Quantification of Lateral Transport of ²³⁰ Th in the Ocean by Boundary Scavenging and Its Implications

Abstract

Thorium‐230 ( ²³⁰ Th) produced in the water column is mostly removed to in situ sediments by sinking particles (proximal scavenging), allowing for the correction of vertical particle fluxes by normalizing accumulated ²³⁰ Th recorded in marine sediments to its production. However, boundary scavenging, which causes lateral ²³⁰ Th transport, potentially challenges the premise of the application of ²³⁰ Th. In this study, we establish an empirical correlation between ²³⁰ Th scavenging intensity and export productivity (EP), enabling the quantification of boundary scavenging effects on the global scale. The contribution of boundary scavenging is quantified to be prominent only in high‐productivity regions (EP > 130 mg C/m ² /day). Since merely 15% of the mid‐low latitude ocean (EP > 200 mg C/m ² /day; mostly marginal seas and coastal upwelling zones) experiences severe ²³⁰ Th lateral transport (>30% of the seawater production), boundary scavenging exerts a negligible impact on ²³⁰ Th normalization in most of the pelagic ocean. The deficits and excesses of observed ²³⁰ Th in the water column relative to predictions are also quantified, in line with the pathways of deep ocean circulation. Our work places important constraints on the lateral transport of ²³⁰ Th in the ocean, and verifies its limited impact on the application of ²³⁰ Th normalization in the mid‐low latitude pelagic ocean.