Metal Sulfide and Pyrite Nanoparticles form in Hydrothermal Vent Waters (Invited)

Using separate analytical schemes on 200 nm filtered solutions followed by induced precipitation of nanoparticles, we discriminated the sulfide and metal composition in metal sulfide nanoparticles. Both acid volatile sulfide (AVS) and chromium reducible sulfide (CRS) as well as hydrochloric acid (analogous to AVS) and nitric acid (analogous to CRS) metal extractions provide evidence that less than 200 nm pyrite particles form in high temperature hydrothermal chimney fluids. In some vents, nanoparticulate pyrite constitutes up to 10% of the total vent-emitted iron. In Lau Basin, this nanoparticulate pyrite may be highly substituted with other metals, including zinc and copper, which can form their own nanoparticulate metal sulfides. These findings have potential implications for the overall ocean budget of metals and sulfur, as metal sulfide nanoparticles are resistant to the rapid precipitation that occurs with larger particles. Also, metal sulfide nanoparticles of Zn, Ag, and Cd exhibit oxidation half lives with dissolved oxygen ranging from one month to one year at 25oC, and these half lives would be four times slower at the ocean bottom temperature of 2oC. Similar slow reactivity is likely for nanoparticulate pyrite but has not been documented yet. Nanoparticulate pyrite is a mechanism by which Fe can avoid rapid Fe(II) oxidation (on the scale of hours) followed by Fe(III) precipitation and/or complexation as well as the rapid precipitation that affects larger micrometer sized pyrite particles. Slow pyrite oxidation would increase the probability of vent-derived Fe to be transported long distances in the deep ocean.