Sulfur geochemistry of hydrothermal waters in Yellowstone National Park, Wyoming, USA. II. Formation and decomposition of thiosulfate and polythionate in Cinder Pool

Cinder Pool is an acid-sulfate;chloride boiling spring in Norris Geyser Bas in, Yellowstone National Park. The pool is unique in that its surface is pa rtially covered with mm-size, black, hollow sulfur spherules, while a layer of molten sulfur resides at the bottom of the pool (18 m depth). The sulfu r speciation in the pool was determined on four different days over a perio d of two years. Samples were taken to evaluate changes with depth and to ev aluate the importance of the sulfur spherules on sulfur redox chemistry. Al l analyses were conducted on site using a combination of ion chromatography and colorimetric techniques. Dissolved sulfide (H2S), thiosulfate (S2O32-), polythionates (SxO62-), and sulfate were detected. The polythionate concentration was highly variable i n time and space. The highest concentrations were found in surficial sample s taken from among the sulfur spherules. With depth, the polythionate conce ntrations dropped off. The maximum observed polythionate concentration was 8 mu M. Thiosulfate was rather uniformly distributed throughout the pool an d concentrations ranged from 35 to 45 mu M. Total dissolved sulfide concent rations varied with time, concentrations ranged from 16 to 48 mu M Sulfate was relatively constant, with concentrations ranging from 1150 to 1300 mu M . The sulfur speciation of Cinder Pool is unique in that the thiosulfate an d polythionate concentrations are significantly higher than for any other a cid-sulfate spring yet sampled in Yellowstone National Park. Complementary laboratory experiments show that thiosulfate is the intermediate sulfoxyani on formed from sulfur hydrolysis under conditions similar to those found in Cinder Pool and that poiythionates are formed via the oxidation of thiosul fate by dissolved oxygen. This last reaction is catalyzed by pyrite that oc curs as a minor constituent in the sulfur spherules floating on the pool's surface. Polythionate decomposition proceeds via two pathways: (1) a reacti on with H2S, yielding thiosulfate and elemental sulfur; and (2) by dispropo rtionation to sulfate and thiosulfate. This study demonstrates that the presence of a subaqueous molten sulfur poo l and sulfur spherules in Cinder Pool is of importance in controlling the p athways of aqueous sulfur redox reactions. Some of the insights gained at C inder Pool may be relevant to acid crater lakes where sulfur spherules are observed and variations in polythionate concentrations are used to monitor and predict volcanic activity. (C) 2000 Elsevier Science B.V. All rights re served.