DETAILED TH-230, TH-232 AND PB-210 FLUXES RECORDED BY THE 1989/90 BOFS SEDIMENT TRAP TIME-SERIES AT 48-DEGREES-N, 20-DEGREES-W

Phytoplankton productivity north of 42 degrees N in the NE Atlantic is dominated by intense blooms in spring and early summer. Detailed analyses of deep-water sediment trap samples over a 17 month period, encompassing two such bloom events, reveal that the removal fluxes of the particle-reactive radionuclides Pb-210 and Th-230 are generally linearly related to mass flux, as observed elsewhere, up to 120 mg m(-2) day(-1). Above mass fluxes of 120 mg m(-2) day(-1), the Th-230 flux is limited close to its water column production rate in a trap at 3100 m, and somewhat in excess of its production rate in a trap at 4465 m (full water column depth 4555 m). The Pb-210 flux is also similarly limited in both traps. These limits are ascribed to the differing behaviour of the faster-settling bloom material, rather than to quantitative scavenging of water column radionuclides. Finite dissolved and particulate activities for both radionuclides are observed in the water column. Comparison of Pb-210/Th-230 activity ratios in surface particulate and trap material indicates that the trap material does not represent surface material which has settled without modification through the water column, Rather the Pb-210/Th-230 activity ratio of the trap material is similar to the activity ratio of particulate material in the integrated water column overlying the shallower trap, but the underlying mechanisms controlling the scavenging limitation remain unclear. Comparison of data from the 3100 m trap in ''clear water'' 1455 m above the sea floor, with those from the second only 90 m above, illustrates that re-entrainment of current-resuspended near-bottom material must contribute to the deep trap. As a result, the intercepted Th-230 in the deep trap regularly exceeds production in the overlying water column throughout the year. The mean CaCO3, (230) Th and Th-232 fluxes to this deep trap agree to within 10% of their long-term fluxes measured in a core from the trap site, although Pb-210 fluxes to the trap are only 70% of the flux recorded by the sediment.