Mercury isotopes identify near-surface marine mercury in deep-sea trench biota

Mercury isotopic compositions of amphipods and snailfish from deep-sea trenches reveal information on the sources and transformations of mercury in the deep oceans. Evidence for methyl mercury subjected to photochemical degradation in the photic zone is provided by odd-mass independent isotope values (Delta Hg-199) in amphipods from the Kermadec Trench, which average 1.57 parts per thousand (+/- 0.14, n = 12, SD), and amphipods from the Mariana Trench, which average 1.49 parts per thousand (+/- 0.28, n = 13). These values are close to the average value of 1.48 parts per thousand (+/- 0.34, n = 10) for methyl-mercury in fish that feed at similar to 500-m depth in the central Pacific Ocean. Evidence for variable contributions of mercury from rainfall is provided by even-mass independent isotope values (Delta Hg-200) in amphipods that average 0.03 parts per thousand (+/- 0.02, n = 12) for the Kermadec and 0.07 parts per thousand (+/- 0.01, n = 13) for the Mariana Trench compared to the rainfall average of 0.13 (+/- 0.05, n = 8) in the central Pacific. Mass-dependent isotope values (delta Hg-202) are elevated in amphipods from the Kermadec Trench (0.91 +/- 0.22 parts per thousand, n = 12) compared to the Mariana Trench (0.26 +/- 0.23 parts per thousand, n = 13), suggesting a higher level of microbial demethylation of the methyl-mercury pool before incorporation into the base of the foodweb. Our study suggests that mercury in the marine foodweb at similar to 500 m, which is predominantly anthropogenic, is transported to deep-sea trenches primarily in carrion, and then incorporated into hadal (6,000-11,000-m) food webs. Anthropogenic Hg added to the surface ocean is, therefore, expected to be rapidly transported to the deepest reaches of the oceans.