Plate tectonics, radiogenic isotopic tracers and paleoceanography - The case of the manganese crusts in the Pacific

摘要

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.