bluuebunny/crossref_metadata_2025 huggingface dataset:
Basal melt beneath Whillans Ice Stream and Ice Streams A and C, West Antarctica
Authors: Joughin Ian R. , Tulaczyk Slawek , Engelhardt Hermann F.
Published: November 2007

We have used a recently derived map of the velocity of Whillans Ice Stream and Ice Streams A and C, West Antarctica, to help estimate basal melt. Ice temperature was modeled with a simple vertical advection–diffusion equation,“tuned” to match temperature profiles. We find that most of the melt occurs beneath the tributaries, where larger basal shear stresses and thicker ice favor greater melt (e.g. 10–20mm a−1). The occurrence of basal freezing is predicted beneath much of the ice plains of Ice Stream C andWhillans Ice Stream. Modeled melt rates for when Ice Stream C was active suggest there was enough meltwater generated in its tributaries to balance basal freezing on its ice plain. Net basal melt for Whillans Ice Stream is greater due to less steep basal temperature gradients. Modeled temperatures on Whillans Ice Stream, however, were constrained by a single temperature profile at UpB. Basal temperature gradients for Whillans branch 1 and Ice Stream A may have conditions more similar to those beneath Ice Streams C and D, in which case, there may not be sufficient melt to sustain motion. This would be consistent with the steady deceleration of Whillans Ice Stream over the last few decades.
The role of lateral and vertical shear in tributary flow toward a West Antarctic ice stream
Authors: Hulbe Christina L. , Wang Weili , Joughin Ian R. , Siegert Martin J.
Published: November 2007

Narrow lateral shear margins are the most distinctive visual feature of the West Antarctic ice streams. Large shear stresses within these layers support the majority of the gravitational driving stress within a fast-flowing ice stream. The present contribution looks upstream, to the tributaries that feed ice-stream onsets, and considers the effects of both horizontal and vertical shear on their flow. Numerical and direct simulations of vertical and horizontal shear are used. Vertical shear, simulated using an anisotropic flow law, is of particular interest. We conclude that by isolating overlying ice from large-amplitude variations in bed elevation,“vertical shear margins” play an important role in sustaining relatively rapid tributary flow.
Recent behaviour of Glaciar Upsala, a fast-flowing calving glacier in Lago Argentino, southern Patagonia
Authors: Skvarca Pedro , Raup Bruce , Angelis Hernan de
Published: November 2007

Sequential optical images of high spatial resolution were used for the first time to derive surface ice velocities of Glaciar Upsala, a fast-moving fresh-water calving glacier in southern Patagonia. Cross-correlation methods applied to four Landsat ETM+ images acquired in 2000–01 yielded average velocities of around 1600 m a−1, similar to values measured in the field in November 1993. The derived velocities show almost no seasonal variation for the analyzed calving termini. During the period of satellite coverage, clear readvances were detected in the autumn–winter period, followed by recessions during summers. Between 24 April 1999 and 14 October 2001, the glacier front has been fluctuating seasonally within about 400 m, in contrast to the previous dramatic recession. During the last 2.5 years, Glaciar Upsala west terminus had a net advance of around 300 m. In addition, the available satellite images allowed us to determine recent calving speeds and confirm the improved calving-rate/water-depth relationship, recently proposed by incorporating new data from Patagonian glaciers.
Surge-front propagation and velocities during the early-1993–95 surge of Bering Glacier, Alaska, U.S.A., from sequential SAR imagery
Authors: Roush James J. , Lingle Craig S. , Guritz Richard M. , Fatland Dennis R. , Voronina Vera A.
Published: November 2007

The initiation and propagation of the 1993–95 surge of Bering Glacier, Alaska, U.S.A., was observed using ERS-1 synthetic aperture radar(SAR) imagery. Images were acquired before and during the surge, between November 1992 and October 1993. Terrain-corrected and co-registered imagery was used to measure the propagation of the surge front. Surface undulations interpreted to be evidence of accelerated flow, indicating surge initiation in late winter, were observed in the 26 March 1993 image. From 19 May to 25 August 1993, the mean propagation velocity of the surge front was 90 m d−1. The surge reached the terminus shortly after 25 August 1993. The central area of the calving terminus then advanced into proglacial Vitus Lake at a mean rate of 19 md−1 between 9 August and 18 October 1993. Feature matching was used to measure discrete velocity vectors between 9 August and 13 September; the vectors were kriged onto a uniform grid and used to compute the principal strain rates. Shattering of the calving front and dramatically increased iceberg calving were accompanied by high compressive strain rates immediately up-glacier from the calving front.
The initiation of glacier surging at Fridtjovbreen, Svalbard
Authors: Murray Tavi , Luckman Adrian , Strozzi Tazio , Nuttall Anne-Marie
Published: November 2007

Glacier surges in Svalbard have long durations and multi-year terminations, but much less is known regarding surge initiation in the archipelago. Fridtjovbreen, a 12 km long glacier in central Spitsbergen, advanced ∼ 2.8 km during a surge in the 1990s at a maximum rate of ∼ 4. 2 m d–1 . Differential dual-azimuth satellite radar interferometry (SRI) is used to produce ten snapshots of three-dimensional surface dynamics and four digital elevation models covering the period October 1991–October 1997. The glacier velocity rose slowly and uniformly until June 1995. It then increased dramatically to a measured maximum of ∼ 2.5 m d–1 during February and May 1996, and by October 1997 it had dropped. We attempt to evaluate errors in the calculated velocities. Systematic errors are evaluated using the apparent displacement of bedrock, ∼0.03 m d–1 . Errors arise from assumptions during processing, for example that ice-flow direction does not change during the surge. Two independent measurements using dual-azimuth processing show the mean absolute change in flow direction was ∼1.2°. This study covers fast-flow initiation and peak flow, but not the deceleration phase. The SRI observations show a progressive acceleration phase to the surge, with no evidence of a surge front propagating down-glacier.
Of isbræ and ice streams
Authors: Truffer Martin , Echelmeyer Keith A.
Published: November 2007

Fast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the “ice-stream” type (represented by the Siple Coast ice streams in West Antarctica) and the “isbræ” type (represented by Jakobshavn Isbræ in Greenland). The typical ice stream is wide, relatively shallow (∼1000 m), has a low surface slope and driving stress (∼10 kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbræ type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zone stend to be wide relative to the isbræ width, and the location of isbræ and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbræs occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.
Submarine melting at the terminus of a temperate tidewater glacier, LeConte Glacier, Alaska, U.S.A.
Authors: Motyka Roman J. , Hunter Lewis , Echelmeyer Keith A. , Connor Cathy
Published: November 2007

Heat, fresh- and sea-water balances indicate that the late-summer rate of submarine melting at the terminus of tidewater LeConte Glacier, Alaska, U.S.A., in 2000 was about 12 m d−1 w.e., averaged over the submerged face. This is 57% of the estimated total ice loss at the terminus (calving plus melting) at this time. Submarine melting may thus provide a significant contribution to the overall ablation of a tidewater glacier. Oceanographic measurements (conductivity–temperature–depth) made 200–500m from the terminus identified an isohaline (27 ppt) and isothermal (7.2°C) layer extending from 130 m depth to the fjord floor. Capping this is a 40 m thick overflow plume, distinguished by high outflow rates, low salinity (22–25 ppt) and lower temperatures (5–6°C). Mixing models indicate that fresh water comprised about 11% of this plume; it originates mostly as subglacial discharge whose buoyancy drives convection at the terminus. Deep, warm saline waters are incorporated into the plume as it ascends, causing substantial melting of ice along the submarine face. The calving terminus undergoes seasonal changes that coincide with changes in subglacial discharge and fjord water temperatures, and we suggest that these fluctuations in terminus position are directly related to changes in submarine melting.
Relating crevassing to non-linear strain in the floating part of Jakobshavn Isbræ, West Greenland
Authors: Prescott Paul R. , Kenneally James P. , Hughes Terence J.
Published: November 2007

Jakobshavn Isbræ is a major ice stream that drains the west-central Greenland ice sheet and becomes afloat in Jakobshavn Isfjord (69° N, 49° W), where it has maintained the world’s fastest-known sustained velocity and calving rate (7 kma−1) for at least four decades. The floating portion is approximately 12 km long and 6 km wide. Surface elevations and motion vectors were determined photogrammetrically for about 500 crevasses on the floating ice, and adjacent grounded ice, using aerial photographs obtained 2 weeks apart in July l985. Surface strain rates were computed from a mesh of 399 quadrilateral elements having velocity measurements at each corner. It is shown that heavy crevassing of floating ice invalidates the assumptions of linear strain theory that (i) surface strain in the floating ice is homogeneous in both space and time, (ii) the squares and products of strain components are nil, and (iii) first- and second-order rotation components are small compared to strain components. Therefore, strain rates and rotation rates were also computed using non-linear strain theory. The percentage difference between computed linear and non-linear second invariants of strain rate per element were greatest (mostly in the range 40–70%) where crevassing is greatest. Isopleths of strain rate parallel and transverse to flow and elevation isopleths relate crevassing to known and inferred pinning points.
The slow advance of a calving glacier: Hubbard Glacier, Alaska, U.S.A.
Authors: Trabant Dennis C. , Krimmel Robert M. , Echelmeyer Keith A. , Zirnheld Sandra L. , Elsberg Daniel H.
Published: November 2007

Hubbard Glacier is the largest tidewater glacier in North America. In contrast to most glaciers in Alaska and northwestern Canada, Hubbard Glacier thickened and advanced during the 20th century. This a typical behavior is an important example of how insensitive to climate a glacier can become during parts of the calving glacier cycle. As this glacier continues to advance, it will close the seaward entrance to 50 km long Russell Fjord and create a glacier-dammed, brackish-water lake. This paper describes measured changes in ice thickness, ice speed, terminus advance and fjord bathymetry of Hubbard Glacier, as determined from airborne laser altimetry, aerial photogrammetry, satellite imagery and bathymetric measurements.The data show that the lower regions of the glacier have thickened by as much as 83 m in the last 41 years, while the entire glacier increased in volume by 14.1 km 3. Ice speeds are generally decreasing near the calving face from a high of 16.5 md −1in1948 to11.5 md −1in 2001. The calving terminus advanced at an average rate of about 16 m a−1 between 1895 and 1948 and accelerated to 32 m a−1 since 1948. However, since 1986, the advance of the part of the terminus in Disenchantment Bay has slowed to 28 m a−1. Bathymetric data from the lee slope of the submarine terminal moraine show that between 1978 and 1999 the moraine advanced at an average rate of 32 m a−1, which is the same as that of the calving face.
An 850 year record of climate and fluctuations of the iceberg-calving Nellie Juan Glacier, south central Alaska, U.S.A.
Authors: Barclay David J. , Wiles Gregory C. , Calkin Parker E.
Published: November 2007

Tree-ring cross-dates of 46 glacially killed trees show that the tidewater Nellie Juan Glacier, Alaska, advanced seaward during the 16th and 17th centuries AD. Ice-scarred trees at the late-Holocene end moraine indicate that the terminus was at its recent maximum from 1842 to 1893. Historical observations and photographs show that subsequent slow retreat changed to rapid iceberg-calving retreat after 1935, and that the tidewater terminus had withdrawn about 3.3 km from the late-Holocene maximum by 1992. Comparison with paleoclimate records from nearby land-terminating glaciers and an 850 year tree-ring-width chronology indicates that the timing of the 19th-century maximum stand of Nellie Juan Glacier was controlled by changes in summer temperature and radiation. However, rapid iceberg-calving retreat did not begin until 40 years of slow retreat had brought the tidewater terminus back from the terminal moraine shoal. Therefore, both the dimensions of the terminal moraine shoal and the magnitude and duration of climate change were important in initiating rapid retreat of this tidewater glacier system.
ERS SAR feature-tracking measurement of outlet glacier velocities on a regional scale in East Greenland
Authors: Luckman Adrian , Murray Tavi , Jiskoot Hester , Pritchard Hamish , Strozzi Tazio
Published: November 2007

Feature tracking, or patch intensity cross-correlation, is used to derive two-dimensional ice-surface velocity fields from 1day and 35 day repeat-pass European Remote-sensing Satellite (ERS) synthetic aperture radar (SAR) data covering a 500 km by 500 km area of central East Greenland. Over regions of fast ice flow, 35 day tracking yields only a slightly lower density of velocity measurements than 1day tracking, and both are broadly in agreement about the spatial pattern of ice velocity except at the glacier termini where tidal effects may dominate. This study suggests that SAR feature tracking may be used to routinely monitor ice-discharge velocities on a regional basis and thereby inform studies of regional mass balance.
Acoustic impedance and basal shear stress beneath four Antarctic ice streams
Authors: Vaughan David G. , Smith Andrew M. , Nath P. Chandrika , Meur Emmanuel Le
Published: November 2007

The acoustic impedance of the subglacial material beneath 7.2 km profiles on four ice streams in Antarctica has been measured using a seismic technique. The ice streams span a wide range of dynamic conditions with flow rates of 35–464 m a–1. The acoustic impedance indicates that poorly lithified or dilated sedimentary material is ubiquitous beneath these ice streams. Meanacoustic impedance across each profile correlates well with basal shear stress and the slipperiness of the bed, indicating that acoustic impedance is a good diagnostic not only for the porosity of the subglacial material, but also for its dynamic state (deforming or non-deforming). Beneath two of the ice streams, lodged (non-deforming) and dilated (deforming) sediment coexist but their distribution is not obviously controlled by basal topography or ice thickness. Their distribution may be controlled by complex material properties or the deformation history. Beneath Rutford Ice Stream, lodged and dilated sediment coexist and are distributed in broad bands several kilometres wide, whileon Talutis Inlet there is considerable variability over much shorter distances; this may reflect differences in the mechanism of drainage beneath the ice streams. The material beneath the slow-moving Carlson Inlet is probably lodged but unlithified sediment; this is consistent with the hypothesis that Carlson Inlet was once a fast-flowing ice stream but is now in a stagnant phase, which could possibly be revivedby raised basal water content. The entire bed beneath fast-flowing Evans Ice Stream is dilated sediment.
Calving speed and climatic sensitivity of New Zealand lake-calving glaciers
Authors: Warren Charles R. , Kirkbride Martin P.
Published: November 2007

Calving speeds and calving mechanisms in fresh water contrast with those in tidewater. We obtained calving speeds for six lake-calving glaciers in New Zealand’s Southern Alps, and surveyed the depths and temperatures of their ice-contact lakes. The glaciers are temperate, grounded in shallow (≤20 m) water, and exhibit compressive flow at their termini. These data increase the global dataset of fresh-water calving statistics by 40%, bringing the total to 21 glaciers. For this dataset, calving rates (uc) correlate positively with water depths (hw) (r2 = 0.83), the relationship being expressed by: uc = 17.4 + 2.3 hw. This is an order of magnitude lower than values of uc at temperate tidewater glaciers. For a subset of 10 glaciers for which ice-proximal water temperature (tw) data are available, uc also correlates positively with tw, supporting a physical relation between calving and melting at and below the water-line. Fluctuations of New Zealand lake-calving glaciers in the period 1958–97 show that although the transition from non-calving to calving dramatically increases frontal retreat rates, the onset of calving does not isolate terminus change from climatic forcing. In terms of climatic sensitivity, lake-calving glaciers occupy an intermediate position between tidewater glaciers (least sensitive) and non-calving glaciers (most sensitive).
Ice flux of Plogbreen, a small ice stream in Dronning Maud Land, Antarctica
Authors: Hedfors Jim , Pohjola Veijo Allan
Published: November 2007

As part of a long-term mass-balance program run by SWEDARP since 1988, a detailed study on Plogbreen, Dronning Maud Land, Antarctica, was undertaken during the austral summer of 2003 to investigate the long-term mass balance. We compare ice outflux, φout, through a cross-sectional gate with ice influx, φin, from the upstream catchment area. The φin is based on calculations of snow accumulation upstream of the gate using data available from published ice-core records. The φout is based on Glen’s flow law aided by thermodynamic modeling and force-budget calculations. Input data from the field consist of measurements of ice surface velocity and ice geometry. The ice surface velocity was measured using repeated differential global positioning system surveying of 40 stakes over a period of 25 days. The ice geometry was determined by 174 km of ground-penetrating radar profiling using ground-based 8MHz dipole antennas. This study presents the collected velocity and geometry data as well as the calculated ice flux of Plogbreen. The results show a negatively balanced system within the uncertainty limits; φout = 0.55 ± 0.05 km3 a–1 and φin = 0.4 ± 0.1 km3 a–1. We speculate that the negative balance can be explained by recent eustatic increase reducing resistive stresses and inducing accelerated flow.
Dry calving processes at the ice cliff of Strandline Glacier northern Victoria Land, Antarctica
Authors: Diolaiuti Guglielmina , Smiraglia Claudio , Vassena Giorgio , Motta Michele
Published: November 2007

This paper describes seasonal variations on Strandline Glacier, Terra Nova Bay, northern Victoria Land, Antarctica, obtained from measurements during two field campaigns (2000/01 and 2002/03). By applying different methodologies and techniques, it was possible to calculate summer changes in thickness (mean decrease over the whole glacier surface of 0.04 mw.e.) and in volume (decrease of 352 μ3 at the terminus). This serves to identify and quantify the processes responsible for these variations (e.g. melting, evaporation, wind erosion/deposition and calving) and to understand the energy transfers at the glacier–air interface. One particular feature of the glacier is the presence of an ice cliff at the terminus, which causes ablation owing to dry calving. This process brought about a summer retreat rate of 1 mweek–1 in the central part of the front during the study period. Qualitative observations carried out during previous field campaigns and quantitative analysis performed during the last two expeditions have provided an understanding of the mechanism underlying the ablation processes, thus confirming the available literature on this topic.
Improved estimation of the mass balance of glaciers draining into the Amundsen Sea sector of West Antarctica from the CECS/NASA 2002 campaign
Authors: Rignot Eric , Thomas Robert H. , Kanagaratnam Pannir , Casassa Gino , Frederick Earl , Gogineni Sivaprasad , Krabill William , Rivera Andrès , Russell Robert , Sonntag John , Swift Robert , Yungel James
Published: November 2007

In November–December 2002, a joint airborne experiment by Centro de Estudios Cientifícos and NASA flew over the Antarctic ice sheet to collect laser altimetry and radio-echo sounding data over glaciers flowing into the Amundsen Sea. A P-3 aircraft on loan from the Chilean Navy made four flights over Pine Island, Thwaites, Pope, Smith and Kohler glaciers, with each flight yielding 1.5–2 hours of data. The thickness measurements reveal that these glaciers flow into deep troughs, which extend far inland, implying a high potential for rapid retreat. Interferometric synthetic aperture radar data (InSAR) and satellite altimetry data from the European Remote-sensing Satellites (ERS-1/-2) show rapid grounding-line retreat and ice thinning of these glaciers. Using the new thickness data, we have reevaluated glacier fluxes and the present state of mass balance, which was previously estimated using ice thicknesses deduced largely from inversion of elevation data assuming hydrostatic equilibrium. The revised total ice discharge of 241 ± 5km3 a–1 exceeds snow accumulation by 81 ± 17 km3 a–1 of ice, equivalent to a sea-level rise of 0.21 ± 0.04 mma–1. This magnitude of ice loss is too large to be caused by atmospheric forcing and implies dynamic thinning of the glaciers. This is confirmed by ice-flow acceleration observed with InSAR. We attribute the flow acceleration and ice thinning to enhanced bottom melting of the ice shelves by a warmer ocean, which reduces buttressing of the glaciers, and in turn accelerates them out of balance.
Stratigraphic continuity in 400MHz short-pulse radar profiles of firn in West Antarctica
Authors: Arcone Steven A. , Spikes Vandy B. , Hamilton Gordon S. , Mayewski Paul A.
Published: November 2007

We track dated firn horizons within 400 MHz short-pulse radar profiles to find the continuous extent over which they can be used as historical benchmarks to study past accumulation rates in West Antarctica. The 30–40cm pulse resolution compares with the accumulation rates of most areas. We tracked a particular set that varied from 30 to 90 m in depth over a distance of 600 km. The main limitations to continuity are fading at depth, pinching associated with accumulation rate differences within hills and valleys, and artificial fading caused by stacking along dips. The latter two may be overcome through multi-kilometer distances by matching the relative amplitude and spacing of several close horizons, along with their pulse forms and phases. Modeling of reflections from thin layers suggests that the – 37 to – 50 dB range of reflectivity and the pulse waveforms we observed are caused by the numerous thin ice layers observed in core stratigraphy. Constructive interference between reflections from these close, high-density layers can explain the maintenance of reflective strength throughout the depth of the firn despite the effects of compaction. The continuity suggests that these layers formed throughout West Antarctica and possibly into East Antarctica as well.
Temporal co-variation of surface and microwave brightness temperatures in Antarctica, with implications for the observation of surface temperature variability using satellite data
Authors: Winebrenner Dale P. , Steig Eric J. , Schneider David P.
Published: November 2007

Satellite observations of microwave emission are a key resource for estimating surface temperatures in Antarctica. Use of these data to examine climate variability, however, relies on the assumption of constancy through time in the relationship between surface temperatures and the proxy brightness temperatures. Thus we are motivated to study the physical relationship between surface and brightness temperature time series, and to seek indicators of possible temporal variability in that relationship. Here we report an initial study using near-surface temperatures from the Byrd Station automated weather station in West Antarctica and 37 GHz, vertically polarized brightness temperatures from the Scanning Multichannel Microwave Radiometer. We begin with the simplest model of the relevant thermal and microwave physics and derive a convolution expression that relates surface and brightness temperatures. The convolution kernel depends on firn thermal diffusivity and the microwave extinction coefficient in a particularly simple way: solely through a single characteristic time-scale. For the Byrd data, we find that the (fractional variation in) observed brightness temperatures can be reproduced by our model in considerable detail, on scales from interannual down to a few days. The time-scale is tightly constrained by minimization of the discrepancy between observed and simulated time series, and the optimized value agrees closely with that derived from independent estimates of firn thermal and microwave parameters. We find no evidence thus far of temporal variability in the relation between surface and brightness temperatures, though investigation across a wider domain in space and time is needed before such variability can be ruled out.
Evidence for subglacial ponding across Taylor Glacier, Dry Valleys, Antarctica
Authors: Hubbard Alun , Lawson Wendy , Anderson Brian , Hubbard Bryn , Blatter Heinz
Published: November 2007

Ice-penetrating radar and modelling data are presented suggesting the presence of a zone of temperate ice, water ponding or saturated sediment beneath the tongue of Taylor Glacier, Dry Valleys, Antarctica. The proposed subglacial zone lies 3–6 km up-glacier of the terminus and is 400– 1000m across. The zone coincides with an extensive topographic overdeepening to 80m below sea level. High values of residual bed reflective power across this zone compared to other regions and the margins of the glacier require a high dielectric contrast between the ice and the bed and are strongly indicative of the presence of basal water or saturated sediment. Analysis of the hydraulic equipotential surface also indicates strong convergence into this zone of subglacial water flow paths. However, thermodynamic modelling reveals that basal temperatures in this region could not exceed –7˚C relative to the pressure-melting point. Such a result is at odds with the radar observations unless the subglacial water is a hypersaline brine.
Pattern of retreat and disintegration of the Larsen B ice shelf, Antarctic Peninsula
Authors: Rack Wolfgang , Rott Helmut
Published: November 2007

The retreat of the Larsen B ice shelf, Antarctic Peninsula, and the collapse of its northern section are analyzed using satellite images acquired between January 1995 and May 2003. Over 1 week during March 2002, after a period of steady retreat since 1995, 2300 km2 of the ice shelf broke up into many small icebergs. This rapid collapse occurred at the end of an exceptionally warm summer, and after a multi-year period of decreasing surface net mass balance, ice thinning, flow acceleration and widening of rifts. The ice-shelf area decreased from 11 512 km2 in January 1995 to 3463 km2 in March 2002, and 2667 km2 in April 2003. ERS synthetic aperture radar (SAR) images were used to identify ice-shelf zones with different surface morphology, which generated icebergs of different sizes and shapes. The pattern of retreat and break-up, similar to that of Larsen A in 1995, suggests that fracturing enhanced by abundant surface melt played a key role. In addition, the recent changes of grounded and residual floating ice north of Larsen B are analyzed by means of Envisat advanced synthetic aperture radar (ASAR) images up to summer 2003, showing significant loss of grounded ice upstream of those ice-shelf sections which disintegrated in 1995 and 2002.
Initial experiments on the effects of particles at grain boundaries on the anelasticity and creep behavior of granular ice
Authors: Song Min , Cole David M. , Baker Ian
Published: November 2007

Experimental observations of the influence of particles at grain boundaries on the anelasticity and creep behavior of granular fresh-water ice are presented. Ice with particle contents of 0–4 wt.% was investigated under both reversed direct-stress and creep loading conditions at –12˚C. The results show that the particles decreased the grain-boundary relaxation by suppressing grain-boundary sliding at higher frequencies (10–1 to 101 Hz). In addition, the modulus increased by up to 30%, and the internal friction decreased by up to 30% at a frequency of 1 Hz. Staged creep tests showed that the particles affected the creep rate substantially. The minimum creep rate of ice containing 1 wt.% particles is 40% higher than that of particle-free ice, indicating that mechanisms besides dislocation glide aid the creep deformation.
Sensitivity of chemical species to climatic changes in the last 45 kyr as revealed by high-resolution Dome C (East Antarctica) ice-core analysis
Authors: Udisti Roberto , Becagli Silvia , Benassai Silvia , De Angelis Martine , Hansson Margareta E. , Jouzel Jean , Schwander Jacob , Steffensen Jørgen P. , Traversi Rita , Wolff Eric
Published: November 2007

To assess the cause/effect relationship between climatic and environmental changes, we report high-resolution chemical profiles of the Dome C ice core (788m, 45 kyr), drilled in the framework of the European Project for Ice Coring in Antarctica (EPICA). Snow-concentration and depositional-flux changes during the last deglaciation were compared with climatic changes, derived by δD profile. Concentration and temperature profiles showed an anticorrelation, driven by changes in source intensity and transport efficiency of the atmospheric aerosol and by snow accumulation-rate variations. The flux calculation allowed correction for accumulation rate. While sulphate and ammonium fluxes are quite constant, Na+, Mg2+ and Ca2+ underwent the greatest changes, showing fluxes respectively about two, three and six times lower in the Holocene than in the Last Glacial Maximum. Chloride, nitrate and methanesulphonic acid (MSA) also exhibited large changes, but their persistence depends on depositional and post-depositional effects. The comparison between concentrations and δD profiles revealed leads and lags between chemical and temperature trends: Ca2+ and nitrate preceded by about 300 years the δD increase at the deglaciation onset, while MSA showed a 400 year delay. Generally, all components reached low Holocene values in the first deglaciation step (18.0–14.0 kyr BP), but Na+, Mg2+ and nitrate show changes during the Antarctic Cold Reversal (14.0– 12.5 kyr BP).
Distribution and pattern of shallow melting at the local glaciers of Terra Nova Bay (Antarctica) coast
Authors: Motta Luigi , Motta Michele
Published: November 2007

Melting is a fundamental process in glacier mass balance as well as in other glacial processes such as ice movement, ice avalanches and snow metamorphism. At Terra Nova Bay, Antarctica, the annual mean temperature is <0˚C but melting is not negligible. Our data show that melting is present up to 1300ma.s.l. The distribution and relative importance of melting change with elevation and exposure. At low elevation and with a southerly exposure, melting is one of the major summer ablation processes, synergetic with dry calving and katabatic wind. Meltwater seepage reaches the glacier substrate. Discharge appears to be linked to irradiation, offset by a few hours relative to it. The frontal area of the glaciers therefore behaves like an aquifer, and its characteristics depend on the morphology of the front. As elevation increases, melting at first becomes limited to the snowpack, then to low-albedo cases, for example when dust is brought in from the areas surrounding the glacier. In this case, melting limits ablation, providing the snow with increased resistance to wind erosion.
Geostatistical estimation from radar altimeter data with respect to morphological units outlined by SAR data: application to Lambert Glacier/Amery Ice Shelf, East Antarctica
Authors: Stosius Ralf , Herzfeld Ute C.
Published: November 2007

The objective of this paper is the comparison of two kriging methods, ordinary kriging and kriging within strata, for calculation of digital elevation models (DEMs) from radar altimeter data, and application to the Lambert Glacier/Amery Ice Shelf system, East Antarctica. Two new DEMs are presented. First, a DEM of the Lambert Glacier/Amery Ice Shelf system is calculated from 1997 European Remote-sensing Satellite-2 (ERS-2) radar altimeter (RA) data using geostatistical interpolation. RA data have high along-track density, but gaps between tracks are several kilometers, depending on the observation mode; this requires interpolation. Because the ice-stream/ice-shelf system is of primary interest in glaciological investigations, in the first approach a variogram characteristic of the Lambert Glacier ice surface is used. The resultant map has low errors for the glacier and the ice shelf. To match the surface characteristics of different morphological units that constitute the Lambert Glacier/Amery Ice Shelf region, a second DEM is constructed as follows: We utilize RADARSAT synthetic aperture radar (SAR) data that were collected in 1997 during the first Antarctic Imaging Campaign and composed into a 125m backscatter-data mosaic by Jezek (1999) and we co-reference the 125m mosaic with the altimetry-derived DEM. The Lambert Glacier/Amery Ice Shelf area is then subdivided into several regions which are homogeneous with respect to characteristic surface-morphological properties identified in the SAR mosaic. For those regions, a problem-oriented complex kriging method known as kriging within strata is performed, and the resulting DEM is compared to the DEM that was derived from kriging without regional subdivision.
Dynamics of the Scharffenbergbotnen blue-ice area, Dronning Maud Land, Antarctica
Authors: Sinisalo Anna , Grinsted Aslak , Moore John
Published: November 2007

Ground-penetrating radar (GPR) surveys in Scharffenbergbotnen valley, Dronning Maud Land, Antarctica, complement earlier, relatively sparse data on the ice-flow dynamics and mass-balance distribution of the area. The negative net surface mass balance in the valley appears to be balanced by the inflow. The flow regime in Scharffenbergbotnen defines four separate mass-balance areas, and about 60 times more ice enters the valley from the northwestern entrance than via the narrow western gate. We formalize and compare three methods of determining both the surface age gradient of the blue ice and the dip angles of isochrones in the firn/blue-ice transition zone: observed and dated radar internal reflections, a geometrical model of isochrones, and output from a flowline model. The geometrical analysis provides generally applicable relationships between ice surface velocity and surface age gradient or isochrone dip angle.
Growth processes of an inland Antarctic ice wedge, Mesa Range, northern Victoria Land
Authors: Raffi Rossana , Stenni Barbara , Flora Onelio , Polesello Stefano , Camusso Marina
Published: November 2007

During the 16th Italian Antarctic Expedition (2000/01) a geomorphological survey of permafrost-related polygons was carried out in the Mesa Range area, upper Rennick Glacier. The investigated site is located in the uppermost reaches of Pain Mesa, which forms the northern sector of the Mesa Range. An ice wedge was found in a volcanic regolith at about 2200ma.s.l. This altitude is below a well-defined erosional trimline, located at about 2380ma.s.l. in this sector of Pain Mesa. The ice was sampled by inserting an ice screw, with an internal diameter of 14 mm, into the ice wedge in vertical sequences. Oriented block samples for thin sections were taken. A co-isotopic study was performed, measuring both oxygen (δ18O) and hydrogen (δD) isotope compositions. Tritium activity was measured, and major cations and anions were determined. The δ18O and δD obtained showed a strong divergence from the snowfalls expected to occur at this elevation, with extremely negative d excess values. Sublimation processes were taken into account to define the origin of the ice forming the wedge. The tritium data obtained suggest that the growth process of the ice wedge might still be active today.
Modelling the isotopic composition of snow using backward trajectories: a particular precipitation event in Dronning Maud Land, Antarctica
Authors: Helsen Michiel M. , Van De Wal Roderik S. W. , Van Den Broeke Michiel R. , Kerstel Erik R. Th. , Masson-Delmotte Valérie , Meijer Harro A. J. , Reijmer Carleen H. , Scheele Marinus P.
Published: November 2007

We consider a specific accumulation event that occurred in January 2002 in western Dronning Maud Land, Antarctica. Snow samples were obtained a few days after accumulation. We combine meteorological analyses and isotopic modelling to describe the isotopic composition of moisture during transport. Backward trajectories were calculated, based on European Centre for Medium-Range Weather Forecasts operational archive data so that the history of the air parcels transporting water vapour to the accumulation site could be reconstructed. This trajectory study showed that the air masses were not (super)saturated along most of the transport path, which is in contrast with assumptions in Lagrangian fractionation models and probably true for most precipitation events in Antarctica. The modelled fractionation along the trajectories was too limited to explain the measured isotopic content of the snow. It is shown that the observed isotopic composition of precipitation resulted from fractionation of initially more depleted water. This lower initial isotopic composition of water vapour might result from atmospheric mixing with more depleted air along the trajectory or from earlier condensation cycles, not captured by the trajectories. This is in accordance with isotope fields resulting from general circulation models, indicating a gradient in isotopic composition from the Equator to Antarctica.
A 700 year record of Southern Hemisphere extratropical climate variability
Authors: Mayewski Paul A. , Maasch Kirk A. , White James W. C. , Steig Eric J. , Meyerson Eric , Goodwin Ian , Morgan Vin I. , Van Ommen Tas , Curran Mark A. J. , Souney Joseph , Kreutz Karl
Published: November 2007

Annually dated ice cores from West and East Antarctica provide proxies for past changes in atmospheric circulation over Antarctica and portions of the Southern Ocean, temperature in coastal West and East Antarctica, and the frequency of South Polar penetration of El Niño events. During the period AD 1700–1850, atmospheric circulation over the Antarctic and at least portions of the Southern Hemisphere underwent a mode switch departing from the out-of-phase alternation of multi-decadal long phases of EOF1 and EOF2 modes of the 850 hPa field over the Southern Hemisphere (as defined in the recent record by Thompson and Wallace, 2000; Thompson and Solomon, 2002) that characterizes the remainder of the 700 year long record. From AD 1700 to 1850, lower-tropospheric circulation was replaced by in-phase behavior of the Amundsen Sea Low component of EOF2 and the East Antarctic High component of EOF1. During the first phase of the mode switch, both West and East Antarctic temperatures declined, potentially in response to the increased extent of sea ice surrounding both regions. At the end of the mode switch, West Antarctic coastal temperatures rose and East Antarctic coastal temperatures fell, respectively, to their second highest and lowest of the record. Polar penetration of El Niño events increased during the mode switch. The onset of the AD 1700–1850 mode switch coincides with the extreme state of the Maunder Minimum in solar variability. Late 20th-century West Antarctic coastal temperatures are the highest in the record period, and East Antarctic coastal temperatures close to the lowest. Since AD 1700, extratropical regions of the Southern Hemisphere have experienced significant climate variability coincident with changes in both solar variability and greenhouse gases.
Towards multi-sensor microwave remote sensing of frost flowers on sea ice
Authors: Kaleschke Lars , Heygster Georg
Published: November 2007

It has been hypothesized that frost flowers could be important for tropospheric chemistry and, as a source of sea-salt aerosol, for the interpretation of ice-core data. Furthermore, frost flowers can cause severe errors in sea-ice products derived from remote-sensing data. Up to now there have been few datasets available relating frost-flower distribution and variability in the sea-ice-covered regions. We present a method for frost-flower detection using a combination of active and passive microwave sensors. The vertical polarized C-band radar backscatter of a young sea-ice region covered with frost flowers can be higher than the signal from multi-year ice or from the wind-roughened ocean. Therefore, the classification result of synthetic aperture radar (SAR) sea-ice images can be defective due to the influence of frost flowers. The ambiguity of the open-water and frost-flower backscatter signal was successfully resolved using additional information from the 85 GHz Special Sensor Microwave/Imager (SSM/I) channels. Results of the method were compared to aircraft measurements and observations in the marginal ice zone near Svalbard.
High-time-resolution profiles of soluble ions in the last glacial period of a Dome Fuji (Antarctica) deep ice core
Authors: Iizuka Yoshinori , Takata Morimasa , Hondoh Takeo , Fujii Yoshiyuki
Published: November 2007

We measured the depth profiles of soluble ions in the Dome Fuji (Antarctica) ice core to search for possible paleoclimate indications of seasonal climate variations in the last glacial period. A 523 mm long core section between 587.65 and 588.18 m depth was selected for this pilot study, and the high-resolution chemical analysis was done on 2 mm thick samples. Our results indicate that anion– cation trapping in ice affects the profiles of the soluble ions and [Na+] in the core may preserve its seasonal signal. Correlation coefficients and the equivalent balances of the soluble ions suggest the following selective coexistences: (1) [Cl–] = [Na+], (2) [NO3–] = 1/2[Ca2+] + [K+], and (3) [SO42–] = 1/2[Ca2+] + [H+] + [Mg2+]. These coexistences are probably due to (1) a sea-salt source of Na+ and Cl–, (2) reaction of NO3– with dust for Ca2+ and NO3–, and (3) anion–cation trapping in ice for SO42– and Ca2+ (Mg2+), respectively.
Initial borehole results from the Amery Ice Shelf hot-water drilling project
Authors: Craven Mike , Allison Ian , Brand Russell , Elcheikh Alan , Hunter John , Hemer Mark , Donoghue Shavawn
Published: November 2007

The Amery Ice Shelf Ocean Research (AMISOR) project aims to examine and quantify processes involved in the interaction between the ice shelf, the interior grounded ice and the oceanic water masses that circulate beneath it. Two boreholes were melted through the shelf, within 100 km of the calving front, to access the ocean cavity. One (AM02) was at a site where it was believed that basal melt was occurring, and the other (AM01) was in a region with accreted marine ice. At both sites the summertime ocean structure revealed meltwater-modified boundary layers up to 100 m thick immediately beneath the shelf. Salinity and temperature data in the upper cavity at AM02 showed a strong seasonal cycle as a result of a combination of ice-shelf basal melt, and the intrusion of ocean water masses modified by sea-ice processes in Prydz Bay. At AM01, a 200m thick layer of marine ice underlay the meteoric ice, and showed an increase in salinity and decrease in stable-isotope fractionation with depth. The lowest 100m of marine ice was highly permeable, with a rectangular banded textural facies. Other preliminary results from this study are also reported.
Sulfur isotopic measurements from a West Antarctic ice core: implications for sulfate source and transport
Authors: Pruett Lee E. , Kreutz Karl J. , Wadleigh Moire , Mayewski Paul A. , Kurbatov Andrei
Published: November 2007

Measurements of δ34S covering the years 1935–76 and including the 1963 Agung (Indonesia) eruption were made on a West Antarctic firn core, RIDSA (78.73˚ S, 116.33˚ W; 1740ma.s.l.), and results are used to unravel potential source functions in the sulfur cycle over West Antarctica. The δ34S values of SO42– range from 3.1‰ to 9.9‰. These values are lower than those reported for central Antarctica, from near South Pole station, of 9.3–18.1‰ (Patris and others, 2000). While the Agung period is isotopically distinct at South Pole, it is not in the RIDSA dataset, suggesting differences in the source associations for the sulfur cycle between these two regions. Given the relatively large input of marine aerosols at RIDSA (determined from Na+ data and the seasonal SO42– cycle), there is likely a large marine biogenic SO42– influence. The δ34S values indicate, however, that this marine biogenic SO42–, with a well-established δ34S of 18‰, is mixing with SO42– that has extremely negative δ34S values to produce the measured isotope values in the RIDSA core. We suggest that the transport and deposition of stratospheric SO42– in West Antarctica, combined with local volcanic input, accounts for the observed variance in δ34S values.
The role of ice thickness and bed properties on the dynamics of the enhanced-flow tributaries of Bailey Ice Stream and Slessor Glacier, East Antarctica
Authors: Rippin David M. , Bamber Jonathan L. , Siegert Martin J. , Vaughan David G. , Corr Hugh F. J.
Published: November 2007

Airborne radio-echo sounding investigations in the upper reaches of Bailey Ice Stream and Slessor Glacier, Coats Land, East Antarctica, have shown that enhanced-flow tributaries are associated with well-defined areas of relatively thicker ice, and are separated from each other by areas of relatively thinner ice. A numerical modelling study has revealed that while internal ice deformation might account for all the observed flow in inter-tributary areas and the majority in the Slessor tributaries, a significant proportion of the flow of Bailey tributary is attributable to basal motion. Further, investigations of depth-corrected basal reflection power indicate that the bed underlying both Bailey and Slessor enhanced-flow tributaries is significantly smoother than in the slower-moving inter-tributary areas. It is thus proposed that enhanced motion within Bailey tributary (and also perhaps Slessor) may be facilitated by a reduction in basal roughness, caused by the accumulation of water and/or sediments within subglacial valleys, or by the erosion and smoothing of bed obstacles.