Trapping efficiencies of sediment traps from the deep Eastern North Atlantic: the Th-230 calibration

Bottom-tethered sediment traps deployed in the deep eastern North Atlantic between 54 degreesN 20 degreesW and 33 degreesN 20 degreesW (L1, L2, L3), a t the European continental margin at 49 degreesN (OMEX) and off the Canary Islands (ESTOC) were investigated for the determination of Th-230 trapping efficiencies. The ratios of Th-230 flux measured in the traps (Fa) to the e xpected Th-230 flux from the production rate of Th-230 in the overlying wat er column (Fp) ranged between 0.09 and 1.26. For the traps with deployment periods > 300 days the interannual variation of Fa/Fp ratios (different yea rs but same location and water depth) were up to 10%, suggesting that the a verage Th-230 flux to the sediment traps did not vary significantly. The in fluence of lateral advection on the Th-230 flux was taken into account eith er by applying a mass balance of Th-230 and Pa-231 or by assuming a constan t removal rate of Th-230 from the water column, an assumption based on simi lar Th-230 concentration-depth profiles observed at most locations investig ated. Th-230 trapping efficiencies were between 9 and 143 %, showing a tren d of increasing efficiencies with increasing water depth. No relation was f ound between current velocities and Th-230 trapping efficiencies. Our inves tigations suggest that the observation of constant or even increasing parti cle flux rates with increasing water depths in several sediment trap arrays investigated may be a result of sediment trap biases. The correction for t he trapping biases is important for the understanding of the regional diffe rences in the particle flux in the eastern North Atlantic. (C) 2001 Elsevie r Science Ltd. All rights reserved.