Thorium Isotopes as Indicators of Export Flux and Particle Dynamics in the Southern Ocean: Joint Global Ocean Flux Study

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961-2761 Cochran This research project is part of the US Joint Global Ocean Flux Study (JGOFS) Southern Ocean Program aimed at (1) a better understanding of the fluxes of carbon, both organic and inorganic, in the Southern Ocean, (2) identifying the physical, ecological and biogeochemical factors and processes which regulate the magnitude and variability of these fluxes, and (3) placing these fluxes into the context of the contemporary global carbon cycle. This work is one of forty-four projects that are collaborating in the Southern Ocean Experiment, a three-year effort south of the Antarctic Polar Frontal Zone to track the flow of carbon through its organic and inorganic pathways from the air-ocean interface through the entire water column into the bottom sediment. The experiment will make use of the RVIB Nathaniel B. Palmer and the R/V Thompson on six process cruises between Oct.-Nov., 1996, and March 1998. This component focuses on the spatial and temporal variations in the distributions and particulate fluxes of thorium isotopes (thorium-234 and thorium-228). These isotopes serve as ocean tracers that yield fundamental information about the rates of biogeochemical processes that govern the production and fate of biogenic particles in the oceanic water column. Measurements of thorium-234 will be made at sea by beta and gamma-counting; measurements of thorium-228 will be made ashore by alpha- spectrometry. The data will be interpreted within the context of biological rate measurements and other data to be collected during the study and will be used to provide estimates of export fluxes of organic matter, carbonate, and silica from the euphotic zone; to quantify transformation processes such as aggregation and disaggregation in the water column; and to test biogeochemical models. Thorium is a particularly useful tracer in this context because its mode of production is well known; it is taken up quickly by biogenic particles, and it s concentration can be measured quite precisely. The removal of thorium by falling particles can therefore be determined, and the inverse problem of inferring the integrated mass of falling particles (the particulate carbon flux) may be solved. *** .