Evidence for Microbial Respiration of Structural Fe3+ from Clay Minerals in an Ancient Diagenetic System: Implications for Authigenic Iron and Silica Products in Mudstones

Gaines, Robert R.¹, John S. Vorhies¹ (1) Pomona College, Claremont, CA

Authigenic silica and iron products in mudstones are typically interpreted to be derived from biogenic or dissolved ionic contributions, rather than from solid-phase minerals. Recently, iron-respiring microbes were found capable of extracting structurally coordinated Fe3+ from clay minerals in vitro in the absence of other electron acceptors. This process has not previously been shown to occur in natural settings, but has important geologic and biogeochemical consequences, as it may trigger dissolution of smectite, resulting in liberation of iron and silica early in diagenesis and the neoformation of illite. Mudstones of the Wheeler and Marjum formations yield a suite of mineral products expected to result from this process. Within host mudstones, sub-micron sized authigenic clays occur around micron-sized pore-filling carbonate cements. Finely intergrown quartz, pyrite and calcite (~50µ) occur within void spaces inside carbonate concretions. These mineral aggregates are cross-cut by septarian cracks, which form prior to compaction. δ18O paleothermometry from quartz phases indicates that precipitation occurred at pre-metamorphic temperatures (26-69 ºC), implicating microbial dissolution of clay minerals. This finding provides evidence that this microbial process operates in natural settings, and is significant in the interpretation of mudstones. Microbially induced dissolution of clay minerals may play a significant role in iron and silica cycling and may be responsible for much of the early silica cementation found within shales prior to the emergence of abundant of siliceous plankton. Primary clay mineralogy may determine the amount of bioavailable iron in sediments, and may be important in the exceptional preservation of fossils as pyrite.