BIOTIC AND ABIOTIC MINERALOGICAL SIGNATURES INDUCED BY EUXINIC METHANE-RICH BRINE SPRING DISCHARGE

Throughout vast durations of Earth’s history, sulfidic saline waters containing anoxygenic photosynthetic organisms and cyanobacteria dominated many low-temperature hydrologic systems. In the present day, increased onshore oil and gas production resulted in the need to understand the behavior of sulfidic and methane-rich brines exposed to surface conditions. We investigated mineralogical signatures of brine-microbe-sediment interactions at Zodletone spring, a sulfide and methane rich anoxic spring in western Oklahoma fed by deep Anadarko Basin brines. At Zodletone spring, euxinic brines discharge into an elevated shallow surface pool, feed a ~20 m channel, then enter a freshwater stream. Microbiological and geochemical processes, including anoxygenic photosynthesis, microbially-influenced barite precipitation, sulfate reduction, and sulfur oxidation have been previously documented at the site. We used X-ray diffraction and Raman laser microspectroscopy to examine the mineralogy of sediment cores, surface sediments, and mineral crusts. Elemental sulfur was present exclusively in samples coincident with carotenoid pigments indicative of photosynthetic bacteria. Although most brine-impacted sediments are black, we observed only traces of pyrite. Synthetic hematite reacted with spring waters and was rapidly converted to iron monosulfide. Carbonate precipitation occurs in crusts and spring-impacted sediments, producing ~5 wt.% calcite near the spring and ~15 wt.% calcite downgradient along the streambank. However, near the transition zone from anoxic to oxic sediment, an absence of calcite was associated with an increase in barite, and the presence of jarosite indicates acidic weathering. Dolomite was also found in various locations at the site; we are further investigating the role of spring discharges in dolomitization. Clay fraction minerology includes illite, chlorite, kaolinite, and abundant smectites, with little difference between oxic and euxinic sediments. These mineralogical signatures provide a baseline for further studies of brine-sediment trace metal signatures, mineral chemistry, and microbiology that will help inform the Proterozoic rock record and the complex biogeochemistry of oilfield produced waters in surface impoundments.