Monomethylmercury (MMHg) is a potent toxin that bioaccumulates and magnifies in marine food webs. Recent studies show abundant methylated Hg in deep oceans (>1000 m), yet its origin remains uncertain. Here we measured Hg isotope compositions in fauna and surface sediments from the Mariana Trench. The trench fauna at 7000–11000 m depth all have substantially positive mass-independent fractionation of odd Hg isotopes (odd-MIF), which can be generated only in the photic zone via MMHg photo-degradation. Given the identical odd-MIF in trench fauna and North Pacific upper ocean (<1000 m) biota MMHg, we suggest that the accumulated Hg in trench fauna originates exclusively from MMHg produced in upper oceans, which penetrates to depth by sorption to sinking particles. Our findings reveal little in-situ MMHg production in deep oceans and imply that anthropogenic Hg released at the Earth’s surface is much more pervasive across deep oceans than was previously thought.

一甲基汞 (MMHg) 是一种强效毒素，可在海洋食物网中生物累积和放大。最近的研究表明深海（>1000米）中存在丰富的甲基化汞，但其来源仍不确定。在这里，我们测量了马里亚纳海沟动物群和表层沉积物中的汞同位素组成。 7000-11000米深度的海沟动物群都具有基本上与质量无关的奇数汞同位素（奇数MIF）的正分馏，这种分馏只能在透光区通过MMHg光降解产生。鉴于海沟动物群和北太平洋上层海洋（<1000 m）生物群 MMHg 具有相同的奇数 MIF，我们认为海沟动物群中积累的汞完全来自上层海洋产生的 MMHg，通过吸附下沉颗粒渗透到深处。我们的研究结果表明，深海中原位MMHg 的产生量很少，这意味着地球表面释放的人为汞在深海中的分布比之前认为的要普遍得多。