210Pb and stable lead through the redox transition zone of an Antarctic lake

Abstract

Despite fully aerobic waters, below 51 m depth in Lake Vanda relatively low Eh values, combined with a lowering of pH, lead to the dissolution of manganese oxides, and the accumulation of dissolved Mn. Just above 61 m oxygen is depleted, and Fe 2+ accumulates. Just below 61 m sulfide accumulates as a result of sulfate reduction. We measured the depth distributions of dissolved and particulate stable lead, 210Pb, and 226Ra, to understand the dynamics of lead cycling through the redox transition zone. Our results indicate that stable lead is released from dissolving manganese oxides in the region between 51 to 61 m, resulting in a dissolved lead maximum at 59 m. Concentration profiles show that dissolved lead diffuses to the top of the zone of aerobic Mn reduction (AMR), and is sequestered onto newly formed manganese oxides. These oxides settle and dissolve, releasing lead back to solution. Lead also diffuses downward into the anoxic-sulfidic waters for permanent removal as insoluble PbS phases. Despite net release of stable lead into solution in the AMR zone, 210Pb and 226Ra results demonstrate active removal of 210Pb at all depths. Removal is fastest at the top of the AMR zone and in the anoxic zone. In the middle of the AMR zone scavenging is less active, though significant. Model calculations demonstrate that scavenging onto particles is the most important removal pathway for stable lead from the AMR zone. Finally, similar timescales of removal are found for 210Pb and stable lead, showing that in this environment, the 210Pb “clock” may be applied to understanding rates of stable lead cycling.