HUNTING FOR NOVEL BIOSIGNATURES: NEW INSIGHTS FROM WEATHERING STUDIES OF ULTRAMAFIC ROCKS BY A TROPICAL MICROBIAL MAT

The quest for novel and quantitative evidence of microbial influence on geologic processes continues to be challenging in modern settings and perhaps daunting in the rock record. Here we present a study of a diverse microbial mat community colonizing a peridotite outcrop on Sulawesi Island, Indonesia. Our objectives were to quantify the effect of the microbial community on the development of biosignatures within the mat (trace element fractionation) and at the mineral-mat interface (leached layers and secondary mineral formation) during weathering reactions. In the field we measured several dissolved redox-active species (Fe (II), Mn(II), oxygen, peroxide, and organo-Fe/Mn complexes) in situ using Au/Hg amalgam voltammetric microelectrodes. Dissolved Fe II/III, phosphate, nitrite and nitrate were measured on site by spectrophotometry. Electrical conductivity, pH, & Eh were determined using combination electrodes. Samples of the peridotite, waters and the mat were collected and prepared for ICP-MS analysis. Mat samples were collected for molecular cloning studies, and were examined microscopically using ESEM. Surfaces and bulks samples of weathered and relatively pristine rocks were compared using XRD, XPS and dynamic SIMS. Voltammetric profiles revealed distinct redox zonation within the mat. Millimolar concentrations of peroxide were detected within the first 10 mm of the atmosphere/mat interface. Up to several micromolar of dissolved Fe(II) and Mn(II) were present in a microaerophilic zone near the bedrock interface. Voltammetric peaks characteristic of both organo-Mn(III/IV) and organo-Fe(III) complexes were detected within this zone implying a photochemical and ligand promoted weathering process within the mat. Trace element (REE, Ni and Mn) distribution within the mat show distinct fractionation from the host mineral phases. The high peroxide concentrations in the mat are likely produced via a photochemical reaction involving the reduction of Fe(III) by UV radiation. We propose that a combination of peroxide and microbially-produced organic ligands enhance the weathering of serpentenized peridotites under microaerophilic conditions, and may be an important mechanism for the development of laminated sedimentary structures and distinct trace element biosignatures.