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The late Neoarchean, ~2.53 to 2.51 Ga Dagushan banded iron formation (BIF), is a typical Algoma-type BIF located in the northeast part of the North China craton. Despite having undergone upper greenschist to lower amphibolite facies metamorphism, the Dagushan BIF retains evidence of varied depositional facies, making it an ideal archive to evaluate the paleomarine environment and the paragenesis of the ore minerals. A transition from oxide to silicate to carbonate facies BIF is evident in a northward direction. The mineralogical composition shifts from magnetite and quartz in the south through a magnetite-quartz-cummingtonite/stilpnomelane assemblage in the transition zone to magnetite-siderite in the north. Such a distinct distribution of mineralogical facies correlates well with the depositional environment of the BIF. The carbonate facies BIFs formed in a near-shore, proximal environment, whereas the oxide and silicate facies BIF assemblages formed in deeper waters, distal to the paleoshoreline. The BIF samples display characteristic seawater-like rare earth element + yttrium (REE + Y) profiles with positive La and Y anomalies and heavy REE enrichment relative to the light REEs when normalized to post-Archean Australian shale. Positive Eu anomalies suggest a high-temperature hydrothermal contribution to the BIF. The absence of a negative Ce anomaly in nearly all samples, coupled with positive δ56Fe in magnetite in all mineralogical facies, indicates a dominantly anoxic water column contemporaneous with deposition of the BIF. At ~2.53 Ga in the Anshan area, seawater was mostly anoxic and rich in ferrous iron. Dissolved ferrous iron in upwelling hydrothermal fluids was oxidized and precipitated as Fe(III) oxyhydroxides in the photic zone leading to BIF formation. Proximal to hydrothermal vents, magnetite formed via the reaction of Fe(III) oxyhydroxides and aqueous Fe(II) supplied from the hydrothermal fluids and microbial dissimilatory iron reduction (DIR) coupled to organic carbon oxidation. Proximal to a paleoshoreline, siderite formed through DIR, as evidenced by the depleted δ13C values and the presence of graphite. Silicates, such as stilpnomelane and cummingtonite, are considered to be the metamorphic products of early diagenetic silicates (e.g., nontronite) that formed in the water column from admixtures of Fe(III) oxyhydroxides and amorphous silica.

Sedimentation velocities of various chemical sediments are typically calculated using Stokes's law. However, applying it to chemical sediments that form in situ in the water column is not ideal because the particle properties do not fulfill many of the assumptions underpinning the applicability of Stokes' law. As a consequence, it has been difficult to predict the sedimentation rate of ancient chemical sediments, such as Precambrian banded iron formations (BIF), because their primary sediments likely comprised aggregates of ferric hydroxides, such as ferrihydrite [Fe(OH)3], and marine bacterial biomass, including cyanobacteria. In this work we use a new experimental method to address the mechanisms by which primary BIF sediment, formed by the oxidation of dissolved Fe(II) by O2 and simultaneously incubated with cyanobacterium Synechococcus sp., were deposited to the Archean ocean. Specifically, we formed the aggregates in situ over a wide range of initial pH and Fe(II) concentrations, continuously recorded the entire settling processes of aggregates under each condition, and then processed the data in MATLAB according to different settling mechanisms. Our results demonstrate that ferrihydrite–cyanobacteria aggregates settled to the ocean floor either through the formation of uniformly descending concentration fronts or through convective plumes. The sedimentation mechanism depended on both initial Fe(II) concentration and the pH. Correspondingly, two algorithms were developed to characterize the sedimentation velocity. These algorithms tracked the alteration of light intensity from low to high as sediments descended from an initially homogeneous state through a water tank, and as well calculated the average light intensity over time, from which vertical time series were constructed allowing calculation of the sedimentation velocity. Our method not only provides an accurate estimation of the in situ sedimentation velocity of cell–mineral aggregates, but also provides new insights into the physical mechanisms by which the primary sediments composing BIF were deposited.

Interactions between microorganisms and clay minerals influence the transport and cycling of metal contaminants in both marine and terrestrial environments. The present study was conducted to quantify the adsorption of dissolved cadmium, Cd(II), under seawater-like conditions to the marine cyanobacterium Synechococcus sp. PCC 7002, three common clay minerals (kaolinite, montmorillonite and illite), as well as cell-clay aggregates. We show here that the Synechococcus-only experiments removed the most Cd above pH 5.5, followed in decreasing order by aggregates of 50% cells:50% individual clays, aggregates of cells and all 3 clays, and individual clays. Electron microscope imaging showed that clays associated in a tangential edge-on orientation to the cells in Synechococcus-clay mineral aggregates. A non-electrostatic surface complexation modeling approach was used to fit Cd adsorption onto Synechococcus cells and individual clay minerals. The resulting Cd binding constants were then used in consort with surface functional group pKa values and site concentrations to accurately predict the extent of Cd adsorption onto the Synechococcus-clay mineral aggregates using the component additivity (CA) approach. We observed that the addition of cyanobacterial cells to clay mineral suspensions led to significantly larger mean aggregate sizes of clay minerals, enhancing the clay sedimentation rate. Although specifically focused on Cd, our study indicates that the ratio of bacterial plankton to clay minerals is an important determinant in terms of understanding the rate with which metals are transferred from the water column to the seafloor.

Biochar (BC) and magnetite (Fe3O4) nanoparticles (MNP) have both received considerable recent attention in part due to their potential use in water treatment. While both are effective independently in the removal of a range of anionic metals from aqueous solution, the efficacy of these materials is reduced considerably at neutral pH due to decreased metal adsorption and MNP aggregation. In addition to synthetic metal oxide–biochar composites for use in treatment and remediation technologies, aggregates may also occur in nature when pyrolytic carbon is deposited in soils. In this study, we tested whether magnetite synthesized in the presence of biochar leads to increased removal efficiency of hexavalent chromium, Cr(VI), at the mildly acidic to neutral pH values characteristic of most natural and contaminated aqueous environments. To do so, magnetite nanoparticles and biochar produced from ground willow were synthesized to form composites (MNP–BC). Batch studies showed that MNP–BC markedly enhanced both adsorption and reduction of Cr(VI) from aqueous solution at acidic to neutral pH as compared to MNP and BC separately, suggesting a strong synergetic effect of hybridizing Fe3O4 with BC. Mechanistically, the Cr(VI) removal processes occurred through both adsorption and intraparticle diffusion followed by reduction to Cr(III). Synchrotron-based X-ray absorption spectroscopy analyses confirmed that Cr(VI) was reduced at the surface of MNP–BC, with electrons derived directly from both biochar and magnetite at low pH, while at near-neutral pH, biochar increased Cr(VI) reduction by inhibiting MNP aggregation. Extended X-ray absorption fine structure fitting results confirmed that the Cr(III) precipitates consist of Cr(OH)3 and chromite (Cr2FeO4) nanoparticles. Our results demonstrate that MNP–BC composites have great potential as a material for the treatment of chromate-containing aqueous solutions across a wide range of pH values, and provide information valuable broadly relevant to soils and sediments that contain biochar.

The Upper Carboniferous Ortokarnash manganese ore deposit in the West Kunlun orogenic belt of the Xinjiang province in China is hosted in the Kalaatehe Formation. The latter is composed of three members: (1) the 1st Member is a volcanic breccia limestone, (2) the 2nd Member is a sandy limestone, and (3) the 3rd Member is a dark gray to black marlstone containing the manganese carbonate mineralization, which, in turn, is overlain by sandy and micritic limestone. This sequence represents a single transgression-regression cycle, with the manganese deposition occurring during the highstand systems tract. Geochemical features of the rare earth elements (REE+Y) in the Kalaatehe Formation suggest that both the manganese ore and associated rocks were generally deposited under an oxic water column with Post-Archean Australian Shale (PAAS)-normalized REE+Y patterns displaying characteristics of modern seawater (e.g., light REE depletion and negative Ce anomalies). The manganese ore is dominated by fine-grained rhodochrosite (MnCO3), dispersed in Mn-rich silicates (e.g., friedelite and chlorite), and trace quantities of alabandite (MnS) and pyrolusite (MnO2). The replacement of pyrolusite by rhodochrosite suggests that the initial manganese precipitates were Mn(IV)-oxides. Precipitation within an oxic water column is supported by shale-normalized REE+Y patterns from the carbonate ores that are characterized by large positive Ce (>3.0) anomalies, negative Y (~0.7) anomalies, low Y/Ho ratios (~20), and a lack of fractionation between the light and heavy rare earth elements ((Nd/Yb)PAAS ~0.9). The manganese carbonate ores are also 13C-depleted, further suggesting that the Mn(II) carbonates formed as a result of Mn(III/IV)-oxide reduction during burial diagenesis.

Methyl tert-butyl ether (MTBE) was used as a replacement for Pb in gasoline before it was found to be harmful, and for this reason, these two contaminants often coexist in groundwater at older fuel...

Casts and molds of soft-bodied organisms in Ediacaran sandstones (“Ediacara-style” fossilization) have played an important role in reconstruction of the emergence and radiation of early complex macroscopic life. However, the preservational processes responsible for the Ediacara fossil record are still vigorously debated. Whereas classic studies proposed fossilization via rapid sulfide mineralization of carcass and matground surfaces, a more recent view posits silica as the key mineral involved in their preservation. We performed experiments in which a variety of soft-bodied organisms were exposed to silica-rich solutions at concentrations considered characteristic of Ediacaran seawater (2 mM). Our results document continuous precipitation of amorphous silica onto the surfaces of these organic tissues under constant and normal marine pH values (7.8). Mineral formation was accompanied by a progressive decrease in the dissolved silica (DSi) concentration of the experimental solution to levels well below amorphous silica saturation. Additionally, we find that the magnitude of silica precipitation is correlated to each organism’s functional-group chemistry, as measured by potentiometric acid-base titrations. We suggest that a wide range of soft-bodied organisms were prone to silicification in Ediacaran marine environments characterized by anactualistically high DSi concentrations. This provides further support for the model that the extraordinary moldic preservation of the Ediacara Biota was promoted by early silica cementation and that this mode of preservation can offer an accurate glimpse into the composition of those early animal ecosystems.

Past changes in the halogen composition of seawater are anticipated based on the differing behavior of chlorine and bromine that are strongly partitioned into seawater, relative to iodine, which is extremely depleted in modern seawater and enriched in marine sediments due to biological uptake. Here we assess the use of chert, a chemical sediment that precipitated throughout the Precambrian, as a proxy for halide ratios in ancient seawater. We determine a set of criteria that can be used to assess the primary nature of halogens and show that ancient seawater Br/Cl and I/Cl ratios can be resolved in chert samples from the 2.5 Ga Dales Gorge Member of the Brockman Banded Iron Formation, Hamersley Group, Western Australia. The values determined of Br/Cl ~2 × 10-3 M and I/Cl ~30 × 10-6 M are comparable to fluid inclusions in hydrothermal quartz from the 3.5 Ga North Pole area, Pilbara Craton, Western Australia, that were the subject of previous reconstructions of ancient ocean salinity and atmospheric isotopic composition. While the similar Br/Cl and I/Cl values indicate no substantial change in the ocean halide system over the interval 2.5–3.5Ga, compared to modern seawater, the ancient ocean was enriched in Br and I relative to Cl. The I/Cl value is intermediate between bulk Earth (assumed chondritic) and the modern seawater ratio, which can be explained by a smaller organic reservoir because this is the major control on marine iodine at the present day. Br/Cl ratios are about 30% higher than both modern seawater and contemporary seafloor hydrothermal systems, perhaps indicating a stronger mantle buffering of seawater halogens during the Archean.

Several Late Devonian sedimentary successions host pyrite with highly positive sulfur isotope values (δ34Spyrite). These anomalous values have been linked to marine anoxia, low sulfate concentrations in seawater, or aerobic re-oxidation of dissolved sulfide within well-oxygenated bottom waters in a local depositional environment. Implicit to these previous models is the assumption of a biogenic genesis from microbial sulfate reduction (MSR) of these pyrites, which, in turn, can be used to understand the biogeochemical sulfur cycle and reconstruct paleoenvironmental conditions. In South China, the Late-Devonian bulk-rock sulfur isotope excursion occurs in the lower Fammenian Wuzhishan Formation of the Youjiang Basin, in both the sedimentary limestone that corresponds to the Late triangularis conodont zone at the Fuhe section and the sediment-hosted Xialei Mn deposit that lies between the Late triangularis and crepida conodont zones. To further constrain the significance of these 34S-enriched pyrites, we carried out detailed textural and in-situ sulfur isotope examinations of pyrites in the Xialei Mn deposit. We found one type of pyrite in siliceous limestone referred to as the host rocks and three distinct pyrite types in Mn ores: Type I – pyrites with Mn-carbonate inclusion-rich cores; Type II – pyrite overgrowths surrounding an “inclusion-rich” core; and Type III – relatively inclusion-poor and mainly subhedral disseminated pyrite grains. Type I pyrites in Mn ores have negative δ34Spyrite values from −7.8‰ to −2.0‰. Types II and III pyrites, and pyrites in the host rock are characterized by highly positive δ34Spyrite values (+4.2‰ to +37.6‰), with some values being higher than that of contemporaneous seawater sulfate. Type I pyrites with negative δ34S values likely formed via MSR in a closed porewater system, driven by increased sedimentation rates during a sea level lowstand. The other pyrites with highly positive δ34S values appear to have formed during secondary hydrothermal alteration, with their sulfur sourced through thermochemical sulfate reduction (TSR) of sulfate from dissolved barites in the underlying Liujiang Formation. This implies that the positive δ34S values of the Late Devonian pyrites from the Wuzhishan Formation represent a later alteration event caused by post-depositional hydrothermal fluids, and may lack a direct biogeochemical connection to the surface sulfur cycle. These results provide an important perspective on the origin of variability in sulfur isotope records and suggest the need for careful petrographic screening and micrometer-scale analysis of sedimentary units used to reconstruct paleoenvironmental conditions.

Ratios of phosphorous (P) to iron (Fe) in Precambrian banded iron formations (BIFs) have previously been used to estimate dissolved seawater phosphate concentrations in the ancient oceans. Such studies rely on an assumed composition of the primary iron minerals, the concentrations of the major ions in seawater, and empirical partitioning coefficients for phosphate sorption to Fe(III) (oxyhydr)oxides. There is limited data, however, regarding the post-depositional stability of phosphate associated with presumed primary BIF iron minerals, such as ferrihydrite under low-grade metamorphic temperature and pressure conditions. Here we experimentally formed ferrihydrite in the presence of silica, which was abundant in the Precambrian oceans, and then incubated it at 170 °C and 1.2 kbar in the presence or absence of organic carbon (Corg; either glucose or microbial biomass) as a proxy for ancient planktonic biomass. We found that the post-metamorphic mineral assemblage resulting from thermochemical Fe(III) reduction of Si-doped ferrihydrite depended on Corg reactivity: In the presence of highly reactive glucose, siderite, magnetite, and vivianite were formed, with less than 1.2 mol% of phosphate (0.5 M NaCl extractable) being mobilized. In contrast, the reaction of Si-doped ferrihydrite with less reactive microbial biomass resulted in the formation of hematite and siderite, but not vivianite, and approximately 10 mol% of phosphate was remobilized into the pore fluids. Collectively, our data suggest that the fidelity with which BIFs record ancient oceanic phosphate concentrations depends on the mineralogy and diagenetic history of individual BIFs but should be reliable within 10%.

The early evolution of metazoans has been reconstructed by studies on exceptionally preserved molds in siliciclastic rocks from the Ediacaran Period. However, there remains considerable controversy regarding the formation mechanisms of this unusual ‘Ediacaran-style’ preservation. Proposed hypotheses usually include early authigenesis of minerals, but evidence for this is scarce. In a recently discovered deposit of Ediacaran biota in Brazil, we show that the classic moldic preservation is related to clay mineral authigenesis. Specifically, these clays originated from the alteration of original pyroclastic sediments, likely enhanced by microbial activity, leading to early illitization and morphological templating of the fossiliferous surfaces at a micrometric scale. Such high-fidelity preservation was made possible by rapid burial during volcanic events and the in-situ templating of tissue by clays via microbially-mediated mineralization. This newly described Lagerstätte demonstrates that a number of minerals can facilitate preservation, and that perhaps ‘Ediacaran-style’ preservation result from different processes leading to the same broad style of preservation.

The Paleoproterozoic Lagoa do Riacho manganese deposit in the Borborema Province is central to unravelling the genesis of Paleoproterozoic manganese deposits, Paleoproterozoic redox evolution and genetic relationship to the West African Craton. With manganese contents averaging 35 wt%, the Lagoa do Riacho manganese unit is lithologically subdivided into oxidized and silicate manganese ores and a garnetite (garnet-rich rocks) subgroup. The oxidized manganese ore consists mainly of pyrolusite, manganese-oxyhydroxide and spessartine. The silicate manganese ore has rhodonite, tephroite and spessartine. Garnetite contains spessartine and minor quartz and rhodonite. The main host rocks include garnet quartzite and graphite-bearing pelitic gneiss. The composition of carbonate inclusions in spessartine-rhodonite and Ca-Mn fractionation between garnet core and rim suggests all these manganese-rich rocks probably formed from protoliths of marl composition. Biogenic, syn-sedimentary graphite in the manganese-rich rocks and their host rocks indicates organic matter was deposited along with the marl protolith. Within this general context and through chemical-petrological relations, we outline an initial environment involving anoxic sediments' deposition (eg., organic matter-rich manganese marl and black shales, and wackes) on the seafloor. The presence of spessartine, rhodonite, and tephroite suggests peak metamorphic conditions reached amphibolite facies conditions (6–7 kbars and 550–700 °C). Amphiboles indicate later retrograde stages and carbonate veins suggest a possible hydrothermal stage after manganese deposition. The metamorphosed manganese-rich rocks were upgraded to medium-grade manganese ores during exposure to surface water. Our results indicated that the manganese-rich rocks and their host rocks were generated in a sedimentary context, possibly in a marine setting with carbonate and organic matter contribution.

This study investigated the biogeochemistry of uranium (U), and the co-occurring elements nickel (Ni) and arsenic (As), in colloids and sediments from two meromictic mine pit lakes that have considerably different depths and geochemistry. In order to characterize the processes controlling metal speciation and cycling in the pits, the distribution and speciation of the elements in colloidal size fractions were analyzed using micro- and ultrafiltration in combination with transmission electron microscopy. Sediment traps collected fresh sediments over the course of one year below the chemocline of the pit lakes and were subsequently analyzed by scanning electron microscopy (SEM) for morphology and chemical digestions. The most common particles found in the shallower pit consisted of Ca-O and Fe-O colloids, while the particles in the deeper pit were composed of Ca-S-O. Filtration results showed a higher abundance of metals in larger colloidal fractions in aged samples, suggesting that colloids can act as metal accumulators. Sediment traps showed the formation of Fe-O, Fe-S, Al-Si, and Ce-P phases, which were observed to sorb U and Ni. The overall U removal was calculated to be 0.9 g/m2/year in both pits, despite considerably different geochemical conditions between the two, and the maximum removal rates for As (shallow pit) and Ni (deeper pit) were 4.7 g/m2/year and 0.6 g/m2/year, respectively. Bottom sediments were also collected from both pits, and characterized using sequential extractions, SEM, synchrotron-based X-ray absorption spectroscopy and Laue diffraction techniques. These techniques showed that the stability of metals in the sediment follows the order Ni

High residual phosphorus in anaerobically digested source-diverted blackwater allows for increased phosphorus recovery through struvite (NH4MgPO4·6H2O) precipitation. Due to the complex matrix of blackwater, recovered products inevitably contain co-precipitates of other wastewater components that have not been well characterized. The potential hazards present include pathogens, antimicrobial resistance genes (ARGs), organic and inorganic compounds. In this study, several important struvite precipitation conditions (pH, Mg2+:PO43− molar ratio and MgCl2 dosing rate) were tested to determine their impact on phosphorus recovery, while taking into account the co-precipitation of enteric bacteria, ARGs and micro- and macronutrients. Results demonstrate that phosphorus recovery efficiency does not correlate to the degree of metal/microbial co-precipitation. Both PO43− recovery and co-precipitates were affected by the operation conditions examined, and the optimal condition for nutrient recovery from blackwater was at pH 9, a 1.5:1 Mg2+:PO43− molar ratio, and a dosing rate of 24 mM min−1. Although previous struvite recovery studies have identified optimal conditions for phosphorus recovery, limited information is available on the process optimization to minimize the potential risks. Downstream application of recovered struvite inevitably contains co-precipitated hazards that if not assessed can result in undesirable public health outcomes. It is necessary to evaluate struvite application and public health exposure to identify key steps that reduce public health impacts. Hence examination of struvite precipitation parameters on enhanced phosphorus recovery, along with co-precipitates, is necessary when considering the public health risk associated with efficiently recovering residual nutrients.

Highlights • The adsorption of Co, Ni, Cu, and Zn to Synechococcus sp PCC 7002 was studied using a surface complexation modelling approach. • A surface complexation model was developed to determine the thermodynamic binding constants of Co, Ni, Cu, and Zn to Synechococcus. • The surface complexation model was able to accurately predict the competitive adsorptionof the four metals to Synechococcus. • Synechococcus could have been an important exit channel for trace elements into ancient sediments such as BIF Marine bacterial plankton play a key role in elemental cycling through their ability to bind, assimilate, metabolize, and modify the redox state of trace metals in seawater. Of those processes, arguably the least studied are the mechanisms underpinning trace metal adsorption to planktonic marine bacteria, despite a plethora of literature pertaining to terrestrial species. Recently, Liu et al. (2015) demonstrated that the marine cyanobacterium Synechococcus sp. PCC 7002 has the capacity to remove appreciable amounts of Cd2+, a proxy for other divalent cations, from seawater by adsorption. In this study, we build on that work and employ a surface complexation modelling (SCM) approach using titration and pH adsorption edge experiments to calculate the thermodynamic binding constants of four bioessential transition metals (Co, Ni, Cu, Zn) to Synechococcus in simulated seawater. Based on the titration results, the major functional groups involved in metal binding were carboxyl groups with a pKa of 5.59 and phosphoryl groups with a pKa of 7.61. Metal adsorption experiments indicate that Synechococcus can bind considerable concentrations of Zn, Cu, Ni, and Co at pH 8. When all four metals are simultaneously added to solution, the same adsorption pattern of Zn > Cu > Ni > Co is maintained, and accurately predicted by the SCM. Based on average marine cell densities and turnover rates of Synechococcus cells in the photic zone, we calculate that Synechococcus, in the absence of competing ligands such as dissolved organic matter (DOM), has the theoretical capacity to remove nearly all of the free metal cations from seawater. These observations highlight the surface reactivity of marine cyanobacteria as a potentially important vector for the transfer of dissolved metals from the photic zone to deeper waters or the seafloor in modernoceans, but they also have implications for the Precambrian oceans as sinking cyanobacteria could have acted as an exit channel for trace elements into ancient sediments including banded iron formations (BIF).

Clays are one of the most abundant minerals on the Earth’s surface, and they play a significant role in global trace element cycling because of their ability to adsorb and incorporate a range of metal cations. However, to what extent clay surface reactivities are affected by changes in pH of the surrounding environment remains poorly understood. To address this uncertainty, we investigated three common clay minerals (kaolinite, illite, and montmorillonite) and compared their surface properties and metal binding capacity prior to and after acidic treatment (pH ranges from 0 to 7). We report that (1) the morphologies and crystal structures of the three studied clay minerals do not differ markedly after exposure to an acidic solution, (2) all three clays display increases in surface area after the pH 0 acidic treatment, and (3) cadmium adsorption to all three clays decreased after the pH 0 acidic treatment. Our results suggest that the variation in metal adsorption capacity is related to acidic modification ...

Development of Archean paleosols and patterns of Precambrian rock weathering suggest colonization of continents by subaerial microbial mats long before evolution of land plants in the Phanerozoic Eon. Modern analogues for such mats, however, have not been reported, and possible biogeochemical roles of these mats in the past remain largely conceptual. We show that photosynthetic, subaerial microbial mats from Indonesia grow on mafic bedrocks at ambient temperatures and form distinct layers with features similar to Precambrian mats and paleosols. Such subaerial mats could have supported a substantial aerobic biosphere, including nitrification and methanotrophy, and promoted methane emissions and oxidative weathering under ostensibly anoxic Precambrian atmospheres. High C-turnover rates and cell abundances would have made these mats prime locations for early microbial diversification. Growth of landmass in the late Archean to early Proterozoic Eons could have reorganized biogeochemical cycles between land and sea impacting atmospheric chemistry and climate., Microbes venturing onto land could have impacted biogeochemical cycles billions of years before terrestrial plants, but insight into this process on ancient Earth has remained elusive. With the discovery and analysis of microbial mats analogous to those of the Precambrian, Finke and colleagues infer how these microbial jungles likely shaped ecology and climate.

Asphalt concrete is one of the most important building materials in the modern world, but the leaching potential of metals from this composite material to the environment is poorly understood. In this study, metals leaching from four hot-mix asphalt samples were analyzed: two fresh samples of low-traffic and high-traffic composition and their weathered equivalents collected from roads in the city of Edmonton, Alberta, Canada. A sequential extraction, based on the Community Bureau of Reference method, was applied to study the speciation and potential mobility of metals and metalloids in those samples. Major trace metals identified in all four samples were Mn, P, Ba, Sr, Zn, V, and Ni, with the highest metals concentrations generally found in weathered asphalt concrete. Of the major trace metals, P, Mn, Sr, and Zn were relatively mobile, having large portions of their total concentrations in the exchangeable/acid-soluble and reducible fractions. When considering the most mobile fraction (exchangeable/acid soluble) and using Canada as a model country, up to 180 t P, 440 t Mn, 50 t Ba, 36 t Sr, 11 t Zn, and 0.11-3.2 t of other metals and metalloids (including Cr, Ni, Cu, As, and Pb) could potentially leach from the top layer of Canada's total of paved public roads. To place these amounts into perspective, they were estimated to make up to 22‰ of Canada's annual release numbers into soil, water and air for these same metals and metalloids. However, they are concentrated in a small area around roads and highways, creating the potential for localized soil and groundwater contamination.

The Jiertieke-Zankan area lies in the Taxkorgan region (West Kunlun Mountains) of Xinjiang province, northwestern China. The area contains the largest commercial iron ore belt in the region. Furthermore, this iron ore belt hosts the youngest banded iron formation identified to date (the Jiertieke iron deposit), which was deposited during the Early Cambrian. Previous studies have mainly provided evidence regarding the depositional environment and formational age of this area, but the tectonic setting and geochronology of the ore deposits remains poorly studied. Here, we provide an integrated examination of high-precision SIMS zircon U-Pb dating, along with the major and trace element geochemistry of the metasedimentary rock (a biotite-quartz schist) that host the iron ores. The Jiertieke area is characterized by bedded metapelites that contain detrital zircons as young as 546 Ma, whereas the Zankan area contains metapsammites, in addition to the metapelites, as young as 541 Ma. Multiple detrital zircons age groups ranging from 700–1100 Ma, 1600–1850 Ma, and 2250–2500 Ma, indicate the input of ancient clastic material to the basin. Based on provenance indicators and Nd isotopic data, they suggest that the metasedimentary rocks are the product of an undifferentiated felsic terrain, likely ancient upper continental crust, being eroded. Given the geochronological and geochemical features of the metasedimentary rocks, and previously identified bimodal volcanics within the region, we suggest that the tectonic setting of the Zankan-Jiertieke area was a back-arc rift basin during the Late Neoproterozoic to Early Cambrian.

International audience; There is longstanding controversy about the genesis of magnetite in banded iron formations (BIFs), particularly concerning whether it is of abiogenic or biogenic origin. The composition of trace elements within magnetite produced by magnetotactic bacteria has been proposed as a promising marker for its biological origin; however further experimental evidence is required to investigate whether this also holds true for iron-metabolizing bacteria. Here, we compared the behavior of zinc (Zn) and nickel (Ni) in magnetite produced either abiogenically (ferrihydrite reacting with dissolved Fe2+) or biogenically (ferrihydrite reacting with dissolved Fe2+ generated via the dissimilatory iron-reducing bacterium - DIRB - Shewanella oneidensis MR-1). These abiotic and biotic incubations were applied for transforming three ferrihydrite (Fh) substrates: (1) Control Fh without added trace elements; (2) Zn-coprecipitated ferrihydrite (ZnFh); and (3) Ni-coprecipitated ferrihydrite (NiFh) in both NaHCO3 and HEPES buffer. X-ray diffraction revealed magnetite as the dominant transformation product for all reaction conditions, while siderite was only detected in experiments containing NaHCO3 as buffer. Based on similar initial dissolved Ni and Zn concentrations, we found opposing behavior of Zn and Ni where Zn showed enrichment in abiogenic magnetite, while Ni was more enriched in biogenic magnetite. Combining supposed Zn and Ni concentrations in Precambrian seawater and magnetite mineral grains from three formations deposited at different ages (i.e. 3.75 Ga Nuvvuagittuq Supracrustal Belt, 2.45 Ga Weeli Wolli Iron Formation, and 1.88 Ga Sokoman Iron Formation), we reconstructed potential Zn and Ni partition coefficients between Precambrian seawater and magnetite minerals. The reconstructed potential Zn and Ni partition coefficients suggested that these magnetite minerals from three formations were abiogenic. These differing behaviors suggest the possibility of using Zn and Ni partition coefficient ratios as a means of determining the presence or absence of DIRB in the formation of ancient magnetite deposits.

The co-contamination of the environment by metals and organic pollutants is a significant concern, and one such example is lead (Pb) and methyl tert-butyl ether (MTBE) due to their historic use as fuel additives. Clinoptilolite is an abundant and efficient zeolite for metal removal, but the potential interference of co-existing organic pollutants on metal removal, such as MTBE, have rarely been discussed. In this study, a combination of batch sorption tests and synchrotron-based X-ray absorption spectroscopic analyses were employed to investigate Pb sorption mechanism(s) onto clinoptilolite in the presence and absence of MTBE. A comparison was made to synthetic ZSM-5 zeolite to gain insights into differences in Pb binding mechanisms between hydrophilic (clinoptilolite) and hydrophobic (ZSM-5) zeolites. Site occupancy and surface precipitation contributed equally to Pb removal by clinoptilolite, while surface precipitation was the main Pb removal mechanism for ZSM-5 followed by site occupancy. Despite the negligible effect of 100 mg/L MTBE on observed Pb removal from solution by both zeolites, a surface-embedded Pb removal mechanism, through the Mg site on clinoptilolite surface, arises when MTBE is present. This study provides an understanding of atomic-level Pb uptake mechanisms on zeolites, with and without co-contaminating MTBE, which aids in their application in water treatment at co-contaminated sites.

Highlights • The adsorption of Cd to Synechocystis sp. PCC 6803 was investigated at both marine and freshwater ionic strength. • The thermodynamic binding constants of Cd to Synechocystis were calculated using a surface complexation modeling approach. • Synechocystis and other planktonic cyanobacteria may be an important vector of trace metals transport to marine settings. Cyanobacteria are abundant in nearly every surface environment on Earth. Understanding their chemical reactivity and metal binding capacity with varying ionic strength (IS) is paramount to understanding trace metal cycling in natural environments. We conducted an investigation on the cell surface reactivity of the freshwater cyanobacterium Synechocystis sp. PCC 6803 at freshwater (0.01 M NaCl) and marine (0.56 M NaCl) IS. Potentiometric titration data were used to develop a multiple discrete site, non-electrostatic surface complexation model (SCM), and corresponding cell surface functional group identities were verified using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Synechocystis cells were best modeled in FITEQL 4.0 using a non-electrostatic 2-site protonation model. Cadmium (Cd) adsorption experiments paired with SCM was utilized to calculate the binding constants of Cd. Synechocystis surface functional groups demonstrated a stronger affinity for Cd across the entire pH range studied (3–9) at freshwater IS, with the greatest difference at circumneutral pH (6–8) where Cd adsorption in freshwater IS was 60% greater than at marine IS. These data combined with the ubiquitous distribution of Synechocystis in freshwater and brackish environments suggest that these organisms could play an important role in trace metal cycling in environments with large salinity gradients, such as estuaries and deltas, and could act as a transport mechanism for trace metals from terrestrial to marine settings.

The Zaonega Formation in northwest Russia (~2.0 billion years old) is amongst the most complete successions that record the middle of the Palaeoproterozoic era. As such, geochemical data from the formation have played a central role in framing the debate over redox dynamics in the aftermath of the Great Oxidation Event (GOE). However, uncertainty over local redox conditions and the degree of hydrographic restriction in the formation has led to contradictory interpretations regarding global oxygen (O2) fugacity. Here, we provide new iron (Fe) isotope data together with major and trace element concentrations to constrain the local physiochemical conditions. The Zaonega Formation sediments show authigenic Fe accumulation (Fe/Al ≫ 1 wt.%/wt.%) and δ56Fe ranging from −0.58‰ to +0.60‰. Many of the data fall on a negative Fe/Al versus δ56Fe trend, diagnostic of a benthic Fe shuttle, which implies that Zaonega Formation rocks formed in a redox-stratified and semi-restricted basin. However, basin restriction did not coincide with diminished trace metal enrichment, likely due to episodes of deep-water exchange with metal-rich oxygenated seawater, as evidenced by simultaneous authigenic Fe(III) precipitation. If so, the Onega Basin maintained a connection that allowed its sediments to record signals of global ocean chemistry despite significant basinal effects.

The ~2.22–2.06 Ga Lomagundi Event was the longest positive carbon isotope excursion in Earth’s history and is commonly interpreted to reflect perturbations in continental weathering and the phosphorous cycle. Previous models have focused on mechanisms of increasing phosphorous solubilization during weathering without focusing on transport to the oceans and its dispersion in seawater. Building from new experimental results, here we report kaolinite readily absorbs phosphorous under acidic freshwater conditions, but quantitatively releases phosphorous under seawater conditions where it becomes bioavailable to phytoplankton. The strong likelihood of high weathering intensities and associated high kaolinite content in post-Great-Oxidation-Event paleosols suggests there would have been enhanced phosphorus shuttling from the continents into marine environments. A kaolinite phosphorous shuttle introduces the potential for nonlinearity in the fluxes of phosphorous to the oceans with increases in chemical weathering intensity., Expanded phosphorus availability possibly triggered a marine bioproduction boom after 2.3 billion years ago, but its delivery mechanisms remain unclear. Here we propose a kaolinite shuttle which efficiently adsorbs phosphorus in continental weathering settings and releases it under marine conditions.

Agreement is often in short supply within the community of researchers asking questions about Earth’s earliest history. One exception, however, is the near-consensus view that oxygen (O2) was mostly absent from the atmosphere and oceans before about 2.4 to 2.3 Ga—the so-called Great Oxidation Event (GOE). It is also widely accepted that the cycles for iron (Fe) and manganese (Mn) in the early oceans are essential themes in our still nascent understanding of the earliest chapters of life’s coevolution with its host environments at Earth’s surface. Yet, despite this community accord over the importance of Fe and Mn, we still know very little about the principal pathways of oxidation for these metals and the implications for early O2. There are three fundamental possibilities: 1) biologically mediated reactions in the presence of even small amounts of O2, 2) anoxygenic photosynthesis with Fe(II) or Mn(II) as electron donors yielding oxidized forms of those metals as by-products, and 3) sunlight-catalyzed photochemical oxidation by ultraviolet (UV) radiation without contributions from life or O2. The latter is perhaps the least appreciated among these three, even though abiotic reactions likely had elevated importance in early oceans as life was finding its first footholds. Liu et al. (1) refocus attention on the photochemical possibility by exploring the details of photooxidation of Mn(II) in the mineral rhodochrosite (MnCO3; Fig. 1) in a series of elegant experiments and by examining the implications of their results in terms of oceanic and atmospheric chemistry during the Archean Eon (4.0 to 2.5 Ga). Fig. 1. Gem-quality crystalline rhodochrosite (MnCO3)—an eye-catching version of the far more mundane small mineral grains that may have settled through the early oceans and accumulated on the seafloor. As the grains settled and accumulated, UV radiation may have oxidized the … [↵][1]1To whom correspondence may be addressed. Email: timothy.lyons{at}ucr.edu. [1]: #xref-corresp-1-1

Given the high surface reactivity of clay minerals, it is assumed that flocculation will lead to metal accumulation in marginal marine settings. However, the degree of metal sorption to clays is impacted by solution pH and ionic strength, and it remains unknown whether riverine clays indeed serve as a metal sink once they encounter seawater where pH and ionic strength markedly increase. Here, we conducted cadmium (Cd) adsorption experiments to three types of common clay minerals – kaolinite, illite and montmorillonite. We found that 20–30% of Cd from illite and montmorillonite surfaces were desorbed when transitioning from freshwater to seawater pH and ionic strength conditions, while kaolinite showed no discernible differences. Synchrotron X-ray adsorption spectroscopy confirmed that Cd release corresponded to a change in bonding from outer- to inner-sphere complexes when clays encountered seawater pH and ionic strength conditions. If other trace nutrients (such as Cu, Zn, Co) adsorbed onto riverine clay minerals behave in a similar manner to Cd, we speculate that their desorption in marginal marine settings should exert a significant impact on the productivity of the biosphere.

The sedimentary fabrics of Precambrian mat-related structures (MRS) represent some of the oldest convincing evidence for early life on Earth. The ca. 2.1 billion-year (Ga) old MRS in the FB2 Member of the Francevillian basin in Gabon has received considerable attention not only because they contain remnants of microbial mats that colonized large areas in oxygenated, shallow-marine settings, but they also contain evidence for ancient multicellular organisms that thrived on these microbial mats using them as a food source. Despite these insights, what remains lacking is a full characterization of the geochemical composition of the MRS to test whether the bulk composition of fossilized MRS is distinct from the host sediments (sandstones and shales). Here, we show that the trace element (TE) content of microbial textures belonging to pyritized MRS, poorly pyritized MRS, and “elephant-skin” textures (EST) is highly variable and differs from that of the host sediments. The poorly pyritized MRS contain a unique matrix with embedded Ti- and Zr-rich minerals and syngenetically enriched in TE. The EST, some of which are developed along the same stratigraphic horizon as the poorly pyritized MRS, display a distinct distribution of TE-bearing heavy minerals, suggesting a local difference in physical conditions during sedimentation. Similarly, high chalcophile-element (CE) content in pyritized MRS relative to the host sediments of the FB2 Member further points to local bacterially influenced enrichments with high rates of microbial sulfate reduction during early diagenesis. The geochemical relationship between the MRS and the Francevillian sediments (e.g., FB, FC, and FD formations) indicates that specific biological pathways for CE enrichments (i.e., microbially controlled accumulation) are not apparent. Our findings highlight bulk-rock TE distinction between the 2.1-billion-year-old MRS and their host sediments, but also indicate that environmental conditions, such as hydrodynamic regime and water-column redox chemistry, may simply overwhelm any potential biological signal. Our data suggest that the microbial impact may have only passively influenced TE enrichment in the studied sediments, implying that TE concentrations in MRS are a poor biosignature. Importantly, this work indicates that bulk TE geochemistry does not unveil specific microbiological processes in the rock record, which is consistent with the observed patterns in modern analogues.

The oceans probably remained well-oxygenated for millions of years after the Palaeoproterozoic Lomagundi-Jatuli Event, according to high concentrations and isotope signatures of redox-sensitive metals in the 2-billion-year-old Zaonega Formation, Russia. The approximately 2,220-2,060 million years old Lomagundi-Jatuli Event was the longest positive carbon isotope excursion in Earth history and is traditionally interpreted to reflect an increased organic carbon burial and a transient rise in atmospheric O-2. However, it is widely held that O-2 levels collapsed for more than a billion years after this. Here we show that black shales postdating the Lomagundi-Jatuli Event from the approximately 2,000 million years old Zaonega Formation contain the highest redox-sensitive trace metal concentrations reported in sediments deposited before the Neoproterozoic (maximum concentrations of Mo = 1,009 mu g g(-1), U = 238 mu g g(-1) and Re = 516 ng g(-1)). This unit also contains the most positive Precambrian shale U isotope values measured to date (maximum U-238/U-235 ratio of 0.79 parts per thousand), which provides novel evidence that there was a transition to modern-like biogeochemical cycling during the Palaeoproterozoic. Although these records do not preclude a return to anoxia during the Palaeoproterozoic, they uniquely suggest that the oceans remained well-oxygenated millions of years after the termination of the Lomagundi-Jatuli Event.

Anammox-based processes and microbial consortia have drawn extensive attention for their use in high-efficiency wastewater treatment technologies. Metals substantially affect the activity of anammox consortia and the quality of wastewater treatment plant effluent. Here, we explored the role of anammox consortia in terms of metals complexation in both single and multi-metal systems. Adsorption edges of single metal cations indicate that the adsorption preference was in the order: Pb(II) > Cd(II) > Cr(VI). A competitive effect was observed in multi-metal cations systems, with Pb(II) being preferably adsorbed and the degree of adsorption somewhat reduced in the presence of either Cd(II) or Cr(VI), while Cd(II) and Cr(VI) were easily exchanged and substituted by other metals. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) further suggest that the adsorption of Pb(II) and Cd(II) are as inner-sphere ion-exchange mechanisms, while Cr(VI) adsorption is mainly by outer-sphere complexation. Density functional theory (DFT) calculations highlight that Cd(II) and Pb(II) have different binding sites compared to Cr(VI), and the order of binding energy (Ebd) of three metal cations were Pb(II) > Cd(II) > Cr(VI). These calculations support the adsorption data in that Pb forms more stable complexes with anammox bacterial surface ligands. Surface complexation modelling (SCM) further predicted both the sorption of single metal cations and competitive adsorption of the three metals to anammox consortia, the exception being Cd at higher loadings. The results of this study highlight the potential role of anammox consortia in removing metal cations from wastewater in treatment systems.

The organic geochemistry of the Paleogene-aged second and first members of the Kongdian Formation (Ek2 and Ek1) and the third member of the Shahejie Formation (Es3) in the Cangdong sag, Bohai Bay Basin, China, are characterized and the hydrocarbon generating potential is evaluated. The Ek2 source rocks have a good to very good oil-generating potential, as indicated by the high abundance and good quality of organic matter within a mature stage. Although the Ek1 mudstones have entered the oil window, they are organic lean and are dominantly composed of Type III kerogen. Accordingly, the Ek1 mudstones are considered to be poor source rocks. The Es3 member is thermally mature and contains several intervals that have high contents of good-quality organic matter (mixed Type I and II kerogens), suggesting that these intervals also have a good potential for oil-generation. Given this potential, new exploration plays sourced from the Es3 member could be possible in the Cangdong sag. Biomarker distributions in the Ek2 and Es3 source rocks are both characterized by a dominance of medium- to high-molecular-weight n-alkanes (n-C18 to n-C29), a low sterane/hopane ratio, and a low diasterane abundance. The Ek2 samples have low Pr/Ph ratios ( 0.1) and β-carotane, trace amounts of C30 4α-methyl-24-ethlycholestanes, and a predominance of C29 regular steranes. Collectively, these characteristics indicate that abundant prokaryotic biomass mixed with some terrigenous organic matter was deposited under reducing conditions within a stratified water column. In contrast, the Es3 source rocks are characterized by variable Pr/Ph ratios, low gammacerane and β-carotane, comparable C27 and C29 regular steranes, and relatively high C30 4α-methyl-24-ethlycholestanes. These properties indicate that the Es3 source rocks contain significant contributions from prokaryotic and algal sources, and were deposited under reducing to suboxic conditions within a relatively less stratified water column.

Ambers—fossilized plant resins—are a rich and unique source of paleoecological data due to their ability to preserve soft body fossils. However, interpretations concerning their environmental context are often hampered by uncertainties in the relationship between assemblages of inclusions and geological context, particularly in the case of secondarily redeposited ambers such as those from the Paleogene of Central Europe. Here we use stable carbon and hydrogen isotope analyses, as well as FTIR spectroscopy, from the northwestern Ukrainian Rovno amber deposit, to provide independent constraints on the geographic and temporal origins of Rovno amber. These analyses address the relationship between the Rovno and Baltic amber deposits as well as German Bitterfeld amber—a subject of considerable debate regarding their provenance. Rovno amber has a δ13C signature of −23.3 ± 0.9‰, similar to both Baltic and Bitterfeld ambers. Since there is a secular decreasing δ13C trend among amber deposits since the Early Eocene, a roughly contemporaneous origin of these deposits in the Eocene can be deduced. However, Rovno amber displays a δ2H signature of −258 ± 9‰, 19‰ more positive than Baltic amber, and directly comparable to Bitterfeld amber. This difference relates to precipitation sources and mean annual temperatures of the amber source regions, and suggests a much more southerly origin of Rovno amber relative to Baltic amber. FTIR spectra of each of these ambers are nearly identical and suggest that resin-producing trees were from similar families, despite contrasting source regions. Thus, we provide the first clear geochemical evidence for the distinct origin of Rovno and Baltic amber deposits, with implications for paleoecological studies involving inclusions from these deposits, and for determining the provenance of archaeological amber finds.

In a system where protons and hydroxide ions are the only aqueous species, the point of zero net proton charge (pHPZNPC) of a mineral defines the pH at which the positively and negatively charged functional groups on its surface are equal ( Drever, 1997 ). Ascertaining the pHPZNPC of clay minerals, a ubiquitous component in soils, sediment and rivers, is useful in predicting its electrostatic interactions with charged aqueous species, colloids, and bacteria. While the pHPZNPC values of most clays have been reported, the variation of pHPZNPC with changing solution ionic strength (IS) and the mathematic relationship between them are not well-understood but critical in assessing the surface reactivity of clays in aqueous solutions ranging from freshwater to brines. To address this gap, we studied the relationship of the pHPZNPC of three clay minerals (kaolinite, illite, and montmorillonite) at seven ionic strengths (from 0.001 to 0.1 M). Titration data for each clay were used to calculate the pHPZNPC by two methods previously documented in literature: (1) using the difference between the blank titration and clay titrations, and (2) by the difference between the number of protons added during titration and the number of protons remaining in solution. The results show that: (1) unlike simple metal oxides (e.g., hematite, gibbsite, quartz), titration curves of clay minerals at different IS do not intersect at a common pHPZNPC value; (2) the pHPZNPC for kaolinite is around 5.6 to 6.6 while the pHPZNPC for illite and montmorillonite is in the range of 9 to10; (3) the pHPZNPC value decreases systematically with increasing IS for all three clay minerals studied; and (4) the change of pHPZNPC is linear with log(IS). A third method using surface complexation modeling (SCM) approach was applied to calculate the pHPZNPC of the clay minerals, and the results match well with Method 2. Our results allow for a more accurate estimation of clay surface charge property in aqueous environment, which, in turn, will improve model predictions of the adsorption of charged species in systems in which ionic strength changes.

This is the peer reviewed version of the following article: Mänd, K., Kirsimäe, K., Lepland, A., Crosby, C. H., Bailey, J. V. Konhauser, K. O., ... Lumiste, K. (2018). Authigenesis of biomorphic apatite particles from Benguela upwelling zone sediments off Namibia: The role of organic matter in sedimentary apatite nucleation and growth, Geobiology, 16(6), 640-658, which has been published in final form at http://doi.org/10.1111/gbi.12309. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. Sedimentary phosphorites comprise a major phosphorus (P) ore, yet their formation remains poorly understood. Extant polyphosphate‐metabolizing bacterial communities are known to act as bacterial phosphate‐pumps, leading to episodically high dissolved phosphate concentrations in pore waters of organic‐rich sediment. These conditions can promote the precipitation of amorphous precursor phases that are quickly converted to apatite—usually in carbonate fluorapatite form [Ca10(PO4,CO3)6F2‐3]. To assess the mechanisms underpinning the nucleation and growth of sedimentary apatite, we sampled P‐rich sediments from the Namibian shelf, a modern environment where phosphogenesis presently occurs. The P‐rich fraction of the topmost centimetres of sediment mainly consists of pellets about 50–400 μm in size, which in turn are comprised of micron‐sized apatite particles that are often arranged into radial structures with diameters ranging from 2 to 4 μm, and morphologies that range from rod‐shapes to dumbbells to spheres that resemble laboratory‐grown fluorapatite–gelatin nanocomposites known from double‐diffusion experiments in organic matrices. The nucleation and growth of authigenic apatite on the Namibian shelf is likely analogous to these laboratory‐produced precipitates, where organic macromolecules play a central role in apatite nucleation and growth. The high density of apatite nucleation sites within the pellets (>109 particles per cm3) suggests precipitation at high pore water phosphate concentrations that have been reported from the Namibian shelf and may be attributed to microbial phosphate pumping. The intimate association of organic material with the apatite could suggest a possible role of biological substrata, such as exopolymeric substances (EPS), in the nucleation of apatite precursors. Importantly, we do not observe any evidence that the apatite particles are actual phosphatized microbes, contradicting some earlier studies. Nevertheless, these results further evidence the potential importance of microbially derived (extracellular) organic matter as a template for phosphatic mineral nucleation in both recent and ancient phosphorites.

In marginal marine settings, understanding the role that clay minerals play in the transfer of metal cations from the water column to the seafloor is important if the composition of modern muds is to be used as a proxy for the precursors of ancient shale deposits. In this study, we conducted potentiometric titrations of three naturally abundant clay minerals (kaolinite, montmorillonite and illite) in 0.56 M NaCl electrolyte solutions to ascertain the surface reactivity of each clay mineral at seawater ionic strength. Our results demonstrate that all three clay minerals were increasingly anionic as pH increases from 3 to 9, with montmorillonite having the highest negative-charge. The potentiometric titrations and adsorption data were modeled using both non-electrostatic (NEM) and electrostatic (constant-capacitance model, CCM) surface complexation models (SCMs). A two-site surface complexation model, that included one basal siloxane surface site (≡X−) and one amphoteric edge site (≡SOH), provided an excellent fit for the protonation data of each mineral. Cd(II) adsorption experiments were conducted to quantify the magnitude of cation adsorption, which showed that 10 g montmorillonite, illite, and kaolinite powder could bind an equivalent of 4.7 × 10−4 g, 4.3 × 10−4 g, and 4.0 × 10−4 g of Cd, respectively, under conditions relevant to seawater (initial Cd concentration of 8.9 × 10−6 M, and pH = 8). To place this value into the context of modern riverine clay inputs to the ocean, 13.5 × 109 tons of total suspended sediment are deposited annually (Milliman and Meade, 1983) of which 10 to 25% is clay (Manheim et al., 1970; Schroeder et al., 2015). The total amount of Cd adsorbed to suspended clay minerals entering the modern oceans could then amount to approximately 104 to 105 tons per year. The results of this study highlight the potential role of clay minerals in transporting metals from the ocean water column to the seafloor.

Biochar has emerged as a useful amendment to release nutrients into agricultural soil, to increase crop productivity, and as a sorbent to remediate metals and organics contamination. Since soils have heterogeneous physical properties across a given crop field, and even over a growing season, it is imperative to select the most appropriate biochar for the intended purpose and in defining the amendment level. In this study, we investigate the adsorption of Cd(II) and Se(VI) as model pollutant cations and anions, respectively, to two agricultural soils amended with a wood pin chip biochar (WPC). The proton reactivity of each sorbent was determined by potentiometric titration, and single-metal, single-sorbent experiments were conducted as a function of pH. The resulting data were modeled using a non-electrostatic surface complexation modeling (SCM) approach to determine the proton and metal binding constants and surface functional group concentrations of each soil and WPC. The SCM approach is a considerable advance over empirical modeling approaches because SCM models can account for changes in pH, ionic strength, temperature, and metal-to-sorbent ratio that may happen over the course of a growing season. The constants derived from the single-metal, single-sorbent experiments were then used to predict the extent of metal adsorption in more complex mixtures of Cd, Se, soil and WPC. Overall the SCM approach was successful in predicting metal distribution in multi-component mixtures. In cases where the predictions were poorer than expected, we identify reasons and discuss future experiments needed to further the application of SCM to sorbent mixtures containing biochar.

Utilizing sophisticated tools in carbonate rocks is crucial to interpretating the origin and evolution of diagenetic fluids from the Upper Jurassic carbonate rocks along the Zagros thrust-fold Belt. The origin and evolution of the paleofluids utilizing in-situ strontium isotope ratios by high resolution laser ablation ICP-MS, integrated with stable isotopes, petrography and fieldwork are constrained. Due to the lack of information on the origin of the chemistry of the fluids, the cements that filled the Jurassic carbonate rocks were analysed from the fractures and pores. This allowed us to trace the origin of fluids along a diagenetic sequence, which is defined at the beginning from the sediment deposition (pristine facies). Based on petrography and geochemistry (oxygen-, carbon- and strontium-isotope compositions) two major diagenetic stages involving the fluids were identified. The initial stage, characterized by negative δ13CVPDB values (reaching −10.67‰), involved evaporated seawater deposited with the sediments, mixed with the input of freshwater. The second stage involved a mixture of meteoric water and hot fluids that precipitated as late diagenetic cements. The late diagenetic cements have higher depleted O–C isotope compositions compared to seawater. The diagenetic cements display a positive covariance and were associated with extra- δ13CVPDB and δ18OVPDB values (−12.87‰ to −0.82‰ for δ18OVPDB and −11.66‰ to −1.40‰ for δ13CVPDB respectively). The distinction between seawater and the secondary fluids is also evident in the 87Sr/86Sr of the host limestone versus cements. The limestones have 87Sr/86Sr up to 0.72859, indicative of riverine input, while the cements have 87Sr/86Sr of (0.70772), indicative of hot fluid circulation interacting with meteoric water during late diagenesis.

Wanning nickel (Ni) abundances in the Neoarchean may have led to a decline in marine methanogenic bacteria due to the loss of a key micronutrient, causing a collapse in atmospheric methane levels and the subsequent rise of atmospheric oxygen – the Great Oxidation Event ∼2.5 Ga. However, this hypothesis currently lacks geological evidence that corroborates the processes that led to a decline in the supply of Ni to the oceans at that time. Here we investigate temporal variations of Ni concentrations in the likely source rocks – various volcanic lithologies - spanning the past 3.5 Byr. By applying a spatially gridded resampling to geochemical data from a global compilation of ∼96,000 continental volcanic rocks, we show that komatiites and basaltic-andesitic rocks largely controlled Ni fluxes to the Earth's surface during the Archean but had a waning influence thereafter due to the secular cooling of the mantle. A new compilation of marine shales (397 samples) and an updated compilation of banded iron formations (2037 samples) further confirms a declining clastic and dissolved supply of terrestrial Ni, respectively. Interestingly, we further observed a stable marine Ni concentration for the last 2 Byr. To explain this unexpected trend, we provide a computational model of mantle melting demonstrating that increasing melting pressures likely inhibited a further decline of Ni concentration in volcanic rocks and led to this relative consistency in Ni supply.

Over the past century a number of models have been developed to explain the vast stratigraphic distribution of authigenic dolomite. These models include hydrogeological, geochemical and geomicrobiological interpretations that shed light on kinetic constraints on low-temperature dolomite formation, and, in particular, how certain microbes can overcome such limitations. In this review, we focus on the direct and indirect mechanisms by which microbes facilitate the nucleation and shallow burial diagenetic stabilization of dolomite in marine sediments. We also consider the likely effects of local and global scale biogeochemical perturbations over the distribution and activities of subsurface microbes, and, thus, potentially over the stratigraphic distribution of dolomite. Considering the many advances that have been made in the fields of geomicrobiology and biogeochemistry during the past 20 years, investigating the important role microbes play in sedimentary systems will undoubtedly contribute to a more complete interpretation of dolomite formation in the rock record.

A modified Community Bureau of Reference (CBR) sequential extraction method was tested to assess the composition of untreated pyrogenic carbon (biochar) and oil sands petroleum coke. Wood biochar samples were found to contain lower concentrations of metals, but had higher fractions of easily mobilized alkaline earth and transition metals. Sewage sludge biochar was determined to be less recalcitrant and had higher total metal concentrations, with most of the metals found in the more resilient extraction fractions (oxidizable, residual). Petroleum coke was the most stable material, with a similar metal distribution pattern as the sewage sludge biochar. The applied sequential extraction method represents a suitable technique to recover metals from these materials, and is a valuable tool in understanding the metal retaining and leaching capability of various biochar types and carbonaceous petroleum coke samples.

Banded iron formations (BIFs) are iron- and silica-rich chemical sedimentary rocks that were deposited throughout much of the Precambrian. The biological oxidation of dissolved Fe(II) led to the precipitation of a ferric oxyhydroxide phase, such as ferrihydrite, in the marine photic zone. Upon burial, ferrihydrite was either transformed into hematite through dehydration or it was reduced to magnetite via biological or abiological Fe(III) reduction coupled to the oxidation of buried microbial biomass. However, it has always been intriguing as to why the oldest BIFs are characteristically magnetite-rich, while BIFs formed after the Neoarchean are dominated by hematite. Here, we propose that some magnetite in early Archean BIF could have precipitated directly from seawater through the reaction of settling ferrihydrite and hot, Fe(II)-rich hydrothermal fluids that existed in the deeper waters. We conducted experiments that showed the reaction of Fe(II) with biogenic ferric iron mats under strict anoxic conditions lead to the formation of a metastable green rust phase that within hours transformed into magnetite. Our model further posits that with the progressive cooling and oxidation of the Earth's oceans, the above reaction shuts off, and magnetite was subsequently restricted to reactions associated with diagenesis and metamorphism.

Banded iron formations (BIFs) are iron- and silica-rich chemical metasedimentary rocks that span much of the Archaean and early Palaeoproterozoic rock record. Given that BIFs precipitated from ambient seawater, they provide an important archive of Earth’s ancient marine environments. Within the Slave craton occurs one of the largest and well-preserved Achaean turbidite basins in the world. Interstratified within the greywacke-mudstone turbidites is BIF that is commonly associated with ferruginous sediments and locally with a felsic tuff bed. Field investigations were conducted at four BIF locations from west to east across the Slave craton. These BIFs are composed of micro- and mesobands of quartz and magnetite, with various amounts of metamorphic grunerite, ferro-actinolite, ferro-hornblende as well as greenalite and stilpnomelane, the latter two are interpreted as retrograde metamorphic phases. As the BIF was deposited in a deep-water turbidite-dominated basin(s), the chemical component was occasionally diluted by clastic detritus, as illustrated by highly variable Al2O3 content, and Al2O3/TiO2 and Cr/Ni that are similar to the clastic sediments. On a layer-by-layer basis, the iron content in the BIF ranges from as low as 4 wt.% and up to ∼75 wt.% Fe2O3, whereas the SiO2 content ranges from 20 wt.% to as high as ∼94 wt.%. This, together with a hyperbolic germanium/silicon distribution, suggests that continentally-sourced silica comprised the background precipitation, which was periodically interrupted by pulses of hydrothermally-sourced iron. High-temperature hydrothermal fluids contributed, with an average shale-normalized positive Eu anomaly of 1.92, which indicates input of reduced Eu2+ to the ambient seawater. The BIF shows characteristic aqueous fluid-derived rare earths + yttrium fractionation (Pr/Yb

Organic-rich, fine-grained sedimentary rocks, such as black shales, are important geochemical archives providing information on the evolution of seawater composition and biological activity over the past 3 billion years. While biological productivity and sedimentation rates greatly affect the organic matter content in these rocks, mechanisms linking these two processes remain poorly resolved. Here, we examine the interactions of clay minerals with the marine planktonic cyanobacterium Synechococcus sp. PCC 7002. We suggest that clays settling through the water column could influence carbon and trace metal burial in three ways: (1) the interaction of reactive clay surfaces with the bacterial cells increases organic matter deposition via mass increase in a seawater growth medium by several orders of magnitude; (2) reactive bacterial cells become completely encased within a clay shroud, enhancing the preservation potential of this organic matter; and (3) the trace metal content of the biomass buried along with metals sorbed to the clay particles contributes to the trace metal concentrations of the black shale precursor sediments. Significantly, our findings imply that the chemical composition of ancient, organic-rich, fine-grained deposits are not only archives of ancient seawater composition and redox state, but they also provide a record of the degree of biological activity in the water column through geological time.

International audience; Precambrian banded iron formations (BIF) are chemical sedimentary deposits whose trace element signatures have been widely used to interrogate the chemical composition and redox state of ancient seawater. Here we investigated trace element signatures in BIF of the 3.22 Ga Moodies Group, Barberton Greenstone Belt (South Africa), which are interbedded with near-shore siliciclastic sedimentary rocks and represent one of the oldest known shallow-water occurrences of BIF. Unusual rare earth element (REE) signatures, notably with pronounced negative Ce anomalies in shale-normalized spectra, have been previously reported for chemical sediments of the Moodies Group, which we confirm here through an expanded dataset for Moodies BIF spanning three different localities. We find negative Ce anomalies as low as 0.2 Ce/Ce* that are associated with unusual enrichment of LREE relative to HREE in the sample set. While total REE abundances and certain REE features appear strongly related to the concentration of detrital indicators (e.g., Zr), and are likely primary, other features, notably LREE enrichment, cannot be explained as a primary feature of the sediment. This is better explained by later addition of REE from a LREE-enriched but Ce-depleted fluid that generated the significant negative Ce anomalies observed in surface samples of Moodies Group BIF. This REE addition event influenced both Sm-Nd and La-Ce isotope systematics, the latter yielding an isochron of 60 +/- 32 Ma, thus constraining the timing of emplacement of the negative Ce anomalies to the past 100 Ma, possibly upon surface exposure of the Barberton Greenstone Belt to wetter conditions during the Cenozoic. Our findings constitute a cautionary tale in that even the most immobile elemental redox proxies may be more sensitive to post-depositional modification than previously thought, and demonstrate the clear advantage offered by paleoredox proxies coupled to radiometric geochronometers to enable the direct dating of ancient signals of Earth surface oxygenation.

Photoferrotrophs repel iron to deposit banded iron formations and contribute methane to the atmosphere during the Archean Eon., Banded iron formation (BIF) deposition was the likely result of oxidation of ferrous iron in seawater by either oxygenic photosynthesis or iron-dependent anoxygenic photosynthesis—photoferrotrophy. BIF deposition, however, remains enigmatic because the photosynthetic biomass produced during iron oxidation is conspicuously absent from BIFs. We have addressed this enigma through experiments with photosynthetic bacteria and modeling of biogeochemical cycling in the Archean oceans. Our experiments reveal that, in the presence of silica, photoferrotroph cell surfaces repel iron (oxyhydr)oxides. In silica-rich Precambrian seawater, this repulsion would separate biomass from ferric iron and would lead to large-scale deposition of BIFs lean in organic matter. Excess biomass not deposited with BIF would have deposited in coastal sediments, formed organic-rich shales, and fueled microbial methanogenesis. As a result, the deposition of BIFs by photoferrotrophs would have contributed fluxes of methane to the atmosphere and thus helped to stabilize Earth’s climate under a dim early Sun.

Banded iron formations are critical to track changes in Archaean to Palaeoproterozoic ocean chemistry, with deposition triggered by water column iron oxidation. Recently, however, it was suggested that reduced iron minerals were the primary precipitates, and these were subsequently oxidized by oxygen-bearing groundwater. If true, this would cast doubt on our understanding of how banded iron formations were deposited and their ability to record early ocean chemistry. Here we present a hydrogeological box model, based on the approximately 2.5 billion year old Hamersley Basin of Western Australia, developed to evaluate the plausibility of secondary iron oxidation. The box model calculates the time required for groundwater to flux enough oxygen through the basin to oxidize a given amount of ferrous iron. Less than 9% of nearly four million model iterations returned oxidation times less than the age of the basin. Successful simulations required simultaneously steep hydraulic gradients, high permeability and elevated oxygen concentrations. Our simulations show that the postdepositional oxidation of large banded iron formations is unlikely, except on a limited scale (that is, during secondary ore formation), and that oxidized iron phases were probably the precursor to large Palaeoproterozoic banded iron formations. Banded iron formations could not have formed by postdepositional oxidation, according to four million hydrogeological box model iterations that failed to reproduce secondary oxidation on reasonable timescales.

International audience; While we appreciate Lascelles’ interest in our work, we find most of the points raised to be either poorly-supported or arising from a significant misunderstanding.

Measuring the metal binding potential and reactivity of bacterial mats is challenging in alkaline and carbonate-rich systems. Traditional methods used to measure these parameters, such as potentiometric titrations and metal adsorption pH edges, are difficult to implement due to the presence of the carbonate minerals that buffer pH and prevent assessment of mat surface reactivity. Additionally, under alkaline conditions metals may form hydroxide and/or carbonate precipitates. In this study we examined the metal binding capacity of four distinct bacterial mats collected from Fairmont Hot Springs, BC, Canada. To prevent metal precipitation, the bacterial mat concentration was varied under a constant initial cadmium (Cd) concentration of 8.89 μM and at pH 8. In addition to the intact bacterial mats, a carbonate mineral sample and two bacterial mats in which the carbonate mineral was removed via acid-treatment, were used as end-members to assess the mechanisms of reactivity in the whole system. Freundlich adsorption isotherms were used to fit metal adsorption data and directly compare surface reactivity among intact mats and mat components. Two of the intact mats exhibited a higher affinity for Cd compared to the mineral at metal equilibrium concentrations above 2.5 μM, while the other two intact mats had lower affinities under all experimental conditions. Generally, we found the acid-treated mats had higher Cd adsorption capacities than the carbonate mineral. When compared to their equivalent intact mats, only one acid-treated mat had a higher affinity for Cd. Further, we modeled whether metal adsorption in the intact mats, containing microbes and carbonate mineral, could be explained by a linear combination of the observed metal uptake by the organic and inorganic components through end-member experiments. Metal adsorption additivity results were mixed. Metal uptake by one intact mat was found to be additive, while for the other mat the additive model significantly underestimated the observed Cd accumulation. Our study demonstrates the potential, as well as the limitations, of using modified metal adsorption edges to determine the metal binding affinity and surface reactivity of bacterial mats in alkaline and carbonate-rich systems.

Two temporally distinct Neoarchean turbidite packages are known to occur in the Slave craton. The older is a greywacke–mudstone succession that includes the renowned Burwash Formation (ca. 2661 Ma). In this study, a previously undated tuff bed is demonstrated to have crystallized at ca. 2650.5 ± 1.0 Ma refining the deposition age of these turbidites between ca. 2661 and 2650 Ma. The younger turbidites are locally distinctive as they contain interstratified banded iron formation (BIF). Previous work demonstrated that the younger turbidites were deposited between ca. 2640 and 2615 Ma, based entirely on maximum depositional ages from detrital zircons. A ∼3 cm thick felsic to intermediate tuff bed was discovered interbedded with these BIF-bearing turbidites. The tuff bed contains a single age population of zircon with a crystallization age of 2620 ± 6 Ma defining the depositional timing of these BIF-bearing turbidites. New U–Pb detrital zircon dates from extensive turbidite sequences in the eastern and central part of the Slave craton are also presented. We use the new and previously published results to recommend nomenclature for these extensive sedimentary rocks in the Slave craton. The ca. 2661–2650 Ma turbidites remain part of the previously ascribed Duncan Lake Group. The younger ca. 2620 Ma turbidites are assigned to the new Slemon Group. Where robust age-data exist, we recommend formation names and include type localities for each.