The sequestration sink of soot black carbon in the Northern European Shelf sediments

[1] To test the hypothesis that ocean margin sediments are a key final repository in the large-scale biogeospheric cycling of soot black carbon (soot-BC), an extensive survey was conducted along the ∼2,000 km stretch of the Swedish Continental Shelf (SCS). The soot-BC content in the 120 spatially distributed SCS sediments was 0.180.130.26% dw (median with interquartile ranges), corresponding to ∼5% of total organic carbon. Using side-scan sonar constraints to estimate the areal fraction of postglacial clay sediments that are accumulation bottoms (15% of SCS), the soot-BC inventory in the SCS mixed surface sediment was estimated at ∼4,000 Gg. Combining this with radiochronological constraints on sediment mass accumulation fluxes, the soot-BC sink on the SCS was ∼300 Gg/yr, which yielded an area-extrapolated estimate for the Northern European Shelf (NES) of ∼1,100 Gg/yr. This sediment soot-BC sink is ∼50 times larger than the river discharge fluxes of soot-BC to these coastal waters, however, of similar magnitude as estimates of atmospheric soot-BC emission from the upwind European continent. While large uncertainties remain regarding the large-scale to global BC cycle, this study combines with two previous investigations to suggest that continental shelf sediments are a major final repository of atmospheric soot-BC. Future progress on the soot-BC cycle and how it interacts with the full carbon cycle is likely to benefit from14C determinations of the sedimentary soot-BC and similar extensive studies of coastal sediment in complementary regimes such as off heavily soot-BC-producing areas in S and E Asia and on the large pan-Arctic shelf.