Total Thorium-234 (Th-234) taken from discrete water column samples collected during CCE Process Cruises (2006 - 2017).

Thorium-234 (Th-234) is the first long-lived (half-life = 24.1 days) daughter particle of Uranium-238 (U-238). While U-238 is conserved in the ocean (it covaries with salinity and has a half-life of 4.5 billion years), Th-234 is scavenged onto particles and hence removed from the surface ocean when those particles sink to depth. By measuring vertical profiles of total Th-234 activity in the euphotic zone and assuming steady-state between production of Th-234 from U-238 decay and removal of Th-234 on sinking particles, we can thus estimate the export of Th-234 from sampled water parcels. These measurements of Th-234 export can be converted to estimates of carbon export by multiplying by the C:Th-234 ratio of sinking particles measured at the same time and location.