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

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. Key Points Ocean margin sediments are a key final repository of soot black carbon The sediment soot‐BC flux reflects atmospheric delivery rather than river flux Large uncertainties remain on the global soot‐BC cycle