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Here we provide the first high-resolution geochemical evidence for microbial metabolism to be preserved in impact-generated materials. This study is unique as not only do we merge complimentary analytical techniques such as high-resolution spectromicroscopy to assess the biogenicity of tubules in impact glasses, but we compare these results to those from co-occurring abiotic quench crystallites as an intrinsic negative control. Scanning transmission X-ray microscopy (STXM) near edge X-ray absorption fine structure spectroscopy (NEXAFS) at the Fe L 3 - and C K-edges revealed iron speciation patterns and organic C associated with tubular features in the impact glass. The high spatial resolution of STXM combined with NEXAFS allowed organic carbon to be localized to the tubule features. The fine energy resolution of NEXAFS allowed for unique populations of organic carbon to be spectrally differentiated between the tubule features and the matrix. The distinct and systematic variation in iron redox states observed is consistent with microbially mediated dissimilatory iron reduction. The Ries tubules comprise the first trace fossil preserved in a substrate unique to the impact process, thus illustrating the potential for microbial metabolism to be preserved in impact materials.

Recent observations of carbonate minerals in ancient Martian rocks have been interpreted as evidence for the former presence of circumneutral solutions optimal for carbonate precipitation. Sampling from surface and subsurface regions of the low-pH system of Río Tinto has shown, unexpectedly, that carbonates can form under diverse macroscopic physicochemical conditions ranging from very low to neutral pH (1.5-7.0). A multi-technique approach demonstrates that carbonate minerals are closely associated with microbial activity. Carbonates occur in the form of micron-size carbonate precipitates under bacterial biofilms, mineralization of subsurface colonies, and possible biogenic microstructures including globules, platelets and dumbbell morphologies. We propose that carbonate precipitation in the low-pH environment of Río Tinto is a process enabled by microbially-mediated neutralization driven by the reduction of ferric iron coupled to the oxidation of biomolecules in microbially-maintained circumneutral oases, where the local pH (at the scale of cells or cell colonies) can be much different than in the macroscopic environment. Acidic conditions were likely predominant in vast regions of Mars over the last four billion years of planetary evolution. Ancient Martian microbial life inhabiting low-pH environments could have precipitated carbonates similar to those observed at Río Tinto. Preservation of carbonates at Río Tinto over geologically significant timescales suggests that similarly-formed carbonate minerals could also be preserved on Mars. Such carbonates could soon be observed by the Mars Science Laboratory, and by future missions to the red planet.

Exceptionally well-preserved pillow lavas and inter-pillow hyaloclastites from the Barberton Greenstone Belt in South Africa contain textural, geochemical, and isotopic biomarkers indicative of microbially mediated alteration of basaltic glass in the Archean. The textures are micrometer-scale tubular structures interpreted to have originally formed during microbial etching of glass along fractures. Textures of similar size, morphology, and distribution have been attributed to microbial activity and are commonly observed in the glassy margins of pillow lavas from in situ oceanic crust and young ophiolites. The tubes from the Barberton Greenstone Belt were preserved by precipitation of fine-grained titanite during greenschist facies metamorphism associated with seafloor hydrothermal alteration. The presence of organic carbon along the margins of the tubes and low d 13 C values of bulk-rock carbonate in formerly glassy samples support a biogenic origin for the tubes. Overprinting relationships of secondary minerals observed in thin section indicate the tubular structures are pre-metamorphic. Overlapping metamorphic and igneous crystallization ages thus imply the microbes colonized these rocks 3.4–3.5 Ga. Although, the search for traces of early life on Earth has recently intensified, research has largely been confined to sedimentary rocks. Subaqueous volcanic rocks represent a new geological setting in the search for early life that may preserve a largely unexplored Archean biomass. D 2005 Elsevier B.V. All rights reserved.