The combined role of near-bed currents and sub-seafloor processes in the transport and pervasive burial of microplastics in submarine canyons

Submarine canyons are important conduits for microplastic transport to the deep sea, but the processes involved in that transport and how faithfully seafloor deposits record trends in pollution remain unclear. We use sediment push cores for microplastic and sediment grain-size analysis from two transects across the Whittard Canyon, UK, to investigate the roles of near-bed flows and sub-seafloor processes in the transport and burial of microplastics and semi-synthetic microfibres. Microplastic and microfibre 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. We calculate sediment accumulation rates from 210 Pb dating and show that microplastic concentrations remain similar at sediment depths down to 10 cm. Throughout the Whittard Canyon there is an observed uniformity in the gradual decline in microfibre concentration with sediment depth, despite the variable sample locations and marked variations in sediment accumulation rates. Furthermore, the huge global increase in plastic production rates over time is not recorded, and microplastics are present in sediments that predate the mass production of plastic. The interaction of turbidity currents, deep tidally driven currents and sub-seafloor processes affects microfibre burial processes in the deep sea and shreds any potential signal that microplastics may provide as indicators of historical plastic production rates; complicating the use of microplastics as fully reliable markers of Anthropocene onset.