Processes controlling radon-222 and radium-226 on the southeastern Bering Sea shelf

Abstract

An investigation was made into the use of radon-222 and radium-226 as tracers of air-sea gas exchange, water column mixing and sediment-water exchange on the southeastern Bering Sea shelf. Further more, a two-dimensional model was developed to unity these three processes into a coherent picture of radon-222 flux out of the sediments, through the water column and into the atmosphere. The best time period to average wind speeds when regressing them against gas transfer coefficients was found to be 3.3 days by a linear regression optimization, approximately the synoptic time scale of storms in the southeastern Bering Sea. A statistically significant relationship between averaged wind speed and transfer coefficients was found at the 80% confidence level. Gas transfer coefficients were found to tie obscured in shallow waters by radon flux from the sediments. Two-dimensional mixing in these continental shelf waters rendered the traditional one-dimensional vertical mixing model of excess radon-222 unable to obtain reliable vertical eddy ditfusivities. Exchange across the sediment-water interface was calculated from the deficiency of radon-222 measured in sediment cores, the standing crop of excess radon-222 in the overlying water column and the radon-222 production rate of sediment surface grab samples. The flux of radon-222 out of the sediments was found to increase in the onshore direction. Biological irrigation appears to be the primary exchange mechanism between the sediment and water column s on this shelf. Distributions in the water column show fine structure reported previously as well as biological removal of radium-226. A chi-square hypersurface search found the optimal horizontal and vertical eddy diffusivities that explained the two-dimensional distribution of radon-222 provided from a kriging estimation exercise on the data measured in this study. This model was essentially a hybrid of a least squares surface fit and a numerical integration of the governing differential equation of radon-222. When considered as a two-dimensional system in the cross-shelf direction, the rates of gas exchange, water column mixing and sediment-water exchange agree with each other to an acceptable degree.