Thorium-234 (²³⁴Th) is widely used to study carbon fluxes from the surface ocean to depth. But few studies stress the relevance of oceanic advection and diffusion on the downward ²³⁴Th fluxes in nearshore environments. Our study in offshore Peru showed strong temporal variations in both the importance of physical processes on ²³⁴Th flux estimates and the oceanic residence time of ²³⁴Th, whereas salinity-derived seawater ²³⁸U activities accounted for up to 40 % errors in ²³⁴Th flux estimates.