Scientists collect yedoma from a newly excavated permafrost tunnel. Plant roots and woody biomass preserved in the permafrost yedoma hang from the ceiling of the tunnel. (Credit: Travis Drake)
As climate change continues, an important question is what will happen when permafrost soils thaw and the large amounts of gases they store is released into the atmosphere as carbon dioxide and methane. Some scientists predict there will be a gradual permafrost carbon release, which will affect the current climate slowly and in a way that plant species adaptation will be able to counter. Other scientists predict the permafrost carbon release will be fast and increase the rate of global temperature rise. This could speed up permafrost thaw even more and have dramatic effects on Arctic ecosystems.
In a recent study, Kim Wickland, a research ecologist at the U.S. Geological Survey, and her colleagues have found evidence that may support the latter hypothesis. Their study of yedoma, ancient permafrost soil from the Pleistocene Era, showed that more than half of the organic carbon in the thawed permafrost degraded and released as carbon dioxide faster than anyone anticipated, one week.
The yedoma soil Wickland studied is wind-deposited, fine-particle soil, the oldest permafrost anyone has ever investigated. The soil was preserved for 30,000 years and contains preserved mammoth bones and plant roots. Over its 30,000-year existence, Wickland notes it may have had many different microbial processes going on in it, even while frozen, at very slow rates. However, Wickland observes, “There apparently wasn’t much bacterial breakdown in this soil or much carbon release, until we thawed it.” Compared to younger permafrost soils, the organic carbon in the yedoma degrades much more rapidly.
Traditional lab experiments exploring carbon dioxide release consist of a jar containing the sample, with a measurement being made of the gas released into the headspace over time. Wickland’s setup is unique, however, in that it is a custom-made bioreactor.
The permafrost was thawed and the dissolved organic carbon was collected by leaching the soil in water. The solution was put into an 11-liter container continuously circulating the water through a series of sensors. A YSI meter measures temperature, pH and fluorescent dissolved organic matter. Membrane exchange is used to measure dissolved carbon dioxide: As water travels through the exchange membrane, the gas comes out, going through an infrared gas analyzer, which measures carbon dioxide. A PP systems model Environmental Gas Monitor provides high-resolution and real-time data of the carbon dioxide that is produced from degrading organic carbon. As for organic carbon, Wickland measures it using discrete samples collected directly.
Wickland and her colleagues also conduct field studies to understand how much carbon is released into the environment when permafrost soils thaw. Carbon from thawing permafrost soils can be dissolved in water and flow into aquatic ecosystems, in addition to being released to the atmosphere.
The field studies are aimed at investigating concentrations of carbon in streams, rivers and lakes in areas where permafrost is known to be thawing and comparing them to non-thawing areas. Measurements of organic carbon and carbon dioxide in streams and rivers are made with aquatic sensors over many years to see if they are increasing with permafrost thaw. Carbon dioxide is measured in these cases using a Vaisala sensor. Even though the sensor is designed for use in the air, it can be adapted for use in water using a membrane. Forerunner waterproof carbon dioxide sensors can also be used.
“In our research, we need high-resolution carbon information from aquatic environments and carbon release measurements in the atmosphere. The measurement of carbon dioxide in water is much more challenging compared to in the air, but technology is improving,” says Wickland. “We hope in the future to also have real-time methane sensors for use in water.”
Dissolved organic carbon released from permafrost soils into streams and rivers can be evaluated in part using their color, as humic compounds tend to contain highly colored dissolved organic species such as phenols. The yedoma soil, however, was almost clear, containing acetates and butyrate, which made up about 80 percent of the soil dissolved organic carbon.
“The yedoma soil nevertheless contained plenty of carbon, despite their lack of color,” Wickland notes. So monitoring for color may not always be effective. Colored material also tends to be subject to photochemical breakdown, a different decomposition process.
In terms of research equipment, Wickland also mentions LGR’s ultra-portable greenhouse gas analyzer, which weighs about 50 pounds and measures carbon dioxide and methane in real time. Scientists use this instrument to measure carbon dioxide and methane released into the atmosphere from soils and aquatic systems by placing a chamber over the soil or floating one on top of the water surface and measuring how quickly the concentrations are changing.
“Most people make their own chambers. We make our own for soil and water measurements,” says Wickland. They aim to make automatic sampling chambers in the future, although some companies such as LI-COR do manufacture them.
In terms of new technology in the area, Wickland says her team would like to see further development of aquatic carbon dioxide and methane sensors. “We’re also always on the lookout for new aquatic sensors; we would like a more robust carbon dioxide measurement and a more rapid response,” she says. Currently there is a 20- to 30-minute time lag in measurements. Wickland would prefer something at 10 minutes or less.
In terms of the research on permafrost itself, Wickland says a major piece still being investigated is “the whole host of microbes” that are involved in permafrost degradation and carbon dioxide release. “A team of USGS microbial ecologists are working on isolating DNA from ancient microbes,” she says. “It’s difficult, because there is often not a lot of DNA to work with.”
In terms of surprises during the course of the yedoma degradation research, Wickland says they were all “very surprised” that the permafrost soil degraded and converted its organic carbon to carbon dioxide in only a week. “Soil degradation is sort of like thinking about candy bars and their wrappers,” says Wickland. “We expected to find mostly wrappers in soil that old…but instead we found there were still plenty of candy bars left!”
Wickland cautions that while the dramatic carbon dioxide release they saw from yedoma soils is concerning, more research is needed to understand the potential impact on a global scale. “We still need to understand thaw in different areas,” she says. “Yes, what we saw was dramatic, but we can’t yet extrapolate those results to all the different sites out there.”
There is also the question of how soil thaws in the lab versus how it thaws in the natural environment. And there is the question of whether dry soil or wet soil is being thawed. “Wet soils have more of a tendency towards producing methane as opposed to carbon dioxide, whereas dry soils tend to produce more carbon dioxide than methane,” says Wickland.
There is also the factor of temperature: Decomposition at increased temperature tends to mean increased carbon dioxide emission. A complicating factor is that, while higher temperatures tend to cause more carbon dioxide emissions, they also tend to cause increased vegetation growth, which can capture some of the carbon.
Wickland explains, “We are diligently trying to understand permafrost carbon release and what its impact might be globally. As a team, and with the larger community of researchers investigating the Arctic, we continue to make great progress on answering that question.”
Wickland’s team consists of graduate students from the University of Colorado and a postdoctoral researcher, and they work closely with colleagues at the USGS and Florida State University. The Yukon River Intertribal Watershed Council has been an important partner in their work. And members of several native Alaskan communities help collect water and permafrost samples to aid the research.
Top image: Scientists collect yedoma from a newly excavated permafrost tunnel. Plant roots and woody biomass preserved in the permafrost yedoma hang from the ceiling of the tunnel. (Credit: Travis Drake)