The Atmospheric Input of Dissolvable Pb Based on the Radioactive 210Pb Budget in the Equatorial Western Indian Ocean

Abstract

To estimate the atmospheric deposition flux of 210Pb in the equatorial western Indian Ocean, we determined the dissolved (<0.45 μm) and particulate 210Pb (>0.45 μm) in the water column. In addition, we calculated the atmosphere-derived dissolvable Pb in seawater using the budget of 210Pb. The dissolved 210Pb and total 210Pb were higher in the surface layer and, overall, showed a decreasing distribution with depth. In particular, radioactive 210Pb activities in the surface-to-upper layer (<1000 m depth) were 1.5 to 2 times higher than those reported in the 1970s (in nearby regions), suggesting that there has been additional 210Pb input in recent years. Based on the mass balance of the total 210Pb budget in the water column, we estimated the atmospheric deposition flux of 210Pb and the residence time of Pb for the first time in this region. The atmospheric deposition flux of 210Pb was estimated to be 0.1–0.5 dpm cm−2 yr−1, and these values agreed with the general global estimations for the major oceans (0.1–0.7 dpm cm−2 yr−1). Considering the residence time of 210Pb (29–41 years) in the water column (estimated from the 210Pb inventory and 234Th-based Pb scavenging rate), the atmospheric input of seawater-dissolvable Pb was quantified to be 0.08–0.1 nmol cm−2 yr−1, which is about eight times higher than the estimated input in the early 1990s in the region. Therefore, these results imply that radioactive 210Pb could be a useful tracer for quantifying Pb flux in seawater.