The generalized (Glen) flow relation for ice, involving the second invariants of the stress deviator and strain-rate tensors, is only expected to hold for isotropic polycrystalline ice. Previous single-stress experiments have shown that for the steady-state flow, which develops at large strains, the tertiary strain rate is greater than the minimum (secondary creep) value by an enhancement factor which is larger for shear than compression. Previous experiments combining shear with compression normal to the shear plane have shown that enhancement of the tertiary octahedral strain rate increases monotonically from compression alone to shear alone. Additional experiments and analyses presented here were conducted to further investigate how the separate tertiary shear and compression strain-rate components are related in combined stress situations. It is found that tertiary compression rates are more strongly influenced by the addition of shear than is given by a Glen-type flow relation, whereas shear is less influenced by additional compression. A scalar function formulation of the flow relation is proposed, which fits the tertiary creep data well and is readily adapted to a generalized form that can be extended to other stress configurations and applied in ice mass modelling.

Sea ice is generally covered with snow, which can vary in thickness from a few centimeters to >1 m. Snow cover acts as a thermal insulator modulating the heat exchange between the ocean and the atmosphere, and it impacts sea-ice growth rates and overall thickness, a key indicator of climate change in polar regions. Snow depth is required to estimate sea-ice thickness using freeboard measurements made with satellite altimeters. The snow cover also acts as a mechanical load that depresses ice freeboard (snow and ice above sea level). Freeboard depression can result in flooding of the snow/ice interface and the formation of a thick slush layer, particularly in the Antarctic sea-ice cover. The Center for Remote Sensing of Ice Sheets (CReSIS) has developed an ultra-wideband, microwave radar capable of operation on long-endurance aircraft to characterize the thickness of snow over sea ice. The low-power, 100 mW signal is swept from 2 to 8 GHz allowing the air/snow and snow/ ice interfaces to be mapped with 5 cm range resolution in snow; this is an improvement over the original system that worked from 2 to 6.5 GHz. From 2009 to 2012, CReSIS successfully operated the radar on the NASA P-3B and DC-8 aircraft to collect data on snow-covered sea ice in the Arctic and Antarctic for NASA Operation IceBridge. The radar was found capable of snow depth retrievals ranging from 10 cm to >1 m. We also demonstrated that this radar can be used to map near-surface internal layers in polar firn with fine range resolution. Here we describe the instrument design, characteristics and performance of the radar.

Consumer-grade digital cameras have become ubiquitous accessories of science. Particularly in glaciology, the recognized importance of short-term variability has motivated their deployment for increasingly time-critical observations. However, such devices were never intended for precise timekeeping, and their use as such needs to be accompanied by appropriate management of systematic, rounding and random errors in reported image times. This study describes clock drift, subsecond reporting resolution and timestamp precision as the major obstacles to precise camera timekeeping, and documents the subsecond capability of camera models from 17 leading manufacturers. We present a complete and accessible methodology to calibrate cameras for absolute timing and provide a suite of supporting scripts. Two glaciological case studies serve to illustrate how the methods relate to contemporary investigations: (1) georeferencing aerial photogrammetric surveys with camera positions time-interpolated from GPS tracklogs; and (2) coupling videos of glacier-calving events to synchronous seismic waveforms.

Marine-terminating glaciers may experience seasonal and short-term flow variations, which can impact rates of ice flux through the glacier terminus. We explore the relationship between variability in the flow of a large tidewater glacier (Belcher Glacier, Nunavut, Canada), the seasonal cycle of surface meltwater production and the rapid drainage of supraglacial lakes. We demonstrate a novel method for analyzing time-lapse photography to quantify lake area change rates (a proxy for net filling and drainage rates) and develop a typology of lake drainage styles. GPS records of ice motion reveal four flow acceleration events which can be linked to lake drainage events discovered in the time-lapse photography. These events are superimposed on a longer pattern of velocity variation that is linked to seasonal variation in surface melting. At the terminus of the glacier, the ice displacement associated with the lake drainage events constitutes ∼10% of the seasonally accelerated displacement or 0.4% of the total annual ice displacement (336 m a−1). While the immediate ice response to these individual perturbations may be small, these drainage events may enhance overall seasonal acceleration by opening and/or sustaining meltwater conduits to the glacier bed.

Freezing temperatures, desiccation and high levels of solar radiation make the surface of the Antarctic ice sheet one of Earth’s harshest habitats. However, our study in the Vestfold Hills area of East Antarctica shows that favourable conditions for microbial production become established just beneath the surface of blue-ice areas, which collectively cover about 2% of the ice-sheet periphery. Their translucent, wind-polished surface allows solar heating to create meltwater in a greenhouse-type environment at depths of up to 1 m. Melting is intensified around dark debris particles, or cryoconite, where we found microbiological activity to be greatest. Rates of photosynthesis (average 2060 ng C (g cryoconite)−1 d−1) were adapted to low light intensities (∼10% of surface irradiance values) and most likely dominated by cyanobacteria and Chloroplastida. A heterotrophic bacterial community was also found to be active within the cryoconite, although average bacterial growth rates (5.7 ng C (g cryoconite)−1 d−1) were far lower than average community respiration (1870 ng C (g cryoconite)−1 d−1). The majority of the respired carbon was most likely associated with the autotrophs and several protists. Therefore, blue-ice areas constitute oases for microbial life around the periphery of Earth’s coldest ice sheet.

The Automated Meteorology–Ice/Indigenous species–Geophysics Observation System (AMIGOS) consists of a set of measurement instruments and camera(s) controlled by a single-board computer with a simplified Linux operating system and an Iridium satellite modem supporting two-way communication. Primary features of the system relevant to polar operations are low power requirements, daily data uploading, reprogramming, tolerance for low temperatures, and various approaches for automatic resets and recovery from low power or cold shutdown. Instruments include a compact weather station, single- or dual-frequency GPS, solar flux and reflectivity sensors, sonic snow gauges, simplified radio-echo sounder, and resistance thermometer string in the firn column. In the current state of development, there are two basic designs. One is intended for in situ observations of glacier conditions. The other supports a high-resolution camera for monitoring biological or geophysical systems from short distances (100 m to 20 km). The stations have been successfully used in several locations for operational support, monitoring rapid ice changes in response to climate change or iceberg drift, and monitoring penguin colony activity. As of August 2012, there are nine AMIGOS systems installed, all on the Antarctic continent or in the surrounding ocean.

We have investigated the isothermal densification of new snow under an external mechanical stress. New snow samples that mimic natural snow were made in the laboratory by sieving ice crystals grown in a snowmaker. This allowed us to assemble homogeneous initial samples with reproducible values of low density and high specific surface area (SSA). Laboratory creep experiments were conducted in an X-ray microtomograph at −20°C for 2 days. We focused on the evolution of density and SSA as a function of constant stress at a single temperature. External mechanical stresses resembled natural overburden stresses of a snow sample at depths of ∼0–30 cm of new snow. We demonstrate that densification increases with higher external stress and lower initial densities. We find that the evolution of the SSA is independent of the density and follows a unique decay for all measurements of the present type of new snow. The results suggest that details of the SSA decrease can be investigated using carefully designed experiments of short duration which are convenient to conduct. Additionally, we calculated the strain evolution and identify transient creep behavior that does not follow the Andrade creep law of denser snow or polycrystalline ice.

Sintering rates in snow were investigated by measuring changes in penetration resistance with time and by using a numerical snow metamorphism model. Periodic Snow Micro Penetrometer (SMP) measurements were performed on uniform snow samples covering a wide range of densities. The mean penetration resistance of snow increased with time according to a power law with an average exponent of 0.18. Simulated changes in the bond-to-grain ratio for simplified spherical ice grains followed a power law with an average exponent of 0.18, showing that the mean penetration resistance, as measured with the SMP, closely relates to bond-to-grain ratio in snow. For lower-density snow samples, consisting mostly of dendritic snow morphologies, the measured increase in penetration resistance was lower. This is likely the result of two competing processes: (1) strengthening of the snow sample due to the creation and growth of bonds and (2) weakening of the snow sample due to the formation of unbonded small rounded particles at the expense of larger dendritic forms. On the other hand, the rate of increase in penetration resistance for snow samples consisting of closely packed depth hoar was similar to that of rounded grains.

Ten ice-sheet models are used to study sensitivity of the Greenland and Antarctic ice sheets to prescribed changes of surface mass balance, sub-ice-shelf melting and basal sliding. Results exhibit a large range in projected contributions to sea-level change. In most cases, the ice volume above flotation lost is linearly dependent on the strength of the forcing. Combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments, suggesting that nonlinear feedbacks are modest. Our models indicate that Greenland is more sensitive than Antarctica to likely atmospheric changes in temperature and precipitation, while Antarctica is more sensitive to increased ice-shelf basal melting. An experiment approximating the Intergovernmental Panel on Climate Change’s RCP8.5 scenario produces additional first-century contributions to sea level of 22.3 and 8.1 cm from Greenland and Antarctica, respectively, with a range among models of 62 and 14 cm, respectively. By 200 years, projections increase to 53.2 and 26.7 cm, respectively, with ranges of 79 and 43 cm. Linear interpolation of the sensitivity results closely approximates these projections, revealing the relative contributions of the individual forcings on the combined volume change and suggesting that total ice-sheet response to complicated forcings over 200 years can be linearized.

Debris-covered glaciers respond to atmospheric conditions in different ways from debris-free glaciers, due to the presence of debris at the surface during the ablation season and at the snow/ice interface during the accumulation season. Understanding the response of debris-covered glaciers to a variety of meteorological conditions in a physically sound manner is essential to quantify meltwater discharge and to predict their response to climate change. To tackle this issue, we developed the Crocus-DEB model as an adaptation of the detailed snowpack model Crocus, to simulate the energy and mass balance of debris-covered glaciers, including periods when debris is covered by snow. Crocus-DEB was evaluated with data gathered during a field experiment using artificial debris covering the snowpack at Col de Porte, France, with very good results in terms of conductive heat flux, both at the surface and at the interface between the debris and the underlying dense snow taken as a surrogate for ice, with and without snow overlying the debris. The model was also evaluated using field data from the debris-covered glacier Changri Nup, Nepal, Himalaya. This paper introduces the design of the model, its performance and its ability to explore relationships between model parameters, meteorological conditions and the critical debris thickness.

Nanospray ionization Fourier transform ion cyclotron resonance mass spectrometry was developed as a tool for examining the dissolved organic matter (DOM) present in ice cores, significantly reducing the sample volume required for extraction compared with previous studies. Using smaller sample volumes is important for resolving temporal changes in ice-core DOM. We demonstrate that a small volume of ice-core meltwater can be extracted and analyzed with sufficient mass spectral sensitivity to describe the molecular composition of the DOM. Two different solid-phase extraction resins (PPL and C18) were compared to examine potential fractionation of the DOM. Over 2500 unique molecular formulas isolated from two different Greenland ice-core samples, dated to approximately AD 1660 and AD 1360, were obtained from the analyses. Method and sensitivity improvements provide an opportunity for organic matter characterization at higher temporal resolution than our previous analyses afforded and allows for examination of ice cores containing low levels of organic carbon.

The evolution of preferred crystal-orientation fabrics is strongly sensitive to the initial fabric and texture. A perturbation in climate can induce variations in fabric and texture in the firn. Feedbacks between fabric evolution and ice deformation can enhance these variations through time and depth in an ice sheet. We model the evolution of fabric under vertical uniaxial-compression and pure-shear regimes typical of ice divides. Using an analytic anisotropic flow law applied to an aggregate of distinct ice crystals, the model evolves the fabric and includes parameterizations of crystal growth, polygonization and migration recrystallization. Stress and temperature history drive the fabric evolution. Using this model, we explore the evolution of a subtle variation in near-surface fabric using both constant applied stress and a stress–temperature history based on data from Taylor Dome, East Antarctica. Our model suggests that a subtle variation in fabric caused by climate perturbations will be preserved through time and depth in an ice sheet. The stress history and polygonization rate primarily control the magnitude of the preserved climate signal. These results offer the possibility of extracting information about past climate directly from ice fabrics.

We observed several swarms of repeating low-frequency (1–5 Hz) seismic events during a 3 week period in May–June 2010, near the summit of Mount Rainier, Washington, USA, that likely were a result of stick–slip motion at the base of alpine glaciers. The dominant set of repeating events (‘multiplets’) featured >4000 individual events and did not exhibit daytime variations in recurrence interval or amplitude. Volcanoes and glaciers around the world are known to produce seismic signals with great variability in both frequency content and size. The low-frequency character and periodic recurrence of the Mount Rainier multiplets mimic long-period seismicity often seen at volcanoes, particularly during periods of unrest. However, their near-surface location, lack of common spectral peaks across the recording network, rapid attenuation of amplitudes with distance, and temporal correlation with weather systems all indicate that ice-related source mechanisms are the most likely explanation. We interpret the low-frequency character of these multiplets to be the result of trapping of seismic energy under glacial ice as it propagates through the highly heterogeneous and attenuating volcanic material. The Mount Rainier multiplet sequences underscore the difficulties in differentiating low-frequency signals due to glacial processes from those caused by volcanic processes on glacier-clad volcanoes.

Repeated Global Navigation Satellite Systems (GNSS) observations were carried out at 50 surface markers in the Vostok Subglacial Lake (East Antarctica) region between 2001 and 2011. The horizontal ice flow velocity vectors were derived with accuracies of 1 cm a−1 and 0.5°, representing the first reliable information on ice flow kinematics in the northern part of the lake. Within the lake area, ice flow velocities do not exceed 2 m a−1. The ice flow azimuth is southeast in the southern part of the lake and turns gradually to east-northeast in the northern part. In the northern part, as the ice flow enters the lake at the western shore, the velocity decreases towards the central lake axis, then increases slightly past the central axis. In the southern part, a continued acceleration is observed from the central lake axis across the downstream grounding line. Based on the observed flow velocity vectors and ice thickness data, mean surface accumulation rates are inferred for four surface segments between Ridge B and Vostok Subglacial Lake and show a steady increase towards the north.