A wide range of seabed-mapping technologies is reviewed in respect to their effectiveness in discriminating benthic habitats at different spatial scales. Of the seabed attributes considered important in controlling the benthic community of marine sands and gravel, sediment grain size, porosity or shear strength, and sediment dynamics were highlighted as the most important. Whilst no one mapping system can quantify all these attributes at the same time, some may be estimated by skilful interpretation of the remotely sensed data. For example, seabed processes or features, such as bedform migration, scour, slope failure, and gas venting are readily detectable by many of the mapping systems, and these characteristics in turn can be used to assist a habitat classification (and monitoring) of the seabed. We tabulate the relationship between “rapid” continental shelf sedimentological processes, the seabed attributes affecting these processes, and the most suitable mapping system to employ for their detection at different spatial scales.
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.
The pollution trend of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the Baltic Sea region was studied based on depth profiles of PCDD/Fs in sediment cores collected from six offshore areas, eight coastal sites impacted by industrial/urban emissions, and one coastal reference site. A general trend was observed for the offshore and coastal reference sites with substantial increase in PCDD/F concentrations in the mid-late 1970s and peak levels during 1985-2002. The overall peak year for PCDD/Fs in Baltic Sea offshore areas was estimated (using spline-fit modeling) to 1994 ± 5 years, and a half-life in sediments was estimated at 29 ± 11 years. For the industrial/urban impacted coastal sites, the temporal trend was more variable with peak years occurring 1-2 decades earlier compared to offshore areas. The substantial reductions from peak levels (38 ± 11% and 81 ± 12% in offshore and coastal areas, respectively) reflect domestic and international actions taken for reduction of the release of PCDD/Fs to the environment. The modeled overall half-life and reductions of PCDD/Fs in offshore Baltic Sea sediment correspond well to both PCDD/F trends in European lakes without any known direct PCDD/F sources (half-lives 30 and 32 years), and previously modeled reduction in atmospheric deposition of PCDD/Fs to the Baltic Sea since 1990. These observations support previous findings of a common diffuse source, such as long-range air transport of atmospheric emissions, as the prime source of PCDD/Fs to the Baltic Sea region. The half-life of PCDD/Fs in Baltic Sea offshore sediments was estimated to be approximately 2 and 4-6 times longer than in semirural and urban European air, respectively. This study highlights the need for further international actions to reduce the levels of PCDD/Fs in Baltic Sea air specifically and in European air in general.
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