A generalized model for the scavenging of trace metals in the open ocean—II. Thorium scavenging

Abstract

A one-dimensional particle cycling model consisting of two size classes, coupled with passive adsorption/desorption of thorium (Th), has been used to simulate dissolved and particulate profiles of 234Th, 228Th, 230Th and 232Th for a generalized “open ocean” case. The results suggest that remineralization is important in sustaining observed upper water column profiles, and that the scavenged flux is correlated to new production. Simultaneous observations of particulate and dissolved profiles of at least two isotopes (e.g. 234Th and 230Th) are needed to accurately determine adsorption and desorption rate constants. Similarly, measurements of both large and small particulate Th fractions should enable aggregation and disaggregation rate constants to be firmly established. An application of the model to particle cycle and 234Th data from the VERTEX 4 site shows that these two kinds of observations can be reconciled successfully within the simple theoretical framework. It also suggests that Th uptake (per unit mass of particles) is enhanced in the euphotic zone relative to deeper waters, possibly due to increased particle surface reactivity.