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Abstract

International Ocean Discovery Program (IODP) Expedition 370 explored the limits of the biosphere in the deep subseafloor where temperature exceeds the known temperature maximum of microbial life (~120°C) at the sediment/basement interface ~1.2 km below the seafloor. Site C0023 is located in the protothrust zone in the Nankai Trough off Cape Muroto at a water depth of 4776 m, in the vicinity of Ocean Drilling Program (ODP) Sites 808 and 1174. In 2000, ODP Leg 190 revealed the presence of microbial cells at Site 1174 to a depth of ~600 meters below seafloor (mbsf ), which corresponds to an estimated temperature of ~70°C, and reliably identified a single zone of elevated cell concentrations just above the décollement at around 800 mbsf, where temperature presumably reached 90°C; no cell count data was reported for other sediment layers in the 70°-120°C range because the detection limit of manual cell counting for low-biomass samples was not low enough. With the establishment of Site C0023, we aimed to detect and investigate the presence or absence of life and biological processes at the biotic-abiotic transition utilizing unprecedented analytical sensitivity and precision. Expedition 370 was the first expedition dedicated to subseafloor microbiology that achieved time-critical processing and analyses of deep biosphere samples, conducting simultaneous shipboard and shore-based investigations.Our primary objective during Expedition 370 was to study the relationship between the deep subseafloor biosphere and temperature. We comprehensively studied the controls on biomass, activity, and diversity of microbial communities in a subseafloor environment where temperatures increase from ~2°C at the seafloor tõ 120°C and thus likely encompasses the biotic-abiotic transition zone. This included investigating whether the supply of fluids containing thermogenic and/or geogenic nutrient and energy substrates may support subseafloor microbial communities in the Nankai accretionary complex. To address these primary scientific objectives and questions, we penetrated 1180 m and recovered 112 cores of sediment and across the sediment/basalt interface. More than 13,000 samples were collected, and selected samples were transferred to the Kochi Core Center by helicopter for simultaneous microbiological sampling and analysis in laboratories with a superclean environment. In addition to microbiological measurements, we determined the geochemical, geophysical, and hydrogeological characteristics of the sediment and the underlying basement and installed a 13-thermistor sensor borehole temperature observatory into the borehole at Site C0023.

Evolution of (Bio‐)Geochemical Processes and Diagenetic Alteration of Sediments Along the Tectonic Migration of Ocean Floor in the Shikoku Basin off Japan

“…The δ 13 C-CH4 values in the methane-rich zone between 200 and 600 mbsf varies between -60 and -65‰ (Figure S2f), which is indicative of a biogenic methane source (Heuer et al, 2020). Furthermore, as evidenced by increasing δ 13 C-CH4 values and concurrently decreasing methane/ethane ratios (C1/C2 < 10 3 ; Figure S2e), thermogenic methane produced by thermal degradation of the organic matter provides an additional source of methane from below (Heuer et al, 2017a;Heuer et al, 2020). These increased methane fluxes, in turn, could cause an up-and downward migration of the shallow and inverse SMT, respectively, and simultaneously slow prograding shifts of the sulfidization fronts (Figure 8c).…”

“…Pisani et al, 2005;Le Pichon et al, 1987;Taira et al, 1992). Site C0023 is located at the seaward-end of this accretionary prism (Heuer et al, 2017a).…”

“…Analysis was carried out using Recoil (University of Ottawa). Based on the prevalent dominance of Fe-rich, clay-sized phyllosilicates (Heuer et al, 2017a), the Lorentzian Site Analysis routine was performed to produce consistent fits and serviceable results that allowed the investigation of Fe(II)/(III) ratios and the comparison with a reference data base (Murad, 1998(Murad, , 2010.…”

“…This 14 m thick greenish layer consists of acidic volcaniclastics with a high abundance of chlorite and smectite. Hyaloclastite deposits with glass and basalts as the dominant clasts characterize the lithologic basement (Heuer et al, 2017a;Taira & Ashi, 1993).…”

“…In order to study long-term variations in biogeochemical and diagenetic processes as well as element fluxes at Site C0023, we combine a unique multi-disciplinary data set that includes shipboard high-resolution pore-water and gas data, organic carbon contents (Heuer et al, 2017a), and sedimentation rates and ages (Hagino & the Expedition 370 Scientists) with our solid-phase data comprising bulk sediment compositions, Fe speciation, and rock magnetic signals. One objective is to evaluate if reactive Fe is still available to serve as a potential energy substrate for microbially mediated processes in the deep and hot subseafloor sediments at Site C0023.…”