The Río Tinto system located in southern Spain is a 100-km long acidic (pH ≈ 2.3) river rich in dissolved iron, sulfate and heavy metals. The red-tinted river, formed as a product of natural acid rock drainage and exacerbated over the years by anthropogenic mining activities, hosts a variety of extremophilic microorganisms that are potential analogues to those found on early Earth and ancient Mars. We compared the potential biosignatures preserved in laminated microbialites versus prevalent sediments deposited along the river. The microbialites are composed of layered structures of Fe(III) minerals with alternating porosities. High resolution imaging and elemental mapping coupled to mineralogical analysis suggest the transformation of K-jarosite [KFe₃(SO₄)₂(OH)₆] to goethite [α-FeOOH], with porosity generated during this process. Meanwhile, riverbed sediments are composed of complex mixtures of schwertmannite (Fe₈O₈(OH)₆(SO₄) · nH₂O), jarosite, goethite and other minor minerals (ferrihydrite, lepidocrocite, hematite). Culture experiments indicate that crystalline jarosite is formed via transformation from poorly crystalline schwertmannite, but that the presence of Acidithiobacillus thiooxidans actually inhibits this transformation. Fe isotopic analyses show no clear variations between layers in the microbialites (δ⁵⁶Fe = -0.85 ± 0.10 ‰), while the riverbed sediments indicate progressively more negative values downstream over a 5-km distance (δ⁵⁶Fe = +0.4 to -0.8‰). The spatial isotopic pattern of the riverbed sediments is consistent with rapid microbial oxidation of Fe²⁺ and inconsistent with the much slower abiotic Fe²⁺ oxidation rate. As a whole, microbialites preserved evidence of biogenicity from its structure but not from its geochemical composition. In contrast, the inconspicuous-looking riverbed sediments provide evidence of biogenicity from their mineralogical and isotopic compositions. This highlights the challenge in detecting life especially when decoupling of morphological and geochemical biosignatures is to be expected in similar systems.
 

Río Tinto,Microbialites,spatial biosignature,Mars,acid rock drainage,schwertmannite,jarosite