Title: Substantial and overlooked greenhouse gas emissions from deep Arctic lake sediment
Authors: Nancy L. Freitas, Katey Walter Anthony, Josefine Lenz, Rachel C. Porras, Margaret S. Torn
Journal: Nature Geoscience
Year: 2025
The Arctic contains vast reserves of carbon in frozen sediments known as permafrost. As the planet warms, the permafrost can thaw and trapped carbon is released into the atmosphere as the greenhouse gasses carbon dioxide (CO2) and methane (CH4). These released gasses will then further contribute to global warming, resulting in a positive feedback that is particularly concerning given recent findings that the Arctic is warming almost four times faster than the rest of the globe.
In a recent publication, Freitas and co-workers note that previous studies of permafrost have focused only on the gradual thawing of surface sediments, a few meters deep. Their study focused on so-called ‘thermokarst lakes’, which can speed up the thawing of permafrost very deep below ground. The lakes are formed when surface permafrost begins to thaw and leaves depressions in the landscape, which fill with water. This water can then transfer heat into the ground resulting in a spreading ‘thaw bulb’ that extends into the deeper permafrost. This process is not currently represented in climate models. In this work, Freitas et al. analysed the greenhouse gas emissions from a sediment core collected from beneath a thermokarst lake in Alaska. The collected data could be used in future modelling to better represent this potential climate feedback process.
In order to comprehensively understand the greenhouse gas emissions, the researchers incubated samples of sediment from different ground depths for one year at different temperatures (4°C, 10°C, and 20°C), and in the presence or absence of oxygen. They then analysed the gasses emitted from the samples using gas chromatography. The first observation made on collecting the core was that the sediment was already naturally thawed all the way down to 20 meters, confirming previous measurements made in the area. The researchers also observed CO2 production from all of the sediment samples collected as soon as they began measurements, indicating that carbon release was already naturally occurring in the deep and cold 20m sediment. Methane emissions from the sediment took 50 to 100 days to begin, as the community of methane-producing microbes needed time to establish in the sediment, but this is still faster than previous work which only observed CH4 emissions after over 600 days.
Previous work has considered greenhouse gas emissions from sediment exposed to oxygen (aerobic conditions) to be higher than those from oxygen-deprived sediment (anaerobic conditions). However, this recent work shows that this isn’t always the case. The team found that while the average production under aerobic conditions was higher, the anaerobic production was increased for deeper sediments and under warmer incubation temperatures. For the deepest sediments, the anaerobic production was often larger than under aerobic conditions. The high anaerobic emissions of deep sediments can be partially attributed to high production of CH4 from these deep sediments, particularly under higher temperatures.
Finally, in order to estimate emission rates at the lake surface, the researchers calculated the greenhouse gas emissions across the whole sediment column. This highlighted that anaerobic greenhouse gas emissions are generally lower than aerobic emission rates but could be much higher than reported in previous literature. They also found that the intermediate depth sediments contributed the majority of emissions in this sample, with deep sediments contributing the second largest fraction.
As the Arctic warms, it is important to represent feedback processes well in climate models. This work suggests that representations of thermokarst lakes, and the associated emission of greenhouse gasses from deep sediments, need to be included in future models.
Featured Image by Jean-Christophe André on Pexels.
