Scavenging, cycling and removal fluxes of 210Po and 210Pb at the Bermuda time-series study site

Abstract

Quantifying relative affinities of Po and Pb in different populations of marine particulate matter is of great importance in utilizing 210Po as a tracer for carbon cycling. We collected and analyzed water samples for the concentrations of dissolved and total 210Po and 210Pb from the upper 600m of the water column at Bermuda Time-series Study site (September 1999–September 2000) to investigate their seasonality of concentrations and their activity ratio (210Po/210Pb activity ratio, AR). Sinking particles collected in sediment traps at depths of 500m, 1500m, and 3200m from the Oceanic Flux Program (OFP) time-series sediment traps were analyzed over a period of 12 months (May 1999–May 2000). The objective was to compare the deficiencies of 210Po with respect to 210Pb in the water column to that measured in the sediment traps and to assess the relative affinities of Po and Pb with different particle pools.Inventories of 210Po in the upper 500m water column varied by a factor of 2, indicating seasonal variations of particulate flux dominated the removal of 210Po. The 210Po/210Pb ARs in the dissolved phase were generally less than the secular equilibrium value (1.0) in the upper 600m, while were generally greater than 1.0 in the particulate phase, indicating higher removal rates of 210Po relative to 210Pb by particulate matter. The measured fluxes of 210Po and 210Pb in the 500m, 1500m, and 3200m traps increased with depth, while the 210Po/210Pb ARs decreased with depth except from May–August 1999. From the measured fluxes of 210Po and 210Pb at these three traps and the concentrations of 210Po and 210Pb in the water column, this region appears to be a sink for 210Pb which is likely brought-in by lateral advection.