Trapping efficiency of bottom-tethered sediment traps estimated from the intercepted fluxes of Th-230 and Pa-231

The trapping efficiency of bottom-tethered deep-sea sediment traps deployed in four oceanic basins was estimated from the intercepted fluxes of Pa-231 and Th-230. The results validate the general use of baffled, conical sedim ent traps for measuring the settling flux of particles in the bathypelagic zone (depth > 1200 m) of the open ocean where current velocity is generally low. At shallower depths, within the mesopelagic zone, trapping efficiency tends to be lower and more erratic, even in areas of low current velocity (< 10 cm s(-1)). It is suggested that depth-related changes in trapping eff iciency reflect changes in the hydrodynamic properties of particles that un dergo multiple cycles of aggregation and disaggregation while settling thro ugh the water column. At the ocean margins and in the Southern Ocean, the u ncertainties in our estimates of trapping efficiency are very large because of the paucity of water column data on the distribution of Th-230 and Pa-2 31. Nonetheless, we could document large undertrapping for a trap deployed at 700 m in the Antarctic Circumpolar Current (ACC) near the Polar Front. N ear continental margins, the flux of Th-230 intercepted by the traps often exceeds the predicted vertical flux, a result that we attribute not to hydr odynamic overtrapping, but to resuspension and lateral transport of slope s ediment into the traps. After correction of the particle flux data obtained during the North Atlantic Bloom Experiment for trapping efficiency, carbon ate fluxes decrease by 30-40% within the bathypelagic zone above the calcit e saturation horizon, possibly a result of calcite dissolution induced by m etabolic CO2 released in microenvironments, or aragonite and magnesian calc ite dissolution below their saturation horizons. Aluminum fluxes remain con stant with depth and indicate minimal lateral input, even in the deeper tra ps. The corrected fluxes indicate that the apparent increase in lithogenic particle flux with depth previously observed in this region is mainly a sam pling artifact resulting from the low trapping efficiency of the traps depl oyed in the mesopelagic zone. (C) 2000 Published by Elsevier Science Ltd.