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

Abstract

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 (Δ¹⁹⁹Hg) in amphipods from the Kermadec Trench, which average 1.57‰ (±0.14, n = 12, SD), and amphipods from the Mariana Trench, which average 1.49‰ (±0.28, n = 13). These values are close to the average value of 1.48‰ (±0.34, n = 10) for methyl-mercury in fish that feed at ∼500-m depth in the central Pacific Ocean. Evidence for variable contributions of mercury from rainfall is provided by even-mass independent isotope values (Δ²⁰⁰Hg) in amphipods that average 0.03‰ (±0.02, n = 12) for the Kermadec and 0.07‰ (±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 (δ²⁰²Hg) are elevated in amphipods from the Kermadec Trench (0.91 ±0.22‰, n = 12) compared to the Mariana Trench (0.26 ±0.23‰, 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 ∼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.

Keywords: deep sea; isotope; mercury; oceanography; trench.

Fig. 1.

Depth of sample collection in meters on log scale versus δ²⁰²Hg of samples. Symbol colors are keyed to the color of text labeling each sample type. Red and green symbols are analyses from this study of snailfish and amphipods from the Mariana (red) and Kermadec (green) trenches, respectively. Blue and black symbols are precipitation and marine particles from the central Pacific Ocean from ref. . Purple x’s are analyses of fish from the central Pacific Ocean from refs. and . See SI Appendix, Table S2 for analytical uncertainty of isotope values.

Fig. 2.

Depth of sample collection in meters on log scale versus Δ²⁰⁰Hg of samples. Symbols are the same as in Fig. 1. See SI Appendix, Table S2 for analytical uncertainty of isotope values.

Fig. 3.

Depth of sample collection in meters on log scale versus Δ¹⁹⁹Hg of samples. Symbols are the same as in Fig. 1. See SI Appendix, Table S2 for analytical uncertainty of isotope values.

Fig. 4.

Δ¹⁹⁹Hg versus δ²⁰²Hg of all samples. Symbols are the same as in Fig. 1. Lines with arrows illustrate the shift in isotopic composition of residual MMHg during important fractionation processes (30). See SI Appendix, Table S2 for analytical uncertainty of isotope values.