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

Thiosulfate (S2O32-), polythionate (SxO62-), dissolved sulfide (H2S), and s ulfate (SO42-) concentrations in thirty-nine alkaline and acidic springs in Yellowstone National Park (YNP) were determined. The analyses were conduct ed on site, using ion chromatography for thiosulfate, polythionate, and sul fate, and using colorimetry for dissolved sulfide. Thiosulfate was detected at concentrations typically less than 2 mu mol/L in neutral and alkaline c hloride springs with low sulfate concentrations (Cl-/SO42- > 25). The thios ulfate 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 con centrations were also typically lower than 2 mu mol/L. However, in some chl oride springs enriched with sulfate (Cl-/SO42- between 10 to 25), thiosulfa te was found at concentrations ranging from 9 to 95 mu mol/L, higher than t he concentrations of dissolved sulfide in these waters. Polythionate was de tected only in Cinder Pool, Norris Geyser basin, at concentrations up to 8 mu mol/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 hydrot hermal water with aerated shallow groundwater, and (3) the oxidation of dis solved sulfide by dissolved oxygen upon discharge of the hot spring. Upon d ischarge of a sulfide-containing hydrothermal water, oxidation proceeds rap idly as atmospheric oxygen enters the water. The transfer of oxygen is part icularly effective if the hydrothermal discharge is turbulent and has a lar ge surface area. Copyright (C) 1998 Elsevier Science Ltd.