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 microelectrodes during 2 contrasting seasons. The measurements were conducted in a wide variety of sediments, including non-depositional areas on the southern shelf as well as depositional areas in the Skagerrak. All measurements were performed within minutes on board at in situ temperature. The curvature of oxygen profiles in sandy sediments on the southern shelf indicated the presence of a surface layer characterised by enhanced diffusion. The occurrence of enhanced diffusion was related to sedimentological and seasonal differences. Quantitative evaluation of the pore water profiles by a diffusion-reaction model indicated that the effective diffusion coefficients in a 0.2 to 16 mm subsurface layer were 1.5 to >100 times higher than the molecular diffusion coefficient. Highest effective diffusion coefficients were reported for non-depositional sediments characterised by low diffusive oxygen fluxes. Oxygen profiles in sediments of the depositional area of the Skagerrak indicated constant diffusivity throughout the sediment column. Diffusive 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 Skagerrak area were minor. It is proposed that near-bottom tidal currents induce enhanced diffusion transport processes in the upper millimetres of the sandy sediments of the southern North Sea, while less energetic hydrodynamical conditions in the depositional area of the Skagerrak favour sediment-water exchange based on molecular diffusion only. Biogeochemical implications of the enhanced diffusivity close to the sediment-water interface are discussed.