ALBERT

Description: Added are 5 seismic reflection data sets of Store Glacier, a tide water glacier in West Greenland Uummannaq Fjord. Two crossing profiles were recorded, 20140513, along the ice flow and 20140514, across the ice flow.

Description: We present ice thickness and bed topography maps with a high spatial resolution (250-500 m) of a land-terminating section of the Greenland Ice Sheet derived from ground-based and airborne radar surveys. The data have a total area of ~12 000 km^2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of Isunnguata Sermia Glacier is overdeepened and reaches an elevation of ~500 m below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The compiled data sets of ground-based and airborne radar surveys cover one of the most studied regions of the Greenland Ice Sheet and can be valuable for detailed studies of ice sheet dynamics and hydrology.

Description: SF6 gas traces were injected into Moulin L41A, and samples were collected in the proglacial river near Leverett Glacier terminus (67.09N, -50.23E). A total of 11 traces were carried out between 16 June and 6 August, 2012. Tracers were injected at ~19:00 hrs, shortly after the peak daily moulin discharge (typically between 15:00 and 19:00 hrs). Tracer concentrations were monitored by collecting water samples from the proglacial river draining Leverett Glacier and were analysed by a Cambridge Scientific 300-series gas chromatograph fitted with an electron capture detector. Analytical errors (estimated from the analysis of duplicate samples) were below 15% and the level of detection of SF6 was below 0.005 parts per trillion by mass in water. The analytical method was more sensitive than that employed in our previous study at this site (Chandler et al., 2013, http://dx.doi.org/10.1038/ngeo1737) and is described in detail by Chandler et el. (In review, Earth and Planetary Science Letters)

Description: Supraglacial river discharge was measured by monitoring water depth with a custom-built pressure sensor, at 1-minute intervals, and converting depth to discharge with a rating curve established from salt dilution gauging. Here we report 1-hour means of the 1-minute measurements. This method is well suited to supraglacial river gauging as the electrical conductivity of supraglacial melt water is very low. The pressure sensor was installed on the bottom of the channel and was able to move downwards with the ice surface as the channel gradually incised. Full details are provided in the online supplement to Wadham et al. (2016) at https://doi.org/10.5194/bg-13-6339-2016.

Description: Surface ablation rates were measured daily using changes in ice surface height at five ablation stakes arranged in a cross configuration at ~2 m separation. The stakes were located in the supraglacial hydrological catchment feeding Moulin L41A. The stakes were installed in holes deeper than the length of the stake (so each measurement of ice surface height was made from the ice surface down to the top of the stake) to avoid the problem of enhanced surface melting caused by solar radiation absorbed by the stake. All ablation measurements were carried out by the same observer to ensure consistency for example in the interpretation of the level of a rough ice surface. A full description was provided by Chandler et al. (2015) at https://doi.org/10.5194/tc-9-487-2015.

Description: Ice surface motion was recorded by five dual-frequency Leica SR520 GPS receivers deployed on poles drilled 2 m into the ice surface, within 700 m of Moulin L41A at 66.97N -49.27E. GPS data were post-processed kinematically (King, 2004, http://dx.doi.org/10.3189/172756504781829747) with Track v.1.27 software (Chen, 1998, Ph.D. thesis, Cambridge MA, USA) against bedrock-mounted reference stations using a precise ephemeris from the International GNSS Service )Dow et al., 2009, http://dx.doi.org/10.1007/s00190-008-0300-3). Reference stations were located 1 km from the terminus of Russell Glacier and at Kellyville, giving baseline lengths less than 41 km. Due to gaps in the time series caused by power outage, we averaged the horizontal velocities recorded at the five stations with the fewest gaps to give a single record. Positions were recorded at 30 s intervals; 1-hr means were then smoothed using a 5-point binomial filter. Since there was generally little difference in velocity between the stakes, the mean velocity across the network gives a better indication of the seasonal pattern of ice motion with fewer gaps than in the individual records. Velocities are centred differences of hourly displacements. GPS stakes required periodic re-drilling as they gradually melted out.

Description: Proglacial river discharge was monitored using stage measurements (collected by a HOBO pressure sensor) and dye dilution gauging at a stable bedrock section near the terminus of Leverett Glacier, 67.09N -50.23E. Here we report hourly means of measurements made at 1 minute intervals. The same method has been used at this site over several melt seasons and is described in detail by Bartholomew et al. (2011, http://dx.doi.org/10.1029/2011GL047063) and Tedstone et al. (2013, http://dx.doi.org/10.1073/pnas.1315843110).

Description: Outlet glaciers of the Greenland Ice Sheet transport ice from the interior to the ocean and contribute directly to sea level rise because because discharge and ablation often exceed the accumulation. To develop a better understanding of these fast flowing glaciers, we investigate the basal conditions of Store Glacier, a large outlet glacier flowing into Uummannaq Fjord in West Greenland. We use two crossing seismic profiles acquired near the centreline, 30 km upstream of the calving front, to interpret the physical nature of the ice and bed. We identify one notably englacial and two notably subglacial seismic reflections on both profiles. The englacial reflection represents a change in crystal orientation fabric, interpreted to be the Holocene–Wisconsin transition. From Amplitude Versus Angle (AVA) analysis we infer that the deepest ∼80 m of ice of the parallel-flow profile below this reflection is anisotropic with an enhancement of simple shear of ∼2. The ice is underlain by ∼45 m of unconsolidated sediments, below which there is a strong reflection caused by the transition to consolidated sediments. In the across-flow profile subglacial properties vary over small scale and the polarity of the ice–bed reflection switches from positive to negative. We interpret these as patches of different basal slipperiness associated with variable amounts of water. Our results illustrate variability in basal properties, and hence ice-bed coupling, at a spatial scale of ∼100 m, highlighting the need for direct observations of the bed to improve the basal boundary conditions in ice-dynamic models.

Description: Marine-terminating outlet glaciers of the Greenland ice sheet make significant contributions to global sea level rise, yet the conditions that facilitate their fast flow remain poorly constrained owing to a paucity of data. We drilled and instrumented seven boreholes on Store Glacier, Greenland, to monitor subglacial water pressure, temperature, electrical conductivity and turbidity along with englacial ice temperature and deformation. These observations were supplemented by surface velocity and meteorological measurements to gain insight into the conditions and mechanisms of fast glacier flow. Located 30km from the calving front, each borehole drained rapidly on attaining ∼600m depth indicating a direct connection with an active subglacial hydrological system. Persistently high subglacial water pressures indicate low effective pressure (180 − 280 kPa), with small amplitude variations correlated with notable peaks in surface velocity driven by the diurnal melt cycle and longer periods of melt and rainfall. The englacial deformation profile determined from borehole tilt measurements indicates that 63-71% of total ice motion occurred at the bed, with the remaining 29-37% predominantly attributed to enhanced deformation in the lowermost 50-100 m of the ice column. We interpret this lowermost 100m to be formed of warmer, pre-Holocene ice overlying a thin (0 − 8 m) layer of temperate basal ice. Our observations are consistent with a spatially-extensive and persistently-inefficient subglacial drainage system that we hypothesize comprises drainage both at the ice-sediment interface and through subglacial sediments. This configuration has similarities to that interpreted beneath dynamically-analogous Antarctic ice streams, Alaskan tidewater glaciers, and glaciers in surge.

Description: Intense rainfall events significantly affect Alpine and Alaskan glaciers through enhanced melting, ice-flow acceleration and subglacial sediment erosion, yet their impact on the Greenland ice sheet has not been assessed. Here we present measurements of ice velocity, subglacial water pressure and meteorological variables from the western margin of the Greenland ice sheet during a week of warm, wet cyclonic weather in late August and early September 2011. We find that extreme surface runoff from melt and rainfall led to a widespread acceleration in ice flow that extended 140 km into the ice-sheet interior. We suggest that the late-season timing was critical in promoting rapid runoff across an extensive bare ice surface that overwhelmed a subglacial hydrological system in transition to a less-efficient winter mode. Reanalysis data reveal that similar cyclonic weather conditions prevailed across southern and western Greenland during this time, and we observe a corresponding ice-flow response at all land- and marine-terminating glaciers in these regions for which data are available. Given that the advection of warm, moist air masses and rainfall over Greenland is expected to become more frequent in the coming decades, our findings portend a previously unforeseen vulnerability of the Greenland ice sheet to climate change.