Conservative behavior of radiogenic neodymium isotopes in the South Pacific interior

• εNd and Nd concentrations behave largely conservatively across the South Pacific. • εNd modified to more radiogenic signatures at eastern and western boundaries. • LCDW retains its εNd signature during northward transport from 55°S to 20°S. The radiogenic neodymium isotope composition (εNd) is widely used to reconstruct past water mass mixing based on the assumption of quasi-conservative behavior in the open ocean. While non-conservative influences from boundary exchange, benthic fluxes, or hydrothermal inputs have been recognized near ocean margins and mid ocean ridges, uncertainties remain regarding the extent to which these processes influence Nd isotope distributions in the Pacific interior, where sluggish circulation and widespread volcanic provinces may lead to the modification of isotopic signatures. Here we present full-depth distributions of εNd and Nd concentrations ([Nd]) from GEOTRACES zonal section GP21 (26–32°S) across the entire South Pacific basin. Using fractional water mass contributions derived from an Optimum Multi-Parameter Analysis, we show that εNd and [Nd] behave conservatively across the oligotrophic South Pacific Gyre, with detectable modifications restricted to the eastern and western boundaries. Combining the GP21 data with other South Pacific zonal transects further demonstrates that intermediate and deep water compositions can be explained by essentially conservative behavior of Nd signatures from subpolar to subtropical latitudes, with minimal alteration (≤2 εNd units) along the major advective pathways. In particular, Lower Circumpolar Deep Water shows no measurable isotopic imprint from benthic fluxes during northward transport from the subpolar South Pacific to ∼20°S. These findings demonstrate that water mass mixing dominates Nd distributions in the central South Pacific, supporting the use of εNd as a robust tracer of past ocean circulation in the basin interior, while highlighting boundary regions as hotspots of isotopic exchange.