Sulfur geochemistry of hydrothermal waters in Yellowstone National Park: I. The origin of thiosulfate in hot spring waters

Thiosulfate (S₂O₃²⁻), polythionate (S_xO₆²⁻), dissolved sulfide (H₂S), and sulfate (SO₄²⁻) concentrations in thirty-nine alkaline and acidic springs in Yellowstone National Park (YNP) were determined. The analyses were conducted on site, using ion chromatography for thiosulfate, polythionate, and sulfate, and using colorimetry for dissolved sulfide. Thiosulfate was detected at concentrations typically less than 2 μmol/L in neutral and alkaline chloride springs with low sulfate concentrations (Cl⁻/SO₄²⁻ > 25). The thiosulfate concentration levels are about one to two orders of magnitude lower than the concentration of dissolved sulfide in these springs. In most acid sulfate and acid sulfate-chloride springs (Cl⁻/SO₄²⁻ < 10), thiosulfate concentrations were also typically lower than 2 μmol/L. However, in some chloride springs enriched with sulfate (Cl⁻/SO₄²⁻ between 10 to 25), thiosulfate was found at concentrations ranging from 9 to 95 μmol/L, higher than the concentrations of dissolved sulfide in these waters. Polythionate was detected only in Cinder Pool, Norris Geyser basin, at concentrations up to 8 μmol/L, with an average S-chain-length from 4.1 to 4.9 sulfur atoms.

The results indicate that no thiosulfate occurs in the deeper parts of the hydrothermal system. Thiosulfate may form, however, from (1) hydrolysis of native sulfur by hydrothermal solutions in the shallower parts (<50 m) of the system, (2) oxidation of dissolved sulfide upon mixing of a deep hydrothermal water with aerated shallow groundwater, and (3) the oxidation of dissolved sulfide by dissolved oxygen upon discharge of the hot spring. Upon discharge of a sulfide-containing hydrothermal water, oxidation proceeds rapidly as atmospheric oxygen enters the water. The transfer of oxygen is particularly effective if the hydrothermal discharge is turbulent and has a large surface area.