Jarosite formation by an AMD sulphide-oxidizing environmental enrichment: Implications for biomarkers on Mars

The formation of jarosite in acid mine drainage (AMD) accompanies microbial oxidation of reduced sulphidic minerals such as pyrite (FeS2) and pyrrhotite (Fe1 − xS) found in mining waste materials. The discovery of jarosite at Meridiani Planum, Mars has been used to interpret past hydrologic conditions as episodically wet, acidic and evaporative based on evidence from jarosite formed abiotically or through microbial iron oxidation. To date, no study has specifically assessed jarosite formation via microbial sulphide oxidation, the most likely pathway in AMD, or the mineral stability of any jarosite formed via this pathway. Here, using both aerobic and anaerobic conditions to control iron- and sulphide oxidation metabolisms, we demonstrate that oxidation of pyrrhotite, a putative Mars mineral, by a sulphur oxidizing environmental AMD enrichment produces crystalline jarosite at a higher pH than previously described. We further show that jarosite produced via microbial sulphide oxidation is stable under the drying, heating and wet-aging conditions shown to impact abiotic- and microbial iron oxidation-derived jarosite. Our results indicate that preservation of microbially produced jarosite via pyrrhotite-associated sulphide oxidation does not require the evaporitic conditions that have been proposed to date. If water is established to have persisted on Mars, then this new evidence signals that the occurrence of stable crystalline jarosite at Meridiani Planum may itself be a clear biomarker indicating that not only water occurred at some time(s) on Mars, but also life.