Semester of Graduation

Summer 2023


Master of Science (MS)


Geology and Geophysics

Document Type



Previous studies of the Whales Deep Basin (WDB) outer continental shelf showed that the paleo-ice shelf fronting the Bindschadler Ice Stream broke up at 12.3 cal kyr BP. A calving cliff was subsequently maintained until at least 11.5 cal kyr BP. Slightly after that time, the grounding line rapidly retreated creating a 200 km embayment of grounded ice over the foredeepened middle continental shelf. The rapid opening of the embayment is recorded by a backstepping succession of small-scale morainal ridges on the middle continental shelf. The overlapping and end-to-end spacing of morainal ridges requires that the retreating grounding line experienced small-scale back-and-forth oscillations that averaged only a few kilometers. Iceberg furrows that cross the ridges in 500-600 m water depths suggest that a calving cliff remained intact during the rapid retreat of grounded ice. The occurrence of cross-cutting iceberg furrows weakens an interpretation of a previous core-based study (McGlannan et al., 2017) that suggested a large ice shelf reformed immediately after the onset of retreat. Distinguishing between an ice shelf and ice-cliff style of retreat has important ramifications for discerning the conditions that preceded collapse, i.e., a rapid and large distance contraction of grounded ice. We evaluated eight cores widely distributed across the WDB middle continental shelf. The cores were acquired during expeditions NBP-9407 and NBP-1502B. Our analyses focused on diatom abundance and diatom assemblage data for the post-glacial sediments. A high abundance of sea-ice diatom assemblages provides strong evidence for the onset of open-marine conditions following the grounding-line retreat. The post-glacial sediment changes at most core stations show a back-and-forth oscillation between open-marine and grounding-line proximal sediments. The grounding-line proximal sediments are represented by low diatom abundances dominated by reworked specimens. Given the morphological constraints requiring that grounding line oscillations were on the order of a few kilometers, these upcore changes are most consistent with retreat of a calving cliff with at most a small ice shelf (<5 >km, measured perpendicular to the retreating grounding line). These findings validate concern that present-day ice shelf thinning and loss could lead to runaway retreat and collapse of West Antarctic Ice Streams.



Committee Chair

Phil Bart

Available for download on Thursday, July 11, 2024