Jeffrey L. Kavanaugh

Abrupt glacier motion and reorganization of basal shear stress following the establishment of a connected drainage system

Three episodes of strong basal motion occurred at Trapridge Glacier, Yukon Territory, Canada, on 11 June 1995 following the establishment of a connected subglacial drainage system. Responses to these spring events are noted in the records for 42 instruments and were recorded throughout the ∼60 000 m study area. Strong basal motion during the events is indicated by ploughmeter, load-bolt and vertical-strain records, and abrupt pressure changes in several transducer records denote damage caused by extreme pressure pulses. These pressure pulses, generated by the abrupt basal motion, also resulted in the failure of seven pressure sensors. Records for pressure, turbidity and conductivity sensors indicate that basal drainage patterns did not change significantly during the events. Geophone records suggest that the episodes of basal motion were precipitated by the gradual failure of a sticky spot following hydraulic connection of part of the study area. This failure resulted in the transfer of basal stress to the unconnected region of the bed during the course of the events. No evidence for strong basal motion is seen in the instrument records for several weeks following the events, suggesting that the mechanical adjustments resulted in a stable configuration of basal stresses. This event illustrates how unstable situations can be quickly accommodated by mechanical adjustments at the glacier bed.

Sublimation and surface energy budget of Taylor Glacier, Antarctica

Taylor Glacier, an outlet of the East Antarctic ice sheet, flows through the Transantarctic Mountains and terminates in the Dry Valleys. Understanding how this glacier fluctuates is important for studies of glacial geology, paleoclimate, ice dynamics and ecology. Sublimation is the primary mass-loss process for most of the glacier. Four years of specific balance measurements from the ablation zone show sublimation rates up to 40 cma−1. We used data from an array of weather stations as inputs to a model for latent heat flux and hence sublimation rate. Calculated and measured ablation rates agree to within uncertainties, indicating that wind speed and vapour pressure gradient (a function of temperature and humidity) are the governing variables, as expected from theory. Measurements and model results together allowed us to examine the spatial and temporal variations of sublimation on the glacier. On average, sublimation is about two times faster in summer than winter. Rapid sublimation occurs during storms and katabatic wind events, but such periods contribute less to the annual total than do slow, persistent losses. Spatially, sublimation reaches a maximum midway along the glacier, where descending surface air currents are focused by the topography of the aptly named tributary, Windy Gully.

Periglacial weathering and headwall erosion in cirque glacier bergschrunds

Glaciers produce cirques by scouring their beds and sapping their headwalls, but evidence to constrain models of these processes has been elusive. We report a suite of environmental measurements from three cirque glacier bergschrunds, including the first temperature series recorded at depth throughout most of an annual cycle. Compared to the ambient air, the bergschrunds were colder in summer and warmer in winter. Freeze-thaw cycles were rare, and relatively stable subfreezing temperatures persisted from November until May. Using a model for rock fracturing driven by ice segregation, we demonstrate that favorable conditions for fracturing occur not only on the headwall above the glacier, but also within the bergschrund, where periglacial weathering and glacial transport can act together to drive cirque headwall retreat. A small (∼3 °C) year-round decrease in temperatures to conditions more typical of the Pleistocene would likely intensify the weathering process. Though so far ignored in all glacial landscape evolution models, the bergschrund likely plays an essential role in the sculpting of alpine landscapes.

Evidence for extreme pressure pulses in the subglacial water system

A suite of subglacial water-pressure records from the 1996 summer field season at Trapridge Glacier, Yukon Territory, Canada, discloses a hydraulic event that cannot readily be explained by known forcings. We suggest that these records indicate covert failure of the pressure sensors caused by at least one large water-pressure pulse. The sign and magnitude of the pulse appears to have varied spatially and the pulse duration was less than the 2 min sampling interval of our data loggers. Laboratory experiments support this interpretation and indicate that the pulse magnitude exceeded 900 m of hydraulic head, roughly 15 times the ice-overburden pressure. Within glaciers, large water-pressure pulses can be generated when abrupt ice motion changes the volume of the subglacial hydraulic system.

Generalized view of source-region effects on δD and deuterium excess of ice-sheet precipitation

Abstract It is generally recognized at present that ice-core deuterium excess measurements are potentially useful for reconstructions of vapor source-region temperature and humidity history, and that such measurements provide a method for correcting isotopic paleothermometers for effects of source temperature variations. Here we use a zonally averaged vapor-transport and isotopic-distillation model to show that deuterium excess of precipitation on the ice sheets is sensitive to a wide variety of source-region climate changes in addition to those changes of temperature and humidity that affect the composition of evaporate. Moreover, it is demonstrated that this wide variety of source-region changes all cause anticorrelated changes in deuterium excess with δD and δ18O over the ice sheets, suggesting that deuteriu 14m excess is a generally more useful tool for correcting isotopic thermometers than is currently recognized.

Dynamics of an alpine cirque glacier

Alpine cirques are excavated by glacial erosion, a process that depends in turn on the movement of ice by basal sliding. Cirque glacier flow is usually depicted as rotational sliding of a rigid block, but this model is based on little evidence and implies unorthodox glacier behavior given typical cirque dimensions. The small (∼1 km2), temperate West Washmawapta Glacier occupies an archetypal overdeepened and “armchair-shaped” cirque in the Canadian Rockies. We measured (1) the annual surface velocity field, (2) ice thickness, (3) sliding and internal deformation at one borehole, and (4) sliding in a marginal cavity. The glacier moves slowly, with surface velocities of 3 to 10 m/yr. The maximum ice thickness (∼185 m) occurs in the center of the cirque basin and roughly coincides with the position of greatest ice flux. Using our field measurements, a standard constitutive relation for ice, and simplifying assumptions related to the depth distribution of strain rates, we approximated the driving and resisting forces acting on sections of the glacier, and inferred the general pattern of basal sliding. Sliding is minimum in the center of the cirque and increases toward the margins, especially up the stoss side of the riegel. Internal deformation accounts for all motion in the cirque center, even if an unusually low viscosity for temperate ice is assumed. Basal shear stresses tend toward 105 Pa everywhere, a typical value for mountain glaciers. Transverse and longitudinal straining are significant in some parts of the glacier. Although a component of rotational flow must occur internally, the glacier does not conform to the rotational sliding model in any essential respect.

Subsurface hydrology of an overdeepened cirque glacier

To investigate the subsurface hydrological characteristics of an overdeepened cirque glacier, nine boreholes were drilled to the bed of West Washmawapta Glacier, British Columbia, Canada, in summer 2007. All holes were surveyed with a video camera, and four were subsequently instrumented with a combination of pressure transducers, thermistors and conductivity sensors. Diurnal pressure and temperature records indicate the presence of a hydraulically connected subglacial drainage system towards the northern glacier margin. Hydraulic jacking in the overdeepening, controlled by changing water volume in the marginal zone, potentially impacts basal ice flow and erosion. The presence of a sediment layer underlying the glacier also likely impacts hydrology and ice dynamics. Influx of warm groundwater into the basal system raises subglacial water temperatures above the pressure-melting point (pmp) and induces diurnal water temperature fluctuations of as much as 0.8◦C; water temperatures above the pmp could affect basal melt rates and the development of subglacial drainage systems. These observations suggest that the characteristics of the subglacial drainage system substantially affect patterns of flow and erosion by this small cirque glacier.

How nonlinear is the creep deformation of polar ice? A new field assessment

Most analyses of glacial systems require a relationship between strain rates and stresses for ice deforming in creep. For conditions relevant to glacier flow, much evidence shows that strain rates increase approximately as deviatoric stress raised to a power n . Field and laboratory experiments suggest that n ≈ 3, but values span a wide range and controversy persists. Most field efforts to determine n seek clarity by examining situations with simple stress states. We instead use a fully three-dimensional model to interpret measurements of flow of an Antarctic outlet glacier that follows a tortuous path through a mountain range. By comparing observed flow to models with different parameter values, we conclude that the effective n must be between 2.6 and 5.1 (99% confidence). The best match occurs with n ≈ 3.5. We conclude that use of the traditional value is appropriate for polar ice deforming at stresses of 0.6–1.2 × 10 5 Pa.

A peak-capturing measurement circuit for detecting and recording short-duration glacial signals

A simple circuit has been developed to allow measurement of brief subglacial water-pressure pulses. This circuit continuously powers a pressure transducer and captures the peak output of the transducer during each measurement interval, thus allowing determination of the maximum pressure attained during the interval. This circuit provides an alternative to setting a data logger to perform rapid repeated measurements, and overcomes some key limitations imposed by rapid measurement. Benefits include significantly lower demands on the data-logger microprocessor, which allows additional instruments to be monitored simultaneously, reduced memory usage and moderately lower power consumption. The reduced microprocessor and memory loads allow older and slower logger models, many of which are still in common use, to be used to obtain data that compare favourably with high-frequency data obtained using newer data loggers.