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.

Description: The deep subseafloor biosphere represents one of the Earth’s largest, but also least understood ecosystems with diverse species and mostly uncharacterized microbial communities. International Ocean Discovery Program (IODP) Expedition 370 (Temperature Limit of the Deep Biosphere off Muroto) established Site C0023 down to 1180 mbsf in the Nankai Trough off Japan to explore the upper temperature limit of microbial life in the deep sedimentary biosphere [1]. Site C0023 is characterized by a complex lithostratigraphic and depositional history with strongly changing sedimentation rates. Volcanic ash layers are ubiquitous in all lithological units. However, the highest abundance of ash layers could be observed between 400 and 700 mbsf. Previous studies have shown that volcanic ashes represent hotspots for microbial life [2] and are commonly characterized by high Fe(III) and Mn(IV) contents [3]. Onboard measurements show a release of dissolved Fe in the depth interval associated with the highest abundance of ash layers [1]. Therefore, we hypothesized that the release is related to microbial Fe reduction fueled by the mineralogy of the volcanic ash. In order to identify the source and reaction pathway of the liberated Fe, we applied sequential extractions of differently reactive Fe oxide pools on mud rock and ash layer samples as well as stable iron isotope (δ56Fe) analyses on pore-water and solid-phase samples. Microbial Fe reduction leads to Fe isotope fractionation with an enrichment of light isotopes in the released Fe and a respective enrichment of heavy isotopes in the residual ferric substrate. Therefore, the δ56Fe signals of different reactive Fe pools and the pore water are used to identify the pools actually involved in microbial respiration processes. Our results show that the total Fe content in mud rock of Site C0023 is relatively constant at ~4.2 wt%. Reactive Fe oxides represent 25% of the total Fe. The bulk Fe content in the ash layers varies between 1.4 and 6.8 wt%. Surprisingly, most ash samples contain less total Fe (3.35 wt% on average) compared to the surrounding mud rock. Similarly, the contents of the reactive Fe oxides are significantly lower. This indicates that either (1) ash layers do not represent the energy substrate for microbial Fe reduction, or (2) reactive Fe in ash samples has already been used up by microbes. The bulk Fe content in recent volcanic material from an active volcano on the Japanese island arc is ~4.4 wt% [4]. The higher Fe content in fresh volcanic material compared to ash samples at Site C0023 might suggest that reactive Fe in ash layers is already reduced. Alternatively, the dissolved Fe release might be related to microbial reduction of structural Fe(III) in smectite promoting the smectite-to-illite transition, which has previously been proposed for Site C0023 [5]. References: [1] Heuer, V.B. et al., 2017. In Proc. IODP Volume 370. [2] Inagaki, F. et al., 2003. AEM 69: 7224-7235. [3] Torres, M.E. et al., 2015. Geobiology 13: 562-580. [4] Vogel, A. et al., 2017. J. Geophys. Res. Atmos. 122: 9485-9514. [5] Kim, J. et al., 2019. Geology 47: 535-539.