Examination of abyssal sea floor and near-bottom water mixing processes using Ra-226 and Rn-222

Abstract

Since Broecker's (1965) original work, extensive studies have been made on abyssal near-bottom water mixing processes using the radioactive parent-daughter pair radium-226 (Ra) - radon-222 (Rn). One assumption critical to all of these studies is that sediments immediately under a given water column are the source of excess radon (= Rn concentration - Ra concentration) found in bottom waters. Since 1965 theoretical works of increasing complexity have tried to explain areal variations of excess radon and radium. However, Key et al. (1979b) have reported the only extensive measurements of radium and radon in bottom water and sediments at the same location. This dissertation is an expansion of that work both in theory and in scope. A diagenetic sediment model based on the work of Schink and Guinasso (1978), Cochran (1979), and Key et al. (1979b) was developed to model Ra-Rn in near-surface abyssal sediments. In order to maximize model application information, the degrees of freedom were minimized by measuring as many of the model parameters as possible. The most glaring discrepancy found was that measured near-surface total radium profiles could not be fit using plutonium-derived bioturbation rates. There is an implication that plutonium profiles modeled with currently accepted bioturbation models do not give a true indication of the real biologically induced mixing process. After adjusting for this problem in the source function, diagenetic theory explains near-surface radon distributions adequately. Using both the "adjusted" diagenetic model and the empirical model developed by Key et al. (1979b), reasonable agreement was found between the sedimentary radon deficit and near-bottom water surplus. Inadequacy of present diagenetic theory makes any attempt to differentiate sedimentary radium sources academic.