ALBERT

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: An International Ocean Discovery Program (IODP) workshop was held at Sydney University, Australia, from 13 to 16 June 2017 and was attended by 97 scientists from 12 countries. The aim of the workshop was to investigate future drilling opportunities in the eastern Indian Ocean, southwestern Pacific Ocean, and the Indian and Pacific sectors of the Southern Ocean. The overlying regional sedimentary strata are underexplored relative to their Northern Hemisphere counterparts, and thus the role of the Southern Hemisphere in past global environmental change is poorly constrained. A total of 23 proposal ideas were discussed, with 12 of these deemed mature enough for active proposal development or awaiting scheduled site survey cruises. Of the remaining 11 proposals, key regions were identified where fundamental hypotheses are testable by drilling, but either site surveys are required or hypotheses need further development. Refinements are anticipated based upon regional IODP drilling in 2017/2018, analysis of recently collected site survey data, and the development of site survey proposals. We hope and expect that this workshop will lead to a new phase of scientific ocean drilling in the Australasian region in the early 2020s.

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: Volcanic ash significantly contributes to marine sediments, especially in regions with active onshore volcanoes. Alteration of volcanic ash releases bicarbonate and cations, which drive precipitation of authigenic carbonate and clay minerals. Furthermore, volcanic ashes are commonly enriched in reactive iron (Fe[III]), suggesting that ash alteration as a source of reactants plays an important role in (bio-)geochemical processes in marine sediments. Volcanic ash layers are ubiquitous in sediments of Site C0023, which was established down to 1180 m below seafloor (mbsf) in the Nankai Trough off Japan during International Ocean Discovery Program Expedition 370. Shipboard measurements show a release of dissolved Fe between 200 and 600 mbsf, coinciding with a high abundance of ash layers [1]. The release of Fe can be related to microbial reduction of structural Fe(III) in smectite promoting the smectite-to-illite transition, as recently proposed [2]. By combining shipboard pore-water data with sequential extractions of reactive Fe pools on ash layers and surrounding mud rock and stable Fe isotope (δ56Fe) analyses, we elucidate the role of ash alteration on (bio-)geochemical cycling at Site C0023. Our results demonstrate that reactive Fe(III) is unexpectedly lower in ash layers compared to the surrounding mud rock (0.6 and 1.2 wt%, respectively). This indicates that (1) Fe(III) originally deposited with tephra has either been used or (2) Fe(III) in tephra is generally lower due to a different chemical composition in the volcanic source material. The δ56Fe signature of hydroxylamine-extracted Fe, which represents easily reducible Fe-oxides and Fe bound in phyllosilicates, is isotopically light (-0.08 to -0.42‰) compared to terrestrial background values (~0.09‰; [3]). This suggests that this pool is diagenetically overprinted by the precipitation of authigenic smectite formed as a result of ash alteration and/or secondary Fe-oxides. Pore-water Fe is extremely negative with δ56Fe 〈-1.5‰, which points to microbial reduction of Fe(III) in authigenic smectite. Our results suggest a coupling between ash alteration, authigenic mineral precipitation, and microbially mediated Fe reduction in sediments of Site C0023. [1] Heuer et al., (2017), In Proc. IODP Volume 370. [2] Kim et al., (2019), Geology 47, 535-539. [3] Beard et al., (2003), Chem. Geol. 195, 87-117.

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.