Quantifying Lithogenic Inputs to the Ocean From the GEOTRACES Thorium Transects in a Data-Assimilation Model

Abstract

The primordial thorium (Th) isotope ²³²Th is delivered to global ocean waters by lithogenic material dissolution, originating from dust deposition on the ocean surface and seafloor sediments around continental margins. Radiogenic ²³⁰Th shares the same particle-scavenging removal processes as ²³²Th but has a simpler source from uniform ²³⁴U decay. The ²³²Th–²³⁰Th isotope system is therefore widely used to jointly constrain source and sink processes and infer lithogenic trace element inputs but has not been leveraged systematically at the global scale. This study utilizes data-assimilation models of the ²³⁰Th and ²³²Th cycles to draw global insights from GEOTRACES section data. Optimization of ²³⁰Th model reveals that scavenging onto biogenic particles is the largest Th removal mechanism globally, but regional-scale losses are dominated by scavenging onto metal oxides near hydrothermal vents, nepheloid layer particles along deep margins, and lithogenic particles beneath dust plumes. Together with large-scale ocean circulation, these processes shape the global distribution of ²³⁰Th. A subsequent ensemble of ²³²Th model optimizations reveals that the observed ²³²Th distribution is consistent with a global source of 2.8 ± 0.54 Mmol/yr from dust dissolution, and implies a higher and more homogeneous solubility of ²³²Th relative to other lithogenic trace elements. An additional source of 5.25 ± 0.86 Mmol/yr from lithogenic sediment dissolution is required to match observed ²³²Th, with a distribution controlled by sediment lithology and accumulation rate. This translates to an oceanic Fe source of 5–50 Gmol/yr, highlighting the need to resolve this process in Fe cycle models that often only consider a reductive sedimentary source.