Nd isotopic variations of Phanerozoic paleoceans

The size of various Phanerozoic paleoceans, determined from paleogeographic maps, provides a framework for assigningε_Nd measurements of primary authigenic marine minerals to individual paleoceans and for establishing theirε_Nd versus age curves. We report newε_Nd data (in the range from 80 to 750 Ma) for the largest Phanerozoic and Late Precambrian paleocean, the Pacific/Panthalassa Ocean (PPO), and limited data on some small Paleozoic paleoceans. These new data and all other published data on paleoceanε_Nd values are used to determine the trends through time ofε_Nd in seven Phanerozoic paleoceans. The variations in paleoceanε_Nd curves are due to changes in the rate of crustal additions and in paleoceanic geometry. From these data we can calculate a meanε_Nd curve and a mean Nd model age curve for Phanerozoic seawater. These curves show large variations with age and reflect the variations ofε_Nd in erosion products from the continents as a function of age. The high erosion rates of young, uplifted areas enhanceε_Nd and Nd model age variations of Phanerozoic seawater. This makes it difficult to quantitatively recover crustal addition rates from these curves; however, it makes the episodic nature of continental additions more evident.