Physico-chemical properties of illite in the organic matter (OM)-rich claystone of Nankai Trough contrast with coeval OM-poor claystone: IODP Expedition 348

Naturally occurring microbially induced smectite-to-illite (Sm-I) reaction is of great interest to understand the control for the conventional Sm-I transformation, the source of bioavailable iron (Fe) in sediments, and the evolution of the microbial community in the deep biosphere. Direct comparisons of physico-chemical properties of illite-like phases including collapsed high-charge smectite and transient phase, and mature illite were performed for the organic matter (OM)-rich claystone in contrast with those of coeval OM-poor claystone (2163 m–2217 m below sea floor) buried at Nankai Trough (Integrated Ocean Discovery Program Site C0002). These clay layers are interbedded with 70°–90° dipping angle, such that they have nearly identical burial conditions of temperature, time, and fluid geochemistry except for OM content, one of the significant controls that can fuel heterotrophic microbial Fe (III) reduction. Unlike the abiotic Sm-I reaction, the local variations in the crystal structure, elemental composition, and oxidation states of Fe for the illite and illite-like phases in the OM-rich claystone suggest that illitization is promoted through the reduction of structural Fe (III) in smectite by microbial activity, however the effects of OM cannot be ruled out.