Voyeykov Ice Shelf in East Antarctica disaggregated in 2007

Large-scale ice shelf retreat is important because it can reduce the back-stress exerted on grounded ice further inland and trigger enhanced ice discharge - a process known as ice shelf de-buttressing.

An important process driving ice shelf retreat is iceberg calving. The frequency of major calving events can be modulated by ice mélange (an amalgamation of sea ice, marine ice, firn and densely packed calved icebergs).

Landfast sea ice, which is sea ice that becomes ‘fastened’ to the coastline, floating ice shelves/tongues, or grounded icebergs can often be a major component of mélange. Landfast ice that persists year-round, known as multiyear landfast ice, can thicken to form extensive, decades-old slabs up to tens of metres thick, which provides stability to floating glacier tongues and ice shelves.

Voyeykov Ice Shelf (66°S 124°E) drains an area of ∼7104 km2 into the 45 km-wide Maury Bay in Wilkes Land. It is a heavily fractured, unconfined ice shelf that terminates in a dense ice mélange composed of calved ice blocks, bound by perennial multiyear landfast sea ice. This sea ice is present year-round in front of the ice shelf rather than breaking up seasonally, as in other parts of Antarctica and Greenland.

Between late March and late May 2007, the combined ice shelf-mélange area lost ∼2445 km2 of ice, of which 9% was intact ice shelf fed by two glaciers. The disaggregation event occurred in two phases:

1. Evacuation of pre-calved blocks and surrounding mélange in front of the ice shelf into the open ocean.
2. Large blocks detached from the ice shelf calving front, following pre-existing rifts.

Here is a summary of the structural changes in the ice shelf prior to, during and following the disaggregation event in 2007:

We used NASA ITS_LIVE annual velocity mosaics to investigate velocity changes on the upper and lower portions of the ice shelves, as well as at the grounding line (the point at which the ice starts to float) before and after the disaggregation event.

We recorded no significant ice flow velocity acceleration in the years immediately prior to or following disaggregation. This suggests the disaggregated portion of the ice shelf and proglacial mélange did not exert a significant buttressing effect and therefore had minimal influence on the dynamics of inland grounded ice.

We observed a major negative sea-ice concentration anomaly between the end of March and the end of April 2007, when mean April sea-ice concentration deviated by 33% from the long-term (2002–2011) April average. This negative anomaly coincided with MYLI breakout and the transition from mélange to open-ocean in front of VIS visible in MODIS and Envisat SAR imagery in April 2007.

Both the mélange/landfast sea ice and the drifting pack ice further offshore experienced a sharp decline after 30 March, returning to concentrations above 90% by 27 April. Sea ice breakout was short-lived and was preceded and followed by greater-than-average mean sea-ice concentrations in March and May 2007.

We suggest the mélange provided some degree of mechanical coupling and enhanced the structural integrity of the ice shelf by filling interconnected rifts and cementing together large, detached ice blocks in the decades prior to disaggregation.

Our observations suggest the mélange provided long-term stability to Voyeykov Ice Shelf until it started to become weakened in the preceding autumn/winter,

The multiyear landfast sea ice adjacent to Voyeykov Ice Shelf is likely to have been weakened and blown offshore by katabatic winds and strong south-easterly winds in the preceding winter, and may have also been weakened by katabatic-induced melt ponding in January 2007.

The sea-ice break-out is also likely related to as a decline in latent-heat polynya extent, which reduced sea-ice production and would have enabled the incursion of warmer surface waters to reduce mélange rigidity and weaken the ice front, making it susceptible to disaggregation.

Our observations show that Voyeykov retains a strong dependency on mélange and multiyear sea ice for its structural integrity, though it does not show signs of imminent disaggregation.

The ice shelf now resembles the present state of other heavily fractured glacier tongues and ice shelves in Antarctica, such as Thwaites Ice Tongue.

Large amounts of mélange could be generated by the retreat of vulnerable marine-based ice shelves into unconfined embayments in future. Damaged areas on glaciers like Thwaites and Pine Island are already generating mélange.

Our observations call for the role mélange in ice shelf stability to be considered in other locations around Antarctica, where unconfined ice shelves cemented with mélange and sea ice provide back-stress to inland ice.

The inclusion of these complex ice shelf-mélange interactions in numerical modelling efforts is crucial for accurately simulating calving dynamics and future sea-level contributions.

The large-scale retreat of Voyeykov Ice Shelf in March–May 2007 comprised ∼2445 km2 of ice loss involving the disaggregation and removal of a complex ice mélange, detached ice blocks and part of the ice shelf.

• The mélange in front of Voyeykov bound the ice shelf together and prevented it disaggregating prior to 2007, although it did not provide significant buttressing to grounded ice because of its unconfined setting.

• The ice shelf was likely preconditioned for disaggregation by the surrounding mélange and landfast sea ice becoming weakened by strong south-easterly winds in the preceding winter and by katabatic wind-driven melting in January 2007.

• The timing of disaggregation coincides with a major negative sea ice anomaly adjacent to Voyeykov, which we link to anomalously warm regional atmospheric conditions and a less persistent latent heat polynya.

• Surface meltwater on the ice shelf itself was not an important trigger of this disaggregation, indicating surface melt pond-driven hydrofracturing is not always a necessary precursor to ice shelf collapse.

Taken together, these processes represent an additional mechanism of ice shelf break-up, whereby the weakening and removal of mélange and multiyear landfast sea ice trigger disaggregation of the structurally-weak ice shelf.

For more information, please check out our paper in Journal of Glaciology:

Arthur JF, Stokes CR, Jamieson SSR, Miles BWJ, Carr JR, Leeson AA (2021). The triggers of the disaggregation of Voyeykov Ice Shelf (2007), Wilkes Land, East Antarctica, and its subsequent evolution. Journal of Glaciology 1–19. https://doi.org/10.1017/jog.2021.45