The ability to reconstruct past ocean currents is essential for determining ocean circulation’s role in global heat transport and climate change. Our understanding of the relationship between circulation and climate in the past allows us to predict the impact of future climate-driven circulation changes. One proposed tracer of past ocean circulation is the neodymium isotope composition (εNd) of ancient water masses. However, ambiguities in what governs the εNd distribution in the modern ocean hamper interpretations of this tracer. Here we present εNd values for marine pore fluids, sediments, and the overlying water column for three sites in the North Pacific. We find that ocean bottom water εNd (εNdBW) in the northeast Pacific lies between the value expected for the water mass (–3.3) and the measured εNd of sediment pore fluid (εNdPW;–1.8). Moreover, εNdPW resembles the εNd of the sediment. Combined, these findings are consistent with recent assessments that sediment pore fluids may be a major source of rare earth elements to the ocean and suggest that the benthic flux of Nd from pore fluids exerts the primary control over the deep ocean distribution of εNd.