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The concentration and isotopic composition of sedimentary molybdenum (Mo) has been used to distinguish different redox environments in modern marine settings and in the geological record. We report Mo concentrations and δ98Mo from porewaters and sediments in three anoxic East Anglian salt marsh pond environments: (1) 'iron-rich' sediments containing high concentrations of dissolved ferrous iron (up to 2 mM), (2) 'sulfide-rich' sediments containing very high concentrations of aqueous sulfide (up to 10 mM) and, (3) sediments that we consider to be intermediate between 'iron-rich' and 'sulfide-rich' conditions. In iron-rich sediments, we suggest that iron speciation and mineralogy controls the concentration and isotopic composition of Mo. Despite similar aqueous sulfide profiles, the intermediate and sulfide-rich pond sediment have different porewater Mo concentrations and δ98Mo. In the sulfide-rich pond sediment, we suggest that the concentration and isotopic composition of Mo is controlled by solubility equilibrium with an Fe-Mo-S mineral species (e.g. FeMoS 4) due to similarities in sediment and porewater δ98Mo throughout the sediment column. In the intermediate pond sediment, we conclude that active breakdown of iron oxides redistributes porewater Mo, observable as a peak of dissolved Mo (>100 ppb), which diffuses within the sedimentary porewaters. The sedimentary δ98Mo is higher in sulfide-rich and intermediate pond sediment (mean = 1.66‰, range = 0.98–1.92‰) than in iron-rich pond sediment (mean = 1.10‰, range = 0.28–1.65‰) with all ponds having sedimentary δ98Mo that is lower than seawater. The maximum sedimentary δ98Mo observed in these anoxic sediments, which is 0.5–0.7‰ lower than seawater, appears to be set by Fe-Mo-S equilibration with ambient thiomolybdate species. We suggest diagenetic overprinting can cause more efficient capture of pond water Mo and causes sediment δ98Mo of originally iron-rich pond sediment to evolve to higher values at progressively higher aqueous sulfide concentrations. [ABSTRACT FROM AUTHOR]

Abstract In this research, sediments from the Sečovlje Salina (northern Adriatic, Slovenia) were geochemically investigated in order to decode the mobility of metal(oid)s in the hypersaline environment. The results demonstrated that the concentrations of As, Co, Cu, Mo, Ni, Pb, Sb, Sn and Zn were comparable to those detected in various similar coastal background areas, as well as in the Sečovlje salt marsh core sample. The estimated mobility potential of metal(oid)s in sediment decreases in the following order: Mo > As > Cu > Sb > Sn > Co > Pb > Ni > Zn. Since the hypersaline sediment (e.g. healing mud) is used as a healing factor (it can be applied directly on human skin), the study of metal(oid) bioavailability in an interaction between the hypersaline sediments and the artificial sweat was also performed. It is significant that the metal(oid) contents are leached in very low concentrations and are treated as nontoxic for humans according to international norms for cosmetic products. The study provides information in order to assess the role of metal mobility on the potential health impact of the application of natural healing mud. Graphical abstract Unlabelled Image Highlights • The transfer of metal(oid)s in a hypersaline environment was investigated. • Availability of metal(oid)s from a health risk perspective was conducted. • Sediment-water physical and geochemical properties induced limited mobility dynamics. • Extraction testing in artificial sweat indicate low mobility of metal(oid)s. [ABSTRACT FROM AUTHOR]

Salt marshes are ecosystems with significant economic and environmental value. However, the accelerating rate of sea-level rise is a significant threat to these ecosystems. Storms significantly contribute to the sediment budget of salt marshes, playing a critical role in salt marsh survival to sea-level rise. There are, however, uncertainties on the extent to which storms contribute sediments to different areas of marsh platforms (e.g., outer marsh vs marsh interior) and on the sediment sources that storms draw on (e.g., offshore vs nearshore). This study uses field analyses from an eight-month field campaign in the Ribble Estuary, North-West England, to understand storms' influence on the sediment supply to different marsh areas and whether storms can deliver new material onto the salt marsh platform which would otherwise not be sourced in fair-weather conditions. Field data from sediment traps indicate that storm activity caused an increase in inorganic sediment supply to the whole salt marsh platform, especially benefitting the marsh interior. Geochemistry and particle size distribution analysis indicate that the majority of the sediment supplied to the salt marsh platform during the stormy periods was generated by an increase in erosion and resuspension of mudflat and tidal creek sediments, while only a minimal contribution was given by the sediments transported from outside the intertidal system. This suggests that, in the long term, storms will promote salt marsh vertical accretion but might simultaneously reduce the overall larger-scale sediment availability with implications for the marsh lateral retreat. • Storms contribute significantly to salt marsh sediment budget. • There are uncertainties as to what extent this contribution benefits the marsh platform. • Field data showed that storms benefit marsh vertical accretion but limit lateral expansion. [ABSTRACT FROM AUTHOR]

In support of efforts to reconstruct relative sea level ( RSL), we investigated the utility of foraminifera, diatoms and bulk-sediment geochemistry (δ13C, C:N and parameters measured by Rock-Eval pyrolysis) as sea-level indicators in Eurasian sub-Arctic salt marshes. At three salt marshes (<15 km apart) in Dvina Bay (White Sea, Russia), we collected surface sediment samples along transects from subtidal to Taiga forest environments. Foraminifera at all sites formed bipartite assemblages, where elevations below mean high higher water ( MHHW) were dominated by Miliammina spp. and elevations between MHHW and the highest occurrence of foraminifera were dominated by Jadammina macrescens and Balticammina pseudomacrescens. Five high-diversity groups of diatoms were identified and they displayed pronounced variability amongst the study sites. Bulk-sediment geochemistry recognized two groups (clastic-dominated environments below MHHW and organic-rich environments above MHHW). As one group included subtidal elevations and the other included supratidal elevations, we conclude that the measured geochemical parameters are not stand-alone sea-level indicators. Core JT2012 captured a regressive sediment succession of clastic, tidal-flat sediment overlain by salt-marsh organic silt and freshwater peat. The salt-marsh sediment accumulated at 2804±52 years before present and preserved foraminifera ( Jadammina macrescens and Balticammina pseudomacrescens) with good analogy to modern assemblages indicating that RSL was +2.60±0.47 m at this time. Diatoms confirm that marine influence decreased through time, but the lack of analogy between modern and core assemblages limited their utility as sea-level indicators. Geochemical parameters also indicate a reduction in marine influence through time. We conclude that RSL reconstructions derived from salt-marsh sediment preserved beneath Eurasian sub-Arctic peatlands can provide valuable insight into the spatio-temporal evolution of the Fennoscandian and Eurasian ice sheets. [ABSTRACT FROM AUTHOR]

Saltmarshes are being lost or degraded as a result of human activity resulting in loss of critical ecosystem services including the provision of wild species diversity, water quality regulation and flood regulation. To compensate, saltmarshes are being restored or re-created, usually driven by legislative requirements for increased habitat diversity, flood regulation and sustainable coastal defense. Yet, there is increasing evidence that restoration may not deliver anticipated ecosystem services; this is frequently attributed to poor drainage and sediment anoxia. However, physical sediment characteristics, hydrology and the sediment geochemical environment are rarely examined in restoration schemes, despite such factors being critical for plant succession. This study presents the novel integration of 3D-computed X-ray microtomography to quantify sediment structure and porosity, with water level and geochemical data to understand the impact of pre-restoration land use and disturbance on the structure and functioning of restored saltmarshes. The study combines a broad-scale investigation of physical sediment characteristics in nine de-embanked saltmarshes across SE England, with an intensive study at one site examining water levels, sediment structure and the sediment geochemical environment. De-embankment does not restore the hydrological regime, or the physical/chemical framework in the saltmarshes and evidence of disturbance includes a reduction in microporosity, pore connectivity and water storage capacity, a lack of connectivity between the sub-surface environment and overlying floodwaters, and impeded sub-surface water flow and drainage. This has significant consequences for the sediment geochemical environment. This disturbance is evident for at least two decades following restoration and is likely to be irreversible. It has important implications for plant establishment in particular, ecosystem services including flood regulation, nutrient cycling and wild species diversity and for future restoration design. [ABSTRACT FROM AUTHOR]

Cryptic sulfur cycling is an enigmatic process in which sulfate is reduced to some lower-valence state sulfur species and subsequently quantitatively reoxidized; the rate and microbial energetics of this process and how prevalent it may be in the environment remain controversial. Here we investigate sulfur cycling in salt marsh sediments from Norfolk, England where we observe high ferrous iron concentrations with no depletion of sulfate or change in the sulfur isotope ratio of that sulfate, but a 5‰ increase in the oxygen isotope ratio in sulfate, indicating that sulfate has been through a reductive cycle replacing its oxygen atoms. This cryptic sulfur cycle was replicated in laboratory incubations using 18 O-enriched water, demonstrating that the field results do not solely result from mixing processes in the natural environment. Numerical modeling of the laboratory incubations scaled to represent the salt marsh sediments suggests that the uptake rate of sulfate during this cryptic sulfur cycling is similar to the uptake rate of sulfate during the fastest microbial sulfate reduction that has been measured in the natural environment. The difference is that during cryptic sulfur cycling, all of the sulfur is subsequently reoxidized to sulfate. We discuss mechanisms for this pathway of sulfur cycling including the possible link to the subsurface iron cycle. [ABSTRACT FROM AUTHOR]

Abstract: Environmental changes during the last 2 millennia in the Minho River tidal marsh (NW Portugal–Spain border) were reconstructed. Changes in the sources of organic matter (OM) delivered to the marsh were evaluated from elemental, isotopic and molecular composition using a 1m sediment core. Carbon isotopic composition (δ13C) and organic carbon to total nitrogen ratio (Corg/N) provided valuable information concerning the origin of the OM. These parameters indicated a major input from land plants, reaching a maximum at 1100–1200 and 1750–1850AD. These periods match with major flood events in the NW of the Iberian Peninsula, as reported by several authors. A significant reduction in the terrestrial signature occurred at 6–4cm (ca. 1960–1985AD), which is contemporaneous with the construction of several major dams on the Minho River. The distribution of selected lipid biomarkers, including n-alkanes, n-fatty acids and n-alkan-2-ones and specific parameters derived from the molecular distributions, were useful for refining bulk geochemical results. Long chain n-alkanes with an odd number of carbons are indicators of soil-and vascular plant-derived terrestrial OM and were dominant throughout the core. In addition, a greater contribution of plankton-derived lipids was observed in the sections corresponding to ca. 1960–1985AD (6–4cm) and ca. 100–200AD (96–94cm). Although different degradation rates for individual compounds might have partly affected biomarker assemblages, the variations could be attributed to a sharp decrease in the freshwater contribution to the Minho River estuary (dam construction) and a possible marine highstand, respectively. In addition, several parameters suggested changes in land use (including deforestation and farming) and probably the effects of mining exploitation during the Roman occupation of the area. [Copyright &y& Elsevier]

Abstract: In order to further elucidate the early diagenetic behavior of Mo, we examined the redox speciation of dissolved Mo in organic rich sediment from a back-barrier salt marsh environment in eastern Long Island (Flax Pond, NY). Total dissolved Mo (ΣMo) was ~80nM in surface nonsulfidic porewater, dominantly as Mo(VI). ΣMo increased up to 150nM at a depth of 3.5cm with low sulfide content (ΣH2S: 25–100μM), and Mo(V) reached 20nM. ΣMo decreased to ~70nM at a depth of 7.5cm in highly sulfidic deep sediment porewater (ΣH2S: >100μM) with Mo(V) accounting for ~10%. Mo(VI) dominated residual ΣMo, likely as MoS4 2−. Averaged in situ Mo speciation patterns are complicated by mixed redox conditions created by biogenic structures and reworking. Serial anoxic incubation of surface sediments revealed reductive redox reaction progression without complications from transport and biogenic microenvironments: Mo(VI) dominated initially, followed by increases in ΣMo (dMo/dt ~7nM/h) and production of Mo(V) under low sulfide conditions (ΣH2S: 25–100μM; Mo(V) as high as 160nM). Mo(V) was subsequently lost rapidly from solution (dMo(V)/dt ~−5nM/h) and residual ΣMo, presumably a mixture of Mo(VI) and a small percentage of Mo(IV), was gradually reestablished under highly sulfidic conditions (ΣH2S >100μM). Mo(V) is clearly produced as a transient dissolved intermediate during reductive redox reaction succession. Mo(V) may react with particles or disproportionate in the presence of polysulfides into Mo(IV), which likely rapidly adsorbs–precipitates as pyritic Mo–Fe–S, sulfidized organic complexes, or perhaps MoS2. Mo(VI), which remains, at least temporarily in solution as thiomolybdate is removed more slowly. In contrast to reductive reactions, reoxidation of reduced sediment results in rapid release of Mo dominated by Mo(VI). Dynamic diagenetic cycling and the existence of Mo(V) as a dissolved reaction intermediate must be accounted for in models of Mo fixation and associated isotopic fractionation in sulfidic deposits. [Copyright &y& Elsevier]

To elucidate the geomicrobiological factors controlling nitrification in salt marsh sediments, a comprehensive approach involving sediment geochemistry, process rate measurements, and quantification of the genetic potential for nitrification was applied to three contrasting salt marsh habitats: areas colonized by the tall (TS) or short (SS) form of Spartina alterniflora and unvegetated creek banks (CBs). Nitrification and denitrification potential rates were strongly correlated with one another and with macrofaunal burrow abundance, indicating that coupled nitrification-denitrification was enhanced by macrofaunal burrowing activity. Ammonia monooxygenase (amoA) gene copy numbers were used to estimate the ammonia-oxidizing bacterial population size (5.6 × 104 to 1.3 × 106 g of wet sediment-1), which correlated with nitrification potentials and was 1 order of magnitude higher for TS and CB than for SS. TS and CB sediments also had higher Fe(III) content, higher Fe(III)-to-total reduced sulfur ratios, higher Fe(III) reduction rates, and lower dissolved sulfides than SS sediments. Iron(III) content and reduction rates were positively correlated with nitrification and denitrification potential and amoA gene copy number. Laboratory slurry incubations supported field data, confirming that increased amounts of Fe(III) relieved sulfide inhibition of nitrification. We propose that macrofaunal burrowing and high concentrations of Fe(III) stimulate nitrifying bacterial populations, and thus may increase nitrogen removal through coupled nitrification-denitrification in salt marsh sediments. [ABSTRACT FROM AUTHOR]

Abstract: Spatial and temporal trends in pore water geochemistry and sediment microbial community structure are compared at three intertidal sites of a saltmarsh on Sapelo Island, GA. The sites include a heavily bioturbated, unvegetated creek bank, a levee with dense growth of Spartina alterniflora, and a more sparsely vegetated ponded marsh site. The redox chemistry of the pore waters ranges from sulfide-dominated at the ponded marsh site to suboxic at the creek bank site. At the three sites, the vertical redox stratification of the pore waters is more compressed in summer than in winter. The trends in redox chemistry reflect opposing effects of sediment respiration and pore water irrigation. Intense and deep burrowing activity by fiddler crabs at the creek bank site results in the efficient oxidation of reduced byproducts of microbial metabolism and, hence, the persistence of suboxic conditions to depths of 50cm below the sediment surface. Increased supply of labile organic substrates at the vegetated sites promotes microbial degradation processes, leading to sharper redox gradients. At the levee site, this is partly offset by the higher density and deeper penetration of roots and macrofaunal burrows. Surprisingly, the microbial community structure shows little correlation with the variable vertical redox zonation of the pore waters across the saltmarsh. At the three sites, the highest population densities of aerobic microorganisms, iron- plus manganese-reducing bacteria, and sulfate reducers coexist within the upper 10cm of sediment. The absence of a clear vertical separation of these microorganisms is ascribed to the high supply of labile organic matter and intense mixing of the topmost sediment via bioturbation. [Copyright &y& Elsevier]