Differing controls over the Cenozoic Pb and Nd isotope evolution of deepwater in the central North Pacific Ocean

Abstract

Dissolved Pb and Nd isotope ratios of seawater, as recorded by chemical marine precipitates such as hydrogenetic ferromanganese crusts, have been used as paleoenvironmental proxy tracers. For the North Pacific, however, all ferromanganese crusts studied so far have either been subject to phosphatization or hydrothermal influence in their old part, or only the young parts have been analyzed. Thus, the Pb and Nd isotope compositions of North Pacific deep waters prior to ∼20 Ma are not well constrained. We present new results for three ferromanganese crusts, one of which (CJ01) shows no phosphatization and is located far away from the EPR. Its age is inferred to be ∼75 Ma and thus provides for the first time an opportunity to trace the Nd and Pb isotope evolution of central North Pacific seawater back to the latest Cretaceous.The three crusts, no matter whether phosphatized or not, display very similar Pb and Nd isotope trends with age, suggesting no modification of the Pb and Nd isotope distribution by post-depositional phosphatization. Our data suggest that dissolved Pb in deep waters of the central North Pacific over the Cenozoic and latest Cretaceous has mainly been derived from eolian dust and only to a minor extent from weathering of island arcs. For Pb these trends broadly resemble the Pb isotope evolution of the eolian silicate dust fraction of core LL44-GPC3 in the central North Pacific. We suggest that the isotope evolution of dissolved Pb in central North Pacific seawater has been mainly controlled by Pb released from eolian dust from North America prior to 50 Ma and after 40 Ma from Asia. In contrast, the Nd isotope time series of the crusts are by no means similar to the Nd isotope evolution of the silicate dust fraction in core GPC3, suggesting a decoupling from the Pb and negligible contributions from dust to the dissolved Nd in the central North Pacific deep water. The rise of Nd isotope ratios of Pacific seawater during the Cenozoic has most likely been caused by the increasing volcanic activity and erosion of the volcanic arcs around the Pacific.