Géomorphologie de Tromelin,océan Indien

Tromelin is a small coral reef island (1 km²) located in the Indian Ocean, approximately 440 km east of Madagascar and 580 km north of La Réunion. Despite the presence of a permanent Météo France weather station on Tromelin since the 1950 s, a detailed geomorhological study of the island has never been undertaken. In this paper, we describe results from the 2008 field season, which enabled us to map and describe seven geomorphological zones on the atoll. (1) Tromelin's bathymetry is characterised by a pronounced submarine slope, which attains depths of 1000 m at only 2.5 km from the island's coastline. This geomorphology is typical of volcanic hotspots. (2) A coral platform borders the emerged crown of the island. This platform is exposed at low tide and has been fashioned in an ancient Pleistocene substratum. The substratum has been eroded by abrasive marine action to its present shape since the stabilisation of relative sea level around 6000 years ago. Erosion of the reef yields most of the biogenic sediment supply to the island. (3) The upper foreshore is characterised by outcrops of beachrock. These formations show a stratfication in slabs and dip gently towards the sea in the same manner as the contemporary clastic sediments. (4) Tromelin's beaches are divided into three sub-units, which reflect the different energy dynamics around the island, in addition to the transfer of sediment from the windward to the leeward side: (i) the southern storm beaches are characterised by coral blocks reworked during episodic high-energy events; they form boulder ramparts. Four perched rampart ridges have been recorded at ∼1 m, ∼3 m, ∼4.5 m and ∼7 m above mean sea level; (ii) transitional beaches are observed between the south and the north, constituting storm blocks and coarse sands and gravels; and (iii) sandy beaches are noted on the northern leeward side, comprising rounded gravels and coarse sands. The northern tip of the island is characterised by a sand spit (∼125 × ∼225 m), whose geometry varies based on the seasonal and meteo-marine contexts. (5) On Tromelin, a number of dune formations are observed. These are best developed in the north of the island due to the south-north translation of clastic sediments by aeolian wind action, and the trapping of sands by the well-developed vegetation. The height of the dunes varies between 10 cm (the southern micro-dunes) to 250 cm in the Northeast of Tromelin. (6) Exceptional waves have deposited storm tracts up to 250 m from the foreshore zone, comprising coarse gravels and coral blocks. The storm tract deposits are most prevalent on the windward side of the island, due not only to the high-energy dynamics, but also to the absence of vegetation cover in this area. (7) The southern depression covers an area of ∼40,000 m². At its lowest point it is only 1.2 m above sea level.     

Accuracy and efficiency of time integration methods for 1D diffusive wave equation

The diffusive wave approximation of the Saint-Venant equations is commonly used in hydrological models to describe surface flow processes. Numerous numerical approaches can be used to solve this highly nonlinear equation. Nonlinear time integration schemes—also called methods of lines (MOL)—were proven very efficient to solve other nonlinear problems in geosciences but were never considered to deal with surface flow modeling with the diffusive wave equation. In this paper, we study the relative performance of different time and space integration schemes by comparing the results obtained with classical approaches and with nonlinear time integration approaches. The results show that (i) the integration method with a higher order in space shows high accuracy regarding an integrated indicator such as the global mass balance error but is less accurate regarding local indicators, and (ii) nonlinear time integration techniques perform better than classical ones. Overall, it seems that integration techniques combining nonlinear time integration and a low spatial order need to be considered when developing hydrological modeling tools owing to their simplicity of implementation and very good performance.     

Calcium,Na, K and Mg Concentrations in Seawater by Inductively Coupled Plasma‐Atomic Emission Spectrometry: Applications to IAPSO Seawater Reference Material,Hydrothermal Fluids and Synthetic Seawater Solutions

We report a measurement procedure to determine simultaneously the major cation concentrations (Na, Ca, K and Mg) of seawater‐derived solutions by inductively coupled plasma‐atomic emission spectrometry. The best results were obtained when the IAPSO (‘standard’) seawater reference material was diluted by thirty times with Milli‐Q^® water. We obtained an average reference value r_K (the ratio of the mass fraction of potassium to that of chlorine, i.e., (g kg^?1)/(g kg^?1)) for IAPSO seawater of 0.0205 ± 0.0006 (2.9% RSD), not significantly different from 0.0206 ± 0.0005 (2.4% RSD) for seawater composition reported in the literature. The measured Na, Ca and Mg concentrations correspond to r_Na, r_Ca and r_Mg values of 0.5406 ± 0.0026 (0.5% RSD), 0.02192 ± 0.00048 (2.2% RSD) and 0.06830 ± 0.00047 (0.7% RSD), respectively, in line with previous values measured by wet‐chemistry and atomic absorption spectrophotometry or wet‐chemical titration. Our measurement procedure was used successfully on synthetic seawater solutions and high‐temperature hydrothermal fluids.     

In situ monitoring of karst springs in Wallonia (southern Belgium)

The implementation of the Water Framework Directive, as well as the forthcoming entry into force of other European water policies focusing on water resources, require a thorough reorganization of groundwater resources monitoring in most European countries. In Wallonia, monitoring programs were initiated in 2005 to control the quantitative and chemical statuses of groundwater bodies, as well as the effectiveness of management plans aimed at achieving environmental objectives. In karst aquifers, springs are preferential monitoring targets even if, as a result of the high heterogeneity of this type of aquifer, the interpretation of time series and spatial data remains a challenge. Since 2006, a “springs” monitoring network has been progressively set up in the southern part of Belgium. Currently, in situ measurements of discharge, temperature, conductivity, turbidity and fluorescence of water are conducted at nine karstic outlets in the Devonian and the Carboniferous limestone aquifers of Wallonia. The main objective of this ongoing research is to evaluate the suitability and the robustness of such standard measurements to discriminate anthropogenic impacts from natural variations, at both short- and long-term scales. Discharge and temperature data revealed being useful in the assessment of regional hydro-climatic trends. Groundwater proved to be a better natural filter for the assessment of long-term climatic changes than mathematical filtering of noisy surface signals. Investigations of such data at a European scale could therefore give additional insight into the impacts of climate change on groundwater resources. Conductivity, turbidity and fluorescence data characterize the particulate, mineral and organic health of water and a better understanding of their natural dynamics could help in the early detection of anthropogenic deviations. However, the current reproducibility of these measurements is too low to ascertain observed trends and fluctuations, and further research is still needed.     

Dissolution–precipitation processes governing the carbonation and silicification of the serpentinite sole of the New Caledonia ophiolite

The weathering of mantle peridotite tectonically exposed to the atmosphere leads commonly to natural carbonation processes. Extensive cryptocrystalline magnesite veins and stock-work are widespread in the serpentinite sole of the New Caledonia ophiolite. Silica is systematically associated with magnesite. It is commonly admitted that Mg and Si are released during the laterization of overlying peridotites. Thus, the occurrence of these veins is generally attributed to a per descensum mechanism that involves the infiltration of meteoric waters enriched in dissolved atmospheric CO₂. In this study, we investigate serpentinite carbonation processes, and related silicification, based on a detailed petrographic and crystal chemical study of serpentinites. The relationships between serpentine and alteration products are described using an original method for the analysis of micro-X-ray fluorescence images performed at the centimeter scale. Our investigations highlight a carbonation mechanism, together with precipitation of amorphous silica and sepiolite, based on a dissolution–precipitation process. In contrast with the per descensum Mg/Si-enrichment model that is mainly concentrated in rock fractures, dissolution–precipitation process is much more pervasive. Thus, although the texture of rocks remains relatively preserved, this process extends more widely into the rock and may represent a major part of total carbonation of the ophiolite.     

A mass‐balance approach to estimate in‐stream processes in a large river

A mass‐balance approach was used to estimate in‐stream processes related to inorganic nitrogen species (NH₄⁺, NO₂^? and NO₃^?) in a large river characterized by highly variable hydrological conditions, the Garonne River (south‐west France). Studies were conducted in two consecutive reaches of 30 km located downstream of the Toulouse agglomeration (population 760 000, seventh order), impacted by modification of discharge regime and high nitrogen concentrations. The mass‐balance was calculated by two methods: the first is based on a variable residence time (VRT) simulated by a one‐dimensional (1‐D) hydraulic model; the second is a based on a calculation using constant residence time (CRT) evaluated according to hydrographic peaks. In the context of the study, removal of dissolved inorganic nitrogen (DIN) for a reach of 30 km is underestimated by 11% with the CRT method. In sub‐reaches, the discrepancy between the two methods led to a 50% overestimation of DIN removal in the upper reach (13 km) and a 43% underestimation in the lower reach (17 km) using the CRT method. The study highlights the importance of residence time determination when using modelling approaches in the assessment of whole stream processes in short‐duration mass‐balance for a large river under variable hydrological conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

Influence of a hedge surrounding bottomland on seasonal soil‐water movement

The influence of a hedge surrounding bottomland on soil‐water movement along the hillslope was studied on a plot scale for 28 months. The study was based on the comparison of two transects, one with a hedge, the other without, using mainly a dense grid of tensiometers. The influence of the bottomland hedge was located in the area where tree roots were developed, several metres upslope from the hedge, and could be observed both in the saturated and non‐saturated zone, from May to December. The hedge induced a high rate of soil drying, because of the high evaporative capacity of the trees. We evaluated that water uptake by the hedge during the growing season was at least 100 mm higher than without a hedge. This increased drying rate led to a delayed rewetting of the soils upslope from the hedge in autumn, of about 1 month compared with the situation without a hedge. Several consequences of this delayed rewetting are expected: a delay in the return of subsurface transfer from the hillslope to the riparian zone, a buffering effect of hedges on floods, already observed at the catchment scale, and an increased residence time of pollutants. Copyright © 2003 John Wiley & Sons, Ltd.     

Estimating Long-Term Solar Irradiance Variability: A New Approach

The detection of solar irradiance variations (both bolometric and at various wavelengths) by satellite-based experiments during the last one-and-a-half decades stimulated modeling efforts to help identify their causes and to provide estimates of irradiance data for those time intervals when no satellite observations exist. In this paper we present estimates of the long-term irradiance changes developed with Fourier and wavelet transforms. The month-to-month irradiance variations, after removing the solar cycle related long-term changes, are studied with the cross-correlation technique. Results of the analysis reveal a significant phase shift at 3 months between the full-disk magnetic field strength and total solar and UV irradiance, with irradiance leading the magnetic field variability. In addition to this time delay between the changes in solar irradiance and the magnetic field, a 10-month phase shift has been found between the UV flux at 280 nm and total solar irradiance corrected for sunspot darkening. The existence of these phase shifts suggests the possibility of a coupling between the physical processes taking place below, in, and above the photosphere.