We report Re–Os isotope and highly siderophile element (HSE) abundance data for komatiites from the Schapenburg Greenstone Remnant (SGR), South Africa, an equivalent to the lowermost formations of ...the Onverwacht Group in the Barberton Greenstone Belt (BGB). The Re–Os isotopic data for 13 whole-rock samples define a regression line with an age of 3549
±
99 Ma, consistent with the ~
3.5 Ga age of the Onverwacht Group. The immobility of Os during alteration and the correct Re–Os age provide evidence that the initial
γ
187Os
=
+
3.7
±
0.3 derived from the regression reflects a time-integrated suprachondritic
187Re/
188Os in the source of the SGR komatiites. The HSE abundances in the emplaced SGR komatiite lavas are 2× to 3× lower than those in other well-studied komatiites. Compared to the Primitive Upper Mantle (PUM) estimate, the calculated mantle source of the SGR komatiites was moderately depleted in HSE and was characterized by a fractionated HSE pattern. The enrichment in
187Os, HSE depletion, and fractionated HSE pattern in the SGR komatiite source could be the result of fluid transport of radiogenic Os from the subducting slab, incorporation in the overlying mantle, and hydrous melting of the modified mantle to produce the komatiites. This would imply that plate tectonic processes operated as early as 3.5 Ga and that at least some komatiites formed via wet melting in island arc settings at relatively shallow depths and temperatures not exceeding 1400 °C. An alternative model would include dry melting of a chemically distinct, majorite-enriched mantle domain formed very early in Earth's history as a result of an initial stratification of the mantle during crystallization of a magma ocean. Preferential partitioning of Re into the majorite-rich domain would have resulted in its acquiring suprachondritic Re/Os. In order to grow the radiogenic Os isotopic composition, the majorite-rich domain would have to have remained isolated from the rest of the mantle for a billion years. If this model is correct, the data would require a deep plume origin for the SGR komatiites at temperatures ~
300 °C higher than the ambient mantle temperatures. Both models have their shortcomings in reconciling the available geochemical and petrologic data for the SGR komatiites. Because BGB komatiites are geochemically closer to island arc tholeiites and boninites than the SGR komatiites, it will be especially important to determine the Re–Os isotope and HSE systematics of the BGB komatiites and of typical boninites.
Rare earth element (REE), dissolved organic carbon (DOC) and trace-element (Al, Mn, Fe, Sr, Ba, U and Th) concentrations were measured in fourteen well water samples (<0.22 μm) and one spring located ...along two transects set up in a catchment from Western Europe (Kervidy/Coët-Dan catchment, France). Previous hydrological and hydrochemical (NO
3
−, SO
4
2−) investigations demonstrated that three chemically and spatially distinct groundwater types are present in this catchment, which is fully confirmed by the REE and DOC results. These include: (i) a shallow, organic-rich groundwater (4.4 < DOC < 34.6 mg/l) from the wetland areas, close to the river network. This first groundwater type, characterized by the development of temporary reducing conditions, records high and variable REE contents (2 < ΣREE < 16 ppb) and displays slight or no negative Ce anomaly (Ce/Ce∗ = 0.8–1.05); (ii) a shallow, organic-poor (DOC < 3 mg/l), NO
3
−-rich groundwater type (86.8 < NO
3
− < 155 mg/l) located in the weathered schists, below the hillslope domains. This second type corresponds to recently recharged, oxidized water and displays also high and variable REE concentrations (2 ppb < ΣREE < 15 ppb), but distinguish from the former by the occurrence of very strong negative Ce anomalies (Ce/Ce∗ = 0.05–0.10); finally (iii) a deep, organic-poor (DOC < 1 mg/l), nitrate-poor (NO
3
− close to 0.2 mg/l; detection limit) groundwater. This third type corresponds to reduced water flowing into the deep fresh schists and yields low to very low REE contents (ΣREE < 0.15 ppb) as well as slight negative Ce anomaly (Ce/Ce∗ = 0.8 to 0.9).
Temporal REE concentration variations were assessed using samples regularly collected over a six month period. Results show that the spatially distributed Ce anomaly and REE pattern signatures are preserved throughout the studied period. By contrast, REE concentrations are quite variable through time, especially in the wetland waters where the REE concentrations are seen to vary in phase with both redox changes and DOC, Fe, U and Th content variations.
Three REE-rich water samples (one DOC-rich and two DOC-poor) were also filtered through membranes of decreasing pore size (100,000 D, 30,000 D, 5,000 D). The results show that between about 40% to 65% of the REE present in the shallow, DOC-poor groundwater samples are controlled by the colloidal fraction, which is likely to consist in these inorganic waters of a mixture of mineral phases. In the wetland groundwaters, the fraction of REE controlled by microparticles is higher than 65%, which confirms the predominant role of organic colloids as major REE carriers in wetland waters.
Using the above data set in conjunction with analyses of soil samples, we show that the deep Ce anomalies found in the upper non organic part of the aquifer are probably not source-rock inherited features: most likely, these anomalies arise from the oxidative precipitation of Ce. The very low REE content displayed by waters flooding the deep fresh schists is interpreted as due to the combined effects of (i) pH variation, (ii) secondary sulfate mineral precipitation and (iii) the trapping of colloids-borne REE by the aquifer-rock pores. Data from wetland groundwaters show that the REE, Fe, U and Th budgets of these waters are mainly controlled by seasonal changes in redox conditions and organic matter content. However, unlike organic-poor waters, it appears difficult to relate the Ce behaviour in these organic-rich waters solely to redox conditions. It is likely that the complexation of Ce by organic colloids in the organic-rich waters may mask redox changes by inhibiting the development of negative Ce anomalies.
Shallow groundwater samples (<0.22μm) collected from a small catchment in Western France (Petit Hermitage catchment) were analyzed for their rare earth elements (REE), dissolved organic carbon (DOC) ...and trace-element (Fe, Mn, Th and U) contents, with the aim to investigate the controlling factors of REE signatures. Two spatially distinct water types are recognized in this catchment based on changes of REE concentrations and variations of Ce anomalies. These include (i) DOC-poor groundwater flowing below the hillslope domains; this type has low REE contents and records conspicuous negative Ce anomalies; (ii) DOC-rich groundwater from the wetland domains, close to the river network; this type displays much higher REE concentrations, and typically lack negative Ce anomalies. Temporal REE concentration variations were assessed in wetland waters. Results show a marked increase of the REE content at the winter–spring transition, along with variations of DOC, Fe and Mn contents and redox potential changes. Using the above data set in conjunction with previously published results on comparable shallow groundwaters, we demonstrate that factors such as aquifer-rock composition or anthropogenic input probably play a minor role in determining the REE signatures of shallow groundwaters. Most likely, the two dominant factors involved are the organic matter content and the redox status of the waters. We suggest that topography might be the ultimate key parameter by its ability to control the DOC content of groundwater at a catchment scale.
A 7-year monitoring period of rare earth element (REE) concentrations and REE pattern shapes was carried out in well water samples from a 450 m long transect setup in the Kervidy/Coët-Dan ...experimental catchment, France. The new dataset confirms systematic, topography-related REE signatures and REE concentrations variability but challenges the validity of a groundwater mixing hypothesis. Most likely, this is due to REE preferential adsorption upon mixing. However, the coupled mixing-adsorption mechanism still fails to explain the strong spatial variation in negative Ce anomaly amplitude. A third mechanism—namely, the input into the aquifer of REE-rich, Ce anomaly free, organic colloids—is required to account for this variation. Ultrafiltration results and speciation calculations made using Model VI agree with this interpretation. Indeed, the data reveal that Ce anomaly amplitude downslope decrease corresponds to REE speciation change, downhill groundwaters REE being mainly bound to organic colloids. Water table depth monitoring shows that the colloid source is located in the uppermost, organic-rich soil horizons, and that the colloid input occurs mainly when water table rises in response to rainfall events. It appears that the colloids amount that reaches groundwater increases downhill as the distance between soil organic-rich horizons and water table decreases. Topography is, therefore, the ultimate key factor that controls Ce anomaly spatial variability in these shallow groundwaters. Finally, the <0.2 μm REE fraction ultimately comes from two solid sources in these groundwaters: one located in the deep basement schist; another located in the upper, organic-rich soil horizon.
Core Ideas
AgrHyS is a long‐term observatory of the agroecosystem.
AgrHyS supports strongly interdisciplinary environmental research.
AgrHyS offers an original experimental setup to explore the ...soil–groundwater–water–plants–atmosphere continuum.
AgrHyS supports original and innovative techniques for environmental monitoring.
The AgrHyS is a long‐term agro‐hydrological observatory dedicated to studying the processes controlling hydro‐chemical fluxes in headwater catchments in response to the effects of agricultural. AgrHyS is composed of instrumented catchments located in western France in a temperate oceanic climate that are characterized by shallow groundwater (<8 m deep) over crystalline bedrocks (granite or schist) and is dominated by intensive agriculture with farming. AgrHyS provides long‐term observations starting in 1990 and supports highly interdisciplinary studies that provide novel contributions to environmental sciences, including hydrology, geochemistry, agricultural and soil sciences, hydrogeology, bioclimatology, and ecology. Here we describe the observatory sites, observation strategy, data management policy, and data access. The objective is to show how AgrHyS has contributed to research in hydrological and environmental sciences through a review of major insights of the research. This analysis highlights the role of AgrHyS in linking, validating, and enriching successive and complementary projects conducted over the last 25 yr. The second objective is to invite new collaborations with a large scientific community for future research.
Soluble organic fractions from soils of two agricultural sites from Brittany (France) have been analyzed to (i) identify the source of polar compounds in soils and (ii) evaluate the impact of organic ...fertilization and crop type on the distribution and concentration of polar compounds in soils. The main sources of polar compounds in soils are higher plants; they represent >70% of the polar compounds from the experimental sites and mainly originate from crop residues and animal manure. Crop type and animal manure application significantly increase the polar compound concentrations in soils. Among polar compounds, fatty acids cannot be used as specific markers because their distributions in soils whatever the crop type or organic fertilization type are the same. On the other hand, analysis of steroids provides interesting information. Cow and poultry manure applications increase only the concentration of steroids. Pig slurry fertilization modifies both the concentration and distribution of steroids. The identified pig slurry steroid fingerprint can persist in the soil for 9 years after the slurry application has been stopped. Those compounds are then robust markers to detect pig slurry contribution in soils.
The development of anoxic conditions in riparian wetland (RW) soils is widely known to release dissolved phosphorus (DP), but the respective roles of reductive dissolution of Fe-oxyhydroxide, pH ...changes and sediment inputs in this release remain debated. This study aimed to identify and quantify these respective roles via laboratory anaerobic/aerobic incubation of RW soils with and without the addition of sediment. The investigated soils came from two RWs with contrasting P status and organic matter (OM) content in their soils, while the added sediment came from an adjacent cultivated field. Results showed that the amount and speciation of the DP released during anaerobic/aerobic incubations were controlled by soil P status and soil OM content. During anaerobic incubation, DP release in the soil with high extractable P and low OM contents was controlled by reductive dissolution of Fe-oxyhydroxides (83%), whereas that released in the soil with low extractable P and high OM contents was controlled by an increase in pH (88%). Anaerobic incubation of a mixture of eroded sediments and RW soils increased the release of DP, dissolved organic carbon and Fe(II) (by 16%, 4% and 18%, respectively) compared to the simple addition of the amounts released during their separate incubations. Management practices should decrease soil erosion from upland fields to avoid deposition of P-rich sediments on RW soils. Management efforts should focus preferentially on RWs whose Fe:P molar ratios in the soil solution during reduction are the lowest, since they indicate a high risk that the DP released will be transferred to watercourses.
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•Dissolved P release increased in wetland soils under anoxic conditions.•Soil properties controlled the amount and speciation of the DP released.•FeO(OH) reduction controlled DP release in high extractable-P but low OM soils.•pH increase controlled DP release in low extractable-P but high OM soils.•Sediment input into RW soils increased its DP release under anoxic conditions.
The complexation of Fe(II) with organic matter (OM) and especially with humic acids (HAs) remains poorly characterized in the literature. In this study, batch experiments were conducted on a pH range ...varying from 1.95 to 9.90 to study HA-mediated Fe(II) binding. The results showed that high amounts of Fe(II) are complexed with HA depending on the pH. Experimental data were used to determine a new set of binding parameters by coupling PHREEPLOT and PHREEQC-Model VI. The new binding parameters (log KMA=2.19±0.16, log KMB=4.46±0.47 and ΔLK2=3.90±1.30) were validated using the LFER (linear free energy relationship) method and published adsorption data between Fe(II) and Suwannee River fulvic acid (SRFA) (Rose and Waite, 2003). They were then put in PHREEQC-Model VI to determine the distribution of Fe(II) onto HA functional groups. It was shown that Fe(II) forms mainly bidentate complexes, some tridentate complexes and only a few monodentate complexes with HA. Moreover, Fe(II) is mainly adsorbed onto carboxylic groups at acidic and neutral pH, whereas carboxy-phenolic and phenolic groups play a major role at basic pH. The major species adsorbed onto HA functional groups is Fe2+; Fe(OH)+ appears at basic pH (from pH 8.13 to 9.9). The occurrence of OM and the resulting HA-mediated binding of Fe(II) can therefore influence Fe(II) speciation and bioavailability in peatlands and wetlands, where seasonal anaerobic conditions prevail. Furthermore, the formation of a cationic bridge and/or the dissolution of Fe(III)-(oxy)hydroxides by the formation of Fe(II)-OM complexes can influence the speciation of other trace metals and contaminants such as As.
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•We measured Fe(II)-humic acid adsorption isotherm and pH sorption edge.•Results were modeled using a coupling of PHREEPLOT–PHREEQC-Model VI.•We determined the binding parameters for Fe(II)-humic acid complex.•We validated the binding parameters using LFERs and published datasets.•We determined the speciation of Fe(II)-humic acid complexes using PHREEQC-Model VI.
Adsorption experiments of rare-earth elements (REE) onto hydrous ferric oxide (HFO) were performed to evaluate the impact of organic complexation on both REE(III) adsorption and the Ce(III) oxidation ...rate. Scavenging experiments were performed at pH 5.2 with NaCl and NaNO
3 solutions containing either free REE (III) or REE(III)–humate complexes. The log
K
d
REE patterns obtained from HFO suspensions exhibit a slight positive Ce anomaly and an M-type lanthanide tetrad effect, in contrast with the partitioning between REE(III)–humate complexes and HFO, which yields completely flat distribution patterns. The “organic” partitioning runs yield log
K
d
REE
organic
/log
K
d
DOC ratios (DOC=dissolved organic carbon) close to 1.0, implying that the REE(III) and humate remain bound to each other during the adsorption experiment. The lack of any positive Ce anomaly or M-type lanthanide tetrad effect in the organic experiments seems to reflect an anionic adsorption of the REE–humate complex. Adsorption onto HFO takes place via the humate side of the REE(III)–humate complexes. The oxidation of Ce(III) by Fe(III) and the proportion of surface hydroxyl groups coordinated to REE(III) at the HFO surface are the two most commonly invoked processes for explaining the development of positive Ce anomalies and the M-type tetrad lanthanide effect. However, such processes cannot proceed since the REE are not in direct contact with the HFO suspensions, the latter being shielded by PHA. The present results further complicate the use of Ce anomalies as reliable paleoredox proxies in natural precipitates. They are also further demonstration that organic matter may inhibit the lanthanide tetrad effect in geological samples.
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