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: 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: The continuous improvement of analytical procedures using multi-collector technologies in ICP-mass spectrometry has led to an increased demand for isotope standards with improved homogeneity and reduced measurement uncertainty. For magnesium, this has led to a variety of available standards with different quality levels ranging from artefact standards to isotope reference materials certified for absolute isotope ratios. This required an intercalibration of all standards and reference materials, which we present in this interlaboratory comparison study. The materials Cambridge1, DSM3, ERM-AE143, ERM-AE144, ERM-AE145, IRMM-009 and NIST SRM 980 were cross-calibrated with expanded measurement uncertainties (95% confidence level) of less than 0.030‰ for the δ25/24Mg values and less than 0.037‰ for the δ26/24Mg values. Thus, comparability of all magnesium isotope delta (δ) measurements based on these standards and reference materials is established. Further, ERM-AE143 anchors all magnesium δ-scales to absolute isotope ratios and therefore establishes SI traceability, here traceability to the SI base unit mole. This applies especially to the DSM3 scale, which is proposed to be maintained. With ERM-AE144 and ERM-AE145, which are product and educt of a sublimation–condensation process, for the first time a set of isotope reference materials is available with a published value for the apparent triple isotope fractionation exponent θapp, the fractionation relationship ln α(25/24Mg)/ln α(26/24Mg).

Description: The Argentina Continental Margin represents a unique geologic setting where fundamental interactions between bottom currents and sediment deposition as well as their impact on biogeochemical processes and element cycling, in particular iron, can be studied. The aims of this study were to investigate 1) the consequences of different depositional conditions on biogeochemical processes and 2) diagenetic cycling of Fe mineral phases in surface sediments. Furthermore, it was 3) studied how sedimentary stable Fe isotope signatures (δ56Fe) are affected during early diagenesis and finally 4) evaluated, under which conditions δ56Fe might be used as proxy for microbial Fe reduction in methanic sediments. During RV SONNE expedition SO260, carried out in the framework of the DFG-funded Cluster of Excellence “The Ocean in the Earth System”, surface sediments from two depositional environments were sampled each using gravity corer and multi corer. One study site is located on the lower continental slope at 3605 m water depth (Biogeochemistry Site), while the other site is situated in a contourite system on the Northern Ewing Terrace at 1078 m water depth (Contourite Terrace Site). Sequential Fe extractions were performed on the collected sediments to determine four operationally defined reactive Fe phases targeting Fe carbonates, (easily) reducible Fe (oxyhydr)oxides and hardly reducible Fe oxides [1]. Purification of extracts for δ56Fe analysis of the Fe carbonates and easily reducible Fe (oxyhydr)oxide fractions followed [2]. The dataset was combined with pore-water data obtained during the cruise and complemented by concentrations and stable carbon isotope signatures of dissolved methane determined post-cruise. The extent of the redox zonation and depth of the sulfate-methane-transition (SMT) differ between the two sites. It is suggested that sedimentation rates at the Biogeochemistry Site are low and that steady state conditions prevail, leading to a strong diagenetic overprint of sedimentary Fe phases. In contrast the Contourite Terrace Site is characterized by high sedimentation rates and a lack of pronounced diagenetic overprint [3]. Reactive Fe phases are subject to reductive dissolution at the SMT. Nevertheless, significant amounts of reactive Fe phases are preserved below the SMT as evidenced by the presence of dissolved Fe in the methanic sediments, and are available for deep Fe reduction possibly through Fe-mediated anaerobic oxidation of methane [4]. In this study, δ56Fe signatures of reactive Fe phases in methanic sediments were determined for the first time. These data suggest significant microbial fractionation of Fe isotopes during deep Fe reduction at the Biogeochemistry Site, whereas at the Contourite Terrace Site the δ56Fe signatures do not indicate remarkable microbial Fe isotope fractionation. It is concluded that the applicability of δ56Fe signatures as tracer for microbial Fe reduction might be sensitive to the depositional regime, and thus may be limited in high sedimentation areas. References: [1]Poulton SW and Canfield DE, 2005. Chemical Geology 214: 209-221. [2]Henkel, S. et al., 2016. Chemical Geology 421: 93-102. [3]Riedinger, N. et al., 2005. Geochimica et Cosmochimica Acta 69: 4117-4126. [4]Riedinger, N. et al., 2014. Geobiology 12: 172-181.

Description: A number of studies have shown that iron reduction in marine sediments is not confined to sulfate- or sulfide-containing depths but may also affect deep methanic intervals. In particular dynamic depositional settings often show the release of dissolved iron below the sulphate-methane transition (SMT). The specific process behind this deep iron release is not well understood. It has been suggested that anaerobic oxidation of methane (AOM) mediated by Fe oxide reduction plays an important role. So there might be a close, so far unaccounted link between the Fe and C cycles in deep marine sediments. Here we present a compilation of inorganic geochemical data including δ56Fe values of pore water and reactive Fe fractions for sediments of the Helgoland mud area (North Sea) for which a coupling between deep iron reduction and AOM has been proposed [1]. The sediments show a shallow SMT and increasing dissolved Fe concentrations of up to 400 µM further below. High sedimentation rates led to a fast burial and preservation of reactive Fe (oxyhydr)oxides, enabling deep iron reduction as we observe it today. Isotopic fractionation of Fe has been demonstrated for DIR in culture experiments and in shallow marine sediments. Such studies build upon the principle that microbes preferentially utilize light Fe isotopes (54Fe) causing a fractionation between solid ferric and dissolved ferrous iron. For alternative biotic Fe reduction pathways in methanic environments, there are practically no data. We hypothesized that any microbially mediated iron reduction process would result in a similar preferential release of 54Fe and, thus, shift pore water δ56Fe towards negative values. Furthermore we hypothesized that the microbial utilization of a specific Fe (oxyhydr)oxide pool would result in a relative enrichment of 56Fe in the residual ferric substrate. Close to the sediment-water interface pore water δ56Fe in the mud area is generally negative and shows a downward trend towards positive values as it can be expected for in-situ dissimilatory iron reduction (DIR) [2]. The Fe isotope signal close to the sulfidic interval is ~1‰ heavier than above and below as Fe sulfide precipitation preferentially removes 54Fe from pore water. A pronounced downward shift of pore-water δ56Fe to more negative values within the methanic zone is a clear indication for microbial Fe reduction coupled to organic matter degradation. However, this shift does not coincide with the main interval of Fe release for which potential for Fe-AOM had been demonstrated [1]. In this deeper interval, the released Fe has an isotopic composition that matches that of the ferric substrates. We conclude that either 1) Fe-AOM plays a subordinate role for Fe release at depth or 2) does not go along with significant Fe isotope fractionation, which might be explained by different ways of electron transfer between microbe and the iron oxide compared to DIR. [1] Aromokeye, D. et al., 2019. Frontiers in Microbiology, doi: 10.3389/fmicb.2019.03041. [2] Henkel, S. et al., 2016. Chemical Geology 421: 93-102.