Biologically induced clay formation in subsurface granitic environments

Cryogenic scanning electron microscopy and X-ray diffraction analysis were used to investigate evidence of in situ biogenic clay formation occurring in experiments set up to simulate microbial–geochemical interactions in deep subsurface low-nutrient granitic environments. Packed-columns and stirred batch reactors contained crushed Äspö granodiorite, artificial groundwater mimicking that from Äspö Underground Research Laboratory (URL), and combinations of three types of subsurface chemolithotrophic bacteria, two of which were indigenous to the Äspö rocks. After 5 days the columns had become impermeable and analysis of the contents revealed that, despite the salinity and nutrient-depleted nature of the media, all of the experiments containing bacteria had produced copious biofilm including unusual biofilamentous networks across porespaces. Furthermore, these biofilaments were encrusted with finer-grained material and surrounded by neoformed fine-grained aluminosilicate deposits. In contrast, non-biological controls showed no evidence of aluminosilicate formation, implying that this process is biologically mediated. Subsequent sets of non-biological experiments designed to assess effects of bulk-scale pH changes and associated mineral dissolution (mimicking one of the possible effects of bacteria), revealed extensive mineral etching along with saturation of soluble species thought to be important in clay formation. However, there was little or no evidence of clay formation except at very high pH values (> pH8) and the etching on mineral surfaces had not appeared previously in the biological reactors. This suggests that the bacteria either actively concentrate relevant chemical species for mineral formation in localised microenvironments rather than affecting the bulk chemical system, or they accelerate clay formation by acting as templates/nucleation points. These findings imply that bacteria indigenous to granitic environments could dramatically affect the hydrological regime, which could be particularly important if repository construction generates new nutrient supplies into the ground water system.