A growing body of Nd isotope data derived from fish debris and Fe-Mn crusts suggests that the Pacific was characterized by deep-water mass formation in both the North and South Pacific during the Early Paleogene. However, the South Pacific source has not been identified to date. Here we present new fossil fish debris neodymium isotope data from the South Pacific and southern tropical Pacific Ocean Drilling Program and Deep Sea Drilling Project Sites 323, 463, 596, 865 and 869 (paleowater depths spanning 1500 to 5000m) to reconstruct the water mass composition over the time interval ~80 to ~24 Ma. The data indicate a relatively unradiogenic South Pacific water mass composition, and the composition of Nd increases with distance northward. The new tropical Pacific data are consistent with existing records from that region. Analyses of detrital sediment Nd isotopic composition, combined with the dissolved Nd composition recorded by fish debris, suggests that the South Pacific water mass convected in the Pacific sector of the Southern Ocean. We designate this water mass South Pacific Deep Water (SPDW). The Nd isotopic composition of SPDW is more radiogenic than initially hypothesized and the relatively small increase in isotopic composition (from ~-6 to ~-4) during the transit from the Southern Ocean to the tropical Pacific suggests a faster rate of overturning circulation during the greenhouse climate interval than previously thought.