A geoscience perspective on transport mechanisms and global accumulation of marine litter in submarine canyons

Marine litter and microplastics have reached every area of the Global Ocean, including remote settings such as the deep seafloor, where the magnitude and impacts of the problem is still largely unknown. Although sequestration of litter in deep-sea sediment is considered its long-term sink, our knowledge about the processes driving the dispersion and the ultimate fate of this pollutant in the marine environment is still limited.

Submarine canyons, acting as preferential conduits connecting shallow to abyssal depths, are widely recognized as main vectors for litter transport from land to the deep sea, and are also focal points for accumulation of litter of maritime origin, primarily derived from fishing gears. The occurrence of fishing-related litter is linked to fishing activities, which can be quite intense in and around canyons due to the high productivity often associated with these features and is mainly observed at the canyon heads and walls, where it can impact ecologically relevant communities that thrive in these habitats. Conversely, transport and accumulation of general waste and plastic along canyon axes can be related to different mechanisms, encompassing enhanced bottom currents, dense water cascading and especially sediment gravity flows. The efficient transfer capacity of gravity flows is supported by ongoing evidence of large litter piles at great water depths and seems to be influenced by the proximity of canyon heads to shore and to riverine inputs, recognized as main entry points for plastic at sea. Along with the transport of plastic at great depths, sediment gravity flows can also determine its burial, which is a still overlooked topic in marine litter research.

Global assessment of canyons exposure to riverine plastic inputs and fishing-related debris indicates varying susceptibility of canyons to litter, with Mediterranean canyons overall more exposed to both riverine and maritime litter contamination when compared to other ocean basins. However, considering the paucity of studies relating litter distribution to transport and depositional processes and the paucity of data from deep sea-setting, along with the evidence that most of the canyons prone to receive large amounts of anthropogenic debris have not yet been surveyed, several knowledge gaps need to be filled to fully understand this issue.