SULFUR CYCLING IN YELLOWSTONE NATIONAL PARK THERMAL FEATURES: A COMPLEX AND DYNAMIC CHALLENGE

Yellowstone National Park is the ultimate natural laboratory for the study of sulfur cycling, many hydrothermal features controlled by a complex hydrologic flow network couple with a wide variety of chemical conditions to create an array of different sulfur-dominated systems to interrogate. Variations in pH, temperature, major ion and metal chemistries, gas delivery, mixing of different waters, and interaction with a complex and tectonically active geological setting create a setting where fluid chemistries change significantly in space and time. Sulfur is a key redox active element that potentially supports a number of different microbial populations in these thermal features through a series of energetically favorable reactions. Sulfur changes in oxidation state between hydrogen sulfide (S^-2) and sulfate (S⁺⁶), as a result of these oxidation, reduction and disproportionation reactions in steps involving a number of key intermediates. These intermediates then participate in reactions including polymerization, nucleophilic displacement, oxygenation, and precipitation of elemental sulfur through aggregation and ripening processes to form one of the 180 polymorphs and allotropes.

Using a combination of in situ and ex situ voltammetry, Raman spectroscopy, and chromatography we have developed a detailed picture of the intermediate sulfur species important in a number of thermal springs in Yellowstone. These intermediates point towards a complex set of reactions where interactions between hydrogen sulfide gas sourced from the magma chamber below is constantly reacting with oxygen sourced from the atmosphere and the intermediate sulfur compounds. Interactions between these sulfur compounds and organics derived from both detritus and microbial sources can significantly contribute to sulfur speciation in a given system via both direct reaction and modifications to the surfaces of (nano)particulate elemental sulfur. As if this chemical complexity was insufficient, voltammetric analyses document a many order of magnitude variability in time at a single point in some springs for sulfide. This variability can be characterized at one extreme as ‘waves’ of high concentrations of dissolved sulfide passing through a fluid in second intervals and the other extreme as one of relative homogeneity.