Enhanced particle scavenging in deep water of the Aleutian Basin revealed by 210Po-210Pb disequilibria

The high sedimentation rate but low primary production in surface ocean raised a question whether particles from local upper water column could support high deposition in the Aleutian Basin. Here we first present large Po-210-Pb-210 disequilibria in deep water of the Aleutian Basin. Dissolved Po-210 and Pb-210 were depleted relative to Pb-210 and Ra-226, respectively, in deep water below 1000 m, as well as decreased with depth, suggesting enhanced particle scavenging in the deep water. The Po-210 residence times (1-2 a) in deep water were comparable to those in the upper water column, indicating that Po-210 scavenging rates were high in deep water of the Aleutian Basin. The export fluxes of Po-210 from the upper 100 m were estimated to be 0.2-0.8 Bq/m(2)/d, much lower than those in the deep water (7-8 Bq/m(2)/d). Similarly, POC export fluxes in deep water (24-80 mmolC/m(2)/d) were higher than those in the upper 100 m (similar to 1 mmolC/m(2)/d). Such a large discrepancy between the upper and deep water suggested that particles from local upper water column could not totally meet the enhanced scavenging in the deep water. Based on mass balance calculations, the extra fluxes of Pb-210 and POC imported to deep water were estimated to be 8-12 Bq/m(2)/d and 22-79 mmolC/m(2)/d, respectively. The ratio of POC to particulate Pb-210 (i.e., POC/PPb) in the extra source was estimated to be 6.5 mmol/Bq, which was lower than that in the Bering Shelf with a mean POC/PPb ratio of 10.9 mmol/Bq, implying that particles in the Bering Shelf could be a potential source for the enhanced particle scavenging in deep water of the Aleutian Basin. However, quantitative and detailed role of ridges and manganese from sediments in particle scavenging in the deep water was unclear, and further studies are necessary.