Sulfur, Sulfates, and Fossils In Acid Saline Deposits In Western Australia and Chile: Clues for Understanding Mars

Acid saline lakes in Western Australia and Chile, analogs for martian sedimentary deposits, are sulfur-rich environments. Sulfur-bearing minerals are found in basement host rocks, in surface waters and shallow groundwaters, and in chemical sediments, suggesting an active sulfur cycle in these systems. Lake and groundwaters (pH 1.5 – 4.0, TDS < 300‰) contain varying and high amounts of dissolved sulfur, sulfate, and likely other sulfur species. Chemical sediments and early diagenetic sulfate minerals include gypsum, alunite, jarosite, basaluminite, native sulfur, and at least three different types of unidentified hydrated sulfate minerals. Analyses of S isotopes, total S, and SO4 concentration in surface and ground waters and chemical precipitates suggests a complex range of factors influencing localized sulfate geochemistry. Gypsum is most abundant and is formed by several different processes such as: (1) direct precipitation from lake water, (2) effluorescent growth by groundwaters wicking to the surface and evaporating, (3) displacive growth from groundwater, (4) physical reworking by winds, and (5) co-precipitation with entrapment of acidophilic, halophilic microorganisms. Some gypsum is encased in halite as solid inclusions, as well as within fluid inclusions in halite. The combination of high salinity and low pH allows for co-precipitation of iron oxides intimately mixed with saline minerals such as gypsum. Our petrographic observations show that gypsum from these two acid saline settings traps microfossils particularly well. Fossils trapped in the different types of gypsum include bacteria/Archea, fungi, algae, pollen, wood and leaf fragments, diatoms, and insects. Because terrestrial acid saline gypsum seems analogous to sulfates on Mars, and because terrestrial acid saline gypsum has high fossilization potential, martian sulfates should be targeted in the search for any past life on Mars.