Sediment in the Yukon River flowing through Alaska contains mercury leached from permafrost, threatening the environment and health of Arctic communities.
The Yukon River flows west through Alaska toward the Bering Sea, eroding Arctic permafrost along its banks and transporting sediment downstream. That sediment contains a toxic metal called mercury. As the Arctic warms four times faster than the global average due to climate change, mercury that has accumulated in permafrost for millennia is being eroded by river water and released into the environment.
In a study published Aug. 15 in the journal Environmental Research Letters, a team from the USC Dornsife College of Arts, Sciences, and Letters presents a more accurate way to measure the amount of mercury released from permafrost through river water and estimate the total amount of mercury waiting to escape. The toxic metal is a threat to the environment and the health of the 5 million people who live in the Arctic region, more than 3 million of whom live in areas where permafrost is expected to disappear entirely by 2050.
“There’s a giant mercury bomb in the Arctic waiting to go off,” said Josh West, a professor of Earth and environmental sciences at USC Dornsife.
Earth’s natural atmospheric circulation often transports pollutants to high latitudes, leading to mercury accumulation in the Arctic, West explains. Permafrost accumulates so much mercury that it can far exceed the amount in the ocean, soil, atmosphere, and biosphere combined.
In the Arctic, plants absorb mercury, then die and become part of the soil, eventually freezing into permafrost. Over thousands of years, mercury concentrations build up in the permafrost until it thaws, a problem that is becoming more common as climate change progresses. The team of experts from the California Institute of Technology (Caltech), the Intertribal Yukon River Basin Council, the Massachusetts Institute of Technology, and Delft University of Technology in the Netherlands focused their research around two villages in Alaska’s Yukon River Basin: Beaver, 100 miles north of Fairbanks, and Huslia, 250 miles west of Beaver.
Previous methods of estimating mercury concentrations used core samples from the top 3 meters of frozen soil, which were four times more inaccurate and limited by the depth of the samples. To increase accuracy, the USC Dornsife-led team analyzed mercury in sediments from riverbanks and estuary sandbars, reaching deeper into the soil. Rivers can rapidly transport large amounts of mercury-laden sediments, said Isabel Smith, a doctoral student at USC Dornsife.
The team found that the mercury levels in the sediments were consistent with estimates from previous studies, confirming that sediment samples provide a reliable measure of mercury concentrations and providing a better understanding of potential risks in permafrost. Additionally, the team used satellite remote sensing data to track how quickly the Yukon River changes course. Changes in the river’s flow are important because they affect how much mercury-laden sediment is eroded from the riverbanks and deposited along sandbars, helping researchers predict the movement of mercury.
Finer-grained silts contained more mercury than coarser-grained silts, suggesting that some soils may pose a greater risk. While environmental mercury released from melting permafrost does not pose an acute toxic risk today, its effects are cumulative over time. Exposure increases as mercury accumulates in the food chain, particularly in fish and animals consumed by humans. Long-term effects could be severe, especially for Arctic communities that rely on hunting and fishing. The team hopes the tool they developed will allow for a more accurate assessment of the “mercury bomb.”