The iron “redox battery” in sandy sediments: Its impact on organic matter remineralization and phosphorus cycling

Permeable sandy sediments cover 50-60% of the global continental shelf and are important bioreactors that regulate organic matter (OM) turnover and nutrient cycling in the coastal ocean. In sands, the dynamic porewater advection can cause rapid mass transfer and variable redox conditions, thus affecting OM remineralization pathways as well as the recycling of iron and phosphorus. In this study, North Sea sands were incubated in flow-through reactors (FTRs) to investigate biogeochemical processes under porewater advection and changing redox conditions. We found that the average rate of anaerobic OM remineralization was 12 times lower than the aerobic pathway, and Fe(III) oxyhydroxides were found as the major electron acceptors during 34 days of anoxic incubation. Abundant reduced Fe in the solid phase (expressed as Fe(II)) was measured before extensive Fe2+ release into porewater, and most of the reduced Fe (~96%) remained in the solid phase throughout the anoxic incubation. Fe(II) retained in the solid phase, either through the formation of authigenic Fe(II)-bearing minerals or adsorption, was easily re-oxidized upon exposure to O2 . Excessive P release (apart from OM remineralization) started at the beginning of the anoxic incubation and accelerated after the release of Fe2+ with a constant P/Fe2+ ratio of 0.26. After 34 days of anoxic incubation, porewater was re-oxygenated and >99% of released P was coprecipitated through Fe2+ oxidation (so-called “Fe2+ curtain”). Our results demonstrate that Fe(III)/Fe(II) in the solid phase can serve as relatively immobile and rechargeable “redox battery” under dynamic porewater advection. Due to frequent oscillation of redox conditions, the Fe “redox battery” is characteristic for permeable sediments and plays an important role in coastal OM turnover. We also suggest that P liberated before Fe2+ release can escape the “Fe2+ curtain” in porewater advection, thus potentially increasing net benthic P efflux from permeable sediments under variable redox conditions

Fe isotopes revealing mineral-specific redox cycling in sediments

Reactive Fe (oxyhydr)oxides preferentially undergo early diagenetic cycling and cause a diffusive flux of dissolved Fe2+ towards the sediment-water interface. The partitioning of sedimentary Fe has traditionally been studied by applying sequential extractions. We modified an existing leaching method [1] in order to enable δ56Fe measurements on specific Fe mineral fractions. Those are siderite/sorbed Fe, ferrihydrite/lepidocrocite, goethite/hematite, and magnetite. The selectivity of extractions was tested by leaching pairs of 58Fe-spiked and unspiked synthetic minerals. Insignificant amounts of goethite and hematite are dissolved in hydroxylamine-HCl targetting ferrihydrite/lepidocrocite. The determination of reducible oxides leached by dithionite was found to be slightly compromised in presence of magnetite. Removal of extraction matrix was achieved by repetitive oxidation, heating, Fe precipitation, and column separation. The new method was applied to a short sediment core from the North Sea. Downcore mineral-specific variations in δ56Fe revealed differing contributions of Fe oxides to redox cycling. Acetic acid soluble Fe and ferrihydrite/lepidocrocite-Fe showed increasing δ56Fe values with depth in accordance with progressive dissimilatory iron reduction (DIR). Low δ56Fe in acetic acid soluble Fe relative to ferric hydrous oxide-Fe is consistent with the fractionation pattern between sorbed Fe(II) and ferric substrate during DIR experiments [2]. Goethite/hematite-and magnetite-Fe do not show δ56Fe trends with depth. The results demonstrate the importance of δ56Fe analysis on individual Fe fractions that differ in origin and reactivity. The developed procedure provides a basis for specific Fe isotope studies in past and present environments that undergo or underwent redox changes. [1] Poulton and Canfield (2005), Chemical Geology 214, 209-221. [2] Crosby et al., Geobiology 5 (2007), 169-189

Benthic element cycling on the Antarctic shelf and its potential control by sea ice cover

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

Marine Ice: A sleeping iron giant in the Southern Ocean?

The Polar Southern Ocean (PSO) provides an excess amount of macro-nutrients but productivity is largely limited by the availability of essential micro-nutrients, namely iron, manganese, zinc and others. Seasonal patches of increased productivity off major ice shelfs around Antarctica suggest that local sources of these deficient micro-nutrients must be present. With this session contribution we present a new study on marine ice from the Filchner-Ronne Ice Shelf (FRIS) as a potential source of iron and other limiting micro-nutrients for the Atlantic sector of the PSO. Marine ice is formed via partial melting of meteoric shelf ice near the grounding line of large ice shelves (e.g. FRIS). During this process small refrozen ice platelets accumulate in a layer of over 100 m thickness underneath the ice shelf to form marine ice containing high amounts of particulate material. In a project funded by the German Research Foundation (DFG) within the priority program SPP1158, we analyse 2 marine ice cores (B13: 62m, B15: 167m of marine ice) recovered in the 1990’s from the FRIS on their geochemical compositions. The coring location of B13 was about 40 km away from the shelf ice edge and B15 was drilled another 136 km further inland along the reconstructed flow line of B13. Due to shelf ice migration over the last 30 years, their locations have shifted about 30 km towards the shelf ice edge. First results show dissolved Fe (dFe) and Mn (dMn) concentrations ranging between 30 and 300 nMol and particulate Fe (pFe) of 20 to 120 µMol (0.2 to 1.4 µMol for pMn). These concentrations are orders of magnitude higher than the ones currently found in the PSO for those elements. Basal melting and ice-berg calving of marine ice with the accompanied release of these essential trace metals could therefore fuel local productivity in regions with large extent of shelf ice. With our study we aim to evaluate marine ice as potentially overlooked source for limiting micro-nutrients that could explain high productivity areas within an otherwise relatively low productive PSO

Iron cycling and stable Fe isotope fractionation in Antarctic shelf sediments, King George Island

Iron (Fe) fluxes from reducing sediments and subglacial environments are potential sources of bioavailable Fe into the Southern Ocean. Stable Fe isotopes (δ56Fe ) are considered a proxy for Fe sources and reaction pathways, 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 present δ56Fe data from pore waters and sequentially extracted sedimentary Fe phases of two contrasting sites in Potter Cove (King George Island, Antarctic Peninsula), a bay that is affected by fast glacier retreat. Sediments close to the glacier front contain more easily reducible Fe oxides and pyrite and show a broader ferruginous zone, compared to sediments close to the icefree coast, where surficial oxic meltwater streams discharge into the bay. Pyrite in sediments close to the glacier front predominantly derives from eroded bedrock. For the high amount of easily reducible Fe oxides proximal to the glacier we suggest mainly subglacial sources, where Fe liberation from comminuted material beneath the glacier is coupled to biogeochemical weathering processes (likely pyrite oxidation or dissimilatory iron reduction, DIR). Our strongest argument for a subglacial source of the highly reactive Fe pool in sediments close to the glacier front is its predominantly negative δ56Fe signature that remains constant over the whole ferruginous zone. This implies in situ DIR does not significantly alter the stable Fe isotope composition of the accumulated Fe oxides. The nonetheless overall light δ56Fe signature of easily reducible Fe oxides suggests pre-depositional microbial cycling as it occurs in potentially anoxic subglacial environments. The strongest 56Fe-depletion in pore water and most reactive Fe oxides was observed in sediments influenced by oxic meltwater discharge. The respective site showed a condensed redox zonation and a pore water δ56Fe profile typical for in-situ Fe cycling. We demonstrate that the potential of pore water δ56Fe as a proxy for benthic Fe fluxes is not straight-forward due to its large variability in marine shelf sediments at small spatial scales (- 2.4‰ at the site proximal to oxic meltwater discharge vs. -0.9‰ at the site proximal to the marine glacier terminus, both at 2 cm sediment depth). The controlling factors are multifold and include the amount and reactivity of reducible Fe oxides and organic matter, the isotopic composition of the primary and secondary ferric substrates, sedimentation rates, and physical reworking (bioturbation, ice scraping). The application of δ56Fe geochemistry may prove valuable in investigating biogeochemical weathering and Fe cycling in subglacial environments. This requires, however (similarly to the use of δ56Fe for the quantification of benthic fluxes), that the spatial and temporal variability of the isotopic endmember is known and accounted for. Since geochemical data from subglacial environments are very limited, further studies are needed in order to sufficiently assess Fe cycling and fractionation at glacier beds and the composition of discharges from those areas

Sulfur cycling in an iron oxide-dominated, dynamic marine depositional system: The Argentine continental margin

The interplay between sediment deposition patterns, organic matter type and the quantity and quality of reactive mineral phases determines the accumulation, speciation, and isotope composition of pore water and solid phase sulfur constituents in marine sediments. Here, we present the sulfur geochemistry of siliciclastic sediments from two sites along the Argentine continental slope—a system characterized by dynamic deposition and reworking, which result in non-steady state conditions. The two investigated sites have different depositional histories but have in common that reactive iron phases are abundant and that organic matter is refractory—conditions that result in low organoclastic sulfate reduction rates (SRR). Deposition of reworked, isotopically light pyrite and sulfurized organic matter appear to be important contributors to the sulfur inventory, with only minor addition of pyrite from organoclastic sulfate reduction above the sulfate-methane transition (SMT). Pore-water sulfide is limited to a narrow zone at the SMT. The core of that zone is dominated by pyrite accumulation. Iron monosulfide and elemental sulfur accumulate above and below this zone. Iron monosulfide precipitation is driven by the reaction of low amounts of hydrogen sulfide with ferrous iron and is in competition with the oxidation of sulfide by iron (oxyhydr)oxides to form elemental sulfur. The intervals marked by precipitation of intermediate sulfur phases at the margin of the zone with free sulfide are bordered by two distinct peaks in total organic sulfur (TOS). Organic matter sulfurization appears to precede pyrite formation in the iron-dominated margins of the sulfide zone, potentially linked to the presence of polysulfides formed by reaction between dissolved sulfide and elemental sulfur. Thus, SMTs can be hotspots for organic matter sulfurization in sulfide-limited, reactive iron-rich marine sedimentary systems. Furthermore, existence of elemental sulfur and iron monosulfide phases meters below the SMT demonstrates that in sulfide-limited systems metastable sulfur constituents are not readily converted to pyrite but can be buried to deeper sediment depths. Our data show that in non-steady state systems, redox zones do not occur in sequence but can reappear or proceed in inverse sequence throughout the sediment column, causing similar mineral alteration processes to occur at the same time at different sediment depths

Postdepositional Behavior of Molybdenum in Deep Sediments and Implications for Paleoredox Reconstruction

Molybdenum (Mo) is a trace element sensitive to oceanic redox conditions. The fidelity of sedimentary Mo as a paleoredox proxy of coeval seawater depends on the extent of Mo remobilization during postdepositional processes. Here we present the Mo content and isotope profiles for deep sediments from the Nankai Trough, Japan. The Mo signature suggests that these sediments have experienced extensive early diagenesis and hydrothermal alteration at depth. Iron (Fe)‐manganese (Mn) (oxyhydr)oxide alteration combined with Mo thiolation leads to a more than twenty‐fold enrichment of Mo within the sulfate reduction zone. Hydrothermal fluids and Mo adsorption onto Fe‐Mn (oxyhydr)oxides cause extremely negative Mo‐isotope values at the underthrust zone. These postdepositional Mo signals might be misinterpreted as expanded anoxia in the water column. Our findings highlight the importance of constraining postdepositional effects on Mo‐based proxies during paleoredox reconstruction

Drivers of Iron Cycling in Sediments of the sub-Antarctic Island South Georgia

Sediments of sub-Antarctic islands have been proposed to be important contributors to natural iron fertilization in the Southern Ocean [1, 2]. This potential contribution depends on biogeochemical processes within the sediment that may result in an iron benthic flux, most likely related to the degradation of organic matter (OM). Yet, the OM degradation pathways vary strongly among different sedimentary settings. We elucidate the role of environmental factors on the prevailing biogeochemical pathways and reaction rates at three contrasting sites of South Georgia, using comprehensive solid-phase and pore-water analyses, as well as transportreaction modelling. Samples were obtained along a transect from a glacial fjord towards the shelf during cruise ANTXXIX/ 4 of RV POLARSTERN in 2013. Oxygen penetration depth at all sites is <1 cm. Sediments recovered within the fjord are dominated by dissimilatory iron reduction (DIR) and show very high dissolved Fe2+ concentrations of up to 760 μM, while sulfide was not detected. In addition, Fe reduction below the sulfate/methane transition was observed. High input of reactive iron phases, possibly enhanced by bioturbation and bubble ebullition, appear to favour DIR as the dominant metabolic process for OM degradation in the basin like fjord. Shelf sediments outside the fjord are sulfidic throughout, with H2S formed primarily by anaerobic oxidation of methane. The conversion of Fe oxides into Fe sulfides significantly alters the initial sediment composition along the shelf, and impact the availability of iron to the water column. OM is of marine origin at all three sites (C:N~7), indicating that Fe oxide availability and reactivity rather than the carbon source determine whether iron or sulfate reduction dominantes. [1] Moore & Braucher (2008) Biogeosciences 5, 631-656. [2] Borrione et al., (2014) Biogeosciences 11, 1981–2001

Influence of Early Low-Temperature and Later High-Temperature Diagenesis on Magnetic Mineral Assemblages in Marine Sediments From the Nankai Trough

Funding Information: This research used samples and data provided by the International Ocean Discovery Program (IODP). The authors thank the Marine Works Japan staff at the Kochi Core Center for support during sampling. This work was supported by the Japan Society for the Promotion of Science Grant-in-Aid for Science Research (grant 17K05681 to Myriam Kars), the German Research Foundation (DFG grants 388260220 to Male Koster and Susann Henkel, and 408178672 to Florence Schubotz), and the Australian Research Council (grant DP200100765 to Andrew P. Roberts). The authors also thank two anonymous reviewers for their constructive comments and Editor Joshua Feinberg for handling the manuscript.Peer reviewedPublisher PD