Submicron scale imaging of soil organic matter dynamics using NanoSIMS - From single particles to intact aggregates

The specific features of the nano-scale secondary ion mass spectrometry (NanoSIMS) technology with the simultaneous analysis of up to seven ion species with high mass and lateral resolution enables us to perform multi-element and stable isotope measurements at the submicron scale. To elucidate the power of this technique, we performed an incubation experiment with soil particles of the fine silt and clay fractions (from an Albic Luvisol), with occluded particulate organic material and intact soil aggregates (from a Haplic Chernozem), using a ¹³C and ¹⁵N labelled amino acid mixture as tracer. Before and during 6-day incubation after the addition of the label, samples were consecutively prepared for NanoSIMS analysis. For this purpose, two different sample preparation techniques were developed: (i) wet deposition and (ii) the sectioning of epoxy resin embedded samples. Single soil particles (fine silt/clay fraction) showed an enrichment of ¹³C and ¹⁵N after label addition that decreased over time. On aggregates of particulate organic matter, re-aggregated during the 6-day incubation experiment, we could show a spatially heterogeneous enrichment of ¹³C and ¹⁵N on the particle surface. The enrichment in ¹⁵N demonstrated the diffusion of dissolved organic matter into intact soil aggregate interiors. The prospects of NanoSIMS for three dimensional studies of stable C and N isotopes in organo-mineral associations is demonstrated by the recorded depth profiles of the organic matter distribution on mineral particles.     

Wrench tectonics flip at oblique subduction. A model from New Zealand

In eastern North Island New Zealand, oblique subduction of the Pacific Plate beneath the Australian Plate is associated with strain partitioning. Dextral along-strike component of displacement occurred first at Early Miocene major faults within the eastern fore-arc domain. These faults were active from Early Miocene to Pliocene times. Since Pliocene times, most of the movement occurs at western faults such as the Wellington Fault. The latter joins the back-arc domain to the north. The jump of wrench faulting is related to the oblique opening of the back-arc domain. Both phenomena are impeded southwards by the Hikurangi oceanic plateau entering the subduction zone. To cite this article: J. Delteil et al., C. R. Geoscience 335 (2003).     

Rivers, chemical weathering and Earth's climate

We detail the results of recent studies describing and quantifying the large-scale chemical weathering of the main types of continental silicate rocks: granites and basalts. These studies aim at establishing chemical weathering laws for these two lithologies, describing the dependence of chemical weathering on environmental parameters, such as climate and mechanical erosion. As shown within this contribution, such mathematical laws are of primary importance for numerical models calculating the evolution of the partial pressure of atmospheric CO₂ and the Earth climate at geological timescales. The major results can be summarized as follow: (1) weathering of continental basaltic lithologies accounts for about 30% of the total consumption of atmospheric CO₂ through weathering of continental silicate rocks. This is related to their high weatherability (about eight times greater than the granite weatherability); (2) a simple weathering law has been established for basaltic lithologies, giving the consumption of atmospheric CO₂ as a function of regional continental runoff, and mean annual regional temperature; (3) no such simple weathering law can be proposed for granitic lithologies, since the effect of temperature can only be identified for regions displaying high continental runoff; (4) a general law relating mechanical erosion and chemical weathering has been validated on small and large catchments. The consequences of these major advances on the climatic evolution of the Earth are discussed. Particularly, the impacts of the onset of the Deccan trapps and the Himalayan orogeny on the global carbon cycle are reinvestigated. To cite this article: B. Dupré et al., C. R. Geoscience 335 (2003).     

Experimental constraints on ultrapotassic magmatism from the Bohemian Massif (durbachite series, Czech Republic)

The equilibrium phase relations of a mafic durbachite (53 wt.% SiO₂) from the Třebíč pluton, representative of the Variscan ultrapotassic magmatism of the Bohemian Massif (338–335 Ma), have been determined as a function of temperature (900–1,100°C), pressure (100–200 MPa), and H₂O activity (1.1–6.1 wt.% H₂O in the melt). Two oxygen fugacity ranges were investigated: close to the Ni–NiO (NNO) buffer and 2.6 log unit above NNO buffer (∆NNO + 2.6). At 1,100°C, olivine is the liquidus phase and co-crystallized with phlogopite and augite at 1,000°C for the whole range of investigated pressure and water content in the melt. At 900°C, the mineral assemblage consists of augite and phlogopite, whereas olivine is not stable. The stability field of both alkali feldspar and plagioclase is restricted to low pressure (100 MPa) at nearly water-saturated conditions (<3–4 wt.% H₂O) and T < 900°C. A comparison between experimental products and natural minerals indicates that mafic durbachites have a near-liquidus assemblage of olivine, augite, Ti-rich phlogopite, apatite and zircon, followed by alkali feldspar and plagioclase, similar to the mineral assemblage of minette magma. Natural amphibole, diopside and orthopyroxene were not reproduced experimentally and probably result from sub-solidus reactions, whereas biotite re-equilibrated at low temperature. The crystallization sequence olivine followed by phlogopite and augite reproduces the sequence inferred in many mica-lamprophyre rocks. The similar fractionation trends observed for durbachites and minettes indicate that mafic durbachites are probably the plutonic equivalents of minettes and that K- and Mg-rich magmas in the Bohemian Massif may have been generated from partial melting of a phlogopite–clinopyroxene-bearing metasomatized peridotite. Experimental melt compositions also suggest that felsic durbachites can be generated by simple fractionation of a more mafic parent and mixing with mantle-derived components at mid crustal pressures.     

Arsenic in an alkaline AMD treatment sludge: Characterization and stability under prolonged anoxic conditions

Lime treatment of acid mine drainage (AMD) generates large volumes of neutralization sludge that are often stored under water covers. The sludge consists mainly of calcite, gypsum and a widespread ferrihydrite-like Fe phase with several associated species of metal(loid) contaminants. The long-term stability of metal(loid)s in this chemically ill-defined material remains unknown. In this study, the stability and speciation of As in AMD sludge subjected to prolonged anoxic conditions is determined. The total As concentration in the sludge is 300 mg kg⁻¹. In the laboratory, three distinct water cover treatments were imposed on the sludge to induce different redox conditions (100%N₂, 100%N₂ + glucose, 95%N₂:5%H₂). These treatments were compared against a control of oxidized, water-saturated sludge. Electron micro-probe (EMP) analysis and spatially resolved synchrotron X-ray fluorescence (SXRF) results indicate that As is dominantly associated with Fe in the sludge. In all treatments and throughout the experiment, measured concentrations of dissolved As were less than 5 μg L⁻¹. Dissolved Mn concentration in the N₂ + glucose treatment increased significantly compared to other treatments. Manganese and As K-edge X-ray absorption near edge structure spectroscopy (XANES) analyses showed that Mn was the redox-active element in the solid-phase, while As was stable. Arsenic(V) was still the dominant species in all water-covered sludges after 9 months of anoxic treatments. In contrast, Mn(IV) in the original sludge was partially reduced into Mn(II) in all water-covered sludges. The effect was most pronounced in the N₂ + glucose treatment, suggesting microbial reduction. Micro-scale SXRF and XANES analysis of the treated sludge showed that Mn(II) accumulated in areas already enriched in Fe and As. Overall, the study shows that AMD sludges remain stable under prolonged anoxic conditions. External sources of chemical reductants or soluble C were needed to induce lower redox state in the systems, and even under these imposed treatments, only weakly reducing conditions (Mn threshold) developed. The results suggest that As(V) in AMD sludge will remain stable under prolonged anoxic conditions as long as Mn(IV) is present and organic matter accumulation is negligible.     

Occurence of abundant diradicaloid moieties in the insoluble organic matter from the Orgueil and Murchison meteorites: a fingerprint of its extraterrestrial origin?

The highly aromatic structure of the macromolecular organic matter (OM) of the Murchison and Orgueil meteorites was recently shown to contain free organic radicals which are concentrated in micro-regions in contrast with terrestrial samples which always show an homogeneous distribution of radicals. An additional signature is revealed, in the present study, by the evolution of the radical concentration with temperature. Whereas in terrestrial samples, this concentration is independent of temperature (Curie magnetism), a significant increase is observed above 150 K in the two meteorites. Based on the electronic structure of organic radicals, calculated by Extended Hückel and Density Functional methods, this behavior was assigned to the occurrence of diradicaloid moieties hosted by aromatic structures of 10 to 15 rings and having a quinoidal structure. They represent 40 and 25% of the total radicals in Orgueil and Murchison, respectively. The search for the cosmochemical interpretation of this unique observation should open a new field of experimental investigations.     

Apports de la méthode sismique réflexion haute résolution à l'identification des structures profondes des formations tertiaires en Médoc (Gironde,France) : implications hydrogéologiques

Near Bordeaux (France), the Oligocene aquifer is a potential target for drinking water supply. A high-resolution seismic campaign and several exploration wells helped to clarify the geometry of this formation and, as a consequence, the Medoc Tertiary aquifers. The major information extracted from these new data is the existence of a soft deformation inducing two different deposit areas, showing different hydrogeological characteristics (thickness, type, hydraulic properties, etc.). The presence of an erosional gap area affecting the Oligocene formations lead us to propose a new image of groundwater flow in the area, in the context of a predicted intensive exploitation of this resource. To cite this article: F. Larroque, A. Dupuy, C. R. Geoscience 336 (2004).     

Biogenic nanoparticulate UO2: Synthesis, characterization, and factors affecting surface reactivity

The surface reactivity of biogenic, nanoparticulate UO₂ with respect to sorption of aqueous Zn(II) and particle annealing is different from that of bulk uraninite because of the presence of surface-associated organic matter on the biogenic UO₂. Synthesis of biogenic UO₂ was accomplished by reduction of aqueous uranyl ions, by Shewanella putrefaciens CN32, and the resulting nanoparticles were washed using one of two protocols: (1) to remove surface-associated organic matter and soluble uranyl species (NAUO2), or (2) to remove only soluble uranyl species (BIUO2). A suite of bulk and surface characterization techniques was used to examine bulk and biogenic, nanoparticulate UO₂ as a function of particle size and surface-associated organic matter. The N₂-BET surface areas of the two biogenic UO₂ samples following the washing procedures are 128.63 m² g⁻¹ (NAUO2) and 92.56 m² g⁻¹ (BIUO2), and the average particle sizes range from 5-10 nm based on TEM imaging. Electrophoretic mobility measurements indicate that the surface charge behavior of biogenic, nanoparticulate UO₂ (both NAUO2 and BIUO2) over the pH range 3-9 is the same as that of bulk. The U L_III-edge EXAFS spectra for biogenic UO₂ (both NAUO2 and BIUO2) were best fit with half the number of second-shell uranium neighbors compared to bulk uraninite, and no oxygen neighbors were detected beyond the first shell around U(IV) in the biogenic UO₂. At pH 7, sorption of Zn(II) onto both bulk uraninite and biogenic, nanoparticulate UO₂ is independent of electrolyte concentration, suggesting that Zn(II) sorption complexes are dominantly inner-sphere. The maximum surface area-normalized Zn(II) sorption loadings for the three substrates were 3.00 ± 0.20 μmol m⁻² UO₂ (bulk uraninite), 2.34 ± 0.12 μmol m⁻² UO₂ (NAUO2), and 2.57 ± 0.10 μmol m⁻² UO₂ (BIUO2). Fits of Zn K-edge EXAFS spectra for biogenic, nanoparticulate UO₂ indicate that Zn(II) sorption is dependent on the washing protocol. Zn-U pair correlations were observed at 2.8 ± 0.1 Å for NAUO2 and bulk uraninite; however, they were not observed for sample BIUO2. The derived Zn-U distance, coupled with an average Zn-O distance of 2.09 ± 0.02 Å, indicates that Zn(O,OH)₆ sorbs as bidentate, edge-sharing complexes to UO₈ polyhedra at the surface of NAUO2 nanoparticles and bulk uraninite, which is consistent with a Pauling bond-valence analysis. The absence of Zn-U pair correlations in sample BIUO2 suggests that Zn(II) binds preferentially to the organic matter coating rather than the UO₂ surface. Surface-associated organic matter on the biogenic UO₂ particles also inhibited particle annealing at 90 °C under anaerobic conditions. These results suggest that surface-associated organic matter decreases the reactivity of biogenic, nanoparticulate UO₂ surfaces relative to aqueous Zn(II) and possibly other environmental contaminants.     

Numerical study of the rectilinear isosceles restricted problem

We study the orbit of a particle in the plane of symmetry of two equal mass primaries in rectilinear keplerian motion. Using the surfaces of section we look for periodic orbits, examine their stability and search for quasi-periodic orbits and regions of escape. For large values of the angular momentumC, we verify the validity of the approximation of two fixed centers. However, we also find irregular families of orbits and resonance zones.For small values ofC, the approximation is no longer valid, but we find invariant curves whose interpretation might be interesting.