The Sources And Evolution Of Radiogenic Nd-Pb-Hf Isotopes In Central North Pacific Deep Water

The radiogenic Nd, Pb, and Hf isotopes have been demonstrated as powerful proxies to study paleocenography and paleoclimate over the last two decades. The oceanic residence time of these radiogenic isotopes are about several hundred years, which allows their long distance transport by ocean currents but prevents complete isotope homogenization in the ocean. Therefore, these radiogenic isotopes could be used to trace ocean currents as well as continental inputs. Nevertheless, in order to use radiogenic Nd-Pb-Hf isotopes to reliably reconstruct past ocean and climate conditions, it is essential to accurately understand the marine geochemical cycling of these elements. Despite the fact that many studies have been done on marine geochemical cycling of radiogenic Nd-Pb-Hf isotopes, there are still some fundamental issues that need to be further explored, including "Nd isotope paradox", the mechanism of formation of the seawater Nd-Hf isotope array, as well as the sources and transport of oceanic Pb in the Pacific. Based on the radiogenic isotope composition recorded in the central North Pacific ferromanganese crusts, the issues mentioned above will be investigated and new findings will be presented as well.In this study, Nd isotopic data of 17 surface scrapings of Fe-Mn crusts from central north Pacific deep water (around 20°N) have been reported. With recently published data on dissolved Nd isotopes of seawater in this region, it is proposed that radiogenic Nd isotopes of North Pacific Intermediate Water (NPIW 300-800m) from the western pacific margin is an important source for the central north Pacific deep water, while surface water of this area has little influence in dominating deep water Nd isotopes. Such a view emphasizes the role of vertical mixing and advection of the marginal ocean currents in balancing Nd isotopes of central north Pacific deep water, and helps to understand oceanic cycling of Nd as well as other elements.Moreover, to clarify the factors controlling Pb isotope evolution in this area over the Cenozoic, this study reports new Pb and Nd isotope time series of 4 ferromanganese crusts (2 from the western Pacific near the Mariana arc and 2 from the central Pacific). Together with previously published records we discuss for the first time the significance of a persistent and systematic Pb isotopic provinciality recorded by central North Pacific crusts over the Cenozoic We propose that globally well mixed stratosphere volcanic aerosols could contribute Pb but have not been the major factors controlling the Pb isotope distribution in the central North Pacific over time. Island arc input (and probably enhanced hydrothermal input between about 45 and 20 Ma) likely controlled the Pb isotope provinciality and evolution prior to ～20 Ma, when coeval Pb isotope records in different crusts showed large differences and atmospheric silicate dust flux was extremely low. After the Eocene, in particular after 20 Ma, Asian dust input has become an isotopically resolvable source, while island arc-derived Pb has remained important to balance the dust input and to produce the observed Pb isotope distribution in the central North Pacific during this period.Nevertheless, quantitative understanding on how Pb isotopes respond to the ocean surface input or other factors is still lacking. Given the strong particle reactivity of Pb and sluggish central Pacific thermolhaline circulation,1-D adsorption-diffusion-advection model which simulates the sensitivity of seawater Pb isotope composition responding to changing adsorption coefficient, external flux and deep water upwelling rate is presented. With reasonable variability of these parameters, it is concluded that Pb isotopes are only sensitive to the surface input and are not greatly influenced by advection from the Southern Ocean.Finally, due to the similarity of compatibility between elemental pair Sm-Nd and Lu-Hf in magmatic processes, Hf and Nd isotopic compositions of most terrestrial rocks display a strong positive correlation. Interestingly, the reported Nd-Hf isotope compositions of authigenic Fe-Mn precipitates as well as dissolved seawater have a different correlation with the terrestrial rocks in the εNd-εHf plot. During the processes of continental weathering and sediment transport, the effects of mineral sorting and different resistance of minerals to weathering (e.g., zircon is a host mineral of Hf and is strongly resistant to weathering) result in Hf isotope fractionation between the weathering product and the bulk rocks. Despite that many studies have suggested continental weathering inputs have contributed to the dissolved oceanic budget of Hf, whether the offset of seawater Nd-Hf isotope compositions from the terrestrial array can be fully generated by incongruent ziron effect of continental rocks (the zircon effect) is still not well constrained. In recent years, with an increasing amount of published combined U-Pb ages and Hf-isotopic compositions of riverine detrital zircons, a new model of the Nd-Hf isotopic compositions of the weathered zircon-free part of the upper continental crust is presented. The results of the model indicate that the Nd-Hf isotopic composition of the weathered zircon-free part of the upper continental crust is not consistent with the seawater isotopic compositions. Therefore the elevated seawater Hf isotope compositions for given Nd isotope compositions cannot be fully explained by incongruent zircon weathering on the continents. This is also supported by a recent study demonstrating the role of incongruent weathering of minerals other than zircon.