Collection and Analysis of Shear Strain Data of Polythermal Ice from
Jarvis Glacier, Alaska
Abstract
We seek to calibrate the flow law for polythermal ice through shear
strain analysis. In a warming climate, increased melting of glaciers and
ice caps play a big role in sea level rise. Approximately 60% of the
current contribution to sea level rise from ice loss is attributed to
glaciers and ice caps, raising the urgency of sharpening mass balance
change predictions in regions of streaming flow. Polythermal glaciers
constitute a significant portion of these contributing glaciers, though
our knowledge of their flow dynamics is incomplete. Thermally complex
polythermal glaciers have both warm and cold ice which lead to weak
wet-based beds, with significant amounts of basal sliding and deformable
till. Consequently, polythermal glaciers experience significant shear
strain as their lateral shear margins sustain the majority of the
resisting stress. Most in-situ and in-lab studies of natural ice over
recent years have focused on bodies of ice with frozen beds that
experience minimal shear strain downglacier and across vertical planes
(with depth) relative to the bed. The lack of studies on wet-based
polythermal glaciers causes uncertainties in the flow law, as
differences in flow law factors between polythermal ice and bodies of
ice with frozen beds have the potential to induce more than an order of
magnitude difference in ice velocity. To improve calibration of the flow
law for polythermal ice, we seek to improve our understanding of their
shear strain regimes. We developed and deployed tilt sensor systems on
the polythermal Jarvis Glacier in Alaska, where we drilled multiple
boreholes close to Jarvis’ shear margin and installed three boreholes
with our tilt sensor systems. The tilt sensors measure gravity, magnetic
and temperature data, and each system consists of multiple sensors
connected along a cable and serially communicating along a common data
bus with a datalogger. We have recently retrieved a year of Jarvis tilt
sensor data and calculated the at-depth shear strain rates in the
boreholes, allowing evaluation of the at-depth shear strain rate regimes
of polythermal ice against theoretical models developed using Glen’s
flow law. We present the development of our data collection methodology
and the results of our shear strain analysis, with suggestions for
potential calibrations of the flow law for polythermal ice.