Differing Controls on the Cycling of Neodymium in the Atlantic and the Pacific

Abstract Neodymium (Nd) isotopes ratios are regarded as a key proxy for past ocean circulation despite unsettled debate on marine Nd cycle controls, which opposes a “top‐down” hypothesis (surface sources dominate) and a “bottom‐up” hypothesis (sedimentary sources dominate), each with important implications for Nd isotope proxy interpretation. We suggest a new, basin‐specific framework for interpreting Nd isotope records. Specifically, while Nd isotope records are reliable water‐mass‐mixing proxies in the Atlantic, they are likely better used as proxies of residence time in the Pacific. Here, we test these hypotheses using a computationally efficient, data‐constrained model of the marine Nd cycle. Our results show Atlantic Nd isotopes are dominated by surface sources, while Pacific Nd isotope ratios are strongly influenced by sedimentary sources. We find that shallow, reactive sediments—especially those with extremely radiogenic or unradiogenic isotopic compositions—are crucial to reproduce observed εNd profiles in the Pacific.