Geochemical Settings Research Articles

Mineral-mediated carbohydrate synthesis by mechanical forces in a primordial geochemical setting

The formation of carbohydrates represents an essential step to provide building blocks and a source of chemical energy in several models for the emergence of life. Formaldehyde, glycolaldehyde and a basic catalyst are the initial components forming a variety of sugar molecules in the cascade-type multi-step formose reaction. While numerous side reactions and even deterioration can be observed in aqueous media, selective prebiotic sugar formation is feasible in solid-state, mechanochemical reactions and might have occurred in early geochemistry. However, the precise role of different basic catalysts and the influence of the atmospheric conditions in the solid-state formose reaction remain unknown. Here we show, that in a primordial scenario the mechanochemical formose reaction is capable to form monosaccharides with a broad variety of mineral classes as catalysts with only minute amounts of side products such as lactic acid or methanol, independent of the atmospheric conditions. The results give insight into recent findings of formose sugars on meteorites and offer a water-free and robust pathway for monosaccharides independent of the external conditions both for the early Earth or an extra-terrestrial setting.

Geochemistry and tectonic setting of the Abu-Marawat metavolcanics, Central Eastern Desert—Egypt

This article discussed the petrographic and geochemical characteristics and tectonic setting of Abu-Marawat metavolcanics as part of the Arabian–Nubian Shield in the Central Eastern Desert of Egypt. They comprise metamorphosed basalt, basaltic andesites, dacite, and rhyolites associated with their volcaniclastics. The presence of large amounts of pyroclastics infers that their magma were erupted in submarine environments. They underwent low-grade regional metamorphism related to greenschist facies during the Pan-African orogeny. The geochemical characteristics revealed that the metavolcanics are characterized by low-medium-k calc-alkaline affinity. Analyses of major and trace elements showed that these rocks are relatively enriched in large ion lithophile elements (LILEs; Ba, Rb, K, Pb, and Sr) and depleted in high field strength elements (HFSEs; Zr, Y, P, and Ti) proving subduction component. The marked depletion of specific trace and major elements from basalts to rhyolites reflected the magmatic fractionation. Chemical characteristics, tectonic discrimination diagrams, and N-MORB normalized spider plots of the present study support that their magmas were erupted in an arc setting.

Biogeochemistry and microbiology of high Arctic marine sediment ecosystems—Case study of Svalbard fjords

AbstractFjord ecosystems of the high Arctic are distinct from fjords of temperate latitudes due to the influence of glaciers, icebergs, sea ice, and the permanently low temperatures. The sediment microbiology and biogeochemical processes were analyzed during an international research program with multiple field studies in Svalbard, situated between the Barents Sea and the Arctic Ocean. We here describe the physical and geochemical setting and the predominant microbiological processes in several fjords. Physiological studies of sediments and pure cultures show how the predominantly psychrophilic bacteria are adapted to the near‐zero temperature. The microbial communities include bacteria responsible for organic matter hydrolytic degradation, fermentation, and terminal oxidation to CO2. These processes drive the cycling of carbon, sulfur, iron, and manganese. The balance between the dominant sediment microbial processes changes along transects out through the fjords, reflecting the varying impact of the glacier‐derived rock flour, rich in metal oxides at the head, and the plankton‐derived, labile, marine organic matter at the mouth. Due to accelerated warming of Arctic ecosystems, glaciers are retreating with impacts on the physical, chemical, and biological functioning of the fjord sediment ecosystems.

Meiofauna Community in Soft Sediments at TAG and Snake Pit Hydrothermal Vent Fields

The risk assessment of Seafloor Massive Sulfides (SMS) mining on meiobenthic organisms, specifically on soft-sediment meiofauna, is impeded by a lack of knowledge on the biology and ecology of these communities. In this study we investigated sediment samples taken in proximity of active vents at Trans-Atlantic Geotraverse (TAG) and Snake Pit, two hydrothermal vent fields of the Mid-Atlantic Ridge, in order to explore metazoan meiofauna, particularly nematode community, and its relation to organic carbon, total nitrogen, total sulfur and dissolved oxygen. Organic carbon and nitrogen contents were low at both sites. High concentrations of total sulfur and low oxygen penetration were found at Snake Pit compared to TAG. Snake Pit showed about four times higher meiofauna and nematode density compared to TAG, as well as a dissimilar nematode community composition. We hypothesize that high sulfur concentrations at Snake Pit may support high microbial growth, which represents one of the main food source for nematodes. Moreover, TAG nematode community mostly consisted of persisters (K-strategists), while Snake Pit one was composed by both persisters (Desmoscolecidae family) and colonizers (r-strategists Metalinhomoeus and Halomonhystera), whose presence can be facilitated by the bioturbation effect of polychaetes observed on the sediment surface. Therefore, food availability, geochemical settings and biotic interactions seem to drive the local meiofauna and nematode community. Our study also draws attention to the opportunity of including meiofauna and specifically nematodes in impact studies conducted in this area in order to assess and monitor the impact of SMS mining.

Serpentinite and the search for life beyond Earth.

Hydrogen from serpentinization is a source of chemical energy for some life forms on Earth. It is a potential fuel for life in the subsurface of Mars and in the icy ocean worlds in the outer solar system. Serpentinization is also implicated in life's origin. Planetary exploration offers a way to investigate such theories by characterizing and ultimately searching for life in geochemical settings that no longer exist on Earth. At present, much of the current context of serpentinization on other worlds relies on inference from modelling and studies on Earth. While there is evidence from orbital spectral imaging and martian meteorites that serpentinization has occurred on Mars, the extent and duration of that activity has not been constrained. Similarly, ongoing serpentinization might explain hydrogen found in the ocean of Saturn's tiny moon Enceladus, but this raises questions about how long such activity has persisted. Titan's hydrocarbon-rich atmosphere may derive from ancient or present-day serpentinization at the bottom of its ocean. In Europa, volcanism or serpentinization may provide hydrogen as a redox couple to oxygen generated at the moon's surface. We assess the potential extent of serpentinization in the solar system's wet and rocky worlds, assuming that microfracturing from thermal expansion anisotropy sets an upper limit on the percolation depth of surface water into the rocky interiors. In this bulk geophysical model, planetary cooling from radiogenic decay implies the infiltration of water to greater depths through time, continuing to the present. The serpentinization of this newly exposed rock is assessed as a significant source of global hydrogen. Comparing the computed hydrogen and surface-generated oxygen delivered to Europa's ocean reveals redox fluxes similar to Earth's. Planned robotic exploration missions to other worlds can aid in understanding the planetary context of serpentinization, testing the predictions herein. This article is part of a discussion meeting issue 'Serpentinite in the Earth System'.

Cenozoic oxidation episodes in West Africa at the origin of the in situ supergene mineral redistribution of the primary uranium orebodies (Imouraren deposit, Tim Mersoï Basin, Northern Niger)

Uranium (U) deposits can undergo oxidizing events, subsequent to which the U may be redistributed or precipitated under distinct mineralogical forms. Occurrences of U6+ minerals can be the result of either (i) in situ oxidation and alteration of previous U4+ minerals or (ii) direct precipitation from U6+-rich solutions. Samples from Imouraren, the largest U6+ore deposit known in the world, located in the Tim Mersoi Basin, Northern Niger, have been studied to determine the nature, distribution, and timing of occurrences of U and related minerals. The host rock is composed of sandstones and analcime-rich siliciclastic rocks of Jurassic age. The U ore is largely dominated by acicular hexavalent U minerals and by a very small amount of uraninite. A comprehensive geochemical model that describes the origin and geochemical setting of this style of economically important mineralization has been developed. The petrographic and mineralogical studies show evidence of in situ uraninite alteration into uranophane and metatyuyamunite, documented by textural relationships and chemical analyses using SEM and electron microprobe. The textural relationship between the U4+ and U6+ minerals is supported by the near-identical REE signatures of the primary uraninite and related secondary U minerals. Ion microprobe U-Pb dating (from 21 to ~ 1 Ma) confirms that primary uraninites were altered to U6+ minerals under supergene conditions during Cenozoic oxidation events in western Africa, in relation with the well-known stages of intense continental weathering that have affected western Africa from the early Cenozoic to recent times.

Metal adaptation strategies of deep-sea Bathymodiolus mussels from a cold seep and three hydrothermal vents in the West Pacific

Deep-sea Bathymodiolus mussels are ubiquitous in most cold seeps and hydrothermal fields, where they have adapted to various toxic environments including high metal exposure. However, there is scarce knowledge of metal accumulation and metal-related biomarkers in B. mussels. Here, we present data for metal concentrations (Ag, Cd, Cr, Cu, Fe, Mn, Pb, and Zn) and metal related biomarkers (superoxide dismutase-SOD, catalase-CAT, glutathione peroxidase-GPX, glutathione-GSH, metallothioneins-MTs, and lipid peroxidation-LPO) in different tissues of B. mussels from four different deep-sea geochemical settings: one cold seep and three vent fields in the West Pacific Ocean.Results showed that mussel gills generally exhibited higher metal enrichment than the mantle. Mussels from hydrothermal vents usually had higher metal concentrations (Fe, Cr, Cd, and Pb) than those from cold seep, which could be related to their higher contents in fluids or sediments. However, despite quite different metals loads among the geochemical environment settings, Mn, Zn, and Cu concentrations varied over a smaller range across the sampling sites, implying biological regulation by deep-sea mussels for these elements. Several statistically significant correlations were observed between SOD, CAT, GSH, MTs, and metal levels in analyzed tissues. Although the vent ecosystem is harsher than the cold seep ecosystem, according to our results their mussels' biomarker levels were not so different. This finding suggests that some adaptive or compensatory mechanisms may occur in chronically polluted deep-sea mussels.Principal component analysis allowed for distinguishing different deep-sea settings, indicating that B. mussels are robust indicators of their living environments. We also compared our results with those reported for coastal mussels. To our best knowledge, this is the first integrated study to report metal accumulation and metal-related biomarkers in the deep-sea B. mussels from the West Pacific.

Boron isotope composition of coexisting tourmaline and hambergite in alkaline and granitic pegmatites

The boron isotopic composition of tourmaline and hambergite (Be2BO3[OH,F]) from peraluminous (n = 12), peralkaline (n = 1), and peralkaline nepheline syenite (n = 16) pegmatites has been measured by secondary ion mass spectrometry, for which a new hambergite reference material was developed. The focus of this study is on nepheline syenite pegmatites from the Larvik Plutonic Complex (Norway) and one peralkaline pegmatite related to the nearby Eikeren-Skrim Complex (Norway), where we investigate the source of boron as being from magmatic vs. external fluids. Tourmaline-hambergite mineral pairs were also analysed from peraluminous pegmatite localities (Russia, Tajikistan, and Pakistan) to test for systematic B-isotope fractionation between these two minerals.Tourmaline and hambergite from peraluminous granitic pegmatites have light boron ratios (δ11B = −12.9to −1.0‰) associated with S-type granites, whereas peralkaline granitic and nepheline syenite pegmatites have boron ratios (δ11B = −1.7 to 11.8‰), which we interpret is a result of heavy‑boron enrichment from external fluids. Our data show that hambergite tracks isotope variations of its geochemical setting and could therefore be used as a proxy mineral in place of tourmaline when geochemical stability favours hambergite.The results suggest a slight but consistent partitioning of B-isotopes between tourmaline and hambergite, with Δ11B = δ11Btourmaline−δ11Bhambergite in the range of approximately −3‰ to −5‰. Boron is in trigonal coordination with oxygen in both of these mineral phases as verified by NMR. Single crystal XRD analyses of tourmaline and hambergite reveal consistent longer <B-O > distances of tourmaline relative to hambergite. We attribute the boron isotopic fractionation to the longer <B-O > bond-lengths in tourmaline compared with hambergite.

Thermodynamic Impact of Mineral Surfaces on Amino Acid Polymerization: Aspartate Dimerization on Goethite.

This article presents a thermodynamic predictive scheme for amino acid polymerization in the presence of minerals as a function of various environmental parameters (pH, ionic strength, amino acid concentration, and the solid/water ratio) using l-aspartate (Asp) and goethite as a model combination. This prediction is enabled by the combination of the surface adsorption constants of amino acid and its polymer, determined from the extended triple layer model characterization of the corresponding experimental results, with the thermodynamic data of these organic compounds in water reported in the literature. Calculations for the Asp-goethite system showed that the goethite surface drastically shifts the Asp monomer-dipeptide equilibrium toward the dipeptide side; when the dimerization of 0.1 mM Asp was considered in the presence of 10 m2 L-1 of goethite, an Asp dipeptide concentration around 105 times larger was computed to be thermodynamically attainable compared with that in the absence of goethite at acidic pH (4-5) and low ionic strength (0.1 mM NaCl). Under this condition, the dipeptide-to-monomer molecular ratio in the adsorbed state reached 20%. In contrast, no significant enhancement by goethite was predicted at alkaline pH (>8), where the electrostatic interactions of the goethite surface with Asp and Asp dipeptide are weak. Thus, mineral surfaces should have had a significant impact on the thermodynamics of prebiotic peptide bond formation on the early Earth, although the influences likely depended largely on the environmental conditions. Future experimental studies for various amino acid-mineral interactions using our proposed methodology will provide a quantitative constraint on favorable geochemical settings for the chemical evolution on Earth. This approach can also offer important clues for future exploration of extraterrestrial life.

Cumulative Sulfate Loads Shift Porewater to Sulfidic Conditions in Freshwater Wetland Sediment.

It is well established that sulfide can be toxic to rooted aquatic plants. However, a detailed description of the effects of cumulative sulfate loads on sulfide and iron (Fe) porewater geochemistry, plant exposure, and ecological response is lacking. Over 4 yr, we experimentally manipulated sulfate loads to self‐perpetuating wild rice (Zizania palustris) populations and monitored increases in the ratio of sulfur (S) to Fe in sediment across a range of sulfide loading rates driven by overlying water sulfate. Because natural settings are complicated by ongoing Fe and S loads from surface and groundwater, this experimental setting provides a tractable system to describe the impacts of increased S loading on Fe–S porewater geochemistry. In the experimental mesocosms, the rate of sulfide accumulation in bulk sediment increased linearly with overlying water sulfate concentration up to 300 µg‐SO4 cm–3. Seedling survival at the beginning of the annual life cycle and seed mass and maturation at the end of the annual life cycle all decreased at porewater sulfide concentrations between 0.4 and 0.7 µg cm–3. Changes to porewater sulfide, plant emergence, and plant nutrient uptake during seed production were closely related to the ratio of S to Fe in sediment. A mass balance analysis showed that porewater sulfide remained a small and relatively transient phase compared to sulfate in the overlying water and Fe in the sediment solid phase. The results illuminate the evolution of the geochemical setting and timescales over which 4 yr of cumulative sulfate loading resulted in a wholesale shift from Fe‐dominated to sulfide‐dominated porewater chemistry. This shift was accompanied by detrimental effects to, and eventual extirpation of, self‐perpetuating wild rice populations. Environ Toxicol Chem 2019;38:1231–1244. © 2019 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals, Inc. on behalf of SETAC.

Significance of the study of sedimental rock-forming evolution in the Earth history for the reconstruction of the changes of geochemical settings on its surface

Significance of the study of sedimental rock-forming evolution in the Earth history for the reconstruction of the changes of geochemical settings on its surface

Spatial variations of microbial communities in abyssal and hadal sediments across the Challenger Deep.

Microbial communities in hadal sediments are least explored in hadal zone (>6,000 m), especially in the Challenger Deep with high pressure (∼110 M pa at the bottom). In this study, we investigated the microbial communities in the sediments of the slope and trench-axis bottom of the Challenger Deep in the Mariana Trench. Classification of the reads of the 16S rRNA gene amplicons showed vertical distribution of prokaryotic microbial inhabitants from the surface to up to 60 centimeter below surface floor (cmbsf). The most dominant phyla were Proteobacteria, Chloroflexi, Actinobacteria, Planctomycetes and candidate phyla Patescibacteria and Marinimicrobia. Distinct dominant groups in the microbial communities were observed in trench-axis sediment (water depth >8,600 m), compared to the slopes of the Challenger Deep. A sampling site at the northern slope was enriched with archaea from mesophilic Euryarchaeota Marine Group II (MGII) as a biomarker of specific geochemical setting. Among archaeal community, Thaumarchaeota represented by Nitrosopumilus were dominant in the upper layers and diminished drastically in the deeper layers. “Ca. Woesearchaeota”, however, became the dominant group in the deeper layers. Overall, our study provides a better understanding on the pattern of the microbial communities in the deepest hadal sediments on Earth, and highlights the extraordinary diversity still waiting to be discovered.

Чевкинит-(Се) и перрьерит-(Се) из островодужных кварцевых габбро-норит-долеритов интрузива Аю-Даг, Горный Крым

Сhevkinite-(Се) and perrierite-(Се) are minerals characteristic for alkaline magmatic rocks and high-temperature alkaline metasomatites, and yet they are typical accessory minerals in Mesozoic island-arc gabbroids of the Rocky Crimea. Early magmatic mineral associations in these low-alkaline rocks of Ayu-Dag intrusion include accessory Ti—Zr—LREE minerals under-saturated in silica: chevkinite-(Се), perrierite-(Се), zirconolite, baddeleyite. Сhevkinite and perrierite occur as isolated shortly prismatic crystals up to 0.1 mm in size and strongly corroded inclusions in center of late-magmatic allanite-(Ce) crystals. Both minerals are the main concentrators of lanthanides in Aju-Dag gabbroids. Distribution of lanthanides in them is as following: Ce ˃> La > > Nd >> Pr > Gd, Sm, Tb, Dy. Contents of lanthanides in chevkinite and perrierite are identical: Ce (54—58 % REE) — La (25—31 %) — Nd (12—20 %). Contents of Zr, Th, Y, Sc in both polymorphs are identical also. Сhevkinite-(Ce) of isolated crystals is magnesium-dominant; its composition is: [Cе 1.5 La 0.8—0.9 Nd 0.3—0.4 Pr 0.1 (Gd+Sm+Tb+Dy) 0.1 Y 0.2 Sc 0.1 Ca 0.7—0.8 Na 0.1 ] 4 (Mg 0.5 Fe 3+ 0.5 ) 1 × (Fe 3+ 1.1—1.2 Ti 0.4—0.7 Zr 0.1—0.2 Al 0.1—0.2 ) 2 Ti 2 [(Si 3.7—4 Al 0.3—0 ) 4 O 22 ]. Сhevkinite-(Ce) from inclusions in allanite is poor in Mg, its formula is: [Cе 1.5—1.7 La 0.6—0.9 Nd 0.5—0.6 Pr 0.1 (Gd+Sm) 0.1 Y 0.1—0.2 Sc 0.1 Ca 0.6—0.8 Na 0.1 ] 4 × (Fe 2+ 0.6—1 Mg 0—0.1 Fe 3+ 0—0.4 ) 1 (Ti 0.8—0.9 Fe 3+ 0.3—0.7 Al 0.2—0.4 Zr 0—0.2 ) 2 Ti 2 [(Si 3.9—4 Al 0.1—0 ) 4 O 22 ]. Perrierite-(Ce) composing idiomorphic crystals is magnesium-dominant, its composition is: [Cе 1.2—1.6 La 0.6—0.8 Nd 0.3—0.4 Pr 0.1 × (Gd+Sm+Tb+Dy) 0.1 Y 0.1 Sc 0.1 Ca 1.0—1.1 Sr 0—0.1 Na 0.1 ] 4 (Mg 0.9—1 Fe 3+ 0.1—0 ) 1 (Fe 3+ 1.0—1.2 Al 0.2—0.7 Ti 0.3—0.5 Zr 0—0.2 ) 2 Ti 2 × [(Si 3.9—4 Al 0.1—0 ) 4 (O 21.3—21.5 OH 0.7—0.5 ) 22 ]. Perrierite-(Ce) occurring inside allanite is poor in Mg, with the formula: [Cе 1.3—1.5 La 0.7—0.8 Nd 0.3—0.5 Pr 0.1 (Gd+Sm) 0.1 Y 0.1 Sc 0.1 Ca 1.1—1.3 Na 0.1 ] 4 (Fe 2+ 0.4—0.5 Fe 3+ 0.2—0.4 Mg 0.2 ) 1 × (Al 0.5—0.7 Ti 0.3—0.8 Fe 3+ 0.3—0.9 Zr 0.1—0.3 ) 2 Ti 2 [(Si 3.9—4 Al 0.1—0 ) 4 (O 21.4—22 OH 0.6—0 ) 22 ]. Presence of chevkinite, perrierite and zirconolite in unusual geochemical settings may be explained perhaps by poor development of late-magmatic processes in near-surface intrusions of Crimeaʼs Mesozoids. Perhaps, the presence of chevkinite and perrierite, as well as zirconolite in unusual geochemical settings can be explained by poor development of late-magmatic processes in near-surface intrusives of the Mountain Crimea mesozoids.

Formation of magnesium‐smectite during lacustrine carbonates early diagenesis: Study case of the volcanic crater lake Dziani Dzaha (Mayotte – Indian Ocean)

AbstractThe volcanic crater lake of Dziani Dzaha in Mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of Mg‐phyllosilicates associated with carbonate rocks. The Dziani Dzaha is characterized by intense primary productivity, volcanic gases bubbling in three locations and a volcanic catchment of phonolitic/alkaline composition. The lake water has an alkalinity of ca 0·2 mol l−1 and pH values of ca 9·3. Cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon–hydrogen–nitrogen elemental analyzer, X‐ray fluorescence spectrometry and X‐ray powder diffraction. In surface sediments, the content of total organic carbon reaches up to 20 weight %. The mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment. Below 30 cm depth, X‐ray diffraction analyses of the &lt;2 μm clay fraction indicate the presence of a saponite‐like mineral, a Mg‐rich smectite. The saponite‐like mineral accumulates at depth to reach up to ca 30 weight %, concurrent with a decrease of the contents of hydromagnesite and organic matter. Thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions. The formation of the saponite‐like mineral instead of Al‐free Mg‐silicates resulted from high aluminum availability, which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting. Supersaturation of the lake water relative to saponite is especially due to high pH values, themselves derived from high primary productivity. This suggests that a genetic link may exist between saponite and the development of organic‐rich carbonate rocks, which may be fuelled by the input of CO2‐rich volcanic gases. This provides novel insights into the composition and formation of saponite‐rich deposits under a specific geodynamic context such as the Cretaceous South Atlantic carbonate reservoirs.

The last universal common ancestor between ancient Earth chemistry and the onset of genetics.

All known life forms trace back to a last universal common ancestor (LUCA) that witnessed the onset of Darwinian evolution. One can ask questions about LUCA in various ways, the most common way being to look for traits that are common to all cells, like ribosomes or the genetic code. With the availability of genomes, we can, however, also ask what genes are ancient by virtue of their phylogeny rather than by virtue of being universal. That approach, undertaken recently, leads to a different view of LUCA than we have had in the past, one that fits well with the harsh geochemical setting of early Earth and resembles the biology of prokaryotes that today inhabit the Earth's crust.

Simple prebiotic synthesis of high diversity dynamic combinatorial polyester libraries

It is widely believed that the origin of life depended on environmentally driven complexification of abiotically produced organic compounds. Polymerization is one type of such complexification, and it may be important that many diverse polymer sequences be produced for the sake of selection. Not all compound classes are easily polymerized under the environmental conditions present on primitive planets, and it is possible that life’s origin was aided by other monomers besides those used in contemporary biochemistry. Here we show that alpha-hydroxy acids, which are plausibly abundant prebiotic monomers, can be oligomerized to generate vast, likely sequence-complete libraries, which are also stable for significant amounts of time. This occurs over a variety of reaction conditions (temperature, concentration, salinity, and presence of congeners) compatible with geochemical settings on the primitive Earth and other solar system environments. The high-sequence heterogeneity achievable with these compounds may be useful for scaffolding the origin of life.

Geochemical Features and Geodynamic Settings of the Formation of Cretaceous Terrigenous Rocks of the West Sakhalin Terrane

The geochemical composition of Cretaceous terrigenous rocks of the West Sakhalin terrane is analyzed and their paleogeodynamic interpretation is suggested. It is revealed that the rocks are characterized by the low maturity of clastic material. They contain fragments of both volcanomictic and sialic material and in composition correspond to graywackes, being petrogenic or “first cycle” rocks. The geochemical data were generalized and interpreted on the basis of their comparison with compositions of the present-day and ancient rocks formed in known geodynamic settings. The obtained results indicate that terrigenous rocks of the terrane were formed in a pull-apart basin at an active continental margin. The source area, which supplied clastic material in this sedimentation basin during the Berriasian–Danian, included a sialic land made up of granite-metamorphic and sedimentary rocks and the mature (deeply dissected) ensialic Moneron–Samarga island arc, which was accreted to the continental margin at the moment of basin initiation. Sediments were accumulated in general along the continent–ocean boundary against the background of large-scale sinistral transform strike-slip movements of the Izanagi Plate relative to the Eurasian continent.

Microbial analysis of Zetaproteobacteria and co-colonizers of iron mats in the Troll Wall Vent Field, Arctic Mid-Ocean Ridge.

Over the last decade it has become increasingly clear that Zetaproteobacteria are widespread in hydrothermal systems and that they contribute to the biogeochemical cycling of iron in these environments. However, how chemical factors control the distribution of Zetaproteobacteria and their co-occurring taxa remains elusive. Here we analysed iron mats from the Troll Wall Vent Field (TWVF) located at the Arctic Mid-Ocean Ridge (AMOR) in the Norwegian-Greenland Sea. The samples were taken at increasing distances from high-temperature venting chimneys towards areas with ultraslow low-temperature venting, encompassing a large variety in geochemical settings. Electron microscopy revealed the presence of biogenic iron stalks in all samples. Using 16S rRNA gene sequence profiling we found that relative abundances of Zetaproteobacteria in the iron mats varied from 0.2 to 37.9%. Biogeographic analyses of Zetaproteobacteria, using the ZetaHunter software, revealed the presence of ZetaOtus 1, 2 and 9, supporting the view that they are cosmopolitan. Relative abundances of co-occurring taxa, including Thaumarchaeota, Euryarchaeota and Proteobacteria, also varied substantially. From our results, combined with results from previous microbiological and geochemical analyses of the TWVF, we infer that the distribution of Zetaproteobacteria is connected to fluid-flow patterns and, ultimately, variations in chemical energy landscapes. Moreover, we provide evidence for iron-oxidizing members of Gallionellaceae being widespread in TWVF iron mats, albeit at low relative abundances.

Effect of sulfide on As(III) and As(V) sequestration by ferrihydrite

The sulfide-induced change in arsenic speciation is often coupled to iron geochemical processes, including redox reaction, adsorption/desorption and precipitation/dissolution. Knowledge about how sulfide influenced the coupled geochemistry of iron and arsenic was not explored well up to now. In this work, retention and mobilization of As(III) and As(V) on ferrihydrite in sulfide-rich environment was studied. The initial oxidation states of arsenic and the contact order of sulfide notably influenced arsenic sequestration on ferrihydrite. For As(III) systems, pre-sulfidation of As(III) decreased arsenic sequestration mostly. The arsenic adsorption capacity decreased about 50% in comparison with the system without sulfide addition. For As(V) systems, pre-sulfidation of ferrihydrite decreased 30% sequestration of arsenic on ferrihydrite. Reduction of ferrihydrite by sulfide in As(V) system was higher than that in As(III) system. Geochemical modeling calculations identified formation of thioarsenite in the pre-sulfidation of As(III) system. Formation of arsenic thioanions enhanced As solubility in the pre-sulfidation of As(III) system. The high concentration of sulfide and Fe(II) in pre-sulfidation of ferrihydrite system contributed to saturation of FeS. This supplied new solid phase to immobilize soluble arsenic in aqueous phase. X-ray absorption near edge spectroscopy (XANES) of sulfur K-edge, arsenic K-edge and iron L-edge analysis gave the consistent evidence for the sulfidation reaction of arsenic and ferrihydrite under specific geochemical settings.

Geostatistics as a tool to improve the natural background level definition: An application in groundwater

The Natural Background Level (NBL), suggested by UE BRIDGE project, is suited for spatially distributed datasets providing a regional value that could be higher than the Threshold Value (TV) set by every country. In hydro-geochemically dis-homogeneous areas, the use of a unique regional NBL, higher than TV, could arise problems to distinguish between natural occurrences and anthropogenic contaminant sources. Hence, the goal of this study is to improve the NBL definition employing a geostatistical approach, which reconstructs the contaminant spatial structure accounting geochemical and hydrogeological relationships. This integrated mapping is fundamental to evaluate the contaminant's distribution impact on the NBL, giving indications to improve it. We decided to test this method on the Drainage Basin of Venice Lagoon (DBVL, NE Italy), where the existing NBL is seven times higher than the TV. This area is notoriously affected by naturally occurring arsenic contamination. An available geochemical dataset collected by 50 piezometers was used to reconstruct the spatial distribution of arsenic in the densely populated area of the DBVL. A cokriging approach was applied exploiting the geochemical relationships among As, Fe and NH4+. The obtained spatial predictions of arsenic concentrations were divided into three different zones: i) areas with an As concentration lower than the TV, ii) areas with an As concentration between the TV and the median of the values higher than the TV, and iii) areas with an As concentration higher than the median. Following the BRIDGE suggestions, where enough samples were available, the 90th percentile for each zone was calculated to obtain a local NBL (LNBL). Differently from the original NBL, this local value gives more detailed water quality information accounting the hydrogeological and geochemical setting, and contaminant spatial variation. Hence, the LNBL could give more indications about the distinction between natural occurrence and anthropogenic contamination.