SEDIMENT ACCUMULATION WITHIN THE SOUTHERN CALIFORNIA BIGHT: PATTERNS, RATES AND PROCESSES

Results from a suite of box cores and vibracores from the Southern California margin between Malibu and Oceanside, California characterize the patterns, rates and processes of sediment accumulation within the Southern California Bight. Sediments on the shelf are generally medium silts near Malibu and in the Oceanside littoral cell, and generally fine sands in the Santa Monica and Orange County areas, reflecting differences in sediment input and regional exposure to oceanographic forcing. In general, the shelf narrows and the continental slope becomes steeper toward the south in response to tectonic forcing, allowing fluvial sediment and anthropogenic discharges to escape the shelf and to be transported to slope and offshore-basin sinks. Sediment accumulation rates decrease offshore, from ~0.7 to 0.05 g/cm²y, suggesting that resuspension and off-shelf transport are important processes of sediment redistribution all along the margin. Sediment partitioning between the shelf, slope and deep basins on the Santa Monica (50% shelf, 30% slope, 20% basin), San Pedro (30% shelf, 40% slope, 30% basin) and Oceanside (25% shelf, 25% slope, 50% basin) margins supports resuspension as a dominant mechanism for off-shelf transport along the Southern California Margin. Sediment budgets for the Southern California Bight are not well-constrained, suggesting the need for better estimates for both sources and sinks, particularly the contribution from sea cliff erosion, which varies significantly along the margin. Anthropogenic contaminants serve as effective tracers for effluent redistribution patterns. Subsurface peaks of ²³⁸U, which are effectively sequestered in reducing outfall sediments found in cores from the Palos Verdes shelf and slope, document historical redistribution of anthropogenic sediments and associated contaminants.