Enrichissement en uranium authigène dans les sédiments glaciaires de l'océan AustralEnrichments in authigenic uranium in glacial sediments of the Southern Ocean

Four sediment cores from the Polar frontal zone and the Antarctic zone in the Indian sector of the Southern Ocean present an increase of authigenic uranium during glacial periods. We show that this increase in uranium is due to a combination of (i) an increase in the lateral transport of organic matter, (ii) a decrease in the oxygen in deep waters, and (iii) a process of diagenesis. It appears that uranium concentration cannot be used as a proxy of palaeoproductivity in the Southern Ocean, as previously suggested by Kumar et al. in 1995. To cite this article: L. Dezileau et al., C. R. Geoscience 334 (2002) 1039–1046.     

Multiple ways of producing intermediate and silicic rocks within Thingmúli and other Icelandic volcanoes

Major and trace element compositions of rocks and coexisting phenocrysts of the Thingmúli volcano suggest a revision of the existing models for the formation of intermediate and silicic melts in Iceland. The new data define two compositional tholeiitic trends with a significant gap between them. A high-iron trend (HFe) contains 6–14 wt% total FeO in silicic rocks with c. 1 wt% MgO, as well as sodic plagioclase and hedenbergite phenocrysts. A low-iron trend (LFe) contains 3–5 wt% FeO at c. 1 wt% MgO, which is typical of Iceland but higher than MORB compositions. The most evolved phenocrysts of the LFe trend do not reach iron-rich end members. The HFe trend is interpreted as a result of fractional crystallization; numerical modelling using the MELTS algorithm suggests that crystallization took place under redox conditions constrained to one-log unit below the fayalite-magnetite-quartz oxygen buffer (FMQ-1). The LFe trend is explained by a combination of mixing between rhyolite and ferrobasalt, assimilation of hydrated crust and fractional crystallization under higher redox conditions (FMQ). The two trends and the gap are best defined in a plot of Mg# versus SiO₂ that is useful to unravel petrogenetic processes. For example, intermediate and silicic rocks of the Holocene volcanic systems of spreading rifts (e.g. Krafla), propagating rifts (e.g. Hekla) and off-rifts (Öræfajökull) also fall into high- and low-iron fields and outline a gap similar to Thingmúli. The identification of two compositional trends in erupted intermediate and silicic volcanic products shows that processes in the deep roots of single volcanic systems are highly diverse and likely controlled by local variations in the thermal gradients and the extend of hydrothermal alteration. Generalizations about the relationship between the compositions of intermediate and silicic rocks and plate tectonic setting, therefore, should be avoided.     

It starts at home? Climate policies targeting household consumption and behavioral decisions are key to low-carbon futures

Through their consumption behavior, households are responsible for 72% of global greenhouse gas emissions. Thus, they are key actors in reaching the 1.5 °C goal under the Paris Agreement. However, the possible contribution and position of households in climate policies is neither well understood, nor do households receive sufficiently high priority in current climate policy strategies. This paper investigates how behavioral change can achieve a substantial reduction in greenhouse gas emissions in European high-income countries. It uses theoretical thinking and some core results from the HOPE research project, which investigated household preferences for reducing emissions in four European cities in France, Germany, Norway and Sweden. The paper makes five major points: First, car and plane mobility, meat and dairy consumption, as well as heating are the most dominant components of household footprints. Second, household living situations (demographics, size of home) greatly influence the household potential to reduce their footprint, even more than country or city location. Third, household decisions can be sequential and temporally dynamic, shifting through different phases such as childhood, adulthood, and illness. Fourth, short term voluntary efforts will not be sufficient by themselves to reach the drastic reductions needed to achieve the 1.5 °C goal; instead, households need a regulatory framework supporting their behavioral changes. Fifth, there is a mismatch between the roles and responsibilities conveyed by current climate policies and household perceptions of responsibility. We then conclude with further recommendations for research and policy.     

Structural investigation of the synthetic CaAn(PO4)2 (An = Th and Np) cheralite-like phosphates

The crystallographic structures of the synthetic cheralite, CaTh(PO₄)₂, and its homolog CaNp(PO₄)₂ have been investigated by X-ray diffraction at room temperature. Rietveld analyses showed that both compounds crystallize in the monoclinic system and are isostructural to monazite LnPO₄ (Ln = La to Gd). The space group is P2₁/n (I.T. = 14) with Z = 2. The refined lattice parameters of CaTh(PO₄)₂ are a = 6.7085(8) Å, b = 6.9160(6) Å, c = 6.4152(6) Å, and β = 103.71(1)° with best fit parameters R _wp = 4.87%, R _p = 3.69% and R _B = 3.99%. For CaNp(PO₄)₂, we obtained a = 6.6509(5) Å, b = 6.8390(3) Å, c = 6.3537(8) Å, and β = 104.12(6)° and R _wp = 6.74%, R _p = 5.23%, and R _B = 6.05%. The results indicate significant distortions of bond length and angles of the PO₄ tetrahedra in CaTh(PO₄)₂ and to a lesser extent in CaNp(PO₄)₂. The structural distortions were confirmed by Raman spectroscopy of CaTh(PO₄)₂. A comparison with the isostructural compounds LnPO₄ (Ln = Ce and Sm) confirmed that the substitution of the large rare earth trivalent cations with Ca²⁺ and Th⁴⁺ introduces a distortion of the PO₄ tetrahedra.     

Geochemical transport modelling of drainage from experimental mine tailings cells covered by capillary barriers

The use of covers with capillary barrier effects (CCBEs) for reducing acid mine drainage (AMD) from sulphidic mine tailings is simulated using the MIN3P finite volume model for coupled groundwater flow, O₂ diffusion and multi-component reactive transport. The model is applied to simulate five pilot-scale in situ test cells containing reactive tailings from the Manitou mine site, Val d’Or, Que., Canada. Four of the cells were constructed with CCBEs over the tailings, while the fifth tailings cell was left uncovered. Observed and simulated discharge from the base of each cell showed that the capillary barrier covers significantly reduced sulphide oxidation and AMD. Compared to acidic discharge from the uncovered cell, discharge from the four CCBE-covered cells had neutral pH levels and 1–7 orders of magnitude lower concentrations of SO₄, Fe, Zn, Cu and Al. The simulations showed that the moisture retaining layer of the CCBEs reduced AMD by inhibiting O₂ diffusion into the underlying reactive wastes. Provided the moisture-retention layer of the CCBE remains close to saturation, its thickness had a relatively minor effect. Under such near-saturated conditions, O₂ availability is limited by its diffusion rate through the bulk porous medium and not by the diffusion rate through the oxidized grain shells. The model is providing important new insights for comparing design alternatives for reducing or controlling AMD.     

Structure and nuclear density distribution in the cheralite—CaTh(PO4)2: studies of its behaviour under high pressure (36?GPa)

The crystal structure of the cheralite—CaTh(PO₄)₂—has been revisited by neutron diffraction and its behaviour under high pressure investigated by X-ray diffraction up to 36?GPa. The neutron diffraction data at ambient pressure gave a more accurate determination of the Ca/Th cation position than previous XRD data, taking advantage that the neutron scattering lengths of calcium and thorium are of same order of magnitude. The nuclear density distribution was also determined using the maximum entropy method (MEM) confirming that the two cations are not located at the same position in the unit cell but are slightly displaced from one another along a specific direction in order to minimize the electrostatic repulsion with the surrounding phosphorus atoms. At high pressure, the compound did not show any phase transition or amorphization. From the evolution of the unit-cell volume as a function of the pressure, the zero-pressure bulk modulus B₀ and its pressure derivative B₀ ^′ have been determined by fitting the experimental compressibility curve to the Birch–Murnaghan equation of state. The results are B₀?=?140(2) GPa and B ₀ ^′ ?=?4.4(4) GPa.     

Southern margin of the Variscan belt: the north-western Gondwana mobile zone (eastern Morocco and Northern Algeria)

Stratigraphic and structural correlations between the Palaeozoic massifs of eastern Morocco and northern Algeria allow three tectonic domains to be distinguished: (1) The cratonic zone, i.e. the West African platform which remained outside the Variscan chain and its peripherical margin (Moroccan Anti-Atlas and Algerian Ougarta); (2) a WSW-ENE trending zone, over 1500 km from Marrakech to Kabylia and Calabria (in their assumed Palaeozoic location). — This zone was characterized during the Late Palaeozoic by a continuous instability indicated by the development of successive turbiditic basins and a major orogeny at the Devonian-Carboniferous boundary; and (3) central and western Morocco, which corresponds to the external zones of the European Hercynides.The Marrakech-Kabylia zone separates the Variscan domain from the stable and undeformed West African craton. During Early Palaeozoic times it began as an extensive or transtensive zone. It has been deformed by the Late Devonian orogeny and by Carboniferous and Permian reactivation. The zone represents the southern limit of the Hercynian chain and is distinguished by its transcurrent regime throughout the Late Palaeozoic. Correspondence to: A. Piqué     

Multivariable variogram and its application to the linear model of coregionalization

In this article, we present the multivariable variogram, which is defined in a way similar to that of the traditional variogram, by the expected value of a distance, squared, in a space withp dimensions. Combined with the linear model of coregionalization, this tool provides a way for finding the elementary variograms that characterize the different spatial scales contained in a set of data withp variables. In the case in which the number of elementary components is less than or equal to the number of variables, it is possible, by means of nonlinear regression of variograms and cross-variograms, to estimate the coregionalization parameters directly in order to obtain the elementary variables themselves, either by cokriging or by direct matrix inversion. This new tool greatly simplifies the procedure proposed by Matheron (1982) and Wackernagel (1985). The search for the elementary variograms is carried out using only one variogram (multivariable), as opposed to thep(p + 1)/2 required by the Matheron approach. Direct estimation of the linear coregionalization model parameters involves the creation of semipositive definite coregionalization matrices of rank 1.     

The Decline and Restoration of a Coastal Lagoon (Lake Veere) in the Dutch Delta

The former tidal inlet Lake Veere was turned into a stagnant brackish lake in 1961. Ever since, the system has shown a continuous degradation. The current study shows the monitoring results for the macrozoobenthic communities and the abiotic conditions for the period 1990–2008. This includes the first step to remediation, the restoration of the exchange between the tidal marine Eastern Scheldt and Lake Veere in 2004. A continuous decline in water clearance co-occurring with decreasing macrofauna densities and richness was observed till 2004. Water quality (e.g., secchi depth, nutrient levels, and oxygen conditions) improved significantly after the measure at a higher salinity level with less variation. But the macrofauna densities, biomass, and diversity did not improve yet. First indications of changes in the benthic communities by arrival of new and returning species are however observed and show that restoration at macrofauna level follows the improved abiotic conditions with a delay of several years.