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"Sources, transport, and degradation of permafrost-derived organic carbon in a warming Arctic: the Siberian Shelf"

Final Report Summary - ARCTIC (Sources, transport, and degradation of permafrost-derived organic carbon in a warming Arctic: the Siberian Shelf)

Due to the magnitude of the pool of organic carbon (OC) stored in coastal and terrestrial permafrost, its potential release into the Arctic Ocean and degradation to CO2, is considered a plausible scenario in a warming climate. However, there is not sufficient information regarding the reactivity of terrigenous material once supplied to the Arctic Ocean. In this study, we address this critical issue by examining the organic composition of surface sediments collected over extensive scales on the East Siberian Arctic Shelf (ESAS) as part of the International Siberian Shelf Study (ISSS). The ESAS represents by far the largest shelf of the Arctic Ocean. Samples were collected from the inner- to the outer-shelf following the sediment transport pathway in a region between the Lena and the Kolyma rivers. The analytical approach includes the characterization of marine and land-derived carbon using a large number of molecular biomarkers obtained by alkaline CuO oxidation such as lignin-phenols, cutin-derived products and solvent-extractable HMW wax lipids (n-alkanoic acids, n-alkanes and n-alkanols). Our analyses focused on different density, size and settling fractions to circumvent the potential bias due to sorting during the sediment transport over the shelf
Our results indicate that the matrix-association of terrigenous OC is extremely variable across the shelf. In the inner-shelf a significant fraction of OC is associated with the light density fraction (<1.8 g/cm3) essentially as plant debris (up to 38% of the bulk OC in this fraction). Partitioning at macromolecular level changes based on the chemistry of the compound analyzed. In the low density fraction for example, lignin phenols can account for almost 70% of the total lignin pool while n-alkanes are mainly associated with the fine dense material (<38um, almost 80%). In the outer-shelf most of the OC is bound to the mineral matrix essentially as fine sediment (<63 um) with low settling velocity. The light fraction is likely retained in the inner shelf because, despite its low density, vascular plant debris is relatively large which result in a high settling velocity. Therefore as the sediment moves across-shelf the relative concentration of the different terrigenous biomarkers changes accordingly. In addition, by focusing on specific size and density fractions, our results quantified the degree of TerrOC degradation at macromolecular level along the sediment transport. The reactivity is occurs as follow and it seems to be a function of the functional groups: lignin phenols>cutin acids>n-alkanoic acids>n-alkanols>n-alkanes. In conclusion, our results at molecular level indicate that both degradation and winnowing of TerrOC severely affect the composition of permafrost once enters into the Arctic Ocean and they occur as a function of intrinsic reactivity and matrix-association.