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Ethan Pierce aboard the Porshild, the research vessel of the Arctic Research Station in Disko, Greenland. (Irina Overeem)

Greenland’s winding, rocky fjords are no strangers to research vessels. Usually, these boats give icebergs a wide berth, because they can roll over unexpectedly.��

That wasn’t the case, though, for the boats carrying INSTAAR fellow and CU associate professor Irina Overeem and her former PhD student during the 2019 and 2022 summer field seasons. They were there for the icebergs.��

Relying on the caution and expertise of Greenlander captains, the scientists sidled up to the floating monoliths aboard small dinghies and carefully chipped off samples before returning to the main boat.

A small dinghy carries Irina Overeem, Tom Marchitto, and Mia, a Greenlandic deckhand, out to sample an iceberg. (Nora Matell)

“We really were relying on the Greenlanders a lot for their sense of what was safe and what was not,” Pierce said.��

“They fish in that environment themselves, so they have a ton of experience doing that risk calculation,” Overeem added.��

Five years later, that calculated risk is paying off. Pierce, Overeem and���Թ��� of Copenhagen associate professor emeritus published a .��

Tom Marchitto paddles a packraft out to sample a dirty iceberg in Nuup Kangerlua (Irina Overeem)
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The investigation is the first to unravel the complex process underlying a phenomenon that has an outsized impact on the Arctic Ocean’s chemistry. The scientists estimate that icebergs account for around one third of all of the sediment leaving Greenland (the rest comes from meltwater). That sediment unloads nutrients into Arctic waters, which support organisms at every scale — from phytoplankton to whales.

Irina Overeem poses with her catch in a Greenlandic fjord. (Tom Marchitto)

Importantly, the scientists were also able to determine the effect of climate change on this process. As the planet warms, icebergs will deposit more and more sediment into the ocean.

“This is the first modern study of this process that can say, ‘with a warming climate we’re going to see more transport of ice-rafted debris,’” Pierce said.

A back-of-the-envelope calculation

Greenland contributes around 15 percent of the sediment that ends up in the ocean each year — an exceptionally high figure for a landmass of its size. Nearly a decade ago, Overeem published�� characterizing the amount of sediment coming from Greenlandic meltwater rivers. She found that these waterways were unloading a vast amount, but they didn’t account for all of Greenland's sediment export.

At the time, Overeem already knew that icebergs might be the missing piece of the puzzle. For years, she had seen dark stripes of debris crisscrossing icebergs in Greenlandic fjords. And, ice-derived debris has long been found in sediment cores pulled from the bottom of the Atlantic Ocean. But, scientists had yet to figure out how exactly the debris got in the ice or how much of it left the continent this way.��

On the flight home from her 2016 field season, Overeem started jotting down rough equations based on previous research from colleagues. The numbers shocked her.

“At the time, I did a back-of-the-envelope calculation,” she said. “I was shocked at how much it could be.” “I pitched the idea to the CU Research and Innovation Office and they funded a pilot project.”

By 2019, Overeem brought Pierce on as a PhD student and put him on the project. Unfortunately, the pandemic delayed multiple field seasons, but Pierce used the extra time to drill down on a mathematical model of how the debris ends up in the ice in the first place.

According to Pierce’s model, the weight of the massive Greenland ice sheet creates pressure points where the ice comes into contact with individual grains of sand on the earth below. These pressure points create heat, which melts the ice. The meltwater then refreezes around the grain of sand. As the ice sheet slides downhill toward the ocean, this process sucks up more and more sediment into the bottom layer of ice. Eventually, the ice reaches the waters edge and breaks off, forming a sediment-laden ice berg.

Pierce's model combined with the field sample data produce the central insight of the new paper — that icebergs contribute about one third of Greenland’s sediment export.

“There are lab experiments and grain-scale models that show that these processes happen, but Ethan is the first person who put together a numerical model that can then be extrapolated out on the scale of the Greenland ice sheet,” Overeem said.��

Powering new research

Now that Overeem’s back-of-the-envelope calculation has grown into a sophisticated model, it’s time for the researchers to pass their data on to other scientists. The model could be useful for myriad other projects, including offering hints into ancient climatic conditions.

Another potential application is more forward looking. As sediment in the Arctic Ocean increases, it will increase the abundance of minerals like iron and silicon. Those are minerals used by phytoplankton, the microscopic foundation of the Arctic marine food web.

“People who are good at observing phytoplankton over the long-term record have seen an uptick in Greenland,” Overeem said. “There’s definitely some interest in this from that community.”

It’s not yet clear exactly how an increase in sediment might affect life in the ocean, but the question could spark further collaborations within the Institute of Arctic and Alpine Research. Tom Marchitto’s laboratory excels at precise measurements of dissolved chemicals, a capability that could further resolve the contents of Greenlandic sediment. On the biological side of things, INSTAAR director Nicole Lovenduski’s lab specializes in modeling phytoplankton blooms in the Arctic.

“This is just one part of a number of potential connections,” Overeem said.

For now, Pierce will move onto other projects as a postdoctoral researcher at Dartmouth, and Overeem will turn her attention to other pressing surface process models. Both can rest easy knowing they placed another puzzle piece in the answer to a question that has loomed over generations of Greenlandic science.