OXYGEN PORE-WATER PROFILES IN CONTINENTAL-SHELF SEDIMENTS OF THE NORTH-SEA - TURBULENT VERSUS MOLECULAR-DIFFUSION

Oxygen pore water profiles in North Sea sediments were measured with m icroelectrodes during 2 contrasting seasons. The measurements were con ducted in a wide variety of sediments, including non-depositional area s on the southern shelf as well as depositional areas in the Skagerrak . All measurements were performed within minutes on board at in situ t emperature. The curvature of oxygen profiles in sandy sediments on the southern shelf indicated the presence of a surface layer characterise d by enhanced diffusion. The occurrence of enhanced diffusion was rela ted to sedimentological and seasonal differences. Quantitative evaluat ion of the pore water profiles by a diffusion-reaction model indicated that the effective diffusion coefficients in a 0.2 to 16 mm subsurfac e layer were 1.5 to >100 times higher than the molecular diffusion coe fficient. Highest effective diffusion coefficients were reported for n on-depositional sediments characterised by low diffusive oxygen fluxes . Oxygen profiles in sediments of the depositional area of the Skagerr ak indicated constant diffusivity throughout the sediment column. Diff usive fluxes calculated from profiles ranged from 5.2 to 8.9 mmol m(-2 ) d(-1) in August 1991, and were between 0.8 and 6.2 mmol m(-2) d(-1) in February 1992. Seasonal differences in sediments located in the Ska gerrak area were minor. It is proposed that near-bottom tidal currents induce enhanced diffusion transport processes in the upper millimetre s of the sandy sediments of the southern North Sea, while less energet ic hydrodynamical conditions in the depositional area of the Skagerrak favour sediment-water exchange based on molecular diffusion only. Bio geochemical implications of the enhanced diffusivity close to the sedi ment-water interface are discussed.