Scientists discovered that while groundwater comprises only a fraction of the total water discharge to the ocean, it delivers an estimated 230 tons of organic carbon per day along the nearly 2,000-kilometer coastline of the Beaufort Sea during the summer. This amount is comparable to the carbon released by the region's free-flowing rivers in the same period.
"This study shows that there's humongous amounts of organic carbon and carbon dioxide released via fresh groundwater discharge in summer," said Cansu Demir, the lead researcher, who conducted the study while completing her doctorate at the UT Jackson School of Geosciences. She is currently a postdoctoral research associate at Los Alamos National Laboratory.
As the Arctic tundra continues to thaw, leading to increased submarine groundwater flow, Demir noted that this process could turn ocean surface waters into a carbon source for the atmosphere. Additionally, carbon dioxide released from groundwater could contribute to ocean acidification.
This study is the first to use direct measurements to confirm the presence of freshwater discharging into the ocean from the coastal submarine environment. Prior to this, the existence of such fresh submarine groundwater discharge in the Arctic was considered minimal, according to Demir.
Furthermore, the research is the first to isolate the fresh component of the groundwater - a mix of rainwater, snowmelt, thawed shallow ground ice, and potentially some permafrost thaw - from the total groundwater discharge. Previous investigations included recirculated saltwater that had infiltrated the ground from the coast.
By employing direct observations, numerical modeling, and thermal and hydraulic techniques, researchers determined that fresh groundwater entering the Beaufort Sea north of Alaska accounts for 3-7% of the total discharge from three major rivers in the region during summer. This volume was unexpectedly large, Demir noted, and comparable to fresh groundwater discharge levels in temperate regions at lower latitudes. Despite its smaller volume, this groundwater carries a carbon load equivalent to that of the larger river systems.
"In that small amount of water, that groundwater carries almost the same amount of organic carbon and nitrogen as rivers," she said.
As groundwater percolates through soil and sediment on its way to the coast, it accumulates organic and inorganic materials along with nutrients. When in contact with permafrost, it can absorb particularly large amounts of carbon. Permafrost functions as a subterranean reservoir, storing water and organic material. As it melts, these elements are released into the groundwater, significantly increasing the carbon content reaching the ocean.
"The Arctic coast is changing in front of our eyes," said Bayani Cardenas, a co-author of the 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."
Beyond its implications for climate change, the influx of carbon and nitrogen could have significant effects on Arctic coastal ecosystems, Demir explained. Ocean acidification could heighten the vulnerability of marine organisms that dwell on or beneath the seafloor, including crustaceans, clams, and snails.
With permafrost thawing expected to accelerate due to climate change, the volume of groundwater flowing to the ocean will likely increase, bringing even more greenhouse gases into coastal waters.
The findings were recently published in Geophysical Research Letters.
Research Report:Coastal Supra-Permafrost Aquifers Of The Arctic And Their Significant Groundwater, Carbon, And Nitrogen Fluxes
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