The scavenging of magnesium and other chemical species by biogenic opal in deep-sea sediments

Abstract

A study of siliceous oozes from the Caribbean Sea and Pacific Ocean shows that considerable quantities of Mg can be adsorbed on to biogenic opal. The process is diffusion limited and requires a hiatus in sedimentation in order that a substantial amount of Mg be absorbed. The adsorption is sensitive to pH, and is virtually inoperative in the absence of carbonate. The Mg apparently comes from seawater, and some well-known pore-water Mg gradients evidently result from this adsorption process. Na is also adsorbed, but the process is not diffusion limited. Fe and Mn may be similarly adsorbed, but, in one case studied, possibly are supplied from the basaltic basement by hydrothermal processes. K apparently does not participate in surface adsorption to any considerable extent. The later conversion of opal-A (amorphous opal) to opal-CT or to quartz evidently creates zones of local high Na and Mg activity which, in turn, is probably responsible for the sepiolite, dolomite, clinoptilolite, and, possibly, much smectite at the level of chert formation in these sediments.