Colloids have been recognized as key vectors of pollutants in aqueous environment. Amongst them, those formed by iron (Fe) and organic matter (OM) are of major importance due to their ubiquity in the ...surface environment and strong affinity for metals. In the recent years, Fe stable isotopes have been increasingly used to elucidate the sources and biogeochemical cycling of Fe in Earth's surface environments. In this study, we aim to elucidate (i) the possible Fe isotopic signature resulting from the Fe/OM colloid formation and (ii) the mechanisms involved in the development of such isotopic signature. For this purpose, Fe-OM associations were synthesized through binding and titration experiments. Various pH levels were used in order to study the isotope behavior of Fe occurring as free species at pH 1, as Fe-OM complexes at pH 2 and as mixed Fe-oxyhydroxide/OM nanoaggregates or particles at pH 6.5. Organic matter-free, Fe-free and OM membrane-deposition experiments were also performed. These suspensions were (ultra)filtered at 0.2 µm, 30 kDa and 5 kDa to evidence the possible Fe isotope fractionation between fractions. This protocol allowed also testing the potential of (ultra)filtration techniques to generate isotope fractionation. The results provided evidence that abiotic Fe precipitation, (ultra)filtration techniques and OM deposition were not able to produce significant Fe isotope fractionation under the experimental conditions. However, at circum-neutral pH, the Fe-OM binding and titration experiments displayed a significant enrichment of heavy Fe isotopes in the <30 kDa fractions relative to the total Fe pool δ56Fe = 0.35 ± 0.05‰ and 0.26 ± 0.05‰ (95% confidence interval, 2σ and relative to international standard IRMM-14), respectively. Mass balance and error propagation calculation showed Fe isotope fractionation in binding and titration experiments between the >30 kDa and <30 kDa fractions for −0.35 ± 0.05‰ and −0.27 ± 0.05‰, respectively. This Fe isotope fractionation could be due to the complexation of Fe by OM in the <30 kDa fractions. At pH 2, the OM-free experiment, the <30 kDa fraction showed Fe isotope ratio δ56Fe = 0.75 ± 0.03‰ with an enrichment in heavy Fe isotopes of δ56Fe’ = 0.14 ± 0.04‰ relative the total Fe pool (δ56Fe’ is δ56Fe value which was corrected by δ56Fe of total fraction). This enrichment in heavy Fe isotopes induced an isotopic fractionation factor of −0.87 ± 0.26‰ between the >30 kDa and <30 kDa fractions produced by the complexation between the heavy Fe isotopes and OH− ligands in the <30 kDa fraction. The natural Fe-OM associations were further investigated through oxidation experiments of a reduced wetland soil solution. The oxidized soil solution was (ultra)filtered at 5 µm, 3 µm, 0.2 µm, 30 kDa and 5 kDa. The highest δ56Fe was obtained in the smallest size fraction, i.e. <5 kDa fraction, yielding a negative isotopic fractionation Δ56Fe >5kDa − <5kDa = −0.23 ± 0.08‰ suggesting that Fe heavy isotopes are preferentially bound to small humic OM molecules in the form of Fe monomers or small clusters. This study highlights the importance of organic matter for metals’ isotopic systems.
The taphonomy of early soft-bodied organisms in Palaeoproterozoic sediments is not yet clearly understood, even though some locations where these fossils are found present all the conditions for ...exceptional fossil preservation. Indeed, the degree of fossil preservation has received attention, but better knowledge of the environmental conditions and associated taphonomic processes is also essential. In the Gabonese Francevillian Basin, the discovery of macrofossils (2.1 Ga) of multicellular organisms in black shales is an outstanding example of this degree of preservation. Indeed, the biological diversity, as a wide variety of fossil morphologies are observed, is associated with two major taphonomic processes – moulding (lenticular-shaped forms) or early pyritization, while the fossil host rocks were not deeply buried and were affected only by weak to moderate diagenesis. However, usually, the mechanisms of this preservation remain difficult to assess, as the original taphonomic processes are impacted by diagenesis and still misunderstood. In this way, by closely observing fossil mineralization in four morphotypes of macrofossils and associated host rocks from mineralogical and textural points of view, this work aims to provide some keys to a taphonomic comprehension of soft-bodied organism preservation. After the deposition of dead organisms on the clayey sediment, an illitization process, which depends on the availability of dissolved K driven by bacterial activity, started from the first stages of preservation by moulding the lenticular-shaped forms and proceeded in the pores of the other macrofossils after their pyritization. At the fossil level, the intensity of illitization is controlled by the mode of preservation and the evolution of the associated permeability. In the nonpyritized lenticular-shaped specimens, illitization was not achieved, preserving I-S mixed-layer minerals, while in the pyritized forms, the illitization degree was more extensive. In comparison, I-S mixed-layer minerals are almost absent in pyritized abiotic concretions. A second process, which occurred later, consists of general chloritization from fluid circulation. Our detailed results show that each specimen behaved like a microsystem with its own physico-chemical and mineralogical evolution during preservation/diagenesis. This finding allows us to propose a conceptual model of the taphonomic history, describing the fossilization stages for each type of specimen.
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•Comparative study of DOM isolated using two filter types and two pore size filters.•No pore size or filter type effect was observed on bulk scale descriptors.•No filter effect was ...observed on the distribution of the 43 target molecules.•Possibility to compare molecular results for different filter type and porosity.•Breakup of colloids due to forces applied during filtration at 0.2µm.
Biogeochemistry of dissolved organic matter (DOM) is important to ecology, ecotoxicology and the carbon cycle. DOM is operationally defined as the OM fraction that passes through filters. Since different filter pore-sizes, ranging from 0.2 to 0.7μm, are commonly used, it is necessary to test if this choice has an effect on the concentration and composition of DOM in order to ensure the comparison between studies using different filter pore sizes and filter types (cellulose acetate vs glass fibre). The concentration and composition of DOM was investigated along a soil-river continuum in a lowland headwater catchment using two filter pore sizes (0.2 and 0.7μm). Dissolved organic carbon (DOC) was quantified, and the DOM composition was investigated using spectroscopic (specific UV absorbance) and isotopic (δ13C) bulk-scale descriptors and semi-quantitative molecular analysis by thermally assisted hydrolysis and methylation with tetramethylammonium hydroxide coupled to a gas chromatography and mass spectrometry (THMGC-MS). No significant differences were detected between DOM<0.2μm and DOM<0.7μm for bulk scale descriptors. Moreover, at the molecular scale, the distribution of the 43 targeted compounds was not impacted. However their concentrations were slightly higher in the <0.2μm fraction. This could be due to a shield effect of mineral phases in the <0.7μm fraction implying a decrease in their recovery. Consequently, the DOM studies using 0.7 and/or 0.2μm filters can be compared. This similarity between those two pore-sizes is suggested to be due to the breakup of colloids by shear forces applying locally during filtration performed at 0.2μm.
Although the behavior of Arsenic (As) under reducing conditions in periods of high water levels in wetlands is well understood and documented, there is a lack of information under oxidizing ...conditions when the water level decreases. In this study, we were interested in the first stage of the oxidizing period, when oxidation products are still in suspension. A soil sample from the Naizin Kervidy wetland (France) was incubated in the laboratory to produce a reduced soil solution. The reduced solution was subsequently oxidized, filtered and ultrafiltered using decreasing pore size membranes (5μm, 3μm, 0.2μm, 30kDa and 5kDa). The distribution of As and Fe was investigated in each size fraction of the oxidized solution and their speciations were studied using XAS, HPLC and SEC-ICP-MS. Organic matter was characterized using thermally assisted hydrolysis and methylation gas chromatography–mass spectrometry (THM-GC–MS) and fluorescence spectroscopy. The majority of the As was present as As(V) but a small amount of As(III) still remained despite the advanced oxidized conditions. In the >0.2μm fractions, the XAS analyses showed that As was associated, in the second shell, with Fe (As–Fe=3.35Å) as bidentate binuclear complexes and C (As–C=2.90Å), suggesting the integration of As in biological objects. In the <30kDa fraction, As was directly bound to C (As–C=1.96Å) in the first shell indicating the presence of organic As species. In the second shell, an As–Fe distance of 3.35Å was found showing that part of the As was still complexed with Fe. The 0.2μm–30kDa fraction was a transitional fraction in terms of the Fe species and OM composition. In this fraction, organic matter exhibited a more humic character (aromatic molecules) inducing an increasing cation binding capacity. As a consequence, in this fraction and in the smallest one, As, Fe and OM seemed to form ternary complexes in which the Fe or nano-oxides in the >30kDa fraction and as monomer, or cluster in <30kDa fraction acted as a bridge. In all of the fractions, a proportion of As(V) was present as organic methylated species. These organic species might be produced by several organisms (animal or plant) via a detoxification process. They seemed to be bound to the particulate and colloidal Fe/OM phases as well as integrated in the remains of the organisms. Mass calculations provided evidence that 90% of the As was contained in the >5μm particulate fraction and thus was hardly mobile. This study showed that although wetlands have been identified as a potential source of As, a number of biological and geochemical trapping mechanisms also favor As stabilization in wetlands.
The evolution of rare earth element (REE) speciation between reducing and oxidizing conditions in a riparian wetland soil was studied relative to the size fractionation of the solution. In all size ...fractions obtained from the reduced and oxidized soil solutions, the following analyses were carried out: organic matter (OM) characterization, transmission electron microscopy (TEM) observations as well as major and trace element analyses. Significant REE redistribution and speciation evolution between the various size fractions were observed. Under reducing conditions, the REEs were bound to colloidal and dissolved OM (<2μm size fractions). By contrast, under oxidizing conditions, they were distributed in particulate (>2μm size fraction), colloidal (<2μm size fraction), organic and Fe-enriched fractions. In the particulate size fraction, the REEs were bound to humic and bacterial OM embedding Fe nano-oxides. The resulting REE pattern showed a strong enrichment in heavy REEs (HREEs) in response to REE binding to specific bacterial OM functional groups. In the largest colloidal size fraction (0.2μm–30kDa), the REEs were bound to humic substances (HS). The lowest colloidal size fraction (<30kDa) is poorly concentrated in the REEs and the REE pattern showed an increase in the middle REEs (MREEs) and heavy REEs (HREEs) corresponding to a low REE loading on HS. A comparison of the REE patterns in the present experimental and field measurements demonstrated that, in riparian wetlands, under a high-water level, reducing conditions are insufficient to allow for the dissolution of the entire Fe nano-oxide pool formed during the oxidative period. Therefore, even under reducing conditions, Fe(III) seems to remain a potential scavenger of REEs.
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•Impact of reducing and oxidizing alternation on the REE speciation and patterns was investigated.•Significant change occurred in the REEs-bearing fractions.•Iron and organic matter play both a major role in the REEs scavenging.•In wetland, Fe oxides are still present under reducing period and partially control REE speciation.
Iron (Fe) isotopes are now recognized as useful tracers of Fe sources and biogeochemical processes in natural environments but many uncertainties remain regarding the mechanisms that control their ...isotopic fractionation. Ultrafiltration techniques applied to separate Fe species could potentially bias Fe isotopic compositions. Here, we investigated frontal centrifugal ultrafiltration. We have set up time-ultrafiltration experiments at low and high Fe concentrations both with and without organic matter (OM) at pH values of 1 and 6.5. The ultrafiltration impact was studied by monitoring the Fe isotope composition in the <30 kDa ultrafiltrates relative to the ultrafiltration time. No Fe isotopes bias resulted from the ultrafiltration technique regardless the Fe and OM concentrations and speciation. This work, therefore, validates the use of the frontal centrifugal ultrafiltration technique to study the signature of Fe isotopes in environmental samples composed of various Fe species sizes such as colloids, nanoparticles, clusters or soluble complexes.
The evidence for macroscopic life during the Palaeoproterozoic era (2.5–1.6 Gyr ago) is controversial. Except for the nearly 2-Gyr–old coil-shaped fossil Grypania spiralis, which may have been ...eukaryotic, evidence for morphological and taxonomic biodiversification of macroorganisms only occurs towards the beginning of the Mesoproterozoic era (1.6–1.0 Gyr). Here we report the discovery of centimetre-sized structures from the 2.1-Gyr-old black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, which we interpret as highly organized and spatially discrete populations of colonial organisms. The structures are up to 12 cm in size and have characteristic shapes, with a simple but distinct ground pattern of flexible sheets and, usually, a permeating radial fabric. Geochemical analyses suggest that the sediments were deposited under an oxygenated water column. Carbon and sulphur isotopic data indicate that the structures were distinct biogenic objects, fossilized by pyritization early in the formation of the rock. The growth patterns deduced from the fossil morphologies suggest that the organisms showed cell-to-cell signalling and coordinated responses, as is commonly associated with multicellular organization. The Gabon fossils, occurring after the 2.45–2.32-Gyr increase in atmospheric oxygen concentration, may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5 Gyr later, in the Cambrian explosion.
Thirty-five samples of cow feces (cowpat and cow manure) and pig slurries subjected to different treatment processes and different storage times before land spreading were extracted and analyzed by ...gas chromatography–mass spectrometry to determine their fecal stanol profiles. The fresh pig slurry data presented here increase considerably the classical range of values obtained for steroid ratios, resulting in an overlap with the range for cow feces. These results lead to the inability to distinguish species source of feces on the basis of steroid ratios alone. The cause of these differences is not known, although it appears likely to be related to differences in the metabolism of animals in relation to their age and/or variations in diet, rather than to secondary mechanisms of steroid degradation during storage or/and treatment of the feces. Nevertheless, the specificity of steroids to serve as a tool to differentiate cow feces from pig slurries is restored by considering the fecal stanol profile, notably, the six most diagnostic stanol compounds, which are 5β-cholestan-3β-ol (coprostanol), 5β-cholestan-3α-ol (epicoprostanol), 24-methyl-5α-cholestan-3β-ol (campestanol), 24-ethyl-5α-cholestan-3β-ol (sitostanol), 24-ethyl-5β-cholestan-3β-ol (24-ethylcoprostanol), and 24-ethyl-5β-cholestan-3α-ol (24-ethylepicoprostanol). In this study, chemometric analysis of the fingerprint of these six stanols using principal components analysis (PCA) distinguished pig slurries from cow feces. The application of PCA to the stanol profiles, as developed in this study, could be a promising tool for identifying the animal source in fecal contamination of waters.
► High-resolution monitoring of stream δ
13C
DOC values in an headwater catchment. ► Precise localization of DOC sources in a headwater catchment. ► Dynamics of DOC sources mobilized during a storm ...event. ► Wetland soils are the dominant DOC source during storm events. ► δ
13C
DOC are an interesting tool for the reconstruction of water pathways in headwater catchments.
Stable carbon isotopes (δ
13C) are assessed in further detail for their potential to (i) trace the relationship between spatial variations in the source of dissolved organic carbon (DOC) in soils and temporal variability of both DOC concentration and composition in streams, and (ii) elucidate water pathway changes during storm events in headwater catchments. For this purpose, we investigated δ
13C
DOC values in a wetland soil (0–50
cm), in deep groundwater (until 6
m) and during a storm flow event with high-resolution monitoring (⩽hourly basis) in a small, lowland catchment in western France (Kervidy-Naizin catchment). The results show a combined increase of stream DOC concentration (from 4 to 14
mg
L
−1) and decrease of stream δ
13C
DOC (from −27 to −29‰) with increasing discharge, suggesting a change in DOC sources between base flow and storm flow periods. Such an interpretation is consistent with the δ
13C
DOC values in soils that show a 6‰ vertical variation, with δ
13C
DOC values of the uppermost soil horizons (0–10
cm) of the wetland domains being close to those measured in the stream channel during the ascending limb of the hydrograph. Overall, the results presented in this study are consistent with a model in which the water-table rise and wetland runoff caused by rainfall lead to a flushing of the DOC stored in the uppermost soil horizons of the wetland domains near the channel network. Subsequently, these wetland soils become the dominant DOC source during storm events (ca. 70% of the total DOC flux). In this way, the stream DOC isotopic composition reflects the combined effects of the vertical variation of soil organic matter composition as well as the changes in water routing through time. This study demonstrates the ability of the stable isotopes of carbon to serve not only as a tool for the location of stream DOC sources in landscapes but also the reconstruction of water pathways in headwater catchments.
The Okavango Delta in North Botswana is a hot‐spot of biodiversity within the semi‐arid central part of the South African plateau. This endorheic ecosystem is highly dependent on the annual flood ...that brings freshwater from the highlands of Angola to the North. However, in many places, the groundwater of the Delta is saline and contains very high concentrations of metal and metalloid elements, making it toxic to the flora and fauna. These saline waters have been largely studied and their formation is generally explained through evaporation and evapotranspiration processes. However, no studies have investigated the geochemical composition of the sediments that host the aquifers. Here, we provide a complete sedimentological and geochemical investigation (major, traces and rare earth elements, carbon and oxygen stable isotopes ratios, Sr and Nd isotope ratios) of the various geomorphological and ecological compartments that form the landscape of the SW Okavango Delta. We demonstrate that the non‐connected underground aquifer is capped by a clay layer corresponding to a major event of dust deposition from the Makgadikgadi pan. We suggest that this aquifer may extend outside the island, below the floodplain and that the hydrological structure of the Nxaraga area is indeed composed of two non‐connected aquifers of different origins. Challenging the model based on evaporation and evapotranspiration, we propose that the composition of that allochtonous clay and the in situ reactions between sediment, water and organic matter in a confined aquifer explain the geochemical enrichment of the water.
Key Points
The hydrological structure of the SW Okavango Delta is composed of two non‐connected aquifers
The high alkalinity of groundwater is linked to bio‐geochemical concentration processes in a confined aquifer
Allochtonous, wind‐blown clay deposits play a major role in the hydro‐geochemical structure of the Okavango alluvial fan
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