Keith A. Echelmeyer

Submarine melting at the terminus of a temperate tidewater glacier, LeConte Glacier, Alaska, U.S.A.

Abstract Heat, fresh- and sea-water balances indicate that the late-summer rate of submarine melting at the terminus of tidewater LeConte Glacier, Alaska, U.S.A., in 2000 was about 12 m d−1 w.e., averaged over the submerged face. This is 57% of the estimated total ice loss at the terminus (calving plus melting) at this time. Submarine melting may thus provide a significant contribution to the overall ablation of a tidewater glacier. Oceanographic measurements (conductivity–temperature–depth) made 200–500m from the terminus identified an isohaline (27 ppt) and isothermal (7.2°C) layer extending from 130 m depth to the fjord floor. Capping this is a 40 m thick overflow plume, distinguished by high outflow rates, low salinity (22–25 ppt) and lower temperatures (5–6°C). Mixing models indicate that fresh water comprised about 11% of this plume; it originates mostly as subglacial discharge whose buoyancy drives convection at the terminus. Deep, warm saline waters are incorporated into the plume as it ascends, causing substantial melting of ice along the submarine face. The calving terminus undergoes seasonal changes that coincide with changes in subglacial discharge and fjord water temperatures, and we suggest that these fluctuations in terminus position are directly related to changes in submarine melting.

Penetration depth of interferometric synthetic‐aperture radar signals in snow and ice

Digital elevation models of glaciated terrain produced by the NASA/Jet Propulsion Laboratory (JPL) airborne interferometric synthetic-aperture radar (InSAR) instrument in Greenland and Alaska at the C- (5.6 cm wave-length) and L-band (24-cm) frequencies were compared with surface elevation measured from airborne laser altimetry to estimate the phase center of the interferometric depth, or penetration depth, δp. On cold polar firn at Greenland summit, δp = 9±2m at C- and 14±4m at L-band. On the exposed ice surface of Jakobshavn Isbrae, west Greenland, δp = 1±2 m at C- and 3±3 m at L-band except on smooth, marginal ice where δp = 15±5 m. On colder marginal ice of northeast Greenland, δp reaches 60 to 120 m at L-band. On the temperate ice of Brady Glacier, Alaska, δp is 4±2 m at C- and 12±6 m at L-band, with little dependence on snow/ice conditions. The implications of the results on the scientific use of InSAR data over snow/ice terrain is discussed.

Glacier motion dominated by processes deep in underlying till

Black Rapids Glacier is a 40 km long surge-type glacier in the central Alaska Range. In spring 1997 a wireline drill rig was set up at a location where the measured surface velocities are high and seasonal and annual velocity variations are large. The drilling revealed a layer of subglacial till, up to 7 m thick, that is believed to be water-saturated. At one location a string of instruments, containing three dual-axis tiltmeters and one piezometer, was successfully introduced into the till. The tiltmeters monitored the inclination of the borehole at the ice-till interface and at 1 and 2 m into the till, for 410 days. They showed that no significant deformation occurred in the upper 2 m of the till layer, and no significant amount of the basal motion was due to sliding of the ice over the till. The measured surface velocity at the drill site is about 60 m a -1 , of which 20-30 m a can be accounted for by ice deformation. Almost the entire amount of basal motion, 30-40 m a -1 , was taken up at a depth of > 2 m in the till, possibly in discrete shear layers, or as sliding of till over the underlying bedrock. We propose that the large-scale mobilization of such till layers is a key factor in initiating glacier surges.

Jakobshavns Isbræ, west Greenland : seasonal variations in velocity ― or lack thereof

The lower 80 km of the fast-moving lakobshavns Isbra:, West Greenland, is subject to significant melting during the summer season. The melt water drains into large supraglacial rivers which pour into moulins or feed into beautiful supraglacial lakes, some of which are observed to drain periodically. Except for a few streams that drain directly off the margins of the ice sheet within the drainage basin of this glacier, the fate of this melt water is unknown. However, a localized upwelling of highly turbid water is often observed during the melt season in the fjord adjacent to the terminal c liff of the glacier, indicating that water from some source does move along the glacier bed. As part of an investigation on the mechanisms of rapid flow on Jakobshavns Isbra!, measurements of surface velocity at several (-25) locations along the ice stream at and below the equilibrium line were made in order to investigate the effects of this seasonally varying input of melt water on the speed of the glacier. No significant seasonal varia tion in speed was found at any location . This indicates that, unlike many other sub-polar and temperate glaciers, surface melt-water production does not affect the motion of th is glacier on a seaso nal basis, and, thus, does not cause a significant temporal vanatIOn in basal sliding . This finding has important ramifications on the mechanisms of flow for this ice stream.

Quantifying the effects of climate and surface change on glacier mass balance

When a mass balance is computed using an outdated map, that computation does not reveal the actual mass change. But older maps often must be used for practical reasons. We present a method by which, with a few additional measurements each year, a mass balance computed with an outdated map can be transformed into an actual mass change. This is done by taking into account the influence of changes in areal extent and changes in the surface elevation of the glacier since the map was made. This method is applied to South Cascade Glacier, Washington, U.S.A., as an example. The computed cumulative mass balance from 1970 to 1997 would have been 16% too negative if the 1970 map had not been updated. While the actual volume change of a glacier is relevant to hydrological studies, the change that would have occurred on a constant (or static) surface is more relevant to certain glacier dynamics problems and most climate problems. We term this the reference-surface balance and propose that such a balance, which deliberately omits the influence of changes in area and surface elevation, is better correlated to climatic variations than the conventional one, which incorporates those influences.

Updated estimates of glacier volume changes in the western Chugach Mountains, Alaska, and a comparison of regional extrapolation methods

and 2001/2004. Average net balance rates ranged between � 3.1 to 0.16 m yr � 1 for the tidewater and � 1.5 to � 0.02 m yr � 1 for the nontidewater glaciers. We tested several methods for extrapolating these measurements to all the glaciers of the western Chugach Mountains using a process similar to cross validation. Predictions of individual glacier changes appear to be difficult, probably because of the effects of glacier dynamics, which on long (multidecadal) timescales, complicates the response of glaciers to climate. In contrast, estimates of regional contributions to rising sea level were similar for different methods, mainly because the large glaciers, whose changes dominated the regional total, were among those measured. For instance, the above sea level net balance rate of Columbia glacier (� 3.1 ± 0.08 km 3 yr � 1 water equivalent (weq) or an equivalent rise in sea level (SLE) of 0.0090 ± 0.0002 mm yr � 1 ) was nearly half of the total regional net balance rate of the western Chugach Mountain glaciers (� 7.4 ± 1.1 km 3 yr � 1 weq or 0.020 ± 0.003 mm yr � 1 SLE between 1950/1957 and 2001/2004). Columbia glacier is a rapidly retreating tidewater glacier that has lost mass through processes largely independent of climate. Tidewater glaciers should therefore be treated separately when performing regional extrapolations.

Of isbræ and ice streams

Abstract Fast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the “ice-stream” type (represented by the Siple Coast ice streams in West Antarctica) and the “isbræ” type (represented by Jakobshavn Isbræ in Greenland). The typical ice stream is wide, relatively shallow (∼1000 m), has a low surface slope and driving stress (∼10 kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbræ type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zone stend to be wide relative to the isbræ width, and the location of isbræ and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbræs occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.

Implications of till deformation on glacier dynamics

The dynamics of glacier motion are governed to a large extent by the properties of the basal interface. In this paper we address the interaction of a glacier with a layer of till at its bed in an attempt to test whether our physical understanding of till is sufficient to explain general features of the observed flow field and changes in geometry of Black Rapids Glacier, Alaska, U.S.A. We also investigate whether or not a till layer has a clear surface-observable signature in the dynamics of the glacier. Towards this end we use a finite-element ice-flow model with a Coulomb failure criterion within the basal till layer. We find that simple till physics can be used to describe decadal, seasonal and short-term (hours to days) velocity variations, and possibly uplift events. Mechanisms for each of these variations involve an increase in the extent of till at failure, a transfer of shear stress across the bed, and a consequent increase in ice deformation. Effective shape factors are calculated that permit a simple incorporation of this boundary condition into glacier response models. Our analyses, however, have not resulted in the identification of a clear and unique signature of a till layer in the surface dynamics of a glacier.

Hydrological discharges and motion of Fels and Black Rapids Glaciers, Alaska, U.S.A.: implications for the structure of their drainage systems

Characteristics of the hydrology and motion of Black Rapids and Fels Glaciers, Alaska, were observed from 1986 to 1989. Hydrological measurements included stage, electrical conductivity and suspended-sediment concentration in the discharge stream of each glacier, and were made at 0.5-1 h intervals continuously through most of the melt seasons. Variations in the glacier speed were monitored through the full year at a number of locations along the length of each glacier using time-lapse photography (1 d time resolution), strain meters (0.5-1 h resolution) and seismometers set up to count acoustic emissions. Both glaciers show similar seasonal, diurnal and short-term event changes in hydrological discharges and ice speed. The hydrological behavior is analyzed in terms of a fast sub-system composed of surface streams, moulins and large tunnels with discharge that responds rapidly and a slow sub-system composed of heterogeneous small passageways through the ice and distributed over the bed that maintain approximately uniform discharge over a day. The timing and amplitude of water discharge in the diurnal cycle indicate that roughly 10-40% of the water is routed directly into the fast system. The remaining 90-60% of the water enters the slow system. Dilution of the solute discharged from the slow system by the variable discharge in the fast system results in changes in water discharge and solute concentration that are approximately equal in relative amplitude and inversely related. A small time lag from discharge maximum (minimum) to solute minimum (maximum) suggests that the fast system is confined to roughly the lowermost 30-40% of the full glacier length. The residence time of water in the fast system is short compared to 1 d. The slow system contains both short- and long-residence time passages. Characteristics of the diurnal cycles are somewhat variable through the melt season, but no systematic evolutionary patterns were discerned even though large changes in the mean discharges of water and solutes occur, which suggests parallel evolution of the variables that control the response of the fast system. Events were characterized by contemporaneous increases in suspended-sediment concentration in the discharge water and distinct changes in straining on the glaciers. Events caused by increases in melt or precipitation related to weather and events related to release from reservoirs internal to the glaciers could be distinguished based on the changes in electrical conductivity of the discharge water. The correlated changes in sediment discharge and motion of the glaciers indicate that the events were associated with temporary modifications of the slow passages distributed over the bed that allowed enhanced sliding and access of basal water flow to erosion products. Hydrological differences between Black Rapids and Fels Glaciers can be explained by differences in the size of the glaciers. If there is a difference in bed structure that explains the difference in dynamics (surge-Black Rapids Glacier-versus non-surge-Fels Glacier), it does not affect the hydrological parameters that were observed.

Measurement of temperature in a margin of Ice Stream B, Antarctica : implications for margin migration and lateral drag

Ice temperature was measured in and around the chaotically crevassed south margin of Ice Stream B, Antarctica, from 1992 to 1994. The temperatures at 30 m depth in the chaotic zone are about 12 K lower than in the adjacent uncrevassed ice, due to the ponding of cold winter air. At depths greater than 150 m, there is clear evidence of internal heating of the ice due to the large shear deformation rate in the marginal zone. Analysis of the depth of cooling below the crevasses and of the internal heating gives two pieces of information. First, over the last half century the lateral shear stress averaged 2.0 x 10 5 Pa in the top third of the margin and, second, the margin moved outward at an average rate of 7.3 m a These values do not involve any assumptions about the flow law of ice. The uncertainties are roughly 20%. The value of lateral shear stress indicates that the most of the drag on the ice stream is along its sides.