Interactions between microbes and minerals have the potential to contribute significantly to the global cycles of various elements, and serve as a link between the geosphere and life. In particular, the microbially mediated cycle of iron within marine sediments is closely tied to the carbon cycle. The dissolved iron that serves as a nutrient is thought to be primarily drawn from well-known pools of highly reactive, bioavailable iron and iron complexes. Iron contained within the crystal lattice of clay minerals, the most abundant materials found at the Earth's surface, is not thought to be part of this pool. Here we analyse the mineral composition of Middle-Cambrian-aged mudstones from the western United States. We find intergrown mineral aggregates of quartz, pyrite and calcite. On the basis of mineral phase relationships and temperatures of crystallization derived from stable isotopes of oxygen, we infer that mineral dissolution driven by microbes released iron and silica to the surrounding sediment pore waters, and led to the subsequent precipitation of the observed minerals. The microbial extraction of structurally coordinated Fe3+ from clay minerals after their deposition in marine sediments may liberate a fraction of iron previously considered unavailable, and may be important in iron and silica cycling in marine sediments.
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