A seafloor experiment to monitor vertical deformation at the Lucky Strike volcano, Mid-Atlantic Ridge

Decades of cruise-based exploration have provided excellent snapshots of the structure of mid-ocean ridges and have revealed that accretion is a mixture of steady-state and quantum events. Observatory-type studies are now needed to quantify the temporal evolution of these systems. A multi-disciplinary seafloor observatory site is currently being set up at the Lucky Strike volcano, in the axial valley of the slow spreading Mid-Atlantic ridge as a part of the MoMAR (monitoring of the Mid-Atlantic Ridge) initiative. The aim of this observatory is to better understand the dynamics of the volcano and the hydrothermal vents hosted at its summit as well as their plumbing systems. In August 2006, the GRAVILUCK cruise initiated an experiment to monitor the deformation of Lucky Strike volcano. A geodetic network was installed, and seafloor pressure, gravity and magnetic data were collected. In this paper, we present the method used to monitor volcanic deformation, which involves measuring relative depth difference between points within a seafloor geodesy network. We show that, taking into account oceanographic variability and measurement noise, the network should be able to detect vertical deformations of the order of 1 cm.     

Long-term evolution (1988–2008) of Zostera spp. meadows in Arcachon Bay (Bay of Biscay)

The spatial variability of seagrass meadows in Arcachon Bay, was studied between 1988 and 2008 using a combination of mapping techniques based on aerial photographs for intertidal dwarf-grass (Zostera noltii) beds and acoustic sonar for permanently submerged eelgrass (Zostera marina) populations. The results show a severe decline over the period for both species, as well as an acceleration of the decline since 2005 for Z. noltii. The total surface regression over the studied period is estimated to be 22.8 km² for Z. noltii and 2.7 km² for Z. marina, which represent declines of 33 and 74% respectively.     

Organic matter dynamics and budgets in the turbidity maximum zone of the Seine Estuary (France)

Organic matter was studied in the turbidity maximum zone (TMZ) of the Seine Estuary during 8 tidal cycles from April to October in 2001, 2002 and 2003, covering a salinity range from 0 to 27. The hydrological conditions were quite varied (extremely wet in 2001, unusually dry in 2003). A particularly striking feature is the high organic matter content in the suspended solids (SS) of the Seine estuary (4–5%).     

Trace element distribution in peridotite xenoliths from Tok, SE Siberian craton: A record of pervasive, multi-stage metasomatism in shallow refractory mantle

Spinel peridotite xenoliths in alkali basalts at Tok, SE Siberian craton range from fertile lherzolites to harzburgites and wehrlites; olivine-rich (70-84%) rocks are dominant. REE patterns in the lherzolites range from nearly flat for fertile rocks (14-17% cpx) to LREE-enriched; the enrichments are positively correlated with modal olivine, consistent with high-permeability of olivine-rich rocks during melt percolation. Clinopyroxene in olivine-rich Tok peridotites typically has convex-upward trace element patterns (La/Nd_PM < 1 and Nd/Yb_PM ? 1), which we consider as evidence for equilibration with evolved silicate liquids (with higher REE and lower Ti contents than in host basalts). Whole-rock patterns of the olivine-rich xenoliths range from convex-upward to LREE-enriched (La/Nd_PM > 1); the LREE-enrichments are positively correlated with phosphorus abundances and are mainly hosted by accessory phosphates and P-rich cryptocrystalline materials. In addition to apatite, some Tok xenoliths contain whitlockite (an anhydrous, halogen-poor and Na-Mg-rich phosphate), which is common in meteorites and lunar rocks, but has not been reported from any terrestrial mantle samples. Some olivine-rich peridotites have generations of clinopyroxene with distinct abundances of Na, LREE, Sr and Zr. The mineralogical and trace element data indicate that the lithospheric mantle section represented by the xenoliths experienced a large-scale metasomatic event produced by upward migration of mafic silicate melts followed by percolation of low-T, alkali-rich melts and fluids. Chromatographic fractionation and fractional crystallisation of the melts close to the percolation front produced strong LREE-enrichments, which are most common in the uppermost mantle and are related to carbonate- and P₂O₅-rich derivatives of the initial melt. Reversal and gradual retreat of the percolation front during thermal relaxation to ambient geotherm (“retrograde” metasomatism) caused local migration and entrapment of small-volume residual fluids and precipitation of volatile-rich accessory minerals. A distinct metasomatic episode, which mainly produced “anhydrous” late-stage interstitial materials was concomitant with the alkali basaltic magmatism, which brought the xenoliths to the surface.     

Delivering time-evolving 3D city models for web visualization

ABSTRACT

Studying and planning urban evolution is essential for understanding the past and designing the cities of the future and can be facilitated by providing means for sharing, visualizing, and navigating in cities, on the web, in space and in time. Standard formats, methods, and tools exist for visualizing large-scale 3D cities on the web. In this paper, we go further by integrating the temporal dimension of cities in geospatial web delivery standard formats. In doing so, we enable interactive visualization of large-scale time-evolving 3D city models on the web. A key characteristic of this paper lies in the proposed four-step generic approach. First, we design a generic conceptual model of standard formats for delivering 3D cities on the web. Then, we formalize and integrate the temporal dimension of cities into this generic conceptual model. Following which, we specify the conceptual model in the 3D Tiles standard at logical and technical specification levels, resulting in an extension of 3D Tiles for delivering time-evolving 3D city models on the web. Finally, we propose an open-source implementation, experiments, and an evaluation of the propositions and visualization rules. We also provide access to reproducibility notes allowing researchers to replicate all the experiments.     

Estimation of crack opening from a two‐dimensional continuum‐based finite element computation

Damage models are capable of representing crack initiation and mimicking crack propagation within a continuum framework. Thus, in principle, they do not describe crack openings. In durability analyses of concrete structures however, transfer properties are a key issue controlled by crack propagation and crack opening. We extend here a one‐dimensional approach for estimating a crack opening from a continuum‐based finite element calculation to two‐dimensional cases. The technique operates in the case of mode I cracking described in a continuum setting by a nonlocal isotropic damage model. We used the global tracking method to compute the idealized crack location as a post‐treatment procedure. The original one‐dimensional problem devised in Dufour et al. [4] is recovered as profiles of deformation orthogonal to the idealized crack direction are computed. An estimate of the crack opening and an error indicator are computed by comparing finite element deformation profiles and theoretical profiles corresponding to a displacement discontinuity. Two estimates have been considered: In the strong approach, the maxima of the profiles are assumed to be equal; in the weak approach, the integrals of each profile are set equal. Two‐dimensional numerical calculations show that the weak estimates perform better than do the strong ones. Error indicators, defined as the distance between the numerical and theoretical profiles, are less than a few percentages. In the case of a three‐point bending, test results are in good agreement with experimental data, with an error lower than 10% for widely opened crack (> 40µm). Copyright © 2011 John Wiley & Sons, Ltd.     

Disproportionation of Fe<Superscript>2+</Superscript> in Al-free silicate perovskite in the laser heated diamond anvil cell as recorded by electron probe microanalysis of oxygen

Experimental evidence is reported for Fe²⁺ disproportionation in Al-free perovskite (Pv), when submitted to large temperature gradients (i.e., under off-equilibrium conditions) in a laser heated diamond anvil cell (LHDAC). To enable this effect, the experimental procedure was designed to produce large radial and axial temperature gradients. In the Pv and ferropericlase (Fp) assemblage synthesized after dissociation of natural olivine, the three chemical states of iron (i.e., Fe⁰, Fe²⁺ and Fe³⁺) could be evidenced by electron probe microanalysis (EPMA), through variations of oxygen contents attached to the Fe cations. Despite inherent difficulties for determination of O-contents and Fe³⁺/ΣFe ratios using EPMA, we recorded significant changes in iron oxidation state across the laser-heated strip. These changes are correlated with variations in composition for the major elements (Fe, Mg, and Si), which evidences that the Pv/Fp assemblage experienced large segregation under the strong temperature gradients. Grains of metallic iron were detected in parts of the laser-heated strip coexisting with a Pv phase with Fe/(Mg + Fe) = 6 at% and most of its iron as Fe³⁺. This Fe²⁺-disproportionation reaction involves insertion of Fe³⁺-defects in the Pv lattice. This Fe³⁺-bearing Pv phase is presumably unstable and decomposes into a mineral assemblage including magnesioferrite, which is detected at the border of the laser-heated strip.     

Experimental reduction of aqueous sulphate by hydrogen under hydrothermal conditions: Implication for the nuclear waste storage

Sulphate reduction by hydrogen, likely to occur in deep geological nuclear waste storage sites, was studied experimentally in a two-phase system (water + gas) at 250-300 °C and under 4-16 bars H₂ partial pressure in hydrothermal-vessels. The calculated activation energy is 131 kJ/mol and the half-life of aqueous sulphate in the presence of hydrogen and elemental sulphur ranges from 210,000 to 2.7 × 10⁹ years at respective temperatures of 90 °C, the thermal peak in the site and 25 °C, the ambient temperature far from the site. The features and rate of the sulphate reduction by H₂ are close to those established for TSR in oil fields. The experiments also show that the rate of sulphate reduction is not significantly affected in the H₂ pressure range of 4-16 bars and in the pH range of 2-5, whereas a strong increase is measured at pH below 2. We suggest that the condition for the reaction to occur is the speciation of sulphate dominated by non symmetric species ( at low pH), and we propose a three steps reaction, one for each intermediate-valence sulphur species, the first one requiring H₂S as electron donor rather than H₂. We distinguish two possible reaction pathways for the first step, depending on pH: reduction of sulphate into sulphur dioxide below pH 2 or into thiosulphate or sulphite + elemental sulphur in the pH range 2-5.     

New experimental constraints on the composition and structure of tholins

Powdered samples of carbon-nitrogen-hydrogen “tholins” that mimic Titan's atmosphere aerosols were produced under levitation conditions in the laboratory with a dusty plasma (PAMPRE experiment) using different initial N₂:CH₄ gas mixtures and studied using UV Raman and infrared spectroscopy, X-ray diffraction and high resolution transmission electron microscopy (HRTEM). Comparison between the tholins produced in the PAMPRE experiments and samples prepared by other techniques reveal that they form a fairly homogeneous family of hydrogenated carbon nitride materials. Wall effects during the PAMPRE deposition experiments and other were found to have little effect on the chemical structure of tholins. The first-order UV Raman bands of the disordered carbonaceous materials point to a large contribution of sp² clusters present compared with olefinic CN or CC groupings, whereas features at 690 and 980 cm⁻¹ suggest C₃N₃ rings are present as a species inserted in the macromolecular network. Diffraction techniques do not indicate the presence of large polyaromatic species in any of the tholins studied, whatever their nitrogen concentration, in disagreement with certain previous observations. This precludes the idea that the nature and degree of absorption in the visible range is controlled by the size of polyaromatic species, as has been observed in series of carbon-based materials obtained via thermal processing. Infrared spectroscopy analysis of the tholins has confirmed earlier identifications of chemical groups present including primary amines, nitriles, and alkyl moieties such as CH₂/CH₃, but has ruled out CH₂/CH₃ branches appearing on secondary or tertiary amines. Similar analyses were also performed on a polymeric (HCN)_x material, which was found to present several similarities with the tholins, hence suggesting similar polymerization processes.     

10Be dating of the Main Terrace level in the Amblève valley (Ardennes,Belgium): new age constraint on the archaeological and palaeontological filling of the Belle‐Roche palaeokarst

It is still disputed whether very old archaeological and palaeontological remains found in the Belle‐Roche palaeocave (eastern Belgium) pertain to the Early (～1 Ma) or Middle (～0.5 Ma) Pleistocene. Here, in situ‐produced cosmogenic ¹⁰Be concentrations from a depth profile in nearby sediments of the Belle‐Roche terrace (Amblève Main Terrace level) are used as an indirect solution of this chronological issue. The distribution of ¹⁰Be concentrations in the upper 3 m of this profile displays the theoretically expected exponential decrease with depth. Assuming a single exposure episode, we obtain a best fit age of 222.5±31 ka for the time of terrace abandonment. However, below 3 m, the ¹⁰Be concentrations show a marked progressive increase with depth. This distinctive cosmogenic signal is interpreted as the result of slow aggradation of the fluvial deposits over a lengthy interval. Modelling of the whole profile thus suggests that the onset of the terrace formation occurred at around 550 ka, with a sediment accumulation rate of ～20 mm ka^?1. Based on two slightly different reconstructions of the geomorphic evolution of the area and a discussion of the temporal link between the cave and Main Terrace levels, we conclude that the fossil‐bearing layers in the palaeokarst pertain most probably to MIS 14–13, or possibly MIS 12–11. This age estimate for the large mammal association identified in the Belle‐Roche palaeokarst and the attribution to MIS 14–13 of a similar fauna found in the lowermost fossiliferous layers of the Caune de l'Arago (Tautavel) are in mutual support. Our results therefore confirm the status of the Belle‐Roche site as a reference site for the Cromerian mammal association in NW Europe.