In Alaska, magnets detect permafrost melt
STANFORD (US) — Researchers are using Earth’s magnetic field to determine if the permafrost beneath lakes is thawing as a result of climate change.
If so, the lakes could become a new source of methane, a global warming gas.
“These measurements go toward a better understanding of how permafrost thaw processes may impact future climate,” says Andrew Parsekian, lead researcher and post-doctoral scholar at Stanford University.
Permafrost refers to ground that stays frozen all year long—approximately a quarter of the Northern Hemisphere. But low spots in permafrost terrain can fill with meltwater, forming small lakes that can in turn melt the permafrost beneath them, forming a pocket of unfrozen soil called a talik.
The melted talik contains plenty of organic material to feed hungry soil microbes, which generate methane as a byproduct of decomposition. Methane is a potent greenhouse gas, and scientists worry that methane released from Arctic permafrost could start an unstoppable feedback loop.
“This is a new carbon pool being turned on under climate warming,” Parsekian says.
As reported in the journal Geophysical Journal Letters, Parsekian and colleagues from the US Geological Survey, Rutgers, University of Alaska Fairbanks, and Leibniz Institute for Applied Geophysics in Germany, used nuclear magnetic resonance, or NMR, to assess the extent of a talik beneath an Arctic lake.
Camped on the frozen surface of Lake Caribou, near Fairbanks, Alaska, they set out loops of wire up to a football field in diameter, which created an electromagnetic pulse in the ground and water below. That pulse caused protons in talik water to wobble in a way Parsekian’s team could detect.
NMR is the same technology used in a medical MRI, which uses magnets 10,000 times more powerful than a refrigerator magnet to perturb atoms. As those atoms return to their rest state, detectors pick up their unique resonance. Since those magnets, weighing several tons, are cumbersome to take into the field, Parsekian, a geophysicist, used the magnetic field of Earth as his magnet.
NMR is the only subsurface imaging method, he says, that is directly sensitive to unfrozen water content. The researchers checked the water and ice in nearby wells to confirm that the NMR measurements were providing accurate information about the situation underground. The team is the first to use NMR to detect taliks.
“By understanding that we have a really good method to make these measurements,” Parsekian says, “we can scale it up into a larger regional study that will be able to change the way we think about Arctic permafrost.”
The team’s work was funded by CUAHSI, NSF Hydrology Program, NASA Carbon Cycle Sciences and the US Geological Survey.
Source: Stanford University
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