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TitleHigh mercury accumulation in deep-ocean hadal sediments
AuthorSanei, H; Outridge, P MORCID logo; Oguri, K; Stern, G A; Thamdrup, B; Wenzhofer, F; Wang, F; Glud, R N
SourceScientific Reports vol. 11, issue 1, 10970, 2021 p. 1-8, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20210142
PublisherNature Research
Mediapaper; on-line; digital
File formatpdf; html
AreaPacific Ocean; Kermadec Trench; Atacama Trench; Peru; Chile; New Caledonia; New Zealand
Lat/Long WENS-177.6667 -176.1667 -30.5000 -32.8333
Lat/Long WENS -72.8333 -70.5000 -19.8333 -24.5000
Subjectsenvironmental geology; marine geology; surficial geology/geomorphology; sedimentology; Nature and Environment; Science and Technology; mercury geochemistry; marine sediments; sea sediment geochemistry; ocean trenches; abyssal plains; organic carbon; modelling; pollutants; metals; marine sediment cores; sedimentation rates; bathymetry; Minamata Convention on Mercury; Oceans
Illustrationslocation maps; geoscientific sketch maps; bar graphs; plots
ProgramEnvironmental Geoscience Program Management
Released2021 05 26
AbstractOcean sediments are the largest sink for mercury (Hg) sequestration and hence an important part of the global Hg cycle1. Yet accepted global average Hg flux data for deep-ocean sediments (> 200 m depth) are not based on measurements on sediments but are inferred from sinking particulates2. Mercury fluxes have never been reported from the deepest zone, the hadal (> 6 km depth). Here we report the first measurements of Hg fluxes from two hadal trenches (Atacama and Kermadec) and adjacent abyssal areas (2-6 km). Mercury concentrations of up to 400 ng g-1 were the highest recorded in marine sediments remote from anthropogenic or hydrothermal sources. The two trench systems differed significantly in Hg concentrations and fluxes, but hadal and abyssal areas within each system did not. The relatively low recent mean flux at Kermadec was 6-15 times higher than the inferred deep-ocean average1,3, while the median flux across all cores was 22-56 times higher. Thus, some hadal and abyssal sediments are Hg accumulation hot-spots. The hadal zone comprises only ~ 1% of the deep-ocean area, yet a preliminary estimate based on sediment Hg and particulate organic carbon (POC) fluxes suggests total hadal Hg accumulation may be 12-30% of the estimate for the entire deep-ocean. The few abyssal data show equally high Hg fluxes near trench systems. These results highlight a need for further research into deep-ocean Hg fluxes to better constrain global Hg models.
Summary(Plain Language Summary, not published)
Ocean sediments are believed to be the largest long-term repositories of the toxic metal mercury (Hg), and permanently retain the Hg removed from the ocean by natural processes. However, the estimates of Hg entering the deep-ocean sediments (>1,000 m depth) have never before been directly measured but instead are inferred from seawater measurements. This study presents the first data on the concentration and fluxes of mercury in sediments deposited into deep-ocean trenches and adjacent abyssal plains. We report that in two Pacific Ocean trenches, the sediment Hg concentrations are the highest ever recorded in unpolluted marine sediments, and approach those in highly contaminated areas. The fluxes of Hg (per unit area per year) are many times the inferred estimates presently used in global Hg models. The deep-ocean may therefore be a Hg accumulation "hotspot". Our findings suggest that the Hg cycle in the oceans is even more poorly understood than is thought, and that the rate of removal of Hg from the planetary environment is much faster than the currently-accepted global Hg budget indicates.

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