Influence of water mass mixing and hydrothermal processes on the distribution of dissolved Nd isotopes and concentrations in the South Pacific

Abstract

The isotope composition of dissolved neodymium (Nd) in seawater, expressed as ε_Nd, serves as a valuable geochemical tracer for the provenance and mixing of water masses in the ocean. However, challenges arise in the application due to additional Nd sources and isotope exchange at ocean boundaries and suspended particles of hydrothermal and lithogenic origin, which are not fully understood. In this study, we present seawater Nd concentrations [Nd] and ε_Nd obtained from a deep-water (>1500 m water depth) zonal transect across the oligotrophic South Pacific gyre sampled during R/V Sonne cruise SO245, GEOTRACES process study GPpr09. Across 8 stations, dissolved [Nd] range from 10.8 to 31.1 pmol/kg with a characteristic pattern of increasing [Nd] at all stations, while our ε_Nd results from -3.9 to -8.5 show pronounced zonal differences between the eastern and western part of the transect. We compare our measured data with values predicted from optimum multiparameter analysis (OMPA) of water masses to quantify the non-conservative influence on ε_Nd and [Nd] along our transect. Our detailed analysis demonstrates that the deep water ε_Nd distribution is dominated by conservative mixing of Upper and Lower Circumpolar Deep Water, Pacific Deep Water and Antarctic Intermediate Water, with 66% of all samples falling within ±0.6 epsilon units (i.e., the propagated uncertainty range derived from endmember compositions and analytical uncertainty) of the ε_Nd predicted from conservative water mass mixing. In contrast, our [Nd] results reveal a pronounced deficit relative to concentrations predicted from water mass mixing (up to ∼10 pmol/kg). By excluding other influencing factors such as surface particle rain and boundary scavenging, we ascribe the [Nd] deficit to enhanced scavenging by hydrothermal particles in particular in PDW. This is consistent with the striking similarity of the [Nd] deficit and the distribution of δ³He as an indicator of hydrothermal activity along our transect. These findings highlight the decoupling of [Nd], modulated heavily by non-conservative processes, from the mainly conservative behaviour of ε_Nd in the deep South Pacific.