Semester of Graduation

Summer 2023

Degree

Master of Science (MS)

Department

Geology and Geophysics

Document Type

Thesis

Abstract

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.

Date

7-12-2023

Committee Chair

Phil Bart

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