Snow accumulation, surface height change, and firn densification at Summit, Greenland: Insights from 2 years of in situ observation

Weekly measurements of surface height change were made at an accumulation forest of 100 stakes at Summit, Greenland, over a 2-year period (17 August 2000 to 8 August 2002). On average, the surface height relative to the stakes increased 64 (±4.8) cm in the first year and 65 (±5.3) cm in the second, identical to the average (65 ± 4.5 cm yr−1) previously reported for the period 1991–1995 in a similar forest 28 km to the southwest. The continuous 2-year data set indicates that the rate of surface rise was not constant, with the summers of 2001 and 2002 both showing markedly slower increases. On-site weather observations suggest that more new snow fell during the summer months than in any other season, consistent with results from previous snow pit and modeling studies yet apparently at odds with the slow rate of height increase. Density profiles from a series of 1-m-deep snow pits sampled monthly reveal that the thickness of the most recent year of accumulated snow (25 cm water equivalent) decreased rapidly between late May and early July, and the layers remained thin through early September. The thinning of the top year is clearly due to compaction in the snowpack. Combining the observed variations in annual layer thickness with a linear height increase based on assumed constant accumulation at 0.18 cm d−1 explains much of the variation in surface height found in the stake measurements. Estimated surface height changes can be forced to exactly match the stake measurements by combining changes in annual layer thickness with a variable accumulation rate over the intervals between pits. This exercise suggests that during the 2 years of this study a consistent seasonal pattern in accumulation was not apparent, rather the intervals indicated to have had enhanced accumulation in the first year (August–October and March–April) apparently had reduced accumulation in the second year

Interannual variability in North American grassland biomass/productivity detected by SeaWinds scatterometer backscatter

We analyzed 2000–2004 growing-season SeaWinds Ku-band microwave backscatter and MODIS leaf area index (LAI) data over North America. Large anomalies in mid-growing-season mean backscatter and LAI, relative to 5-year mean values, occurred primarily in the western Great Plains; backscatter and LAI anomalies had similar spatial patterns across this region. Backscatter and LAI time series data for three ∼103 km2 regions in the western Great Plains were strongly correlated (r2 ∼ 0.6–0.8), and variability in mid-growing season values was well-correlated with annual precipitation (October through September). The results indicate that SeaWinds backscatter is sensitive to interannual variability in grassland biomass/productivity, and can provide an assessment that is completely independent of optical/near-infrared remote sensing instruments

Characteristics of the Bed of the Lower Columbia Glacier, Alaska

An unplanned, but unique, experiment has given an in situ measurement of the strength of deforming subglacial till under the central region of a major valley glacier. We report on both planned and unplanned borehole investigations of the subglacial shear zone of Columbia Glacier, southeast Alaska. Basal samples, coring and down-hole water samples show that the fiord-filling lower reach of the glacier is underlain by a thin, ∼ 7-cm, veneer of rock debris. Fluidized debris intruded at least a meter up the borehole. At a higher site, 13 km from the terminus and above the fiord, probing, samples, and the bending of a drill stem, which was stuck in the basal zone for 5 days, showed that the basal till layer was ∼ 65 cm thick. Horizontal velocity of the till decreased monotonically downward from the ice/till interface. Till at the interface moved with the ice velocity. Plastic deformation of the drill stem gave an estimate of the strength of the basal till, which is normally described as a viscoplastic material. If the till is assumed to be either perfectly plastic or Newtonian viscous, then the strengths are as follows; the plastic yield strength of the till was 5.5×10^3 Pa (0.055 bar) with an upper bound of 1.3 ×10^4 Pa (0.13 bar), while the nominal viscosity was of the order of 2×10^8 Pa s (2×10^9 poise), with an upper bound of 5×10^8 Pa s. In neither case is the till “strength” enough to supply the bulk basal shear stress to resist the glacier flow

Possible Mechanisms for Glacial Earthquakes

The large glacial earthquakes reported on by Ekström et al. (2003, 2006) and Tsai and Ekström (2007) have previously been evaluated in terms of their seismic characteristics. In this paper we attempt to take constraints such as known glacial ice properties, outlet glacier size, calving style, and meltwater variability to construct a self-consistent physical model of the glacial earthquake process. Since many glaciological parameters are poorly constrained, we parameterize a number of important processes and estimate a wide range of possible values for some properties. The range of model outputs is thus fairly large, but it is still difficult to match observational constraints under most conditions. We find that only a small class of models is able to satisfy the major observational constraints. These models are characterized by (1) lost basal resistance coupled to viscoelastic deformation with extensive internal crevassing or with low effective elastic modulus and possibly low effective viscosity or (2) by nonequilibrium calving, such as having large icebergs capsize into the glacier front. Although observational constraints cannot definitively rule out any of the proposed classes of mechanisms, the calving model has much stronger support. Fortunately, the various models make different predictions regarding observables that can potentially be measured in the near future.Earth and Planetary SciencesEngineering and Applied Science

Mechanical and hydrologic basis for the rapid motion of a large tidewater glacier 2. Interpretation

The data presented in part 1 of this paper (Meier et al., this issue) are here used to assess the role of water input/output, water storage, and basal water pressure in the rapid movement of Columbia Glacier, Alaska. Consistently high basal water pressures, mostly in the range from 300 kPa below to 100 kPa above the ice overburden pressure, are responsible in an overall way for the high glacier flow velocities (3.5–9 m d^−1), which are due mainly to rapid basal sliding caused by the high water pressure. Diurnal fluctuation in basal water pressure is accompanied by fluctuation in sliding velocity in what appears to be a direct causal relation at the upglacier observation site. The water pressure fluctuation tracks the time-integrated water input (less a steady withdrawal), as expected for the diurnally fluctuating storage of water in the glacier far from the terminus. At the downglacier site, the situation is more complex. Diurnal peaks in water level, which are directly related to intraglacial water storage as well as to basal water pressure, are shifted forward in time by 4 hours, probably as a result of the effect of diurnal fluctuation in water output from the glacier, which affects the local water storage fluctuations near the terminus. Because of the forward shift in the basal water pressure peaks, which at the downglacier site lead the velocity peaks by 6 hours, a mechanical connection between water pressure and sliding there would have to involve a 6-hour (quarter period) delay. However, the nearly identical nature of the diurnal fluctuations in velocity at the two sites argues for a single, consistent control mechanism at both sites. The velocity variations in nondiurnal “speed-up events” caused by extra input of water on the longer timescale of several days are only obscurely if at all correlated with variations in basal water pressure but correlate well with water storage in the glacier. It appears that small variations in water pressure (≤100 kPa) sufficient to produce the observed velocity variations (15–30%) are mostly masked by pressure fluctuations caused by spontaneous local reorganizations of the basal water conduit system on a spatial scale much smaller than the longitudinal coupling length over which basal water pressure is effectively averaged in determining the sliding velocity. At the achieved level of observation the clearest (though not complication free) control variable for the sliding velocity variations is basal water storage by cavitation at the glacier bed

The Link Between Climate Warming and Break-Up of Ice Shelves in the Antarctic Peninsula

A review of in situ and remote-sensing data covering the ice shelves of the Antarctic Peninsula provides a series of characteristics closely associated with rapid shelf retreat: deeply embayed ice fronts; calving of myriad small elongate bergs in punctuated events; increasing flow speed; and the presence of melt ponds on the ice-shelf surface in the vicinity of the break-ups. As climate has warmed in the Antarctic Peninsula region, melt-season duration and the extent of ponding have increased. Most break-up events have occurred during longer melt seasons, suggesting that meltwater itself, not just warming, is responsible. Regions that show melting without pond formation are relatively unchanged. Melt ponds thus appear to be a robust harbinger of ice-shelf retreat. We use these observations to guide a model of ice-shelf flow and the effects of meltwater. Crevasses present in a region of surface ponding will likely fill to the brim with water. We hypothesize (building on Weertman (1973), Hughes (1983) and Van der Veen (1998)) that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat. A thermodynamic finite-element model is used to evaluate ice flow and the strain field, and simple extensions of this model are used to investigate crack propagation by meltwater. The model results support the hypothesis

Tectonically controlled subglacial lakes on the flanks of the Gamburtsev Subglacial Mountains, East Antarctica

The morphology of surface lakes strongly influences their ecology and limnology (Wetzel, 2001). This morphology is a result of both the geologic processes that produce topographic basins and the regional climatic and local hydrologic processes that control water depth and sediment infilling (Carroll and Bohacs, 1999). Although basin forming processes range from glacial scour to meteorite impacts (Cohen, 2003), the deepest, oldest surface lakes are tectonically controlled (Meybeck, 1995) and contain diverse exotic ecosystems (Rossiterm and Kawanabe, 2000). Subglacial lakes are also thought to be ancient systems that may contain exotic biota (Bulat et al., 2004; Karl et al., 1999; Priscu et al., 1999). Here we present evidence for the scale and configuration of 2 large subglacial lakes in East Antarctica that together with Lake Vostok define a province of major lakes on the flanks of the Gamburtsev Subglacial Mountains. Spatially-defined in the new Moderate Resolution Imaging Spectroradiometer (MODIS) imagery of Antarctica (T. Scambos et al., A MODIS-based mosaic of Antarctica: MOA, submitted to Remote Sensing of Environment, 2005, hereinafter referred to as Scambos et al., submitted manuscript, 2005), these lakes are aligned parallel to Lake Vostok. Other data shows that they are distinguished by distinct gravity lows, flat ice surface slopes and have estimated water depths of at least 900 m. Surface elevation data indicates that large deep subglacial lakes have a profound influence on the regional ice sheet topography and probably ice sheet flow. These deep subglacial lakes with elongate, rectilinear morphology are tectonically controlled features. Unlike the shallow lakes in West Antarctica and beneath Dome Concordia, these deep subglacial lakes remained stable environments through many glacial cycles since their origin 10-35 Ma enabling the development of novel ecosystems

Seasonal and interannual variations in ice melange and its impact on terminus stability, Jakobshavn Isbræ, Greenland

We used satellite-derived surface temperatures and time-lapse photography to infer temporal variations in the proglacial ice melange at Jakobshavn Isbræ, a large and rapidly retreating outlet glacier in Greenland.We used satellite-derived surface temperatures and time-lapse photography to infer temporal variations in the proglacial ice melange at Jakobshavn Isbræ, a large and rapidly retreating outlet glacier in Greenland. Freezing of the melange-covered fjord surface during winter is indicated by a decrease in fjord surface temperatures and is associated with (1) a decrease in ice melange mobility and (2) a drastic reduction in iceberg production. Vigorous calving resumes in spring, typically abruptly, following the steady up-fjord retreat of the sea-ice/ice-melange margin. An analysis of pixel displacement from time-lapse imagery demonstrates that melange motion increases prior to calving and subsequently decreases following several events. We find that secular changes in ice melange extent, character and persistence can influence iceberg calving, and therefore glacier dynamics over daily-to-monthly timescales, which, if sustained, will influence the mass balance of an ice sheet.This research was supported by funds from the Gordon and Betty Moore Foundation (GBMF2627), NASA (NNX08AN74G), the US National Science Foundation (ANT0944193 and ANS0909552) and the New Hampshire Space Grant Consortium (NNX10AL97H). We thank CH2M HILL Polar Services and Air Greenland for logistics support, and PASSCAL (Program for the Array Seismic Studies of theContinental Lithosphere) for the use of seismic instrumentation. Ian Joughin derived TerraSAR-X velocities and terminus positions from images provided by the German (DLR) space agency under NASA grant NNX08AL98A. We acknowledgethe use of Rapid Response imagery from the Land Atmosphere Near-real time Capability for EOS (LANCE) system operated by the NASA/GSFC/Earth Science Data and Information System (ESDIS) with funding provided by NASA HQ. Glacier surface elevations were provided by CReSIS, and bed elevations by CReSIS and Mathieu Morlighem. The manuscript was significantly improved by comments from Tim Bartholomaus and an anonymous reviewer.Ye

Ice flow of Humboldt, Petermann and Ryder Gletscher, northern Greenland

This is the published version. Copyright 1999 International Glaciological SocietyRadar interferometry, ice-penetrating radar profiles and an elevation model are used to determinc the veloeity fields, rates of ice discharge, approximate states of balance and catchment area for three large outlet glaciers in northeast Greenland. Discharge through flux gates is calculated for Humboldt and Petermann Gletscher, which are found to be in balance (at the level that the accumulation is known). A large diflerence between the measured and estimated fluxes for Ryder Gletscher may be a reflection of unsteady flow behavior for this glacier. The patterns of ice flow for the threc glaciers considered are each unique, showing that the nature of ice discharge varies substantially from basin to basin, controlled by bed conditions and the presence of subglacial troughs and obstructions