In situ investigations of allanite hydrothermal alteration: examples from calc-alkaline and anorogenic granites of Corsica (southeast France)

Allanite is, with monazite, the main repository for light rare earth elements (REE) in the continental crust and can be used in U-Th-Pb geochronology. This mineral has been shown to be prone to alteration. The geochemical exchanges occurring between allanite and hydrothermal fluids were explored using backscattered scanning electron microscopy, electron microprobe, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The alteration mechanisms found show the restricted role of metamictization and correspond to the allanite to epidote transformation or to the leaching of the allanite A-crystallographic sites. Hence, the REE are mostly removed from this phase during fluid-mineral interactions, although a heavy rare earth-rich fluid may imprint its geochemical signature on the allanite altered zones. It appears that even if this REE holder presents evidence of alteration, the bulk mineral and whole-rock REE and Sm-Nd isotope signatures were not significantly affected in the cases studied. Alteration results mostly in the input of common lead, thus restricting the usefulness of allanite for U-Th-Pb dating when alteration occurs. Finally, the alteration mechanisms found here confirm the much more limited flexibility of silicates crystalline lattice as compared to phosphates.     

ASSESSMENT OF A SIMPLE METHOD FOR THE DETERMINATION OF NB AND TA AT THE SUB-μG/G LEVEL IN SILICATE ROCKS BY ICP-MS

We report Nb and Ta data for 15 international silicate rock standards, obtained by inductively coupled plasma-mass spectrometry (ICP-MS) measurement following decomposition with HF-HNOs-HCI.⁹⁵ Moand¹⁸⁵Re were used as internal standards, and calibration lines were constructed with artificial solutions. Typical relative standard deviations are better than 10% above 0.2 μg/g, and detection limits for Nb and Ta are 40 and 0.8 ng/g, respectively. The results are generally in good agreement with published data. Moreover, this study points out the Nb-Ta heterogeneity of some standard reference materials on the 100 mg scale.     

Surface rupture of the Cariaco July 09, 1997 earthquake on the El Pilar fault, northeastern Venezuela

This paper discusses the surface rupture of the Cariaco July 09, 1997 Ms 6.8 earthquake in northeastern Venezuela – located at 10.545°N and 63.515°W and about 10 km deep. The field reconnaissance of the ground breaks confirms that this event took place on the ENE–WSW trending onshore portion of the dextral El Pilar fault (between the Gulfs of Cariaco and Paria), which is part of the major wrenching system within the Caribbean–South America plate boundary zone. Dextral slip along this fault was further supported by the structural style of this rupture (en echelon right-lateral R shears connected by mole tracks at restraining stepovers) and by larger geometric complexities (pop-ups at Las Manoas and Guarapiche), as well as by the focal mechanism solutions determined for the event by several authors. This 1997 surface ruptre comprised two distinct sections, from west to east: (a) a main very conspicuous, continuous, 30-km-long, rather straight, 075°N-trending alignment of en echelon surface breaks, with a rather constant, purely dextral coseismic slip of about 25  cm, but reaching a maximum value of 40 cm slightly northwest of Pantoño; and (b) a secondary discontinuous, 10-km-long, boomerang-shaped rupture, with a maximum coseismic slip of 20 cm at Guarapiche. The onshore extent of the surface rupture totalled 36 km, but may continue westward underwater, as suggested by the very shallow aftershock seismicity. This aftershock activity also clearly defined the steep north dip of the fault plane along the western rupture, suggesting tectonic inheritance on this major fault.From many locals' accounts, the rupture seems to have propagated from Pantoño to the west (highly asymmetric bidirectionality). This suggests that earthquake nucleation happened at or near the Casanay–Guarapiche restraining bend and rupture quickly propagated westward, allowing only a small fraction to progress eastwards beyond the bend. Additionally, the large fraction of after-slip (or creep) released is to be related to such restraining bend, which seems to have partly locked slip during rupture.     

Intertidal subsidence and collapse features on wave‐exposed,drift‐aligned sandy beaches subject to Amazon mud: Cayenne,French Guiana

Short‐lived collapse features affecting the intertidal sandy profile were observed on mud‐rich beaches in Cayenne. They were formed in packets of beach sand accumulating over thick (several metres) fluid to under‐consolidated foreshore mud, which adjusts to this sand loading by seaward and longshore migration of fluid mud and subsequent in situ mud dewatering, thus generating subsidence and deformation of the overlying sand sheet. These sandy collapse features required active longshore sand transport and were washed out by wave processes at each rising tide. They finally disappeared permanently as adjustment of the underlying mud enabled stabilization of the beach profile. Copyright © 2006 John Wiley & Sons, Ltd.     

High-resolution atmospheric forcing for regional oceanic model: the Iroise Sea

This study was aimed at modeling, as realistically as possible, the dynamics and thermodynamics of the Iroise Sea by using the Model for Applications at Regional Scale (MARS), a regional ocean 3D model. The horizontal resolution of the configuration in use is 2 km with 30 vertical levels. The 3D model of the Iroise Sea is embedded in a larger model providing open boundary conditions. As regards the atmospheric forcing, the originality of this study is to force the regional ocean model with the high-resolution (6 km) regional meteorological model, Weather Research and Forecasting (WRF). In addition, as the air surface temperature is highly sensitive to the sea surface temperature (SST), this regional meteorological model is improved by taking into account a regional climatologic SST to compute meteorological parameters. By allowing a better coherence between the SST and the temperature of the atmospheric boundary layer while giving a more realistic representation of heat fluxes exchanged at the air/sea interface, this forcing constitutes a noticeable improvement of the Iroise Sea modeling. The different sensitivity tests discussed here pinpoint the importance of entering, in WRF, SST data of sufficiently high quality before the computation of meteorological forcing when the aim is a study of dynamics and thermodynamics far away from the coast. On the other hand, when the target is the reproduction of coastal small-scale features in Iroise Sea modeling, the resolution of the meteorological forcing and the quality of SST are both paramount. The simulation of reference was carried out throughout the Summer and Autumn of year 2005 to allow comparisons with a campaign of surface current measurements by high-frequency radars conducted at the same period.     

The March 25 and 29, 2016 landslide-induced debris flow at Clapar,Banjarnegara, Central Java

The Clapar landslide induced debris flow consisted of the Clapar landslide occurred on 24 March 2017 and the Clapar debris flow occurred on 29 March 2017. The first investigation of the Clapar landslide induced debris flow was carried out two months after the disaster. It was followed by UAV mapping, extensive interviews, newspaper compilation, visual observation and field measurements, and video analysis in order to understand chronology and triggering mechanism of the landslide induced debris flow in Clapar. The 24 March 2016 landslide occurred after 5 hours of consecutive rainfall (11,2 mm) and was affected by combination of fishponds leak and infiltration of antecedent rain. After five days of the Clapar landslide, landslide partially mobilized to form debris flow where the head scarp of debris flow was located at the foot of the 24 March 2016 landslide. The Clapar debris flow occurred when there was no rainfall. It was not generated by rainstorm or the surface erosion of the river bed, but rather by water infiltration through the crack formed on the toe of the 24 March 2016 landslide. Supply of water to the marine clay deposit might have increased pore water pressure and mobilized the soil layer above. The amount of water accumulated in the temporary pond at the main body of the 24 March 2016 landslide might have also triggered the Clapar debris flow. The area of Clapar landslide still shows the possibility of further retrogression of the landslide body which may induce another debris flow. Understanding precursory factors triggering landslides and debris flows in Banjarnegara based on data from monitoring systems and laboratory experiments is essential to minimize the risk of future landslide.     

Modelling larval dispersal and settlement of the reef-building polychaete Sabellaria alveolata: Role of hydroclimatic processes on the sustainability of biogenic reefs

The honeycomb worm Sabellaria alveolata forms biogenic reefs which constitute diversity hotspots on tidal flats. The largest known reefs in Europe, located in the Bay of Mont-Saint-Michel (English Channel), are suffering increasing anthropogenic disturbances which raise the question of their sustainability. As the ability to recover depends partly on the recolonization of damaged reefs by larval supply, evaluating larval dispersal and the connectivity between distant reefs is a major challenge for their conservation. In the present study, we used a 3D biophysical model to simulate larval dispersal under realistic hydroclimatic conditions and estimate larval retention and exchanges among the two reefs of different sizes within the bay. The model takes into account fine-scale hydrodynamic circulation (800×800 m²), advection–diffusion larval transport, and gregarious settlement behaviour. According to the field data, larval dispersal was simulated for a minimal planktonic larval duration ranging from 4 to 8 weeks and the larval mortality was set to 0.09 d⁻¹. The results highlighted the role played by a coastal eddy on larval retention within the bay, as suggested by previous in situ observations. Very different dispersal patterns were revealed depending on the spawning reef location, although the two reefs were located only 15 km apart. The settlement success of the larvae released from the smallest reef was mainly related to tidal conditions at spawning, with the highest settlement success for releases at neap tide. The settlement success of the larvae from the biggest reef was more dependent on meteorological conditions: favourable W and SW winds may promote a ten-fold increase in settlement success. Strong year-to-year variability was observed in settlers’ numbers, with favourable environmental windows not always coinciding with the main reproductive periods of Sabellaria. Settlement kinetics indicated that the ability to delay metamorphosis could significantly improve the settlement success. Although bidirectional exchanges occurred between the two reefs, the highest settlers’ numbers originated from the biggest reef because of its stronger reproductive output. Because of the recent decline of this reef due to increasing anthropogenic disturbances larval supply in the bay may not be sufficient enough to ensure the sustainability of the remarkable habitat formed by Sabellaria alveolata reefs.     

Crustal structure in the Falcón Basin area, northwestern Venezuela, from seismic and gravimetric evidence

The Falcón Basin in northwestern Venezuela has a complex geological history driven by the interactions between the South American and Caribbean plates. Igneous intrusive bodies that outcrop along the axis of the basin have been associated with crustal thinning, and gravity modeling has shown evidence for a significantly thinned crust beneath the basin. In this study, crustal scale seismic refraction/wide-angle reflection data derived from onshore/offshore active seismic experiments are interpreted and forward-modeled to generate a P-wave velocity model for a 450 km long profile. The final model shows thinning of the crust beneath the Falcón Basin where depth to Moho decreases to 27 km from a value of 40 km about 100 km to the south. A deeper reflected phase on the offshore section is interpreted to be derived from the downgoing Caribbean slab. Velocity values were converted to density and the resulting gravimetric response was shown to be consistent with the regional gravity anomaly. The crustal thinning proposed here supports a rift origin for the Falcón Basin.     

Comparison of C and N stable isotope ratios between surface particulate organic matter and microphytoplankton in the Gulf of Lions (NW Mediterranean)

Carbon and nitrogen stable isotope ratios of particulate organic matter (POM) in surface water and 63–200 μm-sized microphytoplankton collected at the fluorescence maximum were studied in four sites in the Gulf of Lions (NW Mediterranean), a marine area influenced by the Rhone River inputs, in May and November 2004. Some environmental (temperature, salinity) and biological (POM, Chlorophyll a and phaeopigments contents, phytoplankton biomass and composition) parameters were also analysed. Significantly different C and N isotopic signatures between surface water POM and microphytoplankton were recorded in all sites and seasons. Surface water POM presented systematically lower δ¹³C (∼4.2‰) and higher δ¹⁵N (∼2.8‰) values than those of microphytoplankton, due to a higher content of continental and detrital material. Seasonal variations were observed for all environmental and biological parameters, except salinity. Water temperature was lower in May than in November, the fluorescence maximum was located deeper and the Chlorophyll a content and the phytoplankton biomass were higher, along with low PON/Chl a ratio, corresponding to spring bloom conditions. At all sites and seasons, diatoms dominated the phytoplankton community in abundance, whereas dinoflagellate importance increased in autumn particularly in coastal sites. C and N isotopic signatures of phytoplankton did not vary with season. However, the δ¹⁵N of surface water POM was significantly higher in November than in May in all sites likely in relation to an increase in ¹⁵N/¹⁴N ratio of the Rhone River POM which influenced surface water in the Gulf of Lions. As it is important to determine true baseline values of primary producers for analysing marine food webs, this study demonstrated that C and N isotopic values of surface water POM cannot be used as phytoplankton proxy in coastal areas submitted to high river inputs.     

Variations in the response of the dune coast of northern France to major storms as a function of available beach sediment volume

A series of airborne topographic LiDAR data were obtained from May 2008 to January 2014 over two coastal sites of northern France (Bay of Wissant and east of Dunkirk). These data were used with wind and tide gauge measurements to assess the impacts of storms on beaches and coastal dunes, and particularly of the series of major storms that hit western Europe during the fall and early winter of 2013. Our results show a high variability in shoreline response from one site to the other, but also within each coastal site. Coastal dune erosion and shoreline retreat occurred at both sites, particularly on the coast of the Bay of Wissant where shoreline retreat up to about 40 m was measured. However, stability or even shoreline advance were also observed despite the occurrence of an extreme water level with a return period >100 years during the storm Xaver in early December 2013. Comparison of shoreline change with variations of coastal dune and upper beach volumes revealed only weak relationships. Our results nevertheless showed that shoreline behavior seems to strongly depend on the initial sediment volume on the upper beach before the occurrence of the storms. According to our measurements, an upper beach volume of about 30 m³ m^?1 between the dune toe and the mean high water level is sufficient at these sites to protect the coastal dunes from storm waves associated with high water levels with return periods >10 years. The identification of such thresholds in terms of upper beach width or sediment volume may represent valuable information for improving the management of shoreline change by providing an estimate of the minimum quantity of sand on the upper beach necessary to ensure shoreline stability in this region. Copyright © 2016 John Wiley & Sons, Ltd.