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Laboratoire d'Ecogéochimie des Environnements Benthiques
UMR 8222

Mathieu Chatelain (Thèse soutenue le 5 juillet 2010)

Mathieu Chatelain

Dissolved fluxes at the sediment-water intreface under oscillating flows

 

Director : Jean-Marc Guarini, Co-directrice : Katell Guizien

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Early diagenesis, a key step of the organic matter recycling in marine ecosystem leading to an accumulation of pore water nutrients, results from dissolved species transfers across the sediment-water interface. This biochemical process is strongly constrained by the oxygen availability and by the physical environment around the sediment-water interface, namely sediment porosity and permeability and hydrodynamic forcing. When the latter is unsteady, a strong coupling may exist with the biochemical dynamics. In non-permeable sediments, exchanges across the interface occur through diffusion. A diffusion-reaction model, describing the dynamics of oxygen diffusive transfers under oscillating flows (swell to tide), shows a stimulation of sediment oxygenation under energetic oscillating flows. For high-frequency flows (swells), the competition between the diffusion time across the diffusive boundary layer and the time granted to this diffusion during a wave cycle regulates the highly non-linear dynamics of oxygen diffusive fluxes at the sediment-water interface. For low-frequency flows (tides), unsteady properties of the flow are transmitted to the pore water and a quasi-steady approach allows a fairly approximation of the oxygen profiles dynamics. In permeable sediments, advective processes gain in importance and play a predominant role in the exchanges across the interface. Thus, the nutrients pools can be made available for primary producers in the water column during strong wave events (subtidal pumping). The quantification of the importance of this pumping for the pore water trapped nutrients release to the water column is crucial to estimate the marine recycling potential. A weekly monitoring of nutrients concentrations in the water column and pore water has beencarried out in two stations within the Bay of Banyuls. Superficial sediment impoverishment and water column enrishment suggest a nutrients release but also a stimulation of the degradation activity after strong wave events. Furthermore, a spatial exploration of the sediment permeability, its grain size distribution and the nutrients pools has been performed in the Bay of Banyuls and widely in the Gulf of Lions. A budget of the nutrients release under wave pumping in shallow coastal waters underlines a non-negligible contribution of this recycling process in the nutrients loads available for primary producers towards the net continental loads, for instance by the Rhône River.

 

Keywords: diagenesis - waves - dissolved oxygen - diffusive boundary layer - subtidal pump - nutrients budget

Mathieu Chatelain (mathieu.chatelain @ obs-banyuls.fr) - 16/03/17

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