The team worked on the assumption that thawing increases the availability of organic carbon for microorganisms to process, in turn releasing vast amounts of carbon dioxide and methane. These gases accelerate the greenhouse effect, leading to further permafrost thawing in a vicious cycle.

Rising temperatures lead to collapse of intact permafrost soils, resulting in landslides and the widespread formation of wetlands. In this latest study, the team investigated what happens to the carbon trapped in the soil when the permafrost thaws out. “The organic material naturally present in the samples accumulated as peat over thousands of years. With permafrost thaw, microbes become active and are able to decompose the peat,” says Kappler. “We also know that iron minerals preserve organic carbon from biodegradation in various environments – and thus they could be a carbon sink even after the permafrost has thawed.” The reactive iron is present as a kind of rust and might be expected to trap the organic material in what the scientists call a “rusty carbon sink.”

The team found that microorganisms are apparently able to use the iron as a food source, thereby releasing the bound organic carbon into the water in the soil. “That means the rusty carbon sink cannot prevent the organic carbon from escaping from the thawing permafrost,” says Andreas Kappler. “Based on data available from elsewhere in the northern hemisphere, we expect that our findings are applicable for permafrost environments worldwide,” says Casey Bryce, who headed the study. 

The lead author of the publication, Monique Patzner, summarizes: “The rusty carbon sink is only found in intact permafrost soils; this sink is lost during permafrost thaw.” Now the researchers are seeking to find out how this facilitates greenhouse gas emissions and thus global warming. “It appears that the previously iron-bound carbon is highly bioavailable and therefore, bacteria could immediately metabolize it into greenhouse gas emissions,” says Patzner. “This is a process which is currently absent from climate-change prediction models and must be factored in.”