Microplastics in the Sediments of Shelves and Basins offshore of Southern California
Surface sediment samples were collected using a multicore from shelves (10-14km offshore; 100-300m deep) and basins (90-130km offshore; 618-997m deep) on two cruises in 2018 on the RV Sally Ride and RV Sproul. For comparison, shallow (2-5m deep) sediments were collected in San Diego Bay. To extract microplastics from the samples, 78-100mL of sediment from the upper 1cm layer of the core was processed by density floatation in Zinc Chloride (1.5 g/cm³). Floating material was then vacuum filtered and identified by counting under a light microscope. Analytical blanks were processed every three samples and methods were further modified to minimize the amount of contaminant plastic found on filters.
The mean [SD] abundance of microplastics in the sediments of San Diego Bay (2.5 [2.3] pieces/ml) was over 4 times greater than in the shelf (0.42 [.11] pieces/ml) and offshore deep basin sediments (0.57 [0.22] pieces/ml). We found microplastic abundances above analytical blanks at all 10 offshore sites including the deep basins. A better understanding of the microplastic distributions in offshore sediments will help us better predict the impact of plastics on deep-sea marine life and can help us estimate future settling rates/patterns of microplastics on the ocean floor.