Gallium isotopes in silicified microbial hot spring deposits: a potential geochemical biosignature

Non-traditional trace metals are increasingly utilized to evaluate potential microbial processes in the search for evidence of ancient life. Recent investigations of modern terrestrial hot spring silica deposits (sinter), as analogs for early life on Earth or Mars (e.g. Homeplate, Gusev crater), have highlighted unique gallium enrichments associated with silicified microbial filaments and microbially mediated rock textures, such as stromatolites. We used new analytical methodologies for in situ and bulk analysis of gallium isotopes in sinter to better understand the observed Ga enrichment. In situ analysis, by Cameca 1280 ion probe, provides the necessary spatial resolution to target individual microbial filaments with a 10 μm ion beam, but with a lesser precision of ~±3 ‰, compared to the ±0.06 ‰ precision via MC-ICPMS bulk analysis. In situ results indicate heterogeneity of δ⁷¹Ga (>10 ‰ variation overall) with silicified microbial filaments on average isotopically lighter than adjacent silica.  Multiple processes may influence the Ga isotopic ratio in sinter including preferential microbial selection, changes in fluid chemistry, and silicification processes. Ongoing experiments on Ga-Si spiked microbial growth and abiotic silica precipitation may further elucidate the cause of isotopic variability as we continue to refine this in situ isotopic methodology and its utility in planetary biosignature detection.