Heavy metals distribution in mangrove sediments along the mobile coastline of French Guiana

The accumulation of nine heavy metals in fine-grained sediments from the mangrove fringed coast of French Guiana is evaluated. The dynamic features of the South American tropical coastline, from the Amazon to the Orinoco Rivers, result in mangrove sediments being alternately submitted to phases of erosion and net sedimentation a few tens of years long. This process influences the distribution of the heavy metals associated with these frequently re-mobilized deposits. Sedimentary cores and mangrove plant samples were collected, at different seasons, in various swamps characterized by different properties (content of sedimentary organic matter, distance from sea water and fresh water). The ranges of measured concentrations expressed in μmol g^− 1 were the following: Cu (0.06 to 0.61), Co (0.12 to 0.68), Pb (0.08 to 0.18), Ni (0.32 to 0.76), Cr (0.61 to 1.40), Zn (1.25 to 5.94), Mn (4.36 to 45.4) and Fe (441 to 1128). No differences were found between sediments from mangroves developing upstream and downstream of urban areas, i.e. Cayenne and Kourou. This suggests that the content of mangrove sediments in heavy metals along the coastline of French Guiana is essentially the result of the continuous alternation of accumulation and transport phases occurring upstream after departing from the Amazon watershed. The sources of this heavy metals content are thus difficult to identify. However it is well known that the alluvium produced by the natural erosion of the Amazonian soils is naturally enriched in mercury. Also, the run-off from gold mining activities is known to contribute to mercury pollution. Ranges in total Hg were between 0.15 and 2.57 nmol g^− 1, with mean values close to 0.41 nmol g^− 1, and were clearly correlated with total organic carbon except for some outstanding high values, which may be a result of rapid geochemical changes. Heavy metal concentrations showed variations with depth. The redox conditions and the decay processes affecting the organic matter control the cycling of iron and manganese, which in turn control the concentrations and associations of heavy metals. These preliminary results suggest that the variations in heavy metal content with depth or between mangrove areas result largely from diagenetic processes rather than changes in metal input resulting from local human activities.     

Diurnal heterogeneity in silicic acid fluxes in shallow coastal sites: Causes and implications

In shallow coastal areas the amplitude and range of benthic silicic acid fluxes can have a significant influence on benthic–pelagic coupling and the functioning of the pelagic system. To explore the oscillation in fluxes over the diurnal cycle and in particular the influence of microphytobenthos (MPB), an experiment was carried out in a shallow subtidal site in the Bay of Brest (France). Benthic chambers were employed over a 48 h period to measure the variability in silicic acid and oxygen fluxes; MPB migration was investigated using a diving Pulse Amplitude Modulated (PAM) fluorometer and uptake rhythms of silicic acid by natural MPB populations were measured using the ³²Si isotope. It was discovered that silicic acid fluxes fluctuated greatly throughout the diurnal period resulting in an oscillation in the availability of this nutrient for phytoplankton communities. The uptake of silicic acid by the MPB was quantified for the first time and highlighted a 2-fold increase in the demand from night to afternoon periods. The combined silicic acid uptake and the concentration of cells at the sediment–water interface, forming a dense biofilm of MPB, were postulated to be the main processes reducing effluxes at midday. Our work highlighted the many processes which influence silicic acid effluxes in shallow coastal areas and the possible interaction between uptake and dissolution processes. The variations in benthic fluxes over the diurnal period were comparable to observations reported at the seasonal scale. Therefore, up-scaling hourly flux observations to daily and annual estimates should be undertaken with caution. Further we suggest that the main processes influencing flux oscillations over the diurnal period should be considered when planning sampling strategies and extrapolating to larger time scales.     

From ACH tomographic models to absolute velocity models

The ACH method, a widely used tomographic inverse method, is characterized by the use of relative residuals in order to avoid possible biases coming from outside the target volume. The ACH method thus does not really retrieve the 3-D structure of the target volume, but instead leads to velocity contrasts relative to the layer average of the velocity, this average value remaining unknown ( Aki et al. 1977 ). Two artefacts derive from this particularity: (1) velocity contrasts are known only in the horizontal direction and it is not possible, in a strict mathematical sense, to estimate the contrasts in the vertical direction with ACH alone; (2) negative anomalies are often interpreted as low velocities, whereas negative anomalies may correspond to high velocities if the average value of the corresponding layer is sufficiently high. The converse is true of positive anomalies. We show with synthetic data how these artefacts can affect the interpretation of tomographic images. We propose to correct the artefacts by reintroducing the 1-D regional average model, and show in synthetic experiments how effective this correction can be.
  The application of this procedure to data recorded in the Kunlun region shows that the retrieval of the absolute values of the 3-D velocity model is helpful for interpreting the tomographic images and better defining which features are anomalous.     

Tracking past mining activity using trace metals,lead isotopes and compositional data analysis of a sediment core from Longemer Lake,Vosges Mountains,France

A 157-cm-long sediment core from Longemer Lake in the Vosges Mountains of France spans the past two millennia and was analyzed for trace metal content and lead isotope composition. Trace metal accumulation rates highlight three main input phases: Roman Times (cal. 100 BC–AD 400), the Middle Ages (cal. AD 1000–1500), and the twentieth century. Atmospheric contamination displays a pattern that is similar to that seen in peat bogs from the region, at least until the eighteenth century. Thereafter, the lake sediment record is more precise than peat records. Some regional mining activity, such as that in archaeologically identified eighteenth-century mining districts, was detected from the lead isotope composition of sediment samples. Compositional data analysis, using six trace metals (silver, arsenic, cadmium, copper, lead and zinc), enabled us to distinguish between background conditions, periods of mining, and of other anthropogenic trace metal emissions, such as the recent use of leaded gasoline.     

Tertiary remagnetization of normal polarity in Mesozoic marly limestones from SE France

The results of our new paleomagnetic investigations on 21 sites in the Cévennes and Lure regions as well as previous studies demonstrate that all Mesozoic marly limestones of SE France exhibit similar paleomagnetic behavior with remagnetization disputed in age. The studied areas have the particularity to have been folded before (Late Eocene), the Alpine folding (Oligo–Miocene). Samples (201 marly limestones) dated from Lower Jurassic to Lower Cretaceous have been demagnetized by thermal treatment. They all present a well-defined component with a normal polarity which was mostly obtained between 200 and 350 °C. Numerous arguments lead from pretectonic to syntectonic widespread remagnetization related to orogenic fluid circulation affecting the whole basin. An Eocene age (between 35 and 40 Ma) is obtained for this remagnetization thanks both to the comparison of the average inclination of all regional paleomagnetic studies (+54.9°/−1.5°) with the expected paleomagnetic inclination and the syntectonic character of remagnetization.     

Enregistrement de la dernière remontée du niveau marin dans l'architecture interne d'une vallée incisée : le pertuis Breton (Charente-Maritime)

Recent very high-resolution seismic profiles ground-truthed by vibrocores allow us to evidence an atypical incised valley fill in a drowned valley segment, the ‘Pertuis Breton’ (Bay of Biscay, France). The sedimentary valley-fill architecture mainly includes five superimposed progradational wedges composed by marine sands. Sandbodies show a landward migration of their depocentres upward and are topped by almost flat unconformities extended by submarine terraces. This sedimentary infill pattern is similar to backstepping wedges, described on continental shelfs. It suggests that this valley fill records sea-level rise during the last transgression. To cite this article: N. Weber et al., C. R. Geoscience 336 (2004).     

The effect of slope aspect on the response of snowpack to climate warming in the Pyrenees

The aim of this study was to analyse the effect of slope aspect on the response of snowpack to climate warming in the Pyrenees. For this purpose, data available from five automatic weather stations were used to simulate the energy and mass balance of snowpack, assuming different magnitudes of an idealized climate warming (upward shifting of 1, 2 and 3 °C the temperature series). Snow energy and mass balance were simulated using the Cold Regions Hydrological Modelling platform (CRHM). CRHM was used to create a model that enabled correction of the all-wave incoming radiation fluxes from the observation sites for various slope aspects (N, NE, E, SE, S, SW,W,NW and flat areas), which enabled assessment of the differential impact of climate warming on snow processes on mountain slopes. The results showed that slope aspect was responsible for substantial variability in snow accumulation and the duration of the snowpack. Simulated variability markedly increased with warmer temperature conditions. Annual maximum snow accumulation (MSA) and annual snowpack duration (ASD) showed marked sensitivity to a warming of 1 °C. Thus, the sensitivity of the MSA in flat areas ranged from 11 to 17 % per degree C amongst the weather stations, and the ASD ranged from 11 to 20 days per degree C. There was a clear increase in the sensitivity of the snowpack to climate warming on those slopes that received intense solar radiation (S, SE and SW slopes) compared with those slopes where the incident radiation was more limited (N, NE and NW slopes). The sensitivity of the MSA and the ASD increased as the temperature increased, particularly on the most irradiated slopes. Large interannual variability was also observed. Thus, with more snow accumulation and longer duration the sensitivity of the snowpack to temperature decreased, especially on south-facing slopes.     

Thermal Fronts Atlas of Canadian Coastal Waters

Oceanic fronts are often associated with enhanced biological activity. Depending on their generation mechanism, they are often linked to specific geographical areas. Here we use 25 years of high-resolution satellite sea surface temperature (SST) daily images to generate maps of SST fronts over Canadian coastal waters. Results show that fronts are ubiquitous features, but some fronts are more persistent than others. We confirmed the location of previously known major fronts, but some new persistent frontal areas were also detected as a result of the use of high-resolution (1.1?km) data and a methodology adapted to detect smaller-scale frontal features. Results also show that some of the frontal areas are associated with enhanced phytoplankton biomass or higher trophic level organisms (whales and birds) confirming the ecological importance of this physical process.