U. MICHIGAN (US)—Chemical tracers in stream water are offering scientists a new way to monitor changes in the Arctic permafrost—soil that normally remains at or below the freezing point for years—as an indicator of global warming.
Details of the new approach are reported in the journal Chemical Geology.
Overlying permafrost is a thin “active layer” that thaws every summer. Increases in the thickness of this layer over the years indicate thawing of permafrost.
Both physical measurements and modeling suggest that active layer thickness has increased in some areas over the 20th century and that if present warming trends continue, increases of up to 40 percent could occur by the end of the 21st century.
Although the full effects of thawing are yet to be determined, coastal erosion and damage to the roads, buildings, and pipelines that have been built on permafrost are likely outcomes.
In addition, thawing permafrost may release the greenhouse gases carbon dioxide and methane into the atmosphere, triggering further warming and more permafrost thawing. Currently, the most commonly used method for determining thaw depth is with a graduate steel probe.
“We were studying the chemistry of soils in the area around Toolik Field Station in northern Alaska, and we found that once we got below the thickness that typically would thaw during summer, the soil chemistry changed dramatically,” says Joel Blum, the John D. MacArthur Professor of Geological Sciences at the University of Michigan.
“Material that has not thawed since it was deposited by glaciers 10,000 to 20,000 years ago is now beginning to thaw, and when it does, it reacts strongly with water, which it’s encountering for the first time. This soil is much more reactive than soils higher up that interact with soil water every summer.”
In particular, the amount of calcium, relative to sodium and barium, is higher in the newly-thawed permafrost, and the ratio of the strontium isotope 87Sr to its counterpart 86Sr is lower. The researchers wondered if these chemical signatures of increasing thaw depth could be seen in local stream water.
George Kling, the Robert G. Wetzel Collegiate Professor of Ecology and Evolutionary Biology, has conducted research at Toolik Lake for many years and obtained stream water samples that had been collected over an 11-year period.
When the samples were analyzed, “we saw really significant changes from year to year that were consistent with what you would predict from increasing thaw depth,” he says.
Although the method can’t reveal precisely how much permafrost thawing is occurring in particular localities, it still can be a useful adjunct to current methods, Blum explains.
“We’d love to be able to say that we see an increase in thickness of, say, 1 centimeter over the entire watershed, but we simply can’t say where in the watershed thawing is occurring.
“Nevertheless, we think it’s important to monitor streams in Arctic regions to keep track of these kinds of changes and follow the rate of change.”
The research was funded by the National Science Foundation and the Geological Society of America.
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