Understanding the Mechanisms of Sulfate Formation in Acidic Hydrothermal Systems Using Field Analogs and Laboratory Experiments

SO₄²⁻ [sulfate] has been found in elevated concentrations on the surface of Mars and linked to oxidation of volcanic sulfur. However, the oxidation mechanisms are unclear due to the lack of molecular O₂ [oxygen] in the Martian atmosphere. To address this uncertainty, I investigated the mechanisms of H₂S [hydrogen sulfide] oxidation to SO₄²⁻ in O₂-depleted acidic hydrothermal systems of Iceland and the United States (Valles Caldera, Lassen, and Yellowstone) as geochemical analogs. Water and sediment samples were collected for chemical and δ¹⁸O [oxygen isotopes] analyses. The measured DO [dissolved oxygen] concentrations were low (0.01 to 1.03 mg/L [milligrams per liter]) and could not account for high concentrations of SO₄²⁻ (100 to 24,461 mg/L). However, high concentrations of leachable Fe [iron] were present in the hydrothermal water and sediment (average of 158 mg/L and 71,302 mg/kg [milligrams per kilogram], respectively), implying that Fe-driven oxidation of H₂S via Fe³⁺ [ferric Fe] reduction to Fe²⁺ [ferrous Fe] is likely involved in SO₄²⁻ formation. Oxygen isotope results show large variation in the δ¹⁸O of SO₄²⁻ (-8.8 to +5.5 ‰ [per mil]) similar to δ¹⁸O of water (-15.5 to +6.3 ‰) in the field samples which might be a result of evaporation and oxygen isotope exchange between SO₄²⁻, SO₃²⁻ [sulfite] and water altering the δ¹⁸O of SO₄²⁻. Therefore, laboratory experiments were conducted to measure the effect of isotopic exchange on the δ¹⁸O of SO₄²⁻ in Mars analog sites. Results suggest that although this oxygen isotope exchange may occur under hydrothermal conditions, it cannot be distinguished from previously established fractionations for both O₂- and Fe-driven oxidation of sulfide. Additionally, experiments were conducted to better understand oxidation mechanism of H₂S gas by Fe and O₂. Results show that O₂ appears to be a slow oxidant, resulting in small amounts of SO₄²⁻. During Fe-driven oxidation even less SO₄²⁻ was formed because of rapid precipitation of insoluble sulfur intermediates. Overall, SO₄²⁻ formation in surface hydrothermal environments is complex, requiring constant supply of oxidants (e.g., Fe, O₂) and changes of hydrological conditions. Nevertheless, this process provides an invaluable alternative explanation for the formation of sulfates in O₂-depleted Martian environments.