Long-term fish and macro-crustacean community variation in a Mediterranean lagoon

The fish and macro-crustacean community of the Vaccarès lagoon (Rhône River delta, South France) was sampled monthly from 1993 to 2002. The lagoon salinity shifted from 15 in 1993 to 5 in 1994–1997 and went back to 15 in 1999–2002. Connections with the sea also varied during the study period with larger openings in 1996–1997. During the study period, the community changed to revert in 2002 to a state similar to 1993. These changes consisted of a sequence of increased and decreased patterns of freshwater species and some marine species. Typical lagoon species tended to resist to salinity changes. Freshwater species colonised the lagoon when the salinity was low. Marine species may have varied both in relation to connections with the sea and to indirect effects of freshwater outflow. This study shows that community changes following environmental variations can be delayed in time, and emphasises the need for long-term studies.     

Modelling the monthly sea surface fCO2 fields in the Indian Ocean

In order to construct monthly fields of sea surface fugacity of carbon dioxide (f_CO2) on a large scale in the Indian Ocean, we use a one-dimensional model which takes into account the main physical and biogeochemical processes controlling f_CO2 variations in the ocean. Physical and biogeochemical processes are constrained by the monthly variations of sea surface temperature, salinity, chlorophyll concentration, wind speed and mixed-layer depth. The model is applied to four locations in the Indian Ocean and it well predicts observed temporal variations in f_CO2 at these locations. Regarding to monthly f_CO2 observations, the model also well simulates the f_CO2 distribution and its temporal variations along a track located between 20 ° and 50 °S with a maximal error of + 10 μatm. The model is also used to predict f_CO2 for 2 ° × 2 ° grids over the entire Indian Ocean and simulates seasonal cycles that are consistent with observations. The monthly f_CO2 fields derived from the model are used to estimate a global air-sea CO₂ flux over the Indian Ocean basin. We estimate a net sink of 0.5 Gt/yr C for the Indian Ocean (20 °N-50 °S), with the main sink located between 20 ° and 50 °S.     

Results from microseismic monitoring,conventional instrumentation,and tomography surveys in the creation and thinning of a burst-prone sill pillar

Located in northern Québec, the Lac Shortt Mine was a small gold mine consisting of a thin subvertical orebody which was mined in three main phases. High stress and rockbursting conditions were experienced when ore was extracted in the upper zone between the surface and a depth of 500 metres during the first two phases of mining. Severe rockbursts were experienced in late 1989 near the shaft and in the footwall development following a deepening of the mine shaft to a depth of 830 m and partial development of footwall drift access for the third phase of mining (the mining of the lower zone starting at a depth of 830 m moving upward toward a depth of 500 m). A 16-channel Electrolab MP250 microseismic system, with a Queen's University Full-Waveform piggy-back system, was installed underground at the site due to these problems.It was expected that the thinning sill would be subjected to an ever-increasing load as the thickness of the 500 m sill pillar decreased in the face of the mining excavation from below. A monitoring program consisting of the microseismic monitoring system, a range of conventional geomechanics monitoring tools as well as the undertaking of periodic seismic tomography surveys to assess the ongoing state of stress and rock mass condition within the sill was therefore warranted.The anomalously high-magnitude stress field and the brittle rockmass created a situation in which rockmass failure was common and violent. In the creation and thinning of the sill pillar, the location of banded microseismic activity was crucial in tracing rockmass failure and the associated ground control problems. Reliable source-location determination enabled the identification of areas of stress increase. The movement of the rockmass failure front could be followed, and was responsible for stope dilution, footwall and orebody development deterioration, and caving.Source-mechanism analyses gave accurate double-couple solutions for approximately forty percent of these events having at least ten recognizable polarities. Results suggested movement along vertical north-south striking or vertical east-west striking features. Underground observation of damaged access points showed that vertical north-south striking joints were experiencing failure.The microseismic activity, which was consistently concentrated close to the southwest and northeast corners of current production stopes, could be explained by a stress field oriented obliquely to the strike of the orebody, as measured prior to shrinkage of the sill pillar byin situ stress measurements and observed borehole overbreaks. The orientations of theP andT axes for the microseismic activity further confirmed that the stress field oriented obliquely to strike.While an increase in compressional-wave velocity of 2.3 percent, corresponding to a measured stress increase of approximately 10 MPa could be measured by repeated tomographic surveys, it was relatively small and only a factor of two or so above the velocity measured uncertainty. The relative insensitivity of thein situ rock mass modulus to the applied stress is believed to be largely due to the rockmass discontinuities being relatively closed prior to stress increase, as substantiated by the small deformations seen by the extensometer and borehole camera. This situation existed because of the very high pre-mining stress level.The experimental demonstration that the rock could not absorb substantially increased load through the mechanism of discontinuity closure or tightening (which would be reflected in the modulus) may be evidence in itself of potentially burst-prone ground, such as encountered at Lac Shortt.     

Apports des méthodes hydrodynamiques à l’étude du fonctionnement des aquifères karstiques de l’Atlas de Beni-Mellal (Maroc)

Résumé

Le massif karstique de Beni-Mellal, dans le Haut Atlas (Maroc), est constitué essentiellement par des calcaires dolomitiques du Lias qui présentent en surface des figures de dissolution spectaculaires. Le massif est situé entre 500 m et 2 400 m d’altitude et alimente plusieurs sources dont la plus importante est celle d’Aïn Asserdoune. La quasi-totalité des sources émergent au niveau des contacts chevauchants qui limitent le Jurassique calcaire des formations tertiaires sousjacentes du piémont. L’étude des débits classés et du tarissement ainsi que l’analyse corrélatoire et spectrale des chroniques pluies – débits du système karstique drainé par la source d’Aïn Asserdoune permettent de caractériser cet aquifère. Il s’agit d’un aquifère inertiel qui assure le flot de base. Cet aquifère est caractérisé par l’existence de réserves dynamiques importantes. Il est influencé par un karst superficiel fonctionnant en période de hautes eaux ainsi que par des apports latéraux provenant des compartiments voisins du massif de Beni-Bellal.     

Mineralogical and geochemical evolution of the 1982–1983 Galunggung eruption (Indonesia)

Pyroclastic deposits from the 1982–1983 eruption of Galunggung volcano (Java, Indonesia) reflect preeruptive magmatic evolution which is of interest because of: (1) its duration of nine months, compared to a few hours or days for most historical eruptions; (2) the diversity of eruptive styles, from ash and scoria flows to phreatomagmatic explosions, and to the strombolian activity that marked the end of the eruption; and (3) the progressive variation in chemical composition with time, from andesite (58 wt.% SiO₂) to high-Mg basalt (47 wt.% SiO₂). The 1982–1983 Galunggung basalts are rather primitive: 10 to 12 wt% MgO, 180 to 200 ppm Ni and 550 to 700 ppm Cr. Despite the presence of about 40% phenocrysts, they may represent the most primitive basalts recognized in western Java. Basalts contain phenocrysts of olivine (Fo_90-80), diopside-salite, and plagioclase (An_95-75). Andesites contain plagioclase (An_80–60), augite, hypersthene (En_67-64), and titanomagnetite. The distribution of mineral compositions in each petrographic type is nearly unimodal, although scarce plagioclase and olivine xenocrysts have been observed. Abundance of gabbroic cumulates associated with the pyroclastic flows and evolution of mineral compositions from high-Mg basalts to andesites support crystal fractionation as the main differentiation mechanism, although magma mixing of basaltic andesite and andesite cannot be excluded. Major and trace element trends, which display rough decreases of MgO, CaO, Ni, Cr with increasing degree of differentiation and also linear positive correlations of hygromagmaphile elements, are compatible with both processes. However, some discrepancies are observed between major and trace element modelling, which may be explained to some extent by the influence of in situ crystallization and/or magma mixing. The constancy of ¹⁴³Nd/¹⁴⁴Nd (0.51286±3), ²³⁰Th/²³²Th (0.65±0.02), Th/U (4.08±0.07) ratios, and to a lesser extent ¹⁸O values (+5.8 to +6.4 % SMOW) and ⁸⁷Sr/⁸⁶Sr ratios (0.70440 to 0.70468) is compatible with a magmatic evolution through fractional crystallization without significant crustal contamination. Nevertheless low-¹⁸O and high ⁸⁷Sr/⁸⁶Sr values in basaltic andesites may be due to the introduction of meteoric fluids into the Galunggung magma.     

Yields of suspended sediment and dissolved solids from the Andean basins of Ecuador

Abstract

Water discharge and suspended and dissolved sediment data from three rivers (Napo, Pastaza and Santiago) in the Ecuadorian Amazon basin and a river in the Pacific basin (Esmeraldas) over a 9-year period, are presented. This data set allows us to present: (a) the chemical weathering rates; (b) the erosion rates, calculated from the suspended sediment from the Andean basin; (c) the spatio-temporal variability of the two regions; and (d) the relationship between this variability and the precipitation, topography, lithology and seismic activity of the area. The dissolved solids load from the Esmeraldas basin was 2 × 10⁶ t year^-1, whereas for the Napo, Pastaza and Santiago basins, it was 4, 2 and 3 × 10⁶ t year^-1, respectively. For stations in the Andean piedmont of Ecuador, the relationship between surface sediment and the total sediment concentration was found to be close to one. This is due to minimal stratification of the suspended sediment in the vertical profile, which is attributed to turbulence and high vertical water speeds. However, during the dry season, when the water speed decreases, sediment stratification appears, but this effect can be neglected in the sediment flux calculations due to low concentration rates. The suspended sediment load in the Pacific basin was 6 × 10⁶ t year^-1, and the total for the three Amazon basins was 47 × 10⁶ t year^-1. The difference between these contributions of the suspended sediment load is likely due to the tectonic uplift and the seismic and volcanic dynamics that occur on the Amazon side.

Editor Z.W. Kundzewicz

Citation Armijos, E., Laraque, A., Barba, S., Bourrel, L., Ceron, C., Lagane, C., Magat, P., Moquet, J.-S., Pombosa, R., Sondag, F., Vauchel, P., Vera, A., and Guyot, J.L., 2013. Yields of suspended sediment and dissolved solids from the Andean basins of Ecuador. Hydrological Sciences Journal, 58 (7), 1478–1494.     

Electron microprobe U–Th–Pb monazite dating of the Transamazonian metamorphic overprint on Archean rocks from the Amapá Block, southeastern Guiana Shield, Northern Brazil

The Amapá Block, southeastern Guiana Shield, represents an Archean block involved in a large Paleoproterozoic belt, with evolution related to the Transamazonian orogenic cycle (2.26 to 1.95 Ga). High spatial resolution dating using an electron-probe microanalyzer (EPMA) was employed to obtain U–Th–Pb chemical ages in monazite of seven rock samples of the Archean basement from that tectonic block, which underwent granulite- and amphibolite-facies metamorphism. Pb–Pb zircon dating was also performed on one sample.Monazite and zircon ages demonstrate that the metamorphic overprinting of the Archean basement occurred during the Transamazonian orogenesis, and two main tectono-thermal events were recorded. The first one is revealed by monazite ages of 2096 ± 6, 2093 ± 8, 2088 ± 8, 2087 ± 3 and 2086 ± 8 Ma, and by the zircon age of 2091 ± 5 Ma, obtained in granulitic rocks. These concordant ages provided a reliable estimate of the time of the granulite-facies metamorphism in the southwest of the Amapá Block and, coupled with petro-structural data, suggest that it was contemporaneous to the development of a thrusting system associated to the collisional stage of the Transamazonian orogenesis, at about 2.10–2.08 Ga.The later event, under amphibolite-facies conditions, is recorded by monazite ages of 2056 ± 7 and 2038 ± 6 Ma, and is consistent with a post-collisional stage, marked by granite emplacement and coeval migmatization of the Archean basement along strike-slip shear zones.     

Carbon and suspended sediment transport in an impounded alpine river (Isère,France)

Carbon and total suspended sediment (TSS) loads were investigated from April 2006 to March 2008 in the mountainous watershed of the Isère River, French Alps (5570 km²). The river bed has been highly impounded for hydroelectricity production during the last century. Hydraulic flushes are managed every year to prevent TSS storage within upstream dams. The Isère River has been instrumented for high‐frequency monitoring of water, TSS by turbidity and carbon (organic, inorganic, dissolved and particulate) in order to evaluate the impact of natural floods and hydraulic flushes on annual loads. Annual TSS load which was estimated between 1.3 and 2.3 MT y^?1 (i.e. 233 to 413 T km^?2 y^?1) highlighted the high erodibility of the Isère watershed. Annual carbon load was estimated between 173 10³ T y^?1 and 199 10³ T y^?1 (i.e 31 to 36 T km^?2 y^?1). About 80% of the annual carbon loads were inorganic. The impact of hydraulic flushes on annual loads appeared limited (less than 3% for annual TSS load and about 1.5% for annual carbon load), whereas the most important natural flood event contributed to 20% of the annual TSS load and 10% of the annual carbon load. Copyright © 2012 John Wiley & Sons, Ltd.     

Deformation of a pervasively molten middle crust: insights from the neoproterozoic Ribeira‐Araçuaí orogen (SE Brazil)

Pervasive melting of the middle crust, as inferred in Tibet and the Altiplano, probably influences the deformation of the lithosphere. To constrain strain distribution in a pervasively molten crust, we analysed the deformation in an eroded analogue of these orogens. The Ribeira‐Araçuaí orogen (SE Brazil) comprises a stack of allochthons containing large volumes of anatectic and magmatic rocks. The upper allochton (∼300 km long, 50–100 km wide and >10 km thick) involves peraluminous diatexites and leucogranites resulting from partial melting of the middle crust. It overlies another allochthon containing huge early‐ to syn‐collisional plutons intruding metasediments. Both anatexites and magmatic intrusions display a pervasive strain‐induced magmatic fabric. Homogeneous strain distribution suggests inefficient localization. U–Pb ages of ∼575 Ma imply that anatexite melting was synchronous to the early‐ to syn‐collisional magmatism. Similarity in ages magmatic and solid‐state fabrics indicates that intrusions and anatexites deformed coherently with solid‐state rocks while still molten, in response to a combination of gravity‐driven and collision‐driven deformation.