Modeling of 231Pa cycle and its implications on particle scavenging in the global ocean

Protactinium-231 ( 231 Pa) is a particle-reactive radionuclide whose distribution integrates the effects of scavenging and ocean transport, making it a powerful tracer of particle dynamics and a potentially useful tracer for ocean circulation. We present the first global 231 Pa inverse model that couples reversible scavenging by biogenic and lithogenic particles with two explicitly parameterized local sinks: bottom scavenging based on satellite-derived eddy kinetic energy (EKE) as a proxy for nepheloid-layer particles, and hydrothermal scavenging linked to 3 He flux as a proxy for hydrothermal particle inputs. Constrained by available 231 Pa observations, the model reproduces global dissolved 231 Pa distributions with a goodness-of-fit R 2 = 0.79, and quantifies the global removal budget: 71 % by biogenic particles, 24 % by bottom scavenging, and 4 % by hydrothermal scavenging. Sensitivity tests reveal that particulate organic carbon (POC) and particulate inorganic carbon (PIC) act as consistent net sinks throughout the water column, whereas biogenic silica (bSi) enhances deep-water 231 Pa through downward transport and release. Bottom scavenging strongly constrains deep-ocean 231 Pa, while hydrothermal scavenging is locally important near active ridges. This mechanistically informed, data-constrained framework advances our understanding of particle–tracer interactions and is directly transferable to other particle-reactive elements, with implications for biogeochemical modeling.