Description: IODP Expedition 370 (Temperature Limit of the Deep Biosphere off Muroto) established Site C0023 down to 1180 mbsf in the Nankai Trough off Shikoku Island, Japan, to explore the upper temperature limit of microbial life in deep subseafloor sediments. Part of the scientific program is to investigate the availability of nutrients and energy substrates and to identify unique geochemical and microbial signatures that differentiate the biotic and abiotic realms and/or their transitions (Heuer et al., 2017). Iron (Fe) reduction is considered one of the most ancient forms of microbial respiration (Vargas et al., 1998). In addition, Fe reducers can grow under high temperature and pressure conditions (Kashefi and Lovley, 2003), suggesting that microbes that use Fe oxides as energy substrates are potential candidates to survive close to the temperature limit of the deep biosphere. In this study, we aim at assessing the role of Fe oxides for microbial respiration and the related diagenetic alterations in deep sediments of Site C0023 by applying sequential extractions of Fe oxide and sulfide minerals. Volcanic ash layers, which are ubiquitous in sediments of Site C0023, are of particular interest as they have been identified earlier as hotspots for microbial life (e.g., Inagaki et al., 2003). Torres et al. (2015) further showed that ash layers at a different site in the Nankai Trough are typically rich in Fe and Mn oxides. Their results support the findings of Treude et al. (2014) who postulate a coupling of microbial processes to mineralogy. In addition, on-board measurements show a release of dissolved Fe into the pore water in the depth interval associated with volcanic ash layers (Heuer et al., 2017), suggesting that the observed liberation of dissolved Fe is related to an alteration of Fe phases in these ash layers. Our results show that the total Fe content in sediments of Site C0023 is relatively constant at ~4.2 wt%. The reactive Fe oxide content represents 25% of the total Fe. Based on sequential extractions, the fraction associated with amorphous Fe oxide such as ferrihydrite and lepidocrocite is the dominant Fe fraction with ~0.7 wt%. Mineralogical analyses are currently conducted in order to determine specific Fe mineral phases within this fraction. The total Fe contents in the ash layer samples strongly vary between 1.4 and 6.8 wt%. However, most samples generally contain less total Fe than the surrounding sediments. Similarly, the contents of the reactive Fe oxides are significantly lower. Thus, reactive Fe oxides in ash layers at Site C0023 do not seem to represent the energy substrate for microbial Fe reduction. As one of the next steps, stable Fe isotope (δ56Fe) analyses will be performed on (1) pore-water samples, the (2) different Fe oxide phases and (3) sediment residues remaining after sequential extractions in order to trace the source and reaction pathway for the observed release of dissolved Fe into the pore water. Diagenetic Fe cycling, in particular the reductive dissolution of Fe oxides driven by the reaction with hydrogen sulfide, may lead to the transformation of reactive Fe oxides to Fe sulfides such as pyrite (e.g., Berner 1970). Fe monosulfide contents are below detection limit in sediments of Site C0023. Pyrite, in contrast, occurs over the whole core interval with strongly varying contents. Three significant peaks with contents up to 0.5 wt% could be observed at 552, 707 and 1033 mbsf. The pyrite profile generally mimics the total sulfur profile, which suggests that most of bulk sulfur is present as pyrite. Fe bound in pyrite (Fepyrite), however, only represents less than 5% of the total Fe pool, except for the interval with elevated pyrite contents where Fepyrite accounts for ~10% of bulk Fe. This indicates that sulfidation does not affect the whole Fe oxide pool in sediments of Site C0023. The reductive dissolution of primary ferrimagnetic Fe oxides and the formation of secondary paramagnetic pyrite is generally known to modify rock magnetic properties such as magnetic susceptibility (e.g., Berner, 1970). Thus, our geochemical results are presented in combination with post-cruise generated magnetic susceptibility data. By combining the geochemical methods, including sequential Fe oxide and sulfide extractions and subsequent δ56Fe analyses, with rock magnetic measurements, we intend to decipher the role of Fe mineral phases in maintaining deep subsurface life at Site C0023. Acknowledgements - This research used samples and data provided by the International Ocean Discovery Program (IODP). We would like to thank all personnel involved in the operations aboard the DV Chikyu during Expedition 370 and the support team at the Kochi Core Center. We further would like to thank the German Research Foundation (DFG) for funding this project (project number: 388260220) in the framework of the priority program 527 (Bereich Infrastruktur – International Ocean Discovery Program). References: Berner, R.A., 1970. Sedimentary pyrite formation. AJS 268: 1-23. Heuer, V.B., Inagaki, F., Morono, Y., Kubo, Y., Maeda, L., and the Expedition 370 Scientists, 2017. Expedition 370 Preliminary Report: Temperature Limit of the Deep Biosphere off Muroto. International Ocean Discovery Program. Inagaki, F., Suzuki, M., Takai, K., Oida, H., Sakamoto, T., Aoki, K., Nealson K.H., Horikoshi, K., 2003. Microbial communities associated with geological horizons in coastal subseafloor sediments from the Sea of Okhotsk. AEM 69: 7224-7235. Kashefi, K., Lovley, D.R., 2003. Extending the upper temperature limit of life. Science 301: 934. Torres, M.E., Cox, T., Hong, W.-L., McManus, J., Sample, J.C., Destrigneville, C., Gan, H.M., Gan, H.Y., Moreau J.W., 2015. Crustal fluid and ash alteration impacts on the biosphere of Shikoku Basin sediments, Nankai Trough, Japan. Geobiology 13: 562-580. Treude, T., Krause, S., Maltby, S., Dale, A.W., Coffin, R., Hamdan, L.J., 2014. Sulfate reduction and methane oxidation activity below the sulfate-methane transition zone in Alaskan Beaufort Sea continental margin sediments: Implications for deep sulfur cycling. GCA 144: 217-237. Vargas, M., Kashefi, K., Blunt-Harris, E.L., Lovley, D.E., 1998. Microbiological evidence for Fe(III) reduction on early Earth. Nature 395: 65-67.

Description: The study investigates the in-situ strength of sediments across a plate boundary décollement using drilling parameters recorded when a 1180-m-deep borehole was established during International Ocean Discovery Program (IODP)Expedition 370, Temperature-Limit of the Deep Biosphere off Muroto (T-Limit). Information of the in-situ strength of the shallow portion in/around a plate boundary fault zone is critical for understanding the development of accretionary prisms and of the décollement itself. Studies using seismic reflection surveys and scientific ocean drillings have recently revealed the existence of high pore pressure zones around frontal accretionary prisms, which may reduce the effective strength of the sediments. A direct measurement of in-situ strength by experiments, however, has not been executed due to the difficulty in estimating in-situ stress conditions. In this study, we derived a depth profile for the in-situ strength of a frontal accretionary prism across a décollement from drilling parameters using the recently established equivalent strength (EST) method. At site C0023, the toe of the accretionary prism area off Cape Muroto, Japan, the EST gradually increases with depth but undergoes a sudden change at ~ 800 mbsf, corresponding to the top of the subducting sediment. At this depth, directly below the décollement zone, the EST decreases from ~ 10 to 2 MPa, with a change in the baseline. This mechanically weak zone in the subducting sediments extends over 250 m (~ 800–1050 mbsf), corresponding to the zone where the fluid influx was discovered, and high-fluid pressure was suggested by previous seismic imaging observations. Although the origin of the fluids or absolute values of the strength remain unclear, our investigations support previous studies suggesting that elevated pore pressure beneath the décollement weakens the subducting sediments.

Description: (Bio-)geochemical processes in subseafloor sediments are closely coupled to global element cycles. To gain an improved understanding of changes in (bio-)geochemical conditions on geological timescales, we investigate sediment cores from a 1180 m deep hole in the Nankai Trough offshore Japan (Site C0023). The sediment cores were taken during International Ocean Discovery Program (IODP) Expedition 370 (Temperature Limit of the Deep Biosphere off Muroto), which aimed at exploring the prerequisites and limits of deep microbial life [1]. Over the past 15 Ma, Site C0023 has moved ~750 km relative to its present-day geographic position from the central Shikoku Basin to the Nankai Trough due to motion of the Philippine Sea plate [2]. During its tectonic migration, Site C0023 has experienced significant changes in depositional and thermal conditions as well as resulting (bio-)geochemical processes. By combining a large set of complementary pore-water, solid-phase and rock magnetic data with sedimentation rates and sediment ages, our aim is to (1) reconstruct the evolution of (bio-)geochemical processes, especially the cycling of iron, along the tectonic migration, and to (2) investigate if iron(III) minerals are still available to serve as energy substrate for microbial respiration in the deep sediments. Our results demonstrate that a transition from organic carbon-starved conditions with predominantly aerobic respiration processes to an elevated carbon burial environment with increased sedimentation occurred at ~2.5 Ma. Higher rates of organic carbon burial as a consequence of an increased nutrient supply and primary productivity likely stimulated the onset of organoclastic iron and sulfate reduction, biogenic methanogenesis and anaerobic oxidation of methane. A significant temperature increase by 50°C across the sediment column associated with trench-style sedimentation since 0.5 Ma potentially increased the bioavailability of organic matter and enhanced biogenic methane production. The resulting shifts in reaction fronts led to a diagenetic transformation of iron (oxyhydr)oxides into pyrite in the lower organic carbon-starved sediments several millions of years after burial. We also show that high amounts of iron(III), which were preserved in the deeply buried sediments due to carbon-starved conditions are still available as energy substrate for microbially mediated processes at Site C0023. Our study emphasizes that depositional and thermal changes ultimately driven by the tectonically induced migration have the potential to strongly influence and control geochemical conditions and (bio-)geochemical processes within the whole sediment column. Such studies are needed to gain a fundamental understanding of the coupling between depositional history, (bio-)geochemical processes and the resulting diagenetic overprint on geological timescales, thereby linking the sedimentary iron, sulfur and carbon cycles. References: [1] Heuer, V.B. et al., 2020. Science 370: 1230-1234. [2] Mahony, S.H. et al., 2011. Bulletin 123: 2201-2223.

Description: Microorganisms in marine subsurface sediments substantially contribute to global biomass.Sediments warmer than 40°C account for roughly half the marine sediment volume, but theprocesses mediated by microbial populations in these hard-to-access environments are poorlyunderstood. We investigated microbial life in up to 1.2-kilometer-deep and up to 120°C hotsediments in the Nankai Trough subduction zone. Above 45°C, concentrations of vegetativecells drop two orders of magnitude and endospores become more than 6000 times more abundantthan vegetative cells. Methane is biologically produced and oxidized until sediments reach 80°to 85°C. In 100° to 120°C sediments, isotopic evidence and increased cell concentrationsdemonstrate the activity of acetate-degrading hyperthermophiles. Above 45°C, populated zonesalternate with zones up to 192 meters thick where microbes were undetectable

Description: Biogeochemical processes in subseafloor sediments are closely coupled to global element cycles. To improve the understanding of changes in biogeochemical conditions on geological timescales, we investigate sediment cores from a 1180 m deep hole in the Nankai Trough offshore Japan (Site C0023) drilled during International Ocean Discovery Program Expedition 370. During its tectonic migration from the Shikoku Basin to the Nankai Trough over the past 15 Ma, Site C0023 has experienced significant changes in depositional, thermal, and geochemical conditions. By combining pore-water, solid-phase, and rock magnetic data, we demonstrate that a transition from organic carbon-starved conditions with predominantly aerobic respiration to an elevated carbon burial environment with increased sedimentation occurred at ∼2.5 Ma. Higher rates of organic carbon burial in consequence of increased nutrient supply and productivity likely stimulated the onset of anaerobic electron-accepting processes during organic carbon degradation. A significant temperature increase by ∼50°C across the sediment column associated with trench-style sedimentation since ∼0.5 Ma could increase the bioavailability of organic matter and enhance biogenic methanogenesis. The resulting shifts in reaction fronts led to diagenetic transformation of iron (oxyhydr)oxides into pyrite in the organic carbon-starved sediments several millions of years after burial. We also show that high amounts of reducible iron(III) which can serve as electron acceptor for microbial iron(III) reduction are preserved and still available as phyllosilicate-bound Fe. This is the first study that shows the evolution of long-term variations of (bio-)geochemical processes along tectonic migration of ocean floor, thereby altering the primary sediment composition long after deposition.

Description: IODP Expedition 370 established Site C0023 down to 1180 mbsf in the Nankai Trough, Japan, to explore the upper temperature limit of microbial life in deep subseafloor sediments. The area is characterized by strongly changing sedimentation rates. The diagenetic iron (Fe) cycling is an important process within the deep biosphere. However, microbial and abiotic alterations of Fe phases in deep subseafloor sediments are poorly understood. Fe (oxyhydr)oxides are important carriers of sedimentary magnetic signals. Diagenetic cycling, especially the reductive dissolution of Fe (oxyhydr)oxides driven by microbial degradation of organic matter and/or by reactions with hydrogen sulfide, may lead to transformations of primary ferrimagnetic Fe (oxyhydr)oxides to secondary Fe sulfides, and thus, to modification of rock magnetic properties. In this study, we aim at assessing the alteration of the primary sedimentary record at Site C0023, including authigenic formation of secondary minerals, pyritization as well as effects on rock magnetic properties. To investigate the Fe mineralogy, sequential extractions of Fe (oxyhydr)oxides and sulfides are combined with rock magnetic analyses and SEM-EDS observations. The reactive Fe pool includes Fe carbonates and Fe (oxyhydr)oxides. Amorphous Fe oxides are the dominant reactive Fe fraction at Site C0023. Fe sulfides, mainly pyrite, are present in all investigated intervals. However, pyritization only affects 5 to 15% of the reactive Fe pool. Rock magnetic properties also show the presence of metastable Fe sulfides in the upper interval between 200 and 450 mbsf. Their preservation might be caused by limited hydrogen sulfide availability, an increase in sedimentation rates, which prevents a complete pyritization by decreasing the time the interval was located in the sulfidic zone, or by recent authigenesis. Combining our geochemical and rock magnetic data improves the understanding of iron cycling in subseafloor sediments and the role of iron minerals in maintaining life in the deep biosphere.

Description: International Ocean Discovery Program (IODP) Expedition 370 aimed to explore the limits of life in the deep subseafloor biosphere at a location where temperature increases with depth at an intermediate rate and exceeds the known temperature maximum of microbial life (~120°C) at the sediment/basement interface ~1.2 km below the seafloor. Drilling Site C0023 is located in the vicinity of Ocean Drilling Program (ODP) Sites 808 and 1174 at the protothrust zone in the Nankai Trough off Cape Muroto at a water depth of 4776 m. ODP Leg 190 in 2000, 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 higher 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 limit of manual cell count for low-biomass samples was not high 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 with 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 by simultaneous shipboard and shore-based investigations. Our primary objectives during Expedition 370 were to study the relationship between the deep subseafloor biosphere and temperature. We aimed to comprehensively study the factors that control biomass, activity, and diversity of microbial communities in a subseafloor environment where temperatures increase from ~2°C at the seafloor to ~120°C at the sediment/basement interface and thus likely encompasses the biotic–abiotic transition zone. We also aimed to determine geochemical, geophysical, and hydrogeological characteristics in sediment and the underlying basaltic basement and elucidate if 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 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 super-clean environment. Following the coring operations, a temperature observatory with 13 thermistor sensors was installed in the borehole to 863 mbsf.

Description: X-ray computed tomography (XCT) can be used to identify lithologies and deformation structures within geological core, with the potential for the identification processes to be applied automatically. However, because of drilling disturbance and other artifacts, the use of large XCT-datasets in automated processes requires methods of quality control that can be applied systematically. We propose a new systematic method for quality control of XCT data that applies numerical measures to CT slices, and from this obtains data reflective of core quality. Because the measures are numerical they can be applied quickly and consistently between different sections and cores. This quality control processing protocol produces downhole radiodensity profiles from mean CT-values that can be used for geological interpretation. The application of this quality control protocols was applied to XCT data from International Ocean Discovery Program (IODP) Expedition 370 Site C0023 located at the toe of the Nankai accretionary complex. The evaluation of core quality based on this protocol was found to be a good fit to standard-evaluations based on the visual description of core, and could be used to select samples free from drilling disturbance or contamination. The quality-controlled downhole mean CT-value profile has features that can be used to identify lithologies within a formation, the presence and type of deformation structures and to distinguish formations.

Description: Molybdenum (Mo) is a trace element sensitive to oceanic redox conditions. The fidelity of sedimentary Mo as a paleoredox proxy of coeval seawater depends on the extent of Mo remobilization during postdepositional processes. Here we present the Mo content and isotope profiles for deep sediments from the Nankai Trough, Japan. The Mo signature suggests that these sediments have experienced extensive early diagenesis and hydrothermal alteration at depth. Iron (Fe)‐manganese (Mn) (oxyhydr)oxide alteration combined with Mo thiolation leads to a more than twenty‐fold enrichment of Mo within the sulfate reduction zone. Hydrothermal fluids and Mo adsorption onto Fe‐Mn (oxyhydr)oxides cause extremely negative Mo‐isotope values at the underthrust zone. These postdepositional Mo signals might be misinterpreted as expanded anoxia in the water column. Our findings highlight the importance of constraining postdepositional effects on Mo‐based proxies during paleoredox reconstruction.