Iceberg A23a has begun to move.

An iceberg known as A23a, considered to currently be the largest in the world, with a surface area of about 4000 km² and a thickness of about 400 m, has begun to move, after remaining grounded within the Weddell Sea for several decades. The iceberg was one of a cluster that broke away from the Filchner Ice Shelf in November 1986, prompting the emergency evacuation of a Russian science station which was located on the breakaway part of the ice, but subsequently became grounded in the shallow waters of the Weddell Sea. The iceberg subsequently moved a short way in 2020, but again became grounded. It has now begun moving again, and appears likely to move out of the Weddell Sea and into the Antarctic Circumpolar Current, which will carry it west towards South Georgia and South Africa.

The current location of Iceberg A23a. Copernicus-Sentinal 3/BBC.

The increased rate of movement and break up of many ice sheets, which leads to the formation of icebergs, is thought to be directly connected to rising global temperatures. However, the movement of icebergs once they have formed is less well understood and it is unclear to what extent these events are climate driven. Icebergs from the Antarctic Peninsula and Ronne Ice Shelf typically become trapped in the Weddell Sea for many years, before migrating into the Antarctic Circumpolar Current. Icebergs in this, slightly warmer, current will tend to melt over time, but can drift a long way first, sometimes becoming stranded off South Georgia or even drifting into the shipping lanes around South Africa.

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Iceberg the size of Majorca calves from the western side of the Ronne Ice Shelf, Antarctica.

An enormous iceberg has calved from the western side of the Ronne Ice Shelf, Antarctica, according to a press release issued by the British Antarctic Survey on Friday 21 May 2021. The iceberg, newly named A-76, measures around 4320 km², and is floating in the Weddell Sea, currently making it the largest iceberg in the world. The calving is considered a natural event and not attributed to climate warming.

Iceberg A-76, which calved from the Ronne Ice Shelf, Antarctica, in May 2021. European Space Agency/Copernicus Sentinel 1.

The ‘megaberg’ was spotted by British Antarctic Survey researchers in recent images captured by the Copernicus Sentinel-1 mission on the Polar View portal. The iceberg is around 170 km in length and 25 km wide, and is slightly larger than the Spanish island of Majorca or the UK county of Somerset.

The enormity of the iceberg makes it the largest in the world, taking first place from the A-23A iceberg (approximately 3880 km² in size) which is also located in the Weddell Sea. In comparison, the A-74 iceberg that broke off the Brunt Ice Shelf, near Halley Reseach Station in February earlier this year, was only 1270 km².

The iceberg was confirmed by the US National Ice Center using Copernicus Sentinel-1 imagery. The Sentinel-1 mission consists of two polar-orbiting satellites that rely on C-band synthetic aperture radar imaging, returning data regardless of whether it is day or night, allowing year-round viewing of remote regions like Antarctica.

Icebergs are traditionally named from the Antarctic quadrant in which they were originally sighted, then a sequential number, then, if the iceberg breaks, a sequential letter.

Kaitlin Naughten, an ocean modeller at the British Antarctic Survey, explained 'Calving is an essential way for ice shelves to stay in balance. Large calving events only occur occasionally, and they can be very dramatic, but they are not necessarily a sign that the system is changing. There is currently no evidence that the Ronne Ice Shelf is calving more often as a result of climate change.'

According to Alex Brisbourne, a glaciologist at the British Antarctic Survey, who has worked on the Ronne Ice Shelf, 'Iceberg A76 is huge, about the size of the county of Somerset in the UK. It wouldn’t make the top 10 list of the biggest known icebergs of all time though. This calving is part of the natural cycle of the Ronne Ice Shelf. The ice shelf is constantly being fed ice from the Antarctic continent, and eventually chunks break off the ice shelf in this way, forming these big flat icebergs and maintaining a balance. Because they are already floating, as they melt, icebergs do not contribute to sea level rise in a significant way. They have been known to eventually get stuck in places like South Georgia in the South Atlantic, disrupting the feeding of Seals and Penguins.'

'We know that the ocean around Antarctica is warming as a result of global heating but the Weddell Sea, where iceberg A76 sits, is not currently experiencing this warming. Elsewhere around the Antarctic continent however, the warming ocean is melting other ice shelves and this is allowing the ice to drain more quickly off the continent, increasing the rate of sea level rise. Of course, this sea level rise isn’t restricted to Antarctica, it affects sea level around the world, including here in the UK, increasing the frequency and severity of storm surges and coastal flooding.'

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Iceberg damage in an Antarctic Clam.

The large Clam Laternula elliptica is found in coastal waters across the Southern Ocean, around the tip of Patagonia and all around the coast of Antarctica. It is considered a keystone species in Antarctic ecosystems, forming an appreciable percentage of the total biomass. It lives infaunally, buried in mud at depths of up to 50 cm, and filtering food from the water column. It faces few predators due to an absence Antarctic animals capable of digging it out, though it is known to be targeted by a type of Fish, Trematomus hansoni, which bites off its siphons.

In a paper published in the journal PLoS One on 28 September 2012, a team of scientists led by Elizabeth Harper of the Department of Earth Sciences at the University of Cambridge discuss the results of a study into damage to the shells of Laternula elliptica, which suggests that iceberg scour is a major source of damage to these animals.

The team sampled Clams from seven sites around Antarctica, and analyzed the shells of living animals for damage and repair, looking for signs of predation or chemical dissolution as well as physical damage. All of the Clams were sampled at depths to great for them to be damaged by storm or tidal turbation.

Map showing the locations from which the Clams were excavated. Harper et al. (2012).

Harper et al. found shells showing signs of recovery from severe crushing at all sites, despite the fact that the Antarctic lacks any known predator which attacks Clams in this way, and the Clams being recovered from sites not targeted by trawl-fishers. The incidence of damaged Clams varied greatly with from 1.7% to 74.2% of the Clams showing signs of having been crushed at different sites.

Typical undamaged Laturnula elliptica shells. Scale bar is 10 mm. Harper et al. (2012).

Laturnula elliptica shells showing signs of damage and repair. Scale bars are 10 mm. Harper et al. (2012).

The damage was most frequent at sited where scouring of the seafloor by icebergs was common, and rarest where permanent pack ice makes scouring by free-floating icebergs unlikely. Clams have been observed being excavated by such scouring and being forced to rebury themselves, suggesting that this scouring was the source of the damage. Harper et al. also noted that Clams in shallower water have thicker shells than those in deeper water, and while the genetics of the Clams is not understood, suggest this is most likely to be as a result of the Clams reacting to their environment by secreting more or less shell, rather than strains of Clams with different shell-thicknesses and depth-preferences being present at numerous sites around Antarctica.

See also The evolution of Galeommatoid Bivalves, Symbiosis and the success of Galeommatoid Bivalves, The biology of pumice rafts, A new species of Scallop from Western Australia and Marine Invertebrates on Sciency Thoughts YouTube.

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