Investigating the Hydrology of Ice Shelves, and Its Implications for Ice-Shelf Stability

• Main Author: Macdonald, Grant John
• Format: Thesis
• Language: English
• Published: The University of Chicago 2019
• Subjects: Geophysics; FOS Earth and related environmental sciences; McMurdo Ice Shelf; Antarc*; Antarctica; Ice Sheet; Ice Shelf; Ice Shelves; Sea ice
• Online Access: https://dx.doi.org/10.6082/uchicago.2097; https://knowledge.uchicago.edu/record/2097

Studies show that surface lakes that form on ice shelves can promote ice-shelf instability, which has consequences for wider ice-sheet stability. Meltwater on ice shelves will become increasingly pervasive due to climate change and it is therefore crucial that we improve understanding of ice-shelf hydrology and its implications for ice-shelf stability. First, we use remote sensing to analyze the seasonal evolution of surface lakes on Peter- mann Glacier’s floating ice tongue (a narrow ice shelf). Lakes start to form on the tongue in June and quickly peak in total number, volume and area in response to increases in air temperatures. However, despite sustained high temperatures, total lake number, volume and area decline through July/August. We suggest that rapid vertical lake drainage events, and evacuation of meltwater from lakes into the ocean via a surface river, limits water storage in lakes on the tongue. Additionally, mean areas of lakes on the tongue are calculated to be ∼20% of those on the grounded ice. Second, we use remote sensing and field data to analyze and document the formation of pedestalled, relict lakes (‘pedestals’) that develop on the debris-covered part of the McMurdo Ice Shelf, Antarctica. Pedestals form when a surface lake that develops in the summer, freezes-over in winter, resulting in the lake-bottom debris being masked by a high-albedo, ice surface. If this ice surface fails to melt during a subsequent melt season, it experiences reduced surface ablation relative to the surrounding debris-covered areas of the ice shelf. We propose that this differential ablation, and resultant hydrostatic and flexural readjustments of the ice shelf, causes the former lake’s surface to become increasingly pedestalled above the surrounding ice shelf. The development of pedestals has a significant influence on the surface-energy balance, hydrology and, potentially, flexure of the ice shelf. Third, we apply an elastic model to investigate the flexural effect of pedestal formation on ice shelves. Our idealized model results suggest that the stresses produced by the effect of pedestal formation on ice-shelf flexure are unlikely to be sufficiently high to cause fracturing or threaten ice-shelf stability. Only in cases where ice-shelf thickness is very low, and surface ablation is sufficiently high (conditions which could become more common due to climate change), does the model suggest that ice-shelf fracturing would occur. Fourth, we use remote sensing to analyze the formation of sea ice ponds from ice-shelf runoff, adjacent to the McMurdo Ice Shelf. Each summer, meltwater flows from the ice shelf onto the sea ice and forms large (up to 9 km2) ponds. This is an undocumented mechanism for the formation of sea ice ponds. These ponds decrease the sea ice’s albedo, thinning it, and we suggest the added mass of ice-shelf meltwater runoff causes the ice to flex, promoting sea-ice instability close to the ice front. As surface melting on ice shelves increases, we suggest that ice-shelf surface hydrology will have a greater effect on sea ice. By improving understanding of ice-shelf hydrology and processes, this thesis will enable the scientific community to better predict the response of the polar ice sheets to climate change.