Thorium-234 as a tracer of the oceanic export of particulate organic carbon

A primary characteristic of the oceanic biological pump is the fixation of CO₂ to particulate organic carbon (POC) and its subsequent removal from the euphotic zone via particle settling. Traditionally, this process has been quantified using sediment traps, which are subject to hydrodynamic biases in the upper ocean and energetic shelf waters. Recently, 234Th (t_1/2 = 24.1 d), a naturally occurring isotope that is produced in situ from 238U (t_1/2 = 4.47 × 109 y), has been applied as a tracer of POC export from the upper ocean. To address several fundamental questions concerning the use of 234Th as a tracer of POC export and the role of POC export in the global carbon cycle, studies were performed in three contrasting marine environments. In the first study, rates of particle-scavenging and particulate organic carbon export were calculated along a transect through the midAtlantic Ocean from 35°S to 10°N in 1996. Vertical profiles of 234Th activities were collected to estimate export fluxes of particulate organic carbon. Using these estimates, approximately 0.29 Gt C y−1 was exported from the Atlantic equator (5°S–5°N, 25°E–5°W); given a recent estimate of average annual production in the eastern tropical Atlantic of 2.3 Gt C y−1, the average e-ratio (particulate export/primary production) for this region is 0.13. The second study involved measurements of POC and 234Th collected during the Canadian Joint Global Ocean Flux Study (JGOFS) program in 1996–1997 along a transect from 125°W to 145°W at 50°N (Ocean Weather Station Papa). In this region, particulate organic carbon export was a factor of two less than new production, suggesting that production and export of dissolved organic carbon may have been important. In the final study, measurements of 234Th, dissolved organic carbon, sediment organic carbon, and primary productivity were used to construct an organic carbon budget for the Gulf of Maine.