The effect of sulfuric acid, abiotic-organic acids, and biotic acids on serpentinite dissolution and trace metal release: Potential biosignatures

Description

Serpentine minerals have been observed or are postulated to exist in multiple locations in the solar system, including impact craters, melange terrains, olivine-rich lithologic units, and subsurface silicate mantle rocks. Serpentine-rich environments are of particular interest in planetary exploration because they host microbial communities on Earth, fueling life via the abiotic reaction of olivine-rich parent rocks with aqueous solutions. On Earth, organic acids produced by biota have been shown to accelerate the dissolution of many minerals including mafic minerals such as olivine. These organic acids can also preferentially release trace elements from the mineral structure, creating potential biosignatures in either reacting solutions or the solid materials. However, organic acids that are formed abiotically have been identified in planetary systems as well as meteorites, complicating the potential usefulness of this tool. In this study, we tested aqueous alteration of serpentinite in three groups of solutions: inorganic acids, organic acids that can be created through abiotic processes (termed “abiotic-organics,” including methanesulfonic, nonanoic, valeric, and α-aminoisobutyric acids), and organic acids that are created through biotic processes (termed “biotic acids,” including acetic, citric, glutamic, formic, fumaric, gluconic, glycolic, lactic, malic, and oxalic acids) over a range of temperatures relevant to conditions on Mars and Europa. A total of 48 batch reactor experiments were carried out at 0°C, 22°C, and 62°C in the presence of 16 different acids at pH 2.6 over 28 days. Additionally, experiments were conducted in sulfuric acid to assess aqueous alteration in sulfate-rich environments. These results show that biotic acids accelerate serpentinite dissolution compared to the control inorganic acid, whereas abiotic acids have no or a very small effect. Interestingly, sulfuric acid also enhances serpentinite dissolution over nitric acid. Twenty elements (Ba, Ca, Ce, Co, Cr, Fe, La, Mg, Mn, Mo, Ni, P, Pb, Pr, Sc, Th, Ti, V, Y, and Zr) are released at higher concentrations only in the presence of biotic acids relative to sulfuric acid. Secondary minerals found in the presence of biotic acids were consistently enhanced in Mn, Ti, and W, and we propose that these preferentially released elements and secondary minerals may be important potential biosignatures. We also show that the release of the major rock-forming elements Mg and Si is correlated with the stability constants for the metal:acid aqueous complex, providing a possible mechanistic interpretation for observed results. These results have important implications for potentially detecting life on other planetary bodies, specifically in acidic and sulfur-rich environments such as Mars and Europa.

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