Stability of banded vegetation patterns under seasonal rainfall and limited soil moisture storage capacity

The delicate equilibrium of soil moisture and biomass may become unstable under water scarcity conditions causing banded vegetation patterns to form on hillsides of semi-arid catchments. Soil related processes that induce instability (namely: soil moisture advection and diffusion), have been evaluated numerically for different rainfall regimes. This study addresses the combined influence of some relevant soil characteristics, and the effect of seasonal precipitation on vegetation patterns, advancing the comprehension of those mechanisms that cause shifts toward banded vegetation patterns or bare states.     

Méthodologie de l'extraction des hydrocarbures aromatiques polycycliques. Application à des sédiments de la lagune de Bizerte (Tunisie)Methodology of extraction of polycyclic aromatic hydrocarbons. Application to sediment from the Bizerte Lagoon (Tunisia)

The marine environment is very complex, with several important chemical, biological, and sedimentological interactions. Sediments constitute a reservoir for numerous pollutants, including polycyclic aromatic hydrocarbons (PAHs). PAHs are known by their mutagenic and/or carcinogenic effect. This study evaluates contamination levels in Bizerte Lagoon with respect to PAHs. The extraction efficiency of PAHs from sediment has been evaluated using Soxhlet or ultrasonic procedures, based on the recovery of an internal standard (9,10-dihydroanthracene). Several PAHs were found, including sixteen that are reported in the priority list of pollutants of the US Environmental Protection Agency (EPA). PAHs analysis was carried out by GC–FID and GC–MS for 16 samples sampled in two seasons: summer and winter. Results permit an assessment of PAHs pollution in the Bizerte Lagoon. To cite this article: N. Mzoughi et al., C. R. Geoscience 334 (2002) 893–901.     

Net autotrophy in a fluvial lake: the relative role of phytoplankton and floating-leaved macrophytes

This study combined water- and sediment flux measurements with mass balances of dissolved gas and inorganic matter to determine the importance of pelagic and benthic processes for whole-system metabolism in a eutrophic fluvial lake. Mass balances of dissolved O₂, inorganic carbon (DIC), nitrogen (DIN), phosphorous (SRP), particulate N (PN) and P (PP) and Chl a were calculated at a nearly monthly frequency by means of repeated sampling at the lake inlet and outlet. Simultaneously, benthic fluxes of gas and nutrients, including denitrification rates, and the biomass of the dominant pleustophyte (Trapa natans) were measured, and fluxes of O₂ and CO₂ across the water–atmosphere interface were estimated from diel changes in outlet concentrations. On an annual scale, Middle Lake exhibited CO₂ supersaturation, averaging 313% (range 86–562%), but was autotrophic with a net O₂ production (6.35 ± 2.05 mol m⁻² y⁻¹), DIC consumption (−31.18 ± 18.77 mol m⁻² y⁻¹) and net export of Chl a downstream (8.38 ± 0.95 mol C m⁻² y⁻¹). Phytoplankton was the main driver of Middle Lake metabolism, with a net primary production estimated at 33.24 mol O₂ m⁻² y⁻¹, corresponding to a sequestration of 4.18 and 0.26 mol m⁻² y⁻¹ of N and P, respectively. At peak biomass, T. natans covered about 18% of Middle Lake’s surface and fixed 2.46, 0.17 and 0.02 mol m⁻² of C, N and P, respectively. Surficial sediments were a sink for O₂ (−14.47 ± 0.65 mol O₂ m⁻² y⁻¹) and a source of DIC and NH₄ ⁺ (18.84 ± 2.80 mol DIC m⁻² y⁻¹ and 0.83 ± 0.16 mol NH₄ ⁺ m⁻² y⁻¹), and dissipated nitrate via denitrification (1.44 ± 0.11 mol NO₃ ⁻ m⁻² y⁻¹). Overall, nutrient uptake by primary producers and regeneration from sediments were a minor fraction of external loads. This work suggests that the creation of fluvial lakes can produce net autotrophic systems, with elevated rates of phytoplanktonic primary production, largely sustained by allochtonous nutrient inputs. These hypereutrophic aquatic bodies are net C sinks, although they simultaneously release CO₂ to the atmosphere.     

Mixed model for interoccurrence times of earthquakes based on the expectation-maximization algorithm

In this paper, the goodness-of-fit test based on a convex combination of Akaike and Bayesian information criteria is used to explain the features of interoccurrence times of earthquakes. By analyzing the seismic catalog of Iran for different tectonic settings, we have found that the probability distributions of time intervals between successive earthquakes can be described by the generalized normal distribution. This indicates that the sequence of successive earthquakes is not a Poisson process. It is found that by decreasing the threshold magnitude, the interoccurrence time distribution changes from the generalized normal distribution to the gamma distribution in some seismotectonic regions. As a new insight, the probability distribution of time intervals between earthquakes is described as a mixture distribution via the expectation-maximization algorithm.     

On the crystal chemistry and elastic behavior of a phlogopite 3<Emphasis Type="Italic">T</Emphasis>

The crystal chemistry and the elastic behavior under isothermal conditions up to 9 GPa of a natural, and extremely rare, 3T-phlogopite from Traversella (Valchiusella, Turin, Western Alps) [(K_0.99Na_0.05Ba_0.01)(Mg_2.60Al_0.20Fe _0.21 ²⁺ )[Si_2.71Al_1.29O₁₀](OH)₂, space group P3₁12, with a = 5.3167(4), c = 30.440(2) Å, and V = 745.16(9) ų] have been investigated by electron microprobe analysis in wavelength dispersion mode, single-crystal X-ray diffraction at 100 K, and in situ high-pressure synchrotron radiation powder diffraction (at room temperature) with a diamond anvil cell. The single-crystal refinement confirms the general structure features expected for trioctahedral micas, with the inter-layer site partially occupied by potassium and sodium, iron almost homogeneously distributed over the three independent octahedral sites, and the average bond distances of the two unique tetrahedra suggesting a disordered Si/Al-distribution (i.e., 〈T1-O〉 ~ 1.658 and 〈T2-O〉 ~ 1.656 Å). The location of the H-site confirms the orientation of the O–H vector nearly perpendicular to (0001). The refinement converged with R ₁(F) = 0.0382, 846 unique reflections with F _O > 4σ(F _O) and 61 refined parameters, and not significant residuals in the final difference-Fourier map of the electron density (+0.77/?0.37 e ^?/ų). The high-pressure experiments showed no phase transition within the pressure range investigated. The P–V data were fitted with a Murnaghan (M-EoS) and a third-order Birch-Murnaghan equation of state (BM-EoS), yielding: (1) M-EoS, V ₀ = 747.0(3) ų, K _T0 = 44.5(24) GPa, and K′ = 8.0(9); (2) BM-EoS, V ₀ = 747.0(3) ų, K _T0 = 42.8(29) GPa, and K′ = 9.9(17). A comparison between the elastic behavior in response to pressure observed in 1M- and 3T-phlogopite is made.     

Deciphering the weathering processes using environmental mineralogy and geochemistry: Towards an integrated model of laterite and podzol genesis in the Upper Amazon Basin

This article reports major results from collaborative research between France and Brazil on soil and water systems, carried out in the Upper Amazon Basin. It reveals the weathering processes acting in the partly inundated, low elevation plateaus of the Basin, mostly covered by evergreen forest. Our findings are based on geochemical data and mineral spectroscopy that probe the crystal chemistry of Fe and Al in mineral phases (mainly kaolinite, Al- and Fe-(hydr)oxides) of tropical soils (laterites). These techniques reveal crystal alterations in mineral populations of different ages and changes of metal speciation associated with mineral or organic phases. These results provide an integrated model of soil formation and changes (from laterites to podzols) in distinct hydrological compartments of the Amazon landscapes and under altered water regimes.