| Summary: | Reconstructions of overturning circulation from IODP Sites covering 40-65 Ma indicate a fundamentally different overturning mode (referred to herein as the Paleogene mode) compared to the modern. Significantly different global climate and plate tectonic boundary conditions during this time interval likely contributed to the different mode of overturning circulation in which each major basin was characterized by distinct and restricted overturning circulations. The modern mode more closely resembles one large continuous Global Overturning Circulation. Here, new Nd isotope data from IODP Site U1438 spanning 27 Ma to the present are used to investigate the timing of the transition from the Paleogene mode to the modern mode. Site U1438 is situated at 4720 m water depth and is ideal to record relative contributions of deep-water masses from the North Pacific and those sourced from the Pacific sector of the Southern Ocean (e.g., Antarctic Bottom Water). The Nd isotopic record collected at U1438 reflects that the shift from the Paleogene mode to the modern mode occurred by 14 Ma, coinciding with the large shift in climate from a relatively warm world to a gradually cooling world that was established by the end of the MMCO. However, the data suggest that at least some part (e.g., 14.8 Ma) of the water mass composition recorded at Site U1438 may have been overprinted by other contributions of dissolved Nd. Given its proximity to contemporaneous volcanic inputs, it is likely that relatively labile volcaniclastic materials partially dissolving at the seafloor and within pore waters potentially affected the εNd composition. This study highlights the need to thoroughly characterize the potential inputs of dissolved Nd to a given location before incorporating a new Nd isotope record into the reconstruction of global deep-water circulation.
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