Our observations show that extreme uplift in southeast Alaska began about 1770 AD, with relative sea level (RSL) change to 5.7 m and current uplift rates to 32 mm/yr. This region experienced widespread glacial melting following the Little Ice Age (LIA), with the collapse of the Glacier Bay Icefield alone equivalent to 8 mm of global sea level rise. Geodynamic modelling links the uplift to post-LIA isostatic rebound, with the extreme uplift signal and a priori knowledge of ice load changes requiring the presence of a low viscosity asthenosphere (3.7 × 1018 Pa s). These crustal deformations are triggered by climate change through glacier wastage.

Extreme uplift rates and sea level changes in southern Alaska have been documented by Global Positioning System (GPS) surveys, tide gauge measurements and studies of raised shorelines. The movements detected in a network of 45 GPS survey points describe a broad pattern of rapid regional uplift. The majority of the study area is uplifting at a rate faster than 10 mm yr(-1), with several sites uplifting more rapidly than 25 mm yr(-1). New tide gauge data presented here consist of repeat occupations of 18 temporary gauge sites. Sea level rates at these sites agree with similar measurements in southern Alaska taken similar to50 yr earlier, and also with the pattern of uplift derived from the GPS data. Raised shoreline studies at 14 sites document total sea level change, with a maximum change in sea level of -5.7 m found in upper Lynn Canal. The start of the ongoing uplift episode that raised these shorelines has been dated with dendrochronology and found to be coincident with the start of the collapse of the Glacier Bay Icefield, at ca. 1750 AD. The pattern of total sea level change is in general agreement with uplift-rate measurements, with greater sea level change found at the sites closest to the peak uplift rates in upper Glacier Bay. We use a viscoelastic earth model subjected to an ice load history built upon observations of glacial change to predict uplift rates at the tide gauge and GPS sites as well as the total uplift at the raised shoreline sites. Our modelling exercises are limited to an ice load model based on independent studies of the region's glacial history over the past 1.7 kyr, to evaluate whether the uplift observations can be explained by simple earth models subjected to this load history. Two-layer earth models, consisting of an elastic crust and a low-viscosity upper mantle half-space, can be adjusted to fit either the raised shoreline data or the combined GPS and tide gauge uplift-rate data, but cannot fit all the data with a single set of earth model parameters. However, all three data sets are consistent with an approximated three-layer earth model. The combined model is constrained by a total of 77 uplift measurements, which at the 95 per cent confidence level require a low-viscosity asthenosphere [eta(A)= (1.4 +/- 0.3) x 10(19) Pa s and thickness 110(-15)(+20) km] beneath a 50(-25)(+30) km thick elastic lithosphere and overlying an upper mantle half-space with a viscosity of 4 x 10(20) Pa s. This earth model achieves a low degree of misfit with the observations (reduced chi-square value of chi(nu)(2)= 2.5), suggesting that glacial isostatic rebound associated with post-Little Ice Age melting can entirely account for the rapid uplift of southern Alaska over the last similar to250 yr.

AbstractFireweed rock glacier is a large rock glacier in south central Alaska, U.S.A. It flows relatively fast, with velocities up to 3.5 ma–1, and exhibits both seasonal and annual velocity variations, some of which are related to periodic terminus calving and increased rainfall. Our analysis reveals that motion is likely concentrated in a pseudo-rectangular channel within the larger parabolic channel with a “shear plane” at ~27 m depth. There is likely motion along the shear plane as well as internal deformation above it. We estimate that the ice—rock mixture is up to seven times softer than clean glacier ice with a temperature of –2°C. Calving at the terminus is an important component of the mass balance of this rock glacier.

AbstractA heavy down-hole hammer actuated from the surface by a light composition rope was used to place instrumented probes into the active, 7m thick, clast-rich till underlying a site on Black Rapids Glacier, Alaska, USA, where the ice is 500m thick. A till penetration of about 2.5m was obtained, and greater depths seem possible. The probes measured pore-water pressure and two axes of tilt, which they broadcasted, without wires, to a receiver just above the ice–till interface.

AbstractGeophysical investigations on rock glaciers are often difficult because rock glaciers are covered by an unconsolidated debris mantle a few meters thick, are typically <50 m thick and are composed of an ice—rock mixture of unknown composition. Transient electromagnetics (TEM) is a method that allows some of these difficulties to be minimized, and data collection is relatively efficient. TEM, with calibration from terminus exposure, was used to determine the thickness (~60 m) of Fireweed rock glacier, Alaska, U.S.A., under complex valley geometry. A conductive layer beneath the rock glacier was identified, and its distribution is consistent with a till-like layer. Seismic refraction, used to resolve the debris-mantle thickness (2–4 m), suggests the presence of a discontinuity at 18–28 m depth within the rock glacier. The discontinuity is also indicated in the radio-echo sounding and the TEM data, but to a lesser extent. This discontinuity is important because the motion of the rock glacier may occur across this as a “shear plane”.

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 in 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 200-500 in from the terminus identified an isohaline (27 ppt) and isothermal (7.2degreesC) layer extending from 130 in depth to the fjord floor. Capping this is a 40 in thick overflow plume, distinguished by high outflow rates, low salinity (22-25 ppt) and lower temperatures (5-6degreesC). 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.

AbstractFast-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.

Hubbard Glacier is the largest tidewater glacier in North America. In contrast to most glaciers in Alaska and northwestern Canada, Hubbard Glacier thickened and advanced during the 20th century. This atypical behavior is an important example of how insensitive to climate a glacier can become during parts of the calving glacier cycle. As this glacier continues to advance, it will close the seaward entrance to 50 km long Russell Fjord and create a glacier-dammed, brackish-water lake. This paper describes measured changes in ice thickness, ice speed, terminus advance and fjord bathymetry of Hubbard Glacier, as determined from airborne laser altimetry, aerial photogrammetry, satellite imagery and bathymetric measurements. The data show that the lower regions of the glacier have thickened by as much as 83 in in the last 41 years, while the entire glacier increased in volume by 14.1 km(3). Ice speeds are generally decreasing near the calving face from a high of 16.5 in d(-1) in 1948 to 11.5 in d(-1) in 2001. The calving terminus advanced at an average rate of about 16 in a(-1) between 1895 and 1948 and accelerated to 32 in a(-1) since 1948. However, since 1986 the advance of the part of the terminus in Disenchantment Bay has slowed to 28 in a(-1). Bathymetric data from the lee slope of the submarine terminal moraine show that between 1978 and 1999 the moraine advanced at an average rate of 32 in a(-1), which is the same as that of the calving face.

Mendenhall Glacier is a dynamic maritime glacier in southeast Alaska that is undergoing substantial recession and thinning. The terminus has retreated 3 km during the 20th century and the lower part of the glacier has thinned 200 m or more since 1909. Glacier-wide volume loss between 1948 and 2000 is estimated at 5.5 km3. Wastage has been the strongest in the glacier's lower reaches, but the glacier has also thinned at higher elevations. The shrinkage of Mendenhall Glacier appears to be due primarily to surface melting and secondarily to lake calving. The change in the average rate of thinning on the lower glacier, <1 m a−1 between 1948 and 1982 and >2 m a−1 since 1982, agrees qualitatively with observed warming trends in the region. Mean annual temperatures in Juneau decreased slightly from 1947 to 1976; they then began to increase, leading to an overall warming of ∼1.6 °C since 1943. Lake calving losses have periodically been a small but significant fraction of glacier ablation. The portion of the terminus that ends in the lake is becoming increasingly vulnerable to calving because of a deep pro-glacial lake basin. If current climatic trends persist, the glacier will continue to shrink and the terminus will recede onto land at a position about 500 m inland within one to two decades. The glacier and the meltwaters that flow from it are integral components of the Mendenhall Valley hydrologic system. Approximately 13% of the recent average annual discharge of the Mendenhall River is attributable to glacier shrinkage. Glacier melt contributes 50% of the total river discharge in summer.

A simple approach to glacier dynamics is explored in which there is postulated to be a relationship between area and volume with three parameters: the time for area to respond to changes in volume, a thickness scale, and an area characterizing the condition of the initial state. This approach gives a good fit to the measurements of cumulative balance and area on South Cascade Glacier from 1970-97; the area time-scale is roughly 8 years, the thickness scale about 123 in, and the 1970 area roughly 4% larger than required for adjustment with volume. Combining this relationship with a version of mass continuity expressed in terms of area and volume produces a theory of glacier area and volume response to climate in which another time constant, the volume time-scale, appears. Area and volume both respond like a damped spring and mass system. The damping of the South Cascade response is approximately critical, and the volume time-scale is roughly 48 years, six times the area time-scale. The critically damped spring and mass analogy reproduces the time dependence predicted by the more complicated traditional theory of Nye.

Abstract Knowledge of iceberg calving is important for understanding instabilities of tidewater glaciers and ice sheets. Since 1995 the terminus of LeConte Glacier, Alaska, U.S.A., has retreated about 2 km and the glacier has thinned approximately 120 m at its 1999 terminus position. Our focus is short-term (hours to weeks) variability of the frequency and magnitude of calving events and calving flux. Both photogrammetric and visual observations are employed in a temporal analysis over a several-week period. We combined these data with measurements of ice speed, tide level, surface water input and water-storage estimates in an attempt to better understand the calving process. Contrary to results obtained over longer time-scales on other glaciers, our results show no correlation between ice speed and the frequency of calving. However, calving events do not appear to occur randomly; often they are a response to measurable changes in other parameters within the terminus region. Caclving can often be attributed to buoyancy perturbations and possibly flexure of the nearly floating terminus. Given the multiple possibilities for buoyancy perturbations, we have found no simple relationship between any specific forcing parameter and calving at short time-scales.

Taku Glacier is one of the few glaciers in Alaska, U.S.A., that has advanced over the last century: 7 km since 1890. This advance slowed substantially during the past decade, but in summer 2001 the glacier terminus began to readvance at a rate of 30 cm d(-1). The advance produced dramatic proglacial sediment deformation up to 200 in in front of the terminus. Two to three large bulges and several secondary bulges developed in the proglacial sediments as a result of glacial compression along a I kin wide portion of the terminus. The bulge nearest the terminus was 10 in high and 65 in wide. The middle bulge (7 in high) advanced at 15 cm d(-1) and the distal bulge (3 in high and 50 in wide) at 9 cm d(-1). Crenulations and prominent fractures developed in the overlying vegetation layer. The frontal lobes of the bulges were steep and overlaid a shear zone, where sediments were being thrust up and over the ground surface. Ice-proximal push moraines, 1-10 in high, formed along much of the 9 km wide terminus, although deformation was minimal at some locations.

Vertical crustal motions at 15 sites along the northern Pacific‐North America plate boundary are determined using relative sea level changes from tide gauge records. Our analysis is based on monthly mean sea levels, from which barometric pressure and seasonal effects are removed. The records are corrected for common‐mode oceanographic variations. These records are statistically examined for non‐linear behavior related to glacial isostatic, tectonic and postseismic effects. To estimate land uplift rates, the local effect of global sea level rise is removed from the relative sea level rates. Slow rates of vertical motion are observed along the southern strike‐slip plate boundary. The extremely rapid uplift of the northern strike‐slip boundary can be attributed entirely to viscoelastic postglacial rebound associated with tidewater‐glacier retreat in Glacier Bay and regional post‐Little Ice Age deglaciation. Isostatic modeling indicates a mantle viscosity of ∼2 × 10 19 –5 × 10 19 Pa s, similar to that found elsewhere along the Pacific‐North America plate margin. At Yakutat, near the transition of plate motion from strike‐slip to subduction, complex non‐linear behavior is evident, with a significant change in uplift rate following the 1979 St. Elias earthquake. Non‐linear uplift rates are predominant within the 1964 Great Alaskan earthquake near‐field. Rapid uplift at Kodiak during a 3.5 year period starting mid‐1964 totaled 47 ± 8 cm. Anchorage, Seward and Seldovia exhibited oscillatory uplift in the period immediately following the earthquake until mid‐1972. Since mid‐1972, uplift rates have increased steadily at Anchorage, Seward, Cordova, and Valdez. During this period Nikiski and Kodiak show decreasing uplift rates.

We have used airborne laser altimetry to estimate volume changes of 67 glaciers in Alaska from the mid-1950s to the mid-1990s. The average rate of thickness change of these glaciers was –0.52 m/year. Extrapolation to all glaciers in Alaska yields an estimated total annual volume change of –52 ± 15 km 3 /year (water equivalent), equivalent to a rise in sea level (SLE) of 0.14 ± 0.04 mm/year. Repeat measurements of 28 glaciers from the mid-1990s to 2000–2001 suggest an increased average rate of thinning, –1.8 m/year. This leads to an extrapolated annual volume loss from Alaska glaciers equal to –96 ± 35 km 3 /year, or 0.27 ± 0.10 mm/year SLE, during the past decade. These recent losses are nearly double the estimated annual loss from the entire Greenland Ice Sheet during the same time period and are much higher than previously published loss estimates for Alaska glaciers. They form the largest glaciological contribution to rising sea level yet measured.

Abstract In 1972 and 1995, shallow ice temperatures were measured at identical locations and depths on polythermal McCall Glacier, Brooks Range, Alaska, U.S.A. Mean annual ice temperatures at 10 m depth have systematically increased by > 1 K for the ablation area (1400–1900 m), while closer to the firn area, where meltwater percolation and refreezing play a role, they remained approximately unchanged. Interpreting these findings in terms of climate change requires careful consideration of the observed thinning of the glacier, which causes lowering of the surface through an existing vertical temperature gradient. Such temperature gradients can be particularly large in the ablation areas of polythermal glaciers; on McCall Glacier they are on the order of 0.2 K m−1. We also study the evolution of a 75 m deep temperature profile measured at one location in 1972 using a one-dimensional heat-diffusion model. We find that this profile was in approximate equilibrium with the mean surface temperature extrapolated from the 1972 data. Using the observed rate of surface lowering and measured rates of vertical advection, we find that both the measured temperature change and the vertical temperature gradient at 10 m depth can be reproduced only if the mean annual surface temperature on McCall Glacier has increased by 1.1 ± 0.3 K between 1972 and 1995. This result is consistent with the observed trend toward more negative mass balances on the glacier in the 1990s.

AbstractThe 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.

Abstract An analysis of motion in the terminus region of LeConte Glacier, Alaska, U.S.A., delineates mechanisms that are important to tidewater glacier stability. This glacier is undergoing rapid retreat. Since 1995 it has retreated 2 km and thinned >125 m at the present location of the terminus. Ice velocities in this region are quite high; at the terminus they exceed 27 m d−1. Our analysis reveals that fluctuations in speed and surface elevation are forced by ocean tides, surface melt and precipitation. The nearterminus ice speed is 180° out of phase with the tide, such that high tide corresponds to low speed. Smaller, melt-forced diurnal variations in speed are present throughout the lowermost 7 km. Speed-ups in excess of 10% of the mean often occur after rainstorms as a result of increased basal water pressure and storage, but the relation is not simple. The velocity does not vary over the spring and summer seasons.

Abstract Glacier response to climate can be characterized by a single time-scale when the glacier changes sufficiently slowly. Then the derivative of volume with respect to area defines a thickness scale similar to that of Jóhannesson and others, and the time-scale follows from it. Our version of the time-scale is different from theirs because it explicitly includes the effect of surface elevation on mass-balance rate, which can cause a major increase in the time-scale or even lead to unstable response. The time constant has a dual role, controlling both the rate and magnitude of response to a given climate change. Data from South Cascade Glacier, Washington, U.S.A., illustrate the ideas, some of the difficulty in obtaining accurate values for the thickness and time-scales, and the susceptibility of all response models to potentially large errors.

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.

AbstractWe employed a commercial wireline drill rig to investigate the subglacial conditions of Black Rapids Glacier, a well-studied surge-type glacier in the central Alaska Range. The four main goals were: to assess the capabilities of the commercial drilling industry for sampling subglacial material, to investigate the basal morphology, to determine the subglacial geology and to emplace borehole instruments. The drilling was done in an area where seasonal and secular variations in speed are large, and where seismic studies suggested the presence of a till layer. Four holes were drilled at three locations to a maximum depth of 620 m. Three holes yielded samples of basal ice and till, although recovery of the latter was generally poor. Bedrock was sampled in one or possibly two of the holes. In the area sampled, t he glacier is underlain by a till layer some 4–7 m thick, confirming the seismic interpretation. It consists of a sandy matrix at least 20–30% of which comprises larger clasts. Limited samples of the matrix indicate that near the top of the till the porosity is 40%, and t hat some of the pore water is frozen. Geologic studies suggest that the drilling area lies to the north of the Denali Fault, a major tectonic boundary followed by the glacier, and that most of the till is locally derived with transport distances of <2 km.