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SPRI Review 2004: Polar Physical Science

Polar Physical Science

Warm ocean triggers ice drawdown from the West Antarctic Ice Sheet

The response of the West Antarctic Ice Sheet to global warming is of great concern because, if it were to melt completely, it would raise sea level by approximately 7 m. Such massive melting is unlikely to occur soon; nevertheless, there is still the potential for a marked increase in the rate of sea level rise due to accelerated ice loss. The great majority of the ice lost today from West Antarctica flows to the sea as ice streams, of which that of Pine Island Glacier is the most important. Pine Island Glacier, and the floating ice shelves at its terminus in Pine Island Bay, have thinned significantly over the past 3 decades. Satellite altimetry has been used to document how the floating ice shelves have thinned, and thinning is attributed to melting caused by the action of ocean currents that are about 0.5°C warmer than freezing. The hypothesis that changes in the grounded glaciers up to 200 km upstream were triggered by this ocean disturbance was then tested using a numerical ice-flow model. The results demonstrate how thinning of coastal ice shelves is transmitted rapidly to the grounded ice streams above, revealing a close coupling between the ice sheet interior and the surrounding ocean. Together, the work shows that recent ocean temperature increases in Pine Island Bay have caused a massive ice drawdown in West Antarctica that constitutes the greatest contribution to present day sea level rise of all Earth's cryosphere. This work is collaborative with Tony Payne of Bristol Glaciology Centre.

Andy Shepherd

Morphology and sedimentary processes on the continental slope off Pine Island Bay, Amundsen Sea, West Antarctica

The nature of the continental slope and shelf break in the isolated Amundsen Sea sector of the Antarctic margin, south of the Pacific Ocean, was examined during a recent cruise of the RRS James Clark Ross. Marine-geophysical methods were used to investigate a 750 km-long section of this margin between 100º and 115º W, beyond the wide continental shelf forming Pine Island Bay. About 300,000 km2 of the modern West Antarctic Ice Sheet drains into this bay, mainly through two major ice streams named Pine Island and Thwaites glaciers. Morphological evidence, in the form of large-scale streamlined sedimentary bedforms, suggests that fast-flowing ice streams extended to the outer shelf edge under full-glacial conditions about 18,000 years ago and have retreated about 350-400 km since then. Networks of gullies and channels dominate the slope adjacent to Pine Island Bay and act as conduits for coarsegrained sediment transfer. Sandy turbidites interbedded with hemipelagic muds occur on the continental rise and adjacent deep-ocean basin. Submarine channels on the upper slope continue into the abyssal plain as far north as about 67ºS. They are separated by sediment drifts and sediment waves resulting from the interaction between downslope turbidity-current processes and along-slope bottom currents. Similar deep-sea sedimentary processes operate along much of the West Antarctic and the western side of the Antarctic Peninsula. The work is collaborative with Prof. J.B. Anderson of Rice University, Houston, Texas.

Julian Dowdeswell, Jeff Evans and Colm Ó Cofaigh

Spatial interpolation of digital elevation models of terrestrial ice masses

Digital Elevation Models (DEMs) of glaciers, ice caps and ice sheets are needed for many purposes. The most obvious is as a means of assessing the mass balance of a glacier through comparison of DEMs acquired at different times. Topographic data suitable for incoroporation into DEMs can be obtained from a variety of techniques including field survey, laser profiling, radar altimetry, and the analysis of aerial photography, visible-wavelength and radar remote sensing data. These techniques have different sampling characteristics which introduces the problem of resampling the data to a common grid. The aim of this project is to carry out a rigorous analysis of the behaviour of different approaches to spatial interpolation, relating this behaviour to the geostatistical properties of glacier surface topography. Once these relationships are properly understood, it will be possible to reanalyse previously obtained topographic datasets in a consistent manner to obtain volume estimates of known accuracy. This will greatly enhance our ability to assess mass balance changes.

Gareth Rees and John Lin

Two Hummock Island in Gerlache Strait, Antarctica
© J.A. Dowdeswell
Image as described adjacent

High resolution topographic mapping of an Arctic glacier

This project represents the current phase of a long-term investigation of the energy and mass balance of the glacier Midre Lovénbreen in Svalbard. Airborne LiDAR were acquired from the glacier in 2003, using the University of Cambridge Unit for Landscape Modelling's Optech LiDAR flown on the NERC Airborne Remote Sensing Facility's aircraft. The data have enabled us to construct the most accurate digital elevation model (DEM) ever produced for this glacier, with a horizontal resolution of 2 m and a vertical accuracy of 5 to 10 cm. In addition to the general morphology of the glacier, the DEM reveals many subtle features on the surface, including active and relic meltwater drainage channels, and crevasses. Many potential applications follow from the ability to map the surface of the glacier in such high detail. These include the possibility of using surface features as markers that can be tracked over time to determine the flow of the glacier, and the ability to infer the physical state of the glacier surface in conjunction with radar imagery, as well as the ability to monitor mass balance. The high resolution DEM is also allows investigation of the role of very small-scale topographic variations on glacier energy and mass balance, partly due to self-shading of the surface at the low solar angles typical of the Arctic, but also because of the anisotropic reflectance of ice and snow at low solar angles, which makes the local variation in solar incidence angle to the surface an important determinant of the surface albedo.

Neil Arnold and Gareth Rees

Topographic controls on glacier mass and energy balance

Solar radiation is generally the most important source of energy for melting glaciers and ice sheets. Calculating the amount of solar radiation available at any point on a glacier surface requires that the height of the sun and its azimuth (direction) in the sky is known, along with the local slope and azimuth of the ice surface and shading of the glacier surface by any surrounding mountain topography. At high latitudes, these topographic controls are especially important, as they can greatly alter the local incidence angle of solar radiation (and hence the energy available at the surface), due to the low solar angles. This is made more important by the anisotropic reflectance properties of ice and snow, which increase the surface albedo at very shallow incidence angles. We have developed a distributed surface energy balance model to calculate these effects.

Neil Arnold

Interactions between snow cover, climate and vegetation

Snow cover, climate and vegetation distribution are tightly linked in the polar regions. Recent research has demonstrated significant trends in the vigour of circum- Arctic vegetation, which is in general increasing although with major spatial variations. These trends are likely to be associated with changes in global climate. One of the mechanisms through which climate affects circumpolar vegetation is through alteration of the duration of snow cover but, until recently, it has been difficult to obtain accurate snow cover data at sufficiently high spatial resolution and over a long enough span of time to investigate the strength of this link. The aim of this project is to generate such a dataset for the Russian North, using archived data from many meteorological stations from the early 1960s to 1990. The data were transcribed manually and contain numerous errors, so quality control of the dataset has been a major issue. However, the data have now been filtered to produce a reliable, internally consistent timeseries for the whole of the Russian mainland north of 60º. The next task is to investigate spatiotemporal correlations between this dataset and time series of vegetation index (derived from satellite data) and temperature. The work is being undertaken in collaboration with Michael Balshi, a former M. Phil. student of the Scott Polar Research Institute now based at the University of Alaska at Fairbanks.

Gareth Rees

Validation measurements for the Cryosat radar altimeter

In preparation for the launch of the Cryosat satellite in 2005 we participated in an international campaign to validate data collected by the new radar altimeter to be carried by Cryosat. A two-person team traversed the Greenland Ice Sheet, measuring snow density profiles in spring and autumn. Colleagues from AWI used an airborne version of the radar altimeter and a laser altimeter to collect elevation data over the traverse line during the two field seasons. Any differences in the results from the two airborne instruments will be interpreted in terms of the ground-based density profiles. A new light-weight automatic profiling system for measurement of snow and ice density using neutron scattering was developed in conjunction with a small UK firm (Geovista) in preparation for the Greenland traverse. This proved very successful and it was possible to measure densification over the summer months in the upper snow layer in detail for the first time. Such changes in snow structure are important as they may influence radar altimeter returns. Theoretical work on neutron scattering in snow to support detailed analysis of the density profiles has continued during the year.

Liz Morris

Climate, mass balance, dynamics and hydrology of New Zealand glaciers

This work is based at Franz Josef Glacier and Brewster Glacier, New Zealand. Unlike many glacierised regions of the world, New Zealand has never had a long-term massbalance monitoring programme, although several glaciers have been observed from the air at the end of each summer since the 1970s to enable their end of summer snowline position to be determined. The overall aim of this project is to monitor the climate, summer and winter mass balance and surface velocity field of two glaciers and to use these data to drive and test a 1-D glacier mass-balance and ice-flow model. The model should be able to reconstruct past glacier mass balance (using the end of summer snow line data as verification) and predict mass balance under future scenarios of climate change. Fieldwork so far has involved: measuring climate variations on the glaciers (using automatic weather stations); monitoring glacier mass balance (using glaciological and geodetic techniques) and monitoring surface velocity variations at a variety of timescales (hourly to yearly). In addition, the hydrology of Brewster Glacier is being studied using surface and bed topography data and GIS techniques to identify the structure of the subglacial drainage system, and dye tracing experiments to investigate the morphology of individual drainage pathways. The work is in collaboration with Andrew Mackintosh and Brian Anderson (Victoria University, Wellington), Wendy Lawson and Becky Goodsell (Canterbury University, Christchurch) and Sean Fitzsimmons (Otago University, Dunedin) all in New Zealand.

Ian Willis

Sediment deformation beneath Icelandic glaciers

Sedimentological techniques are being used to study sediments exposed on recently deglaciated forefields at nine surging and non-surging glaciers in Iceland. The techniques are being used to identify the relative importance of sliding and sediment deformation beneath the glaciers, the styles (ductile vs. brittle) and pervasiveness (depth) of deformation, and whether these vary in space and time. Macro-scale evidence includes particle-size distribution and individual clast shape, angularity and fabric. Micro-scale evidence involves the identification of rotational structures, fold structures and unistrial plasmic fabric (evidence for ductile deformation) and alignment of grains, shear lines and crushed grains (evidence for brittle deformation) in thin sections viewed under a microscope. Provisional results from the surge-type Brúarjökull imply that a lower till unit underwent limited brittle shear at low pore-water pressures during an early surge advance, ice-bed de-coupling was common at the height of the surge due to high pore-water pressures, and the deposition and deformation of an upper till unit occurred at moderate pore-water pressures during the waning stages of the surge and subsequently. This work is being undertaken in collaboration with Colm Ó Cofaigh and Anna Nelson.

Ian Willis

Assemblages of submarine landforms produced by surging tidewater glaciers

In order to interpret the geological record of past glacier and ice-sheet advances, it is important to distinguish between sediments and landforms relating to dynamic behaviour internal to glaciers, surges, and those linked to external climatic factors. The suite of landforms characteristic of past glacier surges is an important inferential tool in this regard. High-resolution swath bathymetry from the marine margins of several Svalbard glaciers shows an assemblage of submarine landforms that appears characteristic of surging glaciers. These landforms are essentially unmodified since their initial deposition over the past hundred years or so because they have not been subjected to subaerial erosion or periglacial activity. A simple descriptive landsystem model for tidewater glaciers of surge-type is derived from these observations. It is the assemblage of landforms in this model that is of diagnostic significance, and individual landform elements, found in isolation in the geological record, are not necessarily indicative of former surge activity. This work is being undertaken in collaboration with Dag Ottesen of the Geological Survey of Norway.

Julian Dowdeswell