Oceanic trace metal scavenging: the importance of particle concentration

Abstract

Adsorption (surface complexation) has long been considered to be the dominant process involved in the oceanic scavenging of many trace metals. Much of what we know about metal removal in the ocean (i.e. rate and extent) is based on measurements of U and Th decay series isotopes. However, the scavenging equations developed from radioactive parent-daughter relationships presume no specific metal removal process and cannot be directly used to verify a particular one.In this paper we examine and compare the phenomenological model of adsorption and oceanic scavenging observations. The formalisms of surface coordination and colloid chemistry are linked to the mechanism-free observations of oceanic trace metal scavenging by the strong similarities in the description of the reaction rates and the influence of particle concentration on those rates and the equilibrium distributions. The correspondence between laboratory sorption data and field scavenging observations as well as the consistency of the hypothesis over a wide range of environmental systems successfully link oceanic trace metal scavenging with surface coordination and colloid aggregation reactions. The merging of descriptions of surface and colloid chemistry and field observations of scavenging provide a framework for interpreting field data and understanding how master variables (e.g. reaction rate, particle concentration, or particle flux) influence metal removal from the oceans.