The frozen soil of the arctic permafrost areas is surprisingly permeable. Because groundwater also collects under these layers of soil and flows underground into the Arctic Ocean near the coast. However, this means that this Arctic groundwater is also a source of organic material and nutrients for the Arctic Sea that has so far been underestimated. As researchers on the coast of the Beaufort Sea in Alaska have determined, twice as much dissolved organic substance enters the sea with the groundwater stream as assumed. This entry makes up around 70 percent of the entries through the rivers.
Groundwater is not only an important resource for our drinking water, the aquifers play a role as transport arteries for trace elements and nutrients, especially near coasts and bodies of water. Much like rivers collect rainwater and its contents from above ground and wash them into the sea, groundwater flows underground from land to sea. “Groundwater is the largest active reservoir in the global water cycle,” said Craig Connolly of the University of Texas at Austin and his colleagues. “Its movement from land to sea represents a significant source of fresh water and nutrients for coastal ecological and biogeochemical processes.” But the contribution of this groundwater inflow into the sea has hardly been recorded for a particularly sensitive region: for the Arctic. There, large amounts of organic carbon and other nutrients are preserved in the form of frozen plant residues and animal carcasses in the permafrost. Typically, a small, superficial layer thaws from this soil only in summer, the rest remains frozen.
Underground material flow
Most of the groundwater in the Arctic climes is below layers of soil that remain frozen throughout the year. For this reason, it has only been partially clarified how much water and organic material seeps into the aquifers from the upper layers – and how much of it reaches the coastal ocean underground. Connolly and his team have now examined this using the example of a coastal strip of the Beaufort Sea in Alaska. For their study, they took soil and water samples at various points in summer and late summer, which they analyzed for their nutrient content and for the carbon isotopes they contain. They also carried out laboratory experiments on permeability and nutrient transport within the soil.
It turned out that contrary to the assumptions, the Arctic groundwater not only contains organic substances from the near-surface layers of permafrost thawed in summer. It also absorbs nutrients from deeper layers hundreds to thousands of years old. “This underlines the assumption that a significant proportion of the organic constituents in the groundwater under permafrost comes from deeper layers of the soil,” say the researchers. These dissolved substances then reach the coastal areas of the sea with the underground groundwater runoff. This inflow accounts for a surprisingly large proportion of the total nutrient inflow into the Arctic Sea, as Connolly and his team determined. “With 1.35 milligrams of carbon per liter and 0.1 grams of nitrogen per liter, the concentrations are one to two orders of magnitude higher than the inflow from nearby rivers in the same period,” they report.
Climate change will increase groundwater transport
According to Connolly and his team, this suggests that Arctic groundwater could be a previously underestimated source of nutrients and organic matter in the Arctic Ocean. “We have to revise our idea of groundwater,” says Connolly’s colleague Jim McClelland. “The water that flows from the rivers into the Arctic Ocean has been well studied and recorded, but until now this has not been the case for the groundwater flowing into the sea there.” That has to change. According to estimates by the research team, a good two thirds as much dissolved organic substance in the Arctic Beaufort Sea could be transported into the sea by underground groundwater flows as by rivers. With the increasing thawing of permafrost due to climate change, this proportion could increase in the future. “The Arctic heats up twice as much as the rest of the planet – and with it the permafrost and aquifere thaws again,” explains Connolly’s colleague Bayani Cardenas. “It is likely that groundwater transport in the Arctic will also become more important in the future.”
Source: Craig Connolly (University of Texas, Austin) et al., Nature Communications, doi: 10.1038 / s41467-020-15250-8