Tropical precipitation regimes and mechanisms of regime transitions: contrasting two aquaplanet general circulation models

The atmospheric general circulation models ARPEGE-climate and LMDz are used in an aquaplanet configuration to study the response of a zonally symmetric atmosphere to a range of sea surface temperature (SST) forcing. We impose zonally-symmetric SST distributions that are also symmetric about the equator, with varying off-equatorial SST gradients. In both models, we obtain the characteristic inter-tropical convergence zone (ITCZ) splitting that separates two regimes of equilibrium (in terms of precipitations): one with one ITCZ over the equator for large SST gradients in the tropics, and one with a double ITCZ for small tropical SST gradients. Transition between these regimes is mainly driven by changes in the low-level convergence that are forced by the SST gradients. Model-dependent, dry and moist feedbacks intervene to reinforce or weaken the effect of the SST forcing. In ARPEGE, dry advective processes reinforce the SST forcing, while a competition between sensible heat flux and convective cooling provides a complex feedback on the SST forcing in the LMDz. It is suggested that these feedbacks influence the location of the transition in the parameter range.     

Speciation and characterization of arsenic in gold ores and cyanidation tailings using X-ray absorption spectroscopy

The knowledge of mineralogy and molecular structure of As is needed to better understand the stability of As in wastes resulting from processing of gold ores. In this study, optical microscopy, scanning electron microscopy, electron microprobe, X-ray diffraction and X-ray absorption fine structure (XAFS) spectroscopy (including both XANES and EXAFS regimes) were employed to determine the mineralogical composition and local coordination environment of As in gold ores and process tailings from bench-scale tests designed to mimic a common plant practice. Arsenic-bearing minerals identified in the ores and tailings include iron (III) oxyhydroxides, scorodite (FeAsO₄·2H₂O), ferric arsenates, arseniosiderite (Ca₂Fe₃(AsO₄)₃O₂·3H₂O), Ca-Fe arsenates, pharmacosiderite (KFe₄(AsO₄)₃(OH)₄·6-7H₂O), jarosite (K₂Fe₆(SO₄)₄(OH)₁₂) and arsenopyrite (FeAsS). Iron (III) oxyhydroxides contain variable levels of As from trace to about 22 wt% and Ca up to approximately 9 wt%.Finely ground ore and tailings samples were examined by bulk XAFS and selected mineral grains were analyzed by microfocused XAFS (micro-EXAFS) spectroscopy to reconcile the ambiguities of multiple As sources in the complex bulk EXAFS spectra. XANES spectra indicated that As occurs as As⁵⁺in all the samples. Micro-EXAFS spectra of individual iron (III) oxyhydroxide grains with varying As concentrations point to inner-sphere bidentate-binuclear arsenate complexes as the predominant form of As. There are indications for the presence of a second Fe shell corresponding to bidentate-mononuclear arrangement. Iron (III) oxyhydroxides with high As concentrations corresponding to maximum adsorption densities probably occur as nanoparticles. The discovery of Ca atoms around As in iron (III) oxyhydroxides at interatomic distances of 4.14-4.17 Å and the coordination numbers suggest the formation of arseniosiderite-like nanoclusters by coprecipitation rather than simple adsorption of Ca onto iron (III) oxyhydroxides. Correlation of Ca with As in iron (III) oxyhydroxides as determined by electron microprobe analysis supports the coprecipitate origin for the presence of Ca in iron (III) oxyhydroxides.The samples containing higher abundances of ferric arsenates released higher As concentrations during the cyanidation tests. The presence of highly soluble ferric arsenates and Ca-Fe arsenates, and relatively unstable iron (III) oxyhydroxides with Fe/As molar ratios of less than 4 in the ore and process tailings suggests that not only the tailings in the impoundment will continue to release As, but also there is the potential for mobilization of As from the natural sources such as the unmined ore.     

Hydrous ferric oxide precipitation in the presence of nonmetabolizing bacteria: Constraints on the mechanism of a biotic effect

We have used room temperature and cryogenic ⁵⁷Fe Mössbauer spectroscopy, powder X-ray diffraction (pXRD), mineral magnetometry, and transmission electron microscopy (TEM), to study the synthetic precipitation of hydrous ferric oxides (HFOs) prepared either in the absence (abiotic, a-HFO) or presence (biotic, b-HFO) of nonmetabolizing bacterial cells (Bacillus subtilis or Bacillus licheniformis, ∼10⁸ cells/mL) and under otherwise identical chemical conditions, starting from Fe(II) (10⁻², 10⁻³, or 10⁻⁴ mol/L) under open oxic conditions and at different pH (6-9). We have also performed the first Mössbauer spectroscopy measurements of bacterial cell wall (Bacillus subtilis) surface complexed Fe, where Fe(III) (10^−3.5-10^−4.5 mol/L) was added to a fixed concentration of cells (∼10⁸ cells/mL) under open oxic conditions and at various pH (2.5-4.3). We find that non-metabolic bacterial cell wall surface complexation of Fe is not passive in that it affects Fe speciation in at least two ways: (1) it can reduce Fe(III) to sorbed-Fe²⁺ by a proposed steric and charge transfer effect and (2) it stabilizes Fe(II) as sorbed-Fe²⁺ against ambient oxidation. The cell wall sorption of Fe occurs in a manner that is not compatible with incorporation into the HFO structure (different coordination environment and stabilization of the ferrous state) and the cell wall-sorbed Fe is not chemically bonded to the HFO particle when they coexist (the sorbed Fe is not magnetically polarized by the HFO particle in its magnetically ordered state). This invalidates the concept that sorption is the first step in a heterogeneous nucleation of HFO onto bacterial cell walls. Both the a-HFOs and the b-HFOs are predominantly varieties of ferrihydrite (Fh), often containing admixtures of nanophase lepidocrocite (nLp), yet they show significant abiotic/biotic differences: Biotic Fh has less intraparticle (including surface region) atomic order (Mössbauer quadrupole splitting), smaller primary particle size (magnetometry blocking temperature), weaker Fe to particle bond strength (Mössbauer center shift), and no six-line Fh (6L-Fh) admixture (pXRD, magnetometry). Contrary to current belief, we find that 6L-Fh appears to be precipitated directly, under a-HFO conditions, from either Fe(II) or Fe(III), and depending on Fe concentration and pH, whereas the presence of bacteria disables all such 6L-Fh precipitation and produces two-line Fh (2L-Fh)-like biotic coprecipitates. Given the nature of the differences between a-HFO and b-HFO and their synthesis condition dependences, several biotic precipitation mechanisms (template effect, near-cell environment effect, catalyzed nucleation and/or growth effect, and substrate-based coprecipitation) are ruled out. The prevailing present view of a template or heterogeneous nucleation barrier reduction effect, in particular, is shown not to be the cause of the large observed biotic effects on the resulting HFOs. The only proposed mechanism (relevant to Fh) that is consistent with all our observations is coprecipitation with and possible surface poisoning by ancillary bacteriagenic compounds. That bacterial cell wall functional groups are redox active and the characteristics of biotic (i.e., natural) HFOs compared to those of abiotic (i.e., synthetic) HFOs have several possible biogeochemical implications regarding Fe cycling, in the photic zones of water columns in particular.     

Weathering of sandstone lotus petals at the Angkor site: a 1,000-year stone durability trial

This article is based on field investigations carried out at Ta Keo and Banteay Srei temples (Angkor, Cambodia), which have similar environmental histories and display the same recurrent decorative pattern (lotus petal) in three different sandstone lithotypes. Based on two original scales of mechanical and chemical weathering, the degree of deterioration of c.2000 sandstone petals was visually assessed, resulting in the proposal of a durability scale. An explanatory scheme was provided, based on laboratory analyses of stone materials (ICP-AES, XRD, SEM-EDS, polarizing microscopy and mercury injection). The key drivers of sandstone durability are both mineralogical and petrophysical. The remarkable preservation of the pink sandstone is due to the quartzose nature of its clasts, the non-swelling clay content of its abundant matrix (kaolinite) and its porosity structure (large connected pores and probable non-connected porosity). The overall good preservation of the green sandstone is due to its extremely low porosity and specific surface area, associated with its strong cementation. The severe deterioration of the grey sandstone is due to its laminated structure (oriented biotites), the susceptibility to hygric swelling and solution of its binding agents (chlorite/smectite and calcite) and its porosity structure, which evolves during a two-stage weathering sequence. The first stage is characterized by a bimodal porosity due to the partial clogging of macropores by swelling clays, and the second stage by a substantial increase in porosity and the penetration of salts (barite and anhydrite). Conversely, oxidation phenomena associated with weathering can lead to a drastic reduction of porosity.     

Coupes équilibrees crustales : méthodologie et application aux Alpes occidentales

Résumé

La construction de coupes équilibrées à l’échelle de la croûte ainsi que leur remise à l’état avant déformation suivent les règles générales de construction des coupes équilibrées (conservation des longueurs et des surfaces entre l’état avant déformation et l’état actuel), mais présentent des caractères qui leur sont propres (nécessité de connaître les mouvements verticaux importants qui accompagnent les déformations à cette échelle ainsi que le mode de déformation de la croûte).

Un exemple de construction de telles coupes est proposé pour les Alpes occidentales. Ces coupes, équilibrées pour le chevauchement crustal des massifs cristallins externes (Burdigalien-Actuel), traduisent un raccourcissement crustal variant de 10 km environ au niveau du Vercors à 55 km environ au niveau des Préalpes. Elles permettent d’intégrer dans un modèle unique un certain nombre de phénomènes comme le raccourcissement crustal, le raccourcissement dans la couverture, la subsidence du bassin molassique, la surrection des massifs cristallins externes, le rétrodéversement des structures en arrière de ces massifs.     

Datation Ar/Ar des leucogranites sous couverture du complexe plutonique de Charroux–Civray (Vienne)Ar/Ar dating of leucogranites in the sediment-covered Charroux–Civray complex (Vienne)

⁴⁰Ar/³⁹Ar dating on muscovites, performed on leucogranitic intrusions of Charroux–Civray plutonic complex, points out the existence of two peraluminous magmatic activities, whose equivalents are known in the Limousin: (1) garnet-bearing leucogranitic veins at ca. 340 Ma; (2) a specialised leucogranite associated with W ± Sn deposits at ca 310 Ma. However, available ⁴⁰Ar/³⁹Ar data do not allow us to provide further data concerning the age and the geometry at depth of a large leucogranitic body identified by geophysics. To cite this article: P. Alexandre et al., C. R. Geoscience 334 (2002) 1141–1148.     

High-pressure yield strength of rocksalt structures using quartz Raman piezometry

The Raman frequencies of quartz are used to evaluate deviatoric stresses in rocksalt-structure media in diamond-anvil cell experiments to pressures up to 20 GPa. The piezospectroscopic effect in quartz is modeled by first-principles calculations. Non-hydrostatic stresses measured in halogen salts give yield strength estimates of 3 GPa in the B1 structure (NaCl), and 4.5 GPa for the B2 structure (KCl and KBr). Raman measurements in MgO show that the yield strength is reached at around 6 ± 1 GPa. Measurements on quartz alone indicate similar yield strength. The estimated yield strength in MgO is thus likely a lower bound, in consistency with former radial X-ray diffraction measurements that gave a yield strength of 8 ± 1 GPa, and lower-pressure large-volume press experiments indicating a yield strength of 6 GPa at 5 GPa. Former values of the yield strength below 2 GPa determined by pressure gradient measurements were underestimated due to unverified assumptions in boundary conditions. The yield strength increases with increasing coordination of ionic solids, likely contributing to increase viscosity at phase transitions near the upper–lower mantle boundary.     

Influence of plankton concentration on gametogenesis and spawning of the black lip pearl oyster Pinctada margaritifera in Ahe atoll lagoon (Tuamotu archipelago, French polynesia)

Pearl culture industry represents one of the dominant business sector of French Polynesia. However, it still entirely relies on unpredictable spat collection success. Our aim was to assess the influence of natural plankton concentration fluctuations on maturation and spawning of the black lip pearl oyster Pinctada margaritifera, during a 4months survey conducted in Ahe atoll lagoon. Plankton concentration was assessed by chlorophyll a extraction and by microscope counts while gonadic index, gonado-visceral dry weights and histology were used to measure pearl oysters reproduction activity. We found that (i) plankton concentration fluctuations were mainly related to wind regime, (ii) gametogenesis rate was mainly related to plankton concentration, (iii) spawning occurred when maximal gonad storage was reached, (iv) plankton concentration was the main spawning synchronizing factor. These results contribute explaining P. margaritifera spat collection variability in French Polynesian atoll lagoon.     

Modélisation de propagation d’écoulement entre lits mineur et majeur sur les fleuves Sénégal et Niger

In the Senegal River valley and Niger Inner Delta, the annual floods inundate a wide floodplain consisting of a complex network of lakes and channels, where topographic information needed by standard hydraulic models is difficult to obtain. To represent the flood propagation between mainstream and floodplain, we use a model designed for flood propagation in river mainstreams with flat bed and large overflow and without topographic data. Depending on the water level in the riverbed, the model calibrated on the levels observed at two stations gives the level in the floodplains and propagation time between stations. Several cases are tested for various types of hydraulic connections between mainstream and floodplain. The model could correctly reproduce the flood rise and fall in the floodplain, even for a lake connected by a single channel to the riverbed or in the case of a strong attenuation of the flood between very distant stations.     

Total organic carbon fluxes of the Red River system (Vietnam)

Riverine transport of organic carbon from terrestrial ecosystems to the oceans plays an important role in the global carbon cycle. The Red River is located in Southeast Asia where river discharge, sediment loads and fluxes of elements (carbon, nitrogen and phosphorus) associated with suspended solids have been dramatically altered over past decades as a result of reservoir impoundment and land use, population, and climate change. Dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations were measured monthly at four stations of the Red River system from January 2008 to December 2010. The results reveal that POC changed synchronically with total suspended solids (TSS) concentration and with the river discharge, whereas no clear trend was observed for DOC concentration. The mean value of total organic carbon (TOC = DOC + POC) flux in the delta of the Red River was 31.5 × 10¹³ ± 4.0 × 10¹³ MgC.yr^?1 (range 27.9–35.8 × 10¹³ MgC.yr^?1 which leads to a specific TOC flux of 2012 ± 255 kgC.km^?2.yr^?1 during this 2008–2010 period. About 80% of the TOC flux was transferred to the estuary during the rainy season as a consequence of the higher river water discharge. The high mean value of the POC:Chl‐a ratio (1585 ± 870 mgC.mgChl‐a^?1) and the moderate C:N ratio (7.3 ± 0.1) in the water column system suggest that organic carbon in the Red River system is mainly derived from erosion and soil leaching in the basin. The effect of two new dam impoundments in the Red River was also observable with lower TOC fluxes in 2010 compared with 2008. Copyright © 2017 John Wiley & Sons, Ltd.