Ice Ablation Research Articles

On post-glacial sea level - II. Numerical formulation and comparative results on spherically symmetric models

SUMMARY Theoretical approaches to computing gravitationally self-consistent sea-level changes in consequence of ice growth and ablation are comprised of two parts. The first is a mapping between variations in global sea level and changes in ocean height (required to define the surface load), and the second is a method for computing global sea-level change arising from an arbitrary surface loading. In Mitrovica & Milne (2003) (Paper I) we described a new, generalized mapping between sea-level change and ocean height that takes exact account of the evolution of shorelines associated with both transgression and regression cycles and time-dependent marine-based ice margins. The theory is valid for any earth model. In this paper we extend our previous work in three ways. First, we derive an efficient, iterative numerical algorithm for solving the generalized sea-level equation. Secondly, we consider a special case of the new sea-level theory involving spherically symmetric earth models. Specifically, we combine our iterative numerical formulation with viscoelastic Love number theory to derive an extended pseudo-spectral algorithm for solving the new sea-level equation. This algorithm represents an extension of earlier methods developed for the fixed-shoreline case to precisely incorporate shoreline migration processes. Finally, using this special case, we quantitatively assess errors incurred in previous efforts to extend the traditional (fixed shoreline) sea-level equation of Farrell & Clark (1976) to treat time-dependent shorelines. We find that the approximations adopted by Johnston (1993) and Milne (1998) to treat transgression and regression at shorelines introduce negligible (∼1 per cent) error into predictions of post-glacial relative sea-level histories. In contrast, the errors associated with the Peltier (1994) sea-level equation are an order of magnitude larger, and comparable to the error incurred using the traditional sea-level theory. Furthermore, our numerical tests verify the high accuracy of the Milne (1998) approximation for treating the influence of grounded, marine-based ice.

A comparison of solute fluxes and sources from glacial and non‐glacial catchments over contrasting melt seasons

AbstractSolute and runoff fluxes from two adjacent alpine streams (one glacial and one non‐glacial) were investigated to determine how the inorganic solute chemistry of runoff responded to seasonal and interannual changes in runoff sources and volume, and to differences in physical catchment properties. Intercatchment differences in solute composition were primarily controlled by differences in catchment geology and the presence of soils, whereas differences in total solute fluxes were largely dependent on specific discharge. The glacial stream catchment had higher chemical denudation rates due to the high rates of flushing (higher specific discharge). The non‐glacial Bow River had higher overall concentrations of solutes despite the greater prevalence of more resistant lithologies in this catchment. This is likely the result of both longer average water–rock contact times, and a greater supply of protons from organic soils and/or pyrite oxidation. Increases in snowpack depth/snowmelt runoff reduced the retention of nitrate in the Bow River catchment (i.e. increased nitrate export), probably by reducing net biological uptake, or by reducing the proportion of runoff that had contact with biologically active soil horizons that tend to remove nitrate.The two streams exhibited opposite solute flux responses to climate perturbations over three melt seasons (1998, 1999, and 2000). The 1998 El Niño event resulted in an unusually thin winter snowpack, and increased runoff and solute fluxes from the glacial catchment, but decreased fluxes from the Bow River catchment. Solute fluxes in the Bow River increased proportionally to discharge, indicating that increased snowmelt runoff in this catchment resulted in a proportional increase in weathering rates. In contrast, the proportional variation in solute flux in the glacial stream was only ∼70–80% of the variation in water flux. This suggests that increased ablation of glacier ice and the development of subglacial channels during the 1998 El Niño year apparently reduced the average water–rock contact time in the glacial catchment relative to seasons when the subglacial drainage system was primarily distributed in character. Copyright © 2005 John Wiley & Sons, Ltd.

Multi-year water and surface energy budget of a high-latitude polythermal glacier: evidence for overwinter water storage in a dynamic subglacial reservoir

AbstractThis paper examines the water budget and surface energy balance of a Svalbard glacier (midre Lovénbreen) over a 6 year period (1997–2002). Fresh-water yields are found to lie between 1.1 and 1.5 m a–1 and reflect variable amounts of glacier ice ablation (0.27 ± 0.15 ma–1) and more consistent amounts of snowmelt and summer precipitation (0.40± 0.10 and 0.49 ± 0.12 m respectively). Between 24% and 36% of the annual runoff is thought to pass through a subglacial drainage system. Although the site is heavily influenced by stable maritime air masses during the summer, surface melting is achieved largely by net shortwave radiation fluxes (74–100% of ablation). Water budget analysis shows that the annual runoff yields may be strongly influenced by water storage within the glacial system. Storage can occur over the winter period and force the early development of the subglacial drainage system during the following summer. Thus annual water fluxes from this glacier cannot be estimated from glacial mass-balance data alone and there is a need to assess the implications of overwinter storage for our understanding of glacial dynamics, annual fluvial process rates and mixing processes in Svalbard’s fjords.

The genesis of the northern Kettle Moraine, Wisconsin

Interpreting past glacial dynamics from the glacial record requires that the depositional environments of glacial sediments and landforms be understood. In the case of interlobate deposits, models that incorporate various components of pro, supra and subglacial deposition have been developed and tested in the northern Kettle Moraine (nKM), Wisconsin; a large interlobate deposit that formed between the Green Bay and Lake Michigan lobes of the Laurentide Ice Sheet during the last deglaciation. In this paper, we interpret a new genesis for the nKM using sediment analysis and distribution along with landform distribution. In Sheboygan County, the nKM consists of two steep-sided, high-relief, hummocky ridges separated by a low elevation and low-relief central axis. Gravel in the bounding hummocky ridges is well-sorted and well-rounded. Some bedding is collapsed. Large, isolated moulin kames are restricted to the axis area and composed of relatively poorly sorted, more angular gravel and diamicton. The distribution of these different sediments and landforms are explained by the accumulation of supraglacial debris that insulated the ice below the axis of the nKM, while the melting of cleaner ice on either side formed channels on the ice surface. As deglaciation proceeded, a substantial thickness of well-rounded, stream-deposited sand and gravel accumulated on ice in the bounding channels. Eventual collapse of this sediment formed the two hummocky ridges. Poorly sorted debris along the axis fell and slid into moulins and larger collapse areas in the ice. Thus, differential debris insulation and ice ablation controlled the mainly supraglacial deposition of this part of the nKM.

Greenland ice sheet surface mass balance 1991–2000: Application of Polar MM5 mesoscale model and in situ data

The Polar Pennsylvania State University–National Center for Atmospheric Research Fifth‐Generation Mesoscale Model (Polar MM5) regional climate model was run over the North Atlantic region for 1991–2000. We analyze 24‐km output over the Greenland ice sheet to evaluate spatial and temporal variability of the surface mass balance and its subcomponents. The model output is compared with 3 years of automatic weather station (AWS) data from 17 sites to identify biases. Using the in situ data, we derive simple corrections for biases in melt energy and in water vapor fluxes from the surface and from blowing snow. The simulated accumulation rate is in agreement with AWS and snow pit observations. Estimates for runoff and the surface mass balance distribution over the ice sheet are produced using modeled melt volume and a meltwater retention scheme. From the decade investigated, the magnitude of interannual variability in surface mass balance components is tentatively established. The largest variability is concentrated along the ice sheet margin, where both accumulation and ablation rates are largest. The simulated interannual fluctuations suggest a large absolute variability, ±187 km3 yr−1 for total ice sheet surface mass balance. Variability in simulated equilibrium line altitude is suggestive of a dominance of thermal variability in the south with increasing importance of accumulation variability with increasing latitude. Empirical functions for the sensitivity of surface mass balance to temperature and precipitation anomalies are presented. The precise locations and regions of maximum and minimum surface energy and mass fluxes are suggested. Using an estimate for iceberg discharge and bottom melting, the total ice sheet mass balance is estimated be −78 km3 yr−1, producing 2.2 mm of eustatic sea level rise over the 1991–2000 decade and contributing 15% to the observed (1.5 mm yr−1) global sea level rise. The more negative mass balance is attributed to including blowing‐snow sublimation loss and to regional warming in the 1990s.

Ice ablation as evidence of climate change in the Alps over the 20th century

Over fifty years of cumulative annual mass balance data for several glaciers in the Alps shows similar fluctuations which seem to provide evidence of a common climatic signal. Separate winter and summer mass balance measurements from the Claridenfirn (glacier in Switzerland) since 1914 and the Sarennes glacier (France) since 1949 show that (1) the annual mass balance is primarily driven by the summer mass balance term and (2) melting rate variations with time are very similar for these two glaciers located 290 km apart. The increase in the ablation rate of 0.5 cm w.e. day−1 between the two periods 1954–1981 and 1982–2002 over these two glaciers corresponds to a 20 Wm−2 rise in the energy flux at the glacier surface. These results suggest that a common summer melting rate change may have affected the Alps as a whole. Detailed observations on the Sarennes glacier show that the origin of this strong increase in summer ablation since 1982 is not only a rise in the summer melting rate, but also an increase in the ablation period during the months of September and October.

Automatic glacier ablation measurements using pressure transducers

AbstractAn instrument is described that automatically records ice ablation while eliminating the need for ablation stakes. A pressure transducer placed at the bottom of a hole drilled into the ice is connected by a hose to a bladder lying on the surface. Ice ablation is detected as a reduction in the hydrostatic pressure measured by the transducer.

Cold surface layer thinning on Storglaciären, Sweden, observed by repeated ground penetrating radar surveys

A characteristic feature of ground penetrating radar (GPR) surveys on polythermal glaciers is an internal reflection presumably caused by the cold temperate transition surface (CTS), hence providing a possible tool for mapping thermal structure with high accuracy. Comparison of detailed temperature measurements in bore holes and GPR profiles at 345 MHz and 800 MHz center frequencies on Storglaciären, Sweden, show that the CTS can be detected and mapped with an accuracy of about ±1 m at both frequencies. A comparison between comprehensive GPR surveys of the cold surface layer, separated by 12 years (1989–2001), shows a substantial and complex thinning of the cold layer. An overall decrease of 8.3 m (22% of average thickness) of the CTS depth is much larger than uncertainties in CTS depth determinations. The stability of the cold surface layer depends on the net ice ablation at the surface and the downward migration of CTS. There is no evidence of substantial increased net ablation between the survey dates that could explain the observed thinning. However, small increase in average winter air temperature, a limiting factor for the temperature gradient through the cold surface layer, may provide a partial explanation. The weaker temperature gradient reduces the transport of latent heat from the CTS, thus slowing down its downward migration.

The role of sea ice in the fate of contaminants in the Arctic Ocean: plutonium atom ratios in the Fram Strait.

Atom ratios of 240Pu to 239Pu in bottom sediments from the Fram Strait are used to provide evidence for the long distance dispersion of Pu in the Arctic Ocean. In particular, low (<0.18) 240Pu/239Pu ratios indicate that plutonium from sources in the Kara Sea and Novaya Zemlya is transported across the basin toward the North Atlantic. The results have implications for the ability of sea ice to incorporate, intercept, and transport contaminants in the Arctic Ocean. They demonstrate that the fates of sea ice and associated contaminants in the Arctic Ocean are closely coupled, with the release of the particulate load and associated chemical species occurring in principal sea ice ablation areas such as the Fram Strait.

Erosional feedbacks and the oscillation of ice masses

An analytical solution to the development of a glacier or ice sheet in a region where the underlying rock rises steadily shows that the ice mass oscillates in size, independefntly of changes in the climate, in response to a feedback in which ice thickness is linked to topography by ice‐induced erosion. The rate of vertical movement of the underlying rock uplift and the parameters governing ice erosion, ablation, and precipitation determine the period of oscillation. Two‐dimensional numerical simulations show that the tectonic uplift rate is the most important control on the period of oscillation and that the feedback period is likely to be between 10 and 100 kyr. An examination of sediment data from the Deep Sea Drilling Project shows evidence of ice mass oscillations in the Chugach‐St. Elias Mountains and in the Himalayas with periods consistent with those produced by the proposed mechanism. Localized changes in glacial extent may not therefore always be the consequence of climate change, and care must be taken in interpreting evidence of glacial advance and retreat.

On post-glacial sea level: I. General theory

SUMMARY Modern analyses of sea level changes due to glacial isostatic adjustment (GIA) are based on the classic sea level equation derived by Farrell & Clark (1976, Geophys. J.R. astr. Soc., 46, 647–667). The connection between global sea level variations and changes to ocean height that is assumed within this equation breaks down in the presence of a time-varying shoreline geometry. We present a generalized sea level equation that overcomes this difficulty. We also derive analytic expressions for, and present schematic illustrations of, the error in the ocean height change over finite time intervals introduced in published efforts to incorporate shoreline evolution into the theory of GIA-induced sea level change. This comparison includes studies of shoreline migration due to either local sea level changes or the growth and ablation of marine-based ice. We conclude that the theories applied by Johnston (1993, Geophys. J. Int., 114, 615–634), Milne (1998, PhD thesis, University of Toronto, Toronto) and co-workers are more accurate than the procedure advocated by Peltier (1994, Science, 265, 195–201; 1998a, Geophys. Res. Lett., 25, 3955–3958; 1998b, Rev. Geophys., 114, 615–634), although an improvement in the latter has recently been reported (Peltier & Drummond (2002, Geophys. Res. Lett., 29, 10.1029/2001GL014273). Our generalized theory is valid for any Earth model. In a companion paper we derive the equations necessary to treat the special case of a spherically symmetric, linear viscoelastic and rotating Earth, and we quantify errors associated with previous work.

High altitude Himalayan climate inferred from glacial ice flux

Glaciological processes are modeled to investigate precipitation patterns and the resulting mass flux of snow and ice across Himalayan topography. Our model tracks the accumulation and ablation of snow and ice and the transport of snow and ice across the topography by glacier motion. We investigate high elevation precipitation on the Annapurna Massif by comparing the existing ice cover with model‐simulated glaciers produced by a suite of different precipitation scenarios. Our results suggest that precipitation reaches a maximum level well below the elevation of the highest peaks. Further, essentially no snow accumulates on the topography above an elevation of 6200–6300 m. Hence, the upper 1000+ m of the massif is a high elevation desert with little flux of snow and ice. Active glaciers are limited to a band of intermediate elevations where a maximum of about 60% of the landscape is covered by moving ice.

Glaciological response to distal tephra fallout from the 1947 eruption of Hekla, south Iceland

AbstractGlacier responses to tephra deposition are shown to be highly variable where wind-transported eruption plumes produce narrow distal fallout zones with steep lateral thickness gradients. Significant but short-lived advances of faster-flowing glaciers can be triggered by deposition from modest eruptions. The 1947 eruption of Hekla, south Iceland, covered nearby glaciers with variable thicknesses of tephra, causing dramatic spatial variations in ablation rate. Relative snow ablation increased by an estimated &gt;80% at the eastern margin of Eyjafjallajökull, but decreased by &gt;54% at the western margin. Relative ice ablation increased by only 4% in the east but decreased by &gt;75% at the western margin, only 15 km distant. The effect on mass balance therefore depends on tephra distribution as well as on the nature of the glacier surface. On Gígjökull, retardation of ablationwas greatest at the terminus, associated with an anomalous advance of ≥328 m between 1947 and 1954. Other glaciers in the fallout zone show either no recognizable response, or accelerated retreat. Advances will be short-lived due to the rapid redistribution of supraglacial tephra in a maritime climate, and the potential for discriminating between volcanically and climatically forced advances in the glacio-geomorphological record is low. The long-term glaciological effect of volcanism is to create thin, low-albedo covers after reworking, which make conditions less favourable for glaciation.

A Terrestrial Analogy for Martian “Accublation Zones” Revealed by Airborne Ice-Penetrating Radar from the East Antarctic Ice Sheet

The surface of the northern martian ice cap is known to contain alternating white–dark scarp–trough features, and one method of their formation involves spatially coherent patterns of ice accumulation and ablation via sublimation. In this so-called “accublation” model, the internal ice sheet layers submerge into the ice sheet across the accumulation zone and emerge to the ice surface across the ablation region. Here we report on an Antarctic analogy to the martian accublation model, which demonstrates that surface mass balance conditions actually affect internal ice-sheet structure as predicted by D. A. Fisher (2000, Icarus 144, 289–294). This analogy shows how it is possible for ancient ice to become exposed at the surface of an ice sheet, allowing the paleoclimate record stored within the ice to be measured without the need for deep ice coring.

The topography of the ice-water interface – its influence on the colonization of sea ice by algae

The ice-water interface constitutes an important habitat for polar organisms, characterized by extreme variability in physical and biological properties, which can range over an order of magnitude on decimeter scales. The porous nature of sea ice allows vertical fluid exchange within the ice and across the ice-water interface in much the same way as the sediment-water interface. The present paper reports on experiments examining the effect of irregular undersides of sea ice on the variability of algal biomass, using flume tanks with constant unidirectional flow. Bulges and depressions at the ice-water interface altered the interfacial pore water flux and affected the spatial distribution and abundance of ice-associated algae substantially. Dye tracer experiments demonstrated that interfacial water fluxes around ice bulges are by a factor of 10–100 higher than published indirect estimates based on algal nutrient demands in flat fast ice. Further, underside relief structures migrated downstream, illustrating their dynamic and transient nature. The presence of a relief fostered algal accumulation compared to a flat underside. Algal growth occurred at bulge sites facing the current, while particles accumulated in the wake further downstream. We infer from our experiments that sea ice is not only a source for algae production, but can also serve as sink for organic material from the water column. We propose that local formation and ablation of ice around underside features is an important process which induces high variations in the sea-ice habitat structure and in growth of sympagic organisms, and could partially explain the high natural variability observed in the abundance and colonization patterns of sea-ice organisms.

The Frontier Mountain meteorite trap (Antarctica)

Abstract— The Frontier Mountain blue ice field is an important Antarctic meteorite trap which has yielded 472 meteorite specimens since its discovery in 1984. Remote sensing analyses and field campaigns from 1993 to 1999 have furnished new glaciological data on ice flow, ice thickness, bedrock topography, ice ablation and surface mass transport by wind, along with detailed descriptions of the field situation at the trap. This solid set of data combined with an updated meteorite distribution map and terrestrial ages available from literature allows us to better describe the nature of the concentration mechanism. In particular, we observe that the meteorite trap forms in a blue ice field (1) located upstream of an absolute and a shallow sub‐ice barriers; (2) characterized by compressive ice flow with horizontal velocities decreasing from 100 to &lt;10 cm/year on approaching the obstacle; (3) undergoing mean ablation rates of 6.5 cm/year; (4) nourished by a limited snow accumulation zone extending ˜20 km upstream of the blue ice area. We also draw the following conclusions: (1) the origin of the meteorite trap can be explained according to the present‐day glaciological situation; (2) the meteorite concentration develops according to the general principles of the “ice flow model”; (3) the accumulation model can be described as “stagnant ice or slow‐moving ice against an absolute and submerged barriers”, according to the descriptive schemes present in literature; (4) the Frontier Mountain ice field is an effective trap for meteorites weighing more than ˜200 g; for smaller masses, the combination of wind and glacial drift may remove meteorites in less than a few tens of thousands of years; (5) although the activation age of the Frontier Mountain trap is not yet constrained, we infer that one of the most important findsites may be as old as 50 ka, predating the last glacial maximum.

Effect of Melt Ponds on First-Year Sea Ice Ablation - Integration of RADARSAT-1 and Thermodynamic Modelling

RÉSUMÉL'évolution de la fonte printanière en Arctique et du bilan d'énergie de surface associé sont des facteurs critiques dans la détermination de la date éventuelle de la désintégration de la glace de première année. La glace de première année étant mécaniquement faible est liée à un profil quasi-isothermique des températures de la glace avec des volumes relativement élevés de saumure. L'évolution des mares de fonte dans la glace de première année joue un rôle critique dans le bilan d'énergie de surface (à travers l'influence de l'albédo) qui détermine la structure thermodynamique de la glace. Un modèle thermodynamique uni-dimensionnel de la glace de mer a été utilisé pour examiner la susceptibilité de la glace de mer de première année à se désintégrer en fonction d'une fraction en pourcentage de mare de fonte. Une surface de glace avec une fraction de mare de 80% peut produire de la glace thermodynamiquement faible plus de trois semaines avant une surface de glace avec une fraction de mare de 10%. Comme prévu, il y a une relation non-linéaire entre le gradient de changement de température de la glace (ITGC) et la fraction de mare permettant d'illustrer les effets de la fraction de mare sur le potentiel de désintégration de la glace de mer de première année. D'autres expériences de modélisation suggèrent que le modèle de glace de mer surestime les taux de fonte de glace de mer de première année, dans ce cas surtout en raison d'une représentation inadéquate de l'albédo au cours de la saison de mares de fonte. Ceci suggère qu'il nous faut soit des meilleures paramétrisations de l'albédo de surface dans les modèles ou la capacité d'utiliser les données de télédétection pour estimer directement les fractions de mares de fonte et leur albédo associé pour utilisation dans les simulations annuelles de glace de première année. L'inversion de l'albédo de surface à partir d'images RADARSAT-1 acquises au-dessus de glace lisse de première année est possible tout au long de la saison de fonte dans ce cas. Les résultats démontrent que l'albédo de surface dérivé de RADARSAT-1 et incorporé dans le modèle thermodynamique de glace de mer améliore les simulations de glace de mer permettant une meilleure prévision de la période de désintégration de la glace de mer de première année.

Indirect measurements of the mass balance of summer Arctic sea ice with an electromagnetic induction technique

AbstractIn the framework of the Surface Heat Budget of the Arctic (SHEBA) study, indirect, non-invasive ice mass-balance measurements were carried out at a drifting station in the northern Chukchi Sea between May and August 1998. Ice thickness was derived from electromagnetic induction (EM) measurements of apparent conductivity along 13 profiles (60−900 m long). As shown through sensitivity studies with a one-dimensional model, the apparent conductivity data from individual points can be inverted to yield estimates of ice thickness and ablation with an accuracy of approximately 0.05 m (for 2 m thick level ice). Ablation rates were 8−18 mm d−1, with total ablation amounting to roughly 0.9−1.2 m. Measurements of thickness and melt rates along different profiles in undeformed multi-year ice corresponded closely, indicating that the sampling statistics are adequate. The roughness of undeformed ice has been found to increase during the summer due to deepening of melt ponds and enhanced bottom melt. Ice under melt ponds was disproportionately thinner, most likely a result of thicker snow cover reducing winter accretion.

The Role of Eolian Processes in Forming Surface Features of the Martian Polar Layered Deposits

The major topographic features of the martian polar layered deposits include the conspicuous spiral troughs, subtle undulations, broad reentrants (chasma), steep, arcuate scarps, and dune fields. A prominent role for eolian processes in the formation of all of these features is proposed. Strong katabatic winds occur on the north polar cap, producing widespread frost streaks. These cold drainage winds are analogous to those on the terrestrial Antarctic cap. In the chasmae convergent katabatic winds create yardangs and erode arcuate scarps in the layered deposits that are also the source for local dune fields migrating outward from the polar cap. The chasmae are postulated to originate by long-term erosion by katabatic wind. Wind plays a role in formation of the troughs and scarps primarily through removal of dust freed from exposed layered deposits by ablation of ice on defrosted equator-facing slopes. The undulations are created by interaction between surface erosion or deposition of perennial ice and standing waves in the shallow katabatic windflow. The south polar layered deposits exhibit topographic features similar to those at the north polar cap, but erosion and depositional processes creating these features may be inactive during the current epoch because of the lack of an extensive perennial ice cover and associated katabatic wind.

Surface properties of Mars' polar layered deposits and polar landing sites

On December 3, 1999, the Mars Polar Lander and Mars Microprobes will land on the planet's south polar layered deposits near (76°S, 195°W) and conduct the first in situ studies of the planet's polar regions. The scientific goals of these missions address several poorly understood and globally significant issues, such as polar meteorology, the composition and volatile content of the layered deposits, the erosional state and mass balance of their surface, their possible relationship to climate cycles, and the nature of bright and dark aeolian material. Derived thermal inertias of the southern layered deposits are very low (50–100 J m−2 s−1/2 K−1), suggesting that the surface down to a depth of a few centimeters is generally fine grained or porous and free of an appreciable amount of rock or ice. The landing site region is smoother than typical cratered terrain on ∼1 km pixel−1 Viking Orbiter images but contains low‐relief texture on ∼5 to 100 m pixel−1 Mariner 9 and Mars Global Surveyor images. The surface of the southern deposits is older than that of the northern deposits and appears to be modified by aeolian erosion or ablation of ground ice.