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

Guangfa Zhong,Xiaotong Peng

doi:10.1130/g48536.1

Abstract

Abstract 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 plastic-dominated 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 upstream-facing 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.

Highlights

Marine plastic pollution is a pressing global challenge because of its deleterious impact on the environment and its ecosystems, on human health, and on social economy
IMPLICATIONS AND CONCLUSIONS While the larger-scale dispersal and accumulation patterns of deep-sea plastic waste remain unclear, our study suggests that the benthic litter accumulated in the scours of the submarine canyon studied here was most likely transported and deposited by gravity flows
The hypothesis of gravity flow–controlled litter dispersion has been raised by several authors, concrete evidence has been lacking; here, we present supporting evidence from mapping the distributions of both the scattered litter items and the litter piles, combined with grain-size and morphodynamic analyses

Summary

Introduction

Marine plastic pollution is a pressing global challenge because of its deleterious impact on the environment and its ecosystems, on human health, and on social economy Litter density in submarine canyons has been reported to be 2–3 times higher than that on adjacent open shelves and slopes (Pham et al, 2014; Cau et al, 2017; Kane et al, 2020). The mechanisms by which benthic litter is dispersed in submarine canyons are, largely unknown. AND METHODS Located in the northwestern SCS, the studied submarine canyon (18.10–18.64°N, 111.86– 112.10°E) is headed at 350 m water depth on the outer shelf, ∼150 km from the nearest coast, and it terminates at ∼2350 m depth on the continental slope. The middle reach of the canyon, the focus of this study, consists of a long stepped chute leading into two successive large scours at the chute toe (Fig. 1B; Fig. S1 in the Supplemental Material). The margin is subject to frequent typhoons and strong internal waves (Alford et al, 2015)

Results

Discussion

Conclusion