Coastal carbon sinks protect permafrost material from decomposition
The Arctic is warming more strongly than any other region on Earth, which leads to serious erosion of coasts where organic matter was frozen in the permafrost for thousands of years. Once eroded material is released into the ocean, microorganisms break down ancient plant remains, releasing sizable quantities of greenhouse gases. Yet, as the latest analyses conducted by AWI experts show, part of the biomass released becomes trapped in deep sinks on the ocean floor off the Arctic coast – where it is protected from microbial decomposition. The outcomes of their analyses are intended to make climate models more accurate.
Today, the soils of the Arctic coasts of Canada, Russia and Alaska, which have been frozen for millennia, are being lost faster than ever before: firstly, because the thawing season in these regions is becoming longer and longer; and secondly, because the ocean remains ice-free longer, and the waves are gnawing away the coasts more rapidly. The ancient organic matter stored in the permafrost soils is released and subsequently broken down by microbes. As a result, the carbon stored in the plants is released as carbon dioxide, reinforcing the greenhouse effect – a vicious circle. In response, climate researchers around the globe are now working towards to more precisely determine how much carbon dioxide these eroding permafrost coasts will actually emit, now and in the future, partly relying on computer models.
Researchers at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) have recently determined that one important factor has been neglected in estimates to date: deep points along the coast of the Arctic known as coastal carbon depocenters. At these locations, the material lost from permafrost soils is only partially broken down. The reason: the material can easily drop straight to the bottom, just like in a water glass, because these depressions are largely free of currents or wave motion. Thanks to this continuous sedimentation process, the organic matter released from the permafrost is rapidly covered by fresher material. Further, in these deposits very little oxygen remains, preventing microbes from breaking down the biomass further. Using biochemical analyses of sediments collected just off the coast of Canada, a team led by AWI researchers Dr Hendrik Grotheer and Dr Michael Fritz determined that, in this way, more than 40 percent of the material eroded from permafrost coasts is protected from bacterial decomposition.
“It’s crucial that these marine depressions are in the immediate vicinity of the shoreline,” explains Hendrik Grotheer. “Only then can the material rapidly sink and become covered up again, which limits microbial decomposition and preserves the organic material.” In contrast, along shallow coasts the wave action and currents ensure that particles remain in the water column longer, where they are largely broken down by bacteria. As tests conducted in a 70-metre-deep basin near Canada’s Herschel Island show, the level of sedimentation in these sinks is tremendously high. Year after year, roughly three millimetres of sediment, including biomass, is deposited in them; further out in the ocean, this could easily take 1000 years. According to Grotheer, the results of the study, which the AWI experts have just released in the journal Geophysical Research Letters, could help to improve mathematical climate models. Since coastal sinks can be found throughout the Arctic, taken together, these sinks have likely preserved tremendous amounts of the organic matter from eroded permafrost soils – just how much is something the researchers plan to determine in future works.
One difficulty that the experts encountered while analysing the sediments: determining whether the biomass in the sinks actually stemmed from permafrost, or from rivers. This meant Hendrik Grotheer and Michael Fritz had to do a bit of detective work. They first had to determine the sediments’ age and the amount of certain molecules (‘biomarkers’) they contained, which indicated their origin. “We can now be sure that the majority of material actually comes from the permafrost – and that roughly 40 percent of the permafrost material remains on the ocean floor, preventing its decomposition and the release of greenhouse gas.” Though these findings are good news, there’s still plenty of reason for concern – the majority of thawing permafrost is still broken down by microbes. Further, if the Arctic Ocean remains ice-free even longer, and the frequency and intensity of storms increase, coastal areas may erode more quickly in the future. Lastly, these changed conditions could potentially stir up even the sediments in the deep ocean floor sinks, sweeping them back into the seawater, where they would be vulnerable to bacterial decomposition.
Grotheer, H., Meyer, V., Riedel, T., Pfalz, G., Mathieu, L., Hefter, J., Gentz, T., Lantuit, H., Mollenhauer, G., Fritz, M., "Burial and origin of permafrost derived carbon in the nearshore zone of the southern Canadian Beaufort Sea", Geophysical Research Letters, 2020, DOI: 10.1029/2019GL085897
NUNATARYUK project website: nunataryuk.org