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.
Persistent organic pollutants (PCBs and PBDEs) were analysed in sediment core samples (0-2 cm) from the southern Mariana Trench at water depths of 7000-11000 m. ∑PCBs concentrations ranged from 931 to 4195 pg/g, far higher than those recorded before in marine sediments from shallower depths. Toxic Equivalence (TEQ) of dl-PCBs ranged from 0.650-14.9 pg/g, which is higher than most marine surficial sediments at <500-2500 m ocean depth, recovered from semi-industrial to industrial areas. However, ∑ 8 PBDEs values (averaging ~136 pg/g) were lower than those in surficial sediments from shelf areas recorded in past studies. Evidently, anthropogenic pollutants have reached the deepest realm on Earth, and the Mariana Trench acts as a repository for POPs amplification. The high concentration of PCBs is an eye-opener, which is directly affecting our deep sea ecosystems, considering their pervasiveness and persistence in trench sediments.
Tectonically induced bending of incoming plates at subduction zones can result in normal faulting in the upper ocean crust. Seismic surveys and numerical models indicate enhanced permeability and fluid circulation when this occurs. Yet, direct geological evidence of such effects on the seafloor is lacking. Here we report Human Occupied Vehicle (HOV) based observations of the existence of fluid discharge features on the seafloor of the incoming plate of the Mariana subduction zone. These features include fluid discharge points and associated pockmarks, which are striking, and occur in abundance in several depth related fields. The existence of Galatheid crabs, a typical seep related organism, also indicates fluid discharge from the seafloor. Alteration of the coexisting basaltic ocean crust is extensive, with iddingsite-rich muds within and overlapping the apparent fluid discharge zones. Our findings are significant in that they suggest that structural deformation of the incoming plate could substantially influence chemical exchange between the upper ocean crust and seawater in a new way. We further suggest that these fluid discharge points may represent previously unknown niches for H 2-based chemolithotrophic life and microbial ecosystems at deep trenches. Observations reported here contrast both chemically and physically with serpentine mud volcano formation associated with the shallower Mariana forearc region.
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