MICROBIAL VS. ABIOTIC OXIDATIVE WEATHERING OF BASALTIC GLASSES: NEW APPROACHES FOR PROBING THE INITIAL STAGES OF DISSOLUTION

Volcanic glasses recovered from the ocean crust commonly show corrosion features considered to be microbial in origin, yet the mechanisms of glass alteration and the phylogeny of microorganisms involved have only recently come under investigation. One class of microorganisms that may be involved in the dissolution of basaltic glasses are Fe-oxidizing chemolithoautotrophs, since basalts contain significant concentrations of Fe(II) to support microbial growth. In turn, such microbial activity may have a significant effect on the rates of exchange between basaltic glasses and seawater. To probe these interactions, we have initiated a series of glass/biofilm experiments using a consortia of Fe-oxidizers we recently enriched from pillow basalts exposed at the submarine hydrothermal system at Loihi Seamount.

One novel approach we have developed is the use of Synchrotron-based x-ray reflectivity and spectroscopy measurements to (1) measure the density and thickness of the “leached” layers that form on the glass surfaces (at a 10-angstrom scale resolution) (2) obtain distribution profiles for a variety of elements (e.g. Fe, Mn, Ca, Ti) in the surface layers vs. the bulk glasses and (3) monitor changes in the redox state of Fe as a function of depth into the substrate during oxidative weathering of the glasses. To date, we have incubated a series of highly polished basaltic glass surfaces in artificial seawater media at 10°C, where one subset of glasses is inoculated with the Fe-oxidizer consortia (which leads to rapid biofilm formation) and the remainder serve as “abiotic controls”. Measurable reaction (10s to 100s angstroms) occurs within the first few weeks, accompanied by Fe-oxidation in the surface layers. Modeling of x-ray reflectivity profiles from the “biotically-altered” surfaces shows that the reacted layer thickness is up to 3-times greater than the abiotic surfaces over the same time period. In addition, fitting of Fe XANES spectra shows that a Fe(III)-front propagates at a much faster rate into the biologically-colonized glasses. Quantitative comparisons between experiments conducted on weeks to several month timescales will be presented.