bluuebunny/crossref_metadata_2025_split huggingface dataset:
Basal sliding of Ice Stream B, West Antarctica
Authors: Hermann Engelhardt , Barclay Kamb 
Published: September 2017

AbstractA “tethered stake” apparatus is used to measure basal sliding in a borehole on Ice Stream B, West Antaretica, about 300 km upstream (east) from its grounding line near the head of the Ross Ice Shelf. A metal stake, emplaced at the top of a laver of unfrozen till underlying the ice, is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding. The measured sliding motion includes any actual slip across the ice–till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface. This 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment, based on features of the record of apparent sliding. Alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm. In any case it is small compared to the seismically inferred till thickness of 9 m. Measured apparent sliding averages 69% of the total motion of 1.2 m d−1 over 26 days of observation if a 3.5 day period of slow apparent sliding (8% of the total motion) is included in the average. The occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c.80% and c. 8% of the total motion, on a time-scale of a few days. However, it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole. If the slow period is therefore omitted, the indicated average basal sliding rate is 83% of the total motion. In either case, basal sliding predominates as the cause of the rapid ice-stream motion. In the last 2 days of observation the average apparent sliding rate reached 1.17 m d−1, essentially 100% of the motion of the ice stream. If till deformation contributes significantly to the ice-stream motion, the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer. These observations, if applicable to the West Antaretic ice sheet in general, pose complications in modeling the rapid ice-streaming motion.
The mass balance of McCall Glacier, Brooks Range, Alaska, U.S.A.; its regional relevance and implications for climate change in the Arctic
Authors: Rabus B.T. , Echelmeyer K. A. 
Published: September 2017

AbstractMcCall Glacier has the only long-term mass-balance record in Arctic-Alaska. Average annual balances over the periods 1958–72 and 1972–93 were –15 and –33cm, respectively; recent annual balances (1993–96) are about –60 cm, and the mass-balance gradient has increased. For an Arctic glacier, with its low mass-exchange rate, this marks a significant negative trend.Recently acquired elevation profiles of McCall Glacier and ten other glaciers within a 30 km radius were compared with topographic maps made in 1956 or 1973. Most of these glaciers had average annual mass balances between –25 and –33 cm, while McCall Glacier averaged –28 cm for 1956–93, indicating that it is representative of the region. In contrast, changes in terminus position for the different glaciers vary markedly. Thus, mass-balance trends in this region cannot be estimated from fractional length changes at time-scales of a few decades.We developed a simple degree-day/accumulation mass-balance model for McCall Glacier. The model was tested using precipitation and radiosonde temperatures from weather stations at Inuvik, Canada, and Barrow, Kaktovik and Fairbanks, Alaska, and was calibrated with the measured balances. The Inuvik data reproduce all measured mass balances of McCall Glacier well and also reproduce the long-term trend towards more negative balances. Data from the other stations do not produce satisfactory model results. We speculate that the Arctic Front, oriented east–west in this region, causes the differences in model results.
The sliding velocity over a sinusoidal bed at high water pressure
Authors: Truffer Martin , Iken Almut 
Published: September 2017

AbstractUnder idealized conditions, when pressurized water has access to all low-pressure areas at the glacier bed, a sliding instability exists at a critical pressure, pc, well below the overburden pressure, p0. The critical pressure is given by , where l is the wave length and a is the amplitude of a sinusoidal bedrock, and T is the basal shear stress. When the subglacial water pressure, pw, approaches this critical value, the area of ice-bed contact, △l, becomes very small and the pressure on the contact area becomes very large. This pressure is calculated from a force balance and the corresponding rate of compression is obtained using Glen’s flow law for ice. On the assumption that compression in the vicinity of the contact area occurs over a distance of the order of the size of this area, Δl, a deformational velocity is estimated. The resultant sliding velocity shows the expected instability at the critical water pressure. The dependency on other parameters, such as wavelength l and roughness a/l, was found to be the same as for sliding without bed separation.
Sensitivity of Rhonegletscher, Switzerland, to climate change: experiments with a one-dimensional flowline model
Authors: Wallinga Jakob , Van De Wal Roderik S.W. 
Published: September 2017

AbstractA one-dimensional time-dependent flowline model of Rhonegletscher, Switzerland, has been used to test the glacier’s response to climatic warming. Mass-balance variations over the last 100 years are obtained from observations of the equilibrium-line altitude (ELA) and a reconstruction of the ELA based on a statistical correlation between temperature and ELA. For the period prior to AD 1882, for which no reliable climate data exist, we chose equilibrium-line altitudes that enabled us to simulate accurately the glacier length from AD 1602.The model simulates the historical glacier length almost perfectly and glacier geometry very well. It underestimates glacier-surface velocities by 1-18%. Following these reference experiments, we investigated the response of Rhonegletscher to a number of climate-change scenarios for the period AD 1990-2100. For a constant climate equal to the 1961-90 mean, the model predicts a 6% decrease in glacier volume by AD 2100. Rhonegletscher will retreat by almost 1 km over the next 100 years at this scenario. At a warming rate of 0.04 K a-1, only 4% of the glacier volume will be left by AD 2100.
Evolution of a Surge-Type Glacier in its Quiescent Phase: Kongsvegen, Spitsbergen, 1964–95
Authors: Melvold Kjetil , Ove Hagen Jon 
Published: September 2017

AbstractKongsvegen is a 102 km2 sub-polar (polythermal) surge-type glacier in northwest Spitsbergen, Svalbard. It surged just before 1948 and is currently in its quiescent phase. Measurements of surface geometry since 1966 show a retreat of the front and strong thinning of up to 75 m in the ablation area, and a build-up of up to 32 m in the accumulation area. Present-day annual velocities along the glacier are low, from 1.4 up to 3.6 m a-1. The measured mean net balance for the period 1987-94 and the balance reconstructed back to 1967 show a weak positive balance of about 0.1 m w.e. The measured actual ice flux is low and the mass transfer down-glacier at the ELA is only about 3-20% of that required for steady state. Thus, the glacier is building up towards a new surge. The total thickening rate on Kongsvegen is somewhat higher than in other cases from Svalbard, but it is small compared with other well-studied surge-type glaciers in Alaska and the Pamirs. This relatively low rate of change is a function of the low accumulation rate and the relatively cold climate compared to other areas and is common for surge-type glaciers in Svalbard.
Ice-shelf dynamics near the front of the Filchner-Ronne Ice Shelf, Antaretica, revealed by SAR interferometry: model/interferogram comparison
Authors: MacAyeal Douglas R , Rignot Eric , Hulbe Christina L 
Published: September 2017

AbstractWe compare European remote-sensing satellite (ERS) synthetic aperture radar interferograms with artificial interferograms constructed using output of a finite-element ice-shelf flow model to study the dynamics of Filchner-Ronne Ice Shelf (FRIS), Antaretica, near Hemmen Ice Rise (HIR) where the iceberg-calving front intersects Berkner Island. We find that the model must account for rifts, mechanically competent sea ice which fills rifts, and ice softening in coastal boundary layers in order to agree with the ice-deformation pattern implied by observed interferograms. Analysis of the stress field in the model experiment that best matches the observed interferograms suggests that: (1) HIR introduces weakness into the ice shelf through the generation of large-scale rifts, and (2) the melange of sea ice and ice-shelf fragments that fills the rifts stabilizes the shelf front by providing mechanical coupling between the fractured shelf front and the adjacent coast. The rift-filling melange could melt more easily than the surrounding ice shelf and thus could represent a vulnerability of the FRIS to climate warming.
The thermal conductivity of seasonal snow
Authors: Sturm Matthew , Holmgren Jon , König Max , Morris Kim 
Published: September 2017

AbstractTwenty-seven studies on the thermal conductivity of snow (Keff) have been published since 1886. Combined, they comprise 354 values ofKeff, and have been used to derive over 13 regression equation and predictingKeffvs density. Due to large (and largely undocumented) differences in measurement methods and accuracy, sample temperature and snow type, it is not possible to know what part of the variability in this data set is the result of snow microstructure. We present a new data set containing 488 measurements for which the temperature, type and measurement accuracy are known. A quadratic equation,whereρis in g cm−3, andKeffis in W m−1K−1, can be fit to the new data (R2= 0.79). A logarithmic expression,can also be used. The first regression is better when estimating values beyond the limits of the data; the second when estimating values for low-density snow. Within the data set, snow types resulting from kinetic growth show density-independent behavior. Rounded-grain and wind-blown snow show strong density dependence. The new data set has a higher mean value of density but a lower mean value of thermal conductivity than the old set. This shift is attributed to differences in snow types and sample temperatures in the sets. Using both data sets, we show that there are well-defined limits to the geometric configurations that natural seasonal snow can take.
Basal-flow characteristics of a linear medium sliding frictionless over small bedrock undulations
Authors: Gudmundsson G. Hilmar 
Published: September 2017

AbstractThe basal deformation of a gravity-driven linear creeping flow sliding frictionless over slowly varying bed undulations in two dimensions is analysed analytically, using results from second-order perturbation theory. One of the key results is that, close to sinusoidal bedrock undulations, up to two different spatial regions of local extrusion flow may arise. The offset and onset of extrusion flow is controlled primarily by the amplitude-to-wavelength ratio. Above the crest of a sinusoidal bed line, a local maximum of the surface-parallel velocity develops for ε : = ak < 0.138, where a is the amplitude and k is the wave number. As ε increases from zerо to this critical value, the vertical position of the velocity maximum moves from kz = 1 to kz ≈ 1.98, where z is the vertical distance above the mean bed line. Within and above the trough of a sinusoid, a region of local minimum of the surface-parallel velocity component develops, which shifts from kz = 1 towards the bed line as ε increases front zero to 1/2. Below this velocity minimum, and for some distance above the velocity maximum, the surface-parallel velocity increases with depth. This type of extrusion flow will cause a reversal of borehole-inclination profiles close to the bedrock.
Some comments on climatic reconstructions from ice cores drilled in areas of high melt
Authors: Koerner Roy M. 
Published: September 2017

AbstractPoor consideration has been given in many Arctic circum-polar ice-core studies to the effect of summer snow melt on chemistry, stable-isotope concentrations and time-scales. Many of these corps are drilled close to the firn line where melt is intense. Some come from below the firn line where accumulation is solely in the form of super-imposed ice. In all cases, seasonal signals are reduced or removed and, in some, time gaps develop during periods of excessive melting which situate the drill site in the ablation zone. Consequently, cross correlations of assumed synchronous events among the cores are invalid, so that time-scales along the same cores differ between authors by factors of over 2. Many so-called climatic signals are imaginary rather than real. By reference to published analyses of cores from the superimposed ice zone on Devon Ice Cap (Koerner, 1970) and Meighen Ice Cap (Koerner and Paterson, 1974), it is shown how melt affects all the normally well-established ice-core proxies and leads to their misinterpretation. Despite these limitations, the cores can give valuable low-resolution records for all or part of the Holocene. They show that the thermal maximum in the circum-polar Arctic occurred in the early Holocene. This maximum, effected negative balances on all the ice caps and removed the smaller ones. Cooler conditions in the second half of the Holocene have caused the regrowth of these same ice caps.
Glaciological and oceanographic evidence of high melt rates beneath Pine Island Glacier, West Antarctica
Authors: Jenkins Adrian , Vaughan David G. , Jacobs Stanley S. , Hellmer Hartmut H. , Keys John R. 
Published: September 2017

AbstractSatellite imagery indicates that the floating terminus of Pine Island Glacier has changed little in extent over the past two decades. Data on the velocity and thickness of the glacier reveal that calving of 28 ± 4 Gta−1 accounts for only half of the ice input near the grounding line. The apparently steady configuration implies that the remainder of the input is lost by basal melting at a mean rate of 12 ± 3 ma−1. Ocean circulation in Pine Island Bay transports +1°C waters beneath the glacier and temperatures recorded in melt-laden outflows show that heat loss from the ocean is consistent with the requirements of the calculated melt rate. The combination of iceberg calving and basal melting lies at the lower end of estimates for the total accumulation over the catchment basin, drawing into question previous estimates of a significantly positive mass budget for this part of the ice sheet.
On the surface energy budget of sea ice
Authors: Wendler Gerd , Adolphs Ute , Hauser Adrian , Moore Blake 
Published: September 2017

AbstractThe surface energy budget was investigated during a cruise through the pack ice in the Southern Ocean. The time of observation was close to mid-summer. Some of the more important findings were:
The mean albedo varied from 11 % for open water to 59% for 10/10 ice cover. Hourly values span the range from 6% (open water) to 76% (total ice cover).The net heat flux into the ocean (B) was on average 109 W m−2, If this energy were used solely for melting of sea ice, 30 mm could be melted each day.For low surface albedos (ice concentration below 7/10), the net radiation increased with decreasing cloudiness. However, the opposite was the case for a high surface albedo.
The last point shows the importance of clouds on the surface energy budget. Not only should their presence or absence be known but also the reflectivity of the underlying surface, as it might change the net radiation in opposite ways.
Simulation of the historical variations in length of Unterer Grindelwaldgletscher, Switzerland
Authors: Schmeits M. J. , Oerlemans J. 
Published: September 2017

AbstractThe historical length variations in Unterer Grindelwaldgletscher have been simulated by coupling a numerical mass-balance model to a dynamic ice-flow model. As forcing functions, we used (partly reconstructed) local climatic records, which were transformed by the mass-balance model into a mass-balance history. The ice-flow model then computes the length variations that have occurred over the course of time.In a model run from AD 1530 to the present, with both seasonal temperature and precipitation variations as forcing functions, the observed maximum length of the glacier around AD 1860 and the subsequent retreat are simulated. The observed AD 1600 maximum, however, does not show up in the simulation. This is probably due to an incorrect reconstruction of the mass balance for this period, as detailed climatic data are available only since 1865. The root-mean-square difference between the simulated and the observed front positions is 0.28 km. The simulated glacier geometry for 1987 fits the observed geometry for that year reasonably well.Because of the success of the historical simulation, an attempt is made to predict future glacier retreat on the basis of two different greenhouse-gas scenarios. For a Business-as-Usual scenario, only 29% of the 1990 volume would remain in AD 2100.
Modelling temperature variations in polar snow using DAISY
Authors: Morris Ε. M. , Bader H. -P. , Weilenmann P. 
Published: September 2017

AbstractA physics-based snow model has been calibrated using data collected at Halley Bay, Antarctica, during the International Geophysical Year. Variations in snow temperature and density are well-simulated using values for the model parameters within the range reported from other polar field experiments. The effect of uncertainty in the parameter values on the accuracy of the predictions is no greater than the effect of instrumental error in the input data. Thus, this model can be used with parameters determined a priori rather than by optimization. The model has been validated using an independent data set from Halley Bay and then used to estimate 10 m temperatures on the Antarctic Peninsula plateau over the last half-century.
Crevasse Deformation and Examples from Ice Stream B, Antarctica
Authors: Vornberger P.L. , Whillans I.M. 
Published: September 2017

AbstractCrevasses, once formed, are subject to rotation and bending according to the velocity field through which they travel. Because of this, crevasse shapes can be used to infer something about the velocity field of a glacier. This is done using a model in which each crevasse opens perpendicularly to the principal extensional strain-rate, when that strain-rate exceeds some specified critical value, and is then deformed according to the same velocity gradients that formed the crevasse. This model describes how crevasses are formed, translated, rotated, bent, and lengthened.Velocity fields are sought for which calculations produce crevasses approximating those found in three example areas on Ice Stream B, Antarctica. The first example is the hook-shaped crevasses that occur just outside the chaotic shear zone at the ice-stream margin. They are used to infer a rate of lateral shearing, and side drag. The second example, a pattern of splaying crevasses, is satisfactorily simulated by a model with side-drag stress varying linearly across the ice stream. This confirms that this region is restrained almost entirely by side drag. The third example is transverse crevasses and their change in orientation, but many different velocity fields can produce the observed pattern. Of these three examples, the shapes of hook-shaped marginal crevasses and splaying crevasses can provide useful information whereas transverse crevasses are less helpful.
Thermal Response of a Small Ice Cap to Climatic Forcing
Authors: Hanson Brian 
Published: September 2017

AbstractTwo-dimensional finite-element calculations of velocity and temperature fields have been applied to the energy balance of a cross-section of Barnes Ice Cap, Baffin Island, Canada. The flow plane currently is cooling near the ice divide and warming near the margin. Long-term simulations show a net warming trend followed by a cooling trend with a steady-state average temperature similar to the present. Sensitivity studies on an idealized version of the flow plane show that the overall temperature responds less than surface-temperature forcing, because a negative feedback in temperature advection is substantially larger than a positive feed-back in strain heating. The response times of the flow plane by itself are somewhat faster but of the same magnitude as response times that would be estimated from one-dimensional modeling. When bedrock-temperature calculations are included, response times increase an order of magnitude, but these do not substantially affect the short-term response.
Subglacial Water Pressures and the Shape of Subglacial Conduits
Authors: Hooke Roger LeB. , Laumann Tron , Kohler Jack 
Published: September 2017

AbstractMeasured subglacial water pressures are frequently higher than theoretical values calculated by assuming that subglacial conduits are straight and either circular or semi-circular in shape. While this may be attributed to errors in the estimates of conduit roughness or ice viscosity, we suggest here an alternative explanation: namely, that the conduits are actually broad and low rather than semi-circular. Good agreement between measured and calculated pressures can be obtained by assuming that the cross-sectional shape of conduits resembles the space between the arc of a circle and its chord. The angle subtended by the arc, θ, is treated as an adjustable parameter. In four cases studied, θ ranged from 2° to 36°.
Instruments and Methods: A Digital Low-Frequency, Surface-Profiling Ice-Radar System
Authors: Wright David L. , Hodge Steven M. , Bradley Jerry A. , Grover Thomas P. , Jacobel Robert W. 
Published: September 2017

AbstractA new short-pulse digital profiling radar system that operates at lower frequencies than most ice radars used in polar regions to date has been designed and built by the U.S. Geological Survey. The transmitter is an avalanche transistor pulser which drives a resistively loaded dipole transmitting antenna. A similar, but separate antenna is connected to the receiver. The receiver has adjustable sensitivity time control (STC) of as much as 60 dB to compensate for attenuation and geometric spreading factors. A fiber-optic cable is used to transmit both control signals and data. The data-acquisition and display system incorporates very high-speed digitizing and signal averaging, real-time profile display, and data storage on standard computer nine-track magnetic tape.The system was successfully used on Ice Stream B in West Antarctica at centre frequencies of 1, 2, 4, 8, and 12.5 MHz. Bottom-return signal-to-noise ratios of more than 40 dB were obtained at 2 MHz through 800 m of ice. Convoluted internal surfaces not related to present bottom topography were resolved within the ice streams and anomalous strong reflections or “bright spots” were identified near the base of the ice. At present, there is no satisfactory glaciological explanation for either of these observations.