Behavior of arsenic and antimony in the surface freshwater reaches of a highly turbid estuary, the Gironde Estuary, France

This study reports on the behavior of two redox-sensitive elements, As and Sb, along the turbidity gradient in the freshwater reaches of the turbid Gironde Estuary. During a 17-month survey, surface water and suspended particulate matter (SPM) were sampled monthly at six sites representing both fluvial branches of the Gironde Estuary. Additionally, two longitudinal high resolution profiles were sampled along the fluvial estuary of the Garonne Branch during two contrasted seasons, i.e. with and without the presence of the maximum turbidity zone (MTZ). Seasonal variability and spatial distribution of dissolved (<0.2 μm; <0.02 μm) and particulate As, Sb and Fe were measured and combined with SPM data to understand metalloid behavior in the estuarine freshwater turbidity gradient.At the two main fluvial entries of the Gironde Estuary, dissolved As and Sb concentrations showed strong (by a factor of 2–4) seasonal variations, that were only partly controlled by discharge-related dilution. Seasonal addition of dissolved As and Sb was attributed to the degradation of particulate As and Sb carrier phases in bottom sediment and/or in the adjacent aquifers, rather than release from SPM. In the surface freshwater reaches of the Gironde Estuary, Sb behaved conservatively under all hydrological conditions. In contrast, As was strongly reactive in the presence of the MTZ, with opposite behaviors in the two fluvial branches of the estuary: in the Garonne Branch As was removed from the dissolved phase, whereas in the Dordogne Branch As was added. Redistribution of As between the dissolved and the particulate phases along the turbidity gradient in estuarine freshwater only affected the <0.02 μm fraction, as the 0.02–0.2 μm fraction remained constant (300 ng L⁻¹ in September 2005). Accordingly, As removal seemed to be decoupled from concomitant “colloidal” (0.02–0.2 μm) Fe flocculation in the turbidity gradient. The contrasting behavior of dissolved As in the fluvial estuaries of the Garonne and Dordogne Branches was attributed to sorption processes during equilibration of river-borne dissolved As with estuarine SPM forming the MTZ. This equilibrium, described by a distinct distribution coefficient K_d(As)  11,000 L kg⁻¹ in the MTZ, resulted in either As release (desorption; Dordogne Branch) or removal (adsorption; Garonne Branch) in the respective fluvial estuaries. A mixing experiment under controlled laboratory conditions tended to support that equilibration between the dissolved phase and MTZ particles may induce both As release and removal in the estuarine freshwater reaches, with As distribution evolving towards a distinct K_d value for increasing SPM concentrations. The long-term survey allowed estimating annual (2004) dissolved fluxes of As and, for the first time Sb, at the main fluvial entries of the Gironde Estuary at 30.7 t a⁻¹ and 3.2 t a⁻¹ (Garonne River) and at 8.0 t a⁻¹ and 2.3 t a⁻¹ (Dordogne River), respectively.     

Processes of India’s offshore summer intraseasonal sea surface temperature variability

Active and break phases of the Indian summer monsoon are associated with sea surface temperature (SST) fluctuations at 30–90 days timescale in the Arabian Sea and Bay of Bengal. Mechanisms responsible for basin-scale intraseasonal SST variations have previously been discussed, but the maxima of SST variability are actually located in three specific offshore regions: the South-Eastern Arabian Sea (SEAS), the Southern Tip of India (STI) and the North-Western Bay of Bengal (NWBoB). In the present study, we use an eddy-permitting 0.25° regional ocean model to investigate mechanisms of this offshore intraseasonal SST variability. Modelled climatological mixed layer and upper thermocline depth are in very good agreement with estimates from three repeated expendable bathythermograph transects perpendicular to the Indian Coast. The model intraseasonal forcing and SST variability agree well with observed estimates, although modelled intraseasonal offshore SST amplitude is undere-stimated by 20–30 %. Our analysis reveals that surface heat flux variations drive a large part of the intraseasonal SST variations along the Indian coastline while oceanic processes have contrasted contributions depending of the region considered. In the SEAS, this contribution is very small because intraseasonal wind variations are essentially cross-shore, and thus not associated with significant upwelling intraseasonal fluctuations. In the STI, vertical advection associated with Ekman pumping contributes to ～30 % of the SST fluctuations. In the NWBoB, vertical mixing diminishes the SST variations driven by the atmospheric heat flux perturbations by 40 %. Simple slab ocean model integrations show that the amplitude of these intraseasonal SST signals is not very sensitive to the heat flux dataset used, but more sensitive to mixed layer depth.     

Using experimental data to reduce the single-building sigma of fragility curves: case study of the BRD tower in Bucharest, Romania

The lack of knowledge concerning modelling existing buildings leads to significant variability in fragility curves for single or grouped existing buildings. This study aims to investigate the uncertainties of fragility curves, with special consideration of the single-building sigma. Experimental data and simplified models are applied to the BRD tower in Bucharest, Romania, a RC building with permanent instrumentation. A three-step methodology is applied： （1） adjustment of a linear MDOF model for experimental modal analysis using a Timoshenko beam model and based on Anderson＇s criteria, （2） computation of the structure＇s response to a large set of accelerograms simulated by SIMQKE software, considering twelve ground motion parameters as intensity measurements （IM）, and （3） construction of the fragility curves by comparing numerical interstory drift with the threshold criteria provided by the Hazus methodology for the slight damage state. By introducing experimental data into the model, uncertainty is reduced to 0.02 considering Sd ） as seismic intensity IM and uncertainty related to the model is assessed at 0.03. These values must be compared with the total uncertainty value of around 0.7 provided by the Hazus methodology.     

Galileo single frequency ionospheric correction: performances in terms of position

For GPS single frequency users, the ionospheric contribution to the error budget is estimated by the well-known Klobuchar algorithm. For Galileo, it will be mitigated by a global algorithm based on the NeQuick model. This algorithm relies on the adaptation of the model to slant Total Electron Content (sTEC) measurements. Although the performance specifications of these algorithms are expressed in terms of delay and TEC, the users might be more interested in their impact on positioning. Therefore, we assessed the ability of the algorithms to improve the positioning accuracy using globally distributed permanent stations for the year 2002 marked by a high level of solar activity. We present uncorrected and corrected performances, interpret these and identify potential causes for Galileo correction discrepancies. We show vertical errors dropping by 56–64 % due to the analyzed ionospheric corrections, but horizontal errors decreasing by 27 % at most. By means of a fictitious symmetric satellite distribution, we highlight the role of TEC gradients in residual errors. We describe mechanisms permitted by the Galileo correction, which combine sTEC adaptation and topside mismodeling, and limit the horizontal accuracy. Hence, we support further investigation of potential alternative ionospheric corrections. We also provide an interesting insight into the ionospheric effects possibly experienced during the next solar maximum coinciding with Galileo Initial Operation Capability.     

Stability analysis of the 2007 Chehalis lake landslide based on long-range terrestrial photogrammetry and airborne LiDAR data

On December 4th 2007, a 3-Mm³ landslide occurred along the northwestern shore of Chehalis Lake. The initiation zone is located at the intersection of the main valley slope and the northern sidewall of a prominent gully. The slope failure caused a displacement wave that ran up to 38 m on the opposite shore of the lake. The landslide is temporally associated with a rain-on-snow meteorological event which is thought to have triggered it. This paper describes the Chehalis Lake landslide and presents a comparison of discontinuity orientation datasets obtained using three techniques: field measurements, terrestrial photogrammetric 3D models and an airborne LiDAR digital elevation model to describe the orientation and characteristics of the five discontinuity sets present. The discontinuity orientation data are used to perform kinematic, surface wedge limit equilibrium and three-dimensional distinct element analyses. The kinematic and surface wedge analyses suggest that the location of the slope failure (intersection of the valley slope and a gully wall) has facilitated the development of the unstable rock mass which initiated as a planar sliding failure. Results from the three-dimensional distinct element analyses suggest that the presence, orientation and high persistence of a discontinuity set dipping obliquely to the slope were critical to the development of the landslide and led to a failure mechanism dominated by planar sliding. The three-dimensional distinct element modelling also suggests that the presence of a steeply dipping discontinuity set striking perpendicular to the slope and associated with a fault exerted a significant control on the volume and extent of the failed rock mass but not on the overall stability of the slope.     

Trace element geochemistry of CR chondrite metal

We report trace element analyses by laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) of metal grains from nine different CR chondrites, distinguishing grains from chondrule interior (“interior grains”), chondrule surficial shells (“margin grains”), and the matrix (“isolated grains”). Save for a few anomalous grains, Ni‐normalized trace element patterns are similar for all three petrographic settings, with largely unfractionated refractory siderophile elements and depleted volatile Au, Cu, Ag, S. All three types of grains are interpreted to derive from a common precursor approximated by the least‐melted, fine‐grained objects in CR chondrites. This also excludes recondensation of metal vapor as the origin of the bulk of margin grains. The metal precursors were presumably formed by incomplete condensation, with evidence for high‐temperature isolation of refractory platinum‐group‐element (PGE)‐rich condensates before mixing with lower temperature PGE‐depleted condensates. The rounded shape of the Ni‐rich, interior grains shows that they were molten and that they equilibrated with silicates upon slow cooling (1–100 K h^?1), largely by oxidation/evaporation of Fe, hence their high Pd content, for example. We propose that Ni‐poorer, amoeboid margin grains, often included in the pyroxene‐rich periphery common to type I chondrules, result from less intense processing of a rim accreted onto the chondrule subsequent to the melting event recorded by the interior grains. This means either that there were two separate heating events, which formed olivine/interior grains and pyroxene/margin grains, respectively, between which dust was accreted around the chondrule, or that there was a single high‐temperature event, of which the chondrule margin records a late “quenching phase,” in which case dust accreted onto chondrules while they were molten. In the latter case, high dust concentrations in the chondrule‐forming region (at least three orders of magnitude above minimum mass solar nebula models) are indicated.     

Crystal size distribution (CSD) in rocks and the kinetics and dynamics of crystallization

Crystal size distributions (CSDs) measured in metamorphic rocks yield quantitative information about crystal nucleation and growth rates, growth times, and the degree of overstepping (T) of reactions during metamorphism. CSDs are described through use of a population density function n=dN/dL, where N is the cumulative number of crystals per unit volume and L is a linear crystal size. Plots of ln (n) vs. L for olivine+pyroxene and magnetite in high-temperature (1000° C) basalt hornfelses from the Isle of Skye define linear arrays, indicating continuous nucleation and growth of crystals during metamorphism. Using the slope and intercept of these linear plots in conjunction with growth rate estimates we infer minimum mineral growth times of less than 100 years at T<10° C, and nucleation rates between 10^–4 and 10^–1/cm³/s. Garnet and magnetite in regionally metamorphosed pelitic schists from south-central Maine have CSDs which are bell-shaped. We interpret this form to be the result of two processes: 1) initial continuous nucleation and growth of crystals, and 2) later loss of small crystals due to annealing. The large crystals in regional metamorphic rocks retain the original size frequency distribution and may be used to obtain quantitative information on the original conditions of crystal nucleation and growth. The extent of annealing increases with increasing metamorphic grade and could be used to estimate the duration of annealing conditions if the value of a rate constant were known. Finally, the different forms of crystal size distributions directly reflect differences in the thermal histories of regional vs. contact metamorphosed rocks: because contact metamorphism involves high temperatures for short durations, resulting CSDs are linear and unaffected by annealing, similar to those produced by crystallization from a melt; because regional metamorphism involves prolonged cooling from high temperatures, primary linear CSDs are later modified by annealing to bell shapes.     

Micro- and nano-textural evidence of Ti(–Ca–Fe) mobility during fluid–rock interactions in carbonaceous lawsonite-bearing rocks from New Zealand

Understanding the mobility of chemical elements during fluid–rock interactions is critical to assess the geochemical evolution of a rock undergoing burial and metamorphism and, more generally, to constrain the geochemical budget of the subduction factory. In particular, determining the behavior and mobility of Ti in aqueous fluids constitutes a great challenge that is still under scrutiny. Here, we study plant fossils preserved in blueschist metasedimentary rocks from the Marybank Formation (New Zealand). Using scanning and transmission electron microscopies (SEM and TEM), we show that the carbonaceous material (CM) composing the fossils contains abundant nano-inclusions of Ti- and Fe-oxides. These nanocrystals are mainly anatase, rutile, and Fe–Ti oxides. The mineral composition observed within the fossils is significantly different from that detected in the surrounding rock matrix. We propose that Ti and Fe might have been mobilized by the alteration of a detrital Ti–Fe-rich protolith during an early diagenetic event under acidic and reducing conditions. Aqueous fluids rich in organic ligands released by the degradation of organic matter may have been involved. Moreover, using mass balance and petrological observations, we show that the contrasted mineralogy between the rock matrix and the fossil CM might be the consequence of the chemical isolation of fossil CM during the prograde path of the rock. Such an isolation results from the early formation of quartz and Fe-rich phyllosilicate layers enclosing the fossil as characterized by SEM and TEM investigations. Overall, this study shows that investigating minerals associated with CM down to the nanometer scale in metamorphic rocks can provide a precious record of early prograde geochemical conditions.