How accurate are the 234Th based particulate residence times in the ocean?

The residence times of colloidal and particulate matter have been estimated often by using naturally occurring Th-234-U-238 disequilibria in the surface ocean. However, we show that the conventional Th-234 multiple-box models underestimate the actual residence time. Using a one-box Th-234 model approach, we calculate about a factor of two to four longer residence times for both colloidal and particulate thorium in the open ocean. This approach also yields very similar residence times between particulate thorium (box model based) and particulate organic carbon (sediment trap based) even with the same data sets previously used for showing a large discrepancy. Our results suggest significant corrections in the application of Th-234 based residence times of the colloids and particulate matter to gauge the removal rates of reactive trace elements and organic carbon in the surface ocean.