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Hydrothermal vent is the one of the main natural Hg sources to the deep ocean. Thus, we investigated which Hg speciation in the sediment core can be the past records for geothermal activities in mid-ocean ridges of the Central Indian Ocean. The result showed that the hydrothermal Hg in the core sediments was mainly associated with Fe-Mn oxides with the elevated concentrations of other hydrothermal-derived trace metals [Co + Zn + Cu]. In addition, the [Sm]/[Nd] and [Rb]/[Sr] ratios and ɛNd CHUR and 87 Sr/ 86 Sr isotopic values supported that the extremely high Hg concentrations were possibly originated from the hydrothermal vent. However, the Hg emitted from submarine volcano was mainly associated with sulfides-organic matters because the volcanos did not release Fe and Mn. Thus, our results showed that the sedimentary Hg is an independent toll for reconstruction of paleodynamics of hydrothermal and/or volcanic activities in deep sea basin of the Central Indian Ocean., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

To geochemically characterize mercury (Hg) in the deep-sea ridge environment, the total concentration, chemical forms (sequential leaching extraction), and isotopic compositions of Hg in surface sediments from the middle portion of the Central Indian Ridge were investigated. Highly elevated Hg concentrations (up to 13,000 ng/g) in sediments near the hydrothermal vent are associated with intense hydrothermal activity driven by serpentinization. The hydrothermal impact on these sediments is also evident in the REE CN fractionation pattern with a remarkably strong positive europium (Eu) anomaly. Most volcanic and hydrothermal Hg in the study area is preferentially precipitated with sulfides; in the hydrothermal vent area, however, scavenging by Fe-Mn hydroxides is another significant removal pathway of Hg. Thus, such precipitation and production of sulfides and hydroxides are a major cause of local enrichment of Hg around the mid-ocean ridge. Most sediments show limited or no mass-independent fractionation (Δ199Hg = +0.02 ± 0.21‰, 2σ, n = 15), indicating that syngenetic magmatic or mantle-derived materials are the dominant Hg source. However, the large variation in mass-dependent fractionation was observed, especially in the vent-distal sediments (δ202Hg = −1.10 ± 0.80‰, 2σ, n = 11), which occurred mainly during the formation of the sulfides and may be associated with preferential precipitation of lighter isotopes. Our study demonstrates that an off-axis high-temperature hydrothermal system driven by exothermic serpentinization of ultramafic mantle rocks may serve as a significant Hg source and provides further insights into grasping the behavior of hydrothermal and volcanogenic Hg in active deep-sea ridge systems. • Extremely high Hg concentrations are observed in the serpentinization-driven hydrothermal vent sediments. • Sequential extraction demonstrates the depositional pathways of Hg in deeps-sea ridge system. • Most volcanic and hydrothermal Hg is sequestered by co-precipitation with sulfides. • Fe-Mn oxides are another prevalent Hg speciation, especially in the hydrothermal sediments. • Significant Hg-MDF may occur during the formation of Hg-sulfide precipitates. [ABSTRACT FROM AUTHOR]