Inhomogeneous substrate analysis using EM300 backscatter imagery

Backscatter reflectivity from multibeam echo-sounders provides a powerful tool to efficiently characterize seafloor substrates. A comprehensive EM300 bathymetric and backscatter survey has been completed of Cook Strait, in central New Zealand. This paper presents a detailed analysis of the realtime corrections applied to the raw EM300 multibeam data and additional corrections required to compute angular variations of the backscatter strength. The corrections, including the local absorption coefficient, the influence of seafloor topography and sound refraction in the water column, are determined for different Cook Strait seafloor substrates. Modifying MB-System software code, we extracted the backscatter signal parameters in order to quantify the raw backscatter strength and apply additional processing. Profiles of backscatter strength versus incidence angle were computed for a variety of sites characterized by flat seafloor and homogeneous substrates, and for which ground-truth data were available. For each homogeneous site, different but characteristic backscatter profiles are observed that can be interpreted in terms of sediment facies. To analyze heterogeneous substrates, we present a statistical technique, based on a 3-dimensional distribution of (incidence angle, backscatter strength) couples that preserves geological information of the substrate components. This analysis, using backscatter data acquired on a submarine volcano, north of New Zealand, clearly differentiates soft sediments and lava flows within a heterogeneous substrate.     

Modelling weathering processes at the catchment scale: The WITCH numerical model

A numerical model of chemical weathering in soil horizons and underlying bedrock (WITCH) has been coupled to a numerical model of water and carbon cycles in forest ecosystems (ASPECTS) to simulate the concentration of major species within the soil horizons and the stream of the Strengbach granitic watershed, located in the Vosges Mountains (France). For the first time, simulations of solute concentrations in soil layers and in the catchment river have been performed on a seasonal basis. The model is able to reproduce the concentrations of most major species within the soil horizons, as well as catching the first-order seasonal fluctuations of aqueous calcium, magnesium and silica concentrations. However, the WITCH model underestimates concentrations of Mg²⁺ and silica at the spring of the catchment stream, and significantly underestimates Ca²⁺ concentration. The deficit in calculated calcium can be compensated for by dissolution of trace apatite disseminated in the bedrock. However, the resulting increased Ca²⁺ release yields important smectite precipitation in the deepest model layer (in contact with the bedrock) and subsequent removal of large amount of silica and magnesium from solution. In contrast, the model accurately accounts for the concentrations of major species (Ca, Mg and silica) measured in the catchment stream when precipitation of clay minerals is not allowed. The model underestimation of Mg²⁺ and H₄SiO₄ concentrations when precipitation of well crystallized smectites is allowed strongly suggests that precipitation of well crystallized clay minerals is overestimated and that more soluble poorly crystallized and amorphous materials may be forming. In agreement with observations on other watersheds draining granitic rocks, this study indicates that highly soluble trace calcic phases control the aqueous calcium budget in the Strengbach watershed.     

Structure cristalline d'une ménéghinite naturelle pauvre en cuivre,Cu0,58Pb12,72(Sb7,04Bi0,24)S24Crystal structure of a Cu-poor natural meneghinite,Cu0.58Pb12.72(Sb7.04Bi0.24)S24

Cu-poor meneghinite from La Lauzière Massif (Savoy, France) has the composition (electron microprobe) (in wt%): Pb 59.50, Sb 20.33, Bi 1.19, Cu 0.87, Ag 0.05, Fe 0.03, S 17.62, Se 0.05, Total 99.64. Its crystal structure (X-ray on a single crystal) was solved with R1=0.0506, wR2=0.1026, with an orthorhombic symmetry, space group Pnma, and a=24.080(5) Å, b=4.1276(8) Å, c=11.369(2) Å, V=1130.0(4) Å³, Z=4. Relatively to the model of Euler and Hellner (1960), this structure shows a significantly lower site occupancy factor for the tetrahedral Cu site (0.146 against 0.25). Among the five other metallic sites, Bi appears in the one with predominant Sb. Developed structural formula: Cu_0.15Pb₂(Pb_0.53Sb_0.47)(Pb_0.46Sb_0.54)(Sb_0.75Pb_0.19Bi_0.06)S₆; the reduced one: Cu_0.58Pb_12.72(Sb_7.04Bi_0.24)S₂₄. The formation of such a Cu-poor variety seems to be related to specific paragenetic conditions (absence of coexisting galena), or to crystallochemical constraints (minor Bi). To cite this article: Y. Moëlo et al., C. R. Geoscience 334 (2002) 529–536.     

Nouvelle interprétation structurale de la « faille Nord- Pyrénéenne » en vallée d'Aspe (Pyrénées-Atlantiques). Remise en question d'un plutonisme ophitique danien dans le secteur de Bedous

In the Aspe Valley (western Pyrenees), the Europe/Iberia boundary corresponds to a complex fracturing zone, called the ‘Bielle–Accous Wrench-Faulting Corridor’, which represents the classical ‘North-Pyrenean Fault’. Located between the High Primary Range and the North-Pyrenean Zone, the BAWC shows different south-verging sheets mainly composed of Triassic materials. The Bedous ophite, associated with Muschelkalk and Keuper sediments, is also Triassic in age and involved in the same Pyrenean thrusting structures. So, contrary to a recent interpretation, this magmatic rock cannot be related to a supposed Danian plutonism inducing metamorphic processes in the surrounding Mesozoic formations. To cite this article: J. Canérot et al., C. R. Geoscience 336 (2004).     

Apparent and real bias in numerical transfer functions in palaeolimnology

We present an evaluation of the procedure by which model prediction bias is examined in palaeolimnological transfer function inference models. We argue that most of the prediction biases commonly reported in the literature are, in fact, fallacious, and are the artificial consequence of the inappropriate manner in which residuals are traditionally examined. We show that the extent of the specious model bias is entirely predictable from first principles and is essentially determined by the strength of the predictive model. We suggest that the analysis of residuals should always be examined as a function of the model's predictions and we discuss the implications of the old and new approaches.     

Dense collection areas and terrestrial alteration of meteorites in the Atacama Desert

In the last 15 years, more than 2700 meteorites have been recovered and officially classified from the Atacama Desert. Although the number of meteorites collected in the Atacama has risen, the physical and climatic properties of the dense collection areas (DCAs) have not been fully characterized. In this article, we compiled the published data of all classified meteorites found in the Atacama Desert to (i) describe the distribution by meteorite groups, (ii) compare the weathering degree of chondrites among different Atacama DCAs and other hot and cold deserts, and (iii) determine the preservation conditions of chondrites in the main Atacama DCAs in relation with the local climatic conditions. The 35 DCAs so far identified in the Atacama Desert are located in three main morphotectonic units: The Coastal Range (CR), Central Depression (CD), and Pre-Andean Range/Basement. A comparison with reported weathering data from other cold and hot deserts indicates that the mean terrestrial weathering of Atacama chondrites (W1–2), displays less alteration than other hot deserts (W2–3) and resembles the weathering distribution of the Antarctic meteorites (W1–2). The highest abundance of Atacama chondrites with low weathering (≤W2) is localized in the CD (78.8%, N = 1435), which is protected from the coastal fog influence and seasonal rainfalls and displays the oldest surfaces in the Atacama Desert. The morphogenetic classification based on present-day temperatures and precipitations of the main Atacama DCAs reveals similar regional/subregional climatic conditions in the most productive areas and a truly productive surface for meteorite recovery between 5% and 58% of the quadrangles formally defined for each Atacama DCA. Our morphogenetic classification lacks consideration of some meteorological parameters such as the coastal fog, so it cannot fully explain the differences in weathering patterns among CR chondrites. Future studies of chondrite preservation in the Atacama DCAs should consider other meteorological variables such as relative humidity, specific humidity, or dew point, in combination with exposure ages of meteorites and its surfaces.     

Properties of unstable waves in the lower St Lawrence Estuary

Abstract

The lower St Lawrence Estuary is an interesting case amongst estuaries in that it is wide enough to accommodate the development of mesoscale unstable waves and eddies. These features are generated by the runoff‐driven jet along this body's south shore. We present data yielding estimates of the length, time and velocity scales of these unstable disturbances. To relate these quantities to the dynamics we employ a 2‐layer quasigeos‐trophic instability model featuring realistic lateral shear. All model runs show short time and length scales, e‐folding periods of less than 10 days and wavelengths less than 50 km.     

A novel approach for estimating karst groundwater recharge in mountainous regions and its application in Switzerland

A pragmatic and simple approach for estimating the groundwater recharge of karst aquifers in mountainous regions by extrapolation of the hydrological regimes of gauged and well‐documented systems is presented. Specific discharge rates are derived using annual precipitation and spring measurements by taking into account catchment size and elevation, which are assumed to be the dominant factors. Reference sites with high data reliability are used for calibration and regional extrapolation. This is performed with normalized values employing spatial precipitation deviations and correlation with the elevation of the catchment areas. A tiered step procedure provides minimum and maximum normalized gradients for the relationship between recharge quantity and elevation for karst regions. The normalized recharge can therefore be obtained and extrapolated for any location using the spatial precipitation variability to provide an estimate of annual groundwater recharge. The approach was applied to Switzerland (approximately 7500 km² of karst terrain situated between 200 and over 4000 m a.s.l.) using annual precipitation data from meteorological stations for the years 2000 to 2011. Results show that the average recharge rates of different Swiss karst domains range from 20 to 46 L/km²s, which corresponds to an infiltration ratio between 0.6 and 0.9 of total precipitation. Despite uncertainties inherent in the approach, these results provide a benchmark for renewable karst groundwater resources in Switzerland of about 8.4 km³/year. The approach can be applied to any other mountainous karst region, that is, where a clear relationship between elevation, precipitation and recharge can be assumed. Copyright © 2015 John Wiley & Sons, Ltd.