Release of ballast material during sea-ice melt enhances carbon export in the Arctic Ocean

• Published in: PNAS Nexus
• Main Authors: Swoboda, Steffen, Krumpen, Thomas, Nöthig, Eva-Maria, Metfies, Katja, Ramondenc, Simon, Wollenburg, Jutta, Fahl, Kirsten, Peeken, Ilka, Iversen, Morten
• Other Authors: Amon, Cristina, DFG-Research Center, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, German Helmholtz Association, German Federal Ministry of Education and Research, Helmholtz Excellence Network
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2024
• Subjects: Arctic; Arctic Ocean; Fram Strait; Sea ice
• Online Access: http://dx.doi.org/10.1093/pnasnexus/pgae081; https://academic.oup.com/pnasnexus/advance-article-pdf/doi/10.1093/pnasnexus/pgae081/56697366/pgae081.pdf; https://academic.oup.com/pnasnexus/article-pdf/3/4/pgae081/57336229/pgae081.pdf

Abstract Globally, the most intense uptake of anthropogenic carbon dioxide (CO2) occurs in the Atlantic north of 50°N, and it has been predicted that atmospheric CO2 sequestration in the Arctic Ocean will increase as a result of ice-melt and increased primary production. However, little is known about the impact of pan-Arctic sea-ice decline on carbon export processes. We investigated the potential ballasting effect of sea-ice derived material on settling aggregates and carbon export in the Fram Strait by combining 13 years of vertical flux measurements with benthic eDNA analysis, laboratory experiments, and tracked sea-ice distributions. We show that melting sea-ice in the Fram Strait releases cryogenic gypsum and terrigenous material, which ballasts sinking organic aggregates. As a result, settling velocities of aggregates increased ≤10-fold, resulting in ≤30% higher carbon export in the vicinity of the melting ice-edge. Cryogenic gypsum is formed in first-year sea-ice, which is predicted to increase as the Arctic is warming. Simultaneously, less sea-ice forms over the Arctic shelves, which is where terrigenous material is incorporated into sea-ice. Supporting this, we found that terrigenous fluxes from melting sea-ice in the Fram Strait decreased by >80% during our time-series. Our study suggests that terrigenous flux will eventually cease when enhanced sea-ice melt disrupts trans-Arctic sea-ice transport and thus, limit terrigenous-ballasted carbon flux. However, the predicted increase in Arctic primary production and gypsum formation may enhance gypsum-ballasted carbon flux and compensate for lowered terrigenous fluxes. It is thus unclear if sea-ice loss will reduce carbon export in the Arctic Ocean.