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Accueil du site > Séminaires > Séminaires 2018 > Séminaire MSC. 19 février 2018. Ian R. Jenkinson (Agence de Conseil et de Recherche Océanographiques & Academy of Sciences Institute of Oceanology ) : "Rheological properties of natural waters.".

Séminaire MSC. 19 février 2018. Ian R. Jenkinson (Agence de Conseil et de Recherche Océanographiques & Academy of Sciences Institute of Oceanology ) : "Rheological properties of natural waters."

Sauf mention contraire, les séminaires et les soutenances se déroulent à 11h30 en salle 454A du bâtiment Condorcet.

Rheological properties of natural waters : genomic control of climate using organic matter.

Ian R. Jenkinson Agence de Conseil et de Recherche Océanographiques, 19320 La Roche Canillac, France & Chinese Academy of Sciences Institute of Oceanology, State Key Laboratory of Marine Ecology and Environmental Sciences, 266071 Qingdao, P.R. China.,

Abstract :

Bulk-phase 3D rheology. Organic matter (OM) in natural waters is produced mostly by phytoplankton. Rheology of phytoplankton cultures made in oscillating Couette geometry have shown mostly shear-thinning viscosity, with viscosity η up to 400 times that of culture medium at a shear rate of 0.002 s-1 , as well as measurable elasticity G’. Bulk-phase seawater also generally shows a shear-thinning excess viscosity ηE due to OM, plus the Newtonian viscosity ηW due to water and salts. ηE is generally positively correlated to phytoplankton abundance. Measurements of viscosity in phytoplankton cultures at low Reynolds number using capillaries surprisingly gave some negative values of ηE. This has been provisionally ascribed to superhydrophobic drag reduction acting on sculptured hydrophobic surfaces (glycocalyxes) of the cells or the organic aggregates, or both.

Surfaces of natural waters. The water-atmosphere surface is the selective “gate” through which all fluxes pass between the ocean and atmosphere. The OM in the microlayer is in exchange with that in underlying water, thus also under control by the plankton ecosystem. This surface comprises a 60-µm layer thick microlayer distinct from underlying water in properties including pH and OM content. Much of this OM is in the form of a more-or-less broken up gel. The microlayer is overlain by an extremely hydrophobic nanolayer of thickness 5 nm.

Foams and whitecaps. Breaking waves force air bubbles into the water. When the bubbles rise to the surface they adsorb OM. In some case the bubbles burst releasing aerosol particles into the atmosphere, where winds can carry them to high altitudes where they change cloud formation dynamics. In other cases, interaction of the bubbles with OM produces foam on the sea surface. This foam increases the reflectivity of the sea surface, thus reducing heating. Some of it is also carried into the atmosphere by winds, producing further aerosols of a different nature.

Conclusions. Genes in aquatic organisms produce OM. This OM controls many ocean-atmosphere fluxes including those of O2, CO2 and other greenhouse gases. Shifts in the taxonomic composition of marine ecosystems may thus abruptly change ocean rheology, and produce unexpected feedbacks with global climate change.

Contact : Équipe séminaires / Seminar team - Published on / Publié le 6 février