Boundary Controls on Radium‐223 and Radium‐228 Distributions Along GEOTRACES GP17‐OCE: Implications for Trace Element Transport in the Distal South Pacific Ocean

Abstract

In regions far removed from major atmospheric dust sources, such as the South Pacific and Southern Ocean, boundary exchange processes play a dominant role in regulating the supply of trace elements and their isotopes (TEIs) to the upper ocean, with important consequences for ecological stoichiometry and productivity. Here, we use radium (Ra) isotopes ( ²²³ Ra, ²²⁶ Ra, ²²⁸ Ra) as tracers of boundary exchange along the US GEOTRACES GP17‐OCE Pacific and Southern Ocean transect, providing the first full ocean‐basin section of excess ²²³ Ra. Activities of ²²⁸ Ra exhibit a prominent subsurface maximum (mean ≈0.21 dpm/100 L) between 20°S and 55°S that persists to ≈800 m. Hydrographic context and GLORYS12 velocity fields link this maximum to Subantarctic Mode Water sourced over the shallow Campbell Plateau. We infer source activities of 0.22–0.31 dpm/100 L, implying ventilation and export on sub‐annual to 2‐year timescales. Near‐bottom inventories of ²²⁸ Ra and excess ²²³ Ra reveal spatially variable benthic fluxes, and zonal integration of ²²⁸ Ra indicates that fluxes in the North and South Pacific are similar (2.8 × 10 ¹⁶ and 3.5 × 10 ¹⁶  dpm/yr, respectively), and comparable in magnitude to coastal ²²⁸ Ra inputs to the Pacific Ocean. Above the Pacific–Antarctic Ridge, elevated excess ²²³ Ra, ²²⁴ Ra, and ²²⁸ Ra, together with activity ratios, indicate entrainment of low‐temperature hydrothermal fluids that are weeks to a month old. Collectively, these results identify mode waters formed over the Campbell Plateau as a dominant conveyor of shelf‐derived Ra—and associated TEIs—to the South Pacific interior and document an active Pacific–Antarctic Ridge hydrothermal contribution that is underrepresented in current TEI models.