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Juan Carlos Silva Tamayo, University of Bern (Switzerland)
Thomas Naegler, University of Bern (Switzerland)
Afonso Nogueira, Universidade Federal do Para (Brazil)
Igor Villa, University of Bern (Switzerland)
Claudio Riccomini, Universidade de Sao Paulo (Brazil)
Alcides Sial, Universidade Federal de Pernambuco (Brazil)
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Recently, we reported significant variations in the Ca isotope stratigraphy between post-glacial carbonate successions following the ca 0.74 Ga Rapitian and the ca 0.64 Ga Marinoan global glaciations (Silva et al., 2007). On the basis of two Rapitian (NW Canada, NW Brazil) and one Marinoan (NW Canada) sequences, and of the comparison with the Marinoan sequence studied by Kasemann et al. (2005) in NW Namibia, we had proposed that the two deglaciation events occurred in very different ways. Therefore, Ca isotope stratigraphy can be a reliable tool to differentiate and correlate post-glacial Neoproterozoic carbonate successions.
Our new data on carbonate successions overlying 0.64 Ga glacial deposits in SW Brazil (Mirasol d'Oeste and Guia Fms, Araras Group) reproduce the stratigraphic Ca isotope variations displayed by the coeval carbonate successions in NW Canada and in Namibia. These results support the applicability of Ca isotope stratigraphy as a tool to correlate cap-carbonate successions worldwide. They further support our interpretation of widespread perturbations in the Ca isotope composition of the ocean in the aftermath of the Marinoan global glaciation. We propose a two-step post-Marinoan deglaciation process, and performed simple box modeling to explain the data. First, partial and rapid retreat of glaciers may have exposed pre-glacial carbonatic and siliciclastic shelves, which became the principal source of Ca to the ocean. This stage allowed the rapid (less than 100 ka) deposition of the basal postglacial cap dolostone successions. It was followed by the final retreat of glaciers, which resulted in an increased Ca influx to the postglacial ocean (up to 100 times today's influx). This increase produced a short-lived (ca. 40 ka) major perturbation in the oceans' Ca isotope composition, a pronounced negative Ca isotope excursion displayed by the lowermost part of the limestone successions overlying the basal cap dolostones. The stabilization of this transient Ca isotope signal according to our model requires ca 1 Ma. As the studied limestone succession only reach a steady-state Ca isotopic composition in its uppermost part, we argue that it was deposited in approximately 1 Ma. Such fast deposition is in agreement with the ca. 3 Ma U-Pb zircon age differences of ash beds interbedded with post-Marinoan carbonates in SE China: 635.2 ± 0.6 to 632.4 ± 1.3 Ma (Condon et al., 2005).
Silva, J.C et al., 2007. Geophys. Res. Abstr. 9, 01980. Kasemann S. et al., 2005. Earth Planet. Sci. Lett. 231, 73-86. Condon D. et al., 2005. Science 308, 95-98.
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