An Optimal Approach for Assessment of Dust Fluxes to the Ocean Using 232Th and 230Th

Quantifying fluxes of mineral dust to the ocean surface is important for understanding ocean biogeochemical cycles and climate. Geochemical methods can provide dust flux estimates, including through the application of 232Th/230Th. This use of thorium isotopes has been hindered, however, by the ubiquitous presence of strong vertical increases in calculated 232Th (and therefore dust) fluxes with water depth. Such increases cannot realistically reflect changing dust fluxes with depth, so demonstrate a limitation in the thorium isotope approach. To investigate the origins of apparent increases in flux with depth, we have applied a one-dimensional reversible scavenging model to produce idealized profiles of 230Th and 232Th, and have used that model to test existing approaches to flux assessment and to develop an improved method. Our model and field data indicate that 232Th is released from dust over the upper few hundred meters of the water column. In this depth range, traditional use of integrated 230Th residence times (τ230) to calculate 232Th fluxes may be reasonable because the source terms of both isotopes are similar and approximately constant with depth. At greater water depths, integrated τ230 overestimates the 232Th flux by a factor that approaches 2, due to the different depth distributions of 232Th and 230Th addition. We suggest a new approach to calculate τ230, which relies on in situ concentrations of 230Th at each depth, rather than an integrated value. Using this “volumetric” approach yields 232Th fluxes that are broadly constant with depth and which more accurately capture the true 232Th flux.