Arctic permafrost contains one billion metric tons of carbon, some of which is emitted into the atmosphere as a result of microbial activity. By the end of this century, experts estimate that between 5 and 15 percent of the organic carbon stored in permafrost will be released as carbon dioxide, elevating global warming by an additional 0.3 to 0.4 degrees Celsius.
However, a new study indicates that this estimate is too low because it fails to account for another natural process that releases CO2.
Researchers at the University of Michigan have shown that after thawing permafrost enters lakes and rivers, organic carbon can be oxidized by sunlight in a process known as photomineralization.
The team demonstrated that organic carbon from thawing permafrost is highly susceptible to photomineralization by ultraviolet and visible light, and has the potential to convert 14 percent of the organic carbon stored in permafrost into carbon dioxide.
Study lead author Rose Cory is an aquatic geochemist and a professor of Earth and Environmental Sciences.
“Only recently have global climate models included greenhouse gases from thawing permafrost soils. But none of them contain this feedback pathway,” said Professor Cory.
“To get a number on how much carbon could be released from permafrost soils through oxidation, we have to understand what are the processes and what is the timescale: maybe this carbon is just so resistant to oxidation that, even if thawed out, it would just flow into the Arctic ocean and be buried in another freezer.”
It is challenging to measure the potential effects of photomineralization because the process is influenced by specific wavelengths of light. In order to measure each wavelength’s effect on soil organic carbon, study co-author Collin Ward built a new instrument that uses LED lights to mimic different wavelengths of the sun.
“This new LED-based method makes it far easier and cheaper to figure out how light-driven reactions vary for different wavelengths of the sun,” said Ward. “After I built the instrument I immediately called Rose and told her that I wanted to first use it on permafrost samples.”
The researchers used soil samples collected from six Arctic locations and subjected the organic carbon to LED light. After the light exposure, the team used a mass spectrometer to measure the age and amount of carbon dioxide obtained from the soil carbon.
The analysis revealed that the amount of carbon dioxide released was not only affected by the wavelength of sunlight, but also by the amount of iron in the sample. Iron acted as a catalyst, increasing the reactivity of the soil.
“What we have long suspected is that iron catalyzes this sunlight-driven process, and that’s exactly what our results show,” said Professor Cory. “As the total amount of iron increases, the amount of carbon dioxide increases.”
The team set out to show that the oxidation of organic carbon was not just taking place in permafrost that thaws annually, which would release much smaller amounts of CO2, but was also occurring in ancient permafrost. Carbon dating revealed that the soil samples were between 4,000 and 6,300 years old, confirming that old permafrost carbon can be converted to CO2.
“Not only do we have the first wavelength specific measurement of this sunlight-driven reaction but we have verification that it’s old carbon that is oxidized to carbon dioxide,” said Professor Cory.
“We can put to rest any doubt that sunlight will oxidize old carbon and we show what is controlling this process – it’s the iron that catalyzes the sunlight oxidation of ancient (or old) carbon.”
The study is published in the journal Geophysical Research Letters.