bluuebunny/crossref_metadata_2025_split huggingface dataset:
Ephemeral grounding as a signal of ice-shelf change
Authors: Schmeltz Marjorie , Rignot Eric , MacAyeal Douglas R. 
Published: November 2007

AbstractInterferometric synthetic aperture radar (InSAR) observations of ice-shelf tidal deformation reveal the wide transition between grounded and floating ice as well as local areas where the ice shelf is only grounded at low tide, a condition that we call ephemeral grounding. Ephemeral grounding creates a subtle, local disturbance on the vertical motion field of the ice-shelf surface in response to changes in oceanic tide which is detected with millimetric precision using InSAR. These ice-shelf features are, however, not expected to produce a noticeable disturbance on the ice-shelf velocity field. To illustrate the influence of ephemeral grounding on ice-shelf creep flow, we use a finite-element model in which ephemeral grounding is incorporated through a variable basal friction coefficient. The results show that while ice rises (permanently grounded areas) have a pronounced influence on the ice-shelf velocity field, areas of ephemeral grounding have a vanishingly small influence. What is thus of most interest is the capacity for observations of ephemeral grounding to reveal subtle changes in ice-shelf thickness over time. We discuss an example in the Thwaites Glacier area, West Antarctica, where multi-year data show how ice rises become ephemeral grounding and subsequently disappear. This result is consistent with the grounding-line retreat and ice thinning of Thwaites Glacier.
Anomalous heat and mass budget of Glaciar Zongo, Bolivia, during the 1997/98 El Niño year
Authors: Wagnon P. , Ribstein P. , Francou B. , Sicart J. E. 
Published: November 2007

AbstractDuring El Niño–Southern Oscillation (ENSO) warm events, outer tropics glaciers usually experience a deficit of precipitation, an increase of air temperature and a strongly negative mass balance. At Glaciar Zongo, Bolivia, this was particularly striking during the vigorous 1997/98 El Niño event, one of the strongest of the century, and which resulted in an annual depth of runoff two-thirds higher than normal. We compare the energy balance on the glacier between two contrasting cycles, 1996/97 (La Niña year) and 1997/98 (El Niño year). Due to a 1.3°C increase of annual mean air temperature, the sensible-heat flux slightly increases from 6.1 to 9.8 W m−2 During the El Niño year, sublimation is reduced, leaving more energy for melting (LE = −18.1 W m−2 in 1996/97 and LE = −11.6 W m−2 in 1997/98). The main factor responsible for the dramatic increase in melting is the net all-wave radiation, which is three times higher in 1997/98 than in 1996/97 (48.7 and 15.8 W m−2, respectively). This sharp increase of net all-wave radiation is related to the decrease of albedo due to the precipitation deficit.
Patterns of wind-drifted snow on the Alaskan arctic slope, detected with ERS-1 interferometric SAR
Authors: Li Shusun , Sturm Matthew 
Published: November 2007

AbstractWe tested whether spaceborne interferometric synthetic aperture radar (InSAR) could be used to reveal patterns of redistribution of wind-drifted snow in arctic Alaska. Based on a simple geometric model, we found that lateral variations in new-snow (assuming a density of 0.3 g cm−3) accumulation of > 11 cm, or redistribution of the existing snow into dunes of half this height, could produce decorrelation of C-band interferograms. Comparison of interferograms with field observations for two periods from winter 1993/94, one with wind but little new snow, and the second with wind and new snow, indicates the interferograms delineated areas where the snow depth had changed due to drifting. Striking patterns of windward scouring and leeward deposition were revealed. The interferograms also showed that during one high-wind event, conspicuous interferometric bands a few kilometers wide and 30 km long were formed downwind of a mountain ridge. We speculate that these bands were caused by large-scale alterations in the concentrations of moving snow particles, a finding consistent with ground observations of alternating bands of clear air and blizzard for the same area and similar to phenomena observed with the Advanced Very High Resolution Radiometer and the Geodetic Earth Observing Satellite during blizzards in North Dakota and Iowa, U.S.A.
Spatial and temporal variability of snow accumulation using ground-penetrating radar and ice cores on a Svalbard glacier
Authors: Pälli Anja , Kohler Jack C. , Isaksson Elisabeth , Moore John C. , Pinglot Jean Francis , Pohjola Veijo A. , Samuelsson Håkan 
Published: November 2007

AbstractA 50 MHz ground-penetrating radar was used to detect horizontal layers in the snowpack along a longitudinal profile on Nordenskjöldbreen, a Svalbard glacier. The profile passed two shallow and one deep ice-core sites. Two internal radar reflection layers were dated using parameters measured in the deep core. Radar travel times were converted to water equivalent, yielding snow-accumulation rates along the profile for three time periods: 1986–99, 1963–99 and 1963–86. The results show 40–60% spatial variability in snow accumulation over short distances along the profile. The average annual accumulation rate for 1986–99 was found to be about 12% higher than for the period 1963–86, which indicates increased accumulation in the late 1980s and 1990s.
Dating firn cores by vertical strain measurements
Authors: Hawley Robert L. , Waddington Edwin D. , Morse David L. , Dunbar Nelia W. , Zielinski Gregory A. 
Published: November 2007

AbstractWe have developed a system for measuring a vertical strain-rate profile in the firn on polar ice sheets using a readily available video camera to detect metal bands inserted in an air-filled hole. We used this system in 1995 and 1996 at Taylor Dome, Antarctica. We use density measurements combined with our strain rates to infer vertical velocities. From our velocities we calculate a steady-state depth–age scale for the firn at Taylor Dome. The age of a visible ash layer from 79.1 m is 675 ± 25 years; this ash can be correlated with ash found at 97.2 m in a recent ice core at Siple Dome, West Antarctica.
InSAR observations of the 1993–95 Bering Glacier (Alaska, U.S.A.) surge and a surge hypothesis
Authors: Fatland Dennis R. , Lingle Craig S. 
Published: November 2007

AbstractTime-varying accelerations were observed on Bagley Icefield during the 1993–95 surge of Bering Glacier, Alaska, U.S.A., using repeat-pass synthetic aperture radar interferometry. Observations were from datasets acquired during winter 1991/92 (pre-surge), winter 1993/94 (during the surge) and winter 1995/96 (post-surge). The surge is shown to have extended 110 km up the icefield from Bering Glacier to within 15 km or less of the flow divide. Acceleration and step-like velocity profiles are strongly associated with an along-glacier series of central phase bull’s-eyes with diameters of 0.5–4 km. These bull’s-eyes are interpreted to represent glacier surface rise/fall events of ∼3–30 cm during 1–3 day observation intervals and indicate possible migrating pockets of subglacial water. We present a surge hypothesis that relates late-summer climate to englacial water storage and thence to the subglacial water dynamics — pressurization, hydraulic jacking, depressurization and migration — suggested by our observations.
Tank study of physico-chemical controls on gas content and composition during growth of young sea ice
Authors: Tison Jean-Louis , Haas Christian , Gowing Marcia M. , Sleewaegen Suzanne , Bernard Alain 
Published: November 2007

AbstractDuring an ice-tank experiment, samples were taken to study the processes of acquisition and alteration of the gas properties in young first-year sea ice during a complete growth–warming–cooling cycle. The goal was to obtain reference levels for total gas content and concentrations of atmospheric gases (O2, N2, CO2) in the absence of significant biological activity. The range of total gas-content values obtained (3.5–18 mL STP kg−1) was similar to previous measurements or estimates. However, major differences occurred between current and quiet basins, showing the role of the water dynamics at the ice–water interface in controlling bubble nucleation processes. Extremely high CO2concentrations were observed in all the experiments (up to 57% in volume parts). It is argued that these could have resulted from two unexpected biases in the experimental settings. Concentrations in bubbles nucleated at the interface are controlled by diffusion both from the ice–water interface towards the well-mixed reservoir and between the interface water and the bubble itself. This double kinetic effect results in a transition of the gas composition in the bubbles from values close to solubility in sea water toward values close to atmospheric, as the ice cover builds up.
Unsteady flow inferred for Thwaites Glacier, and comparison with Pine Island Glacier, West Antarctica
Authors: Bamber Jonathan , Rignot Eric 
Published: November 2007

AbstractWe present a comparison of surface velocities in 1996, derived from interferometric synthetic aperture radar, with an estimate of the long-term, depth-averaged velocity based on the assumption of steady-state flow for both Pine Island Glacier and its neighbour, Thwaites Glacier, West Antarctica. The results show that the former was close to balance conditions in 1996 (within 9%). The ice flux and velocity of the latter is significantly different in magnitude and distribution from that required to maintain the basin in a state of mass balance over the long term. The balance flux was found to be 32 ± 19% less than the measured outgoing flux. We conclude that the mass imbalance and dramatic difference in the pattern of flow is most likely due to a substantial change in the flow regime of Thwaites Glacier in the recent past.
Measurement of vertical strain and velocity at Siple Dome, Antarctica, with optical sensors
Authors: Zumberge Mark A. , Elsberg Daniel H. , Harrison William D. , Husmann Eric , Morack John L. , Pettit Erin C. , Waddington Edwin D. 
Published: November 2007

AbstractAs part of a larger program to measure and model vertical strain around Siple Dome on the West Antarctic ice sheet, we developed a new sensor to accurately and stably record displacements. The sensors consist of optical fibers, encased in thin-wall stainless-steel tubes, frozen into holes drilled with hot water, and stretched from the surface to various depths (up to 985 m) in the ice sheet. An optical system, connected annually to the fibers, reads out their absolute lengths with a precision of about 2 mm. Two sets of five sensors were installed in the 1997/98 field season: one set is near the Siple Dome core hole (an ice divide), and a second set is on the flank 7 km to the north (the ice thickness at both sites is approximately 1000 m). The optical-fiber length observations taken in four field seasons spanning a 3 year interval reveal vertical strain rates ranging from −229 ± 4 ppm a−1 to −7 ± 9 ppm a−1. In addition to confirming a non-linear constitutive relationship for deep ice, our analysis of the strain rates indicates the ice sheet is thinning at the flank and is in steady state at the divide.
On the numerical resolution in a thermodynamic sea-ice model
Authors: Cheng Bin 
Published: November 2007

AbstractThe numerical integration of the heat-conduction equation is one of the main components in a thermodynamic sea-ice model. The spatial resolution in the ice normally varies from a minimum of three layers up to a few tens of layers. The temporal resolution varies from a few minutes up to hours. In this paper the impact of numerical resolution on the prediction of a one-dimensional thermodynamic ice model is studied. Analytical solutions for idealized cases were derived and compared with the numerical results. For the full ice model, groups of simulations were made, applying average climatic weather-forcing data corresponding to the ice-freezing, ice-thermal equilibrium and ice warm-up seasons. Special attention was paid to the effect of model spatial resolution. Early in the freezing season, the influence of resolution on model predictions is not significant. When the shortwave radiation becomes large, its absorption within the ice or snow cover was found to modulate the effect of numerical resolution on predictions of ice temperature and surface heat fluxes (e.g. the model run with a coarse spatial resolution yielded large daily variations in surface temperature). Resolution also affects the in-ice temperature profile. For process studies, a two-layer scheme for the solar radiation penetrating into the ice is suitable for a fine-spatial-resolution ice model.
Evidence for basal cavity opening from analysis of surface uplift during a high-velocity event: Haut Glacier d’Arolla, Switzerland
Authors: Mair Douglas W. F. , Sharp Martin J. , Willis Ian C. 
Published: November 2007

AbstractHigh rates of surface uplift and horizontal velocities were measured during a hydrologically induced spring speed-up event. Spatial patterns of surface uplift are analyzed to estimate components of vertical motion due to flow along an inclined bed and vertical strain. Areas are identified where surface uplift was most likely due in part to the opening or enlargement of subglacial cavities by bed separation. Results suggest a widespread enlargement of subglacial cavities during the event, and survival of residual cavities after the event. The spatial pattern of cavity enlargement closely matches previously identified axes of preferential subglacial drainage. It is suggested that localized cavity opening along axes of preferential drainage may constitute the initial stage in the seasonal development of channelized subglacial drainage. It is concluded that spatial and temporal variations in glacier motion may play an active role in determining the structure and rate of development of subglacial drainage during the summer melt season.
Extension of Glacier de Saint-Sorlin, French Alps, and equilibrium-line altitude during the Little Ice Age
Authors: Lliboutry Louis 
Published: November 2007

AbstractGlacier de Saint-Sorlin, French Alps, left terminal moraines at 1.3, 2.9 and 3.7 km ahead of the present terminus. According to proxy data and to historical maps, these were formed in the 19th, 18th and 17th centuries, respectively. A plateau at 2700–2625 m was then surrounded by ice but never became an accumulation area. This fact shows that the equilibrium-line altitude (ELA) on the glacier never dropped below 2300 m. The following simple models apply sufficiently to yield reliable estimations of past ELA: (1) a uniform and constant vertical gradient of the mass balance, down to the terminus; and (2) a plane bed, with a slope of 8.5° and a uniform width. Then in a steady situation the accumulation–area ratio is 1/2. Compared to the mean for 1956–72, at the onset of the Little Ice Age the balances were higher by 3.75 m ice a−1, and the ELA was 400 m lower. Correlations between 1956–72 balances and meteorological data suggest that during the melting season the 0°C isotherm was about 800 m lower, while the winter precipitation at low altitudes did not change. These correlations may have been different in the past, but an equal lowering of the ELA and of the 0°C isotherm, as assumed by several authors, seems excluded.
Isochrones and isotherms beneath migrating ice divides
Authors: Nereson Nadine A. , Waddington Edwin D. 
Published: November 2007

AbstractWe use simple numerical and analytical models of ice flow and heat flow to characterize the shape of isochrones and isotherms beneath moving ice divides. Both nonlinear ice flow and reduced accumulation (wind scouring) at a divide can cause reduced downward flow in a region about one ice thickness wide under a divide. Greater downward velocities on the flanks cause isochrones and isotherms to become arched at depth. The magnitudes and shapes of these arches depend on the history of divide position. Arch amplitudes decrease by approximately e−1 for each increase in migration rate of 3–5 times the accumulation rate, the arches become asymmetric, with steeper leading edges and more gentle trailing edges, and the arch apex lags behind the divide. Isochrone and isotherm shapes can be used to infer past divide motions. In advection-dominated ice sheets, isochrone shapes record a longer history of divide position than do isotherm shapes. The opposite is true for diffusion-dominated ice sheets, in which a spatial array of ice-temperature measurements might extend the recorded history of divide position.
Temporal variability of accumulation at Neumayer station, Antarctica, from stake array measurements and a regional atmospheric model
Authors: Schlosser Elisabeth , Lipzig Nicole van , Oerter Hans 
Published: November 2007

AbstractIn this study we investigate the ability of the Regional Atmospheric Climate MOdel (RACMO) to represent temporal variations of Antarctic accumulation using weekly stake array measurements from Neumayer Station. The model uses European Centre for Medium-Range Weather Forecasts re-analysis data to force the atmospheric variables at the lateral boundaries of the model domain. Accumulation is defined as precipitation minus sublimation. Generally the model represents reasonably well the synoptic situations that lead to precipitation. The amounts of accumulation are, however, usually lower in the model than in the measurements. It cannot be determined whether the model underestimates precipitation or whether this effect is due to the redistribution of snow by the wind, which is not taken into account in the model but affects the accumulation at the measuring site. Significant differences between model and measurements also occur in cases of net ablation due to wind erosion or when accumulation at the stakes was due to snowdrift from the southwest without precipitation being observed.
Density measurements in ice boreholes using neutron scattering
Authors: Morris Elizabeth M. , David Cooper J. 
Published: November 2007

AbstractThis paper describes the use of a neutron probe to measure detailed stratigraphy in ice and snow. The Wallingford neutron probe, developed for measurement of soil moisture, consists of an annular radioactive source of fast neutrons around the centre of a cylindrical detector for slow (thermal) neutrons. In snow and ice, the fast neutrons lose energy by scattering from hydrogen atoms, and the number of slow neutrons arriving at the detector (the count rate) is related to the density of the medium. Calibration equations for count rate as a function of snow density and borehole diameter have been derived. Snow-density profiles from boreholes obtained using the probe show that, despite the smoothing produced by the neutron-scattering process, annual variations in density can be resolved. The potential contribution of the neutron probe to improvements in mass-balance monitoring is discussed.
Short-term variations in calving of a tidewater glacier: LeConte Glacier, Alaska, U.S.A.
Authors: O’Neel Shad , Echelmeyer Keith A. , Motyka Roman J. 
Published: November 2007

AbstractKnowledge of iceberg calving is important for understanding instabilities of tidewater glaciers and ice sheets. Since 1995 the terminus of LeConte Glacier, Alaska, U.S.A., has retreated about 2 km and the glacier has thinned approximately 120 m at its 1999 terminus position. Our focus is short-term (hours to weeks) variability of the frequency and magnitude of calving events and calving flux. Both photogrammetric and visual observations are employed in a temporal analysis over a several-week period. We combined these data with measurements of ice speed, tide level, surface water input and water-storage estimates in an attempt to better understand the calving process. Contrary to results obtained over longer time-scales on other glaciers, our results show no correlation between ice speed and the frequency of calving. However, calving events do not appear to occur randomly; often they are a response to measurable changes in other parameters within the terminus region. Caclving can often be attributed to buoyancy perturbations and possibly flexure of the nearly floating terminus. Given the multiple possibilities for buoyancy perturbations, we have found no simple relationship between any specific forcing parameter and calving at short time-scales.
Hydrological controls on patterns of surface, internal and basal motion during three “spring events”: Haut Glacier d’Arolla, Switzerland
Authors: Mair Douglas , Willis Ian , Fischer Urs H. , Hubbard Bryn , Nienow Peter , Hubbard Alun 
Published: November 2007

AbstractThree early-melt-season high-velocity events (or “spring events”) occurred on Haut Glacier d’Arolla, Switzerland, during the melt seasons of 1998 and 1999. The events involve enhanced glacier velocity during periods of rapidly increasing bulk discharge in the proglacial stream and high subglacial water pressures. However, differences in spatial patterns of surface velocity, internal ice deformation rates, the spatial extent of high subglacial water pressures and in rates of subglacial sediment deformation suggest different hydrological and mechanical controls. The data from two of the events suggest widespread ice–bed decoupling, particularly along a subglacial drainage axis creating the highest rates of basal motion and “plug flow” in the overlying ice. The other event showed evidence of less extensive ice–bed decoupling and sliding along the drainage axis with more mechanical support for ice overburden transferred to areas adjacent to decoupled areas. We suggest that: (1) plug flow may be a common feature on glaciers experiencing locally induced reductions in basal drag; (2) under certain circumstances, enhanced surface motion may be due in part to non-locally forced enhanced bed deformation; and (3) subglacial sediment deformation is confined to a depth of the order of centimetres to decimetres.
Bed topography and lubrication inferred from surface measurements on fast-flowing ice streams
Authors: Thorsteinsson Throstur , Raymond Charles F. , Hilmar Gudmundsson G. , Bindschadler Robert A. , Vornberger Paul , Joughin Ian 
Published: November 2007

AbstractObservations of surface elevation (s) and horizontal velocity components (u and v) are inverted to infer the topography (b) and lubrication (c) at the bed of an ice stream, based on a linearized perturbation theory of the transmission of flow disturbances through the ice thickness. Synthetic data are used to illustrate non-uniqueness in the inversion, but also demonstrate that effects of b and c can be separated when s, u and v are specified, even with added noise to simulate measurement errors. We have analyzed prominent short-horizontal-scale (∼2 km) features in topography and velocity pattern in a local 64 km by 32 km area of the surface of Ice Stream E,West Antarctica. Our preferred interpretation of bed conditions beneath the most prominent features on the surface identifies a deep trough in the basal topography with low lubrication in the base of the trough.