Matanuska Glacier Research Articles

Seasonal variability in hydrologic-system response to intense rain events, Matanuska Glacier, Alaska, U.S.A.

AbstractTwo rain events at Matanuska Glacier illustrate how subglacial drainage system development and snowpack conditions affect hydrologic response at the terminus. On 21 and 22 September 1995, over 56 mm of rain fell in the basin during a period usually characterized by much drier conditions. This event caused an 8-fold increase in discharge and a 47-fold increase in suspended-sediment concentration. Peak suspended-sediment concentration exceeded 20 kg m —3, suggesting rapid evacuation of stored sediment. While water discharge returned to its pre-storm level nine days after the rain ceased, suspended- sediment concentrations took about 20 days to return to pre-storm levels. These observations suggest that the storm influx late in the melt season probably forced subglacial water into a more distributed system. In addition, subglacially transported sediments were supplemented to an unknown degree by the influx of storm-eroded sediments off hillslopes and from tributary drainage basins.A storm on 6 and 7 June 1997, dropped 28 mm of rain on the basin demonstrating the effects of meltwater retention in the snowpack and englacial and subglacial storage early in the melt season. Streamflow before the storm event was increasing gradually owing to warming temperatures; however, discharge during the storm and the following week increased only slightly. Suspended-sediment concentrations increased only a small amount, suggesting the drainage system was not yet well developed, and much of the run off occurred across the relatively clean surface of the glacier or through englacial channels.

Glaciohydraulic supercooling: a freeze-on mechanism to create stratified, debris-rich basal ice: I. Field evidence

AbstractDebris-laden ice accretes to the base of Matanuska Glacier, Alaska, U.S.A., from water that supercools while flowing in a distributed drainage system tip the adverse slope of an overdeepening. Frazil ice grows in the water column and forms aggregates, while other ice grows on the glacier sole or on substrate materials. Sediment is trapped by this growing ice, forming stratified debris-laden basal ice. Growth rates of >0.l ma−1of debris-rich basal ice are possible. The large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings, including Heinrich events and the till sheets south of the Laurentian Great Lakes.

Glaciohydraulic supercooling: a freeze-on mechanism to create stratified, debris-rich basal ice: I. Field evidence

AbstractDebris-laden ice accretes to the base of Matanuska Glacier, Alaska, U.S.A., from water that supercools while flowing in a distributed drainage system tip the adverse slope of an overdeepening. Frazil ice grows in the water column and forms aggregates, while other ice grows on the glacier sole or on substrate materials. Sediment is trapped by this growing ice, forming stratified debris-laden basal ice. Growth rates of >0.l ma−1 of debris-rich basal ice are possible. The large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings, including Heinrich events and the till sheets south of the Laurentian Great Lakes.

Preliminary results of tritium analyses in basal ice, Matanuska Glacier, Alaska, U.S.A.: evidence for subglacial ice accretion

The stratified-facies ice of the basal zone of Matanuska Glacier, Alaska. U.S.A., contains significant concentrations of anthropogenic tritium, whereas unaltered englacial-zone ice is devoid of tritium. Supercooled water flowing through subglacial conduits during the melt season likewise contains tritium, as does frazil and other platy ice that nucleates and grows within this subglacially flowing water. These initial results demonstrate net accretion of more than 1.4 m of stratified basal-zone ice since initiation of above-ground, thermonuclear bomb testing in 1952. Furthermore, these results support a theory of basal ice formation by ice accretion and debris entrainment from supercooled water within a distributed subglacial drainage system.

Preliminary results of tritium analyses in basal ice, Matanuska Glacier, Alaska, U.S.A.: evidence for subglacial ice accretion

The stratified-facies ice of the basal zone of Matanuska Glacier, Alaska. U.S.A., contains significant concentrations of anthropogenic tritium, whereas unaltered englacial-zone ice is devoid of tritium. Supercooled water flowing through subglacial conduits during the melt season likewise contains tritium, as does frazil and other platy ice that nucleates and grows within this subglacially flowing water. These initial results demonstrate net accretion of more than 1.4 m of stratified basal-zone ice since initiation of above-ground, thermonuclear bomb testing in 1952. Furthermore, these results support a theory of basal ice formation by ice accretion and debris entrainment from supercooled water within a distributed subglacial drainage system.

An investigation of the deforming layer/debris-rich basal-ice continuum, illustrated from three Alaskan glaciers

AbstractThree small Alaskan glaciers with different bed conditions were studied: Exit Glacier had a thin deforming layer and produced subglacial and proglacial glaciotectonic land forms; Childs Glacier also had a thin deforming layer but the upper part was frozen to the ice; Matanuska Glacier had no deforming layer but had subglacial debris-rich ice. Since it has been shown that sediment at the base can account for the majority of ice movement, it is suggested that there is a deforming bed/debris-rich continuum whereby similar processes occur throughout the different subglacial environments. These include: similar longitudinal deformation patterns (compression at the margin, extension and simple shear up-glacier); similar vertical deformation patterns, increase in deformation (and fabric strength) upwards through the sequence (leading to the attenuation of stratified ice into dispersed ice); and similar and interrelated incorporation processes. The major differences were that the processes occurred at a much faster rate within the deforming layer and that probably only the deforming layer will be recorded in the geological record.

An investigation of the deforming layer/debris-rich basal-ice continuum, illustrated from three Alaskan glaciers

AbstractThree small Alaskan glaciers with different bed conditions were studied: Exit Glacier had a thin deforming layer and produced subglacial and proglacial glaciotectonic land forms; Childs Glacier also had a thin deforming layer but the upper part was frozen to the ice; Matanuska Glacier had no deforming layer but had subglacial debris-rich ice. Since it has been shown that sediment at the base can account for the majority of ice movement, it is suggested that there is a deforming bed/debris-rich continuum whereby similar processes occur throughout the different subglacial environments. These include: similar longitudinal deformation patterns (compression at the margin, extension and simple shear up-glacier); similar vertical deformation patterns, increase in deformation (and fabric strength) upwards through the sequence (leading to the attenuation of stratified ice into dispersed ice); and similar and interrelated incorporation processes. The major differences were that the processes occurred at a much faster rate within the deforming layer and that probably only the deforming layer will be recorded in the geological record.

Short-pulse radar wavelet recovery and resolution of dielectric contrasts within englacial and basal ice of Matanuska Glacier, Alaska, U.S.A.

AbstractWavelets transmitted by short-pulse radar are recovered from continuous profiles and used to determine interfacial dielectric contrasts within the englacial and basal ice at the terminus area of Matanuska Glacier, Alaska, U.S.A. The field studies were in the ablation region, where radar horizons could, at some point, be identified with interfaces between clear ice and air, water or basal ice, and were performed in early spring before drainage fully developed. The profiles used closely-spaced antennas with bandwidths centered near 50 and 400 MHz. Transmitted wavelets reflected from interfaces of known dielectric contrasts are used to establish a phase reference for other events from interfaces between unknown contrasts. Migration and Fourier-transform filtering are then applied to the profiles and shown to recover these wavelets from diffractions and reflections. Interfacial dielectric contrasts are determined from the relative phase of the wavelets. Near the terminus lake, some basal ice events above 8 m depth are interpreted as voids. Further up-glacier, most englacial events are interpreted as voids, but deeper localized reflectors and horizons to 90 m depth and within the basal zone are interpreted as voids, water or debris. Phase cannot be determined for the basal-substrate transition reflections. Recommendations are made for improving the wavelet recovery process and the quality of GPR migration.

Short-pulse radar wavelet recovery and resolution of dielectric contrasts within englacial and basal ice of Matanuska Glacier, Alaska, U.S.A.

AbstractWavelets transmitted by short-pulse radar are recovered from continuous profiles and used to determine interfacial dielectric contrasts within the englacial and basal ice at the terminus area of Matanuska Glacier, Alaska, U.S.A. The field studies were in the ablation region, where radar horizons could, at some point, be identified with interfaces between clear ice and air, water or basal ice, and were performed in early spring before drainage fully developed. The profiles used closely-spaced antennas with bandwidths centered near 50 and 400 MHz. Transmitted wavelets reflected from interfaces of known dielectric contrasts are used to establish a phase reference for other events from interfaces between unknown contrasts. Migration and Fourier-transform filtering are then applied to the profiles and shown to recover these wavelets from diffractions and reflections. Interfacial dielectric contrasts are determined from the relative phase of the wavelets. Near the terminus lake, some basal ice events above 8 m depth are interpreted as voids. Further up-glacier, most englacial events are interpreted as voids, but deeper localized reflectors and horizons to 90 m depth and within the basal zone are interpreted as voids, water or debris. Phase cannot be determined for the basal-substrate transition reflections. Recommendations are made for improving the wavelet recovery process and the quality of GPR migration.

Mobilization, Movement and Deposition of Active Subaerial Sediment Flows, Matanuska Glacier, Alaska

Subaerial sediment flow is the predominant process depositing diamictons at the terminus of Matanuska Glacier. Flows originate where sediments overlie glacier ice. Ablation of ice exposed in slopes disaggregates the overlying sediment and mixes it with meltwater and debris released simultaneously. This material generally flows only after its strength is further reduced by excess pore pressures and seepage pressures generated by meltwater from thawing ice. Moving sediment flows show reasonably systematic changes in physical attributes such as dimensions, texture, flow rates, density and erosional action, and in grain support and transport mechanisms that can be related to changes in the water content of their matrix material. At lowest water contents, flows support grains by their strength and move through shear in a thin zone at their base. Increased thickness of the zone in shear and deformation of other types accompany increased water contents, with grain interference and collisions, localized liquefact...

Distinguishing characteristics of Diamictons at the Margin of the Matanuska Glacier, Alaska

The origins of diamictons deposited at the Matanuska Glacier are identified in stratigraphic sequences mainly by the presence or absence of a pebble fabric, internal structure, and variation in gravel-size clast distribution. These properties correlate with major differences in depositional mechanisms and source material. Melt-out till mostly inherits fabric, internal structure, and grain-size distribution from its debris-laden basal ice source. Sediment flow deposits and ice-slope colluvium (deposited by ablational slope processes) have properties developed by resedimentation mechanisms. Melt-out till ranges from structureless to stratified with interspersed lenses and discontinuous laminae, and generally possesses a well-defined pebble fabric. Sediment flow deposits show various combinations of six sedimentologic units with distinct sedimentary features and clast dispersion. Pebble fabric is absent or poorly defined, depending upon the unit's origin. Ice-slope colluvium is usually featureless, except for randomly-dispersed laminated lenses and irregularly-shaped gravelly zones in an otherwise disarrayed assemblage of particles, and is without a pebble fabric.

A Comparison of the Pebble Orientations in Ice and Deposits of the Matanuska Glacier, Alaska

Depositional processes and their sediment source determine the orientation of pebbles in the deposits of the Matanuska Glacier and the relationship of this orientation to the direction of ice flow. Pebble fabrics in ice-derived deposits differ from those in resedimented deposits. The plunge direction of the long axes of prolate-shaped pebbles and the ab-plane dip of blade- and plate-shaped pebbles parallel the local ice flow direction in basal zone ice and in melt-out till derived from that ice. The pebble fabric in deposits from sediment flow, ablation of exposed basal zone ice, and the slumping and spalling of ice-cored slopes does not correspond to the ice flow direction, but is developed by these depositional processes. Pebbles in basal ice and melt-out till show a unimodal distribution of orientations with individual observations only slightly dispersed about the mean axis. Pebble fabrics in other deposits are polymodal with a significantly larger amount of dispersion about the mean axis. The regional pattern of mean axes of basal zone ice and melt-out till pebble fabrics approximates the local and regional trends of ice flow, but pebble imbrication in ice and sediment does not necessarily indicate the direction from which the glacier flowed. A small number of measurements of pebble orientations at many sites and the analysis of these data by the eigenvalue method appear to be suitable techniques for examining the pebble fabric of glacial deposits, but additional sedimentological data are needed to define the origins of these deposits.

Characteristics and Origins of the Debris and Ice, Matanuska Glacier, Alaska

Abstract The physical characteristics of the ice and debris of the Matanuska Glacier, Alaska (lat. 61° 47’, long. 147° 45’) and their consistent variation in a vertical stratigraphic column of the glacier indicate multiple origins for both. The two zones of ice recognized in the glacier are the upper englacial zone, which comprises the bulk of the glacier (est. 300 m thick), and the thin (3 to 15 m thick) basal zone, which contains an estimated 97% of the sediment transported in the glacier. The physical characteristics of the ice and the quantity and mode of distribution of debris in it define two ice facies in each zone. The englacial zone consists mainly of the debris-poor (0.002 volume per cent) diffused facies. White coarse-bubbly ice and blue coarse-clear ice (2 to 10 cm diameter) comprise most of this facies. The bubble-rich banded facies (debris bands) contains abundant debris (25 to 50 volume per cent), varies in lateral and vertical extent, and occurs randomly in the diffused facies.

Characteristics and Origins of the Debris and Ice, Matanuska Glacier, Alaska

AbstractThe physical characteristics of the ice and debris of the Matanuska Glacier, Alaska (lat. 61° 47’, long. 147° 45’) and their consistent variation in a vertical stratigraphic column of the glacier indicate multiple origins for both. The two zones of ice recognized in the glacier are the upper englacial zone, which comprises the bulk of the glacier (est. 300 m thick), and the thin (3 to 15 m thick) basal zone, which contains an estimated 97% of the sediment transported in the glacier. The physical characteristics of the ice and the quantity and mode of distribution of debris in it define two ice facies in each zone. The englacial zone consists mainly of the debris-poor (0.002 volume per cent) diffused facies. White coarse-bubbly ice and blue coarse-clear ice (2 to 10 cm diameter) comprise most of this facies. The bubble-rich banded facies (debris bands) contains abundant debris (25 to 50 volume per cent), varies in lateral and vertical extent, and occurs randomly in the diffused facies.

An Oxygen Isotope Investigation of the Origin of the Basal Zone of the Matanuska Glacier, Alaska

An analysis of the oxygen isotope content of ice of the englacial and basal zones of the Matanuska Glacier at its terminus reveals the origin of the ice and entrained debris. The decrease with depth in the $$\delta O^{18}$$ values of ice of the diffused facies of the englacial zone and the dispersed facies of the basal zone is consistent with previous studies and indicates this ice originates in the accumulation area. Characteristics of the ice and debris of the dispersed facies indicate a subglacial source for most of the debris. The sharp increase of more than 4‰ in the $$\delta O^{18}$$ values of ice of the lower, stratified facies of the basal zone and its young radiocarbon age indicate this facies formed by subglacial freezing of isotopically enriched meltwater, probably surface-derived, to the glacier sole. The bubble-poor, fine-grained ice, thickness, stratification, rounded pebbles, and undisturbed sedimentary structures in this facies support this conclusion. The location, extent, and rates of subglacial ice formation and sediment entrainment vary. The Matanuska Glacier is therefore thermally complex, with zones of ice at the glacier sole that are at or below the pressure-melting point.

Late Wisconsin and Recent History of the Matanuska Glacier, Alaska

Reports studies of this glacier in the upper Matanuska valley in 1953, 1954, and 1957, as part of the U.S. Geological Survey terrain and permafrost investigations in Alaska. Advances and retreats of the glacier, indicated by moraines and marginal channels, were traced. The results are outlined amplifying the preliminary report, No. 56057.