THORIUM ISOTOPES AS INDICATORS OF PARTICLE DYNAMICS IN THE UPPER OCEAN - RESULTS FROM THE JGOFS NORTH-ATLANTIC BLOOM EXPERIMENT

Measurements of Th-234 and Th-228 in suspended and sinking particles m ade during the 1989 JGOFS North Atlantic Bloom Experiment permit estim ation of the rates of particle cycling. Using a simple model of thoriu m-particle interactions applied to water column and floating sediment trap data at 150 and 300 m, the rate constant, beta2, for aggregation of small suspended particles into large rapidly sinking (approximately 150 m day-1) particles increases from approximately 0 to approximatel y 30 y-1 over the course of the bloom. The rate constant for disaggreg ation of sinking particles, beta-2, similarly increases from approxima tely 100 to approximately 300 y-1 over the same period. These values s uggest that small particle residence times (relative to packaging or a ggregation) decrease to approximately 15 days and that large particle residence times (relative to disaggregation) decrease to approximately 1 day as the bloom progresses. Late in the bloom, particles are cycle d such that aggregation of suspended particles (approximately 2 mug l- 1 day-1) is comparable to particle break-up (approximately 3 mug l-1 d ay-1). Errors on the rate constants, calculated by propagating estimat ed errors on the individual terms in the model, are large and arise pr incipally from uncertainty in the gradient in activity and mass fluxes between the two trap depths. However, the values calculated independe ntly from the two tracers (Th-234 and Th-238) generally agree to withi n 30%. The Th-234 balance for the upper water column (BUESSELER et al. , Deep-Sea Research, 39, 1115-1137, 1992) suggests that a substantial portion of the thorium and mass flux is not recorded by the traps. If it is assumed that this flux is carried on more slowly sinking particl es (approximately 50 m day-1) that are not trapped efficiently, and th ese particles directly interact with the suspended particles pool in t he same fashion as the trapped sinking particles, calculation of aggre gation and disaggregation rate constants late in the bloom shows a hig her value for beta2 but a comparable value for beta-2 relative to the values determined for the trapped particles. This suggests that the sl owly sinking material (e. g. marine snow) is more effective at aggrega ting small, suspended particles than are the rapidly sinking particles . Temporal increases in beta2 and beta-2 for the trapped particles are matched by increases in the rate constants for decomposition of parti culate organic carbon and nitrogen (2-35 y-1 for C; 4-40 y-1 for N), s uggesting that increases in microbial activity are directly reflected in rates of particle aggregation and disaggregation.