Distribution of 210Pb and 210Po in the Arctic water column during the 2007 sea-ice minimum: Particle export in the ice-covered basins

Pb-210 and Po-210 are naturally occurring radionuclides that are commonly used as a proxy for particle and carbon export. In this study, the distribution of the Po-210/Pb-210 pair was investigated in the water column of the Barents, Kara and Laptev Seas and the Nansen, Amundsen and Makarov Basins in order to understand the particle dynamics in the Arctic Ocean during the 2007 sea-ice minimum (August-September). Minimum activities of total Pb-210 and Po-210 were found in the upper and lower haloclines (approx. 60-130 m), which are partly attributed to particle scavenging over the shelves, boundary current transport and subsequent advection of the water with low Pb-210 and Po-210 activities into the central Arctic. Widespread and substantial (> 50%) deficits of Po-210 with respect to Pb-210 were detected from surface waters to 200 m on the shelves, but also in the basins. This was particularly important in the Makarov Basin where, despite very low chlorophyll-a levels, estimates of annual new primary production were three times higher than in the Eurasian Basin. In the Nansen, Amundsen and Makarov Basins, estimates of annual new primary production correlated with the deficits of Po-210 in the upper 200 m of the water column, suggesting that in situ production and subsequent export of biogenic material were the mechanisms that controlled the removal of Po-210 in the central Arctic. Unlike Po-210, Th-234 deficits measured during the same expedition were found to be very small and not significant below 25 m in the basins (Cai et al., 2010), which indicates, given the shorter half-life of Th-234, that particle export fluxes in the central Arctic would have been higher before July-August in 2007 than later in the season.