『Abstract
Hydrogenous ferromanganese crust deposits on seafloor and volcanic
seamounts are archives of past isotopic compositions of the ocean.
Isotopic compositions of long-life radiogenic isotopes that are
dissolved in the ocean such as Nd and Pb, are mixtures of different
erosion products (continents, volcanics arcs, ridges etc...) transported
by oceanic currents or winds. Isotopic studies on Mn crusts are
thus potential paleogeographic and paleoceanographic tracers.
However, global paleogeography can drastically evolve over the
period of growth of crust, as well as the latitude, longitude
and water depth of the Mn crust. Thus each crust sampled a succession
of geographic sites related to its spatial evolution through time.
For this reason, valid information about the past ocean isotopic
composition can only be extracted from Mn crust under the condition
that the successive paleogeographic and paleodepth positions can
be reconstructed for each Mn crust.
We followed this approach for six hydrogenous Mn crusts from
the Pacific Ocean that have recorded Pb and Nd isotopic signals
for the past 80 Myr. The evolution of the isotopic data was considered
within a proper geodynamical framework that takes into account
the paleopositions of the Pacific plate and surrounding continents,
of the hot spots and ridges as well as the paths of the six crusts.
The differences and similarities between the Pb and Nd revealed
by our analysis emphasize their complementarity as paleoceanographic
tracers. The Nd isotopes are strongly influenced by the exchange
between Indian and Pacific waters, specifically by the progressive
closure of the Indonesian Gateway. The closed central Pacific
characteristics are defined from the extrapolation of the isotopic
trend. This highlights the important role played by erosion of
volcanic arcs in the Nd isotopic value of seawater. Our approach
also stresses the influence of local volcanic activity (ridges
or active hot spot) for lead isotopes.
Keywords: Pb; Nd; radiogenic isotopes; Indian Ocean; Pacific Ocean;
Indonesian Gateway; seawater; paleogeography; manganese crust;
hot spots』
1. Introduction
2. Summary of the samples and isotope data
2.1. The Pacific ferromanganese crusts
2.2. Isotopic data
2.2.1. Lead isotopes
2.2.2. Nd isotopes
3. Paleogeographic evolution of Fe-Mn crusts environment
4. Factors controlling the Nd and Pb isotopic variations
4.1. Ocean water stratification with depth
4.2. Neodymium isotopes and paleoceanography
4.3. Lead isotopes and paleoceanography
5. Complementarity between lead and neodymium isotopic messages
Acknowledgements
Appendix A. Supplementary data
References