Simulating marine neodymium isotope distributions using Nd v1.0 coupled to the ocean component of the FAMOUS–MOSES1 climate model: sensitivities to reversible scavenging efficiency and benthic source distributions

Abstract. The neodymium (Nd) isotopic composition of seawater is a
widely used ocean circulation tracer. However, uncertainty in quantifying
the global ocean Nd budget, particularly constraining elusive
non-conservative processes, remains a major challenge. A substantial
increase in modern seawater Nd measurements from the GEOTRACES programme,
coupled with recent hypotheses that a seafloor-wide benthic Nd flux to the
ocean may govern global Nd isotope distributions (εNd),
presents an opportunity to develop a new scheme specifically designed to
test these paradigms. Here, we present the implementation of Nd isotopes
(143Nd and 144Nd) into the ocean component of the FAMOUS coupled
atmosphere–ocean general circulation model (Nd v1.0), a tool which can be
widely used for simulating complex feedbacks between different Earth system
processes on decadal to multi-millennial timescales. Using an equilibrium pre-industrial simulation tuned to represent the
large-scale Atlantic Ocean circulation, we perform a series of sensitivity
tests evaluating the new Nd isotope scheme. We investigate how Nd
source and sink and cycling parameters govern global marine εNd distributions and provide an updated compilation of 6048 Nd
concentrations and 3278 εNd measurements to assess model
performance. Our findings support the notions that reversible scavenging is
a key process for enhancing the Atlantic–Pacific basinal εNd gradient and is capable of driving the observed increase in Nd
concentration along the global circulation pathway. A benthic flux
represents a major source of Nd to the deep ocean. However, model–data
disparities in the North Pacific highlight that under a uniform benthic
flux, the source of εNd from seafloor sediments is too
non-radiogenic in our model to be able to accurately represent seawater
measurements. Additionally, model–data mismatch in the northern North
Atlantic alludes to the possibility of preferential contributions from
“reactive” non-radiogenic detrital sediments. The new Nd isotope scheme forms an excellent tool for exploring global
marine Nd cycling and the interplay between climatic and oceanographic
conditions under both modern and palaeoceanographic contexts.