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

Kim et al. (1999) pose the question: “How accurate are the Th based particulate residence times in the ocean?” Kim et al. present their newly derived calculation of particulate Th residence times and provide some examples of how these rates vary in space and time. They compare these residence times to estimated particulate organic carbon (POC) residence times and find apparent agreement. Using only four samples as evidence, these authors conclude that their calculations of particulate Th residence times are accurate and provide an improved approach for estimating POC residence times and hence, fluxes. The intent of this commentary is to point out the difficulty one has confirming the accuracy of their Th residence time approach when the validity test is a limited comparison between two methods with large uncertainties, namely shallow sediment traps and this new model. Furthermore, we suggest that there is ample evidence to show that the residence time of POC is not the same as the naturally occurring tracer Th. As such, the calculation of Th residence times, even if accurate, would not provide a direct estimate of POC turnover and export in the upper ocean. Alternative techniques using calculated Th fluxes multiplied by the measured ratio of POC/Th do not suffer from these assumptions (as summarized in Buesseler, 1998).