Tracing Water Mass Mixing From the Equatorial to the North Pacific Ocean With Dissolved Neodymium Isotopes and Concentrations

The sluggish water mass transport in the deeper North Pacific Ocean complicates the assessment of formation, spreading and mixing of surface, intermediate and deep-water masses based on standard hydrographic parameters alone. Geochemical tracers sensitive to water mass provenance and mixing allow to better characterize the origin and fate of the prevailing water masses. Here, we present dissolved neodymium (Nd) isotope compositions (epsilon(Nd)) and concentrations ([Nd]) obtained along a longitudinal transect at similar to 180 degrees E from similar to 7 degrees S to similar to 50 degrees N. The strongest contrast in Nd isotope signatures is observed in equatorial regions between surface waters (epsilon(Nd) similar to 0 at 4.5 degrees N) and Lower Circumpolar Deep Water (LCDW) prevailing at 4500 m depth (epsilon(Nd) = -6.7 at 7.2 degrees N). The Nd isotope compositions of equatorial surface and subsurface waters are strongly influenced by regional inputs from the volcanic rocks surrounding the Pacific, which facilitates the identification of the source regions of these waters and seasonal changes in their advection along the equator. Highly radiogenic weathering inputs from Papua-New-Guinea control the epsilon(Nd) signature of the equatorial surface waters and strongly alter the epsilon(Nd) signal of Antarctic Intermediate Water (AAIW) by sea water-particle interactions leading to an epsilon(Nd) shift from -5.3 to -1.7 and an increase in [Nd] from 8.5 to 11.0 pmol/kg between 7 degrees S and 15 degrees N. Further north in the open North Pacific, mixing calculations based on epsilon(Nd), [Nd] and salinity suggest that this modification of the AAIW composition has a strong impact on intermediate water epsilon(Nd) signatures of the entire region allowing for improved identification of the formation regions and pathways of North Pacific Intermediate Water (NPIW). The deep-water Nd isotope signatures indicate a southern Pacific origin and subsequent changes along its trajectory resulting from a combination of water mass mixing, vertical processes and Nd release from seafloor sediments, which precludes Nd isotopes as quantitative tracers of deep-water mass mixing. Moreover, comparison with previously reported data indicates that the Nd isotope signatures and concentrations below 100 m depth essentially remained stable over the pest decades, which suggests constant impacts of water mass advection and mixing as well as of non-conservative vertical exchange and bottom release.

Automated summary

The study utilizes dissolved neodymium isotopes to track the formation and mixing of water masses in the North Pacific, revealing significant contrasts in neodymium isotope signatures between equatorial surface waters and deep waters, influenced by weathering inputs from Papua-New-Guinea. In the deep-water region, neodymium isotopes are not effective quantitative tracers of water mass mixing.