Mechanical Degradation and Creep Behaviour of Tuff by Indigenous Microbial Weathering

While physical and chemical weathering processes on and within volcanic tuff have been extensively investigated, the role of natural microbial communities in its mechanical degradation remains insufficiently quantified. This study evaluates the mechanical responses of tuff to microbial weathering after 28 days of soaking in a microbial culture medium by combining uniaxial compressive strength (UCS) tests, stepwise creep tests, microstructural observations, and measurements of organic ligand production. In this study, indigenous microorganisms and tuff were collected from a tuff outcrop in southwestern Hokkaido, Japan. The specimens were classified in three categories: initial, immersed for 28 days in a microbial medium and a sterilized control medium, respectively. The UCS of microbially soaked specimens decreased by more than 40% compared with intact and sterilized-medium specimens, indicating that microbial activity substantially enhances strength degradation beyond water–rock interaction alone. Although the macroscopic creep strain evolution showed no clear separation among conditions, specimens exposed to microbial soaking exhibited earlier and higher cumulative acoustic emission activity during creep, suggesting enhanced microcrack activity. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy analyses further indicated carbon signals concentrated along internal crack surfaces, particularly in iron-rich domains, consistent with preferential microbial attachment within fracture networks. In addition, time-series measurements showed an early increase in siderophore production followed by a later-stage increase in oxalate production as dissolved iron declined, implying ligand-regulated iron acquisition that may promote the mobilization and redistribution of iron-bearing phases. Overall, the results suggest that indigenous microbial communities can colonize tuff fractures and accelerate chemical weathering processes that lead to pronounced strength loss and increased microstructural damage activity.