BIOSIGNATURES IN METABASALTIC GLASS: HAS THE SUBSEAFLOOR BEEN A HABITAT FOR MICROBIAL LIFE ACROSS EARTH HISTORY?

Abundant microbiological and geochemical data from the in-situ oceanic crust supports the existence of a microbial subseafloor biosphere [e.g. 1]. Candidate textural evidence includes mineral encrusted cells [e.g. 2], also tubular and granular cavities argued to record endolithic microbes in metavolcanic glass [e.g. 3]. The question of whether the subseafloor has provided a habitat for life in deep time has relied largely upon the analysis of microtextures in altered volcanic glass from the in-situ oceanic crust, ophiolites and greenstone belts [3]. However, metamorphic processes in the oceanic crust due to transitions in tectonic environment can modify and/or destroy biosignatures. Furthermore, distinguishing abiotic microtextures from those related to possible bioalteration of metavolcanic glass has proven to be a major challenge. For example, new in-situ U-Pb ages, morphological analysis, metamorphic and nano-SIMS mapping has strongly questioned the syngenicity and biogenicity of filamentous titanite in Archean pillow lavas as a reliable biosignature of earliest subseafloor life [4, 5, 6]. Thus, we caution that full characterization of abiotic scenarios, especially metamorphic and hydrothermal processes is required in variably overprinted Precambrian greenstone belt pillow lavas, before claims for early subseafloor life can be made.

Metamorphic facies transitions in basaltic glass have the potential to modify candidate biosignatures and to introduce preservational biases that may mask geobiological information. We thus present a multi-pronged re-evaluation of candidate biosignatures in metavolcanic glass of varying age and geodynamic setting by developing a hierarchical scheme for assessing the probability of a biogenic origin. Characterizing and distinguishing the products of abiotic from possible biotic alteration in the oceanic crust through time will help us to better understand critical geobiological transitions in the subseafloor through Earth history.

[1] Lever et al. (2013) Science 339: 1305-1308.

[2] McLoughlin et al. (2011). Chem. Geol 289: 154-162.

[3] Staudigel et al. (2008) Earth Sci. Rev. 89: 156-176.

[4] McLoughlin et al. (2012) Geology 40: 1031-1034.

[5] Grosch & McLoughlin (2014) PNAS 111: 8380-8385.

[6] Grosch et al. (2014) Astrobiology 14: 216-228.