234Th:238U disequilibria within the California Current1

Abstract

Profiles of dissolved and particulate 234Th were determined at several stations within the California Current. Modeling of the disequilibria between the 234Th and 238U within the surface waters provides for estimates of the residence time of dissolved thorium with respect to particle scavenging (τd varies from 6 to 50 days), the particle residence time (τp varies from 2 to 20 days), and the particulate 234Th flux exiting the surface layer. The model‐derived, first‐order scavenging rate constant for dissolved thorium is observed to be proportional to the rate of primary production. Particle residence times seem to be governed by the rate of zooplankton grazing and the types of zooplankton present. Model‐derived particulate 234Th fluxes are in good agreement with direct measurements by sediment traps.