The Arctic is the fastest-warming region on Earth, and increasing temperatures are thawing permafrost, releasing more carbon into the atmosphere, and accelerating warming. These feedbacks concern scientists because roughly 850 gigatons of carbon—representing 25%–50% of all soil organic carbon on Earth—are believed to be stored in the permafrost at present.
Arctic rivers receive carbon both from the seasonally thawing top layer of the soil and from eroding riverbanks. Besides the ongoing permafrost thaw, warming of the region also affects the rivers by extending the ice-free season and by changing the way water flows through the landscape and interacts with carbon in the soil.
In a new study, Bröder et al. analyze water from two sites in the Kolyma River watershed in northern Siberia. The Kolyma River flows northward across the easternmost part of Russia, eventually draining into the East Siberian Sea; its watershed is the largest on Earth that is entirely underlain by continuous permafrost. For half of the year, the river is covered by ice; its flow peaks after snowmelt in early summer. The researchers collected water from two separate locations near the town of Cherskiy, Russia. One batch came from the main stem of the Kolyma River, and the other was taken from a small headwater stream known as Y3. Samples were collected every 4–7 days during the ice-free periods of 2013 and 2015.
The team analyzed both water sources for suspended particulate organic carbon (POC) and dissolved organic carbon (DOC); they also conducted isotope analyses to help understand where the carbon was coming from. The carbon in both sample sites followed a typical pattern, with the highest concentrations showing up in the first few weeks after the ice breakup and tapering off later in the summer. Overall, the main stem of the Kolyma contained more POC, as well as older carbon, than the Y3 headwater stream. Conversely, the Y3 stream had higher DOC than the Kolyma.
The researchers say these findings indicate that POC in the main Kolyma comes from both recent vegetation and permafrost, whereas the POC in the Y3 stream comes primarily from younger plants. The researchers attribute the increased POC concentration in the main Kolyma to active riverbank erosion, which would be less of a factor in the smaller stream. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2019JG005511, 2020)
—David Shultz, Science Writer