Research Highlight: After the Break

The calving of an iceberg the size of Delaware from Antarctica’s Larsen C ice shelf announced on July 12 made headlines around the world, more than a year after scientists first noticed that the separation had begun.

But for Antarctic researcher Maria Vernet, a polar ecologist at Scripps Institution of Oceanography at the University of California San Diego, this event is just the beginning of the story.

Vernet studies phytoplankton, microscopic plants that make up the base of the marine food web, and she’s particularly interested in polar regions, including Antarctica. Her science starts now, as the Larsen C iceberg ­– measuring 5,700 square kilometers (2,200 square miles) – begins to drift away from the ice shelf, creating a new ecosystem around it and in the space it leaves behind.

The Larsen ice shelf has been slowly breaking up for decades, in a pattern that appears to be migrating progressively southwards. The northernmost portion, Larsen A, a 2,000-square-kilometer (770-square-mile) chunk disintegrated in 1995, and Larsen B, a piece roughly 3,250 square kilometers (1,250 square miles) in area, collapsed in 2002. Vernet and her former PhD student, Mattias Cape, now a postdoctoral scholar at the University of Washington, studied the ecological aftermaths of those two break-ups, and they offer some clues of what could happen after this Larsen C calving event.

For starters, the iceberg will seed new areas of the Weddell Sea and beyond with iron, nitrates, and other nutrients trapped in the dust and sediments as the ice shelf from which it calved formed on land. This type of ocean fertilizer will encourage primary productivity at the base of the food web.

It will take some time for this iron-seeding effect to have maximum impact, because the iceberg is still close to the continental shelf, where the waters are more nutrient-rich, the researchers said.

By July 18, less than a week after the final separation was announced, the iceberg was already nearly three kilometers (1.8 miles) away from Larsen C at its south end. Janet Sprintall, a physical oceanographer at Scripps who has studied the physical environment associated with icebergs, said it is unknown exactly how long the iceberg’s journey to open water will take.  

“It depends on winds, ocean currents, Earth's rotation and surrounding ice conditions, to name just a few factors,” she said. “But it will probably follow a well-travelled route known as ‘iceberg alley’ that flows north along the east side of the Antarctic Peninsula toward the South Orkney Islands before being entrained into the eastward flowing Antarctic Circumpolar Current that circles Antarctica.”

In the meantime, the open embayment that the iceberg leaves behind will also become more fertile. Cape had observed the transformation when he looked at the area of open water left behind by the disintegrated pieces of Larsen B as they drifted away from the continent.

“Larsen B was present for thousands of years, and was hundreds of meters thick, so it completely blocked sunlight from reaching the surface of the ocean,” he said. “So underneath this ice shelf there was no photosynthesis, no phytoplankton growing, which means there weren’t many food sources for organisms beneath the ice shelf.”

But when this huge ice cover is removed, “you’re suddenly exposing an area of the ocean the size of a small U.S. state to sunlight,” Cape said, “and now you have plankton growth, and potentially have fish, krill, and whales using that habitat.”

It’s a “complete change in the ecosystem of that region,” he added.

Vernet said the removal of the ice cover “basically makes the ocean a little bit bigger.”

Both these effects—the opening of a new ocean ecosystem in the uncovered water left behind, and the deposit of iron into nutrient-limited open ocean as the iceberg melts—have the effect of increasing photosynthesis in the water, and thus increasing carbon removal from the atmosphere.  In that sense, Vernet said the break-up of the ice shelves provide negative feedback loops counteracting the effects of climate change, sucking up carbon dioxide that would otherwise contribute to global warming. But she and Cape agree that this effect isn’t likely to have a significant impact on climate change overall.

“Even if you remove all the surface area of all of those ice shelves, I don't think there would be much of an effect because of the strong seasonal fluctuations in the area, and the presence of relatively permanent sea ice cover in the region,” Cape said.

Antarctic scientists like Cape, Vernet, and Sprintall are intensely interested in the aftermath of the Larsen C break-up, because, as Sprintall pointed out, there are still so many unanswered questions about icebergs in general, let alone one this big.

“There have been only a few studies, and the biological and physical effects do not tend to scale with the size of the iceberg, which makes their study difficult,” Sprintall said. Although scientists including Cape and Vernet have documented that icebergs are hotspots of chemical and biological enrichment, it’s not clear to what extent the currents and other physical factors around the iceberg directly impact the biology.

“The role of the ocean currents and physical environment in dispersing the chemical inputs and biological productivity associated with icebergs is very much an open question,” Sprintall said.

But for the most part, oceanographers will have to attempt to answer these questions from afar.

“It's going to be really challenging to evaluate all these effects,” Vernet said, “This area of Antarctica is very difficult to study because of the amount of sea ice.”

She has tried on three different research expeditions to reach Larsen C; the first two times sea ice blocked the way. The third time, her team made it but the ship was only there for a few days before the wind shifted. The opening in the sea ice they had come in through began to close forcing a hasty retreat, and she hasn’t been back since.

So with the relative scarcity of field data on Larsen C and the surrounding region, its evolution and the ecosystems it creates will be mostly monitored by satellite, which scientists can learn a lot from, Cape said.

“When you combine data from satellites and sparse measurements from the field, you can use computer simulations to try to look at what potentially might be the future trajectory of these ice shelves and glaciers” to try to figure out how the system will evolve—and what’s in store for Antarctica’s future, he said.

The researchers’ study of the Larsen ice shelves has been carried out with funding from the National Science Foundation Office of Polar Programs.

• Mallory Pickett

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