Congratulations to Mark Robbins who defended his dissertation on Thursday, December 5, 2019

Advisor: Joel Blum

Abstract:

Existence, configuration, and efficiency of subglacial hydrologic systems have great implications for both glacial dynamics and expressed chemistry in proglacial waters. These networks may exist in poorly connected inefficient configurations, fast flowing highly competent configurations, an intermediary configuration, or all options in different regions of the glacial bed. Directly studying the configuration and development of these networks is understandably difficult as they are obscured by ice, yet important in glaciological research as configuration of these networks control spatial distribution and residence time of subglacial water. Broad subglacial distribution of meltwater may decrease effective ice pressure, enhancing basal sliding. Furthermore, subglacial network efficiency controls subglacial water-rock interaction time affecting chemical weathering reactions and thus solute type and concentration expressed in proglacial outflow. Previous research into the configuration of subglacial hydrologic networks is limited in both temporal and spatial resolution, as field research frequently occurs in the more affable summer months and is limited to more easily accessible glaciers. This dissertation investigates seasonal changes in subglacial hydrologic networks as evidenced by changing meltwater chemistry in late-summer at both an alpine glacier and an outlet glacier from the Greenland Ice Sheet. I undertook multi-month field campaigns at each location, during which I collected samples and in situ measurements to correlate changes in chemical constituents carried within melt to changes in seasonality, improving understanding this understudied time in seasonal glacial development.

This dissertation uses laboratory experiments with sediment samples collected at glacial termini to evaluate the use of radon-222 activity concentrations, an intermediary in the uranium-238 decay chain, as a proxy for subglacial water residence time. These measurements are compared to in field Rn-222 activity concentration measurements at sediment collection locations. Results show Rn-222 activity concentration serves as a subglacial water residence time proxy but also reflects mineralogical sources of its parent isotope, radium-226. Field measurements of Rn-222 activity concentrations as a proxy will be more robust and reliable if supported with laboratory leachate experiments with site-specific sediment samples, addressing likely lithological and sediment-size controls on Ra-226 concentrations.

I undertook a three-month field study of the alpine Athabasca Glacier in the Canadian Rockies in August through October, 2014. Both in situ and elemental chemistry of pro-glacial meltwaters are investigated relative to water discharge fluxes, air temperatures, and precipitation events to see how the subglacial network changes in response to changes in these variables during this time of year. Results reveal immediate changes in subglacial configuration, evidenced by different rates of subglacial chemical weathering, to weather and seasonal change, showing the subglacial environment to be dynamic and very responsive. Methods used at the Athabasca Glacier were then scaled to Kiattuut Sermiat, an outlet glacier from the Greenland Ice Sheet to investigate possible differences in hydrology between alpine and outlet glaciers. Direct comparisons proved impossible owing to an absence of similar environmental forcings during field campaigns, however Kiattuut Sermiat results suggest the possible existence of an interannual subglacial drainage system evacuating waters sourced from further up into the Greenland Ice Sheet. This dissertation presents new measurements of glacial chemistry from an understudied period in seasonal glacial evolution, with interpretations unique to each glacier investigated.