TH-230-TH-232 SYSTEMATICS IN THE CENTRAL PACIFIC-OCEAN - THE SOURCES AND THE FATES OF THORIUM

Th-232 and Th-230 of unfiltered and filtered sea water samples were me asured along a depth profile of the central Pacific Ocean using isotop ic dilution and thermal ionization mass spectrometry. The Th-232 conce ntration is rather constant from below 25 m down to over 2000 m (45-60 x 10(6) atom cm(-3)) and it then increases rapidly towards the bottom (119-360 X 10(6) atom cm(-3) at 4000 m). The Th-230 concentration inc reases linearly with depth from 3 X 10(3) atom cm(-3) at 25 m to 110 X 10(3) atom cm(-3) at 4000 m. The reproducibility of replicate analyse s and the regularity of the profiles demonstrate the reliability of th e procedure, However, some lack of reproducibility among different sam ples from the same depth has to be explained by natural variability or by contamination during sample collection. Due to the very low Th-232 concentrations and relatively high Th-230 concentrations, the Th-230/ Th-232 ratios can be as high as 1.6 X 10(-3). The ratio of the amount of Th on particles (> 0.2 mu m) to the amount of Th in solution is sig nificantly higher for Th-232 (approximate to 0.37) than for Th-230 (ap proximate to 0.18). These data are interpreted in terms of a model tha t specifically treats particle transport assuming local equilibrium be tween particles and sea water. The relatively high Th-230 concentratio ns and Th-230/Th-232 ratios measured at 25 m are consistent with a mix ed layer of about 100 m. The linear increase in Th-230 with depth impl ies that the effect of eddy diffusion is negligible for most of the wa ter column over the Th removal time scale. This profile is consistent with transport of Th-230 by reversible uptake on particles settling wi th a constant velocity. The Th-232 data are inconsistent with this mod el and they suggest that particles carrying Th-232 experience 70% of m ass loss between the surface and 4000 m and must follow different tran sport rules. This difference in the transport of Th-230 and Th-232 dem onstrates the distinctive behavior of these isotopes during particle f ormation and subsequent remineralization and repackaging in the water column. Different speciation of Th-230 and Th-232 may be explained by their distinctive sources in the ocean (as Th-230 is produced in solut ion and Th-232 is carried by detrital material) and by their respectiv e packaging mechanisms, which are not yet known.