Emanation of radon-222 from marine sediments

The behavior of radon in the sea-floor region provides a useful test of theories which describe mass transport in sediments. We have made measurements of Rn-222 and Ra-226 in near-bottom waters and near-surface sediments at the same location. The distribution of radon in sediments can be described by a simplified diagenetic equation when advection, adsorption, and bioturbation are ignored. Sediment measurements show a radon deficit relative to radium emanation. A reasonable balance is found between integrated radon deficit in sediment and radon surplus in the overlying water.In most cores radium increased with depth in the top 10 cm of sediment. This implies that bioturbation and other mixing processes do not homogenize the radium concentration in the zone of diffusive radon loss, and that radium is diffusing out of the sediments.Radon leakage is less than that predicted by previous authors. Radon leakage depends upon the physical distribution of radium in marine sediments. We present a model that predicts leakage of 30–40% for normal deep-sea sediments, in agreement with measured values.Radon surplus in near-bottom waters depends upon the radium distribution, radon leakage, and effective diffusion coefficients. These in turn depend on the properties of the sediment, such as composition, accumulation rate, and porosity. As we learn how these factors interact we may be able to infer sedimentary features from measurements of radon in overlying waters.