230Th [sbnd]232Th systematics in the central Pacific Ocean: The sources and the fates of thorium

Abstract

232Th and 230Th of unfiltered and filtered sea water samples were measured along a depth profile of the central Pacific Ocean using isotopic dilution and thermal ionization mass spectrometry. The 232Th concentration is rather constant from below 25 m down to over 2000 m (45–60 × 10 6 atom cm −3) and it then increases rapidly towards the bottom (119–360 × 10 6 atom cm −3 at 4000 m). The 230Th concentration increases linearly with depth from 3 × 10 3 atom cm −3 at 25 m to 110 × 10 3 atom cm −3 at 4000 m. The reproducibility of replicate analyses and the regularity of the profiles demonstrate the reliability of the procedure. However, some lack of reproducibility among different samples from the same depth has to be explained by natural variability or by contamination during sample collection. Due to the very low 232Th concentrations and relatively high 230Th concentrations, the 230Th/ 232Th ratios can be as high as 1.6 × 10 −3. The ratio of the amount of Th on particles (> 0.2 μm) to the amount of Th in solution is significantly higher for 232Th (≈ 0.37) than for 230Th (≈ 0.18). These data are interpreted in terms of a model that specifically treats particle transport assuming local equilibrium between particles and sea water. The relatively high 230Th concentrations and 230Th/ 232Th ratios measured at 25 m are consistent with a mixed layer of about 100 m. The linear increase in 230Th with depth implies that the effect of eddy diffusion is negligible for most of the water column over the Th removal time scale. This profile is consistent with transport of 230Th by reversible uptake on particles settling with a constant velocity. The 232Th data are inconsistent with this model and they suggest that particles carrying 232Th experience 70% of mass loss between the surface and 4000 m and must follow different transport rules. This difference in the transport of 230Th and 232Th demonstrates the distinctive behavior of these isotopes during particle formation and subsequent remineralization and repackaging in the water column. Different speciation of 230Th and 232Th may be explained by their distinctive sources in the ocean (as 230Th is produced in solution and 232Th is carried by detrital material) and by their respective packaging mechanisms, which are not yet known.