Fe and S isotope variations in cyanobacterial mats: modern analogues of ancient stromatolites

Iron and sulfur isotope variations in modern microbial mats from the hypersaline ponds of the Guerrero Negro salt works, Baja California Sur, Mexico have been investigated. Cyanobacteria are the primary producers of this mat ecosystem. The oxygen concentrations in the surface 2 mm of the mats alternate between oxygen-rich and oxygen-free over a diel cycle. Previous work indicates that heterotrophic metabolism is dominated by sulfate reducing bacteria whereas direct metabolic processing of Fe, such as dissimilatory Fe(III) reduction or photoautotrophic Fe(II) oxidation, is negligible. Extremely high rates of sulfate reduction are observed near the mat surface, which coincides with the highest δ ³⁴S values of sedimentary sulfides (total inorganic sulfide, TRIS). The overall δ ³⁴S values of TRIS are -19 to -46 ‰ decreased relative to seawater sulfate. The absence of significant S isotope variations in sedimentary sulfide below the surface 1 cm indicates that sulfate does not become limiting within the microbial mats. Iron isotope compositions of pyrite, expressed as δ ⁵⁶Fe and normalized to average igneous rocks, varies between -1.9 and -0.2 ‰ . These values are in the range of previously reported Fe isotope compositions of sedimentary pyrites from Archean shales, Banded Iron Formations and modern continental margin sediments (-2.5 to -0.5 ‰ ). Similar to S isotopes, the highest values in δ ⁵⁶Fe are observed in the surface 1 cm of the mat. The dominant processes that control S isotope compositions are microbial processing of S, including bacterial sulfate reduction and S disproportionation; inorganic fractionations, e.g. during conversion of H₂S, are negligible. In contrast, Fe isotope variations are the cumulative expression of multiple microbial and inorganic reactions, including reductive dissolution, inter-mineral fractionations, ligand-promoted dissolution and redox reactions. In this study we examine how these fundamentally different controls on S and Fe isotope fractionations can be related to the isotope variations in a modern microbial mat. The aim is to provide a framework for the interpretation of S and Fe isotope compositions in ancient sedimentary environments.