Rare earth element and neodymium isotopes of the eastern US GEOTRACES Equatorial Pacific Zonal Transect (GP16)

Abstract

Dissolved rare earth element ([REE]) and neodymium isotopic (εNd) data from the US GEOTRACES Eastern Equatorial Pacific Transect (EPZT) are presented. These data are compared to watermass distributions (Peters et al., 2018), particulate data (Lam et al., 2018) and carbonate species data (Bates, 2018) to evaluate present theories of REE and εNd geochemistry, given that the Pacific has been notably intractable in relation to these ideas (Jones et al., 2008). The [REEs] have typical depth profile distributions (e.g., [La] ranges from ∼5 to 15 pmol/kg in the surface, increasing with depth to values of ∼40 pmol/kg), as does εNd (ranging from ∼-1 to -6 εNd; similar to previous Pacific data). However, despite the unsurprising nature of these data, several apparent inconsistencies with respect to current models for REE/εNd geochemistry arise. These are: (1) While εNd remarkably maintains watermass distribution information, there is a significant (+1.4 εNd) offset in the data compared to published end-member compositions, indicating a strong but largely consistent non-conservative component; (2) Reversible scavenging seems an unlikely mechanism for redistribution of [REE] and εNd in the water column; (3) There does not appear to be any correlation of [REE] to carbonate ion concentration, and; (4) There are indications of influence from a benthic flux, but the evidence is ambiguous.