Description: The flux of methane, a potent greenhouse gas, from the seabed is largely controlled by anaerobic oxidation of methane (AOM) coupled to sulfate reduction (S-AOM) in the sulfate methane transition (SMT). S-AOM is estimated to oxidize 90 % of the methane produced in marine sediments and is governed by a consortium of anaerobic methanotrophic archaea (ANME) and sulfate reducing bacteria. An additional methane sink, i.e., iron oxide coupled AOM (Fe-AOM), has been suggested to be active in the methanic zone of marine sediments. Geochemical signatures below the SMT such as high dissolved iron, low to undetectable sulfate and high methane concentrations, together with the presence of iron oxides are taken as prerequisites for this process. So far, neither has Fe-AOM been proven in marine sediments nor have the governing key microorganisms been identified. Here, using a multidisciplinary approach, we show that Fe-AOM occurs in iron oxide-rich methanic sediments of the Helgoland Mud Area (North Sea). When sulfate reduction was inhibited, different iron oxides facilitated AOM in long-term sediment slurry incubations but manganese oxide did not. Especially magnetite triggered substantial Fe-AOM activity and caused an enrichment of ANME-2a archaea. Methane oxidation rates of 0.095 ± 0.03 nmol cm 3 d 1 attributable to Fe-AOM were obtained in short-term radiotracer experiments. The decoupling of AOM from sulfate reduction in the methanic zone further corroborated that AOM was iron oxide-driven below the SMT. Thus, our findings prove that Fe-AOM occurs in methanic marine sediments containing mineral-bound ferric iron and is a previously overlooked but likely important component in the global methane budget. This process has the potential to sustain microbial life in the deep biosphere.

Description: Numerous studies have provided compelling evidence that the Pacific Ocean has experienced substantial glacial/interglacial changes in bottom-water oxygenation associated with enhanced carbon dioxide storage in the glacial deep ocean. Under postulated low glacial bottom-water oxygen concentrations (O2bw), redox zonation, biogeochemical processes and element fluxes in the sediments must have been distinctively different during the last glacial period (LGP) compared to current well-oxygenated conditions. In this study, we have investigated six sites situated in various European contract areas for the exploration of polymetallic nodules within the Clarion-Clipperton Zone (CCZ) in the NE Pacific and one site located in a protected Area of Particular Environmental Interest (APEI3) north of the CCZ. We found bulk sediment Mn maxima of up to 1 wt% in the upper oxic 10 cm of the sediments at all sites except for the APEI3 site. The application of a combined leaching protocol for the extraction of sedimentary Mn and Fe minerals revealed that mobilizable Mn(IV) represents the dominant Mn(oxyhydr)oxide phase with more than 70% of bulk solid-phase Mn. Steady state transport-reaction modeling showed that at postulated glacial O2bw of 35 µM, the oxic zone in the sediments was much more compressed than today where upward diffusing pore-water Mn2+ was oxidized and precipitated as authigenic Mn(IV) at the oxic-suboxic redox boundary in the upper 5 cm of the sediments. Transient transport-reaction modeling demonstrated that with increasing O2bw during the last glacial termination to current levels of ~ 150 µM, (1) the oxic-suboxic redox boundary migrated deeper into the sediments and (2) the authigenic Mn(IV) peak was continuously mixed into subsequently deposited sediments by bioturbation causing the observed mobilizable Mn(IV) enrichment in the surface sediments. Such a distinct mobilizable Mn(IV) maximum was not found in the surface sediments of the APEI3 site, which indicates that the oxic zone was not as condensed during the LGP at this site due to two- to threefold lower organic carbon burial rates. Leaching data for sedimentary Fe minerals suggest that Fe(III) has not been diagenetically redistributed during the LGP at any of the investigated sites. Our results demonstrate that the basin-wide deoxygenation in the NE Pacific during the LGP was associated with (1) a much more compressed oxic zone at sites with carbon burial fluxes higher than 1.5 mg Corg m-2 d-1, (2) the authigenic formation of a sub-surface mobilizable Mn(IV) maximum in the upper 5 cm of the sediments and (3) a possibly intensified suboxic-diagenetic growth of polymetallic nodules. As our study provides evidence that authigenic Mn(IV) precipitated in the surface sediments under postulated low glacial O2bw, it contributes to resolving a long-standing controversy concerning the origin of widely observed Mn-rich layers in glacial/deglacial deep-sea sediments.

Description: The Antarctic continental shelf represents roughly 11% of the world’s continental-shelf area and exhibits the highest area-based primary production rates in the Southern Ocean. On the shelf, primary production strongly varies depending on light conditions, sea ice cover, mixing depth and nutrient availability. In regions impacted by global warming, such as the Antarctic Peninsula, these conditions are changing. The retreat of sea ice and the availability of previously ice covered areas for marine primary production has important repercussions on nutrient and carbon fluxes. In this study, we investigated benthic remineralization processes along a cryopelagic productivity gradient from year-round heavy ice conditions through the marginal ice zone to mainly ice-free conditions at the Western shelf of the Weddell Sea (East Antarctic Peninsula). Carbon mineralization rates were derived from pore-water profiles of oxygen, nitrate, ammonium, dissolved manganese and dissolved iron. Pore water samples were obtained from sediment cores retrieved by multi-coring at water depths between 330 to 455 m. Two deep stations (3000 m depth) were sampled for comparison. While yearly sea ice cover decreased from 80 to 30% between the stations, benthic carbon oxidation rates increased from 1.0 to 7.6 mmol C m-2 d-1 and the total organic carbon contents ranged from 0.15 to 1.5 wt.%. The low rates at heavy ice covered shelf stations were comparable to those of deep sea stations further north. Carbon mineralization rates showed that aerobic respiration accounted for 60-95% of the total carbon degradation. Anaerobic degradation was dominated by denitrification and iron reduction at stations with high sea ice cover, while sulfate reduction was present only at stations with less sea ice cover. Pore water Fe2+ concentrations reached up to 50 μmol/L near the sediment surface and up to 670 μmol/L at about 4 cm depth, which can lead to a substantial release of Fe2+ to the water column and to a subsequent increase of the iron limited primary production. In summary, the results indicate that future sea ice retreat may lead to a significant increase of benthic carbon mineralization with a subsequent enhancement of the benthic iron efflux.

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: 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: Polar regions are critical for future climate evolution, and they experience major environmental changes. A particular focus of biogeochemical investigations in these regions lies on iron (Fe). This element drives primary productivity and, thus, the uptake of atmospheric CO2 in vast areas of the ocean. Due to the Fe-limitation of phytoplankton growth in the Southern Ocean, Antarctica is a key region for studying the change of iron fluxes as glaciers progressively melt away. The respective climate feedbacks can currently hardly be quantified because data availability is low, and iron transport and reaction pathways in Polar coastal and shelf areas are insufficiently understood. We show how novel stable Fe isotope techniques, in combination with other geochemical analyses, can be used to identify iron discharges from subglacial environments and how this will help us assessing short and long term impacts of glacier retreat on coastal ecosystems. Stable Fe isotopes (δ56Fe) may be used to trace Fe sources and reactions, but respective data availability is low. In addition, there is a need to constrain δ56Fe endmembers for different types of sediments, environments, and biogeochemical processes. δ56Fe data from pore waters and sequentially extracted solid Fe phases at two sites in Potter Cove (King George Island, Antarctica), a bay affected by fast glacier retreat, are presented. Close to the glacier front, sediments contain high amounts of easily reducible Fe oxides and show a dominance of ferruginous conditions compared to sediments close to the ice-free coast, where surficial oxic meltwater discharges and sulfate reduction dominates. We suggest that high amounts of reducible Fe oxides close to the glacier mainly derive from subglacial sources, where Fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering. A strong argument for a subglacial source is the predominantly negative δ56Fe signature of reducible Fe oxides that remains constant throughout the ferruginous zone. In situ dissimilatory iron reduction (DIR) does not significantly alter the isotopic composition of the oxides. The composition of the easily reducible Fe fraction therefore suggests pre-depositional microbial cycling as it occurs in subglacial environments. Sediments influenced by oxic meltwater discharge show downcore trends towards positive δ56Fe signals in pore water and reactive Fe oxides, typical for in situ DIR as 54Fe becomes less available with increasing depth. We found that a quantification of benthic Fe fluxes and subglacial Fe discharges based on stable Fe isotope geochemistry will be complicated because (1) diagenetic processes vary strongly at short lateral distances and (2) the variability of δ56Fe in subglacial meltwater has not been sufficiently well investigated yet. However, isotope mass balance models that consider the current uncertainties could, in combination with an application of ancillary proxies, lead to a much better quantification of Fe inputs into polar marine waters than currently available. This would consequently allow a better assessment of the flux and fate of Fe originating from the Antarctic Ice Sheet. Henkel et al. (2018) Diagenetic iron cycling and stable Fe isotope fractionation in Antarctic shelf sediments, King George Island. GCA 237, 320-338.

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.

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.

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.