Microplastics contaminate the deepest part of the world’s ocean

doi: 10.7185/geochemlet.1829 Millions of metric tons of plastics are produced annually and transported from land to the oceans. Finding the fate of the plastic debris will help define the impacts of plastic pollution in the ocean. Here, we report the abundances of microplastic in the deepest part of the world’s ocean. We found that microplastic abundances in hadal bottom waters range from 2.06 to 13.51 pieces per litre, several times higher than those in open ocean subsurface water. Moreover, microplastic abundances in hadal sediments of the Mariana Trench vary from 200 to 2200 pieces per litre, distinctly higher than those in most deep sea sediments. These results suggest that manmade plastics have contaminated the most remote and deepest places on the planet. The hadal zone is likely one of the largest sinks for microplastic debris on Earth, with unknown but potentially damaging impacts on this fragile ecosystem. Received 22 July 2018 | Accepted 29 October 2018 | Published 27 November 2018 1. Deep Sea Science Division, Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya, Hainan 572000, China 2. Institute of Marine Geology and Resources, Ocean College, Zhejiang University, Zhoushan 316021, China * Corresponding author (email: chans@idsse.ac.cn) Letter Plastics are worldwide marine pollutants, accumulating in seawater and sediments (Hammer et al., 2012; Cózar et al., 2014; Ivar do Sul and Costa, 2014). It was estimated that between 4.8 and 12.7 million metric tons of plastic waste entered the ocean in 2010 and this mass could increase by one order of magnitude by 2025 (Jambeck et al., 2015; Geyer et al., 2017). Besides the ocean surface (Thompson et al., 2004; Barnes et al., 2009; Van Sebille et al., 2015; Chae and An, 2017), potential sinks for plastics include deep sea biota (Oliveira et al., 2012), the water column (Courtene-Jones et al., 2017; Kanhai et al., 2018) and sediments (Bergmann et al., 2017), where broken plastics exist as microplastics (<5 mm in size) (Arthur et al., 2009; Hidalgo-Ruz et al., 2012). So far, however, microplastics in the deepest ocean remain largely unexplored. The hadal zone, which is the deepest region (6000-11000 m) of the oceans lying within trenches, represents 1-2 % of the global benthic area (Jamieson et al., 2010). Although it was reported that toxic anthropogenic pollutants (e.g., persistent organic pollutants) have reached the deepest ocean on Earth (Jamieson et al., 2017; Dasgupta et al., 2018), little is known about the nature of anthropogenic microplastics in this deep and remote environment. To evaluate the abundance, distribution, and fate of microplastics in the hadal zone, we collected bottom water samples and sediment samples at depths of 2500-11000 m and 5500-11000 m, respectively, from the southern Mariana Trench, where the Challenger Deep, the deepest point on Earth, is situated (Fujioka et al., 2002) (Fig. 1). Identification by optical microscope and Raman spectrometer confirmed that microplastics are abundant in hadal bottom water (Fig. S-1). The microplastics are fibrous, rod-like, and roundish in shape, and mostly blue, red, white, green, and purple in colour. Plastic microfibres dominate in all the microplastics and are commonly 1-3 mm in length in seawater samples and mostly 0.1-0.5 mm in sediment samples (Table S-4). The microplastic abundances in bottom waters range from 2.06 to 13.51 pieces per litre and become more concentrated with depth (Fig. 2) with one exception at depth of 6802 m, reaching 13.51 pieces per litre. At 10903 m, the microplastic abundance reaches 11.43 pieces per litre, which is four times higher than that reported in the subsurface water of open seas, including the NE Pacific Ocean (Desforges et al., 2014), South Pacific subtropical gyre (Eriksen et al., 2013), North Pacific Gyre (Goldstein, 2012), North Atlantic Ocean (Courtene-Jones et al., 2017), and the Arctic Ocean (Bergmann et al., 2017; Kanhai et al., 2018) (Table 1). The high abundance of microplastics in hadal bottom water is also comparable to that reported in coastal waters, for example, in the Yangtze River and the Strait of Georgia, which are regarded as heavily polluted by microplastics (Desforges et al., 2014; Zhao et al., 2014). Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2018) 9, 1-5 | doi: 10.7185/geochemlet.1829 2 Figure 1 Sampling location map of Mariana Trench seawater (in red triangles) and sediments (in yellow circles). Please see Tables S-1 and S-2 for sampling details. Figure 2 Profile of microplastic abundances and compositions in water samples from Mariana Trench. Pie charts represent the microplastic compositions and numbers in the bracket are the microplastic abundances with units of pieces per litre. PVC-polyvinyl chloride, PA-polyamide, Ra-rayon, ABS-acrylonitrile butadiene styrene, PP-polypropylene, PE-polyethylene, PS-polystyrene, aPAaromatic polyamide, PET-polyethylene terephthalate, Pe-polyester, PU-polyurethane. The X-axis corresponds to the crossline from point A (12 oN, 142.5 oE) to point B (9.8 oN, 141.43 oE) in Figure 1.