bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractMass-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

AbstractThe 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

AbstractOne 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

AbstractA 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

AbstractNew 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

AbstractAnalysis 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

AbstractSurface 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

AbstractNumerous 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

AbstractWe 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

AbstractPeroxide 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

AbstractMicrofabrics 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

AbstractSnowmelt 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

AbstractThe 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

AbstractLack 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

AbstractWhen 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

AbstractWe 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

AbstractOxygen 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.