Ocean tides trigger ice shelf rift growth and calving

Abstract

Tabular iceberg calving reduces ice-shelf extent, affecting ocean circulation and ice-sheet stability. Here we present detailed observations of a rift on the Brunt Ice Shelf, East Antarctica, from 2017-2023 and its behaviour in the lead up to calving in January 2023. The timing of rift propagation was controlled by the rate of change of ocean tide height, wind speed, and an iceberg collision in August 2021, as well as the long-term ice dynamics. A viscoelastic rheological model is used to estimate the relative magnitude of stresses acting on the rift and to determine a critical threshold for fracture, which was exceeded during a sequence of propagation events in early 2019. The eventual calving on 22nd January 2023 occurred at the peak of a spring tide, supporting the conclusion that tides directly influenced the timing.

Fig. 1. Time series of rift growth on the Brunt Ice Shelf.

a Overview of the Brunt Ice Shelf in the Weddell Sea; b the location of panels (c–h); c–h Landsat imagery of Chasm-1 from 2017 to 2023, showing the location of instruments and the rift tip; i Rift width at fixed GNSS baselines NN00-OO00 & TT01-KK00 (right axis) and an ApRES moved annually (left axis), showing the shorter time periods covered by other figures and the timing of A-74 collision and A-81 calving; j Satellite-derived positions of the rift tip relative to the McDonald Ice Rumples (MIR), with reference to the timing of panels (c–h).

Fig. 2. Crack opening angle and estimated stress during 2019.

a during propagation and b during stagnation with a least-squares fit to the observations; c and d the six strongest of the thirteen terms that sum to produce the fitted opening rate (${\mathrm{\psi }}_{\mathrm{Ew}}$ and ${\mathrm{\psi }}_{\mathrm{V}{\mathrm{t}}^2\mathrm{w}}$ are small and hidden behind other terms); e, f a separation of the three forcing terms, converted to stresses at the tip.

Fig. 3. Timing of rift propagation relative to the modelled stress during early 2019.

a Overview of location of the rift tip, the same as in Fig. 1c–h;b modelled variability in stress showing that rift propagation coincides with periods when a threshold (here 240 kPa) is exceeded; c–l TerraSAR-X backscatter images from before and after each propagation event.

Fig. 4. Iceberg collision with the ice shelf in 2021.

a, b Sentinel-1 imagery showing collision between A-74 iceberg and the Brunt Ice Shelf; c–f frames from animation (Supp. 1) showing movement of the iceberg over a period of 2 days; g GNSS data showing 7 m of widening of Chasm-1 at the TT01-KK00 baseline on top of a background rate of 0.10 m/day and 3 m of widening at the NN00-OO00 baseline on top of a higher background rate of 0.44 m/day (See Fig. 1g for baseline locations).

Fig. 5. Behaviour in the month leading up to calving.

a the ice shelf before calving from Landsat-9, same region as Fig. 1c-h;b the ice shelf after calving from Sentinel-2; c the tidal amplitude at Halley (left) and relative displacement of the iceberg at ZZ06 (right) throughout January 2023; d the same as (c) covering the four days around the calving.