Showing papers in "Annals of Glaciology in 2012"

Characteristics of ocean waters reaching Greenland's glaciers

Abstract: Interaction of Greenland's marine-terminating glaciers with the ocean has emerged as a key term in the ice-sheet mass balance and a plausible trigger for their recent acceleration. Our knowledge of the dynamics, however, is limited by scarcity of ocean measurements at the glacier/ocean boundary. Here data collected near six marine-terminating glaciers (79 North, Kangerdlugssuaq, Helheim and Petermann glaciers, Jakobshavn Isbrae, and the combined Sermeq Kujatdleq and Akangnardleq) are compared to investigate the water masses and the circulation at the ice/ocean boundary. Polar Water, of Arctic origin, and Atlantic Water, from the subtropical North Atlantic, are found near all the glaciers. Property analysis indicates melting by Atlantic Water (AW; found at the grounding line depth near all the glaciers) and the influence of subglacial discharge at depth in summer. AW temperatures near the glaciers range from 4.58C in the southeast, to 0.168C in northwest Greenland, consistent with the distance from the subtropical North Atlantic and cooling across the continental shelf. A review of its offshore variability suggests that AW temperature changes in the fjords will be largest in southern and smallest in northwest Greenland, consistent with the regional distribution of the recent glacier acceleration.

Tropical forcing of Circumpolar Deep Water Inflow and outlet glacier thinning in the Amundsen Sea Embayment, West Antarctica

Abstract: Outlet glaciers draining the Antarctic ice sheet into the Amundsen Sea Embayment (ASE) have accelerated in recent decades, most likely as a result of increased melting of their ice-shelf termini by warm Circumpolar Deep Water (CDW). An ocean model forced with climate reanalysis data shows that, beginning in the early 1990s, an increase in westerly wind stress near the continental shelf edge drove an increase in CDW inflow onto the shelf. The change in local wind stress occurred predominantly in fall and early winter, associated with anomalous high sea-level pressure (SLP) to the north of the ASE and an increase in sea surface temperature (SST) in the central tropical Pacific. The SLP change is associated with geopotential height anomalies in the middle and upper troposphere, characteristic of a stationary Rossby wave response to tropical SST forcing, rather than with changes in the zonally symmetric circulation. Tropical Pacific warming similar to that of the 1990s occurred in the 1940s, and thus is a candidate for initiating the current period of ASE glacier retreat.

Numerical experiments on subaqueous melting of Greenland tidewater glaciers in response to ocean warming and enhanced subglacial discharge

Abstract: The largest dischargers of ice in Greenland are glaciers that terminate in the ocean and melt in contact with sea water. Studies of ice-sheet/ocean interactions have mostly focused on melting beneath near-horizontal floating ice shelves. For tidewater glaciers, melting instead takes place along the vertical face of the calving front. Here we modify the Massachusetts Institute of Technology general circulation model (MITgcm) to include ice melting from a calving face with the freshwater outflow at the glacier grounding line. We use the model to predict melt rates and their sensitivity to ocean thermal forcing and to subglacial discharge. We find that melt rates increase with approximately the one-third power of the subglacial water flux, and increase linearly with ocean thermal forcing. Our simulations indicate that, consistent with limited field data, melting ceases when subglacial discharge is shut off, and reaches several meters per day when subglacial discharge is high in the summer. These results are a first step toward a more realistic representation of subglacial discharge and of ocean thermal forcing on the subaqueous melting of tidewater glaciers in a numerical ocean model. Our results illustrate that the ice-front melting process is both complex and strongly time-dependent.

Sensitivity of the ice-shelf/ocean system to the sub-ice-shelf cavity shape measured by NASA IceBridge in Pine Island Glacier, West Antarctica

Abstract: Two high-resolution (1 km grid) numerical model simulations of the Amundsen Sea, West Antarctica, are used to study the role of the ocean in the mass loss and grounding line retreat of Pine Island Glacier. The first simulation uses BEDMAP bathymetry under the Pine Island ice shelf, and the second simulation uses NASA IceBridge-derived bathymetry. The IceBridge data reveal the existence of a trough from the ice-shelf edge to the grounding line, enabling warm Circumpolar Deep Water to penetrate to the grounding line, leading to higher melt rates than previously estimated. The mean melt rate for the simulation with NASA IceBridge data is 28 ma–1, much higher than previous model estimates but closer to estimates from remote sensing. Although the mean melt rate is 25% higher than in the simulation with BEDMAP bathymetry, the temporal evolution remains unchanged between the two simulations. This indicates that temporal variability of melting is mostly driven by processes outside the cavity. Spatial melt rate patterns of BEDMAP and IceBridge simulations differ significantly, with the latter in closer agreement with satellite-derived melt rate estimates of ~50ma–1 near the grounding line. Our simulations confirm that knowledge of the cavity shape and its time evolution are essential to accurately capture basal mass loss of Antarctic ice shelves.

Ice-shelf basal melting in a global finite-element sea-ice/ice-shelf/ocean model

Abstract: The Finite Element Sea ice Ocean Model (FESOM) has been augmented by an ice-shelf component with a three-equation system for diagnostic computation of boundary layer temperature and salinity. Ice shelf geometry and global ocean bathymetry have been derived from the RTopo-1 dataset. A global domain with a triangular mesh and a hybrid vertical coordinate is used. To evaluate sub-ice shelf circulation and melt rates for present-day climate, the model is forced with NCEP reanalysis data. Basal mass fluxes are mostly realistic with maximum melt rates in the deepest parts near the grounding lines and marine ice formation in the northern sectors of Ross Ice Shelf and Filchner-Ronne Ice Shelf. Total basal mass loss for the ten largest ice shelves reflects the importance of the Amundsen Sea ice shelves; Getz Ice Shelf is shown to be a major melt water contributor to the Southern Ocean. Despite their modest melt rates, the ``cold water' ice shelves in the Weddell Sea are still substantial sinks of continental ice in Antarctica. Discrepancies between the model and observations can partly be attributed to deficiencies in the forcing data or to (sometimes unavoidable) smoothing of ice shelf and bottom topographies.

The supply of subglacial meltwater to the grounding line of the Siple Coast, West Antarctica

Abstract: Recent satellite studies have shown that active subglacial lakes exist under the Antarctic ice streams and persist almost to their grounding lines. When the lowest-lying lakes flood, the water crosses the grounding line and enters the sub-ice-shelf cavity. Modeling results suggest that this additional freshwater influx may significantly enhance melting at the ice-shelf base. We examine the spatial and temporal variability in subglacial water supply to the grounding lines of the Siple Coast ice streams, by combining estimates for lake volume change derived from Ice, Cloud and land Elevation Satellite (ICESat) data with a model for subglacial water transport. Our results suggest that subglacial outflow tends to concentrate towards six embayments in the Siple Coast grounding line. Although mean grounding line outflow is ~60m3 s–1 for the entire Siple Coast, maximum local grounding line outflow may temporarily exceed 300 m3 s–1 during the synchronized flooding of multiple lakes in a hydrologic basin. Variability in subglacial outflow due to subglacial lake drainage may account for a substantial portion of the observed variability in freshwater flux out of the Ross Ice Shelf cavity. The temporal variability in grounding line outflow results in a net reduction in long-term average melt rate, but temporary peak melting rates may exceed the long-term average by a factor of three.

Spreading of warm ocean waters around Greenland as a possible cause for glacier acceleration

Abstract: We examine the pattern of spreading of warm subtropical-origin waters around Greenland for the years 1992–2009 using a high-resolution (4km horizontal grid) coupled ocean and sea-ice simulation. The simulation, provided by the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project, qualitatively reproduces the observed warming of subsurface waters in the subpolar gyre associated with changes of the North Atlantic atmospheric state that occurred in the mid-1990s. The modeled subsurface ocean temperature warmed by 1.5˚C in southeast and southwest Greenland during 1994–2005 and subsequently cooled by 0.5˚C; modeled subsurface ocean temperature increased by 2–2.5˚C in central and then northwest Greenland during 1997–2005 and stabilized thereafter, while it increased after 2005 by <0.5˚C in north Greenland. Comparisons with in situ measurements off the continental shelf in the Labrador and Irminger Seas indicate that the model initial conditions were 0.4˚C too warm in the south but the simulated warming is correctly reproduced; while measurements from eastern Baffin Bay reveal that the model initial conditions were 1.0˚C too cold in the northwest but the simulated ocean warming brought modeled temperature closer to observations, i.e. the simulated warming is 1.0˚C too large. At several key locations, the modeled oceanic changes off the shelf and below the seasonal mixed layer were rapidly transmitted to the shelf within troughs towards (model-unresolved) fjords. Unless blocked in the fjords by shallow sills, these warm subsurface waters had potential to propagate down the fjords and melt the glacier fronts. Based on model sensitivity simulations from an independent study (Xu and others, 2012), we show that the oceanic changes have very likely increased the subaqueous melt rates of the glacier fronts, and in turn impacted the rates of glacier flow.

Estimating ocean heat transports and submarine melt rates in Sermilik Fjord, Greenland, using lowered acoustic Doppler current profiler (LADCP) velocity profiles

Abstract: Submarine melting at the ice-ocean interface is a significant term in the mass balance of marine-terminating outlet glaciers. However, obtaining direct measurements of the submarine melt rate, or the ocean heat transport towards the glacier that drives this melting, has been difficult due to the scarcity of observations, as well as the complexity of oceanic flows. Here we present a method that uses synoptic velocity and temperature profiles, but accounts for the dominant mode of velocity variability, to obtain representative heat transport estimates. We apply this method to the Sermilik Fjord-Helheim Glacier system in southeastern Greenland. Using lowered acoustic Doppler current profiler (LADCP) and hydrographic data collected in summer 2009, we find a mean heat transport towards the glacier of 29 � 10 9 W, implying a submarine melt rate at the glacier face of 650 m a -1 .T he resulting adjusted velocity profile is indicative of a multilayer residual circulation, where the meltwater mixture flows out of the fjord at the surface and at the stratification maximum.

Oceanic mechanical forcing of a marine-terminating Greenland glacier

Abstract: Dynamics of marine-terminating major outlet glaciers are of high interest because of their potential for drawing down large areas of the Greenland ice sheet. We quantify short-term changes in ice flow speed and calving at a major West Greenland glacier and examine their relationship to the presence of the sea-ice melange and tidal stage. A field campaign at the terminus of Store Gletscher (70.40˚N, 50.55˚W) spanning the spring and summer of 2008 included four broadband seismometers, three time-lapse cameras, a tide gauge, an automatic weather station and an on-ice continuous GPS station. Sub-daily fluctuations in speed coincide with two modes of oceanic forcing: (1) the removal of the ice melange from the terminus front and (2) tidal fluctuations contributing to speed increases following ice melange removal. Tidal fluctuations in ice flow speed were observed 16km from the terminus and possibly extend further. Seismic records suggest that periods of intensive calving activity coincide with ice-flow acceleration following breakup of the melange in spring. A synchronous increase in speed at the front and clearing of the melange suggests that the melange directly resists ice flow. We estimate a buttressing stress (~30–60 kPa) due to the presence of the ice melange that is greater than expected from the range of observed tides, though an order of magnitude less than the driving stress.

The role of Pine Island Glacier ice shelf basal channels in deep-water upwelling, polynyas and ocean circulation in Pine Island Bay, Antarctica

Abstract: Several hundred visible and thermal infrared satellite images of Antarctica's southeast Amundsen Sea from 1986 to 2011, combined with aerial observations in 2009, show a strong inverse relation between prominent curvilinear surface depressions and the underlying basal morphology of the outer Pine Island Glacier ice shelf. Shipboard measurements near the calving front reveal positive temperature, salinity and current anomalies indicative of melt-laden, deep-water outflows near and above the larger channel termini. These buoyant plumes rise to the surface and are expressed as small polynyas in the sea ice and thermal signatures in the open water. The warm upwellings also trace the cyclonic surface circulation in Pine Island Bay. The satellite coverage suggests changing modes of ocean/ ice interactions, dominated by leads along the ice shelf through 1999, fast ice and polynyas from 2000 to 2007, and larger areas of open water since 2008.

Dependence of the water retention curve of snow on snow characteristics

Abstract: The water retention curve (WRC), which shows the relationship between the volumetric liquid water content, θ v , and suction, h, is a fundamental part of the characterization of hydraulic properties. Therefore, the formulation of the WRC as a function of snow characteristics is essential for establishing a model of water movement through the snow cover. In this study, we measured the WRC of several snow samples, which had different characteristics (grain size, bulk dry density and grain type), using a gravity drainage column experiment and then analysed these data using the Van Genuchten soil physics model (VG model). The shape of the WRC depended strongly on both the sample grain size, d, and bulk dry density, p. Therefore, we introduced the parameter ρ/d to model the WRC of snow. The relationships between the parameters α and n of the VG model and ρ/d change with grain type. For melt forms, a, which is related to the inverse value of the air-entry suction, increases quickly as ρ/d decreases, whereas n, which is related to the gradient of θ v vs h, increases with ρ/d. Conversely, neither of these parameters of the VG model for rounded grains showed obvious dependence on ρ/d. These results suggest that water movement through snow cover can be modelled using grain size, bulk dry density and grain type based on the soil physics model.

Basal crevasses and associated surface crevassing on the Larsen C ice shelf, Antarctica, and their role in ice-shelf instability

Abstract: We identify a series of basal crevasses along a 31 km transect across the northern sector of the Larsen C ice shelf, Antarctica, using in situ ground-penetrating radar. The basal crevasses propagate from a region of multiple, shallow basal fractures to form widely spaced (0.5–2.0 km) but deeply incised (70–134 m) features. Surface troughs, observed in visible imagery, exist above the basal crevasses as the ice vertically shears to reach hydrostatic equilibrium, while widespread surface crevassing occurs along the crests and on the flanks of the undulations, primarily aligned with the topography. We suggest, based on the location of the surface crevasses and the along-flow evolution of the basal crevasses, that the former are induced by a bending stress created by gradients in hydrostatic forces. Using a linear elastic fracture mechanics model, we investigate the sensitivity of basal crevasse propagation to observed trends of ice-shelf thinning and acceleration. Basal crevasses are large-scale structural weaknesses that can both control meltwater ponding and induce surface crevassing. Together, these features may represent an important mechanism in both past and future ice-shelf disintegration events on the Antarctic Peninsula.

Glaciar Jorge Montt (Chilean Patagonia) dynamics derived from photos obtained by fixed cameras and satellite image feature tracking

Abstract: Tidewater calving glaciers can undergo large fluctuations not necessarily in direct response to climate, but rather owing to complex ice-water interactions at the glacier termini. One example of this process in Chilean Patagonia is Glaciar Jorge Montt, where two cameras were installed in February 2010, collecting up to four glacier photographs per day, until they were recovered on 22 January 2011. Ice velocities were derived from feature tracking of the geo-referenced photos, yielding a mean value of 13 � 4md -1 for the whole lower part of the glacier. These velocities were compared to satellite- imagery-derived feature tracking obtained in February 2010, resulting in similar values. During the operational period of the cameras, the glacier continued to retreat (1 km), experiencing one of the highest calving fluxes ever recorded in Patagonia (2.4 km 3 a -1 ). Comparison with previous data also revealed ice acceleration in recent years. These very high velocities are clearly a response to enhanced glacier calving activity into a deep water fjord.

Adjoint sensitivities of sub-ice-shelf melt rates to ocean circulation under the Pine Island Ice Shelf, West Antarctica

Abstract: We investigate the sensitivity of sub-ice-shelf melt rates under Pine Island Ice Shelf, West Antarctica, to changes in the oceanic state using an adjoint ocean model that is capable of representing the flow in sub-ice-shelf cavities. The adjoint code is based on algorithmic differentiation (AD) of the Massachusetts Institute of Technology’s ocean general circulation model (MITgcm). The adjoint model was extended by adding into the AD process the corresponding sub-ice-shelf cavity code, which implements a three-equation thermodynamic melt-rate parameterization to infer heat and freshwater fluxes at the ice-shelf/ocean boundary. The inferred sensitivities reveal dominant timescales of 30–60 days over which the shelf exit is connected to the deep interior via advective processes. They exhibit rich three-dimensional time-evolving patterns that can be understood in terms of a combination of the buoyancy forcing by inflowing water masses, the cavity geometry and the effect of rotation and topography in steering the flow in the presence of prominent features in the bedrock bathymetry. Dominant sensitivity pathways are found over a sill, as well as ‘shadow regions’ of very low sensitivities. To the extent that these transient patterns are robust they carry important information for decision- making in observation deployment and monitoring.

Sensitivity experiments for the Antarctic ice sheet with varied sub-ice-shelf melting rates

Abstract: Ice-sheet modelling is an important tool for predicting the possible response of ice sheets to climate change in the past and future. An established ice-sheet model is SICOPOLIS (Simulation COde for POLythermal Ice Sheets), and for this study the previously grounded-ice-only model was complemented by an ice-shelf module. The new version of SICOPOLIS is applied to the Antarctic ice sheet, driven by standard forcings defined by the SeaRISE (Sea-level Response to Ice Sheet Evolution) community effort. A crucial point for simulations into the future is to obtain reasonable initial conditions by a palaeoclimatic spin-up, which we carry out over 125 000 years from the Eemian until today. We then carry out a set of experiments for 500 years into the future, in which the surface temperature and precipitation are kept at their present-day distributions, while sub-ice-shelf melting rates between 0 and 200 m a(-1) are applied. These simulations show a significant, but not catastrophic, sensitivity of the ice sheet. Grounded-ice volumes decrease with increasing melting rates, and the spread of the results from the zero to the maximum melting case is similar to 0.65 m s.l.e. (metres sea-level equivalent) after 100 years and similar to 2.25 m s.l.e. after 500 years.

Partitioning effects from ocean and atmosphere on the calving stability of Kangerdlugssuaq Glacier, East Greenland

Abstract: We use the 7 km retreat of Kangerdlugssuaq Glacier (KG), East Greenland, to examine the mechanisms, interactions and relative significance of atmospheric forcing and ice/ocean interactions. Hydrographic data from 1991, 1993 and 2004 show that subtropical waters are common in Kangerdlugssuaq Fjord (KF), and that surface waters were warm in 2004 relative to 1991 and 1993. The main water column was nonetheless warmest in 1991. We contend that while flow of subtropical waters into fjords provides a setting in which rapid glacier retreat can occur, the triggering of retreat depends on additional environmental factors. The climatic variables standing out in our study of KG and KF are air temperature and katabatic winds. Both had strong positive anomalies during winter 2004/05, when KG retreated. We show that proglacial ice melange was absent and that fjord freeze-up did not occur until 11 April 2005, due to warm and windy conditions. We demonstrate that this setting is unusual and hypothesize that exposure to open water in winter months caused the retreat. Calculation of ice-front melt rates shows that discharge of basal meltwater, first from runoff and subsequently from frictional basal heating, should intensify the interaction between glacier and fjord.

Testing the effect of water in crevasses on a physically based calving model

Abstract: A new implementation of a calving model, using the finite-element code Elmer, is presented and used to investigate the effects of surface water within crevasses on calving rate. For this work, we use a two-dimensional flowline model of Columbia Glacier, Alaska. Using the glacier's 1993 geometry as a starting point, we apply a crevasse-depth calving criterion, which predicts calving at the location where surface crevasses cross the waterline. Crevasse depth is calculated using the Nye formulation. We find that calving rate in such a regime is highly dependent on the depth of water in surface crevasses, with a change of just a few metres in water depth causing the glacier to change from advancing at a rate of 3.5 km a –1 to retreating at a rate of 1.9 km a –1 . These results highlight the potential for atmospheric warming and surface meltwater to trigger glacier retreat, but also the difficulty of modelling calving rates, as crevasse water depth is difficult to determine either by measurement in situ or surface mass-balance modelling.

Response of the Ross Ice Shelf, Antarctica, to ocean gravity-wave forcing

Abstract: Comparison of the Ross Ice Shelf (RIS, Antarctica) response at near-front seismic station RIS2 with seismometer data collected on tabular iceberg B15A and with land-based seismic stations at Scott Base on Ross Island (SBA) and near Lake Vanda in the Dry Valleys (VNDA) allows identification of RIS-specific signals resulting from gravity-wave forcing that includes meteorologically driven wind waves and swell, infragravity (IG) waves and tsunami waves. The vibration response of the RIS varies with season and with the frequency and amplitude of the gravity-wave forcing. The response of the RIS to IG wave and swell impacts is much greater than that observed at SBA and VNDA. A spectral peak at near-ice-front seismic station RIS2 centered near 0.5 Hz, which persists during April when swell is damped by sea ice, may be a dominant resonance or eigenfrequency of the RIS. High-amplitude swell events excite relatively broadband signals that are likely fracture events (icequakes). Changes in coherence between the vertical and horizontal sensors in the 8-12 Hz band from February to April, combined with the appearance of a spectral peak near 10 Hz in April when sea ice damps swell, suggest that lower (higher) temperatures during austral winter (summer) months affect signal propagation characteristics and hence mechanical properties of the RIS.

Resolution requirements for grounding-line modelling: sensitivity to basal drag and ice-shelf buttressing

Abstract: Simulations of grounding-line migration in ice-sheet models using a fixed grid have been shown to exhibit poor convergence at achievable resolutions. We present a series of ‘shelfy-stream’ flowline model experiments using an idealized set-up. We assess the performance of a range of grounding-line parameterizations (GLPs) over a large input space by varying bedrock gradient, rate factor, basal drag coefficient and net accumulation. The relative performance of GLPs is similar to Gladstone and others (2010a) except at low basal drag, in which case the grounding-line errors are very small for all GLPs. We find that grounding-line errors are far more sensitive to basal drag than to the other inputs or to choice of GLP. We then quantify grounding-line errors as a function of resolution while varying basal drag and channel width (using a parameterization to represent buttressing). Reducing either basal drag or channel width reduces the errors associated with the grounding line. Our results suggest that a structured fixed-grid shelfy-stream ice-sheet model would need to run at a horizontal resolution of ~1–2km to accurately simulate grounding-line positions of marine ice-sheet outlet glaciers such as Pine Island Glacier, Antarctica.

Radar characterization of the basal interface across the grounding zone of an ice-rise promontory in East Antarctica

Abstract: Radar power returned from the basal interface along a 42 km long profile over an ice-rise promontory and the adjacent Roi Baudouin ice shelf, Dronning Maud Land, East Antarctica, is analyzed to infer spatial variations in basal reflectivity and hence the basal environment. Extracting basal reflectivity from basal returned power requires an englacial attenuation model. We estimate attenuation in two ways: (1) using a temperature-dependent model with input from thermomechanical ice-flow models; and (2) using a radar method that linearly approximates the geometrically corrected returned power with ice thickness. The two methods give different results. We argue that attenuation calculated using a modeled temperature profile is more robust than the widely used radar method, especially in locations where depth-averaged attenuation varies spatially or where the patterns of basal reflectivity correlate with the patterns of the ice thickness.

Tidewater glacier fluctuations in central East Greenland coastal and fjord regions (1980s–2005)

Abstract: Summer 2000/01 ASTER and Landsat 7 scenes and semi-automated digitization were used to compile a glacier inventory for local glaciers of the Geikie Plateau region, central East Greenland. Of the 332 glaciers (41 591 km2), 120 are tidewater-terminating and drain 90% of the glacierized area. Differencing the 2000/01 tidewater margins from the 1980s GEUS map database ice polygons (113 glaciers) and from 2004/05 ASTER tidewater margins (78 glaciers) shows a cumulative tidewater terminus width decrease from 196 km to 183 km between the 1980s and 2000s, with a corresponding areal loss of ~31 km2 and an effective length change of –14.3 km. Between 2000/01 and 2004/05, areal loss was 26 km2. Average margin retreat rate increased two- to threefold, from 1.7–2.1km2 a–1 (1980s– 2000) to 3.9–5.7km2 a–1 (2000–05). Advances are negligible, apart from two surges, of which one was previously undetected. Coastal, ‘outer’ fjord-terminating, glaciers have two to three times larger areal and effective length retreat rates than ‘inner’ fjord-terminating glaciers. We investigate possible causes and hypothesize that, in addition to ocean temperature and sea ice, changes in sea fog may affect coastal-terminating more than inner fjord-terminating glaciers.

Observing calving-generated ocean waves with coastal broadband seismometers, Jakobshavn Isbræ, Greenland

Abstract: We use time-lapse photography, MODIS satellite imagery, ocean wave measurements and regional broadband seismic data to demonstrate that icebergs that calve from Jakobshavn Isbrae, Greenland, can generate ocean waves that are detectable over 150 km from their source. The waves, which are recorded seismically, have distinct spectral peaks, are not dispersive and persist for several hours. On the basis of these observations, we suggest that calving events at Jakobshavn Isbrae can stimulate seiches, or basin eigenmodes, in both Ilulissat Icefjord and Disko Bay. Our observations furthermore indicate that coastal, land-based seismometers located near calving termini (e.g. as part of the new Greenland Ice Sheet Monitoring Network (GLISN)) can aid investigations into the largely unexplored, oceanographic consequences of iceberg calving.

Ice-flow sensitivity to boundary processes: A coupled model study in the vostok subglacial lake area, antarctica

Abstract: Several hundred subglacial lakes have been identified beneath Antarctica so far. Their interaction with the overlying ice sheet and their influence on ice dynamics are still subjects of investigation. While it is known that lakes reduce the ice-sheet friction towards a free-slip basal boundary condition, little is known about how basal melting and freezing at the lake/ice interface modifies the ice dynamics, thermal regime and ice rheology. In this diagnostic study we simulate the Vostok Subglacial Lake area with a coupled full Stokes 3-D ice-flow model and a 3-D lake-circulation model. The exchange of energy (heat) and mass at the lake/ice interface increases (decreases) the temperature in the ice column above the lake by up to 10% in freezing (melting) areas, resulting in a significant modification of the highly nonlinear ice viscosity. We show that basal lubrication at the bottom of the ice sheet has a significant impact not only on the ice flow above the lake itself, but also on the vicinity and far field. While the ice flow crosses Vostok Subglacial Lake, flow divergence is observed and modelled. The heterogeneous basal-mass-balance pattern at the lake/ice interface intensifies this divergence. Instead of interactive coupling between the ice-flow model and the lake-flow model, only a single iteration is required for a realistic representation of the ice/water interaction. In addition, our study indicates that simplified parameterizations of the surface temperature boundary condition might lead to a velocity error of 20% for the area of investigation.

Basal melting at the Ekstrom Ice Shelf, Antarctica, estimated from mass flux divergence

Abstract: We characterize the basal mass balance of the Ekstr ¨om Ice Shelf, Dronning Maud Land, Antarctica, using interferometrically derived surface velocities and ice thickness measurements from radio-echo sounding (RES). The surface velocities are based on data from European Remote-sensing Satellites-1 and -2 (ERS-1/2) during 1994–97. The ice thickness grid consists of 136 RES profiles acquired between 1996 and 2006. Mass fluxes are calculated along selected RES profiles where possible, to reduce uncertainties from ice thickness interpolation. Elsewhere large-scale mass fluxes are calculated using interpolated ice thickness data. Themass flux into the Ekstr ¨om Ice Shelf from themain grounded drainage basins is estimated to be 3.19±0.4Gt a−1. The mass flux near the ice shelf front is 2.67±0.3Gt a−1. Assuming steady state, and based on the equation of continuity, we interpret the residual mass flux as a combined effect of snow accumulation and subglacial melting/refreezing. Using net snow accumulation rates from previous studies, we link the mass flux divergence in irregular-shaped polygons to processes beneath the ice shelf. The highest subglacial melt rates of ∼1.1ma−1 are found near the grounding zone of two main inflow glaciers, and around the German station Neumayer III. The detection of unlikely refreezing in a small area ∼15 km west of Neumayer III is attributed to both dataset inaccuracies and a (possibly past) violation of the steady-state assumption. In general, the method and input data allow mapping of the spatial distribution of basal melting and the results are in good agreement with several previous studies.

Optical-televiewer-based identification and characterization of material facies associated with an antarctic ice-shelf rift

Abstract: We have drilled 13 boreholes within and around a through-cutting rift on the (unofficially named) Roi Baudouin Ice Shelf, East Antarctica. Logging by optical televiewer (OPTV) combined with core inspection has resulted in the identification and characterization of several material facies. Outside the rift, OPTV-imaged annual layering indicates 150 years of accumulation over the 66 m length of one of the boreholes. Luminosity analysis of this image also reveals the presence of numerous dark melt layers as well as a systematic decrease in background luminosity, interpreted in terms of a progressive increase in light transmission during firnification. We identify four material facies within the rift: snow, granular ice, marine ice and unconsolidated platelets. We interpret the granular ice facies as snow that has been saturated by percolating sea water, and the underlying marine ice as compacted buoyant platelets that have adhered to the rift base. Core sections reveal the presence of tubular channels within the marine ice, indicating that it is macroporous and permeable to sea water. The lower boundary of this facies merges into a mushy layer of unconsolidated platelets that were successfully imaged by OPTV, revealing irregular sub-horizontal layering similar to that reported previously on the basis of (directional) borehole video.

Passive underwater acoustic evolution of a calving event

Abstract: Direct measurements of processes occurring at the ice–ocean boundary are difficult to acquire because of the dangerous and dynamic nature of the boundary, yet these processes are among the least well understood in glaciology. Because sound travels well through water, passive underwater acoustics offers a method to remotely sense activity at this boundary. Here we present passive acoustic measurements and spectral analysis of the evolution of a subaerial calving event and the subsequent mini-tsunami and seiche at Meares Glacier, Alaska, USA. Using two hydrophones to record sound from 1 to 40 000 Hz, we find that each phase of a calving event has distinctive spectral characteristics. An event begins with an infrasound rumble (1–20 Hz), then the ice fractures (20–100 Hz), falls and impacts the water (200–600 Hz). High-frequency (>10 000 Hz) sound increases in intensity quickly as the iceberg oscillates, creating turbulence, spray and waves. Within 10 s, the low-frequency audible sound dissipates and the mini-tsunami and seiche sounds dominate (infrasound plus high frequencies) and continue for over 10 min. The specific frequencies and duration of each phase of a calving event depend on its size and location and the glacier and fjord characteristics.

Formation of ice-shelf moraines by accretion of sea water and marine sediment at the southern margin of the McMurdo Ice Shelf, Antarctica

Abstract: A combination of ground-penetrating radar surveys, physical sedimentology and ice composition measurements has been used to characterize ice and sediment accreted to the southern margin of the McMurdo Ice Shelf, Antarctica. The radar data and surface observations show that the ice-shelf margin consists of strongly layered debris-rich ice that contains marine sediment and fossils. A modified Rayleigh-based distillation system has been used to model the isotopic fractionation from sea water to ice in a closed system. The model of ice formation is consistent with formation during almost complete freezing of a sea-water reservoir. By contrast, ice on the upstream side of the grounding line has formed during the early stages of freezing in which a small fraction of the sea-water reservoir has frozen. The model results and the presence of delicate, well-preserved marine fossils are interpreted as evidence of anchor ice formation on the seabed, and rafting of glaciomarine sediment into the bottom of the ice shelf. We argue that repeated accretion of sea water and marine sediment has produced a stacked sequence of ice and glaciomarine debris that forms shore-parallel ice-cored moraines.

The influence of ice melange on fjord seiches

Abstract: We compute the eigenmodes (seiches) of the barotropic and baroclinic hydrodynamic equations for an idealized fjord having length and depth scales similar to those of Ilulissat Icefjord, Greenland, into which Jakobshavn Isbrae (also known as Sermeq Kujalleq) discharges. The purpose of the computation is to determine the fjord's seiche behavior when forced by iceberg calving, capsize and melange movement. Poorly constrained bathymetry and stratification details are an acknowledged obstacle. We are, nevertheless, able to make general statements about the spectra of external and internal seiches using numerical simulations of ideal one-dimensional channel geometry. Of particular signifi- cance in our computation is the role of weakly coupled ice melange, which we idealize as a simple array of 20 icebergs of uniform dimensions equally spaced within the fjord. We find that the presence of these icebergs acts to (1) slow down the propagation of both external and internal seiches and (2) introduce band gaps where energy propagation (group velocity) vanishes. If energy is introduced into the fjord within the period range covered by a band gap, it will remain trapped as an evanescent oscillatory mode near its source, thus contributing to localized energy dissipation and ice/melange fragmentation.

Rock debris in an Antarctic ice shelf

Abstract: We have discovered a band of stones and coarse sand in the Ronne Ice Shelf, Antarctica, some 60m above the ice shelf’s base, 40km from its seaward edge and 420km from the point where the ice originally went afloat. A study of ice-sounding radar data from across the Ronne Ice Shelf has revealed other areas likely to contain debris in significant quantities. It appears that basal debris at the margins of ice streams feeding the ice shelf can be buried in the ice shelf by sea water freezing-on at the ice-shelf base. These findings are evidence for a mechanism active in a present-day ice-sheet/shelf system, which enables icebergs to transport large volumes of ice-rafted debris, and which also provides a potential mechanism for the formation of ice rises near ice fronts. We anticipate that a seismics study of debris melted from the ice shelf and deposited beneath will provide a valuable control on the history of ice-shelf–ocean interactions.

Relationships between iceberg plumes and sea-ice conditions on northeast Devon Ice Cap, Nunavut, Canada

Abstract: This study investigates the impact of sea-ice conditions on the production of iceberg plumes from two tidewater glaciers on Devon Ice Cap, Nunavut, Canada. These effects are quantified using a 12 year RADARSAT-1 satellite record from 1997–2008 that contains imagery from approximately every 1–2 weeks in the winter and every 1–4 days in the summer. Iceberg plumes identified in this record are verified against terrestrial time-lapse photography of Belcher Glacier from 2007–08. Results suggest a strong relationship between iceberg plumes and the retreat of sea ice from the glacier termini, with the plumes caused by both the release of previously calved icebergs (ice melange) and new glacier calving. Iceberg plumes are also sometimes observed at other times in the summer and in midwinter (occasionally on both glaciers simultaneously), with these events likely due to new glacier calving alone. Analysis of tides and air temperatures suggests that they provide a minor influence on the timing of iceberg plumes. Instead, it appears that changes in the presence of sea ice are dominant on seasonal timescales, although internal glacier dynamics likely play a significant role for winter plume events that occur when substantial thicknesses of landfast sea ice are present.