During severe storms, the stratigraphic waters of the fjords mix, oxygenating the bottoms. But storms also mean that atmospheric methane emissions from fjords are increasing. Researchers from the University of Gothenburg have estimated that total greenhouse gas emissions are significant from fjords as well as from all deep sea regions on Earth combined.
The fjords of the world were created when the ice sheet retreated and is a fairly rare natural form that makes up 0.13 percent of the total sea surface. According to researchers from the University of Gothenburg, methane emissions from the water surface in the straits are comparable to those emitted in aggregate from all areas of the deep sea, which represents 84 percent of the sea surface on Earth. It was reported in a scientific article in the prestigious journal Limnology and Oceanography Letters.
It is already known that many fjords have anoxic environments near the bottom and that methane gas is formed in the sediments. Normally a small portion of the gas reaches the atmosphere as it decomposes on its way up, in the oxygen-rich waters closest to the surface. But in our research, we’ve recorded significant methane emissions when water mixes in a fjord during storms, for example, says Stefano Bonaglia, a marine geochemistry researcher in the Department of Marine Sciences at the University of Gothenburg.
Oxygen-free bottoms produce methane
Tracking and calculating atmospheric methane emissions is important so you can calculate future climate. Researchers estimate that methane emissions cause about 30 percent of the global warming effect. In this context, the contribution of oceans to methane emissions is seen as much less than that of land areas, but human activity has increased eutrophication in coastal marine areas, creating larger anoxic areas at the bottoms. In the fjords, this becomes very evident and despite the fact that they make up only 1.3 per mile of the total ocean, they account for about half of the methane emissions to the atmosphere.
– This is because in the fjords, rich bottom sediments of flora and fauna, as well as material from the surrounding lands are deposited through the waterways that flow into the fjords. Since these waters are relatively protected from ocean currents, they remain in layers with different temperatures, salt and oxygen content. Closer to the bottom, says Stefano Bonaglia, oxygen-free zones are created where methane is formed when material in the sediment decomposes.
Agriculture leads to eutrophication in fjords
Researchers from the University of Gothenburg studied the Byfjorden near Uddevalla during 2009-2021 and conducted field studies to measure methane production in the fjord. The Byfjorden region suffers from hypoxia and is affected by eutrophication, among other things because the Bavian River flows into the fjord and brings many nutrients from farms in the area. It was clear that at times of mixing water in the fjord, methane emissions into the atmosphere increase. Then the oxygen-free water at the bottom quickly rises to the surface and takes methane with it, which can be released into the atmosphere.
million tons of methane
– Methane emissions were significant, and American researchers witnessed the same events in the fjords of Canada. We estimate that emissions from all of the world’s fjords are in the same order of magnitude, about 1 teragram (teragram) or million tons/year, as emissions from all deep seas are estimated. This is because there is a much shorter bottom-to-sea surface in the fjord than in the deep sea, which gives more organic matter deposition in the sediments and methane not enough time to decompose on the way to the surface, says Stefan Bonaglia, adding that climate change is leading to more From extreme weather events, methane emissions can increase, but only to a certain extent.
If we had a sharp increase in the number of storms, methane emissions would decrease, because the oxygen-free environments at the bottom of the fjord disappear when the waters mix more often.
Scientific material: High methane emissions from anoxic fjords driven by mixing and oxygen
Contact: Stefan Bonaglia, Associate Professor of Marine Geochemistry in the Department of Marine Sciences at the University of Gothenburg. phone: 0737-41 24 97And the E-mail [email protected]
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