Description: The Argentina Continental Margin represents a unique geologic setting to study interactions between bottom currents and sediment deposition as well as their impact on (bio)geochemical processes, particularly the cycling of iron (Fe). Our aim was to determine (1) how different depositional conditions control post-depositional (bio)geochemical processes and (2) how stable Fe isotopes (δ56Fe) of pore water and solid phases are affected accordingly. Furthermore, we (3) evaluated the applicability of δ56Fe of solid Fe pools as a proxy to trace past diagenetic alteration of Fe, which might be decoupled from current redox conditions. Sediments from two different depositional environments were sampled during RV SONNE expedition SO260: a site dominated by contouritic deposition on a terrace (Contourite Site) and the lower continental slope (Slope Site) dominated by hemipelagic sedimentation. Sequentially extracted sedimentary Fe [1] and δ56Fe analyses of extracts and pore water [2,3] were combined with sedimentological, radioisotope, geochemical and magnetic data. Our study presents the first sedimentary δ56Fe dataset at the Argentina Continental Margin. The depositional conditions differed between and within both sites as evidenced by variable grain sizes, organic carbon contents and sedimentation rates. At the Contourite Site, non-steady state pore-water conditions and diagenetic overprint occurs in the post-oxic zone and the sulfate-methane transition (SMT). In contrast, pore-water profiles at the Slope Site suggest that currently steady-state conditions prevail, leading to a strong diagenetic overprint of Fe oxides at the SMT. Pore-water δ56Fe values at the Slope Site are mostly negative, which is typical for on-going microbial Fe reduction. At the Contourite Site the pore-water δ56Fe values are mostly positive and range between -0.35‰ to 1.82‰. Positive δ56Fe values are related to high sulfate reduction rates that dominate over Fe reduction in the post-oxic zone. The HS- liberated during organoclastic sulfate reduction or sulfate-mediated anaerobic oxidation of methane (AOM) reacts with Fe2+ to form Fe sulfides. Hereby, light Fe isotopes are preferentially removed from the dissolved pool. The isotopically light Fe sulfides drive the acetate-leached Fe pool towards negative values. Isotopic trends were absent in other extracted Fe pools, partly due to unintended dissolution of silicate Fe masking the composition of targeted Fe oxides. Significant amounts of reactive Fe phases are preserved below the SMT and are possibly available for reduction processes, such as Fe-mediated AOM [4]. Fe2+ in the methanic zone is isotopically light at both sites, which is indicative for a microbial Fe reduction process. Our results demonstrate that depositional conditions exert a significant control on geochemical conditions and dominant (bio)geochemical processes in the sediments of both contrasting sites. We conclude that the applicability of sedimentary δ56Fe signatures as a proxy to trace diagenetic Fe overprint is limited to distinct Fe pools. The development into a useful tool depends on the refining of extraction methods or other means to analyse δ56Fe in specific sedimentary Fe phases. References: [1]Poulton and Canfield, 2005. Chemical Geology 214: 209-221. [2]Henkel et al., 2016. Chemical Geology 421: 93-102. [3]Homoky et al., 2013. Nature Communications 4: 1-10. [4]Riedinger et al., 2014. Geobiology 12: 172-181.

Description: Colonization of newly ice-free areas by marine benthic organisms intensifies burial of macroalgae detritus in Potter Cove coastal surface sediments (Western Antarctic Peninsula). Thus, fresh and labile macroalgal detritus serves as primary organic matter (OM) source for microbial degradation. Here, we investigated the effects on post-depositional microbial iron reduction in Potter Cove using sediment incubations amended with pulverized macroalgal detritus as OM source, acetate as primary product of OM degradation and lepidocrocite as reactive iron oxide to mimic in situ conditions. Humic substances analogue anthraquinone-2,6-disulfonic acid (AQDS) was also added to some treatments to simulate potential for electron shuttling. Microbial iron reduction was promoted by macroalgae and further enhanced by up to 30-folds with AQDS. Notably, while acetate amendment alone did not stimulate iron reduction, adding macroalgae alone did. Acetate, formate, lactate, butyrate and propionate were detected as fermentation products from macroalgae degradation. By combining 16S rRNA gene sequencing and RNA stable isotope probing, we reconstructed the potential microbial food chain from macroalgae degraders to iron reducers. Psychromonas, Marinifilum, Moritella, and Colwellia were detected as potential fermenters of macroalgae and fermentation products such as lactate. Members of class deltaproteobacteria including Sva1033, Desulfuromonas, and Desulfuromusa together with Arcobacter (former phylum Epsilonbacteraeota, now Campylobacterota) acted as dissimilatory iron reducers. Our findings demonstrate that increasing burial of macroalgal detritus in an Antarctic fjord affected by glacier retreat intensifies early diagenetic processes such as iron reduction. Under scenarios of global warming, the active microbial populations identified above will expand their environmental function, facilitate OM remineralisation, and contribute to an increased release of iron and CO2 from sediments. Such indirect consequences of glacial retreat are often overlooked but might, on a regional scale, be relevant for the assessment of future nutrient and carbon fluxes.

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: 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: Antarctic shelf regions are potential carbon and nutrient cycling hotspots where rapid climatic changes are projected to affect seasonal sea ice cover, water column stratification, and thus surface primary production and associated fluxes of organic carbon to the seafloor. Here, we report on surface sediment oxygen profiles and respective fluxes in combination with pore water profiles of dissolved iron (DFe) and phosphate (PO43-) from 7 stations along a 400 mile transect with variable sea ice cover and water column stratification from the East Antarctic Peninsula to the west of South Orkney Islands. Our results show that sea ice concentrations and stratification of the upper water column decreased across the transect. We defined a marginal sea ice index of 5-35% sea ice cover which was positively correlated with the benthic carbon mineralization rate. C-mineralization rates increased gradually between the heavy ice-covered station and the marginal sea ice stations from 1.1 to 7.3 mmol C m-2 d-1, respectively. The rates decreased again to 1.8 mmol C m-2 d-1 at the ice-free station, likely attributed to a deeper water column mixed layer depth, which decreases primary production and thus organic carbon export to the sediment. Iron cycling in the sediment was elevated at the marginal sea ice stations where Fe-reduction led to DFe fluxes in the pore water of up to 0.379 mmol DFe m-2 d-1, while moderate (0.068 mmol DFe m-2 d-1) and negligible fluxes were observed at ice-free and ice-covered stations, respectively. In pore waters, concentrations of DFe and PO43- were significantly correlated with almost identical flux ratios of 0.33 mol PO43- per mol DFe for most of the stations, indicating a strong control of the iron cycling on the phosphate release to the water column. The high benthic DFe and PO43- fluxes highlight the importance of sediments underlying the marginal ice zone as source for limiting nutrients to the shelf waters.

Description: Diagenesis can have a major impact on sedimentary mineralogy. Primary magnetic mineral assemblages can be modified significantly by dissolution or by formation of new magnetic minerals during early or late diagenesis. At International Ocean Discovery Program Site C0023, which was drilled in the protothrust zone of the Nankai Trough during Expedition 370, offshore of Shikoku Island, Japan, non-steady state conditions have produced a complex sequence of magnetic overprints. Detailed rock magnetic measurements, which characterize magnetic mineral assemblages in terms of abundance, grain size, and composition, were conducted to assess magnetic mineral alteration and diagenetic overprinting. Four magnetic zones (MZs) are identified down-core from ∼200 to 1100 meters below sea floor based on rock magnetic variations. MZ 1 is a high magnetic intensity zone that contains ferrimagnetic greigite, which formed at shallow depths and is preserved because of rapid sedimentation. MZs 2 and 4 are low magnetic intensity zones with fewer magnetic minerals, mainly coarse-grained (titano-)magnetite and hematite. This magnetic mineral assemblage is a remnant of a more complex assemblage that was altered diagenetically a few million years after deposition when the site entered the Nankai Trough. MZ 3 is a high magnetic intensity zone between MZs 2 and 4. It contains authigenic single-domain magnetic particles that probably formed from fluids that circulated through faults in the accretionary prism. Varying sediment supply and organic matter input through time, burial temperature, and tectonic fluid circulation are the primary drivers of magnetic mineral assemblage variations.