Spatial Variation of Sea-Spray Fluxes over a Mediterranean Coastal Zone Using a Sea-State Model

We first deal with sea-spray flux estimates for short fetch conditions in coastal Mediterranean areas. To this end, a sea-state dependent model for the whitecap fraction was included in three different formulations for the sea-spray source function. A comparison with the sea-spray fluxes, calculated on the basis of aerosol size distributions measured at the island of Porquerolles located south off the French Riviera, evaluates the predictions of different whitecap dependant flux formulations. Then we deal with the spatial distribution of the whitecap fraction and the sea-spray fluxes in the study area. To achieve this, a whitecap dependant flux formulation was forced by a wave numerical model that was implemented in the study area. Experimental results on wave conditions have been used to adjust the model in the Mediterranean coastal area. Numerical simulations of wave and whitecap coverage have been carried out during typical regional wind events, and they show a nonhomogeneous distribution of the sea-surface production over the northern Mediterranean as a consequence of the spatial variation of the sea state. In particular, we note the occurrence of a narrow band of high sea-surface production following the northern coast and along the east part of the Gulf of Lions.     

Multi-tracer tests to evaluate the hydraulic setting of a complex aquifer system (Brévilles spring catchment, France)

For good management of groundwater resources, and to comply with European and national regulations, a detailed understanding of an aquifer’s hydraulic setting is required. In order to better characterize a sandy aquifer that is affected by diffuse pollution (Brévilles spring catchment, Val d’Oise, France), and to quantify the transfer time in the saturated zone, a multi-tracer test involving a new technique, the ‘finite volume point dilution method’, has been performed in natural flow conditions. In November 2005, injections of four different tracers took place in four piezometers involving different locations and depths in the aquifer. Recovery of the tracers was observed at two different places near the aquifer outlet. A particularly long and unusual monitoring exercise (27 months) demonstrated the existence of several different velocities within the sandy layer, which seems to be linked to the decrease of hydraulic conductivity with depth. The new insight and parameter quantification brought by interpretation of these tests contribute to a better characterization of the saturated zone. The particularly long-term monitoring exercise also gives new information to understand and forecast the trend and persistence of groundwater contamination by pesticides in the catchment.     

Organic matter in sediments of canyons and open slopes of the Portuguese,Catalan, Southern Adriatic and Cretan Sea margins

We describe the quantitative and compositional (phytopigment, protein, carbohydrate and lipid) patterns of sedimentary organic matter along bathymetric gradients in seven submarine canyons and adjacent open slopes located at four European regions: one along the NE Atlantic and three along the Mediterranean continental margins. The investigated areas are distributed along a putative longitudinal gradient of decreasing primary production from the Portuguese (northeastern Atlantic Ocean), to the Catalan (western Mediterranean Sea), Southern Adriatic (central Mediterranean Sea) and Southern Cretan (eastern Mediterranean Sea) margins. Sediment concentrations of organic matter differed significantly between the Portuguese margin and the Mediterranean regions and also from one study area to the other within the Mediterranean Sea. Differences in quantity and composition of sediment organic matter between canyons and open slopes were limited and significant only in the eutrophic Portuguese margin, where the differences were as large as those observed between regions (i.e. at the mesoscale). These results suggest that the overall trophic status of deep margin sediments is controlled mostly by the primary productivity of the overlying waters rather than by the local topography. Moreover, we also report that the quantity and nutritional quality of sediment organic matter in canyons and adjacent open slopes do not show any consistent depth-related pattern. Only the Nazaré and Cascais canyons in the Portuguese margin, at depths deeper than 500 m, displayed a significant accumulation of labile organic matter. The results of our study underline the need of further investigations of deep margins through sampling strategies accounting for adequate temporal and spatial scales of variability.     

C chronology for ice retreat and inception of Champlain Sea in the St. Lawrence Lowlands, Canada

AMS radiocarbon cross-dating of plant debris and marine shells trapped in a lake basin on Mount St. Hilaire (Québec, Canada) provides a direct assessment of a reservoir effect totaling ca. 1800 ¹⁴C years during the early stage of Champlain Sea. Pollen-based extrapolation of bottommost ages on terrestrial plant macrofossils in sediments of this lake, and of another lake nearby support an estimate of 11,100 ± 100 ¹⁴C yr B.P. for marine invasion in the Central St. Lawrence River Lowlands. Results indicate a 400–1000 years younger regional chronology of ice retreat, now congruent with the one inferred from the New England varve chronology. This is a summary of a longer paper to be published in French.     

Large-eddy Simulation of the Dispersion of Solid Particles in a Turbulent Boundary Layer

A large-eddy simulation (LES) with the dynamic Smagorinsky-Germano subgrid-scale (SGS) model is used to study the dispersion of solid particles in a turbulent boundary layer. Solid particles are tracked in a Lagrangian way. The instantaneous velocity of the surrounding fluid is considered to have a large-scale part (directly computed by the LES) and a small-scale part. The SGS velocity of the surrounding fluid is given by a three-dimensional Langevin model written in terms of SGS statistics at a mesh level. An appropriate Lagrangian correlation time scale is considered in order to include the influences of gravity and inertia of the solid particle. Inter-particle collisions and the influence of particles on the mean flow are also taken into account. The results of the LES are compared with the wind-tunnel experiments of Nalpanis et al. (1993 J Fluid Mech 251: 661–685) and of Tanière et al. (1997 Exp in Fluids 23:463–471) on sand particles in saltation and in modified saltation, respectively.     

Nutrients limiting the algal growth potential (AGP) in the Po River plume and an adjacent area, Northwest Adriatic Sea: Enrichment bioassays with the test algaeNitzschia closterium andThalassiosira pseudonana

From April 1993 to March 1994, 135 samples were collected at two sites in the coastal Adriatic Sea, both near (14.5 km) and far (63 km) from the Po River delta. The nutrient(s) limiting algal growth potential (AGP) were estimated by bioassay usingNitzshia closterium (local isolate) andThalassiosira pseudonama (strain 3-H). Estimates were also made by comparing the nutrient molar ratios, ΣN:P and ΣN:Si (whereΣN=NO₃ ^?+NO₂ ^?+NH₄ ⁺+urea), to the Redfield Ratio (16∶1, 1∶1). According to the bioassay results, phosphorus was the sole nutrient limiting AGP in 2% of the samples and was the most limiting nutrient in 69% of the samples; nitrogen was sole limiting nutrient in 18% of the samples. In 11% of samples, nitrogen and phosphorus were equally co-limiting. Omission of phosphorus from spike enrichments allowed, on average, only a 1.6-fold increase in biomass over that in the unenriched controls. Similar omission of nitrogen allowed a 4-fold increase, while silicon, iron, and micronutrients resulted in 14-fold, 18-fold, and >20-fold increases, respectively. In most of the samples, ΣN:P was much greater than 16, indicating a marked phosphorus deficiency, while ΣN:Si values suggested that silicon was the third most limiting nutrient in 35% of samples. In water collected for from the Po delta, the yield ofN. closterium was not limited by any nutrients other than the three major ones: P, N and Si. In these same waters,T. pseudonana was also potentially limited by iron and, to a lesser extent, by vitamins. The role of iron varied. In samples collected near the Po delta, iron acted as the third most limiting nutrient forN. closterium in June and September; it appeared 29 times out of 78 on the list of potentially limiting nutrients forT. pseudonana, including 5 times as the most limiting. Altogether, comparison with published results suggests that the roles of iron and silicon in AGP limitation have increased during the past three decades, and could become even more important if eutrophication in the Adriatic Sea continues to increase.     

Volcans de la chaîne des Puys (Massif central,France) : point sur la chronologie Vasset–Kilian–Pariou–Chopine

On the basis of radiocarbon ages obtained on wood burned by base surges and tephrochronological observations, it has been possible to assess that the Puy Chopine, aged ca 9700 yr is older by a few centuries than both the Vasset and Kilian volcanoes. The last two have nearly the same age, ca 9300–9400 yr. Layers of trachytic tephra, most probably originated from Kilian or Vasset, can be observed overlapping those from the Pariou. This last volcano had been previously thought to be the younger of the area on the basis of the observation of trachytic tephra beneath its own tephra. However, new observations allow us to assume that those trachytes originated from the acid phase of the Pariou itself. Therefore, it can be assessed that the four volcanoes erupted according to the following sequence: Chopine/Pariou/?Vasset?/Kilian; the chronological situation of Vasset lies on poor arguments and it should be confirmed. To cite this article: D. Miallier et al., C. R. Geoscience 336 (2004).