Since it snapped off the Larsen C ice shelf in July 2017, the trillion-tonne iceberg known as A68 has spent most of its time stuck in the mud. Now, new satellite data reveals that the ‘berg made its biggest move yet over the austral winter — a dramatic counterclockwise rotation that shows no signs of stopping.
About a quarter the size of Wales or half the size of Jamaica, A-68 is the sixth largest iceberg on record according to the UK-based Antarctic research group Project MIDAS.
Its break from the Larsen C ice shelf following months of build-up captivated the world last year. But since then, our attention has drifted elsewhere as the iceberg idled around aimlessly, repeatedly becoming pinned to the seabed, or grounded, in a shallow area known as the Bawden Ice Rise.
But in July, under the cloak of darkness that is the Antarctic winter, A-68 began to swing northward. Polar oceanographer Mark Brandon recently spotted the action using brightness temperature data collected by the Suomi NPP satellite. This data produces false-colour images where warmer areas, such as ocean water, shine more brightly than cooler regions, such as ice-covered surfaces.
The images clearly show A-68 making a big counterclockwise swing starting in early July. By the end of August, the ‘berg had rotated itself about 90 degrees, becoming nearly perpendicular with the shelf. NASA glaciologist Christopher Shuman told us he estimates that A-68 has rotated 10 degrees in the past two weeks alone, with almost 30 degrees of rotation in the two weeks prior.
What set A-68 in motion? Nobody’s sure. “One of the problems we have here is that the seabed in this area is fairly poorly mapped,” glaciologist and former Project MIDAS collaborator Martin O’Leary told us via email. “It’s usually clogged with sea ice, which makes it tough to get boats in there.”
Indeed, thick sea ice foiled a British Antarctic Survey-led attempt to reach the giant iceberg and study that seabed back in March.
Still, O’Leary says the iceberg’s recent rotation probably indicates that it’s come loose from whatever it had been pinned against for the past year.
“It might have been shaken loose by winds or ocean currents, or it might be that the natural thinning process (from both melting and the flow of the ice) has lifted the bottom of the iceberg off the sea bed,” he said. “In any case, it looks like the berg is now a lot more free to move about, so it will probably continue to rotate, and to move out to sea.”
Brandon suspects that A-68 will continue on its present course, eventually colliding with the Larsen C shelf. “It has a spectacular amount of momentum and it’s not going to stopped easily,” he wrote in a blog post. While O’Leary says a collision is “certainly possible” he doubts it would have much of an effect on either ‘berg or shelf given the slow-motion speed at which it would occur.
Eventually, O’Leary says the prevailing ocean currents will push the iceberg northwards and eastwards into the Southern Ocean “where it will probably break up and melt”.
We’ll have to wait and see what’s in store next for A-68. But it seems that this iceberg hasn’t finished making a spectacle of itself.