Going against the flow: Pinning points in the Ross Ice Shelf, Antarctica

• Main Author: Still, Holly Rebecca
• Other Authors: Hulbe, Christina
• Format: Thesis
• Language: English
• Published: University of Otago 2020
• Subjects: Antarctic glaciology; ice dynamics; ice mechanics; ice shelves; pinning points; ice rise; ice sheet model; Antarctic; Bindschadler Ice Stream; Ross Ice Shelf; Shirase Coast; The Antarctic; West Antarctica; Antarc*; Antarctica; Ice Sheet; Ice Shelf; Ice Shelves
• Online Access: http://hdl.handle.net/10523/10149

The ice shelves surrounding much of the Antarctic continent are supported by pinning points, sites of localised grounding where the floating ice runs aground on the seafloor. Pinning points regulate ice shelf flow and thickness by generating flow resistance, and can in turn, modify grounding line position and tributary glacier dynamics. Ice rises and rumples, the surface topographic expressions of pinning points, are common features in the Ross Ice Shelf (RIS), West Antarctica. Large ice rises in the RIS are known to contribute to flow resistance and ice shelf stability but are unlikely to change over the coming decades, while smaller-scale ice rumples are more vulnerable to changes in the degree of basal contact. This thesis examines how pinning points regulate the present-day flow of the RIS. Two separate studies are undertaken to elucidate the full range of ice mechanical and dynamical effects associated with smaller-scale pinning points that are often overlooked in numerical models of the ice sheet-ice shelf system. The first study presents a mechanical inventory of 15 pinning points in the RIS. A force budget technique is applied to quantify the magnitude and direction of resistive forces generated by individual pinning points. Basal drag inferred from the pinning point force budgets varies by two orders of magnitude, implying that variations in the subglacial material directly affect the flow resistance generated by each feature. Of all the RIS pinning points, a collection of smaller-scale, lightly-grounded ice rumples are remarkable for generating flow resistance comparable to large ice rises. These ice rumples are investigated in more detail in the second study. The second study uses a numerical model of RIS and tributary ice stream flow to examine how the Shirase Coast Ice Rumples (SCIR) in the eastern RIS regulate the behaviour of the interconnected ice shelf-ice stream system. Two configurations are compared: (1) the present-day RIS with the SCIR included and (2) a perturbed model with the SCIR removed from the model domain. Differences between the two simulations demonstrate how the SCIR modify ice flow, thickness, grounding line position, mass flux, and the distribution of stresses resisting ice flow. The SCIR promote a slower-flowing eastern RIS, a more seaward grounding line position, and a decrease in mass flux through the MacAyeal and Bindschadler Ice Stream outlets of 2.3% and 3.4%, respectively, in comparison to an RIS configuration without the SCIR. When the SCIR are removed, the flow resistance generated by other grounded features increases to maintain the balance of forces acting on ice shelf flow. This mechanism limits the magnitude of the RIS speed-up. The mechanical and dynamical effects of pinning points are timely mechanisms to investigate given current observations of rapid mass loss along west Antarctic coastal margins triggered by ocean warming. This thesis quantifies both the immediate and multi-decadal consequences of changes in pinning point configuration and elucidates why it is important to include even small-scale pinning points in model inversions for ice material properties, as well as in model projections of variability in the ice sheet-ice shelf system.