Chimie des eaux interstitielles en équilibre avec les sédiments des marais salants de Salin-de-Giraud (Sud de la France)

Interstitial and phreatic waters were sampled in different pans of the solar salt-works of Salin-de-Giraud (Bouches-du-Rhône). A new technique allows to extract and to filter these waters in the field. In each pan interstitial and phreatic waters are in equilibrum with free waters and concentrations in Cl-, Na+, K+, Mg++ and Ca++ ions are not perturbed by the different sedimentary environ¬ ments : ancient sediments of the Rhône delta or recent sediments due to salt-work activities. Sulphate ion concentrations are the only ones which could be modified by bacterial sulphate reduction. On the contrary, the dissolved silica content of the interstitial and phreatic brines and even of the free waters are affected 1) by ben-thic diatoms biological activity in gypsum undersaturated pans, 2) by the sediments where gypsum is precipitated. The benthic diatoms by creating a cycle of silica constitue a blocking and a stocking of dissolved silica in these gypsum under-saturated pans, preventing any migration towards the pans with supersaturated brines.Des eaux interstitielles et phréatiques ont été prélevées dans divers bassins du marais salant de Salin-de-Giraud (Bouches-du-Rhône). Une technique originale permet d'extraire et de filtrer ces eaux directement sur les lieux d'observation. Dans chaque bassin, les eaux interstitielles et phréatiques tendent à s'équilibrer avec les eaux libres, et les concentrations en ions Cl-, Na+, K+, Mg++ et Ca++ ne sont pas perturbées par les contextes sédimentaires différents : anciens sédiments du delta du Rhône ou sédiments actuels dûs à l'activité du salin. Seules les concentrations en ions sulfate peuvent être modifiées par les réductions sulfato-bactériennes. Par contre, les teneurs en silice dissoute des saumures interstitielles et phréatiques, de même que celles des eaux libres sont conditionnées par l'activité biologique des diatomées benthiques, dans les bassins sous-saturés en gypse, et par les sédiments dans les bassins où le gypse précipite ; les diatomées benthiques, en créant un cycle de la silice, constituent un blocage et un stockage de la silice dissoute dans ces bassins sous-saturés en gypse, empêchant tout passage vers les bassins contenant des saumures sur-saturées.Gouleau Dominique. Chimie des eaux interstitielles en équilibre avec les sédiments des marais salants de Salin-de-Giraud (Sud de la France). In: Géologie Méditerranéenne. Tome 9, numéro 4, 1982. Nature et genèse des faciès confinés. pp. 349-367

Migrations précoces des sels dissous à l'interface air/eau/sédiment dans les vasières émergentes des côtes atlantiques françaises

Gouleau Dominique. Migrations précoces des sels dissous à l'interface air/eau/sédiment dans les vasières émergentes des côtes atlantiques françaises. In: Phénomènes de transport de matière dans l'écorce terrestre. ATP - CNRS. Action thématique Programmée du Centre National de la Recherche Scientifique. Résultats scientifiques. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1979. pp. 49-52. (Sciences Géologiques. Mémoire, 53

Migrations précoces des sels dissous à l'interface air/eau/sédiment dans les vasières émergentes des côtes atlantiques françaises

Gouleau Dominique. Migrations précoces des sels dissous à l'interface air/eau/sédiment dans les vasières émergentes des côtes atlantiques françaises. In: Phénomènes de transport de matière dans l'écorce terrestre. ATP - CNRS. Action thématique Programmée du Centre National de la Recherche Scientifique. Résultats scientifiques. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1979. pp. 49-52. (Sciences Géologiques. Mémoire, 53

Aspects minéralogiques et chimiques des sédiments et des sols des mangroves du Sénégal

Lucas Jacques, Kalck Yves, Gouleau Dominique. Aspects minéralogiques et chimiques des sédiments et des sols des mangroves du Sénégal. In: Phénomènes de transport de matière dans l'écorce terrestre. ATP - CNRS. Action thématique Programmée du Centre National de la Recherche Scientifique. Résultats scientifiques. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1979. pp. 53-56. (Sciences Géologiques. Mémoire, 53

Nutrient fluxes on an intertidal mudflat in Marennes-Oléron Bay, and influence of the emersion period

Fluxes of nutrients (NH4+, NO3-, PO43- and Si(OH)4) were studied on an intertidal mudflat in Marennes-Oléron Bay, France, at two different seasons and at different times of the emersion period. Fluxes through the sediment-water interface were both calculated from vertical profiles of nutrient concentration in pore-water (diffusive fluxes, JD) and measured in light and dark benthic mini-chambers (measured fluxes, J0). Results indicate that ammonia was mainly released in summer while nitrate was mainly taken up in late winter. This uptake from the overlying water was probably due to the coupling of nitrification-denitrification within the sediment. The J0/JD ratio further indicates that bioturbation likely enhanced ammonia release in summer. Concerning phosphate, the comparison of diffusive and measured fluxes suggests that PO43- could be assimilated by the biofilm in winter while it was released in summer at a high rate due to both bioturbation and desorption because of the relative anoxic conditions in summer. Silica was always released by the sediment, but at a higher rate in summer. Statistically significant differences in measured fluxes were detected in dark chambers at different times of low tide, thus suggesting a short-term variability of fluxes. Microphytobenthos preferred ammonia to nitrate, but assimilated nitrate when ammonia was not available. It also turned out that benthic cells could be limited in nitrogen during low tide in late winter. In summer, ammonia was not limiting and microphytobenthic activity significantly decreased the measured flux of NH4+ in the middle of low tide when its photosynthetic capacity was highest

Rapport II.2 Les mécanismes d'envasement de certaines zones littorales

Silting mechanics of certain littoral zones. The paper analyses the mechanics of mud sedimentation specific to three tidal seashore sites : littoral mud-banks and marshes, channels adjacent to estuaries, and ostreicultural areas. They share numerous analogies in the factors that goven their silt-up : -the settling-lag effect (Van Straaten) , difference between the floating silt and the carrying-back in suspension by the ebb : phenomenon that is normal in marshy areas but amplified in estuary-neighbouring channels ; -the biological factors : diatoms or reeds on sea-shore marshes, molluscs for shellfish culture ; -human activities : part played by the sluices and the conchological installations.On analyse les mécanismes de la sédimentation des vases propres à trois milieux littoraux particuliers situés dans les mers à marée : les vasières littorales et les roselières, les étiers adjacents aux estuaires, et les zones ostréicoles. Elles présentent de nombreuses analogies dans les facteurs qui régissent leur envasement : -le settling lag effect (Van Straaten), différence entre les apports en flot et la remise en suspension par le jusant, phénomène normal sur les vasières, amplifiés dans les étiers, -les facteurs biologiques : diatomées ou roseaux sur les vasières ou étiers, mollusques pour la conchyliculture, -activités humaines : rôle des écluses et des installations conchylicoles.Diara Mariline, Sornin Jean-Marc, Ottmann François, Nikodic Jean, Gouleau Dominique. Rapport II.2 Les mécanismes d'envasement de certaines zones littorales. In: L'hydraulique et la maitrise du littoral. Problèmes côtiers posés par le mouvement des sédiments et la pollution. Dix-huitièmes journées de l'hydraulique. Marseille, 11-13 septembre 1984. Tome 2, 1984

Short- and long-term sedimentation on Montportail–Brouage intertidal mudflat, Marennes–Oléron Bay (France)

International audienceThe intertidal mudflat of Montportail-Brouage is 4 km wide and extends over about 30% of intertidal area of the Bay of Marennes-Oléron. The sampling area was a cross-shore transect (east–west) in the mid-part of the mudflat, so as to overlap the different geomorphological features, from the shore to the central channel. Particularly distinctive were zones of shore-oblique ridges and runnels, and small channels. The short-term erosion-sedimentation processes related to these bedforms, and their effects on the long-term stability of these structures are poorly known. Thus, sediment budget is difficult to assess in the long term, due to temporal variations. Monthly sedimentological surveys were performed from March 1997 to May 1998. Six stations were sampled by coring for determination of water content, dry density, grain size and carbonate content of the sandy fraction, measured on the topmost 5 cm. Within the ridge and runnel zone, this sampling was done on both structures along with the determination of depth profiles of macrofauna species. Radiographic profiles to a depth of 50 cm were made at each of the 6 stations for description of the sedimentary facies, and radioisotope profiles () were made at four stations. Wet bulk density appeared to be roughly constant (1.39–1.44 kg m−3) over the whole mudflat below 2 or 3 cm. For the levels 0–1 and 1–2 cm, the bulk density showed much more variation, in particular in the runnels. These variations were due to episodic deposits of fluid mud (wet bulk density <1.25 kg m−3). The sand content decreased from the lower part of the mudflat to the upper part, particularly in the fluid mud. On the contrary, the carbonate content of the sand fraction increased in the upper part by accumulation of foraminifera associated with shell lag deposits. 7 Be measurements revealed a short-term sedimentation of fluid mud (up to 21.04 cm yr−1), particularly in runnels from the upper mudflat, thus indicating a temporary deposit, easily resuspended by wave action. 210 Pbexc measurements showed a long-term sedimentation rate which was 4 times higher in the lower part (0.97 cm yr−1) of the mudflat (near oyster reefs and mussel “bouchots”) than in the upper part (0.26 cm yr−1). In the middle mudflat, ridges and runnels exhibited a large difference of the long-term sedimentation rates, 0.70 cm yr−1 on ridges and only 0.07 cm yr−1 in runnels, that could be considered as a passageway for fluid mud transport. In the middle mudflat, where ridges and runnels occurred, synopsis of observations indicated that a 30 yr old and 18 cm thick ridge layed on a more than 100 yr old basement mudflat. This bedform is maintained through short- and long-term sedimentations. In the runnels, always flooded by water even at low tide, the short-term sedimentation was 21.04 cm yr−1, but these sediments can be easily resuspended by tidal currents, inducing a weak long-term sedimentation (0.07 cm yr−1). In the ridges, the short-term sedimentation was 7.27 cm yr−1; however, due to potential impacts of drying processes, along biostabilization by epipelic diatoms during subaerial exposure at low tide, the short-term sedimentation contributed to a long-term sedimentation of 0.70 cm yr−1

Nutrients (organic C, P, N, Si) in the eutrophic River Loire (France) and its estuary

The Loire estuary has been surveyed from 1982 to 1985 by 13 isochronous longitudinal profiles realized at low tide. Nutrient (SiO2, NO3-, NH4+, PO3-4, particulate organic carbon or POC) patterns are very variable depending on the season, the estuarine section [river, upper-inner estuary, upstream of the fresh-water-saline-water interphase FSI, the lower-inner estuary characterized by the high turbidity zone (HTZ), the outer estuary] and the river discharge. Biological processes are dominant. In the eutrophied River Loire (summer pigment > 100 μg l-1), the high algal productivity (algal POC > 3 mg l-1) results in severe depletion of SiO2, PO43-, NO3-. The enormous biomass (55 000 ton algal POC/year) is degraded in the HTZ where bacterial activity is intense. As a result, there is generally a regeneration of dissolved SiO2 and PO43-, a marked NH4+ maximum, while NO3- is conservative or depleted when the HTZ is nearly anoxic. Other processes can be considered including pollution from fertilizer plans (PO43-, NH4+) and from a hydrothermal power plant (NH4+). In the less turbid outer estuary, nutrients are generally conservative. Major variations of concentrations are observed in the lowest chlorinity section (Cl- < 1 g kg-) and also upstream the FSI, defined here as a 100% increase in Cl-. Nutrient inputs to the ocean are not significantly modified for SiO2 and NO2-, but are increased by 70% and 180% for PO43- and NH4+ and depleted by 60% for POC. Odd hydrological events, especially some floods, may perturbate or even mask the usual seasonal pattern observed in profiles. © 1988.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

The influence of long emersion on biota, ammonium fluxes and nitrification in intertidal sediments of Marennes-Oléron Bay, France

International audienceA comparative study between waterlogged and reflooded intertidal sediments was undertaken in March and June 1999 through statistical analysis of selected sediment parameters (biota, salinity, O2, Eh), pool sizes and benthic fluxes of nutrients (NH4+, NO2−, NO3−) and nitrification rates. In March samples, absence of polychaetes and oligochaetes from upper sediment horizons were due to erosional events sweeping away surface sediments. Presence of richer annelid assemblages in June samples indicated more stable hydrodynamic conditions that favoured the development of benthic microalgae biofilms. Dewatering of sediments during a 3-day emersion period promoted a salinity rise on top layers, migration of pore water ions towards the sediment surface, and created sediment fissures that accelerated water exchange on reflooding. Reflooded and waterlogged sediment systems were comparable with respect to the release of NH4+ to overlying water but were different with respect to nitrification rates. Sediment-water NH4+ fluxes were higher (P=0.011) in March (3.3 mmol m−2 day−1) compared to June (1.4 mmol m−2 day−1) due to higher macrofauna biomasses and lower benthic microalgae concentrations in March samples. Potential nitrification rates (range from 19 to 60 mmol NO3− m−2 day−1) were not statistically different between March and June. A thinner oxic layer in reflooded compared with waterlogged systems reflects a decrease of O2 diffusion into sediment at high salinities which resulted in the fall of the actual nitrification rates (P<0.05). Our data suggest that long term dessication of intertidal sediments may depress the nitrification process at the ecosystem level