The Carboniferous to Permian volcanic-sedimentary succession shown by the LY-F core from the Lucenay-lès-Aix area, in the northern part of the Massif Central, has been studied in order to obtain both ...landscape reconstructions (sedimentological analyses) and geochronological constraints (U-Pb dating on zircon and apatite). The lowermost part of the core consists mainly of lacustrine deposits with Gilbert-type delta and volcaniclastic-rich fan delta deposits including several altered volcanic ash layers (tonstein). In contrast, in the uppermost part of the core, playa-lake deposits dominate. LA-ICP-MS U-Pb analyses were performed on both zircon and apatite grains from interbedded tonsteins. This coupled U-Pb dating approach allows to assess potential reworking of volcanic material or the occurrence of non-volcanic grains, such as xenocrysts, in order to provide better evaluations for the depositional ages of the tonsteins. These investigations reveal that sedimentation took place between the late Gzhelian and the late Sakmarian (i.e., between c. 301 and 290 Ma). This sedimentary succession can therefore be compared to those from adjacent basins for which geochronological constraints are available (i.e., Autun and Lodève basins, resp. located north and south of the Massif Central). This study provides a reference section for future comparisons with similar sections from other Carboniferous to Permian basins, in France as well as elsewhere in Europe.
•The LY-F section contains lacustrine deposits with interbedded tonstein layers.•The lower part of the core consists of Gilbert-type deltas deposits.•They are interbedded with volcaniclast-rich delta deposits.•The upper part of the core consists of playa-lake deposits.•The depositional age of the LY-F section is late Gzhelian to late Sakmarian.
► Mass-balance and carbon isotopic signature of dissolved organic carbon in an eutrophic reservoir. ► Identification and quantification of allochthonous sources relative to autochthonous sources. ► ...No DOC sources derived from phytoplankton degradation. ► No evidence of organic matter desorption from sediments.
A combined mass-balance and stable isotope approach was set up to identify and quantify dissolved organic carbon (DOC) sources in a DOC-rich (9
mg
L
−1) eutrophic reservoir located in Western France and used for drinking water supply (so-called Rophemel reservoir). The mass-balance approach consisted in measuring the flux of allochthonous DOC on a daily basis, and in comparing it with the effective (measured) DOC concentration of the reservoir. The isotopic approach consisted, for its part, in measuring the carbon isotope ratios (
δ
13C values) of both allochthonous and autochthonous DOC sources, and comparing these values with the
δ
13C values of the reservoir DOC. Results from both approaches were consistent pointing out for a DOC of 100% allochthonous origin. In particular, the
δ
13C values of the DOC recovered in the reservoir (−28.5
±
0.2‰;
n
=
22) during the algal bloom season (May–September) showed no trace of an autochthonous contribution (
δ
13C in algae
=
−30.1
±
0.3‰;
n
=
2) being indistinguishable from the
δ
13C values of allochthonous DOC from inflowing rivers (−28.6
±
0.1‰;
n
=
8). These results demonstrate that eutrophication is not responsible for the high DOC concentrations observed in the Rophemel reservoir and that limiting eutrophication of this reservoir will not reduce the potential formation of disinfection by-products during water treatment. The methodology developed in this study based on a complementary isotopic and mass-balance approach provides a powerful tool, suitable to identify and quantify DOC sources in eutrophic, DOC-contaminated reservoirs.
The primary aim of this study was to differentiate immersed wood samples from dry wood samples based on chemical analysis. The method has been developed to be applied to wood found in archaeological ...sites to distinguish between driftwood and wood that was cut in the forest tundra and then transported to the sites. The results of our research show that Na concentrations in the immersed samples were much higher than in the dry samples for coniferous and deciduous wood samples. Principal components analysis (PCA) based on the element concentrations normalized to the total cation concentrations show that the data from the immersed wood samples and the dry wood samples clustered into two separate groups.
Dolomite concretions from Paleoproterozoic organic-rich sediments of the Franceville Basin (Francevillian) in southeastern Gabon are studied. These concretions, belonging to one of the rare ...well-preserved Proterozoic sediments in the world, are mainly observed in the shaly levels of the Francevillian B1 Formation. The concretions often show a central pyrite-rich layer. The decrease in the carbonate content from the centre to the edges, and the carbon isotopic signal of carbonate within the concretions indicate a concentric growth of the concretions prior to compaction and precipitation of carbonate associated with the degradation of organic matter during early diagenesis. From its geochemical signature and texture, dolomite was not recrystallized. Moreover, the interior of the concretions shows well-preserved accumulations of microbial mats. These concretions thus appear to represent an exceptional record of the biogeochemical signature of the crucial period when the oxygen rose for the first time in Earth's atmosphere. If their rare-earth element pattern does not seem to reflect that of Paleoproterozoic seawater, they appear as potential tools for estimating the original oceanic temperature in surface sediments.
Chemical erosion rates have been determined on two upland granitic catchments under agricultural pressure in Brittany, France. Intensive agriculture has been carried out for at least 30 years in this ...region. The influence of geochemical processes related to agriculture on the chemistry of streamwaters is determined through a geochemical mass balance. The elemental export fluxes from these two agricultural catchments are then compared with other catchments around the world.
The volume and concentrations of the precipitation are taken into account, as well as the inputs of organic and chemical fertilizers, groundwaters and streamwaters, to estimate the relative influence on export fluxes, and then evaluate the elemental fluxes released by weathering. The relatively high Si flux of about 1.8
±
0.9 kmol ha
−
1
yr
−
1
is directly attributed to the chemical weathering of soil and rock in the catchment system. However, the Si flux remains comparable to values found in both small and large-sized catchments under temperate and tropical conditions. On the other hand, extremely high fluxes of major cations (Ca, Na and Mg) are observed, ranging from 4.2
±
2.6 to 8.0
±
4.9 kmol ha
−
1
yr
−
1
, which can be attributed to chemical weathering. These fluxes remain dramatically higher than those found in granitic catchments worldwide.
Despite an integrated agriculture, the soil acidification induced by fertilizer application leads mainly to a release of major cations from the system, by processes of soil ion-exchange leaching as well as weathering of soil and rock.
Metal contaminants were found in a soil amended with a compost produced from household waste that included plastic debris. A strong correlation between the microplastics (MPs) distribution and the ...metal concentrations in the soil profile. Metals in the highest concentrations corresponded to the most significant plastic additives. As the total amount of plastic debris and the loss of metals and plastic particles were unknown, it was not possible to conclude that plastic debris is responsible for all of the metal contamination. Amount of calcium (Ca) in MPs (24.5 g kg-1 of MPs) are high in response to it use as filler in plastic formulation. As strontium (Sr) is an analogous of Ca, the potential of 87Sr/86Sr ratios to quantify MPs and nanoplastics (NPs) was tested. Elemental concentrations (Ca, Cd, Cr Pb, Ni and Sr) coupled with Sr isotopic ratios were compared in both amended soil and a reference soil without amendment. The 87Sr/86Sr ratios of the amended soil were less radiogenic than for the reference soil (0.724296 ±0.000010 against 0.726610 ±0.00009 for the 0-5 cm soil layer, respectively). The Sr isotopic ratio of MPs was also significantly less radiogenic (0.711527 ±0.000010 for the 0-5 cm soil layer) than for soils. The MPs< 2 mm occurred in the ploughed soil depth with concentration varying from 1.19 to 0.09 mg kg-1. The NPs concentration stayed quite constant from 0 to 55 cm at around 0.25 µg kg-1. The presence of NPs until 55 cm soil depth was attested by the detection of polypropylene NPs by Py-GCMS in the soil solution <0.8 µm. These results highlighted, for the first time, the NPs mobility throughout the soil depth and their ability to reach hydrosystems. It also demonstrated that Sr could be a potential tracer of the MPs<2 mm and NPs amount occurring in soils.
This study deals with the chemical characterization of the biogeochemical processes occurring in a shallow aquifer in crystalline rocks. The influence of rock heterogeneity and the related physical ...processes on the aquifer biogeochemistry have been investigated. A hydrochemical survey (major anion and cation analysis) shows that rock heterogeneity leads to a stronger spatial than temporal variability. Some rapidly recharged and low-mineralized waters are present at the soil/rock interface. However the pumped well intersects a preferential flow path and pumps nitrate-rich water. Sulfur and oxygen isotope data from sulfates in the pumped water clearly show sulfide oxidation with only 20-30% of the oxygen atoms in sulfates formed by sulfide oxidation coming from atmospheric oxygen. This low contribution of molecular oxygen in sulfide oxidation, associated with the drastic decrease in nitrate concentration, involves a marked relationship between the nitrogen and sulfur cycles through denitrification, coupled with sulfide oxidation. Conversely, for rapidly recharged waters, the rock physical heterogeneity allows sulfide oxidation by molecular oxygen indicated by a contribution of atmospheric oxygen of nearly 70% in the newly formed sulfate. As the aquifer biogeochemistry is controlled by the physical characteristics of the rocks, pumping may overcome the natural flux pattern described previously. This anthropogenic disturbance leads to a modification of water pathways (spatial mixing or relative contribution of the fracture/matrix waters to the global fluxes) and, consequently, to a modification of the physical and biogeochemical processes occurring in the aquifer.
Predicting the behavior and fate of redox-sensitive trace elements (TEs; e.g. As, U, Cu, Cr) in natural systems is challenging. Colloids have been reported to control TEs speciation and catalyze TEs ...redox reactions in many aquatic environments. We hypothesize that the lack of accurate thermodynamic models that account for the role of colloids in TEs speciation explains our inability to predict their redox state distribution in the environment. The slow evolution of the colloidal compartment in response to the prevailing bio/hydro/pedo/climatological conditions need to be decoupled from the fast TEs redox reactions promoted by colloidal surfaces. Further progress is hampered by experimental and theoretical challenges associated with capturing the extreme physical and chemical heterogeneity of colloids, their metastable structures, and their dynamic transformation behavior.
Display omitted
Re-Os analyses were performed on five soil profiles developed on silicate lithologies in the Narayani drainage basin of the Himalayas, as well as on several altered black shales and associated ...alteration phases. The results indicate that all of these samples have lost large amounts of Re. This Re loss approached 100% in the black shale-derived soils, which also most probably lost substantial quantities of Os. The near constancy of Os and Re concentrations with depth indicates that this loss occurred quite early in the alteration process, rather than progressively during the course of soil development. A slight decrease in
187Os/
188Os was observed toward the surface of the two black shale soil profiles, possibly indicating a minor preferential mobility of radiogenic
187Os for this lithology. In the black shale-derived saprolite and soils, the loss of Re and Os was related to the loss of organic carbon. The importance of this phase is underscored by the fact that organic material separated from a black shale sample accounted for nearly all of the Os contained in the whole rock. Analysis of alteration phases from the surface of a weathered black shale showed that Os liberated during alteration was partially reprecipitated in Fe oxides, with Os isotopic compositions very similar to that of the original rock, whereas Re was removed with the fluid phase.
These results demonstrate the disproportionately large role that black shale weathering may play in determining the Re and Os contents of continental runoff and, thus, ultimately of seawater, but they also emphasize the possible complexity of this process. The data suggest that a large fraction of the Os contained in black shales is released during chemical alteration, which is apparently not the case for most other silicate rocks. Therefore, the potential contribution of black shales to the dissolved Os budget of runoff and river water may be even greater than that expected from the high Os concentrations of these rocks. The extent to which this Os will affect the seawater Os composition depends on how efficiently it is recaptured by secondary phases such as Fe-Mn oxides in situ or during transport. These results also underscore the high mobility of Re with respect to Os at every stage in the erosional process, thus explaining the order of magnitude increase in Re/Os ratio from the black shale source rocks to seawater.
The Late Cretaceous–early Paleogene interval is globally associated with transient to long-term changes in the stable carbon isotopic composition of marine carbonates (δ13Ccarb). Based on ...biostratigraphic reconstruction, this critical period of Earth's history is thought to coincide with the deposition of world heritage Paleocene phosphate deposits (phosphorites) in northwestern Morocco. However, the detailed stratigraphy of the Gantour basin, one of the most important Moroccan phosphate deposits, has not yet been constrained. For instance, the former “Montian” Stage has been used to tentatively approximate the Danian, whereas the succeeding Selandian Stage remains to be identified. Here, we develop a detailed organic carbon isotopic (δ13Corg) curve from phosphorus-rich horizons of the western Gantour sedimentary sequence in an attempt to constrain their stratigraphic placement and depositional age model. Upsection, these strata host long-term negative and positive δ13Corg trends that tend to correlate with global δ13Ccarb records of the Cretaceous–Paleogene and mid-Thanetian transitional boundaries. The data support the presence of Danian and Selandian rocks in the Gantour basin, which are succeeded by strata containing characteristic signatures of the well-known Cenozoic δ13C maximum at 58–57.5 Ma (the Paleocene Carbon Isotope Maximum). Our results shift the previously proposed Cretaceous–Paleogene transition in the Gantour basin further down into the older sediment C2M layer without interfering with recorded massive biological turnover in faunal diversity and abundance. Moreover, the refined stratigraphy suggests that the deposition of the Gantour phosphorites spanned ~8.5 Myr. Our results confirm the utility of δ13Corg chemostratigraphy for dating and correlating phosphate-bearing deposits of the Tethyan province. They have important implications for deciphering Paleocene phosphogenesis, the co-evolution of associated vertebrate groups, and for prospecting phosphorus-rich mineral deposits.
PDF
