bluuebunny/crossref_metadata_2025 huggingface dataset:
The accumulation regime of Blue Glacier, U.S.A., 1914–96
Authors: Rasmussen L. A. , Conway H. , Hayes P. S.
Published: November 2007

Precipitation at 2050 m on Blue Glacier, U.S.A., was measured daily from August 1957 through July 1958. Its correlation with a nearby lowland station with a good, long-term (1914–96) record is used to estimate precipitation on the glacier over that entire period. Average annual precipitation on Blue Glacier is 4500 mm w.e. Snowfall depends on the joint distribution of precipitation and temperature. Over the period 1948–96, for which twice-daily radiosonde observations are available, temperature at any elevation on the glacier is interpolated in the radiosonde profile to partition the precipitation as either rain or snow. Daily partitioning is preferred, especially during spring and autumn storms when averaging over longer periods may substantially under- or overestimate snowfall on the glacier. Prior to 1948, snowfall is estimated from the mean over 1948–96, in a particular month and elevation, of the fraction of the precipitation falling as snow. The standard error in the October–May snowfall at 2100 m is estimated to be 250 mm w.e. during the radiosonde era (1948–96) and 350 mm prior to that. For the first 10 years or so after mass-balance measurements began at Blue Glacier (1957), precipitation increased and winter temperature at 850 mbar (about 1450 m) decreased, but since then the trends have reversed. The combined effect, increasing snowfall until 1965 and decreasing since, closely parallels measured mass changes of Blue Glacier. When the average vertical profile of total annual snowfall is subjected to a hypothetical 1 K warming, the resulting reduction in snowfall is greatest at the glacier terminus and decreases up-glacier; the average over the entire glacier is 300 mm w.e.
Mass balance of the northeast sector of the Greenland ice sheet: a remote-sensing perspective
Authors: Rignot Eric , Buscarlet Guillaume , Csathó Beáta , Gogineni Sivaprasad , Krabill William , Schmeltz Marjorie
Published: November 2007

Synthetic-aperture radar interferometry data and airborne ice-sounding radar (ISR) data are employed to obtain modern estimates of the inland ice production from Nioghalvfjerdsbræ (NB) and Zachariae Isstrøm (ZI), the two largest glaciers draining the northeast sector of the Greenland ice sheet. Ice fluxes are measured at the grounding line (14.2 ±1 km3ice a−1for NB and 10.8 ±1 km3ice a−1for ZI) with an ice thickness deduced from ice-shelf hydrostatic equilibrium, and along an ISR profile collected upstream of the grounding line (14.3 ± 0.7 km3ice a−1for NB and 11.6 ± 0.6 km3ice a−1for ZI). Balance fluxes calculated from a map of snow accumulation and model predictions of surface melt are 11.9 ± 2 km3ice a−1for NB and 10.0 ± 2 km3ice a−1for ZI at the grounding line, and 12.2 and 10.3 km3ice a−1, respectively, at the ISR line. The two glaciers therefore exhibit a negative mass balance equivalent to 14% of their balance flux, with a ±12% uncertainty. Independently, we detect a retreat of the grounding line of NB between 1992 and 1996 which is larger at the glacier center (920 ± 250 m) than on the sides (240 ± 50 m). The corresponding ice-thinning rates (2 ± 1 m a−1at the glacier center and 0.6 ± 0.3 m a−1on the sides) are too large to be accommodated by temporal changes in ablation or accumulation, and must be due to dynamic thinning.
A model of ablation-dominated medial moraines and the generation of debris-mantled glacier snouts
Authors: Anderson Robert S.
Published: November 2007

Medial moraines form striking dark stripes that widen non-linearly, steepen laterally and increase in relief down-glacier from the equilibrium line. Coalescence of these low-ablation-rate features can feed back strongly on the mass balance of a glacier snout. Ablation-dominated medial moraines originate from debris delivered to glacier margins, producing a debris-rich septum between tributary streams of ice below their confluence. Emergence of this ice below the equilibrium line delivers debris to the glacier surface, which then moves down local slopes of evolving morainal topography. A quantitative description of moraine evolution requires specification of the debris concentration field within the glacier, treatment of the melt-rate dependence on debris thickness, and characterization of processes that transport debris once it emerges onto the ice surface. Debris concentration at glacier tributary junctions scales with the erosion rates and the lengths of the tributary-valley walls, and inversely with the tributary ice speeds. Melt rate is damped exponentially by debris, with a ∼10 cm decay scale. Debris flux across the glacier surface scales with the product of debris thickness and local slope. Analytical and numerical results show that medial moraines should develop cross-glacier profiles with parabolic crests and linear slopes, and should widen with age and hence distance down-glacier. Debris should be both thin and uniform over the moraine. Observed faster-than-linear growth of moraine widths with distance reflects the increasing ablation rate down-glacier. Increase in medial moraine cover reduces the local average ablation rate, allowing the glacier to extend further down-valley than meteorology alone would suggest. This feedback is especially effective when moraines merge.
Dynamic behaviour analysis of glacier de Saint Sorlin, France, from 40 years of observations, 1957–97
Authors: Vincent C. , Vallon M. , Reynaud L. , Le Meur E.
Published: November 2007

Mass-balance and dynamic measurements carried out on glacier de Saint Sorlin since 1957 provide a good opportunity to study the dynamics of this glacier. Ice-flow analysis shows that dynamic changes have been important over the last 40 years and that these changes are not consistent with the concepts usually used in glacier modelling. Present velocities are larger than the 1960 velocities, although the thickness decreased everywhere (10–30 m in the ablation zone). A simple numerical ice-flow model which does not include longitudinal stress gradients has been used to investigate these phenomena. This model allows us to infer the sliding velocity from observed surface and calculated deformation velocities. We conclude that: (1) the sliding velocity cannot be described by Weertman analysis or empirical relations which link the sliding to the thickness and surface slope; (2) the inferred sliding velocity is uniform over at least half of the glacier; and (3) there is no clear link between the sliding process and the quantity of water coming from surface ablation. Furthermore, it may not be reasonable to calibrate model flow parameters from geometry changes because the surface geometry is relatively insensitive to velocity changes over some decades.
A comparison of radio-echo sounding data and electrical conductivity of the GRIP ice core
Authors: Hempel Ludwig , Thyssen Franz , Gundestrup Niels , Clausen Henrik B. , Miller Heinz
Published: November 2007

The depth of reflecting layers in Arctic ice sheets has been determined by electromagnetic echo sounding, using a varying distance between transmitter and receiver to determine the radar wave velocity. The depth of the radar reflecting layers is compared with a profile of electrical conductivity measurements (ECMs) from the Greenland Ice Core Project (GRIP) ice core, in order to determine the velocity of the radar waves in the ice cap. By using several reflecting layers, it is possible to isolate the firn correction of the wave velocity and to estimate the accuracy of the calculated electromagnetic wave velocity. The measured firn correction is compared with the correction calculated from the density profile, and a comparison between the depth profiles of ECM and radar based on the corrected electromagnetic wave velocity is presented. This profile shows that acid layers, which originate from major volcanic eruptions, show up as reflecting radar horizons.
Glaciochemical dating of an ice core from upper Grenzgletscher (4200 m a.s.l.)
Authors: Eichler Anja , Schwikowski Margit , Gäggeler Heinz W. , Furrer Verena , Synal Hans-Arno , Beer Jürg , Saurer Matthias , Funk Martin
Published: November 2007

One crucial condition for the interpretation of ice-core records is the establishment of an accurate time-scale. This task is especially difficult for glacier sites in a complex topography such as the Alps, due to the often irregular deposition of fresh precipitation. In this work, dating techniques were applied to an Alpine ice core from upper Grenzgletscher, Monte Rosa massif (4200 m a.s.l.), representing about two-thirds of the total glacier thickness. They are based on (i) the radioactive decay of the isotope 210Pb, (ii) seasonally varying signals such as the concentrations of NH4+ and the isotopic ratio δ18O, and (iii) stratigraphic markers from Saharan dust falls, atmospheric nuclear weapon tests and the reactor accident in Chernobyl. From the combined application of these dating methods, a time period of 1937–94 covered by the ice core was derived. Dating uncertainty is <1 year for the period 1970–94 and ± 2 years for the period 1937–69. The observed thinning of the annual layers as a function of depth could be well described by a simple kinematic glacier flow model.
The use of bulk and profile methods for determining surface heat fluxes in the presence of glacier winds
Authors: Denby Bruce , Greuell W.
Published: November 2007

A one-dimensional second-order closure model and in situ observations on a melting glacier surface are used to investigate the suitability of bulk and profile methods for determining turbulent fluxes in the presence of the katabatic wind-speed maximum associated with glacier winds. The results show that profile methods severely underestimate turbulent fluxes when a wind-speed maximum is present. The bulk method, on the other hand, only slightly overestimates the turbulent heat flux in the entire region below the wind-speed maximum and is thus much more appropriate for use on sloping glacier surfaces where katabatic winds dominate and wind-speed maxima are just a few meters above the surface.
Geophysical investigations of ice-sheet internal layering and deformation in the Dome C region of central East Antarctica
Authors: Hodgkins Richard , Siegert Martin J. , Dowdeswell Julian A.
Published: November 2007

New maps are presented of three internal ice-sheet radio-echo sounding (RES) layers in the region 73.5–75.75° S, 120–127° E (56 000 km2) around Dome C, central East Antarctica. These layers represent horizons of enhanced acidity resulting from volcanic aerosol deposition, identified from analogue RES data. They are continuous over the entire mapped area, and constitute deformation markers in the ice column. Internal RES layers were initially identified from discrete radar power reflection coefficient profiles and subsequently digitized directly from prints of ice-sheet cross-sections, acquired by continuous RES profiling. Georeferenced vector data are used to generate a 5 km gridcell raster of depth for each internal RES layer, as a basis for contour mapping. Ice deformation in the Dome C region is significant because this is the location of the European Project for Ice Coring in Antarctica. Since internal layers are isochronous, the one-dimensional ice-core data at Dome C can be correlated over the survey area to produce a three-dimensional context.
Reconstructing past atmospheric CO2 concentration based on ice-core analyses: open questions due to in situ production of CO2 in the ice
Authors: Tschumi Jürg , Stauffer Bernhard
Published: November 2007

Analysis of air extracted from bubbles of polar ice showed the anthropogenic increase of atmospheric CO2 concentration during the past few hundred years (preindustrial concentration 280 ppmv) and, unexpectedly, that the concentration also increased due to natural causes from 200 to 280 ppmv during the transition from the last glacial epoch to the Holocene. However, more detailed reconstructions based on ice-core analyses suggested that some of the additional observed variations were actually due to the modification of CO2 concentration in the bubbles by chemical reactions between impurities in the ice. Detailed analyses of acidity, the carbonate concentration, the concentration of oxidation agents like hydrogen peroxide (H2O2), and organic compounds (e.g. formaldehyde (HCHO)) along short intervals of ice cores, representing only a few annual layers, allowed us to investigate these chemical reactions. The records are not conclusive but they indicate that the oxidation of organic compounds is at least as important as acid–carbonate reactions. Ice containing a low mean carbonate and a low H2O2 concentration, as well as a small scattering of CO2 results obtained on several adjacent samples representing a few annual layers at most, is at present the best guarantee for reliable results of the atmospheric CO2 concentration.
The onset area of Ice Stream D, West Antarctica
Authors: Bindschadler Robert , Chen Xin , Vornberger Patricia
Published: November 2007

Surface flow in a 10 000 km2 expanse of the onset area of Ice Stream D, West Antarctica, was measured by repeat, precise global positioning system surveys over a 1 year interval. The pattern of velocity and strain rate shows the development of Ice Stream D, the major flow into which originates south of Byrd station and follows the course of a deep bed channel. Plotting of the driving stress vs the ratio of velocity and ice thickness identifies the onset of streaming flow (roughly 140 km downstream of Byrd station) as a transition between deformation flow and sliding flow. Along the kinematic center line of the developing ice stream, the ice rheology is linear at stresses below 0.6 bar, and appears temperate at the base well before the onset of streaming is reached. The onset corresponds to a maximum driving stress of 0.8 bar. It occurs downstream of a slight increase in longitudinal strain rate where stronger along-flow lineations are apparent in Landsat imagery, and after the ice has passed the center of an overdeepening in the bed channel. No current deviation from equilibrium is detected in this region, but a set of flow stripes misaligned with present flow indicates significant changes in flow have occurred in the past.
Natural and anthropogenic levels of tritium in a Canadian Arctic ice core, Agassiz Ice Cap, Ellesmere Island, and comparison with other radionuclides
Authors: Kotzer Thomas G. , Kudo Akira , Zheng James , Workman Wayne
Published: November 2007

Numerous studies of the ice caps in Greenland and Antarctica have observed accumulations of transuranic radionuclides and fission products from nuclear weapons testing, particularly during the period 1945–75. Recently, the concentrations of radionuclides in the annually deposited surface layers of Agassiz Ice Cap, Ellesmere Island, Canadian Arctic, from 1945 to the present have been measured and have demonstrated a continuous record of deposition of 137Cs and 239,240Pu in ice and snow. In this study, 3He-ingrowth mass spectrometry has been used to measure the low levels of tritium (3H) in some of these samples. Pre-nuclear-bomb tritium levels in ice-core samples were approximately 12 TU in high-latitude meteoric waters and 3–9 TU in mid-latitude meteoric waters. Comparisons of 3H levels and 3H/137Cs + 239,240Pu ratios, which were quite low during the earliest fission-bomb detonations (1946–51) and substantially higher during thermonuclear hydrogen-fusion bomb testing (1952–64), provide a clear indication of the type of nuclear device detonated. This finding accords with the results from other ice-core studies of the distribution of anthropogenic radionuclides from bomb fallout.
Bed properties of Siple Dome and adjacent ice streams, West Antarctica, inferred from radio-echo sounding measurements
Authors: Gades A. M. , Raymond C. F. , Conway H. , Jagobel R. W.
Published: November 2007

We have used ground-based radio-echo sounding (RES) profiles to reveal the spatial distribution of basal and internal ice properties across Siple Dome, West Antarctica, and under the dormant ice streams on its flanks. The RES-detected bed-reflection power, corrected for the effects of instrumentation and ice-thickness variation, is nearly constant across Siple Dome at a value suggesting spatially homogeneous basal properties of ice frozen to bedrock. Till, if present under the dome, must be thin (<0.1 m). The high basal reflectivity measured under now dormant “Siple Ice Stream” (SIS) and Ice Stream C suggests that they are underlain by either a thin (<0.05 m) water layer or a thick (>1 m) thawed or frozen till layer. The evidence that the dormant SIS is not frozen directly to underlying bedrock (but is separated by a water or till layer) is a further indication that it was once an active ice stream, and suggests that streaming motion may have ceased before the basal layer was frozen. The absence of a thick till layer beneath Siple Dome is consistent with its apparent stability as an inter-ice-stream ridge in the past and may suggest that it will remain as a stable limitation of ice-stream width in the future.
Formaldehyde and peroxide concentrations in Law Dome (Antarctica) firn and ice cores
Authors: Gillett R. W. , van Ommen T. D. , Jackson A.V. , Ayers G. P.
Published: November 2007

Peroxide speciation and formaldehyde measurements have been made on ice cores retrieved from Law Dome, Antarctica. Measurements were made for ice deposited during four different periods: modern, pre-industrial Holocene, early Holocene and Last Glacial Maximum (LGM). The data show modern peroxide levels >50% above pre-industrial levels (at ∼1.6 μmol L−1) and an absence of methyl hydroperoxide (down to a detection threshold of 0.003 μmol L−1). Formaldehyde levels show a 40% increase from pre-industrial to modern times (rising from ∼0.07 μmol L−1 to ∼0.10 μmol L−1), with a further increase and possible seasonality near the surface which we associate with post-depositional processes. Peroxide levels in LGM ice are low, but formaldehyde concentrations are high (at ∼0.13 μmol L−1) relative to modern levels. Similar high levels of formaldehyde are seen in early Holocene ice (∼6900 years BP).
Subglacial deformation: evidence from microfabric studies of particles and voids in till from the upper Ürümqi river valley, Tien Shan, China
Authors: Chaolu Yi , Zhijiu Cui
Published: November 2007

Microfabrics formed by flat particles and voids in till samples from the upper Ürümqi river valley, Tien Shan, northwestern China, were analyzed using three mutually perpendicular thin sections. Chi-squared tests were used to determine significance levels of the orientations of both particles and voids. The basal tills in an end moraine, on the stoss sides of two roche moutonnées, on the stoss sides of a drumlin with a rock core, and on the lee side of the same drumlin far from the rock core have strong particle and void microfabrics. Field evidence suggests that these tills were deformed in a subglacial setting. Thus, consistent with recent laboratory studies of till fabric development, deformation is believed to be responsible for the strong particle and void microfabrics. In the same end moraine, particle microfabrics in an overlying till were strong, but void microfabrics were weak. This till is believed to have been formed by dumping and rolling of debris from hill slopes and of ablation moraine from the glacier surface. The weak void microfabric is interpreted to indicate that the till was not consolidated and hence not sheared under the weight of the glacier, despite the strong particle microfabric. Basal tills on the lee sides of the roche moutonnees, and on the lee side of the drumlin but near its core, have weak microfabrics.
Detection of snowmelt regions on the Greenland ice sheet using diurnal backscatter change
Authors: Nghiem S.V. , Steffen K. , Kwok R. , Tsai W. Y.
Published: November 2007

Snowmelt regions on Greenland ice are mapped daily with the SeaWinds wideswath Ku-band (13.4 GHz) scatterometer on the QuikSCAT satellite. The approach exploits the high temporal resolution of SeaWinds/QuikSCAT data for the melt mapping using diurnal backscatter change independent of the absolute calibration. The results reveal several pronounced melting and refreezing events, and effects of topography are evident in the melt patterns. The spatial resolution is sufficient to identify melt features on the Sukker-toppen Iskappe west of the main ice sheet. An anomalous warming event, caused by down-ward mixing of warm air, is detected in late September 1999 over the west flank of the southern Greenland ice sheet. Time-series images of melt regions are presented over the period from summer to the fall freeze-up. The satellite observations are verified with in situ measurements from the Greenland Climate Network stations.
Flow dynamics of tidewater glaciers: a numerical modelling approach
Authors: Vieli Andreas , Funk Martin , Blatter Heinz
Published: November 2007

The dynamics of grounded tidewater glaciers is investigated with a time-dependent numerical flow model, which solves the full equations for the stress and velocity fields and includes a water-pressure-dependent sliding law. The calving criterion implemented in the model shifts the calving front at each time-step to the position where the frontal ice thickness exceeds flotation height by a prescribed value. With this model, the linear relation between calving rate and water depth proposed on empirical grounds is qualitatively reproduced for the situation of a slowly retreating or advancing terminus, but not for situations of rapid changes. Length changes of tidewater glaciers, i.e. especially rapid changes, are dominantly controlled by the bed topography and are to a minor degree a direct reaction to a mass-balance change. Thus, accurate information on the near-terminus bed topography is required for reliable prediction of the terminus changes due to climate changes. The results also confirm the suggested cycles of slow advance and rapid retreat through a basal depression. Rapid changes in terminus positions preferably occur in places where the bed slopes upwards in the ice-flow direction.
Potential for stratigraphic folding near ice-sheet
centers
Authors: Waddington Edwin D. , Bolzan John F. , Alley Richard B.
Published: November 2007

Lack of agreement between the deep portions of the Greenland Icecore Project (GRIP)
and Greenland Ice Sheet Project II (GISP2) ice cores from central Greenland suggests
that folds may disrupt annual layering, even near ice divides. We use a simple
kinematic flow model to delineate regions where slope disturbances
(“wrinkles”) introduced into the layering could overturn into recumbent
folds, and where they would flatten, leaving the stratigraphic record intact.
Wrinkles are likely to originate from flow disturbances caused internally by
inhomogeneities and anisotropy in the ice rheological properties, rather than from
residual surface structures (sastrugi), or from open folds associated with transient
flow over bed topography. If wrinkles are preferentially created in anisotropic ice
under divides, where the resolved shear stress in the easy-glide direction can be
weak and variable, then the deep intact climate record at Dye 3 may result from its
greater distance from the divide. Alternatively, the larger simple shear at Dye 3 may
rapidly overturn wrinkles, so that they are not recognizable as folds. The ice-core
record from Siple Dome may be intact over a greater fraction of its depth compared to
the central Greenland records if its flat bedrock precludes fluctuations in the
stress orientation near the divide.
Quantifying the effects of climate and surface change on glacier mass balance
Authors: Elsberg D. H. , Harrison W. D. , Echelmeyer K. A. , Krimmel R. M.
Published: November 2007

When a mass balance is computed using an outdated map, that computation does not reveal the actual mass change. But older maps often must be used for practical reasons. We present a method by which, with a few additional measurements each year, a mass balance computed with an outdated map can be transformed into an actual mass change. This is done by taking into account the influence of changes in areal extent and changes in the surface elevation of the glacier since the map was made. This method is applied to South Cascade Glacier, Washington, U.S.A., as an example. The computed cumulative mass balance from 1970 to 1997 would have been 16% too negative if the 1970 map had not been updated. While the actual volume change of a glacier is relevant to hydrological studies, the change that would have occurred on a constant (or static) surface is more relevant to certain glacier dynamics problems and most climate problems. We term this the reference-surface balance and propose that such a balance, which deliberately omits the influence of changes in area and surface elevation, is better correlated to climatic variations than the conventional one, which incorporates those influences.
A mean net accumulation pattern derived from radioactive layers and radar soundings on Austfonna, Nordaustlandet, Svalbard
Authors: Pinglot Jean Francis , Hagen Jon Ove , Melvold Kjetil , Eiken Trond , Vincent Christian
Published: November 2007

We present the snow-accumulation distribution over Austfonna, Nordaustlandet, Svalbard, based on 29 shallow ice cores that were retrieved from this ice cap during 1998 and 1999. Mean annual net accumulation is deduced from radioactive layers resulting from the 1954–74 atmospheric nuclear tests (maximum in 1963) and the Chernobyl accident (1986). The Chernobyl layer was located in 19 ice cores in the accumulation area, and the nuclear test layer was located in two deeper ice cores. In addition, the spatial variation of the depth of winter 1998/99 snowpack was mapped using snow probing, ground-penetrating radar methods and pit studies. The altitudinal gradient of the mean annual net mass balance and the altitude of the mean equilibrium line are determined along five transects ending at the top of the ice cap. The mean annual net mass balance and the equilibrium-line altitudes show a high degree of asymmetry between the western and eastern parts of Austfonna, in accordance with the distribution of winter accumulation. Large interannual variations of the accumulation exist. However, the study of the mean annual net mass balance shows no trend for two different time periods, 1963–86 and 1986 to the date of the drillings (1998/99).
Oxygen isotopic and soluble ionic composition of a shallow firn core, Inilchek glacier, central Tien Shan
Authors: Kreutz Karl J. , Aizen Vladimir B. , DeWayne Cecil L. , Wake Cameron P.
Published: November 2007

Oxygen isotopic and soluble ionic measurements made on snow-pit (2 m depth) and firn-core (12.4 m depth) samples recovered from the accumulation zone (5100 m) of Inilchek glacier (43° N, 79° E) provide information on recent (1992–98) climatic and environmental conditions in the central Tien Shan region of central Asia. The combined 14.4 m snow-pit/firn-core profile lies within the firn zone, and contains only one observed melt feature (10 m temperature = −12°C). Although some post-depositional attenuation of the sub-seasonal δ18O record is possible, annual cycles are apparent throughout the isotope profile. We therefore use the preserved δ18O record to establish a depth/age scale for the core. Mean δ18O values for the entire core and for summer periods are consistent with δ18O/temperature observations, and suggest the δ18O record provides a means to reconstruct past changes in summer surface temperature at the site. Major-ion (Na+, K+, Mg2+, Ca2+, NH4+, Cl−, NO3−, SO42−) data from the core demonstrate the dominant influence of dust deposition on the soluble chemistry at the site, and indicate significant interannual variability in atmospheric-dust loading during the 1990s. Anthropogenic impacts on NH4+ concentrations are observed at the site, and suggest a summer increase in atmospheric NH4+ that may be related to regional agricultural (nitrogen-rich fertilizer use) activities.
Estimating South Cascade Glacier (Washington, U.S.A.) mass balance from a distant radiosonde and comparison with Blue Glacier
Authors: Rasmussen L. A. , Conway H.
Published: November 2007

A simple flux model using twice-daily measurements of wind, humidity and temperature from standard upper-air levels in a distant radiosonde estimated winter balance of South Cascade Glacier, Washington, U.S.A., over 1959–98 with error 0.24 m w.e. Correlation between net and winter balance is strong; the model estimates net balance with error 0.53 m w.e. Over the past 40 years, average net balance of South Cascade Glacier has been strongly negative (−0.46 m w.e.), and it has been shrinking steadily. In comparison, 200 km west-southwest at Blue Glacier, the average balance has been less negative (−0.13 m w.e); that glacier has undergone little change over the 40 years. Balance histories of the two glaciers are positively correlated (r = +0.54), and South Cascade has been more out of balance than Blue, presumably because it is still adjusting to climate change since the Little Ice Age. Recent warming and drying has made the net balance of both glaciers strongly negative since 1976 (−0.84 m w.e. at South Cascade, −0.56 m w.e. at Blue). If South Cascade Glacier were in balance with the 1986–98 climate, it would be about one-quarter of its present area.
Abrupt glacier motion and reorganization of basal shear stress following the establishment of a connected drainage system
Authors: Kavanaugh Jeffrey L. , Clarke Garry K. C.
Published: November 2007

Three episodes of strong basal motion occurred at Trapridge Glacier, Yukon Territory, Canada, on 11 June 1995 following the establishment of a connected subglacial drainage system. Responses to these “spring events” are noted in the records for 42 instruments and were recorded throughout the ∼60 000 m2 study area. Strong basal motion during the events is indicated by ploughmeter, load-bolt and vertical-strain records, and abrupt pressure changes in several transducer records denote damage caused by extreme pressure pulses. These pressure pulses, generated by the abrupt basal motion, also resulted in the failure of seven pressure sensors. Records for pressure, turbidity and conductivity sensors indicate that basal drainage patterns did not change significantly during the events. Geophone records suggest that the episodes of basal motion were precipitated by the gradual failure of a “sticky spot” following hydraulic connection of part of the study area. This failure resulted in the transfer of basal stress to the unconnected region of the bed during the course of the events. No evidence for strong basal motion is seen in the instrument records for several weeks following the events, suggesting that the mechanical adjustments resulted in a stable configuration of basal stresses. This event illustrates how unstable situations can be quickly accommodated by mechanical adjustments at the glacier bed.
Interannual variability of elevation on the Greenland ice sheet: effects of firn densification, and establishment of a multi-century benchmark
Authors: Cuffey K. M.
Published: November 2007

In order to interpret measurements of ice-sheet surface elevation changes in terms of climatic or dynamic trends, it is necessary to establish the range of stochastic variability of elevation changes resulting from interannual fluctuations of accumulation rate and firn density. The analyses presented here are intended to facilitate such interpretations by defining benchmarks that characterize elevation-change variability in central Greenland, in the current climate and over the past millennium. We use a time- dependent firn-densification model coupled to an ice- and heat-flow model, forced by annual accumulation rate and temperature reconstructions from the Greenland Ice Sheet Project II (GISP2) ice core, to examine the elevation changes resulting from this climatic forcing. From these results, effective firn densities are calculated. These are factors that convert water-equivalent accumulation-rate variability to surface elevation variability. A current-climate benchmark is defined by applying this conversion to Van der Veen and Bolzan’s water-equivalent statistics, and to a 50 year accumulation variability estimate from the GISP2 core. Elevation-change statistics are compiled for the past millennium to define longer-term benchmarks, which show that multi-century variability has been substantially larger than current variability estimated by Van der Veen and Bolzan. It is estimated here that the standard deviation of net elevation change over 5 and 10 year intervals has been 0.27 and 0.38 m, respectively. An approximate method for applying these quantitative results to other dry-snow sites in Greenland is suggested.
Internal ice-sheet radar layer profiles and their relation to reflection mechanisms between Dome C and the Transantarctic Mountains
Authors: Siegert Martin J. , Fujita Shuji
Published: November 2007

Causes of ice-sheet layering at ice depths greater than about 900 mina transect between Dome C and the Transantarctic Mountains are examined using 60 MHz radar data, collected in the 1970s by the U.K.–U.S.–Danish collaboration. Normally, a dual-frequency technique is required for accurate determination of internal reflection mechanisms. However, by extracting the depth-related features of 60 MHz radar profiles and comparing them with the dual-frequency data collected by the Japanese Antarctic Research Expedition, we have identified a simple method to estimate internal reflection mechanisms. Two zones can be distinguished: (1) the CA zone, where change in electrical conductivity due to variation in acidity is the major cause of internal reflection, and (2) the PCOF zone, where change in dielectric permittivity due to crystal-orientation fabrics is the major cause of internal reflections. Our analysis shows that the radar data reveal the development of PCOF zones in regions where large amounts of ice shearing are expected. This analysis shows how a similar interpretation of the full radar-data archive may reveal information on internal reflection mechanisms across a large part of the East Antarctic ice sheet.
Spatial patterns of snow stability throughout a small mountain range
Authors: Birkeland K.W.
Published: November 2007

This research investigates snow stability on the eastern side of a small mountain range in southwest Montana, U.S.A., on one mid-season day and one late-season day during the 1996/97 winter. Although previous research has addressed snow stability at smaller spatial scales, this is the first field-based study to investigate snow stability (as measured by stability tests) over a mountain range in order to better understand its spatial distribution and the implications for predicting dry-slab avalanches. Using helicopter access, six two-person sampling teams collected data from over 70 sites on each of the two sampling days. Variables for terrain, snowpack, snow strength and snow stability were generated from the field data, and analyzed using descriptive statistics, correlation analysis and multiple regression. Results from the first sampling day show stability is only weakly linked to terrain, snowpack and snow-strength variables due to consistently stormy weather conditions leading up to that day. The second field day’s results demonstrate a stronger relationship between stability and the other variables due to more variable weather conditions that ranged from periods of sunshine to storms. On both days stability decreased on high-elevation, northerly-facing slopes. The data-structure complexity provides insights into the difficulties faced by both scientists and conventional avalanche forecasters in predicting snow avalanches.
Coupled marine-ice-sheet/Earth dynamics using a dynamically consistent ice-sheet model and a self-gravitating viscous Earth model
Authors: Meur E. Le , Hindmarsh Richard C. A.
Published: November 2007

We use a self-gravitating viscoelastic model of the Earth and a dynamically consistent marine ice-sheet model to study the relationships between marine ice-sheet dynamics, relative sea level, basal topography and bedrock dynamics. Our main conclusion is that sea-level change and lithospheric coupling are likely to have played limited roles in the postglacial retreat of marine ice sheets. The postglacial rise in sea level would only have caused at the most around 100 km of grounding-line retreat for an ice sheet of similar dimensions to the West Antarctic ice sheet, compared with the several hundred km of retreat which has occurred in the Ross Sea. There is no evidence that reverse slopes lead to instability. Incorporating coupling with lithospheric dynamics does not produce markedly different effects. The implication of these studies is that marine ice-sheet retreat is the result of physical mechanisms other than lithospheric coupling and sea-level rise.
Formation processes of basal ice at Hamna Glacier, Sôya Coast, East Antarctica, inferred by detailed co-isotopic analyses
Authors: Iizuka Yoshinori , Satake Hiroshi , Shiraiwa Takayuki , Naruse Renji
Published: November 2007

Debris-laden basal ice is exposed along an ice cliff near Hamna Glacier, Sôya Coast, East Antarctica. The basal ice is about 6.8 m thick and shows conspicuous stratigraphic features. The upper 5.5 m consists of alternating layers of bubble-free and bubbly ice. δ values of the bubble-free ice layers are enriched by 2.4 ±1.0‰ (standard deviation) for δ18O compared to values of neighboring bubbly ice layers above, and slopes of δ18O vs δD are close to 8. Such layers are suggested to have been formed by refreezing of meltwater in an open system. In contrast, part of the bubbly ice layers shows neutral profiles for stable isotopes, suggesting that these ice masses are undisturbed ice-sheet ice which was not affected by melting and freezing. The massive alternating layers are thus considered to have been formed by folding of refrozen and non-melted layers. The lower 1.3 m consists predominantly of bubble-free massive ice. The profile of co-isotopic values shows a change of about 3.0‰ for δ18O at the interface between bubble-free and bubbly ice. Since the isotopic change occurred over a wider thickness than the upper 5.5 m, the basal ice is suggested to have been formed by refreezing of meltwater on a larger scale than the upper 5.5 m.
Blue-ice areas in Antarctica derived from NOAA AVHRR satellite data
Authors: Winther Jan-Gunnar , Jespersen Martin Nørman , Liston Glen E.
Published: November 2007

We have mapped Antarctic blue-ice areas using the U.S. National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) Antarctica cloud-free image mosaic established by the United States Geological Survey. The mosaic consists of 38 scenes acquired from 1980 to 1994. Our results show that approximately 60 000 km2of blue ice exist for each of the two main types of blue ice: “melt-induced” and “wind-induced”. Normally, the former type is located on slopes in coastal areas where climate conditions (i.e. persistent winds and temperature), together with favourable surface orientation, sustain conditions for surface and near surface melt. The latter blue-ice category occurs near mountains or on outlet glaciers, often at higher elevations, where persistent winds erode snow away year-round, and combined with sublimation creates areas of net ablation. Furthermore, we have identified an additional area of 121 000 km2as having potential for blue ice. However, in these areas features such as mixed pixels, glazed snow surfaces, crevasses and/or shadows make interpretation more uncertain. In conclusion, a conservative estimate of Antarctic blue-ice area coverage by this method is found to be 120 000 km2(∼0.8% of the Antarctic continent), with a potential maximum of 241 000 km2(∼1.6% of the Antarctic continent).
Investigations of an “ice plain” in the mouth of Pine Island Glacier, Antarctica
Authors: Corr H. F. J. , Doake C. S. M. , Jenkins A. , Vaughan D. G.
Published: November 2007

We present newly acquired airborne radar data showing ice thickness and surface elevation for Pine Island Glacier, Antarctica. These data, when combined with earlier measurements, suggest the presence of a lightly grounded area immediately above the grounding line of Pine Island Glacier. We identify this region as an “ice plain”. It lies close to the centre line of the glacier, has an elevation above buoyancy of <50 m and extends inland for >28 km. The upstream edge of the ice plain is defined by a “coupling line”. The configuration of the ice plain implies that nearby thinning of the ice stream would result in substantial grounding-line retreat. We suggest that the grounding-line retreat of Pine Island Glacier, observed between 1992 and 1996, probably commenced sometime after 1981.
Implications of till deformation on glacier dynamics
Authors: Truffer Martin , Echelmeyer Keith A. , Harrison William D.
Published: November 2007

The dynamics of glacier motion are governed to a large extent by the properties of the basal interface. In this paper we address the interaction of a glacier with a layer of till at its bed in an attempt to test whether our physical understanding of till is sufficient to explain general features of the observed flow field and changes in geometry of Black Rapids Glacier, Alaska, U.S.A. We also investigate whether or not a till layer has a clear surface-observable signature in the dynamics of the glacier. Towards this end we use a finite-element ice-flow model with a Coulomb failure criterion within the basal till layer. We find that simple “till physics” can be used to describe decadal, seasonal and short-term (hours to days) velocity variations, and possibly uplift events. Mechanisms for each of these variations involve an increase in the extent of till at failure, a transfer of shear stress across the bed, and a consequent increase in ice deformation. “Effective shape factors” are calculated that permit a simple incorporation of this boundary condition into glacier response models. Our analyses, however, have not resulted in the identification of a clear and unique signature of a till layer in the surface dynamics of a glacier.
The seasonal cycle of sublimation at Halley, Antarctica
Authors: King J. C. , Anderson P. S. , Mann G.W.
Published: November 2007

We have used micrometeorological data collected at Hailey Research Station, Antarctica, to estimate monthly totals of snow sublimation. Direct sublimation from the snow surface is calculated using bulk-transfer formulae, while the sublimation of blowing snow is estimated using a model for suspended-particle number density and individual particle sublimation rates. During the winter months, sublimation losses are negligible, but between November and March sublimation removes around 25% of the snowfall. Surface sublimation and sublimation of blowing snow make roughly equal contributions to this total. Estimates of sublimation using micrometeorological data agree well with estimates made from daily snow-stake measurements.
Multiphase formation of superimposed ice during a mass-balance year at a maritime high-Arctic glacier
Authors: Wadham Jemma L. , Nuttall Anne-Marie
Published: November 2007

Meteorological and snow-temperature data from midtre Lovénbreen, Spitsbergen, Svalbard, indicate two distinct annual phases of rapid snowpack warming and superimposed-ice formation in 1998/99. Short periods of positive air temperatures in early winter, lasting up to 36 hours and often coinciding with rainfall, caused rapid glacier-wide melting. Percolating water froze to form superimposed ice on the lower half of the glacier, and wetted–refrozen snow and ice lenses at higher altitudes. The second period of superimposed-ice formation commenced in May/June 1999 and continued for 5 weeks at low altitudes and throughout the summer at high altitudes. These observations at midtre Lovénbreen are typical of Spitsbergen glaciers and reflect the unique climatology of the region. They contrast with those from glaciers in more continental climatic settings where superimposed-ice formation is confined to a single period during summer. There are significant implications for glacier mass balance, with superimposed ice locally comprising up to 20% of winter balances and accounting for ∼16–25% of the annual accumulation. Since projected climatic warming is greatest during the winter months in Arctic regions, superimposed ice may become an increasingly important component of the winter, and potentially the net, balance of Spitsbergen glaciers.
Observations of a reversal in vertical and horizontal strain-rate regime during a motion event on Unteraargletscher, Bernese Alps, Switzerland
Authors: Gudmundsson G. Hilmar
Published: November 2007

During a motion event on Unteraargletscher, Bernese Alps, Switzerland, in spring 1996, surface velocities were measured up to eight times a day at four different locations along the central flowline using global positioning system equipment. In addition, accumulated vertical strains over the uppermost 50 and 100 m were measured at a location where the total ice thickness is 260 m. The motion event was accompanied by high horizontal and vertical strain rates as compared to annual mean values. A reversal in strain regime was observed, with horizontal strain rates changing to extension while vertical strain rates became compressive. This strain-rate reversal coincided, within the temporal resolution of the data, with a maximum in vertical ice displacement at the surface.Within a day, variations in vertical strain from 0.04 a−1 to −0.06 a−1 were observed over the uppermost 100 m. Vertical stretching is estimated to have contributed to at least 20% of the anomalous vertical ice movement at the surface. There were significant differences between measured longitudinal strain, averaged over a distance corresponding to a few ice thicknesses, and measured vertical strain. In spring 1997 a similar, but more detailed, set of measurements was collected at the same measuring site, and vertical strain rates were found to vary non-uniformly with depth, with the largest values closest to the surface.
Sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions: a model study
Authors: Schmeltz Marjorie , Rignot Eric , Dupont Todd K. , MacAyeal Douglas R.
Published: November 2007

We use a finite-element model of coupled ice-stream/ice-shelf flow to study the sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions. By tuning a softening coefficient of the ice along the glacier margins, and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream, we are able to match model velocity to that observed with interferometric synthetic aperture radar (InSAR). We use the model to investigate the effect of small perturbations on ice flow. We find that a 5.5–13% reduction in our initial ice-shelf area increases the glacier velocity by 3.5–10% at the grounding line. The removal of the entire ice shelf increases the grounding-line velocity by > 70%. The changes in velocity associated with ice-shelf reduction are felt several tens of km inland. Alternatively, a 5% reduction in basal shear stress increases the glacier velocity by 13% at the grounding line. By contrast, softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity. Hence, both the ice-shelf buttressing and the basal shear stress contribute significant resistance to the flow of Pine Island Glacier.