South Pacific dissolved Nd isotope compositions and rare earth element distributions: Water mass mixing versus biogeochemical cycling

Abstract

Despite its enormous extent and importance for global climate, the South Pacific has been poorly investigated in comparison to other regions with respect to chemical oceanography. Here we present the first detailed analysis of dissolved radiogenic Nd isotopes (εNd) and rare earth elements (REEs) in intermediate and deep waters of the mid-latitude (∼40°S) South Pacific along a meridional transect between South America and New Zealand. The goal of our study is to gain better insight into the distribution and mixing of water masses in the South Pacific and to evaluate the validity of Nd isotopes as a water mass tracer in this remote region of the ocean. The results demonstrate that biogeochemical cycling (scavenging processes in the Eastern Equatorial Pacific) and release of LREEs from the sediment clearly influence the distribution of the dissolved REE concentrations at certain locations. Nevertheless, the Nd isotope signatures clearly trace water masses including AAIW (Antarctic Intermediate Water) (average εNd=−8.2±0.3), LCDW (Lower Circumpolar Deep Water) (average εNd=−8.3±0.3), NPDW (North Pacific Deep Water) (average εNd=−5.9±0.3), and the remnants of NADW (North Atlantic Deep Water) (average εNd=−9.7±0.3). Filtered water samples taken from the sediment–water interface under the deep western boundary current off New Zealand suggest that boundary exchange processes are limited at this location and highlight the spatial and temporal variability of this process.These data will serve as a basis for the paleoceanographic application of Nd isotopes in the South Pacific.