The diffusive boundary layer (DBL) is a thin (< 1 mm) film of warer that covers the sea floor, and through which molecular diffusion is the dominatn 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, dhe 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 acorss the sediment/water interface was about 2.5 times higher than that calculated from a simple one-dimensional diffusion model. These results indicate the benthic oxygen consumption and other fluxes can be studied by direct measurement of DBL microgradients at the undisturbed seabed floor.
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