A relatively small amount of groundwater trickling through Alaska's tundra is releasing huge quantities of carbon into the ocean, where it can contribute to climate change, according to new research out of The University of Texas at Austin.
Researchers found that although the groundwater only makes up a fraction of the water discharged to the sea, it's liberating an estimated 230 tons of organic carbon per day along the almost 2,000-kilometer coastline of the Beaufort Sea in summer. This quantity of carbon is on par with what free-flowing rivers in the area release during summer months.
"This study shows that there's humongous amounts of organic carbon and carbon dioxide released via fresh groundwater discharge in summer," said Cansu Demir, who led the research while she was completing her doctoral degree at the UT Jackson School of Geosciences. She is now a postdoctoral research associate at Los Alamos National Laboratory.
The research was published recently in Geophysical Research Letters .
As the tundra continues to thaw and the flow of submarine groundwater ratchets up, Demir said that the outflow of carbon from shore to sea could effectively make ocean surface waters a carbon source to the atmosphere. The CO2 released via groundwater could also contribute to ocean acidification.
The study is first to use direct observations to show that fresh water is being discharged into the submarine environment ocean where the coast meets the sea. Before this research, the existence of fresh submarine groundwater discharge in this area of the Arctic was thought to be very limited, Demir said.
The study is also the first to isolate freshwater — which could be made up of rainwater, snow melt, thawed shallow ground ice, and potentially some permafrost thaw — from the total groundwater discharge. Previous studies of groundwater discharge in the Artic included recirculated saltwater, which seeped into the ground from the coast.
Using direct observations, numerical modeling, thermal and hydraulic techniques, researchers found that during the summer, fresh groundwater entering the Beaufort Sea north of Alaska is equal to 3-7% of the total discharge from three major rivers in that area. This volume of water is surprisingly high, according to Demir, who said it's comparable to fresh groundwater discharge amounts in the temperate regions of lower latitudes. And although the volume of groundwater is proportionally small to the overall river flow, it holds a comparable amount of carbon.
"In that small amount of water, that groundwater carries almost the same amount of organic carbon and nitrogen as rivers," she said.
Groundwater travels beneath the surface through soils and sediments as it makes its way to the coast, picking up organic matter, inorganic matter, and nutrients on its journey. When it interacts with permafrost, it can receive especially large volumes of carbon. Permafrost is akin to a subterranean estuary – holding large volumes of water and organic matter. When the ice melts and becomes part of the groundwater flow, it can bring a huge quantity of carbon along with it.
"The Arctic coast is changing in front of our eyes," said Bayani Cardenas, a co-author of this study and professor at the Jackson School's Department of Earth and Planetary Sciences. "As permafrost thaws, it turns into coastal and submarine aquifers. Even without this thawing, our studies are among the first to directly show the existence of such aquifers."
In addition to contributing to global climate change, this huge influx of carbon and nitrogen could have major impacts for Arctic coastal ecology, Demir said. For example, ocean acidification could lead to increased vulnerability of some of the organisms that live on and under the seafloor, such as crustaceans, clams, and snails.
As permafrost continues to thaw under climate change, the amount of freshwater making its way to the sea underground will potentially increase, delivering even more greenhouse gases into coastal waters.
