Arsenic in gases from shallow-water hydrothermal systems

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Marine shallow-water hydrothermal systems (MSWHS), as opposed to deep-sea hydrothermal systems, occur in shallow-water nearshore settings where they can have a considerable impact on the coastal ocean through discharge of reduced, hot hydrothermal fluids that often contain arsenic (As). The emission of fluids is generally accompanied by the extensive discharge of hydrothermal gases. Nothing is known about the arsenic concentrations in those gases, although arsenic partitions into the gas phase in hydrothermal systems and is known to be present in comparable land-based volcanic gases.Why arsenic has not been measured in the gas phase in MSWHS is unknown, although it is likely that the collection of sufficient sample volume and the transport to a suitable laboratory were a deterrent. To collect anywhere from 10 to 25 L of gas from hydrothermal vents on the seafloor is not trivial, and shipping such volumes would be problematic.With this in mind, the objective of the proposed project will be to develop and test a procedure for trapping and measuring volatile arsenic in the gas phase in MSWHS. The method will be based on an established method of underwater gas collection into gastight Tedlar® bags initially developed for mercury (Hg). Following collection, arsenic will be extracted from the gas by either trapping onto a silicon resin or into a NaOCl solution. First, the trapping methods will be tested and optimized under laboratory conditions. Second, they will be applied and tested in the field by sampling hydrothermal gases from the MSWHS in Paleochori Bay on the south side of Milos, Greece. There, elevated concentrations in the hydrothermal fluids lead to an annual flux of 1.5 × 10^4 kg arsenic from the Bay into the Mediterranean Sea. Still, this estimation is missing for the volatile arsenic fraction and assuming concentrations similar to land-based hydrothermal systems, the arsenic flux via the gas phase would be 4.5 × 10^4 kg, three times the estimated flux via the water phase. Thus, a field test in Paleochori Bay should provide sufficient arsenic concentrations in the gas phase and provide first data for this arsenic-rich MSWHS. Once successfully applied, the newly developed method will provide a suitable field procedure for sampling arsenic in gases from MSWHS worldwide.