Does 234Th/238U disequilibrium provide an accurate record of the export flux of particulate organic carbon from the upper ocean?

The magnitude of the flux of biogenic particulate organic carbon (POC) exported from the surface waters of the world ocean and remineralized at depth is critical to constraining models of the global carbon cycle, yet remains controversial. The use of upper ocean sediment traps is still one of the primary tools for determining this export flux, although trap fluxes have been shown to vary significantly because of hydrodynamic and sample collection biases. Over the past decade, 234Th increasingly has been used as a tracer to estimate POC export from the euphotic zone by multiplying the depth-integrated 234Th flux by the POC/234Th ratio of sinking particles. The accuracy of this technique is highly dependent on the natural variability in the POC/ 234Th ratio and 234Th flux, yet the significance of this variability to estimates of POC export remains uncertain. Based on an analysis of new 234Th and POC data from the Labrador Sea and a review of 25 previous independent field studies, we report that POC export fluxes can vary 2–10 times or more solely because of variability in the POC/234Th ratio and procedures used to estimate the 234Th flux. Recommended improvements include studies of the biological, chemical, and physical mechanisms controlling 234Th-organic matter interactions in seawater; detailed comparisons of POC/234Th ratios in size-fractionated and sediment trap material; increased spatial and temporal sampling density of 234Th; and more standardized procedures to calculate the 234Th export flux.