Local and Regional Inputs of Lead in Pacific Deep Water Over the Past 20 Myr

Lead has a residence time in the oceans that is much shorter than the global mixing time. Therefore, the limited Pb isotope variability in Pacific deep water has been interpreted to reflect either a uniform supply and provenance of Pb over the last 20 Myrs or a very efficient thermocline mixing (von Blanckenburg & Igel, 1999). However, the history of Pacific deep water is based on time-series data for ferromanganese crusts that only come from equatorial regions. We present the first high-resolution Pb isotope time-series of ferromanganese crusts from the northern (Gulf of Alaska and NW-Pacific) and southern (Tasman Basin and Bauer Basin) Pacific Ocean, measured at high precision by MC-ICPMS. The crusts recorded deep and intermediate water isotope compositions over the past 10 to 20 Myrs. Dating was obtained from ¹⁰Be/⁹Be ratios calculated by combining AMS-¹⁰Be profiles and ICP-MS Be measurements obtained for the same aliquots. Ages beyond 10 Ma were extrapolated from the growth rates of the ¹⁰Be/⁹Be -dated sections. The two northern crusts (NW-Pacific and Gulf of Alaska) show variations in deep water ²⁰⁶Pb/²⁰⁴Pb over the past 10 Ma that exceed the amplitude of the so far known equatorial Pacific records about threefold. Moreover, these two northern records are strikingly similar, even though the crusts are separated by more than 3000 km along the Aleutian Arc and sources other than the Aleutian volcanics have probably contributed Pb to the deep North Pacific in the past. The two records provide powerful evidence that local inputs and sources dominate the Pb budgets in the northern Pacific Ocean. The ferromanganese crust from the Tasman Basin is in a perfect location to provide a monitor of the inflow of Circum Antarctic water masses into the Pacific Ocean. The whole profile shows higher ²⁰⁶Pb/²⁰⁴Pb values than the central Pacific time-series over the past 28 Myrs. The past 8 Myrs show comparable trends to the equatorial Pacific records, whereas the difference prior to 8 Ma is much larger. This is consistent with weaker water mass exchange and mixing between these regions prior to this time. Reference: von Blanckenburg, F. and Igel, H., Earth Planet. Sci. Lett. 169, 113 (1999)