Description: The central Baltic Sea is a marginal brackish basin which comprises anoxic bottom waters and is surrounded by geological source terrains with a wide variety of compositions and ages. This allows the investigation of water mass mixing using radiogenic isotope compositions of Nd and Hf as well as their geochemical cycling across varying redox conditions in the water column. In this study, we present the distribution of Nd and Hf concentrations and their isotopic compositions for 6 depth profiles and 3 surface water sites obtained during a cruise in the central Baltic Sea onboard the RV Oceania as a part of the international GEOTRACES program. The results obtained indicate that Nd isotopes effectively trace the mixing between more radiogenic saline waters from the south and unradiogenic fresh waters from the north, which helps to understand the reliability of Nd isotopes as water mass tracer in the open ocean. In surface waters, Nd shows higher concentrations and less radiogenic isotope compositions at the northern stations, which are progressively diluted and become more radiogenic to the south, consistent with the counterclockwise circulation pattern of central Baltic Sea surface waters. In contrast to the variable Nd concentrations, Hf shows much less variability. At the Gotland Deep station, the Nd concentrations of the euxinic waters are higher by a factor 〉10 than those of the overlying oxygen-depleted waters, whereas Hf only shows small concentration variations. This indicates faster removal of Hf from the water column than Nd. Moreover, the dissolved Hf isotope signatures document great variability but no consistent mixing trends. Our explanation is that Hf has a lower residence time than Nd, and also that the Hf isotope signatures of the sources are highly heterogeneous, which is attributed to their differing magmatic and tectonic histories as well as incongruent post-glacial weathering around the central Baltic Sea.

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: Glaciers and ice sheets export significant amounts of silicon (Si) to downstream ecosystems, impacting local and potentially global biogeochemical cycles. Recent studies have shown Si in Arctic glacial meltwaters to have an isotopically distinct signature when compared to non-glacial rivers. This is likely linked to subglacial weathering processes and mechanochemical reactions. However, there are currently no silicon isotope (d30Si) data available from meltwater streams in Antarctica, limiting the current inferences on global glacial silicon isotopic composition and its drivers. To address this gap, we present dissolved silicon (DSi), d30SiDSi, and major ion data from meltwater streams draining a polythermal glacier in the region of the West Antarctic Peninsula (WAP; King George Island) and a cold-based glacier in East Antarctica [Commonwealth Stream, McMurdo Dry Valleys (MDV)]. These data, alongside other global datasets, improve our understanding of how contrasting glacier thermal regime can impact upon Si cycling and therefore the d30SiDSi composition. We find a similar d30SiDSi composition between the two sites, with the streams on King George Island varying between -0.23 and C1.23h and the Commonwealth stream varying from -0.40 to C1.14h. However, meltwater streams in King George Island have higher DSi concentrations, and the two glacial systems exhibit opposite DSi–d30SiDSi trends. These contrasts likely result from differences in weathering processes, specifically the role of subglacial processes (King George Island) and, supraglacial processes followed by instream weathering in hyporheic zones (Commonwealth Stream). These findings are important when considering likely changes in nutrient fluxes from Antarctic glaciers under climatic warming scenarios and consequent shifts in glacial thermal regimes.

Description: Banded iron formations (BIF), deposited prior to and concurrent with the Great Oxidation Event (GOE) at ~2.4 Ga, record changes in the oceanic and atmospheric chemistry during this critical time interval. Three previously unstudied drill-cores from the western Transvaal Basin, South Africa, capturing the rhythmically mesobanded Kuruman BIF and the overlying granular Griquatown BIF, were sampled every ~20 m. along core depth. These samples were analysed for mineralogy, geochemistry and bulk Fe and C-isotopes. Bulk Fe-isotopic values of 50 samples show an apparent relationship with mineralogy. The lower δ56Fe values (〈 -1.3) correlate with carbonate-rich samples, whereas higher δ56Fe values (〉0.0) correspond to samples rich in bulk modal magnetite. To further investigate this relationship, a 3-step sequential extraction protocol was developed to separate the three main Fe-hosting fractions (Fe-carbonates, Fe-oxides and Fe-silicates). Rare Earth Element (REE) patterns were resolved for the individual fractions and using the leachate destruction protocol of Henkel et al. [1] we were able to measure for the first time species specific Fe-isotopes of bulk-BIF samples. Species specific Fe-isotopes are probably a better proxy for the Palaeoproterozoic ocean than bulk-rock values, since the latter are strongly influenced by the modal mineralogy of each sample. We used bulk-rock C-isotope data combined with the species specific REE and δ56Fe to argue that the Fe-carbonates (and possibly Fe-silicates) in the Transvaal BIFs record primary chemical signatures. It follows that chemical signatures can be preserved, through changes of the textural appearance of minerals in BIF during diagenesis and low-grade metamorphism [2]. Preliminary data indicate that the Fe-oxides (dominated by magnetite) are probably formed by recycling and mixing of precursor Fe-(oxy)hydroxides and ferrous sea- or pore-waters, since their positive δ56Fe values deviate strongly and consistently from the negative ones of the other fractions. The post-GOE Fe-oxides of the stratigraphically higher Hotazel Formation have negative δ56Fe values, which supports a basin-wide Rayleigh fractionation of isotopically heavy-Fe [3]. References: [1] Henkel et al., (2016) Chemical Geology, 421, 93-102 [2] Frost et al., (2007) Contributions to Mineralogy and Petrology 153, 211-235 [3] Tsikos et al., (2010) Earth and Planetary Science Letters, 298, 125-134

Description: Iron fluxes from reducing sediments and subglacial environments are potential sources of bioavailable iron into the Southern Ocean. Stable iron isotopes are considered a proxy for Fe sources, but respective data are scarce and Fe cycling in complex natural environments is not understood sufficiently to constrain respective δ56Fe “endmembers” for different types of sediments, environmental conditions and biogeochemical processes. We show δ56Fe data from pore waters and sequentially leached solid Fe (for method see [1]) of two contrasting sites in a bay of King George Island that is affected by fast glacier retreat. Sediments close to the glacier front contain more reactive Fe oxides and pyrite compared to those close to the ice-free beach and show a broader ferruginous zone. Since sulfate reduction (SR) is almost negligible at this site, the pyrite likely derives from eroded bedrock. Interestingly, 56Fe depletion in pore water and most reactive Fe oxides is more pronounced close to the ice-free beach where SR was observed at shallow sediment depth. Downcore δ56Fe variability close to the glacier front is limited to surface-reduced Fe, whereas it also occurs in the ferrihydrite-lepidocrocite fraction station close to the ice-free beach. The ferrihydrite-lepidocrocite fraction is at least 0.5‰ lighter than goethite-hematite and magnetite at both sites indicating that it incorporates Fe that previously underwent redox cycling. High amounts of easily reducible Fe oxides, esp. at the glacier site, stimulate dissimilatory iron reduction (DIR) and prevent the use of less reactive Fe oxides. We infer that pyrite oxidation (subglacially or within the deposited sediment in the bay) and/or Fe2+ supply from subglacial environments promote Fe cycling and that DIR-dominated sediments do not necessarily result in isotopically lighter Fe fluxes compared to SR-dominated sediments. [1] Henkel et al. (2016), Chem. Geol. 421, 93-102.