234Th and particle cycling in the central equatorial Pacific

US JGOFS-EgPac234Th data sets for 1992 boreal spring (Survey I, TT007) and fall (Survey 11, TT011) cruises from 12°N to 12°S along 140°W were used to determine rates of234Th and particle cycling using a thorium sorption model and three coupled particle-thorium models. Sampling methodology had a large impact on model results — estimates of particulate organic carbon varied by a factor of 3 between bottle and in-situ filtration techniques. Adsorption rate constants and residence times from the thorium sorption model showed strong depth, latitudinal and seasonal variability which we were able to attribute to changes in particle concentration. A reevaluation of the `particle concentration effectʹ on the adsorption rate constant, k′, showed that our values of k′ increased with particle concentration and were consistent with other study sites with similar particle concentrations. Recycling of particulate organic carbon in the euphotic zone of the central equatorial Pacific was 2–10 times faster than sites previously studied. Calculations of adsorption rate constants from the thorium sorption, coupled particle-234Th and phytoplankton models were extremely dependent on the model treatment of remineralization. Results from the coupled particle-234Th model, where particles have a constant ]ability, suggested that234Th recycled three to four times between the dissolved and paticulate phases before being removed from the euphotic zone. Aggregation rate constants and sinking rates in the central equatorial system were compared with other sites using the size-fractionated model developed by Clegg and Whitfield (1991,Deep-Sea Research,38, 91–120). Removal of particles by sinking from the equatorial euphotic zone depended on a mechanism of differential recycling of organic matter in the euphotic zone in which only a fraction of the particles are remineralized and the more refractory particles sink.