Surface Water 234Th/238U Disequilibria: Spatial and Temporal Variations of Scavenging Rates within the Pacific Ocean
Dissolved and particulate 234Th were determined on several surface transects within the Pacific Ocean. These transects, spanning major oceanographic regimes, ranged from oligotrophic subtropical gyres to eutrophic eastern boundary and equatorial divergence zones. Modeling of the disequilibria between 234Th and 238U within the surface waters provides estimates for the residence time of dissolved thorium with respect to particle scavenging, the residence time of particulate 234Th, and the particulate 234Th flux from the surface layer. The results provide a broad spatial view of the intensity and temporal variability of these scavenging processes. Within the oceanic surface waters dissolved 234Th residence times vary from 6 to 220 days and the scavenging rate of dissolved 234Th onto particles appears to be proportional to primary production. The scavenging intensity of dissolved 234Th is strongly correlated with particulate organic carbon flux throughout a wide range of marine environments ranging from estuaries to the deep sea. Including these environments, which represent end-members for 234Th scavenging intensity, the residence time for dissolved 234Th is shown to vary from 0.3 days to 2 years depending on the particle flux. Particulate 234Th residence times in oceanic surface waters are of the order of weeks and appear to be governed by the rate of zooplankton grazing. Model-derived particulate 234Th fluxes can be used to constrain estimates based upon surface sediment trap collections.
KeywordsParticulate Organic Carbon California Current Scavenge Rate Particulate Organic Carbon Flux Oceanic Surface Water
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