Joe MacGregor

Research Interests

I am interested in the glaciogical settings of modern ice sheets and glaciers. Why do they flow quickly in some areas but not others? How can we observe the properties that control fast flow? These are outstanding questions for large sectors of both the Greenland and Antarctic ice sheets and are critical for predicting their behavior using ice-flow models. These questions often involve the nature of the ice bottom, where the ice meets hard bedrock, soft till, water, or some combination of those materials, so that's a common theme in my work.

These interests broadly include the following topics: glacier and ice-sheet dynamics; subglacial conditions and geology of Greenland and Antarctica; active and passive geophysical methods; glaciochemistry; dielectric properties of ice; crevasses; geophysical inverse theory; ice-sheet-climate interactions; climate change.

Current and Recent Work

• Basal conditions and margin migration of Thwaites Glacier, West Antarctica

  Thwaites Glacier is one the fastest and fastest-changing glaciers in Antarctica today. During the 2008-9 austral summer, I participated in a PSU-led traverse from the ice-core site near the West Antarctic ice divide between the Ross and Amundsen Sea embayments, to upper Thwaites Glacier to investigate basal conditions there. We used several radar systems, active seismic surveys and GPS surveys to investigate the relationship between ice flow and basal conditions there.

  At UTIG, I am investigating the behavior of the margins of Thwaites Glacier. We want to determine how its fast-flow margins have migrated in the past (centennial to millenial scale) to places its current rapid changes in context. The basal conditions at these margins will almost certainly play a role in these migrations. To accomplish this study, we will be collecting new radar data and analyzing the existing AGASEA dataset, collected by UTIG's airborne geophysics group.

• Deep ice over subglacial Lake Vostok, East Antarctica

  Lake Vostok is a very large subglacial lake in the middle of the slow-moving East Antarctic plateau, and has anomalous basal conditions compared to most of the rest of the inland ice sheet. There is both melting and accretion of ice occurring over the lake, and the spatial distribution of this basal mass balance may further influence the dynamics of the ice-sheet flow over the lake. To paint a better picture of this behavior, we studied the nature of the surmised reflector near the boundary of the meteoric ice (snow that fell on the surfce and eventually densified into ice) and accreted ice (supercooled water that froze onto the ice-sheet bottom from the lake) that was observed in the Vostok 5G ice core, I've also modeled millennially averaged accumulation rates for the Lake Vostok region and studied the horizontal spatial variation of englacial radar attenuation rates over the lake. All of that work is described in two published papers and one forthcoming paper. Finally, along with others, I'll be developing a full picture of the radar-inferred nature of the ice-lake interface.

• Englacial radar attenuation

  Understanding the relationship between ice chemistry, temperature and radar attenuation is important for studies of echo intensities in radar data. Radar attenuation is often poorly constrained but it must be estimated to calculate, for example, the bed reflectivity, which is diagnostic of a frozen or thawed basal condition. To model radar attenuation, I synthesized an temperature-, chemistry- and density-dependent ice-conductivity model, based on published measurements of the dielectric properties of ice. This model was initially tested using ice-core and radar data from Siple Dome, West Antarctica.
  
  Matlab scripts and functions for modeling ice-sheet radar-attenuation rates

  
  

Publications

Jacobel, R.W., B.C. Welch, D. Osterhouse, R. Pettersson and J.A. MacGregor, 2009, Spatial variation of radar-derived basal conditions on Kamb Ice Stream, West Antarctica, Annals of Glaciology, 50(51), 10-16.

MacGregor, J.A., K. Matsuoka, M.R. Koutnik, E.D. Waddington, M. Studinger and D.P. Winebrenner, 2009, Millennially averaged accumulation rates for the Vostok Subglacial Lake region inferred from deep internal layers, Annals of Glaciology, 50(51), 25-34. pdf

MacGregor, J.A., K. Matsuoka and M. Studinger, 2009, Radar detection of accreted ice over Lake Vostok, Antarctica, Earth and Planetary Science Letters, 282, 222-233. pdf

MacGregor, J.A., D.P. Winebrenner, H. Conway, K. Matsuoka, P.A. Mayewski and G.D. Clow, 2007, Modeling englacial radar attenuation at Siple Dome, West Antarctica, using ice chemistry and temperature data, Journal of Geophysical Research, 112, F03008. pdf

O'Neal, M.A., M.E. O'Mansky and J.A. MacGregor, 2005, Modeling the natural degradation of earthworks, Geoarchaeology, 20(7), 739-748.