Evidence for a differentiated ignimbritic activity ending the building-stage of Tahiti-Nui (French Polynesia)

A thick columnar-jointed ignimbrite-like formation occupies the upper part of the Orohena Massif on Tahiti-Nui. Differentiated products related to the ignimbrite were sampled at the base of the Orohena cliff and have been dated using the unspiked K/Ar Cassignol technique at $502\pm 7\text{ka}$. These pyroclastic volcanic rocks show chemical similarities with Ne-syenitic rocks from a shallow magma reservoir presently exposed farther south because of the dissection of the island by deep erosion. The presence of less-evolved magma blebs in the ignimbrite products suggests an incomplete magma mixing process. This likely indicates a late injection of a more basic magma into the reservoir, probably triggering the ignimbritic eruption, unique in the generally non-explosive eruptive history of Tahiti-Nui. To cite this article: A. Hildenbrand, P.-Y. Gillot, C. R. Geoscience 338 (2006).     

Fluage du sel gemme sous très faibles charges

Long-term creep tests have been performed under very small uniaxial loadings ($\sigma =0.02$ to 0.1 MPa). Testing devices were set in an underground gallery, where temperature fluctuations are in the order of one hundredth of a Celsius degree. The mechanical loading was provided by dead weights. The displacements were measured through special sensors whose resolution is $\mathrm{\Delta }?={10}^{\mathrm{?8}}$. Strain rates as small as $\dot{?}={10}^{\mathrm{?12}}{\text{s}}^{\mathrm{?1}}$ were measured. The tests prove that constitutive laws deduced from tests performed on rock salt samples under standard mechanical loadings cannot be extrapolated to very small loadings. To cite this article: P. Bérest et al., C. R. Geoscience 336 (2004).     

Âge 40K/40Ar,Carbonifère inférieur,du magmatisme basique filonien du synclinal paléozoïque de Tin Serririne,Sud-Est du Hoggar (Algérie)

Palaeozoic formations of the Tassilis Oua-n-Ahaggar (southeastern Hoggar) include magmatic rocks in the Tin Serririne syncline. Slight contact metamorphism of the overlying bed and studies of anisotropy of magnetic susceptibility of these rocks show that the latter correspond to sills and NW–SE or north–south dykes. ⁴⁰K/⁴⁰Ar dating of separated feldspars and whole rock for one sample and of whole rock for two other samples give a mean age of $347.6\pm 16.2\text{Ma}$ (at the 2-σ level), thus corresponding to a Lower Carboniferous (Tournaisian) age. Taking into account both the age of this magmatism and the stratigraphic and structural data for this region suggests that dolerites were emplaced within distensive zones that are related to the reactivation of Panafrican faults. To cite this article: H. Djellit et al., C. R. Geoscience 338 (2006).     

Kinetics of oil group-type generation and expulsion: An integrated application to Dongying Depression,Bohai Bay Basin,China

Pyrolysis of two kerogens isolated from the ${\text{E}}_{2\text{-}3}{\text{s}}_3^3$ and ${\text{E}}_{2\text{-}3}{\text{s}}_4^1$ source rocks in the Niuzhuang sag, Dongying Depression, Bohai Bay Basin, China, was performed in a confined system. The products were extracted with solvent and separated using micro-column chromatography into group-type fractions (saturates, aromatics, resins and asphaltenes) with the kerogen residue in each case undergoing swelling with a variety of solvents. The kinetics for generation and retention of crude oil and its group-type fractions from the kerogens were studied and the kinetic parameters applied to modeling generation and retention of crude oil and its fractions from the ${\text{E}}_{2\text{-}3}{\text{s}}_3^3$ and ${\text{E}}_{2\text{-}3}{\text{s}}_4^1$ source rocks on the basis of burial and thermal history of the Niuzhuang sag. The results show that the “normal oil” was generated at about 4.26 Ma and 24.85 Ma ago, but expelled at about 3.96 Ma and 17.46 Ma ago, respectively, from ${\text{E}}_{2\text{-}3}{\text{s}}_3^3$ and ${\text{E}}_{2\text{-}3}{\text{s}}_4^1$ source rocks. The current proportions of the expelled saturates, aromatics and NSOs are about 60%, 15% and 25%, respectively.     

Isotopic tracing of perchlorate sources in groundwater from Pomona,California

The groundwater of Pomona, California, is contaminated with perchlorate (${\text{ClO}}_4^{-}$). This water is treated to reduce the ${\text{ClO}}_4^{-}$ concentration to less than 6 μg L⁻¹ for compliance with California Department of Public Health drinking water regulations. A study of the isotopic composition of oxygen and chlorine in ${\text{ClO}}_4^{-}$ has been conducted to determine the source of the contamination. Isotopic compositions were measured for ${\text{ClO}}_4^{-}$ samples extracted from 14 wells, yielding ranges of δ¹⁸O values from −10.8‰ to −8.0‰, Δ¹⁷O values from +4.6‰ to +7.5‰, and δ³⁷Cl values from −12.8‰ to −8.9‰. Evaluation of mixing proportions using published isotopic data for three ${\text{ClO}}_4^{-}$ end-members (synthetic, Atacama, and indigenous natural ${\text{ClO}}_4^{-}$) indicates that contamination is dominantly (85–89%) Atacama ${\text{ClO}}_4^{-}$ derived from past use of imported Chilean nitrate fertilizer in citrus cultivation. This interpretation is consistent with (1) aerial photography archives showing extensive citrus fields surrounding Pomona in the early- to mid-20th century, (2) mass-balance estimates for ${\text{ClO}}_4^{-}$, and (3) numerical hydrologic models yielding travel-times for ${\text{ClO}}_4^{-}$ from fields to wells that are in the range of 15 to >100 years. The hydrologic models predict that ${\text{ClO}}_4^{-}$ contamination of Pomona groundwater will persist for decades into the future.     

Geochemistry of surface and ground water in Guiyang,China: Water/rock interaction and pollution in a karst hydrological system

The chemical compositions of the surface/ground water of Guiyang, the capital city of Guizhou Province, China are dominated by Ca²⁺, Mg²⁺, ${\text{HCO}}_3^{-}\text{and}{\text{SO}}_4^{2-}$, which have been derived largely from chemical weathering of carbonate rocks (limestone and dolomite). The production of ${\text{SO}}_4^{2-}$ has multiple origins, mainly from dissolution of sulfate evaporites, oxidation of sulfide minerals and organic S in the strata, and anthropogenic sources. Most ground water is exposed to soil CO₂ and, therefore, the H₂CO₃ which attacks minerals contains much soil C. In addition, the H₂SO₄ produced as a result of the oxidation of sulfides in S-rich coal seams and/or organic S, is believed to be associated with the chemical weathering of rocks. The major anthropogenic components in the surface and ground water include K⁺, Na⁺, Cl⁻, ${\text{SO}}_4^{2-}\text{and}{\text{NO}}_3^{-}$, with Cl⁻ and ${\text{NO}}_3^{-}$ being the main contributors to ground water pollution in Guiyang and its adjacent areas. The seasonal variations in concentrations of anthropogenic components demonstrate that the karst ground water system is liable to pollution by human activities. The higher content of ${\text{NO}}_3^{-}$ in ground water compared to surface water during the summer and winter seasons, indicates that the karstic ground water system is not capable of denitrification and therefore does not easily recover once contaminated with nitrates.     

New data on the age of the installation of Urgonian-type carbonates along the northern Tethyan margin: biostratigraphy of the Chopf Member (Helvetic Alps,eastern Switzerland)

The Chopf Member is a glauconitic, phosphate-bearing succession that occurs in the distal part of the Helvetic Alps (eastern Switzerland). The recent discovery of age-diagnostic ammonites within this horizon allows for its attribution to the lower part of the Gerhardtia sartousiana zone (middle Late Barremian). This new age corresponds to a maximal age for the onset of the Schrattenkalk Fm. in this area, and is coeval with the onset of the Urgonian facies in other parts of the western Tethyan realm. This new age allows also for a more precise dating of Late Barremian ${\delta }^{13}\text{C}$ curves. To cite this article: S. Bodin et al., C. R. Geoscience 338 (2006).     

Bulk modulus of Fe-rich olivines corrected for non-hydrostaticity

In situ X-ray diffraction was used to measure the isothermal bulk modulus at room conditions (K_T0) of synthetic olivines with different iron contents. The chemical formulae of the olivine samples were ${({\mathrm{Fe}}_x,{\mathrm{Mg}}_{1?x})}_2\mathrm{Si}{\mathrm{O}}_4$ with x = 0.45; 0.64; 0.82; 1, with 1% standard deviation (referenced as Fa45, Fa64, Fa82 and Fa100, respectively). All experiments were performed in the multi-anvil apparatus installed at NSLS beamline X17B2, to pressures up to about 7 GPa. Unit-cell volumes under hydrostatic conditions and differential stresses present in the samples were calculated using the method developed by Singh et al. (1998), and pressures measured using NaCl as a standard were then corrected for these stresses. Using a second-order Birch–Murnaghan equation of state, we obtained the isothermal bulk modulus of each composition: $K_{T0}^{\mathrm{Fa45}}=131.4\pm 2.6$ GPa, $K_{T0}^{\mathrm{Fa64}}=132.1\pm 3.1$ GPa, $K_{T0}^{\mathrm{Fa82}}=136.3\pm 1.7$ GPa and $K_{T0}^{\mathrm{Fa100}}=134.8\pm 1.4$ GPa. These values combined with data available in the literature show that the K_T0 of Fe-rich olivines increases very slowly with the Fe content, but possibly not in a simple linear trend.     

Absolute paleointensity of the Earth's magnetic field during Jurassic: case study of La Negra Formation (northern Chile)

We carried out a detailed rock-magnetic and paleointensity study of the ～187-Ma volcanic succession from northern Chile. A total of 32 consecutive lava flows (about 280 oriented standard paleomagnetic cores) were collected at the Tocopilla locality. Only 26 samples with apparently preserved primary magnetic mineralogy and without secondary magnetization components were pre-selected for Thellier paleointensity determination. Eleven samples coming from four lava flows yielded reliable paleointensity estimates. The flow-mean virtual dipole moments range from 3.7±0.9 to $\text{7.1±0.5}\text{(10}{}^{22}$ A m²). This corresponds to a mean value of (5.0±1.8)×10²² A m², which is in reasonably good agreement with other comparable quality paleointensity determinations from the Middle Jurassic. Given the large dispersion and the very poor distribution of reliable absolute intensity data, it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field. To cite this article: J. Morales et al., C. R. Geoscience 335 (2003).     

Model reconstruction of nitrate pollution of riverbank filtration using 15N and 18O data,Karany, Czech Republic

Stable isotopes of O (δ¹⁸O) in water and N (δ¹⁵N) in ${\text{NO}}_3^{-}$ have been used as natural indigenous groundwater tracers for sources of water and of ${\text{NO}}_3^{-}$ at two riverbank filtration (RBF) water supply systems. Both RBF systems (Skorkov and Sojovice) have wells in unconsolidated Quaternary sediments close to the Jizera River (Czech Republic) that have been affected by increasing ${\text{NO}}_3^{-}$ concentrations. The area is underlain by Turonian sandstones and marls that form a deeper bedrock aquifer. Sources of ${\text{NO}}_3^{-}$ are local sewerage systems and landfills (point sources) and seasonal application of manure and inorganic fertilizers (diffuse sources).At RBF Skorkov recharge to wells can be modelled using a two-component model with 60% river water contribution and 40% of very shallow groundwater with an average residence time of one month. During periods of abundant precipitation, groundwater originates entirely from the unsaturated zone of the Quaternary aquifer; extensive pumping for over 40 a has created new, bypassing flow paths that preferentially drain the contaminated unsaturated zone. During dry periods, wells are recharged by longer residence time groundwater from the Quaternary aquifer.At RBF Sojovice there is an additional recharge component of groundwater from the Turonian aquifer, which is sandier at this locality; this contains denitrified ${\text{NO}}_3^{-}$ with highly positive δ¹⁵N values.     

Remediation of coal-mine drainage by a sulfate-reducing bioreactor: A case study from the Illinois coal basin,USA

This study reports changes in coal-mine drainage constituent concentrations through an anaerobic SO₄-reducing bioreactor monitored over a 3-a period. The purpose of the study was to identify and monitor over time the biogeochemical mechanisms that control the attenuation of toxic compounds in the mine drainage. This information is needed to investigate bioreactor performance and longevity. The water treated at the case example site, the Tab-Simco Mine, was highly acidic with an average pH of 2.9, a net acidity of 1674 mg/L CaCO₃ equivalent-CCE, and high levels of dissolved ${\text{SO}}_4^{2-}$, Al, Fe and Mn. The results of this study indicated that the treatment system increased the pH of the acid mine drainage (AMD) to 6.2 and decreased the median acidity to 22.7 mg/L CCE, ${\text{SO}}_4^{2-}$ from 2981 to 1750 mg/L, Fe from 450.6 to 1.76 mg/L, Al from 113 to 0.42 mg/L, and Mn from 36.4 to 23.3 mg/L. Geochemical modeling indicates that the bioreactor discharge is saturated with respect to the minerals alunite, gibbsite, siderite, rhodochrosite, jarosite, and Fe hydroxide precipitates. The observed trends also include seasonal variations in ${\text{SO}}_4^{2-}$ reduction and a general decline in the amount of alkalinity produced. The average δ³⁴S value of the ${\text{SO}}_4^{2-}$ in the untreated AMD was +7.3‰. In the bioreactor, δ³⁴S value of ${\text{SO}}_4^{2-}$ increased from an average of +6.9‰ to +9.2‰, suggesting the presence of bacterial SO₄ reduction processes. Preliminary results of a bacterial community analysis show that DNA sequences corresponding to bacteria capable of SO₄ reduction were present in the bioreactor outflow sample. However, these sequences were outnumbered by sequences similar to bacteria capable of reoxdizing reduced sulfur species. This study illustrates the dynamic nature of metal removal in SO₄-reducing bioreactor-based treatment systems.     

Rôle des héritages latéritiques et du niveau des nappes dans l'évolution des modelés et des sols en zone intertropicale forestière humide

Analysis of landscapes and soil covers from the outer to the inner part of a $75\text{km}\times 60\text{km}$ watershed in the South-Cameroon rainforest zone, characterized by important remnants of inherited ferricretes and stable hydrostatic base levels, indicates that the development of present landforms has not involved significant erosion by slope retreat because of the relative preservation of downslope residual ferricrete and that soil covers have vertically decayed under in situ geochemical process. To cite this article: D. Bitom et al., C. R. Geoscience 336 (2004).     

Estimation of subsidence using GPS measurements,and related hazard: the Pingtung Plain,southwestern Taiwan

Four years of GPS measurements were done since 1996 in southwestern Taiwan in order to investigate crustal deformation and land subsidence. The network of 48 stations revealed the velocity field in the Pingtung Plain. Horizontal velocities range from 31 to 54 mm?yr^?1 towards azimuths 247° to 273°. They show clear anti-clockwise deviation in the coastal area, consistent with tectonic transtension related to lateral escape at the transition collision–subduction. The deformation resembles a particle flow towards a free boundary, and is not a rigid rotation. Vertical velocities range from +13 to $?25\text{mm}{\text{yr}}^{\mathrm{?1}}$. Twenty stations show fast subsidence in the coastal area, with rates averaging $?16\text{mm}/\text{yr}$. The comparison with Holocene subsidence suggests that about 75% of the present-day subsidence result from decreasing groundwater level induced by over-pumping, adding significant short-term component to the natural risk resulting from long-term tectonic subsidence. To cite this article: C.-S. Hou et al., C. R. Geoscience 337 (2005).     

Neotectonics and seismic hazard assessment in Hengchun Peninsula,southern Taiwan

Neotectonic data may indicate whether stored elastic energy will be dissipated or released destructively. The Hengchun Peninsula of Taiwan is the southern extremity of the Central Range of Taiwan and thus an emergent part of the accretionary wedge resulting from subduction of Eurasia beneath the Philippine Sea plate at the Manila Trench. Radiocarbon dating of fossil shorelines on the peninsula shows that it has been uplifted at an average rate of 3.8 mm/yr during the Holocene. About $1/3$ of the uplift is due to deformation along the Hengchun reverse fault but, in contrast with the Chelungpu and other low angle reverse faults west of the Central Range, it accommodates strain principally by aseismic creep. To cite this article: C. Vita-Finzi, J.-C. Lin, C. R. Geoscience 337 (2005).