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

The trapping efficiency of bottom-tethered deep-sea sediment traps deployed in four oceanic basins was estimated from the intercepted fluxes of ²³¹Pa and ²³⁰Th. The results validate the general use of baffled, conical sediment 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 efficiency reflect changes in the hydrodynamic properties of particles that undergo multiple cycles of aggregation and disaggregation while settling through the water column. At the ocean margins and in the Southern Ocean, the uncertainties in our estimates of trapping efficiency are very large because of the paucity of water column data on the distribution of ²³⁰Th and ²³¹Pa. Nonetheless, we could document large undertrapping for a trap deployed at 700 m in the Antarctic Circumpolar Current (ACC) near the Polar Front. Near continental margins, the flux of ²³⁰Th intercepted by the traps often exceeds the predicted vertical flux, a result that we attribute not to hydrodynamic overtrapping, but to resuspension and lateral transport of slope sediment into the traps. After correction of the particle flux data obtained during the North Atlantic Bloom Experiment for trapping efficiency, carbonate fluxes decrease by 30-40% within the bathypelagic zone above the calcite saturation horizon, possibly a result of calcite dissolution induced by metabolic CO₂ released in microenvironments, or aragonite and magnesian calcite dissolution below their saturation horizons. Aluminum fluxes remain constant with depth and indicate minimal lateral input, even in the deeper traps. The corrected fluxes indicate that the apparent increase in lithogenic particle flux with depth previously observed in this region is mainly a sampling artifact resulting from the low trapping efficiency of the traps deployed in the mesopelagic zone.