QUANTIFYING THE ROLES OF CHEMICAL AND MICROBIAL WEATHERING IN ACID-SULFATE HYDROTHERMAL SYSTEMS

Abstract

Although the roles of microbial and chemical processes are relatively well-studied in neutral-chloride hydrothermal systems, very few studies have addressed these processes in acid-sulfate hydrothermal systems. This study aims to survey the roles of chemical and microbial weathering in acid-sulfate hydrothermal systems in order to provide greater understanding of the geochemical processes operating in low pH (2-4) and relatively high temperature (43-90oC) enviroments. These data provide insight into both modern and ancient life in extreme environments, as well as which processes are abiotically controlled. Field microcosm experiments indicate initial dissolution in Las Pailas hydrothermal system, located on the southwest flank of Rincón de la Vieja, Costa Rica, is likely driven by microorganisms. These microorganisms increase the short-term volumetric weathering rate of anorthoclase containing Fe-oxide and apatite mineral inclusions by an order of magnitude relative to abiotic controls. However, weathering of other silicates by microorganisms appeared to be relatively similar to abiotic controls. These results indicate that microbially induced silicate dissolution facilitates phosphate solubulization in acid-sulfate hydrothermal systems. These results are similar to previous research conducted in low temperature (T), circum-neutral pH systems, despite the higher reaction rates due to increased T and acid attack in this extreme environment. The net result of increased weathering is the mobilization of trace metals into solution. Hydrothermal fluid fluxes contain abundant trace metals, however, these metals preferentially partition into the sediments at Las Pailas. In other hydrothermal systems and acid mine drainage environments, trace metals preferentially bind to iron oxides. Microorganisms in these systems typically facilitate the formation of Fe-oxides to which trace metals bind. In circum-neutral hydrothermal systems, associated with low-sulfidation epithermal ore deposits, microorganisms form shallow epithermal ore deposits. Sequential extraction of Las Pailas sediments indicates microorganisms also concentrated trace metals, particularly copper, gold and silver in the Las Pailas sediments, despite the acidic pH. However, microorganisms in this acid-sulfate system appear to sequester trace metals by binding them to microbial cell surfaces, exopolymeric substances, and iron oxides produced and entrained within biofilm. These data suggest microorganisms may create shallow/surficial indicators of epithermal Au-Ag ore formation at depth. Moreover, the association of microbial biomarkers and influences on the isotopic record suggest microorganisms may play a role in ore formation that occurs below the limit for life (~121oC) and that microorganisms may have been involved in ore formation throughout geologic time. Not all processes in acid-sulfate hydrothermal systems, however, are microbially controlled. Weathering not only concerns itself with the dissolution of primary mineral phases, but also the formation of secondary mineral phases, particularly nontronite and kaolinite formation. Pailas de Agua I, one of the hot springs in the Las Pailas hydrothermal field, contains abundant clay minerals. To assess the influence of microorganisms on secondary mineral formation in Las Pailas, a model hydrothermal solution, based on the solution geochemisty of Pailas de Agua I, was created. Experiments using this solution were performed at high (80oC) and low (25oC) temperatures, with and without the addition of fluoride and microbial surrogates to determine the influence of temperature, Al-complexation by fluoride and microbial processes on clay formation. Results indicate that high temperature experiments form nontronite and kaolinite regardless of experimental conditions. However, in low temperature solutions, fluoride plays a key role in Al-complexation and aids in authigenic nontronite precipitation. Microbial surrogates play little role in clay formation in acidic pH systems, in contrast to, clay mineral formation in many circum-neutral pH systems, which is microbially influenced. Acid-sulfate hydrothermal systems have been proposed as an analog for Mars because of mineralogical similarities between the two systems. These data indicate that while clay minerals on Mars may be good indicators of water in Mars' history, they do not specifically indicate an environment of formation, nor should they be used as an indicator of past life on Mars. Moreover, these data suggest that the kickstarting of the "clay mineral factory" on early Earth may not be the result of microbial processes. These results indicate that many microbial processes, including microbially induced mineral dissolution and trace metal immobilization, may be ubiquitous in nature regardless of whether exceptional preservation of microbial structures occurs. However, the mechanisms that underpin these processes may differ between environments. Most importantly, despite the common association between microorganisms and clay minerals in modern environments, authigenic clay formation may occur in the absence of microbial surrogates, if/when Al-complexing ligands are present in solution. Both abiotic and biological processes influence weathering in acid-sulfate hydrothermal systems and these processes may likely be differentiable in the rock record through examination of associations between biomarker associations with sediments, even in the absence of exceptional preservation.