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

• Published in: PNAS nexus Vol. 3; no. 4; p. pgae081
• Main Authors: Swoboda, Steffen, Krumpen, Thomas, Nöthig, Eva-Maria, Metfies, Katja, Ramondenc, Simon, Wollenburg, Jutta, Fahl, Kirsten, Peeken, Ilka, Iversen, Morten
• Format: Journal Article
• Language: English
• Published: US Oxford University Press 01.04.2024
• Subjects: Aggregates; Analysis; Atmospheric carbon dioxide; Carbon dioxide; Carbon dioxide fixation; Carbon sequestration; Earth Sciences; Environmental aspects; Exports; Fluctuations; Fluxes; Gypsum; Human influences; Ice formation; Melting; Ocean; Oceanography; Physical Sciences and Engineering; Primary production; Sciences of the Universe; Sea ice; Settling velocity; Straits; Terrigenous sediments; Arctic Ocean; Germany; Arctic region; Fram Strait
• ISSN: 2752-6542; 2752-6542
• DOI: 10.1093/pnasnexus/pgae081

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.