RUNAWAY SULFUR-CYCLING IN EPHEMERAL ACID SALINE ENVIRONMENTS OF WESTERN AUSTRALIA

Acid brine systems in Western Australia are an example of a sulfur-rich end-member environment with intense sulfur-cycling. In these extreme environments, sulfur compounds and sulfur minerals exist in great abundance and diversity. Lake waters and groundwaters of the Yilgarn Craton of Western Australia are Na-Cl-SO₄-Mg-rich brines with pH as low as 1.7 and sulfuric acid as the dominant acid. Measured concentrations of total S range from 500 to 19,800 ppm, and SO₄ ranges from 692 to 35,169 ppm. The difference between S and SO₄ suggest that other S species are present in these waters. Rainwaters, which have been suggested as an input of seaspray aerosol sulfate to the lake systems yielded 7 to 8 ppm SO₄. Sulfur isotope analysis shows that δ³⁴S for lake waters is 17.0 to 19.1 ‰, for groundwaters is 17.0 to 18.6 ‰, for gypsum is 20.2 to 20.4 ‰, and for alunite is 18.9 ‰. These values are consistent with a combination of S sources and sinks. Sulfur cycling processes include sulfide oxidation, evapoconcentration, mineral precipitation, dissolution, and bacterial reduction and oxidation. Field, petrographic, and molecular analyses of microorganisms in these acid saline environments suggest that some are S-oxidizing and S-reducing microorganisms. Sulfur-bearing minerals in the area include both sulfides and sulfates. Sulfides, including pyrite, have been observed in sediments, veins, and disseminated in surface exposures and shallow cores of Archean igneous and metamorphic host rocks. Sedimentary sulfate minerals include gypsum, bassanite, anhydrite, and rozenite that precipitate directly from lake waters, and gypsum, anhydrite, jarosite, alunite, natroalunite, basaluminite, hydrobasaluminite, and rozenite that grow diagenetically from very shallow groundwaters. These sedimentary minerals are subject to physical reworking and to chemical dissolution. Sulfur cycling amongst the lithosphere, hydrosphere, biosphere, and atmosphere in acid-saline environments is dynamic. Driving forces are rock weathering, water geochemistry, and climate. Flooding - evapoconcentration - desiccation cycles accelerate short-term local and regional sulfur cycling. The combination of low pH, highly saline waters and dynamic changes in environment make sulfur arguably the most important element in these environments.