Global Ca isotope variations in ca. 0.7 Ga old postglacial carbonate successions

• Published in: Terra Nova
• Main Authors: Silva Tamayo, JC, Nägler, TF, VILLA, IGOR MARIA, Kyser, K, Vieira, LC, Sial, AN, Narbonne, GM, James, NP
• Other Authors: Silva Tamayo, J, Nägler, T, Villa, I, Vieira, L, Sial, A, Narbonne, G, James, N
• Format: Article in Journal/Newspaper
• Language: English
• Published: Blackwell Scientific Publications 2010
• Subjects: Ca isotopes; cap carbonates; global glaciation; pre-Marinoan; Neoproterozoic; CO2 drawdown; GEO/08 - GEOCHIMICA E VULCANOLOGIA; Canada; glacier*
• Online Access: http://hdl.handle.net/10281/18246; https://doi.org/10.1111/j.1365-3121.2010.00933.x

To discriminate and correlate Neoproterozoic post-glacial carbonate successions, isotopic stratigraphy has been attempted in recent years. We report here the development of a new, promising discriminant, the 44Ca/40Ca isotope ratio. Two well-preserved carbonate successions overlying c. 0.7 Ga old glaciogenic deposits in central Brazil and NW Canada display similar Ca isotope time series. The Ca isotope stratigraphic profiles of these are similar to that of a weakly metamorphosed c. 0.7 Ga old carbonate succession in NE Brazil, suggesting robust preservation of the original Ca isotopic compositions. The Ca isotopic record is a reliable archive of changes in the oceanic Ca isotopic composition. It suggests rapid glacier melting and large increase in the Ca input to the ocean immediately after deglaciation, followed by progressive increase in carbonate precipitation, balancing the large initial Ca input. The global δ44Ca pattern recorded by c. 0.7 Ga old cap carbonates monotonically increases from ∼0‰ to 1‰, very different from the oscillation between ∼0‰ and 2‰ of the c. 0.64 Ga old global glaciation. This implies a difference in Ca mass balance evolution amongst the two deglaciation events. A possible explanation is that the high pCO2 built up during the first glaciation was consumed quickly by silicate weathering, while after the subsequent glaciation, the recently formed cap carbonates were redissolved first, followed only later by silicate weathering. The different Ca isotopic evolutions support Ca isotope stratigraphy as a reliable tool to discriminate amongst Neoproterozoic post-glacial carbonate successions.