A high-resolution Os-Nd-Pb isotopic record of a hydrogenous Fe[sbnd]Mn crust from the Northwestern Pacific and its response to plate motion

Abstract

Ferromanganese crusts are ideal archives of the isotopic composition of ambient deep waters. The cause of the long-term variations of Nd and Pb in the North Pacific deep water has not been well constrained because of a lack of reliable dating method. In this study, we documented the long-term Sr-Os-Nd-Pb isotopic evolution and major elements chemistry of a ferromanganese crust that grew in Northwestern Pacific deep water. The crust was dated using Os isotopic stratigraphy and has a maximum age of 86 Ma. In the early Cenozoic, εNd values increased from −5.3 to −4.0, and the Pb isotope ratios decreased, which can be explained by an input from the margin of South America that contributed both Nd and Pb to the Pacific deep water. The synchronous variations of Nd and Pb isotopic compositions were relate to the reduction of the source. Since Miocene, εNd value trended to more radiogenic values of −4.8 to −3.3 with oscillation owe to the decreased and pulsed input of the southern component water into the Pacific caused by the closure of the Indonesian seaway. The Pb isotope ratios increased after 35 Ma but remained lower than those of central North Pacific crusts. The Pb isotopic compositions did not respond to the one order of magnitude increase in dust flux and increase of Al content along with Pb isotope ratios since 10 Ma, which support that the arc-derived Pb dominated the Pb isotopic signature as the crust moved from the southern to northern Pacific, although Asian dust may have contributed to the Pb budget.