Particle–Seawater Interaction of Neodymium in the North Atlantic

Dissolved
neodymium (Nd) isotopes (expressed as εNd) have been
widely used as a water mass tracer in paleoceanography.
However, one aspect of the modern biogeochemical cycle of Nd that
has been sparsely investigated is the interplay between dissolved
and particulate phases in seawater. We here present the first regional
data set on particulate Nd isotope compositions (εNdp) and concentrations ([Nd]p) from five stations
in the western North Atlantic Ocean along the GEOTRACES GA02 transect,
in conjunction with previously published dissolved Nd isotope compositions
(εNdd) and concentrations ([Nd]d)1. Key observations and interpretations from our new
particulate data set include the following: (1) Low fractional contributions
of [Nd]p to the total Nd inventory per volume unit of seawater
(∼5%), with significant increases of up to 45% in benthic boundary
layers. (2) Increasing Nd concentrations in suspended particulate
matter ([Nd]SPM) and fractions of lithogenic material with
water depth, suggesting the removal of Nd poor phases. (3) Different
provenances of particulates in the subpolar and subtropical gyres
as evidenced by their Nd isotope fingerprints reaching from εNdp ≈ −20 near the Labrador Basin
(old continental crust), over εNdp ≈
−4 between Iceland and Greenland (young mafic provenance),
to values of εNdp ≈ −13
in the subtropics (similar to African dust signal). (4) Vertical heterogeneity
of εNdp as well as large deviations from
ambient seawater values in the subpolar gyre indicate advection of
lithogenic particles in this area. (5) Vertically homogeneous εNdp values in the subtropical gyre, indistinguishable
from εNdd values, are indicative of predominance
of vertical particulate supply. The process of reversible scavenging
only seems to influence particulate signatures below 3 km. Overall,
we do not find evidence on enhanced particle dissolution, often invoked
to explain the observed increase in dissolved Nd in the North Atlantic.