Untangling microfibres: Pervasive plastic pollution in submarine canyons

Submarine canyons are important conduits for microplastic transport to the deep sea via turbidity currents. However, other near-bed oceanographic flows and sub-seafloor processes may play an important role in the transport and burial of microplastics. We use sediment push-cores for microplastic and sediment grain-size analysis from two transects across the Whittard Canyon, UK, to show that complex process-interactions control the transport and burial of microplastics and semi-synthetic microfibres in the thalweg and on the canyon flanks. Microplastic pollution is pervasive across the canyon at both transects, from the thalweg and from 500 m higher on the flanks, despite turbidity currents being confined to the canyon thalweg. Furthermore, we calculate sediment accumulation rates from 210Pb dating and show that microplastic concentrations remain similar at sediment depths down to 10 cm. This reveals that the huge global-increase in plastic production rates over time is not recorded, and that microplastics are present in sediments that pre-date the mass-production of plastic. The interaction of turbidity currents, deep-tidally-driven currents, and sub-seafloor processes shreds any potential signal that microplastics may provide as indicators of historical plastic production rates, which undermines the utility of microplastics as reliable markers of the onset of the Anthropocene.