Inter-Basin Comparison of Boundary Scavenging Expressed in Dissolved and Particulate Thorium and Protactinium Along GEOTRACES GN01, GP16, and GA03 Transects

Over the past decade the U.S. GEOTRACES program has investigated and documented how biogeochemical processes affect the cycling of trace elements and their isotopes on a basin scale. We use dissolved and particulate ²³⁰Th and ²³¹Pa data along three U.S. GEOTRACES transects to establish an inter-basin comparison of boundary scavenging in the Arctic Ocean (GN01 Transect), South Pacific Ocean (GP16 Transect), and North Atlantic Ocean (GA03 Transect).

The conceptual model of classical boundary scavenging is characterized by two end-member scavenging regimes that have a particle flux gradient from the ocean margin to the ocean interior. It predicts lower isotopic concentrations at the margins due to particle flux, and resulting scavenging intensity, decreasing with distance from the margin to the interior. To first order, all three ocean basins support this concept with a lateral gradient in dissolved xs²³⁰Th and xs²³¹Pa concentrations. Concentration gradients in dissolved xs²³¹Pa are weaker than those observed in dissolved xs²³⁰Th due to the longer ocean residence time of ²³¹Pa. Dissolved xs²³¹Pa/xs²³⁰Th activity ratios in all three basins are higher at the margins than the interior and an order of magnitude higher than those observed in the particulate phase due to the preferential scavenging of ²³⁰Th relative to ²³¹Pa. Lateral gradients in isotopic concentrations and activity ratios are much weaker in the particulate phase than in the dissolved phase. Notably, particulate xs²³¹Pa/xs²³⁰Th activity ratios in the Canada Basin of the Arctic Ocean show no conclusive evidence for the boundary scavenging signal observed in the dissolved phase and nearly all particulate xs²³¹Pa/xs²³⁰Th activity ratios are below the production ratio (0.093). We utilize particle composition data from all three basins to explore how particle chemistry, not just particle flux, affects boundary scavenging and how it is expressed in particulate xs²³¹Pa/xs²³⁰Th activity ratios.