The Association of Carbonate Minerals to Acidic Environments: a Possible Biosignature for Mars

Introduction: Carbonate production is thermodynamically inhibited under low-pH conditions. More specifically, in most terrestrial environments, neither abiotic or biotic pathways favor the production of carbonates below a pH of ~4.5. However, carbonate minerals have been recognized associated to extremely acidic environments of Río Tinto, Spain. The mineralogical similarity of these deposits at Rio Tinto and those in the Meridiani region of Mars suggest that carbonates may be more widespread on Mars than originally thought. Carbonates in the subsurface of Rio Tinto: (1) Carbonates occur in subsurface deposits associated with hydrothermal sulfides (primarily pyrite) in two boreholes (4 and 8) that feed the acidic headwaters of Peña de Hierro [1]. At these locations, highly-fractured Carboniferous volcanosedimentary deposits (sedimentary shale and hydrothermal silica) host platy-like rounded carbonate structures covered in some cases by spheroidal objects, as well as rhomboidal cements. EDS microanalyses of these materials show that they are composed of phosphorus-bearing Fe-carbonate. (2) Carbonates of ankerite-like composition (Fe-Ca-Mg carbonates) have been found associated with fractured shales (borehole 1) that cover the hydrothermal complex sourcing of the microbially dominated acidic waters of Río Tinto [1]. In some cases, the fractures are filled by carbonate-bearing aggregates having fibrous microtextures, but isolated rombohedral carbonate crystals composed of Fe, Mg and Ca (Fig. 1) and infilling microcavities have also been observed. (3) Carbonate structures have been recognized in the 2 Ma old terrace deposits of the Río Tinto fluvial system. They are comprised of rhombohedral crystals that fill the porous regions of biological remains. Microscope observations show that filamentous structures are occluded in the crystals and that these are sub-perpendicular to crystal faces. The occurrence of carbonates in the three different subsurface settings suggests mildly acidic to neutral pH (~ 5-7) and somewhat reducing (Eh < 0) conditions. Water samples obtained in the Peña de Hierro aquifers support the existence of mildly acidic to neutral ferrous solutions that have seasonally high concentrations of carbon dioxide, which favor the precipitation of Fe-bearing carbonates. In addition, the