Description: The occurrence of microbially induced smectite-to-illite (S-I) reaction has challenged both the notions of solely inorganic chemical control for this reaction and the conventional concept of a semiquantitative illite geothermometer for the reconstruction of the thermal and tectonic histories of sedimentary basins. Here, we present evidence for a naturally occurring microbially induced S-I transition, via biotic reduction of phyllosilicate structural Fe(III), in mudstones buried at the Nankai Trough, offshore Japan (International Ocean Discovery Program Site C0023). Biotic S-I reaction is a consequence of a bacterial survival and growth strategy at diagenetic temperatures up to 80 °C within the Nankai Trough mudstones. These results have considerable implications for petroleum exploration, modification of fault behavior, and the understanding of microbial communities in the deep biosphere.

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: Iron reduction is one of the most ancient forms of microbial respiration. This and the observation that iron reducers can grow under high temperature and pressure conditions suggests that they may play an important role in the deep biosphere. We will use stable Fe isotopes to disentangle microbial and abiotic processes involved in deep Fe cycling at IODP Site C0023 in the Nankai Trough. This will help to reach the goal of Expedition 370: ”T-Limit of the Deep Biosphere off Muroto” – the assessment of how microbial communities change with increasing sediment depth and temperature, by which factors changes are controlled, and where microbial life ceases. Dissolved iron was found at Site C0023 only within the methanic zone from 400 to 600 mbsf. The total drilling depth was 1180 mbsf. Is the Fe2+ release coupled to microbial activity? If yes, is it confined to the 200 m thick interval due to presence of reactive Fe minerals or because the microbes cannot cope with the temperatures prevailing in deeper sediments? Microbial iron reduction is known to cause pronounced enrichments of 54Fe in pore water, which should also be reflected by authigenic Fe minerals. The residual Fe pool, in contrast, becomes progressively enriched in 56Fe. Kinetic reactions of iron with sulfide enrich 56Fe in pore water, which allows a discrimination between microbial reduction and abiotic iron - sulfur interactions based on δ56Fe. As a result of different origins of incorporated Fe and different reactivities towards microbial reduction and sulfidation, Fe minerals in sediments possess different δ56Fe signatures and may show geochemical indications for microbial life. By analyzing δ56Fe of pore water and sequentially leached reactive and refractive Fe phases from Site C0023 sediments we will gain insight into the processes driving Fe2+ liberation at depth and hopefully assess links between the microbial activity and mineralogy (the presence of electron acceptors) as well as temperature.

Description: Sediments of sub-Antarctic islands have been proposed to be important contributors to natural iron fertilization in the Southern Ocean [1, 2]. This potential contribution depends on biogeochemical processes within the sediment that may result in an iron benthic flux, most likely related to the degradation of organic matter (OM). Yet, the OM degradation pathways vary strongly among different sedimentary settings. We elucidate the role of environmental factors on the prevailing biogeochemical pathways and reaction rates at three contrasting sites of South Georgia, using comprehensive solid-phase and pore-water analyses, as well as transportreaction modelling. Samples were obtained along a transect from a glacial fjord towards the shelf during cruise ANTXXIX/ 4 of RV POLARSTERN in 2013. Oxygen penetration depth at all sites is 〈1 cm. Sediments recovered within the fjord are dominated by dissimilatory iron reduction (DIR) and show very high dissolved Fe2+ concentrations of up to 760 μM, while sulfide was not detected. In addition, Fe reduction below the sulfate/methane transition was observed. High input of reactive iron phases, possibly enhanced by bioturbation and bubble ebullition, appear to favour DIR as the dominant metabolic process for OM degradation in the basin like fjord. Shelf sediments outside the fjord are sulfidic throughout, with H2S formed primarily by anaerobic oxidation of methane. The conversion of Fe oxides into Fe sulfides significantly alters the initial sediment composition along the shelf, and impact the availability of iron to the water column. OM is of marine origin at all three sites (C:N~7), indicating that Fe oxide availability and reactivity rather than the carbon source determine whether iron or sulfate reduction dominantes. [1] Moore & Braucher (2008) Biogeosciences 5, 631-656. [2] Borrione et al., (2014) Biogeosciences 11, 1981–2001.

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: Specific trace metal contents that can record the environment at the time of deposition are commonly applied as tracers (proxies) to reconstruct ancient oceanic conditions. However, microbial processes can alter the primary trace metal signal of the sediments during sediment burial. To investigate trace metal cycling during early diagenesis, geochemical analyses were performed via bag-incubations on samples collected from two giant box corers retrieved during RV SONNE Expedition SO260, funded by the MARUM-Center for Marine Environmental Sciences at the University of Bremen. The cores were retrieved off-shore Argentina, one from the head of the Mar del Plata Canyon and the other from a coral mound. Collected sediments are dominantly silt to fine grained sand but include dropstones and coral fragments as well. Our data show strong changes in the pore-water trace metal concentrations in the samples from the Mar del Plata Canyon. For example, molybdenum (Mo) increases by more than 5000 nM within 8 months. In samples from the coral mound box core, pore-water Mo increases by more than 1000 nM in the first 4 months before decreasing again likely due to the onset of sulfate reduction and, consequently, the formation of hydrogen sulfide leading to the (co-)precipitation of Mo. Our data indicate that the reductive dissolution of iron and manganese oxides leads to the release of associated trace metals at different time points for each site. The observed changes are likely related to sediment composition and physical properties. Sediments sampled at the coral mound site include coral fragments, increasing overall porosity and permeability providing more space for fluid circulation and to host microbial communities. Therefore, our data suggest that trace metal cycling is closely related to physical properties including pore space, permeability, and grain size that affect how much area is available for microbial communities.

Description: Rare earth elements (REE), such as the lanthanides, yttrium and scandium together with neodymium isotopes (εNd) are useful tracers in geochemistry to characterise sediment provenances. Furthermore, they can help to comprehend other elemental fluxes from land to sea. However, input mechanisms of REE into seawater are still being investigated to better understand REE cycling. Particularly, shelves in glacial areas are subject to environmental transformations due to modern climate change. Retreating glaciers expose relatively reactive sediments which are ideal to study aquatic geochemical processes governing REE distribution. We have collected filtered (0.45 µm) seawater from the Kongsfjord (Svalbard) and meltwater samples from the glaciers draining into the fjord for REE and εNd. Samples were analysed for REE on a ThermoFisher® Element2 after an offline pre-concentration using a seaFAST®. The REE were quantified with a known amount of thulium (Tm) as an internal standard prior to pre-concentration. With a neglectable fractionation amongst REE on the seaFAST column, this method allows an efficient and accurate quantification of such elements. Neodymium isotopes will be analysed at a later stage to better understand the provenance of the meltwater distributaries. The distributions of REE in the meltwater show clear enrichment of MREE and low HREE/LREE, in particular close to one land terminating glacier front on the Brogger-peninsula, typical for freshwater. Patterns from the fjord show a seawater distribution with high HREE/LREE and low Ce/Ce* in the deep outer fjord. Lower salinity surface waters are enriched in REE and show lower HREE/LREE than deep waters. Surface samples in the outer fjord show highest REE concentrations, suggesting that at this location REE inputs from the large tidewater glacier Kronebreen at the head of Kongsfjorden are subordinated to the inputs by smaller glaciers draining meltwater over proglacial sediments from the Brogger-peninsula.