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.

Mercury Isotope Fractionation by Internal Demethylation and Biomineralization Reactions in Seabirds: Implications for Environmental Mercury Science

“…where A is a n × m matrix of the spectroscopic f(Spi)t values, n is the number of tissue samples (22)(23), and m is the number of Hg species (3). x and b are two column vectors associated with the unknown δ 202 Spi values and the measured δ 202 Hgt values, respectively.…”

“…8 Aside from demethylation, other processes that fractionate Hg isotopes have not been documented within biota, including mass-independent fractionation (MIF; expressed as Δ 199 Hg, Δ 200 Hg, Δ 201 Hg, Δ 204 Hg) from kinetic processes or equilibrium isotope effects, the latter driven by both MDF and nuclear volume effect (NVE). [20][21][22] A fundamental understanding of the internal processes controlling the isotopic fractionation of Hg in organisms is essential to fully utilize isotopic values for environmental applications, such as identifying the dietary origins of MeHg, 11,23 climate change effects, 24 and response to changes in Hg sources. 25,26 To address these science needs, the isotopic fractionation of Hg between the three prominent chemical forms of Hg (MeHg, Hg(Sec)4, HgSe) was quantified using diverse tissues of giant petrels.…”

Geology, environment, and life in the deepest part of the world’s oceans

“…They argued that the isotopic composition of THg in each sample was representative of the isotopic composition of MeHg in that sample and that deep ocean MeHg is supplied from the upper ocean by particles sinking rather than by in situ microbial methylation. This argument is further supported by Blum et al., 162 who suggested that anthropogenic Hg in the marine food web at∼500 m is transported to deep-sea trenches primarily through carrion in the water column, and integrated within the endemic fauna in hadal trenches. Liu et al.…”

“…They argued that the isotopic composition of THg in each sample was representative of the isotopic composition of MeHg in that sample and that deep ocean MeHg is supplied from the upper ocean by particles sinking rather than by in situ microbial methylation. This argument is further supported by Blum et al, 162 who suggested that anthropogenic Hg in the marine food web at500 m is transported to deep-sea trenches primarily through carrion in the water column, and integrated within the endemic fauna in hadal trenches. Liu et al 34 measured the THg and MeHg in amphipods from the Mariana, Massau, and New Britain Trenches, at depths of 6,990 to 10,840 m. They found that THg and MeHg concentrations were higher in trench amphipods than in amphipods from coastal and freshwater environments and suggested that Hg bioaccumulation in the hadal trenches depends on food availability, which is closely related to the surface ocean's biogeochemical characteristics, such as lateral transport, methylation, and MeHg bioaccumulation in food webs.…”

Profile of Joel D. Blum

“…His research group has also developed methods for making high-precision measurements of metallic element isotope ratios in terrestrial, aquatic, and atmospheric systems. Elected to the National Academy of Sciences in 2020, Blum used mercury isotopic signatures to investigate the world's deepest ocean trenches, and the findings are reported in his Inaugural Article (1). Blum and his colleagues found that anthropogenic mercury reaches these remote areas primarily via sinking fish carcasses, an insight that could aid global mercury pollution reduction efforts.…”