Environmental mineralogy – Understanding element behavior in ecosystems

Environmental Mineralogy has developed over the past decade in response to the recognition that minerals are linked in many important ways with the global ecosystem. Minerals are the main repositories of the chemical elements in Earth's crust and thus are the main sources of elements needed for the development of civilization, contaminant and pollutant elements that impact global and local ecosystems, and elements that are essential plant nutrients. These elements are released from minerals through natural processes, such as chemical weathering, and anthropogenic activities, such as mining and energy production, agriculture and industrial activities, and careless waste disposal. Minerals also play key roles in the biogeochemical cycling of the elements, sequestering elements and releasing them as the primary minerals in crustal rocks undergo various structural and compositional transformations in response to physical, chemical, and biological processes that produce secondary minerals and soils. These processes have resulted in the release of toxic elements such as arsenic in groundwater aquifers, which is having a major impact on the health of millions of people in South and Southeast Asia. The interfaces between mineral surfaces and aqueous solutions are the locations of most chemical reactions that control the composition of the natural environment, including the composition of natural waters. The nuclear fuel cycle, from uranium mining to the disposition of high-level nuclear waste, is also intimately related to minerals. A fundamental understanding of these processes requires molecular-scale information about minerals, their bulk structures and properties such as solubility, their surfaces, and their interactions with aqueous solutions, atmospheric and soil gases, natural organic matter, and biological organisms. Gaining this understanding is further complicated by the presence of natural, incidental, and manufactured nanoparticles in the environment, which are becoming increasingly important due to the rapidly developing field of nanotechnology. As a result of this complexity, Environmental Mineralogy requires the use of the most modern molecular-scale analytical and theoretical methods and overlaps substantially with closely related fields such as Environmental Sciences, low-temperature Geochemistry, and Geomicrobiology. This paper provides brief overviews of the above topics and discusses the complexity of minerals, natural vs. anthropogenic inputs of elements and pollutants into the biosphere, the role of minerals in the biogeochemical cycling of elements, natural nanoparticles, and the Environmental Mineralogy of three major potential pollutant elements (Hg, As and U).     

Variation and temporal patterns in the composition of the surface ichthyoplankton in the southern Bay of Biscay (W. Atlantic)

From September 2000 to December 2006, surface plankton samples were collected on a monthly basis, from a station located in the southern Bay of Biscay (43°37N; 1°43W France), near the deep Capbreton canyon. In this paper, the results for the ichthyoplankton assemblage are presented. Among the 62 taxa recorded, only 35 were present in the larval stage whilst only 10 were represented by their eggs. Taxa represented by both stages (eggs+larvae; N=17) had the highest abundance. The presence in the surface plankton assemblage of species, at either or both stage, is interpreted within the context of the bathymetric distribution of species. The maxima in abundance and diversity occurred in February–March, for eggs, and May–June, for larvae. This 3-month time-lag between the stages is proposed to be related to the timing of egg spawning and larval recruitment to the pelagic environment. Mean egg abundances (82.4±29.8 eggs/10 m²) were 10-fold higher than the larval abundances (7.1±1.8 larvae/10 m²). Despite pronounced monthly variability, no statistically significant decrease in either egg or larvae abundance was observed during this 6-year study period. Compared with previous published studies, this study shows that the peak in ichthyoplankton diversity occurred two months earlier. In addition, the spawning period occurred over the whole year, even during autumn and winter. Using ordination techniques, the annual sequence appearance of taxa are described at the study site: Gadiforms, Ammodytidae and Pleuronectiforms were present during the winter whilst Sparidae, Blennidae, Labridae and Gobiidae, formed the summer group. Only three species, European anchovy Engraulis encrasicolus, European pilchard Sardina pilchardus and Atlantic horse mackerel Trachurus trachurus were recorded throughout the year.     

The 1958�C2009 Greenland ice sheet surface melt and the mid-tropospheric atmospheric circulation

In order to assess the impact of the mid-tropospheric circulation over the Greenland ice sheet (GrIS) on surface melt, as simulated by the regional climate model MAR, an automatic Circulation type classification (CTC) based on 500?hPa geopotential height from reanalyses is developed. General circulation correlates significantly with the surface melt anomalies for the summers in the period 1958?C2009. The record surface melt events observed during the summers of 2007?C2009 are linked to the exceptional persistence of atmospheric circulations favouring warm air advection. The CTC emphasizes that summer 500?hPa circulation patterns have changed since the beginning of the 2000s; this process is partly responsible for the recent warming observed over the GrIS.     

Textural and geochemical constraints on eruptive style of the 79 <Emphasis Type="SmallCaps">ad</Emphasis> eruption at Vesuvius

The 79 ad Plinian eruption of Vesuvius produced first a white pumice fallout from a high steady eruptive column, and then a grey pumice fallout originating from an oscillatory eruptive column with several partial column collapse events after which there was a total column collapse. This first total collapse was followed by renewed Plinian activity and produced the last grey pumice (GP) fallout deposit of the eruption. Textural characteristics (vesicularity and microcrystallinity) of a complete sequence of the pumice fallout deposits are presented along with the major element compositions and residual volatile contents (H₂O, Cl) to constrain the degassing processes and the eruptive dynamics. Large variations in residual volatile contents exist between the different eruptive units. Textural features also strongly differ between white and grey pumices, but also within the grey pumices. The degassing processes were thus highly heterogeneous. We propose a new model of the 79 ad eruption in which pre-eruptive conditions (H₂O saturation, magma temperature and viscosity) are the critical controls on the diversity of the syn-eruptive degassing processes and hence the eruptive dynamics. Cl contents measured in melt inclusions show that only the white pumice and the upper part of the grey pumice magma were H₂O saturated prior to eruption. The white pumice eruptive units represent a typical closed-system degassing evolution, whereas the first grey pumice one, stored under similar pre-eruptive saturation conditions, follows a particular open-system degassing evolution. We suggest that the oscillatory regime that dominated the grey pumice eruptive phase is linked to pre-eruptive water undersaturation of most of the grey magma, and the associated time delays necessary for H₂O exsolution. We also suggest that the high residual H₂O content of the last grey pumice, deposited after the renewal of Plinian activity following the first total column collapse event, is due to syn-eruptive saturation of GP magma and reduced H₂O exsolution efficiency resulting from speciation of dissolved H₂O in the melt.     

Tectonique superposée de la zone de faille d’Arakapas (Massif de Troodos,Chypre)

Résumén

Une analyse tectonique du couloir faillé d’Arakapas est menée à partir de la mesure de plus de 3 000 plans striés relevés dans les formations ophiolitiques du Troodos et du Kellaki. Des mesures ont également été effectuées dans les formations du Crétacé terminal — Paléogène et du Néogène, afin de préciser l’âge des différents stades de la déformation. Elles ont été traitées par les méthodes des dièdres droits et du calcul du tenseur des contraintes. Il est montré que le couloir d’Arakapas a été le siège de plusieurs déformations successives. Au Néogène règne un régime en extension qui succède à un jeu décrochant dextre contemporain des compressions d’âge Crétacé terminal à Miocène. Un jeu décrochant antérieur peut-être mis en évidence. Il serait compatible avec le fonctionnement en faille transformante océanique proposé par divers auteurs.     

Evolution and degradation of flat‐top mesas in the hyper‐arid Negev,Israel revealed from 10Be cosmogenic nuclides

Mesas are ubiquitous landforms in arid and semiarid regions and are often characterized by horizontal stratified erodible rocks capped by more resistant strata. The accepted conceptual model for mesa evolution and degradation considers reduction in the width of the mesa flat‐top plateau due to cliff retreat but ignores possible denudation of the mesa flat‐top and the rates and mechanism of erosion. In this study we examine mesas in the northeastern hyperarid Negev Desert where they appear in various sizes and morphologies and represent different stages of mesa evolution. The variety of mesas within a single climatic zone allows examination of the process of mesa evolution through time. Two of the four sites examined are characterized by a relatively wide (200–230 m) flat‐top and a thick caprock whereas the other two are characterized by a much narrower remnant flat‐top (several meters) and thinner caprock. We use the concentration of the cosmogenic nuclide ¹⁰Be for: (a) determining the chronology of the various geomorphic features associated with the mesa; and (b) understanding geomorphic processes forming the mesa. The ¹⁰Be data, combined with field observations, suggest a correlation between the width of flat‐top mesa and the denudation and cliff retreat rates. Our results demonstrate that: (a) cliff retreat rates decrease with decreasing width of the flat‐top mesa; (b) vertical denudation rates increase with decreasing width of the flat‐top mesa below a critical value (~60 m, for the Negev Desert); (c) the reduction in the width of the flat‐top mesa is driven mainly by cliff retreat accompanied by extremely slow vertical denudation rate which can persist for a very long time (>10⁶ Ma); and (d) when the width of the mesa decreases below a certain threshold, its rate of denudation increases dramatically and mesa degradation is completed in a short time. Copyright © 2014 John Wiley & Sons, Ltd.     

The Transfiguration continental red-bed Cu–Pb–Zn–Ag deposit,Quebec Appalachians,Canada

The Transfiguration Cu–Pb–Zn–Ag deposit, enclosed within reduced grey sandstone, is associated with continental red beds of the Lower Silurian Robitaille Formation in the Quebec Appalachians, Canada. The Robitaille Formation rests unconformably on foliated Cambro-Ordovician rocks. The unconformity is locally cut by barite veins. The basal unit of the Robitaille Formation comprises green wacke and pebble conglomerate, which locally contain calcite nodules. The latter have microstructures characteristic of alpha-type calcretes, such as “floating” fabrics, calcite-filled fractures (crystallaria) and circumgranular cracks. Massive, grey sandstone overlies the basal green wacke and pebble conglomerate unit, which is overlain, in turn, by red, fine-grained sandstone. Mineralisation occurred underneath the red sandstone unit, chiefly in the grey sandstone unit, as disseminated and veinlet sulphides. Chalcopyrite, the most abundant Cu sulphide, replaced early pyrite. Calcrete, disseminated carbonate and vein carbonate have stable isotope ratios varying from −7.5‰ to −1.1‰ δ¹³C and from 14.7‰ to 21.3‰ δ¹⁸O. The negative δ¹³C values indicate the oxidation of organic matter in a continental environment. Sulphur isotope ratios for pyrite, chalcopyrite and galena vary from −19‰ to 25‰ δ³⁴S, as measured on mineral concentrates by a conventional SO₂ technique. Laser-assisted microanalyses (by fluorination) of S isotopes in pyrite show an analogous range in δ³⁴S values, from −21‰ to 25‰. Negative and positive δ³⁴S values are compatible with bacterial sulphate reduction (BSR) in systems open and closed with respect to sulphate. We interpret similarly high δ³⁴S values for sulphide concentrates (25.1‰) and for vein barite (26.2‰) to result from rapid and complete thermochemical reduction of pore-water sulphate. Two early to late diagenetic stages of mineralisation best explain the origin of the Transfiguration deposit. The first stage was characterised by the ponding of groundwater over the Taconian unconformity, recorded by calcrete and early pyrite formation via BSR in grey sandstone. Early pyrite contains up to 2 wt.% Pb, which is consistent with Pb fixation by sulphate-reducing bacteria. The second stage (II) is defined by the replacement of early pyrite by chalcopyrite, as well as by sulphide precipitation via either BSR or thermochemical sulphate reduction (TSR) in grey sandstone. This event resulted from the synsedimentary fault-controlled percolation and mixing of (1) an oxidising, sulphate-bearing cupriferous fluid migrating per descensum from the red-bed sequence and (2) a hydrocarbon-bearing fluid migrating per ascensum from the Cambro-Ordovician basement. Mixing between the two fluids led to sulphate reduction, causing Cu sulphide precipitation. The positive correlation between Cu and Fe³⁺/Fe²⁺ bulk rock values suggests that Fe acted as a redox agent during sulphate reduction. Stage II diagenetic fluid migration is tentatively attributed to the Late Silurian Salinic extensional event.     

Nouvelles données pétrographiques et structurales sur le magmatisme du Sud du bassin de Morondava (Sud-Ouest de Madagascar)

New petrological and geochemical data are presented on basic igneous rocks from the south of the Morondova basin located in the southwestern part of Madagascar. The structural setting is well defined and, with the aid of a schematic map, the spatial relationships between the magmatic centres and major faults can be deduced from seismic data. On the other hand, new petrographic and geochemical data enable the different types of basaltic magmatism manifested in this area to be distinguished. It appears that tholeiitic magmatism evolved towards transitional magmatism from the south to the north of the Morondava basin. This geochemical evolution of basaltic magmatism can be linked to the initiation of rifting between Madagascar and Africa along the Mozambique channel and the length of the Davie ridge associated with the opening of the Indian ocean. These observations, together with structural data and recently obtained geochronological data, contribute to a better understanding of the mechanism and evolution of Madagascan rifting.