Quantifying lithogenic inputs to the Southern Ocean using long-lived thorium isotopes

Thorium (Th) isotopes were applied to quantify the supply of lithogenic inputs from the Kerguelen Plateau to the Southern Ocean. The dissolved concentrations of ²³²Th and ²³⁰Th were measured following a novel pre-concentration method using the Nobias resin from 10 stations above and on the slopes of the plateau. Elevated ²³²Th/²³⁰Th ratios in the upper 500 m of the water column confirm the input of lithogenic material from islands, glaciers and the resuspension of shelf-deposited sediments. ²³⁰Th concentrations were used to calculate a scavenging residence time for Th, which was then applied to calculate the flux of dissolved ²³²Th required to match the observed concentrations of dissolved ²³²Th. The ²³²Th content of the lithogenic material from the Kerguelen Plateau was used with the solubility of Th reported in the literature, to estimate a lithogenic particle flux of 7 to 810 mg m^–2 day^–1. This flux is comparable to fluxes obtained using sediment traps (8–777 mg m^–2 day^–1) in other continental-margin zones of the Southern Ocean. The flux of dissolved iron, generated by the dissolution of particles, was also calculated (4,189–6,800 nmol m^–2 day^–1) based on the iron/thorium ratio in the material from the Kerguelen Plateau. This is higher than previous estimates (1,342 nmol m^–2 day^–1), suggesting the dissolution of particles as the missing source of iron, thus closing the iron budget of the region. This study confirms the utility of long-lived Th isotopes to quantify lithogenic inputs from continental margin settings.