Neil F. Humphrey

Physical Conditions at the Base of a Fast Moving Antarctic Ice Stream

Boreholes drilled to the bottom of ice stream B in the West Antarctic Ice Sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1.6 bars of the ice overburden pressure. These conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick. The mechanics of ice streaming plays a role in the response of the ice sheet to climatic change.

Chemical weathering in glacial environments

Do glaciers enhance or inhibit chemical weathering rates relative to other environments? The importance of glaciers in the global carbon cycle and climate change hinges on the answer. We show that catchments occupied by active alpine glaciers yield cation denudation rates greater than the global mean rate but do not exceed rates in nonglacial catchments with similar water discharge. Silica denudation rates are distinctly lower in glacier-covered catchments than in their nonglacial counterparts. Because sediment yields are high from glaciers, this suggests that water flux, rather than physical erosion, exerts the primary control on chemical erosion by glaciers. Potassium and calcium concentrations are high relative to other cations in glacial water, probably due to dissolution of soluble trace phases, such as carbonates, exposed by comminution, and cation leaching from biotite. Preferential weathering of biotite may result in higher 87 Sr/ 86 Sr in glacial runoff than expected from whole-rock compositions. Thus, although glaciers do not influence total chemical denudation rates at a given runoff, they may yield compositionally distinctive chemical fluxes to the oceans. Disruption of mineral lattices by grinding increases dissolution rates; this and high surface area should make glacial sediments exceptionally weatherable. Weathering of glacial erosion products in environments beyond the glacier margin deserves attention because it may figure prominently in global chemical cycles.

Hydrology, erosion and sediment production in a surging glacier: Variegated glacier, Alaska, 1982-83

Outlet streams of Variegated Glacier, Alaska, U .S.A ., were observed before, during and after the surge of 1982-83. Measurements of discharge, suspended sediment and dissolved load in the outlet streams are presented for the years 198284, and comparisons are made with data from previous years. The data are interpreted to yield characteristics of the basal hydraulic system. The surging region of the glacier was underlain by a basal hydraulic zone of low water velocity and high water storage, inferred to be a distributed-flow system. The ice down-glacier of the propagating surge front was underlain by a highvelocity, low-storage zone, inferred to be a conduit system. The volume of water stored above the surge front was the major hydraulic control on the surge. Basal bedrock erosion during the surge was extreme in comparison to non-surging glaciers. The sediment output was directly proportional to the basal sliding, with a dimensionless erosion rate (meters eroded from the bed divided by meters of sliding) of order 1.0 x 10--4. Total erosion during the 20 year surge cycle was on the order of 0.3 m of bedrock, with approximately two-thirds occurring during the 2 years of the surge peak, and the bulk of this during the peak 2 months .

Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn

Surface melt on the Greenland ice sheet has shown increasing trends in areal extent and duration since the beginning of the satellite era. Records for melt were broken in 2005, 2007, 2010 and 2012. Much of the increased surface melt is occurring in the percolation zone, a region of the accumulation area that is perennially covered by snow and firn (partly compacted snow). The fate of melt water in the percolation zone is poorly constrained: some may travel away from its point of origin and eventually influence the ice sheet’s flow dynamics and mass balance and the global sea level, whereas some may simply infiltrate into cold snow or firn and refreeze with none of these effects. Here we quantify the existing water storage capacity of the percolation zone of the Greenland ice sheet and show the potential for hundreds of gigatonnes of meltwater storage. We collected in situ observations of firn structure and meltwater retention along a roughly 85-kilometre-long transect of the melting accumulation area. Our data show that repeated infiltration events in which melt water penetrates deeply (more than 10 metres) eventually fill all pore space with water. As future surface melt intensifies under Arctic warming, a fraction of melt water that would otherwise contribute to sea-level rise will fill existing pore space of the percolation zone. We estimate the lower and upper bounds of this storage sink to be 322 ± 44 gigatonnes and  gigatonnes, respectively. Furthermore, we find that decades are required to fill this pore space under a range of plausible future climate conditions. Hence, routing of surface melt water into filling the pore space of the firn column will delay expansion of the area contributing to sea-level rise, although once the pore space is filled it cannot quickly be regenerated.

Mechanical and hydrologic basis for the rapid motion of a large tidewater glacier: 1. Observations

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.

Basal Drainage System Response to Increasing Surface Melt on the Greenland Ice Sheet

Draining Through Ice Water formed by surface melting of the Greenland Ice Sheet is transferred rapidly to the underlying bedrock, but how the water is then dispersed is less clear. This question is important because how the ice-rock interface is lubricated affects how fast the ice sheet moves. Existing conceptual models are based on observations of mountain glaciers, but Meierbachtol et al. (p. 777; see the Perspective by Lüthi) now show that those ideas may not be applicable to the Greenland Ice Sheet. Measuring water pressures in a transect of 23 boreholes revealed that drainage structures differ between the edge, where large melt channels form, and further inland, where more distributed pathways are found. Basal drainage structures at the edges of the Greenland ice sheet differ from those found farther in the interior. [Also see Perspective by Lüthi] Surface meltwater reaching the bed of the Greenland ice sheet imparts a fundamental control on basal motion. Sliding speed depends on ice/bed coupling, dictated by the configuration and pressure of the hydrologic drainage system. In situ observations in a four-site transect containing 23 boreholes drilled to Greenland’s bed reveal basal water pressures unfavorable to water-draining conduit development extending inland beneath deep ice. This finding is supported by numerical analysis based on realistic ice sheet geometry. Slow meltback of ice walls limits conduit growth, inhibiting their capacity to transport increased discharge. Key aspects of current conceptual models for Greenland basal hydrology, derived primarily from the study of mountain glaciers, appear to be limited to a portion of the ablation zone near the ice sheet margin.

River incision or diversion in response to bedrock uplift

INTRODUCTIONMany researchers have examined the long-term behavior of alluvialrivers (i.e., Schumm et al., 1987; Snow and Slingerland, 1990; Paola et al.,1992). On long time scales, alluvial rivers act as sediment-transport systemsthat broadly deposit sediment over low areas in their basins (i.e.,Tucker andSlingerland, 1996). The behavior of bedrock rivers is less well understood,although most workers agree that bedrock erosion is some function of streampower (Howard and Kerby, 1983; Seidl and Dietrich, 1992; Howard et al.,1994). Accordingly,researchers addressing a river’s response to bedrock up-lift equate incision with relatively high stream power and diversion with rela-tively low stream power (e.g., Burbank et al., 1996). However, these studiesassume that the erosion rate of the bedrock channel across the mountainrange controls the eventual outcome; they have not examined the behavior ofthe alluvial river upstream and downstream of the uplift. We propose to lookat the entire erosional and depositional river system and examine the long-term response of a coupled alluvial-bedrock river system to tectonic uplift.Our motivation is general and aims at a greater understanding offluvial and tectonic interactions, but our model is necessarily both specificand simplified:the tectonic uplift of a bedrock massif into the path of a later-ally unconstrained alluvial river. Such a scenario has two possible out-comes: either the river maintains its original course and incises through theuplift, or the river diverts and forms a new course around the uplift. A sig-nificant complication in the analysis of this scenario is that the evolvingriver system is partial alluvial and partial bedrock channeled. Because thebehaviors of these two river types are different, modeling requires couplingdiverse systems. We construct an analytical model to perform a first-orderanalysis of this coupled system. Our objective is to identify the specific fac-tors that determine the final outcome of river diversion or incision.Our model focuses on long time scales (10

Natural oscillations in coupled geomorphic systems: An alternative origin for cyclic sedimentation

Internally oscillating cycles of cutting and filling occur naturally in many coupled geomorphic systems. As a demonstration, we have modeled stream entrenchment and backfilling where alluvial systems couple to uplifting mountain blocks. This simple model tracksbedrockstreamerosionandtransportofalluviumtobaselevel.Internaloscillations arise from coupling of a kinematic wave model for mountain erosion to a diffusion model for the alluvial basin. Decaying periodic cutting andfilling occur on time scales of 10 5 yr; the largest magnitudes are found at the coupling point. Model results suggest that cyclic sedimentation may result from the tintinnabulation of single perturbations applied to natural systems and need not record cyclic changes in climate, tectonics, or base level.

Rock glacier dynamics and paleoclimatic implications

Many rock glaciers contain massive ice that may be useful in paleoclimate studies. Interpreting geochemical ice-core records from rock glaciers requires a thorough understanding of rock glacier structure and dynamics. High-precision surface-velocity data were obtained for the Galena Creek rock glacier, Absaroka Mountains, Wyoming. Surface velocities range from 0 to 1.00 m/yr and vary across the rock glacier in a manner similar to true glaciers. We used Glens flow law to calculate the thickness of the deforming ice layer. The modeled ice thickness ranges from 0 to 50 m, and is confirmed by direct observations. This agreement shows that rock glacier movement can be entirely explained by deformation of massive ice within the rock glacier; neither basal sliding nor deformation of basal debris is necessary. Recovered ice cores (to depths of 25 m) contain thin debris layers associated with summer ablation in the accumulation zone. The ages of four samples of organic material removed from several debris layers inthe southern half of the rock glacier range from 200 ± 40 to 2250 ± 35 14C yr B.P., demonstrating that the rock glacier formed well before the Little Ice Age and may contain ice dating to the middle Holocene or earlier.

Vertical extension of the subglacial drainage system into basal crevasses.

Water plays a first-order role in basal sliding of glaciers and ice sheets and is often a key constituent of accelerated glacier motion. Subglacial water is known to occupy systems of cavities and conduits at the interface between ice and the underlying bed surface, depending upon the history of water input and the characteristics of the substrate. Full understanding of the extent and configuration of basal water is lacking, however, because direct observation is difficult. This limits our ability to simulate ice dynamics and the subsequent impacts on sea-level rise realistically. Here we show that the subglacial hydrological system can have a large volume of water occupying basal crevasses that extend upward from the bed into the overlying ice. Radar and seismic imaging combined with in situ borehole measurements collected on Bench Glacier, Alaska, reveal numerous water-filled basal crevasses with highly transmissive connections to the bed. Some crevasses extend many tens of metres above the bed and together they hold a volume of water equivalent to at least a decimetre layer covering the bed. Our results demonstrate that the basal hydrologic system can extend high into the overlying ice mass, where basal crevasses increase water-storage capacity and could potentially modulate basal water pressure. Because basal crevasses can form under commonly observed glaciological conditions, our findings have implications for interpreting and modelling subglacial hydrologic processes and related sliding accelerations of glaciers and ice sheets.