Deep-Sea Research II, Vol. 44, No. 9-10, pp 2049-2083, 1997 0 1998 Else&r Science Ltd. All rights resewed Pergamon PII: SO967-0645(97)00063-5 Printed in Great Britam 0967-0645/98 $19.00+0.00 234Th and particle cycling in the central equatorial Pacific JOHN P. DUNNE,* JAMES W. MURRAY,* JENNIFER YOUNG,* LAURIE S. BALISTRIERI* and JAMES BISHOP? Abstract-US JGOFS-EqPac 234Th data sets for 1992 boreal spring (Survey I, TT007) and fall (Survey II, zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA TTOl 1) cruises from l2”N to 12”s along 14O”W were used to determine rates of 23?h 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 re- evaluation of the ‘particle concentration effect’ 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-23?h and phytoplankton models were extremely dependent on the mode1 treatment of remineralization. Results from the coupled particle-23‘?‘h model, where particles have a constant lability, suggested that 23?h recycled three to four times between the dissolved and particulate 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, zyxwvutsrqponmlkji 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. 0 1998 Elsevier Science Ltd. All rights reserved INTRODUCTION Importance of studying particle cycling dynamics Integrated physical, chemical and biological processes control particle cycling in the upper ocean. The cycle of input of nutrients, production, sinking and remineralization of particles controls oceanic distributions of organic carbon, major nutrients and a suite of trace elements. The equatorial oceans are important places to study these processes because of their large geographical areas and the intensity of particle cycling. 234Th often has been used as an effective tracer of particle cycling to estimate residence times of trace metals and particulate carbon and to calculate the sinking flux of particulate organic carbon (Coale and Bruland, 1985; Murray et al., 1989; Buesseler et al., 1992; Murray et al., 1996). The most basic particle-234 Th cycling model is a steady-state mass balance in which the vertical flux of 234Th equals the difference between the rates of in-situ production of 23‘?h (t l/,=24.1 days) from its long-lived, conservative parent 238U (tlj2 =4.47 x IO9 years) and in-situ radioactive decay of 234Th. Conversion of the *School of Oceanography, Box 357940, University of Washington, Seattle, WA, 981957940, USA. 7 School of Earth and Ocean Sciences, University of Victoria, PO Box 1700 MS 4015, Victoria, BC, Canada. 2049