238U is a long-lived isotope (half-life is approximately the age of the earth), that is conserved in the ocean - meaning that it behaves like a salt and we can estimate its concentration from salinity. Its first long-lived daughter particle (234Th) has a half-life of only 24.1 days, thus in a closed system, we would expect 238U and 234Th to reach secular equilibrium (meaning that 234Th has reached a steady state in which it decays away at the same rate that it is produced from 238U). However, unlike 238238U, 234Th scavenges onto particles and hence is removed from the surface ocean when these particles sink. Thus by measuring the deficiency of 234Th relative to 238U, and making assumptions of steady-state and minimal advection, we can estimate the export of 234Th from the surface ocean. Measurements of the C:234Th ratio of sinking particles then allow us to convert to carbon export rates.
While there are many issues associated with the assumptions of steady-state and no lateral or vertical inputs of 234Th, 234Th can serve as a complementary method in conjunction with traditional sediment traps. Furthermore, the true power of 234Th deficiency measurements is that, unlike with sediment traps, POC export measurements can be made from water column samples. 234Th thus allows for spatially explicit measurements of carbon export and remineralization patterns. I have used 234Th measurements to constrain carbon export in regions across the globe. Most recently, 234Th carbon export measurements made during transects across a frontal system in the California Current Ecosystem have allowed me to show that fronts are regions of heightened biogeochemical fluxes.
