Mercury (Hg) in marine shallow-water hydrothermal systems - an overlooked source of Hg in the global cycle

• Applicant: Professor Dr. Thomas Pichler
• Subject Area: Oceanography ; Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management ; Mineralogy, Petrology and Geochemistry
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 391059775

• Final Report Year: 2023

Final Report Abstract

At all three sites, the Hg concentrations in hydrothermal fluids were higher than those in seawater (up to 3000-times). Hydrothermal fluids had a lower pH, higher temperature, elevated concentrations of Ca, K, Li, Mn, Si, H2S, THg, and Hgdiss, and lower concentrations of Cl, Br, Na, and Sr than the background samples. The same was observed when seawater samples collected in the area of hydrothermal venting were compared to seawater samples away from the area of hydrothermal venting. Methylated Hg species were not found in the hydrothermal fluids at any of the three sites. No linear relationship was observed between THg and Hgdiss. Neither was there any correlation with H2S, although higher Hg concentrations were generally found when H2S was low. This could indicate that H2S plays a role in removing Hg from the hydrothermal fluid via possible precipitation of a Hg-sulfide. The Hg concentrations in the hydrothermal gases ranged over several orders of magnitude from 0.7 to 2791 nmol/m3. There seems to be a slight relationship between THg in the pore fluids and Hg in the gases. More strikingly, the sediment cover at the venting site seemingly has a large effect on the Hg concentrations of the hydrothermal gases. With a few exceptions, the concentration in gas samples collected from unsedimented sites was up to three orders of magnitude higher. This leads to the conclusion that likely gas-water or gas-sediment reactions in the sediment cover remove Hg from the hydrothermal gases. In summary, emissions of Hg are generally determined by three basic principles. First, the concentration of Hg in source material (e.g., subsurface magmatic vapor) determines the limits of Hg emission from any given system. Second, direct, high temperature, high volume flow maintains the most Hg emission within gases and fluids within the subsurface. Third, sedimentation plays a crucial role in filtering Hg from gases and fluids. Methylation of Hg from MSWHS was not found to occur within these systems, however, transport of inorganic species increases the likelihood of integration into local food webs. The observations reported here indicate that MSWHS contribute Hg to the global Hg cycle. Indeed, large mercury deposits are strongly associated with hydrothermalism. While the most significant contributor to the global mercury cycle continues to be anthropogenic influences, MSWHS are points of natural Hg emission. The dynamic interplay between environmental conditions (e.g., temperature, sedimentation, pH, hydrothermal flow) alters the fate of that emitted Hg. Continued observations of MSWHS will be necessary to characterize the individual processes appropriately and to better define the extent of impact.

Publications

• Speciation analysis of methylmercury via species specific isotope dilution GC-ICP-MS. ThermoFisher, Technical Note 30465, pp. 6
  Brombach, C.-C., Froellje, H., Pichler, T., Guzzonato, A., and Lindemann, T.
  
• Determination of ultra-low volatile mercury concentrations in sulfur-rich gases and liquids. Talanta, 199, 277-284.
  Brombach, Christoph-Cornelius & Pichler, Thomas
  
• Mercury in the hydrothermal fluids and gases in Paleochori Bay, Milos, Greece. Marine Chemistry, 233, 103984.
  Roberts, Hannah; Price, Roy; Brombach, Christoph-Cornelius & Pichler, Thomas
  
• Hg in the hydrothermal fluids and gases in Baia di Levante, Vulcano, Italy. Marine Chemistry, 244, 104147.
  Roberts, Hannah & Pichler, Thomas