for the glacier Midre Lovénbreen, NW Spitsbergen, using mass balance data from 1968–2001, and climate data from the period 1958–2001. The results of the complete mass balance model reproduce the measured surface mass-balance patterns well. We also undertook fieldwork in the spring of 2009 on Midre Lovénbreen to investigate the role played by re-freezing of meltwater (by processes known as superimposed ice formation and internal accumulation) on the mass balance of Arctic glaciers, processes now included in the model. This fieldwork involved taking shallow cores from the glacier accumulation area to measure snow and firn stratigraphy, density and temperature, and also making shallow Ground Penetrating Radar (GPR) surveys over and between the boreholes. This work has been carried out with SPRI doctoral student, Cameron Rye, and in collaboration with Dr Jack Kohler of the Norwegian Polar Institute.

Neil Arnold and Ian Willis

Iceberg offshore of Illulissat, West Greenland

Iceberg offshore of Illulissat, West Greenland