Diffusion coefficients calculated using Cs-137 profiles applied to Pb-210 dating in lake core sediments

Several processes change the radionuclide profiles in sediment cores. Two important processes are sediment mixing by worms and microbes, and molecular diffusion in the pore waters. Simple diffusion is used to estimate these influences upon the Lead-210 distribution using an analytic solution to the Constant Flux, Constant Sedimentation model. Diffusion coefficients are calculated by fitting Gaussian curves to Cesium-137 distributions before diffusion (bases on fallout measured in precipitation during nuclear testing) and after diffusion (distributions measured in sediments cores). These diffusion coefficients change the ages calculated by the Constant Flux, Constant Sedimentation model by as much as 50%. Because diffusion is occurring in all sediment to some degree, these corrections will always cause the Constant Flux, Constant Sedimentation model to incorrectly calculate the ages of sediment as younger than they actually are. Computer programs were written to simulate diffusion over a range of different diffusion constants and sedimentation rates. These simulations were compared to the theoretical method of fitting simple Gaussian curves to see how accurate this simplification is. Over a large range of diffusion constants and sedimentation rates the computer simulations suggest that the Gaussian approximations made were accurate. The average correction to the Constant Flux, Constant Sedimentation model was 31% using data from 21 lakes dated by the University of Maine Departments of Physics.