Ice-sheet dynamics drive glacial-interglacial shifts in North Atlantic seawater neodymium isotopes.

Radiogenic neodymium (Nd) isotopic composition (εNd) of seawater is widely used to reconstruct past ocean circulation, assuming quasi-constant end-member signatures throughout the Quaternary. However, the εNd signature of Glacial North Atlantic Intermediate Water deviated markedly from its modern value, challenging this assumption, and the driving mechanisms remain debated. Here, we show that the upper Glacial North Atlantic Intermediate Water was consistently more radiogenic (-7.2) than interglacial values (-13.4) over the past 230,000 years and that pronounced glacial-interglacial εNd variability closely followed global ice-volume fluctuations (r = 0.80). This variability was primarily reconciled with Icelandic volcanic Nd inputs, modulated by the advance and retreat of the Icelandic Ice Sheet. A forward model reproduces the observed εNd evolution and reveals a nonlinear response of Nd export to ice-sheet dynamics, yielding a mean glacial Icelandic Nd input flux of 2.4 × 108 grams per year. Our results highlight the pivotal role of high-latitude ice-sheet systems in regulating North Atlantic seawater chemistry during glacial periods.