The Signature of Sulfur: Geochemical Characterization of Hydrothermal S-rich Deposits in Terrestrial Mars Analogs

Abstract:

The Spirit rover found localized hydrothermal/fumarolic deposits enriched in Fe-, Ca-, and Mg-sulfate within Gusev crater. However, it did not find conclusive evidence for the presence of reduced S (e.g., sulfides, elemental S), which dominates analogous terrestrial hydrothermal settings. Consequently, the sulfate (SO₄) enrichment and apparent absence of reduced S in Gusev sediments raises questions about the formation and oxidation mechanisms of S in acidic hydrothermal systems. To address these questions, we collected sediment and water samples from highly-acidic hot springs, mud pots, fumaroles, and drainages with elevated H₂S emissions in four analog sites, including Yellowstone, Valles Caldera, Lassen, and Iceland. The method of Sulfur Sequential Extraction (SSE) was used to determine oxidation states and measure quantities and S isotope compositions of sulfides (S^2-, S^-), elemental S (S⁰), and sulfates (S⁶⁺). Results show that S⁰ was highly abundant in most sediment samples (0.3 - 20 wt.% S, but up to ~70 wt.% S), followed by S^- (0.2 - 4.4 wt.% S), with significantly lower S⁶⁺ in the sediment and water column (0.07 - 1.6 wt.% S). In the majority of samples, the δ³⁴S of S⁶⁺ was lower (-3 to +3‰) compared to emitted H₂S (-2 to +6‰), but similar to S^- and S⁰ precipitated in the hydrothermal sediments (-7 to +3‰), suggesting the importance of subsequent step-oxidation of the reduced S to sulfate. Our results indicate that surface hydrothermal systems are capable of producing large quantities of reduced S, and could explain high-S deposits on Mars. However, the reported S contents for Gusev crater are lower in range (0.4 - 5.6 wt.% S) and more oxidized (mainly sulfate) compared to the studied analog sites (0.2 - 24 wt.% S, mainly elemental S and sulfides). The negligible amounts of reduced S in Gusev may be a result of subsequent oxidation to SO₄, and the overall smaller amount of S might reflect removal of SO₄ by an active hydrological cycle during formation or later on over several billion years. Our previous study showed that ferric iron (Fe³⁺) reduction participates in the step-oxidation of hydrothermal H₂S. This is especially compelling given the high concentrations of Fe³⁺ iron and Fe-sulfates detected in Gusev, and thus provides new context for the formation of sulfate in Martian oxygen-depleted surface environments.