Seeing sea ice formation


The structure of an ice core tells a story about its life cycle; you can take a look and read it like a timeline. Geophysicist Andy Mahoney, assistant research professor in the University of Alaska Fairbanks Geophysical Institute, extracted a sea ice core offshore from Barrow, Alaska. He described how sea ice takes form.

When a freeze begins in open water it creates a mixture of grainy ice crystals suspended near the surface of the water, not yet solidified into solid ice. The mixture is known as frazzle ice. “Frazzle is the word we use for the kind of loose soup of unconsolidated ice crystals that float around in the ocean as it is freezing,” Mahoney explained. The granular crystals “Float to the surface and they congeal,” creating ice characterized by lots of trapped gas bubbles called inclusions. Mahoney: “Once this solidifies, it caps the ocean and you get less wind influence, less waves, you have a much quieter regime under the ice.” Frazzle ice forms in open ocean water as the beginnings of new sea ice.

Once the ocean is capped by a layer of solidified frazzle ice and creates that “quieter regime,” the ice starts to grow downward-growing columnar ice. Mahoney said “The rejection of salt between the ice crystals creates a constitutionally super cooled layer underneath the ice. So any crystal that is protruding down into it, or any crystal that is vertically sticking down into that layer has a growth advantage.” Ice crystals that grow downward grow faster than ones sticking out to the side. “Pretty quickly all the crystals start aligning.” Mahoney can point out the columnar ice on the ice core he extracted.


Then, on this particular core, the columnar ice is interrupted. Besides forming in open ocean water in the fall, frazzle ice also forms when sea ice already exists but large cracks called leads have spread the ice apart to reveal open water. Cold temperatures prompt frazzle ice formation atop water bared by the cracks. Then, strong winds (which likely encouraged cracks to form in the first place) are able to blow the frazzle ice hard against the ice, and actually push it underneath pack ice edges. Mahoney can sometimes link frazzle ice he finds in cores to past time when he witnessed cracks form in sea ice, which he monitors using radar mounted in Barrow. “We can date where this happened. This happened in late December when there was a great break out with the landfast ice, strong eastward winds pushed the ice out,” Mahoney said. There was a memorable break out event in the landfast ice off Barrow’s coast. Cold temperatures promoted new frazzle ice growth. Then the wind reversed and blew west again, forcing the frazzle ice under the landfast ice.

Mahoney continues to read the ice’s history as he points out features moving down the core. After the December frazzle ice there’s more columnar ice. It is flawed by a pattern that looks like a tree shape was removed from the ice: tiny pathways joining each other and becoming larger as they move down. Mahoney explained “This is a dendritic brine drainage channel. As the ice is expelling the salt from the ice… As the ice cools down and brine volume decreases, and pores constrict, you get expulsion of brine from within the ice. And that’s what’s happening here, that’s a channel that’s worn, and you can see all the branches feed into one main channel.” Salty water is expelled from ice through drainage pathways into the ocean below. “That tells us something about how fluids are transported in these layers.” Mahoney said “From an oil percolation point of view, that is clearly not a barrier to percolation. Oil could migrate all the way up through that.” So oil spilled into water that encountered these brine drainage pathways would use them to infiltrate through the ice. Mahoney took another photo for documentation.

All the way down at the bottom where the ice core’s ice met ocean water the ice is different. It’s still full of briny salt water. A discoloration betrays the presence of life. “That is actually algae, phytoplankton that is living,” said Mahoney. The ice crystals were still growing, and here where they touched the water their alignment appears more horizontal because flowing currents in the ocean push at the crystals they touch. “The currents underneath the ice instill a preferred orientation into the ice crystals.” Near-coast currents are aligned with the coast, and so they also align the growing ice crystals parallel with the coast. It’s just one more thing guiding the formation of the ice microstructure. And even before reaching the lab, a sea ice scientist like Mahoney can read telltale signs.

Laura Nielsen 2016

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