60 Myr records of major elements and Pb-Nd isotopes from hydrogenous ferromanganese crusts: Reconstruction of seawater paleochemistry

Abstract

The time series of major element geochemical and Pb- and Ni-isotopic composition obtained for seven hydrogenous ferromanganese crusts from the Atlantic, Indian, and Pacific Oceans which cover the last 60 Myr are compared. Average crust growth rates and age-depth relationships were determined directly for the last about 10 Myr using sup(10)Be/ sup(9)Be profiles. In the absence of other information these were extrapolated to the base of the crusts assuming constant growth rates and constant initial sup(10)Be/ sup(9)Be ratios due to the lack of additional information. Co contents have also been used previously to estimate growth rates in Co-rich Pacific and Atlantic seamount crusts (Puteanus and Halbach, 1988). A comparison of sup(10)Be/ sup(9)Be- and Co-based dating of three Co-rich crusts supports the validity of this approach and confirms the earlier chronologies derived from extrapolated sup(10)Be/ sup(9)Be-based growth rates back to 60 Ma. The data show that the flux of Co into Co-poor crusts has been considerably lower. The distribution of Pb and Nd isotopes in the deep oceans over the last 60 Myr are expected to be controlled by two main factors: (a) variations of oceanic mixing patterns and flow paths of water masses with distinct isotopic signatures related to major paleogeographic changes and (b) variability of supply rates or provenance of detrital material delivered to the ocean, linked to climate change (glaciations) or major tectonic uplift. In this study Co is the only element which shows a relationship to Pb and Nd isotopes in Pacific crust. A possible link to changes of Pacific deep water properties associated with an enhanced northward advection of Antarctic bottom water from about 14 Ma is consistent with the Pb but not with the Nd isotopic results. The self-consistent profiles of the Pb and Nd isotopes suggest that postdepositional diagenetic processes in hydrogenous crusts, including phosphatization events, have been insignificant for particle reactive elements such as Pb, Be, and Nd. Isotope time series of Pb and Nd show no systematic relationships with major element contents of the crusts, which supports their use as tracers of paleo-seawater isotopic composition