Marie-Francoise Huault

Resuspension and advection processes affecting suspended particulate matter concentrations in the central English Channel

Suspended particulate matter (SPM) measurements obtained along a cross-section in the central English Channel (Wight-Cotentin transect) indicate that the area may be differentiated into: (1) an English coastal zone, associated with the highest concentrations; (2) a French coastal zone, with intermediate concentrations; and (3) the offshore waters of the Channel, characterised by a very low suspended-sediment load. The SPM particle-size distribution was modal close to the English coast (main mode 10-12 mu m); the remainder of the area was characterised by flat SPM distributions. Examination of the diatom communities in the SPM suggest:; that material resuspended in the intertidal zone and the estuarine environments was advected towards the offshore waters of the English Channel.

Continental/marine ratio changes in suspended and settled matter across a macrotidal estuary (the Seine estuary, northwestern France)

Abstract Suspended matter movements in the macrotidal Seine estuary (France) are mainly controlled by tidal currents. During high river spates, suspended matter is carried out of the estuary into the English Channel. A large part of the estuarine suspended matter comes from continental inputs. Throughout the year, suspended matter is trapped in the Turbidity Maximum Zone and on the estuarine tidal flats. Marine particles which are concentrated in the Turbidity Maximum Zone can be transported upstream into fresh water conditions. Physical and chemical measurements, as well as grain size analysis associated with scanning electron microscopy techniques, are used to study the characteristics of silty sediments and suspended matter. In the Turbidity Maximum Zone of the Seine estuary, particulate tracers, such as diatoms, indicate a continental/marine ratio more than 90% in the suspended matter. These results are similar with those from radiotracers. Sedimentary analyses on fine deposits from the estuarine tidal flats show close similarities with suspended matter coming from the Turbidity Maximum Zone. The sedimentological processes of the Seine estuary are compared with those in some other estuaries.

Diatoms as particulate tracers in the water column in the eastern english channel

Abstract In the English Channel, diatoms constitute an important part of the suspended particulate matter (SPM). In this study, the diatom contents (allochthonous and autochthonous) in surface waters are used as biogenic tracers to define the sources of suspended material and the exchange of particles between the different water masses in the eastern English Channel. Such an approach is especially worthwhile in this area where diatoms are very numerous. A factor analysis applied to the counts of dead and living diatoms showed that the following zones could be distinguished in order of importance: a. a central zone in which primary production of marine planktonic diatoms is preponderant (autochthonous diatoms); b. a zone with numerous tychoplanktonic marine diatoms, easily resuspended in shallow waters (allochthonous diatoms); c. a deeper zone with a high ratio of marine to marine-brackish benthic diatoms, which indicate particulate input from the Bay of the Seine and the Seine Estuary (allochthonous diatoms); and d. a coastal zone with numerous tidal-zone diatoms (allochthonous diatoms) which originate from littoral erosion (especially along the French coast). The combined analysis of dead and living diatoms allows a description of the passive transport of particles. A spatial study during an annual survey within the Strait of Dover showed the reproducibility of the hydrodynamical response of the allochthonous diatom content for tracing SPM. Such a study in the eastern English Channel allowed the detection of the flux of estuarine particulate input to the open sea.

Holocene evolution of a wetland in the Lower Seine Valley, Marais Vernier, France

Estuaries like that of the Seine River in NW Europe developed in incised fluvial valleys after the last glacial maximum. Since the 1940s, several authors have studied the largest wetland of the Seine estuary, the Marais Vernier, to understand depositional environments during Holocene infilling. We reinterpret previous research based on new and published data (for example fill thickness and material source) to (1) describe facies and depositional environments; (2) reconstruct palaeoenvironmental evolution; (3) show the influence of local and global forcing on depositional environments. Before 7000—6000 cal. BC, terrestrial material was deposited because of catchment erosion related to changes in climate. Just before 7000—6000 cal. BC, estuarine material began to be deposited in low-lying areas in response to sea-level rise, while terrestrial material still settled at higher elevations. After this, but before 5850—5710 cal. BC, estuarine material areas began to accumulate at both high and low elevations. This marked a general flooding of the Marais Vernier, synchronous with that at the Seine estuary mouth. Soon after, peat accumulated over a wide area as a response to a local change in accommodation and a worldwide drop in sea level. A tidal channel developed to the west of the Marais Vernier, providing minerogenic material. After 1130—900 cal. BC, human influence becomes increasingly clear in the record. This record of regional change during the Holocene can serve as a reference for further studies in the area.