Modelling neodymium isotopes to investigate the marine neodymium cycle

The neodymium (Nd) isotope composition (εNd) of seawater is used to trace ocean circulation. However, there remains uncertainty in quantifying oceanic budgets of Nd. For example, a recently proposed benthic Nd flux complicates traditional interpretations of the seawater εNd signal by introducing a major non-conservative process.

This thesis implements marine Nd isotopes into a fast General Circulation Model (FAMOUS); revisiting the sources, sinks and transformation of Nd within the ocean. A statistical emulator is applied to explore three key parameters, and to optimise the scheme’s ability to represent modern seawater measurements. Altogether, the calibrated simulation captures the influence of the physical/biogeochemical processes to which global εNd distributions are known to be sensitive (i.e., inputs/sinks, internal cycling, and water mass structure and mixing). Of the total Nd sourced to the ocean, 64% comes from the seafloor sediment flux, 30% from rivers, and 6% from dust, and simulated Nd has an estimated residence time of 727 years. Sensitivity tests reveal that vertical Nd transport via reversible scavenging maintains water provenance signals by enhancing basinal εNd signatures. Model-data inconsistencies in the North Pacific and northern North Atlantic imply that a spatially uniform seafloor-wide bulk sediment benthic Nd flux does not capture the mobile particle-seawater exchange in all instances. Furthermore, simulations exploring the mobilisation of ‘reactive’ sediment Nd demonstrate sluggish Pacific waters are sensitive to benthic flux alterations, whereas the well-ventilated North Atlantic displays a much weaker response.

In closing, there are distinct regional differences in how seawater acquires its εNd signal. More careful interpretations to set apart the interlinked roles of major Nd sources (εNd composition, spatial extent, and reactivity), from ocean structure (water mass presence, convection, and ventilation), and regional environmental conditions (biological productivity, pH, redox conditions) are required for the robust applications of εNd as an ocean circulation tracer.