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Early Paleogene neodymium (Nd) isotopic reconstructions of ocean circulation in the Pacific suggest a bipolar mode of overturning circulation characterized by convection of deep-water masses from 70 to at least 40 Ma. This mode is fundamentally different from the "sluggish, diffusive" mode that characterizes the modern North Pacific. Therefore, major reorganization of Pacific circulation must have occurred between 40 Ma and the modern. To investigate the potential timing of such a reorganization, we present new Nd isotope data from drill sites spanning 40 to 10 Ma. Here, we use Nd isotopic analyses to identify the sources of dissolved Nd to the Pacific in order to evaluate the application of the Nd proxy for ancient water mass composition. We combine analyses of fossil fish debris, sequentially leached oxide coatings and leach residues to further refine our understanding of Nd marine geochemical cycling and to contribute new constraints on the paleoceanographic history of the Pacific. Overall, these data support previous interpretations of the convection of deep water in the Pacific sector of the Southern Ocean and surface-ventilated North Pacific Deep Water formation from similar to 70 to 40 Ma. The new Nd isotope data suggest a gradual transition between 36 and 25 Ma, a time period characterized by global cooling. This transition consists of a change from North Pacific Nd isotopic signatures controlled by convection and water mass mixing to values heavily influenced by overprinting from other sources of Nd to the deep Pacific. Such a transition is consistent with a change in overturning circulation from a "Paleogene mode" to a mode dominated by sluggish diffusive mixing. After 25 Ma the data indicate a significant contribution of radiogenic dissolved Nd to the deep waters of the North Pacific. The most likely source of radiogenic dissolved Nd would have been labile volcanic ash that altered the composition of the water mass as it slowly advected through the North Pacific. A slowing of overturning circulation in the North Pacific could have resulted in diminished poleward heat transport, increasing equator to pole thermal gradients and contributing to a cooling global climate. (C) 2019 Elsevier B.V. All rights reserved.