Variations in Sr and C isotopes and Ce anomalies in successions from China: evidence for the oxygenation of Neoproterozoic seawater?

samples of marine carbonates, phosphatic fossils, phosphorites and manganes e deposits were systematically collected from the Neoproterozoic Sinian Sys tem in the Yangtze Region, South China, and data of C and Sr isotopes and C e-anom values were obtained and investigated. These data, combined with Nd isotopic data (Yang et al. 1997a, Chem. Geol. 135, 127-133; Yang et al. 199 7b, Chinese Sci. Bull. 42, 2072-2075), provide a detailed record of isotope s and Ce anomaly variations in Chinese seawater during the Sinian and early Cambrian periods, and show significant changes: delta(13)C: -4 similar to + 6 parts per thousand; epsilon(Nd)(T): -4.5 similar to -7.6; Sr-87/Sr-86: 0.7077 similar to 0.7087; Ce-anom: + 0.20 similar to -0.65, respectively. T he changes exceed or match the largest in seawater throughout the Phanerozo ic. The generally high values of delta(13)C during the Sinian may reflect an in crease in the C-org burial rate. It, in turn, may be attributed to high pro ductivity at the surface of the ocean and a rise in the sedimentary rate, w hich resulted from enhanced tectonism and a global temperature rise. A tren d of declining Ce-anom values may also reflect an increase in organic produ ctivity at the ocean surface and a rise in O-2 concentration in the atmosph ere and the upper oceans. The most important point revealed by the Nd and S r isotopic studies is that the strongest and most widespread tectonism took place towards the end of the Proterozoic. The tectonism caused a paleogeog raphic reordering, rapid rise in the global sedimentary rate and great chan ges in ocean chemistry, and ultimately led to the variations in C, Sr and N d isotopes and Ce anomaly in the Sinian seawater. (C) 1999 Elsevier Science B.V. All rights reserved.