Hydrology is important for glacier dynamics, but it is difficult to monitor the subsurface drainage systems of glaciers by direct observations. Since meltwater drainage generates seismic signals, ...passive seismic analysis has the potential to be used to monitor these processes. To study continuous seismic radiation from the drainage, we analyze geophone data from six stations deployed at the Kaskawulsh Glacier in Yukon, Canada, during the summer of 2014 using ambient noise cross‐correlation techniques. We locate the noise sources by backprojecting the amplitude of the cross correlation to the glacier surface. Most of the ambient noise sequences are found in two clusters, with each cluster located in the vicinity of a moulin identified at the surface. Stronger seismic radiation is observed during the day, consistent with expected variability in melt rates. We demonstrate that the sparse seismic network array with 2 km station separation has the ability to detect moulins within the array with a precision of 50 m. We confirm that seismic activity is correlated with air temperature, and thus, melt, on a diurnal timescale, and precipitation correlates with the activity at longer timescales. Our results highlight the potential of passive seismic observations for monitoring water flow into subglacial channels through moulins with an affordable number of seismic stations, but quantification of water flow rates still remains a challenge. The cross‐correlation backprojection technique described here can also potentially be applied to any localized source of ambient noise such as ocean noise, tectonic tremor, and volcanic tremor.
Key Points
We locate ambient noise sources in glaciers by backprojecting cross‐correlation time series
Two major moulins were located, and both have diurnal activity related to melting in the daytime
The correlation between precipitation and seismic activity is significant at timescales longer than a day
With an extensive ice cover and rich display of glacier behaviour, the St. Elias Mountains continue to be an enviable natural laboratory for glaciological research. Recent work has been motivated in ...part by the magnitude and pace of observed glacier change in this area, which is so ice-rich that ice loss has a measurable impact on global sea level. Both detection and attribution of these changes, as well as investigations into fundamental glacier processes, have been central themes in projects initiated within the last decade and based at the Kluane Lake Research Station. The scientific objectives of these projects are (1) to quantify recent area and volume changes of Kaskawulsh Glacier and place them in historical perspective, (2) to investigate the regional variability of glacier response to climate and the modulating influence of ice dynamics, and (3) to characterize the hydromechanical controls on glacier sliding. A wide range of methods is being used, from ground-based manual measurements to space-based remote sensing. The observations to date show glaciers out of equilibrium, with significant ongoing changes to glacier area, volume, and dynamics. Computer models are being used to generalize these results, and to identify the processes most critical to our understanding of the coupled glacier-climate system. Grâce à leur importante couverture de glace et au riche étalage de comportement des glaciers, les monts St. Elias continuent de servir de laboratoire naturel enviable pour la recherche glaciologique. Des études récentes ont été motivées, en partie, par la magnitude et la vitesse des changements observés dans les glaciers de l'endroit, qui sont riches en glace au point que la perte de glace a une incidence mesurable sur le niveau général de la mer. La détection et l'attribution de ces changements de même que les recherches à l'égard des processus des glaciers ont servi de thème central à des projets qui ont été mis en œuvre au cours de la dernière décennie à la station de recherche du lac Kluane. Les objectifs scientifiques de ces projets consistent (1) à quantifier les changements récents relativement à l'aire et au volume du glacier Kaskawulsh, puis à les mettre dans une perspective historique, (2) à faire enquête sur la variabilité générale de la réaction du glacier vis-à-vis du climat et de l'influence modulatrice de la dynamique de la glace, et (3) à caractériser le contrôle hydromécanique par rapport au glissement du glacier. Une vaste gamme de méthodes est employée pour parvenir à ces fins, allant des mesures manuelles sur le terrain à la télédétection spatiale. Jusqu'à maintenant, les observations indiquent que les glaciers ne sont pas en équilibre et que d'importants changements se produisent quant à l'aire, au volume et à la dynamique du glacier. Des modèles informatiques sont utilisés pour généraliser ces résultats ainsi que pour cerner les processus les plus critiques à notre compréhension du système couplé glacier-climat.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Bed-reflection power (BRP) from ice-penetrating radar has been used to make inferences about subglacial conditions and processes, yet is subject to confounding influences, including englacial ...attenuation and bed geometry. We use radar data collected in 2008–11 from a polythermal glacier to compute BRP with the aim of relating BRP to basal conditions. We examine the relationship between raw BRP and ice thickness, apparent bed slope and thickness of the englacial scattering layer as a proxy for internal reflection power. We then analyze a corrected form of the BRP with a graph-segmentation algorithm to delineate areas of high and low reflection power. Low corrected BRP values are found near the glacier terminus where the bed is most likely to be cold, while high corrected BRP is found in the region thought to be undergoing a slow surge. We find a spatial correlation between high BRP and high values of subglacial hydraulic upstream area, suggestive of a hydrological control on BRP. Whereas in dominantly cold glaciers BRP seems to distinguish cold from temperate regions of the bed, BRP in a polythermal glacier with a substantial volume of temperate ice may be a more complex product of thermal and hydrological conditions.
The distribution of cold and temperate ice within glaciers and ice sheets affects processes relevant to englacial and basal hydrology, sliding, and material rheology. Thermal regimes, in turn, are ...shaped by glacier and ice sheet dynamics, as well as environmental setting. We investigate the thermal structures of two small (<7 km2) neighboring glaciers in the St. Elias Mountains of southwestern Yukon, Canada, using ice‐penetrating radar and borehole temperature measurements. Our data reveal polythermal regimes in both glaciers that are strongly influenced by accumulation zone meltwater entrapment, suggesting a climatic control on thermal structure. Differences in hypsometry and glacier dynamics nevertheless result in observed variations in the distribution of temperate ice between the two sites. Experiments with a thermomechanically coupled flow band model corroborate the strong control of meltwater entrapment on thermal structure and suggest a generally minimal role for strain heating. An exception to this occurs where localized basal sliding produces lateral shearing and thus enhanced heat generation. Time‐dependent model simulations suggest that the future thermal evolution of the two glaciers may differ, and therefore simple parameterizations of thermal response based on regional climate may not capture realistic variability between individual glaciers. Despite these differences, both glaciers are ultimately expected to become fully cold prior to disappearing under negative mass balance conditions.
Key Points
Polythermal structure mapped in two neighboring glaciers
We simulate observed meltwater-dominated thermal structure
Differences in the observed thermal structure related to glacier dynamics
Basal hydrology is acknowledged as a fundamental control on glacier dynamics, especially in cases where surface meltwater reaches the bed. For many glaciers at midlatitudes, basal drainage is ...influenced by subaerial, englacial, and subsurface water flow. One of the major shortcomings of existing basal hydrology models is the treatment of the glacier bed as an isolated system. We present theoretical and computational models that couple glacier surface runoff, englacial water storage and transport, subglacial drainage, and subsurface groundwater flow. Each of the four model components is represented as a two‐dimensional, vertically integrated layer that communicates with its neighbors through water exchange. Governing equations are derived from the law of mass conservation and are expressed as a balance between the internal distribution of water and external sources. The numerical exposition of this theory is a time‐dependent finite difference model that can be used to simulate glacier drainage. In this paper we outline the theory and conduct simple tests using an idealized glacier geometry. In the companion paper, the model is tailored to Trapridge Glacier, Yukon Territory, Canada, where results are compared with measurements of subglacial water pressure.
Temperature-index models are popular tools for glacier melt-modeling due to their minimal data requirements and generally favorable performance. We examine the effects of temperature forcing ...provenance and extrapolation on the performance of one such model applied to a small glacier in the Saint Elias Mountains of northwestern Canada. The model is forced with air temperatures recorded (a) on two glaciers, (b) at two nearby ice-free locations, and (c) by two low-elevation valley stations. We extrapolate these temperatures using constant lapse rates and assess model performance by comparing measured and modeled cumulative summer ablation at a network of stakes over five melt seasons. When the model is calibrated individually for each temperature forcing and lapse rate, the variation in model performance is modest relative to inter-annual variations associated with melt-season conditions and calibration data quality. Despite <30% variation in estimated summer ablation arising from the combined influences of temperature forcing and lapse rate, the resulting variations in estimated annual mass balance can be significant (>100% in some cases). While model parameters calibrated in this way suffer from error compensation and exhibit equifinality, the lapse rates associated with minimum model error exhibit inter-annual variation that can be related to prevailing meteorological conditions. When the model is instead calibrated at the point scale without employing a lapse rate, and the resulting parameters are paired with an arbitrary temperature forcing, lapse rates associated with minimum model error vary widely between forcing types and years. Low-elevation stations distal from the study site sometimes outperform the calibration station, but the prescribed lapse rate becomes critical in this case. With either calibration method, lapse rates that minimize model error for the valley stations are generally steeper than the measured environmental lapse rates.
We apply a point-scale energy-balance model to a small polythermal glacier in the St Elias Mountains of Canada in order to investigate the applicability and limitations of different treatments of the ...glacier surface temperature and subsurface heat flux. These treatments range in complexity from a multilayer subsurface model that simulates snowpack evolution, to the assumption of a constant glacier surface temperature equal to 0°C. The most sophisticated model includes dry densification of the snowpack, penetration of shortwave radiation into the subsurface, internal melting, refreezing of percolating meltwater and generation of slush layers. Measurements of subsurface temperature and surface lowering are used for model validation, and highlight the importance of including subsurface penetration of shortwave radiation in the model. Using an iterative scheme to solve for the subsurface heat flux as the residual of the energy-balance equation results in an overestimation of total ablation by 18%, while the multilayer subsurface model underestimates ablation by 6%. By comparison, the 0°C surface assumption leads to an overestimation of ablation of 29% in this study where the mean annual air temperature is about −8°C.
We investigate the dynamics of a small surge-type valley glacier as part of a study to characterize glacier response to climate in the Donjek Range, southwest Yukon, Canada. Pole displacements were ...measured using kinematic GPS techniques during three consecutive summer field seasons. Measured surface velocities range from <10 m a−1 over the lower 1500 m of the 5 km long glacier to a maximum of ∼25–35 m a−1 over the upper 3500 m. Basal velocities along an approximate flowline are reconstructed from the measured surface velocities using inverse methods. Control tests are used to validate the inversion scheme, and sensitivity tests are performed to evaluate the influence of the flow-law coefficient, shape factor and longitudinal averaging length. Inversion of the real data shows that basal motion accounts for 50–100% of the total surface motion along the flowline. Based on these results, and several other lines of evidence, we suggest this glacier may be undergoing a slow surge.
Lacustrine sediment cores from proglacial lake Hvítárvatn, central Iceland, reveal a detailed chronology of Holocene sedimentation, from which environmental conditions and the attendant fluctuations ...of Langjökull ice cap have been interpreted. We apply a numerical ice-sheet model to determine the climatic conditions under which the empirical reconstruction is glaciologically plausible. Modelling constraints are derived from core records of diatom concentration, benthic diatom fraction, and ice-rafted debris occurrence, as well as lake bottom morphology and the present-day ice-cap geometry. Holocene simulations driven by the NGRIP δ18O record that are consistent with the empirical constraints show the most extensive advance of Langjökull ice cap to be its most recent, beginning somewhere between 5 and 3kaBP. Ice advance in response to the 8.2kaBP cold event is followed by several thousand years of nearly ice-free conditions in the mid-Holocene. All simulations suggest that the maximum Holocene stand of the ice cap occurred during the Little Ice Age (LIA); those consistent with the constraints show little to no ice advance into Hvítárvatn before ∼1kaBP and indicate the lake area occupied by ice was much greater during the LIA than at any previous time. The most plausible simulation results were obtained for a maximum Holocene warming of 3–4∘C relative to the 1961–1990 reference, twice the Arctic average, and for Holocene Thermal Maximum (HTM) precipitation amounts comparable to or slightly greater than the modern. Reconciling the simulated subglacial discharge record to the empirically derived sediment volumes and emplacement times requires mean Holocene sediment concentrations of 0.8–1.6kgm-3. These estimates increase to 1.4–2.0kgm-3 when sedimentation rates are highest.
The association between basal hydrology and glacier sliding has become nearly synonymous with the early work of Almut Iken and colleagues. Their research published in the Journal of Glaciology from ...1981 to 1986 made an indelible impact on the study of glacier hydromechanics by documenting strong correlations between basal water pressure and short-term ice-flow variations. With a passion for elucidating the physics of glacier-bed processes, Iken herself made fundamental contributions to our theoretical and empirical understanding of the sliding process. From the theoretical bound on basal shear stress, to the inferences drawn from detailed horizontal and vertical velocity measurements, the work of Iken and colleagues continues to inform the interpretation of data from alpine glaciers and has found increasing relevance to observations from the ice sheets.
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