Pathways and processes controlling the distribution of anthropogenic lead in the interior of the Southern Ocean

Since the onset of industrialisation, anthropogenic emissions have been the primary source of lead (Pb) to the surface ocean. Over time, this pollutant Pb has spread into the ocean interior, predominately through water mass transport, but also by reversibly scavenging off sinking particles, particularly in areas of high productivity. In the Southern Ocean, biogeochemical provinces with distinct particle flux conditions may variably influence the pathways of anthropogenic Pb to the deep ocean. To investigate this, this study determined the dissolved Pb concentrations and Pb isotope compositions of seawater depth profiles from GEOTRACES section GS01 between Australia and Antarctica. The meridional section reveals that the highest anthropogenic Pb signatures ( 206 Pb/ 207 Pb = 1.147–1.152, 208 Pb/ 207 Pb = 2.423–2.426) occur at intermediate depths of the Subantarctic Zone (SAZ). This pollutant Pb enters the Southern Ocean interior via deep convection associated with Subantarctic Mode Water formation in the Southern Ocean and zonal advection of Antarctic Intermediate Water from the Indian Ocean. In the High Nutrient Low Chlorophyll region close to Antartica, strong vertical gradients in Pb isotope ratios suggest that low particle fluxes are insufficient to modify the natural Pb isotope composition of deep waters ( 206 Pb/ 207 Pb = 1.168–1.179, 208 Pb/ 207 Pb = 2.443–2.463) advected by the Antarctic Circumpolar Current. In contrast, the SAZ exhibits near-homogeneous isotope depth profiles with deep-sea Pb isotope compositions ( 206 Pb/ 207 Pb = 1.150–1.163, 208 Pb/ 207 Pb = 2.426–2.440) that cannot be explained by circulation. Mixing models that consider the Pb isotope signatures of SAZ surface waters and advected deep waters from the Indian Ocean suggest that vertical particle transport may account for ∼ 5–75% of the SAZ deep-water Pb inventory. Paradoxically, no deep-sea Pb isotope anomalies are observed in the Polar Frontal Zone despite higher rates of primary production in this region compared to the SAZ. This discrepancy may reflect differences in the nature of the sinking particles between the two regions. In the SAZ, carbonate-rich particles produced by coccolithophores undergo dissolution below the calcite saturation horizon in the deep ocean, while in the Polar Frontal Zone, diatom-dominated production generates opal ballast that rapidly remineralises its Pb inventory in the water column, thereby restricting the vertical transport of Pb to shallower depths. These findings highlights that particle composition can play a key role in determining the pathways of pollutant Pb to the deep ocean.