Examining the role of biotic iron reduction as a life-sustaining process at the potential temperature limit of the deep subseafloor biosphere (IODP Expedition 370) (RESPIRE)

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This project will contribute to the research questions of IODP Expedition 370: T-Limit of the Deep Biosphere off Muroto. Temperatures at the Drill Site C0023 in the Nankai Trough (Japan) increase to ~120°C in 1.2 km depth and thus reach the maximum that can potentially be tolerated by microbes. However, a more reasonable T estimate for sterilisation of nutrient-poor deep sediments is 80-90°C. Aim of Expedition 370 was to assess how microbial communities change with depth, by which factors the changes are controlled and where microbial life ceases. It is part of the scientific program to investigate energy substrates available at depth and to identify unique geochemical and microbial signatures that differentiate the biotic and abiotic realms and/or their transitions. High-resolution and high-precision pore water data were produced enabling an identification of reaction fronts, potential microbial activity and hydrothermal alteration. A large part of the cored interval was methanic and sulfate-free. Microbial activity hence depends on electron acceptors other than sulfate. Recent studies indicate that the classical redox sequence needs to be complemented by Fe and Mn reduction in methanic sediments and that biogeochemical processes in natural systems show a stronger link to mineralogy than to a strict vertical sequence of reactions according to calculated energy yields. Fe(III) reduction is one of the most ancient forms of microbial respiration and iron reducers can grow under high T and pressure conditions, which suggests that Fe(III) reducers are potential candidates to survive close to T limit of the deep biosphere.We identified Fe and Mn reduction zones in methanic sediments of Site C0023. By applying sequential extractions we aim at assessing which Fe and Mn phases are available as electron acceptors and how strongly primary minerals have been diagenetically altered. Of particular interest are ash layers as those have been identified earlier as hotspots for microbial life. Ash layers are ubiqous in C0023 sediments and are typically rich in Fe and Mn.Microbial Fe reduction enriches 54Fe in pore water and, thus, authigenic Fe minerals (e.g. siderite, magnetite), whereas abiotic reactions with sulfide lead to more 56Fe in the dissolved phase. We aim at using stable Fe isotopes of dissolved and reactive solid Fe to discriminate microbial and abiotic drivers of Fe reduction. The d56Fe composition of solid Fe will be measured on Fe-carbonates, ferrihydrite + lepidocrocite, goethite + hematite and magnetite. The extent of sulfidation that has implications for the interpretation of magnetic property data will be determined by extracting acid volatile sulfur and chromium reducible sulfur. It is the aim of this project to assess the role of iron oxides for microbial respiration and the related diagenetic alterations in deep sediments of Site C0023.