Mercury proxy for hydrothermal and submarine volcanic activities in the sediment cores of Central Indian Ridge
Introduction
The geothermal activities including hydrothermal vent and submarine volcano in the Mid-Ocean Ridges (MOR) have been suggested to be an important natural source of mercury (Hg) to the ocean (Cox and McMurtry, 1981; Racki et al., 2018). Generally, hydrothermal vents form black smoke chimney by minerals dissolved in the vent fluids, which contains sulfides, various metals (e.g., iron, manganese, copper, zinc and etc.) and gases (hydrogen, carbon dioxide and methane). A recent study has found significantly high Hg concentrations in hydrothermal fluids, ranging from 4 to 16 pM, which indicated that the entire ocean has a potential Hg flux of 0.1 to 0.4 Mmol yr−1 (Fitzgerald et al., 2007) comparable to the amount of 0.25 to 0.5 Mmol yr−1 emitted from terrestrial volcano activity (Pyle and Mather, 2003). The results in the Hellenic Arc in Greece showed that the concentration of gaseous Hg (Hg0) emanated from hydrothermal vents was roughly 10 times higher than that of the terrestrial volcanic Hg, highlighting an importance of hydrothermal input for natural Hg sources (Rizzo et al., 2019). However, the magnitude of Hg emission from hydrothermal vents is still highly uncertain, even though considerable contributions from natural Hg sources are suspected. Furthermore, discrimination of hydrothermal- and volcanic-sourced Hg in the deep-sea environments has not been well constrained despite the differences in gas compositions between them. In this respect, fractionation of Hg species in the sediments can provide insights into the identification of Hg source in complex geothermal environments of the deep sea, as well as the estimation of the deep sea-sourced Hg flux. In addition, the Hg can be considered to be a more useful hydrothermal tracer than the conventional tracers (e.g., Fe and Mn), because of its low background value as well as uncommon sources in the deep sea.
The Central Indian Ridge (CIR) is the slow spreading MOR traversing from north to south in the Indian Ocean. The CIR has the great potentials for active hydrothermal vents, and the evidences of 18 hydrothermal vent existences have been reported in this area (German et al., 1998; German and Von Damm, 2004; Kattoju et al., 2015). Recently, the new hydrothermal vents named as Onnuri Vent Field (OVF) have been discovered near to the CIR by vessel RV Isabu operated by KIOST (the Korea Institute of Ocean Science and Technology) (Ivanenko et al., 2019), implying that the CIR is still likely to have various hydrothermal vents coupled with active volcanic eruptions along the ridge systems. Thus, this MOR in the Central Indian Ocean is the challenging place to investigate the influence of hydrothermal vent and submarine volcano on Hg flux and formation.
Since Hg in the sediments is present in a large variety of chemical forms that are controlled by various physiochemical factors, chemical determination of Hg species is of great relevance to better understand Hg source and its depositional mechanism. For examples, the Hg associated with Fe-Mn oxides can indicate which type of geothermal activities influenced the Hg deposition, and further their temporal records in the sediment core can indicate the magnitude and intensity of geothermal activities in deep sea basins such the MOR. Thus, based on the above-mentioned backgrounds, we investigated how the Hg from geothermal activities were conserved in the deep-sea sediments, and of which existing forms of Hg can be proxy for hydrothermal or submarine volcanic activities in the sediment cores. To address these questions, three sediment cores were collected from the CIR affected by geothermal activities (Table 1 and Fig. 1). For this study, the Hg speciation including sulfide and organic matter-bound Hg (sulfide-organic-bound Hg), Fe and Mn oxide-bound Hg (Fe-Mn oxide-bound Hg) and residual Hg (residue-Hg) were analyzed, and additionally total organic carbon (TOC) and sulfur (TS), several mantle-derived trace metals (i.e., Co, Cu, Zn, Cd, Sn, Mo and Pb), proxy tracer ratio ([Mg]/[Ca]), and rare earth elements (REE; Sm, Nd, Rb and Sr) were analyzed after removing the biogenic carbonates to examine the influence of geothermal activities on sediment Hg flux and speciation.
Section snippets
Sediment core sampling
Three short sediment cores (MC1705, MC1706, and MC1707) up to 28 to 50 cm in length were collected on the rift valley (3454 to 4317 m water depth), ~3 km from ridge axis of segment 1 of the northern CIR (Table 1 and Fig. 1A and B) using a multiple corer on board of the R/V Isabu during the CIR hydrothermal exploration cruise in June 2017. The sediment cores were split lengthways, photographed, and logged in detail by visual examination (Fig. 1C), and then subsampled at 1 to 3 cm intervals for
Reference value for sedimentary Hg concentrations in CIR sediments
The total Hg (THg) concentration of the MC1707 core ranged from 3 to 25 ng g−1, showing a slightly increasing trend over the last 4.25 kyr (Fig. 2A). More than 80% of THg in most sediments of the core was associated with sulfide-organic-bound Hg (Fig. 2B). THg mass accumulation rates (Hg MAR) also showed a significantly upward increase, ranging from 0.12 to 0.91 μg m−2 yr−1 (Fig. 2C). TS contents also tended to increase to the top of the core in line with Hg influx (Figs. 2A and 3B). Notably,
Conclusion
The purpose of this study was to find a better index for hydrothermal and submarine volcano activities using the sequential Hg extraction in the sediment cores collected from the Central Indian Ocean. For attaining this purpose, the Hg speciation was compared with other conventional hydrothermal proxies including other trace metals, their ratios, and Sr-Nd isotopic compositions. Characteristically, the Fe-Mn oxide-bound Hg in the sediments of the cores was a reliable evidence for an increased
CRediT authorship contribution statement
Dhongil Lim: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Supervision, Project administration, Funding acquisition. Haryun Kim: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing, Visualization. Jihun Kim: Investigation. Dohyun Jeong: Investigation. Dongsung Kim: Conceptualization, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
“Understanding the deep-sea biosphere on seafloor hydrothermal vents in the Indian Ridge (NO. 20170411)” was funded by the Ministry of Oceans and Fisheries, Korea. Partial supports were provided by the National Research Foundation of Korea (NRF) grants (2019R1F1A1059106) funded by the Ministry of Science and ICT of Korean government. Lastly, We thanked to the Library of Marine Samples for supplying the samples.
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