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Sea ice is not just solid frozen water. It’s riddled with tiny pockets and channels of liquid brine. Whether those pockets connect to form pathways determines whether seawater, nutrients and gases can move through the ice, according to decades of research by University of Utah mathematician Ken Golden.
In a new study, Golden and colleagues focus on granular sea ice, a type made of small, randomly oriented ice grains that is becoming more common as the polar regions continue warming. The scientists wanted to know when this type of ice becomes porous enough for fluids to flow vertically through it.
They found a clear tipping point. In columnar ice, characterized by orderly crystals, fluid starts flowing when 5% of the ice volume is brine. But with granular ice, that threshold is twice as high, about 10%, indicating that in this type of ice, the brine phase is far less interconnected. This difference has major implications for microbial communities that form the base of the robust sea ice ecosystem and for various geophysical processes.
“Going from 5% to 10% means that you need twice the porosity, twice the brine volume fraction to get flow. If algae are living in columnar ice versus living in granular ice, then there are quite different conditions under which they’ll get their food and nutrients,” said Golden, a distinguished professor mathematics. “It’s much harder to get it in granular ice. And there are other microorganisms, viruses and bacteria and nematodes and all sorts of other critters, that would be in the same boat.”
In this study, Golden collaborated with Cynthia Furse, a Utah professor of electrical and computer engineering, to measure various properties of sea ice in the Arctic and Antarctic. Their findings appear in Scientific Reports.
Sea ice microstructures matter
Like bone, sea ice is a multi-scale composite material, but where the host is pure ice, and the inclusions are brine.
“The geometry, the connectivity, and the volume fraction of these inclusions depend dramatically on temperature,” Golden said. “The way that the fluid is arranged within the ice depends strongly on the polycrystalline structure. In other words, the conditions under which the ice is formed, which is the main distinction between columnar ice versus granular ice.”
When sea ice forms in turbulent conditions, as often seen in the Antarctic, it is more likely to have a granular structure as opposed to a columnar structure.
As planetary warming reshapes sea ice, making it thinner, younger, and more granular, its internal plumbing system also changes. This study shows that we can’t view all sea ice the same: its microscopic structure has planet-scale consequences.
“Granular ice has a very different permeability structure. Many processes depend on the fluid permeability, such as nutrient replenishment, snow-ice production in the Antarctic and melt pond evolution in the Arctic; all these kinds of things depend on fluid flow,” Golden said. “So, when are the different scenarios triggered? When do the nutrients shut off? When do they turn on? When do the melt ponds drain? When can seawater percolate, flood the surface and then freeze? A quarter of the ice pack in the Antarctic depends on this mode of formation. Whether it’s granular or columnar can influence how much ice might be produced.”
In previous studies, Golden borrowed percolation theory from physics to develop his famous “Rule of Fives” to characterize fluid movement through columnar sea ice. It becomes permeable at 5% porosity, which happens when temperatures reach minus 5 degrees Celsius for a typical bulk sea ice salinity of 5 parts per thousand.
The spread of granular ice and its planetary consequences
Golden originally conjectured the higher threshold for granular ice in the paper where the Rule of Fives for columnar ice was first proposed. Over the years, he saw evidence that this rule may indeed be different for granular ice, the type that was becoming more common in the Arctic during his decades of field research. The new study arises from field measurements Golden led in the Antarctic aboard the Australian research vessel Aurora Australis.
The researchers discovered that below the 10% porosity threshold, the brine pockets are too disconnected, so the ice acts like a barrier. This shows that the growing prevalence of granular ice may be disrupting many natural processes associated with sea ice, such as gas exchange between the ocean and atmosphere and melt pond drainage. This, in turn, undermines the accuracy of current models that scientists rely on to forecast the fate of Earth’s sea ice packs and the implications of receding sea ice.
“In granular ice, it’s harder for CO2 to move through the ice. There are different conditions under which you get transport up or transport down. That’s also important for microbial critters,” he said.
Likewise, meltwater on the surface of the ice will have greater difficulty draining, so seasonal ponds forming on the ice may be larger, so that the sea ice albedo decreases, meaning it absorbs more of the sun’s heat.
“The surface albedo might be very different because you might have 60% coverage versus 40% coverage depending on the ability to drain,” Golden said. In other words, the more granular the sea ice, the more heat it will absorb, potentially making it melt faster.
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