Natural input of arsenic into a coral reef ecosystem by hydrothermal fluids and its removal by Fe(III) oxyhydroxides

The coral reef that circles Ambitle island, Papua New Guinea, is exposed to the discharge of a hot, mineralized hydrothermal fluid. The hydrothermal f luids have a pH of similar to 6 and are slightly reducing and rich in As. S even individual vents discharge an estimated 1500 g of As per day into an a rea of approximately 50 x 100 m that has an average depth of 6 m. Despite t he amount of As released into the bay, corals, clams, and fish do not show a response to the elevated values. We analyzed hydrothermal precipitates fo r their chemical and mineralogical composition in order to determine As sin ks. Two mechanisms efficiently control and buffer the As concentration: (1) dilution by seawater and (2) incorporation in and adsorption on Fe(III) ox yhydroxides that precipitate when the hydrothermal fluids mix with ambient seawater. Fe(III) oxyhydroxides contain up to 76 000 ppm As, by an order of magnitude the highest As values found in a natural marine environment. Fol lowing adsorption, As is successfully retained in the Fe(III) oxyhydroxide deposits because oxidizing conditions prevail and high As activity allows f or the formation of discrete As minerals, such as claudetite, arsenic oxide , and scorodite.