TH-234 AND PARTICLE CYCLING IN THE CENTRAL EQUATORIAL PACIFIC

US JGOFS-EqPac Th-234 data sets for 1992 boreal spring (Survey I, TT00 7) and fall (Survey II, TT011) cruises from 12 degrees N to 12 degrees S along 140 degrees W were used to determine rates of Th-234 and part icle cycling using a thorium sorption model and three coupled particle -thorium models. Sampling methodology had a large impact on model resu lts - estimates of particulate organic carbon varied by a factor of 3 between bottle and in-situ filtration techniques. Adsorption rate cons tants and residence times from the thorium sorption model showed stron g depth, latitudinal and seasonal variability which we were able to at tribute to changes in particle concentration. A reevaluation of the 'p article concentration effect' on the adsorption rate constant, k(1)', showed that our values of k(1)' increased with particle concentration and were consistent with other study sites with similar particle conce ntrations. Recycling of particulate organic carbon in the euphotic zon e of the central equatorial Pacific was 2-10 times faster than sites p reviously studied. Calculations of adsorption rate constants from the thorium sorption, coupled particle-Th-234 and phytoplankton models wer e extremely dependent on the model treatment of remineralization. Resu lts from the coupled particle-Th-234 model, where particles have a con stant lability, suggested that Th-234 recycled three to four times bet ween the dissolved and particulate phases before being removed from th e euphotic zone. Aggregation rate constants and sinking rates in the c entral equatorial system were compared with other sites using the size -fractionated model developed by Clegg and Whitfield (1991, Deep-Sea R esearch, 38, 91-120). Removal of particles by sinking from the equator ial euphotic zone depended on a mechanism of differential recycling of organic matter in the euphotic zone in which only a fraction of the p articles are remineralized and the more refractory particles sink. (C) 1998 Elsevier Science Ltd. All rights reserved.