Detailed 230Th, 232Th and 210Pb fluxes recorded by the 1989/90 BOFS sediment trap time-series at 48°N, 20°W

Phytoplankton productivity north of 42°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 210Pb and 230Th 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 230Th 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 210 Pb flux is also similarly limited in both traps. These limits are ascribed to the differing behaviour of the fastersettling 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 210Pb23Th 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 210Pb230Th 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 3 100 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 230Th in the deep trap regularly exceeds production in the overlying water column throughout the year. The mean CaCO3 230Th and 232Th fluxes to this deep trap agree to within 10% of their long-term fluxes measured in a core from the trap site, although 210Pb fluxes to the trap are only 70% of the flux recorded by the sediment.