Long-term observations of terminus position change, structural glaciology and velocity at Ninnis Glacier, George V Land, East Antarctica (1963-2021)

Abstract

Over the last four decades, some major East Antarctic outlet glaciers have undergone rates of retreat, thinning and acceleration in response to ocean-climatic forcing. However, some major East Antarctic outlet glaciers remain unstudied in the recent past. Ninnis Glacier is one East Antarctic outlet glacier that is potentially vulnerable to future ocean-climate change and requires monitoring. This thesis quantifies and analyses long-term (1963-2021) changes in terminus position, structural glaciology and velocity at Ninnis Glacier. The results of this study show that Ninnis underwent three major calving events (in 1972-1974, 1998 and 2018), characterised by a 20–25-year periodicity and indicative of a naturally occurring cycle. Each respective calving event created a large-scale tabular iceberg and formed a new terminus position at similar locations up-ice relative to Ninnis’ 1992 grounding line position. The major calving events in 1998 and 2018 were controlled by the development of a central rift system that appears in the same location on Ninnis’ tongue, reinforcing the notion of a predictable calving cycle. Ice flow velocity trends before the 2018 calving event (2017-2018) revealed no discernible change in velocity immediately up-ice (+0.2 %) and down-ice (>0 %) of the 1992 grounding line, suggesting that rifting took place within a ‘passive’ sector of Ninnis’ ice tongue. Between 2018 and 2021, Ninnis underwent a pervasive deceleration up-ice (-2.1 %) and down-ice (-1.4 %) of the 1992 grounding line and on the distal ice tongue (-18.7 %). This indicated that the 2018 calving event did not result in the loss of dynamically important ice. Although Ninnis has previously been deemed a sector at risk of retreat, it is concluded that Ninnis is not currently undergoing Marine Ice Sheet Instability and is not currently sensitive to external forcing. This is consistent with low basal melt rates, negligible grounding line retreat and low thermal forcing temperatures in the coastal waters observed at Ninnis.