Transport and accumulation of plastic litter in submarine canyons?The role of gravity flows

rine canyons are, however, largely unknown. It has been suggested that certain underflows, including internal tides (Schlining et al., 2013; van den Beld et al., 2017) and various types of gravity flows (Wei et al., 2012; Tubau et al., 2015; Daly et al., 2018; Kane and Clare, 2019; Pierdomenico et al., 2019; Pohl et al., 2020), might be responsible. Here, we report the results of manned submersible dives that found focused accumulations of benthic litter in a submarine canyon of the South China Sea (SCS). We investigated the transport and depositional processes that formed these accumulations by documenting litter distribution together with grain-size and morphodynamic analyses. Manned submersible dives discovered plastic litter accumulations in a submarine canyon located in the northwestern South China Sea, ?150 km from the nearest coast. These plasticdominated litter accumulations were mostly concentrated in two large scours in the steeper middle reach of the canyon. Plastic particles and fragments generally occurred on the upstreamfacing sides of large boulders and other topographic obstacles, indicating obstruction during down-valley transportation. Most of the litter accumulations were distributed in the up-valley dipping slopes downstream of the scour centers. This pattern is tentatively linked to turbidity currents, which accelerated down the steep upstream slopes of the scours and underwent a hydraulic jump toward the scour centers before decelerating on the upstream-facing flank. Associated seabed sediment consisted of clayey and sandy silts, with unimodal or bimodal grain-size distributions, which are typical for turbidites. The focused distribution of the litter accumulations is therefore linked to turbidity currents that episodically flush the canyon. Our findings provide evidence that litter dispersion in the deep sea may initially be governed by gravity flows, and that turbidity currents efficiently transfer plastic litter to the deeper ocean floor.

Automated summary

The study found focused accumulations of plastic litter in a submarine canyon of the South China Sea, concentrated mainly in two large scours in the steeper middle reach of the canyon. The distribution of litter accumulations is linked to turbidity currents, indicating that litter dispersion in the deep sea may be governed by gravity flows initially.