Transport and accumulation of litter in submarine canyons: a geoscience perspective

Abstract

Marine litter is one of the most pervasive and fast-growing aspects of contamination in the global ocean, and has been observed in every environmental setting, including the deep seafloor where little is known about the magnitude and consequences of the problem. Submarine canyons, the main conduits for the transport of sediment, organic matter and water masses from shallow to abyssal depths, have been claimed to be preferential pathways for litter transport and accumulation in the deep sea. This is supported by ongoing evidence of large litter piles at great water depths, highlighting efficient transfer via canyons. The aim of this article is to present an overview of the current knowledge about marine litter in submarine canyons, taking a geological, process-based point of view. We evaluate sources, transport mechanisms and deposition of litter within canyons to assess the main factors responsible for its transport and accumulation in the deep sea. Few studies relate litter distribution to transport and depositional processes; nevertheless, results from available literature show that canyons represent accumulation areas for both land-based and maritime-based litter. Particularly, accumulation of fishing-related debris is mainly observed at the canyon heads and walls and is related to fishing activities carried out in and adjacent to canyons, while 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 turbidity currents, and is related to the proximity of canyons to shore. Global assessment of canyons exposure to riverine plastic inputs and fishing-related debris indicates varying susceptibility of canyons to litter, also highlighting that most of the canyons prone to receive large amounts of anthropogenic debris have not yet been surveyed. Considering that litter research in canyons is still in its infancy, several knowledge gaps need to be filled before the role of canyons as litter traps and the implication for benthic ecosystems can be fully understood.

Large volumes of microplastics are transported to the deep sea by turbidity currents

“…As there are >5000 land-detached canyons globally, occurring on all of the world's continental slopes (42) (fig. S6), the high microplastic contents reported here suggest that such canyons are globally important pathways and repositories for microplastics, and that land-attached canyons that are more efficiently connected to terrestrial outflows of pollution may be equally, if not more, important, as demonstrated for macrolitter (35).…”

“…These voluminous, powerful and often-destructive flows typically originate on or near to the continental shelf-edge and transfer sediment through submarine canyons directly or indirectly connected to rivers, or through land-detached canyons fed by river-derived and coastal sediment transported vast distances along the continental shelf by ocean currents (33). Thus, microplastics supplied by polluted rivers may plausibly be carried from the continental shelves to the deep sea via turbidity currents passing through submarine canyons (29,(34)(35). However, a paucity of direct deep-sea monitoring and in-situ sampling means that the role of turbidity currents in microplastic transport has never been definitively demonstrated.…”

Marine macro-litter mass outweighs biomass in trawl catches along abyssal seafloors of Sardinia channel (Italy)

“…The three most contaminated locations represented almost half of all items collected (48.8%) and 70.9% of all the weight collected in the survey when density values were standardized by the swept area. These hauls were scattered (SE, S, and NW), thus hindering the establishment of one single accumulating area and once again supporting the idea that marine litter distribution on the seafloor is highly heterogeneous and driven by several factors, among which (i) intrinsic features of macro-litter items, which eventually influence their transport, sink, and persistence in the environment, combined with (ii) local circulation patterns (e.g., upwelling or downwelling); (iii) the presence of peculiar geomorphological features such as seamounts, submarine canyons, or shelf areas (Woodall et al 2015 ; Pierdomenico et al 2019a , b , 2023 ; Cau et al 2022 ); and nonetheless, (iv) anthropogenic variables (Galli et al 2023 ). On the other hand, the remaining hauls showed a much smaller density of marine litter (< 90 items and 20 kg km −2 on average), which would support Sardinia as being a less impacted area compared to other particular regions of the Mediterranean (i.e., NW Mediterranean Sea or Adriatic Sea) (Alvito et al 2018 ; Galimany et al 2019 ; Soto-Navarro et al 2020 ).…”

Extending morphometric scaling relationships: the role of bankfull width in unifying subaquatic channel morphologies

“…Submarine canyons and channels, among the largest morphological features on Earth (Harris and Whiteway, 2011), are shaped by the longest sediment flows, termed turbidity currents, fostering immense scientific interest (Talling et al, 2022). These turbidity currents bear a significant role in transporting sediment, organic carbon, nutrients, and pollutants to the deep-sea (Galy et al, 2007;Kane and Clare, 2019;Pierdomenico et al, 2023). They also pose a potential hazard to critical infrastructure like undersea cables, which requires an indepth understanding of their behaviors (Piper et al, 1988;Talling et al, 2022).…”