Modern sedimentary processes in the Santa Monica, California continental margin: sediment accumulation, mixing and budget

Sediment input to SMB appears to be associated with at least two point sources on the shelf, with Malibu Creek and the Hyperion sewage outfall being the most significant. Sediment contributions are sufficient to support apparent mass accumulation rates near these sources up to 1.8 g/cm2 year, which with distance decrease to 0.5 g/cm2 year near the shelf break ( 80–100 m water depth). Sequestering of material on the shelf and decreasing sediment supply to the slope is evident as rates decrease between 100 and 200 m water depths to less than 0.2 g/cm2 year. Below 100–200 m water depth, rates are relatively slow throughout a broad region of the slope (0.07–0.14 g/cm2 year). These slower rates are in general agreement with rates determined on the flanks of the California Borderland basins. Sediment texture fines from 3.5 phi to 7 phi with distance offshore. Texture does not exhibit significant changes from surficial values with depth in the seabed at any given site or between sites on the slope. This similarity in rates and downcore texture over such a broad extent suggests that hemipelagic sedimentation is the dominant mechanism of sediment delivery in water depths >200 m. Seabed distributions of radionuclides suggest that apparent accumulation rates in SMB may be twice the actual accumulation rates. A sediment budget documents that over the past century at least, SMB has served as a sink for 50–100% of the natural and anthropogenic inputs to the coastal ocean.