Vertical flux of particulate Al, Fe, Pb, and Ba from the upper ocean estimated from 234Th/238U disequilibria

The vertical sinking flux of particulate Al, Fe, Pb, and Ba from the upper 250 m of the Labrador Sea has been estimated from measurements of 234Th/238U disequilibrium and the respective metal/234Th ratios in >53 μm size particles. 234Th-derived particulate metal fluxes include in situ scavenged metals, labile lithogenic metals, and metals derived from external input (e.g., atmospheric supply). In contrast to the POC/234Th ratio, particle size-fractionated (0.4-10 μm, 10-53 μm, and >53 μm) Al/234Th, Fe/234Th and Pb/234Th, and Ba/234Th ratios generally increase with depth and exhibit no systematic change with particle diameter. Sinking fluxes of particulate Al (2.47-22.3 μmol m−2 d−1), Fe (2.69-16.3 μmol m−2 d−1), Pb (2.85-70 nmol m−2 d−1), and Ba (0.13-2.1 μmol m−2 d−1) at 50 m (base of the euphotic zone) and 100 m (base of the mixed layer) are largely within the range of previous sediment trap results from other ocean basins. Estimates of the upper ocean residence time of Al (0.07-0.28 yr) and Pb (0.8-2.9 yr) are short compared to previously reported values. The settling rate of >53 μm particles calculated from the 234Th data ranges from 14 to 38 m d−1.