Effects of Past Nd Seawater Concentrations on Nd-Isotope Paleocirculation Reconstructions: A Bayesian Approach

Neodymium (Nd) isotope ratios are potentially valuable for paleo-ocean circulation reconstructions because they trace water-mass transport and mixing in the present-day oceans. Moreover, the Atlantic and Pacific global end-member Nd-isotope values can be constrained through time, and at in-between sites their proportions vary with past climate changes. However, an important source of uncertainty about Nd-isotopes' applicability for paleo-circulation studies arises from the inability to constrain past Nd-concentrations of the end-members. Here we address this paleo-[Nd] problem through a Bayesian analysis. Results show that even large variability in end-member Nd-concentrations over the Pleistocene is unlikely to significantly impact their concentration ratio, therefore end-member concentration changes likely have only small impacts on Nd-isotope ratios at in-between locations. The results support their applicability to reconstruct past Atlantic paleo-circulation. In addition, a Nd-isotope mass balance for Antarctic Bottom Water shows that its present-day Nd-isotope values are consistent with the input from its sources. Ocean circulation transfers heat around the Earth and characterizing its past variability is critical for understanding climate changes. Neodymium (Nd) isotopes are often used to trace past ocean circulation because they mimic deep ocean water mass mixing, and the North Atlantic and North Pacific global ocean mixing end-member water mass compositions can be constrained through time. Therefore, temporal changes in Nd-isotope ratios at intermediate locations like the Equatorial and South Atlantic are assumed to approximate changes in the proportions contributed by the global end-members and thus reflect ocean circulation changes. However, Nd-isotope ratios at intermediate locations are also sensitive to changes in Nd-concentrations, which cannot be constrained in the global ocean end-member water masses. This has caused the applicability of Nd-isotope ratios for tracing paleo-ocean circulation to be questioned. Our study examines the sensitivity of Nd-isotope ratios at locations in-between the global end-members to changes in their Nd-concentrations over recent interglacial-glacial cycles. Results show that even substantial changes in their Nd-concentrations likely have little impact on the Nd-isotope ratios at intermediate locations, indicating that given the right intermediate location choices, Nd-isotope ratios indeed reflect past deep ocean mixing, thus supporting their use to reconstruct past ocean circulation. Neodymium (Nd) isotopes trace modern water mass mixing, but unconstrained end-member Nd concentrations limit their paleocirculation useA Bayesian approach tests effects of Nd-concentration changes in global water mass end-members on Nd-isotopes at intermediate locationsResults predict small impacts on Nd-isotopes and constrain projected uncertainties in past ocean end-member mixing reconstructions

Automated summary

Neodymium (Nd) isotope ratios are valuable for paleo-ocean circulation reconstructions, tracing water mass transport and mixing. Bayesian analysis shows that changes in end-member Nd concentrations have little impact on intermediate locations' Nd-isotope ratios, supporting their applicability for reconstructing past Atlantic paleo-circulation.