Mercury (Hg) geochemistry of mid-ocean ridge sediments on the Central Indian Ridge: Chemical forms and isotopic composition

Mercury Isotopes in Deep-Sea Epibenthic Biota Suggest Limited Hg Transfer from Photosynthetic to Chemosynthetic Food Webs

“…THg concentrations in the studied massive sulfides range over 3 orders of magnitude from 7 to 1.8 × 10 4 ng g −1 (dry weight), with an average of 2690 ± 4689 ng g −1 (mean ± 1SD, n = 14) (Table S1 and Figure 2). These high and large scatters of Hg concentrations are consistent with the previously reported values in massive and chimney sulfides and sulfide minerals of Duanqiao and Yuhuang HFs from SWIR (350−4.4 × 10 4 ng g −1 ) and hydrothermally altered sediments in CIR (17−1.3 × 10 4 ng g −1 ), 15,60 corroborating the viewpoint that Hg discharged from hydrothermal vent fluids is preferably removed by coprecipitated sulfides. 60,61 The Hg MDF and odd-MIF values (δ 202 Hg = −0.77 ± 0.25‰, −1.17 to −0.34‰ ; Δ 199 Hg = 0.05 ± 0.06‰, −0.06 to 0.16‰, n = 14) (Table S1 and Figure 3) of our massive sulfides are also comparable (P > 0.05) to those of previously reported SWIR sulfides (δ 202 Hg = −0.59 ± 0.30‰, −1.23 to −0.05‰ ; Δ 199 Hg = 0.05 ± 0.06‰, −0.10 to 0.20‰, n = 42).…”

“…These high and large scatters of Hg concentrations are consistent with the previously reported values in massive and chimney sulfides and sulfide minerals of Duanqiao and Yuhuang HFs from SWIR (350−4.4 × 10 4 ng g −1 ) and hydrothermally altered sediments in CIR (17−1.3 × 10 4 ng g −1 ), 15,60 corroborating the viewpoint that Hg discharged from hydrothermal vent fluids is preferably removed by coprecipitated sulfides. 60,61 The Hg MDF and odd-MIF values (δ 202 Hg = −0.77 ± 0.25‰, −1.17 to −0.34‰ ; Δ 199 Hg = 0.05 ± 0.06‰, −0.06 to 0.16‰, n = 14) (Table S1 and Figure 3) of our massive sulfides are also comparable (P > 0.05) to those of previously reported SWIR sulfides (δ 202 Hg = −0.59 ± 0.30‰, −1.23 to −0.05‰ ; Δ 199 Hg = 0.05 ± 0.06‰, −0.10 to 0.20‰, n = 42). 15 The even-MIF values (Δ 200 Hg = 0.01 ± 0.01‰, −0.01 to 0.03‰; Δ 204 Hg = 0.00 ± 0.03‰, −0.07 to 0.08‰) of our massive sulfides are mostly within their analytic uncertainty (Figure S1).…”

“…71,72 Based on the mean Hg concentration (4475 ± 6400 ng g −1 , n = 56) of massive sulfides in four HFs (Table 1), we calculated that the seafloor hydrothermal sulfides deposited 2685 ± 3840 Mg of Hg, in which ∼1300 Mg was from seawater according to our binary mixing model. A recent study estimated that the total Hg budget in global mid-ocean ridge regions could reach ∼320 Mg yr −1 , 60 implying that seawater and magmatic/mantle Hg, each of ∼160 Mg yr −1 , are immobilized in seafloor sulfide deposits. The seawater Hg sink fluxes via seafloor hydrothermal sulfides are thus equally important as those via trench sediments, also estimated as ∼160 Mg yr −1 .…”

“…The Hg isotopic compositions in submarine hydrothermal sulfides have been interpreted to reflect a mixture of primordial (magmatic/mantle) Hg and seawater Hg under a regime of hydrothermal leaching. , The magmatic/mantle Hg is characterized by zero MIF, , and the seawater Hg is characterized typically by small and positive MIF. , Using a binary mixing model that assumes magmatic/mantle and open seawater have mean Δ 199 Hg values of 0.00 ± 0.05‰ and 0.11 ± 0.09‰ (1SD, represented by Atlantic open seawater), respectively, we estimate that on average ∼50% of the Hg in CIR sulfides (Edmond and Kairei: 46%, 34–73%) and SWIR sulfides (Duanqiao: 46%, 40–59%; Longqi: 51%, 37–75%; Yuhuang: 49%, 33–82%; respectively) sourced from seawater. We acknowledge that the calculated seawater Hg contributions to the sulfides are biased by the representativeness of magmatic/mantle and open seawater end-members and potentially small MIF during the abiotic, hydrothermal processes .…”

“…Δ 199 Hg versus δ 202 Hg in biota (including two samples from the Site F cold seep, abbreviated as SFCS, South China Sea), sulfides, and sediments from the Indian Ocean Ridge reported in this and previous studies. 15,60 Also shown are Hg isotope values (mean ± 1SD) of chimney pieces and fluid precipitate from the Guaymas Basin (filled red-white circle), 19 Earth′s silicate crust (empty red diamond), 62 primordial mantle (red cross), 64 seawater (filled blue circle), 33,69 deep-sea (<1000 m) sediments (filled gray circle), 33,38,73,95−98 trench fauna (filled brown-yellow triangle), 38,40 and upper marine fishes (filled purple-pink triangle). 33−37 The inset shows the enlarged area of the figure, and the ellipses are used to outline the variation ranges of Hg isotopic compositions for sulfides/sediments and biota.…”