234Th as a tracer of particulate organic carbon export in the subarctic northeast Pacific Ocean

Profiles of particulate and dissolved 234Th (t1/2=24.1 d) were collected during the Canadian JGOFS program (February, May, and August 1996 and February 1997) along a transect from 125°W to 145°W at 50°N (Ocean Station Papa – OSP). Persistent features included a deficit of 234Th relative to its parent 238U in the upper ∼75 m and occasionally an excess of total 234Th (particulate+dissolved) at ∼150–200 m attributed to redissolution of the particulate-bound nuclide. To calculate POC export fluxes, a steady-state model of 234Th export was combined with measurements of the POC/234Th ratio determined on particles >1-μm collected by in situ filtration. Along the transect, e-ratios (POC export/primary production) calculated for the 0.1% light level were similar during February 1997 and May 1996, 0.06–0.08 (avg. POC flux =2.8–7.1 mmol C m−2 d−1). During August 1996, e-ratios reached a maximum, averaging 0.13 (avg. POC flux=7.6 mmol C m−2 d−1). Particulate 234Th fluxes measured by drifting cylindrical sediment traps were 3-fold greater than the flux calculated for the water column 234Th deficit. Finally, to study changes in 234Th and POC export on short time-scales, some stations were sampled twice within 36 hours. During February 1997, 234Th increased nearly 50% from 60–100 m in a 36 h period. Hydrographic data suggest that this change was due to advective processes.