Microstructure of diffusive boundary layers and the oxygen uptake of the sea floor

THE diffusive boundary layer (DBL) is a thin (≲1 mm) film of water that covers the sea floor, and through which molecular diffusion is the dominant transport mechanism for dissolved material. The diffusive fluxes are a measure of the rate of remineralization of organic matter in the sea bed, and of the dissolution or precipitation of minerals such as carbonates or metal oxides. Here we report detailed in situ analyses of chemical microgradients within the DBL, using a microelectrode profiling instrument with a spatial resolution of 25–50 µm. Over a Danish coastal sediment at 15 m water depth, the DBL was 0.5-0.7 mm thick and showed both stochastic fluctuations of oxygen distribution owing to boundary-layer turbulence and harmonic oscillations resulting from surface waves. A three-dimensional mapping of the DBL and the corresponding sediment surface showed that the DBL was spatially well defined and followed surface contours, but smoothed out sediment microtopographic features smaller than 100 µm. The three-dimensional oxygen diffusive flux across the sediment/water interface was about 2.5 times higher than that calculated from a simple one-dimensional diffusion model. These results indicate that benthic oxygen consumption and other fluxes can be studied by direct measurement of DBL microgradients at the undisturbed sea floor.