Felix Ng

Thermally controlled glacier surging

Bakaninbreen in Svalbard and Trapridge Glacier in Yukon Territory, Canada, are two prominent examples of surging glaciers which are thought to be controlled by their thermal regime. Both glaciers have developed large bulges which have propagated forward as travelling wave fronts, and which are thought to divide relatively stagnant downstream cold-based ice from faster-moving warm-based upstream ice. Additionally, both glaciers are underlain by a wet, metres thick layer of deforming till. We develop a simple model for the cyclic surging behaviour of these glaciers, which interrelates the motion of the ice and till through a description of the subglacial hydrology, We find that oscillations (surges) can occur if the subglacial hydrological transmissivity is sufficiently low and the till layer is sufficiently thin, and we suggest that these oscillations are associated with the development and propagation of a travelling wave front down the glacier. We therefore interpret the travelling wave fronts on both Trapridge Glacier and Bakaninbreen as manifestations of surges. In addition, we find that the violence of the surge in the model is associated with the resistance to ice flow offered by undulations in the bed, and the efficiency with which occasional hydrological events can release water accumulated at the glacier sole.

A glacier respires: Quantifying the distribution and respiration CO2 flux of cryoconite across an entire Arctic supraglacial ecosystem

Hodson, A., Anesio, A. M., Ng, F., Watson, R., Quirk, J., Irvine-fynn, T., Dye, A., Clark, C., McCloy, P., Kohler, J., Sattler, B. (2007). A glacier respires: Quantifying the distribution and respiration Co2 flux of cryoconite across an entire Arctic supraglacial ecosystem. Journal of Geophysical Research, 112 (G4).

Fast-growing till over ancient ice in Beacon Valley, Antarctica

We analyze published cosmogenic 3He depth profiles through the till that covers relict glacier ice in Beacon Valley, Antarctica, in order to derive rigorous constraints on the till thickness history, and on the amount and rate of ice loss by sublimation. The till is a residue of debris-laden ice that sublimed. The 3He profiles show that the lower 80% of the till formed in the past 310–43 k.y. under sublimation rates averaging >7 m·m.y.−1 (meters per million years). Such rapid recent growth of the till contradicts previous interpretations that it is older than 8.1 Ma at an adjacent site, where it encloses volcanic ash of this age. We question whether the ash provides a valid age constraint for the ice. Cosmogenic nuclide analysis of the till where the ash was collected for dating should resolve this question.

Fast-flow signature in the stagnated Kamb Ice Stream, West Antarctica

Among the major ice streams that drain West Antarctica, Kamb Ice Stream (formerly called Ice Stream C) is unique in that it stagnated ∼150 yr ago, but its former fast-flow conditions are virtually unknown. Here we presentsurface-based radar profiles of the ice streams undulating internal stratigraphy, which records these conditions. Our analysis of the profiles indicates that pre-stagnation flow velocities, averaged over a period <740 yr, exceeded 350 m.yr - 1 in the trunk of the ice stream. This velocity constraint would be lower if the ice had been thickening (higher if thinning), but suggests mass loss from the ice-stream catchment that is of sufficient magnitude to reverse the gain estimated for todays Siple Coast region. Analysis of other ice streams would allow comparison of velocities over millennial time scales with observations of present-day velocities, useful for evaluating how West Antarctic ice drainage has evolved.

Temporal dynamics of a jokulhlaup system

Recurring jokulhlaups from ice-dammed lakes often form irregular time sequences that are seemingly unpredictable. Using the flood dates of Merzbacher Lake, Kyrgyzstan, as an example, we study these sequences through a model of lake filling and drainage where flood events initiate at a threshold water depth. Even with a constant threshold, model simulation can explain key aspects of the Merzbacher flood sequence. General analysis of model dynamics reveals a pacing mechanism that links one flood to the next, and which may be represented mathematically as an iterative map. This theory clarifies how environmental factors govern the long-term pattern of flood timings and their frequency distribution in the year. A reconstruction of the past level of Merzbacher Lake also suggests that its flood-initiation threshold decreases with the rate of lake-level rise. These results may help us understand how to forecast future outbursts from jokulhlaup lakes.

Coupled ice-till deformation near subglacial channels and cavities

Previous models of ice-till deformation near subglacial channels or cavities neglect the fact that the motions of the two materials are coupled, and thus the interface between ice and till may not remain stationary. Here, we analyze in succession two models which address the effect of such coupling via specification of appropriate continuity conditions for stress and velocity across the interface. The modelled scenario is that of a shallow channel-cavity, with its long axis parallel to the principal ice-flow direction, overlying actively deforming till sediments. By applying asymptotic techniques, we investigate how the pattern and velocity of the creep flow depend generally on the ratio between the ice and till viscosities, and on the deforming-till thickness. A more sophisticated, non-linear rheology for till sediments is then introduced. It reveals that the two-way interaction between water percolation and deformation in the till will enhance the localization of sediment flow near the channel margins. The length scale over which transition of effective stress in the till takes place-from its relatively high, far-field value to the low, channel value-is found to depend critically on a dimensionless permeability parameter (A). In any case, coupled deformation causes sediment (and ice) flow towards the channel, subsidence of the ice-till interface just outside the channel, and extension of the area over which the ice is in contact with till. Apart from having direct implications for subglacial sediment transport, these results indicate that coupled deformation can contribute significantly to the spatial evolution of stress distribution under a glacier, and thus its incorporation into future sliding and drainage theories for a soft bed should be considered essential.

Modelling the coupling of flood discharge with glacier flow during jökulhlaups

Abstract We explore a mathematical model that couples together a thermomechanically evolving subglacial channel, distributed cavity drainage, and basal sliding along a subglacial flood path fed by a jökulhlaup lake. It allows water transfer between channel and cavities and a migrating subglacial water divide or ‘seal’ to form between floods. Notably, it accounts for full coupling between the lake and subglacial drainage in terms of both discharge and pressure, unlike models that neglect the pressure coupling by imposing a known history of lake discharge at the channel inlet. This means that flood hydrographic evolution and its impact on glacier motion are consistently determined by our model. Numerical simulations for a model alpine lake yield stable limit cycles simulating repeating jökulhlaups, with the channel drawing water from the cavities at a varying rate that modulates basal sliding during each flood. A wave of fast sliding propagates down-glacier at flood initiation, followed by deceleration as the growing channel sucks water from the cavities. These behaviours cannot be correctly simulated without the full coupling. We show that the flood’s peak discharge, its initiation threshold and the magnitude of the ‘fast sliding’ wave decrease with the background water supply to the cavities.

Ice-marginal sediment delivery to the surface of a high-arctic glacier: Austre Broggerbreen, Svalbard

Abstract. Enhanced delivery of water‐saturated, ice‐marginal sediments to the glacier surface is a response to glacier thinning that has the potential to increase both levels of sediment transfer through the glacier hydrological system and total basin sediment yields. Preliminary observations made during summer 2007 at Austre Brøggerbreen, Svalbard, confirm that ice‐marginal debris flows in the upper reaches of the glacier are actively delivering sediments to the glacier surface, which may then be flushed into the glaciers hydrological system. During a four‐day observation period, several stochastic pulses in water turbidity were observed at a single portal where solely supra‐ and englacial drainage emerge at the glacier margin. The erratic suspended sediment fluxes were hypothesized to originate from ice‐marginal sources. Quantitative analysis of continuous turbidity and discharge data confirm that discharge is not driving these turbidity pulses and, combined with observational data, that the most likely origin is the delivery of water‐saturated sediments to the glacier surface from ice‐marginal, debris flows with subsequent transfer to the portal via the glacial drainage system. These observations illustrate the potential importance of the paraglacial component to the overall sediment cascade of deglaciating basins and highlight the need for careful interpretation of turbidity records, where stochastic pulses in turbidity may be attributed to sources and processes other than ice‐marginal sediment inputs.

Ice-dammed lake drainage evolution at Russell Glacier, west Greenland

Glaciological and hydraulic factors that control the timing and mechanisms of glacier lake outburst floods (GLOFs) remain poorly understood. This study used measurements of lake level at fifteen minute intervals and known lake bathymetry to calculate lake outflow during two GLOF events from the northern margin of Russell Glacier, west Greenland. We used measured ice surface elevation, interpolated subglacial topography and likely conduit geometry to inform a melt enlargement model of the outburst evolution. The model was tuned to best-fit the hydrograph’s rising limb and timing of peak discharge in both events; it achieved Mean Absolute Errors of < 5 %. About one third of the way through the rising limb, conduit melt enlargement became the dominant drainage mechanism. Lake water temperature, which strongly governed the enlargement rate, preconditioned the high peak discharge and short duration of these floods. We hypothesize that both GLOFs were triggered by ice dam flotation, and localised hydraulic jacking sustained most of their early-stage outflow, explaining the particularly rapid water egress in comparison to that recorded at other ice-marginal lakes. As ice overburden pressure relative to lake water hydraulic head diminished, flow became confined to a subglacial conduit. This study has emphasised the inter-play between ice dam thickness and lake level, drainage timing, lake water temperature and consequently rising stage lake outflow and flood evolution.

Greenland Ice Sheet surface topography and drainage structure controlled by the transfer of basal variability

Ice flow can transfer variations in basal topography and basal slipperiness to the ice surface. Recent developments in this theory have made it possible to conduct numerical experiments to predict mesoscale surface topographical undulations and surface relief on an ice sheet-scale. Focussing here on the contemporary Greenland Ice Sheet (GrIS), we demonstrate that the theory can be used to predict the surface relief of the ice sheet from bed topography, ice thickness and basal slip ratio datasets. In certain regions of the GrIS our approach overestimates, while in others underestimates, the observed surface relief. The magnitude and spatial pattern of these mismatches correspond with the theory’s limitations and known uncertainties in the bed topography and basal slip ratio datasets. Our prediction experiment establishes that the first-order control on GrIS surface relief is basal topography modulated by ice thickness, surface slope and basal slip ratio. Additional analyses show that the surface relief, which is controlled by the bed-to-surface transfer of basal topography, preconditions the large scale spatial structure of surface drainage, with other factors such as surface runoff modulating the actual drainage system through influencing the temporal evolution of meltwater features. It follows that the spatial structure of surface drainage depends strongly on the transfer of basal topography to the ice surface. These findings represent an important step towards investigating and understanding the net long-term (>102 years) effect of surface drainage on ice sheet mass balance and dynamics during deglaciation events.