Dating the 840-544 Ma Neoproterozoic interval by isotopes of strontium, carbon, and sulfur in seawater, and some interpretative models

M. R. Walter*, J. J. Veevers, C. R. Calver, P. Gorjan, A. C. Hill

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

346 Citations (Scopus)

Abstract

We construct a time scale for the 840-544 Ma Neoproterozoic interval from isotopic variation of δ13C(carbonate) and δ13C(organic), 87Sr/86Sr, and δ34S(sulfate) in seawater measured from reference columns in Canada and Australia. We distinguish 18 features (Z-I) in the δ13C(carbonate) and δ13C(organic) curves: two intervals of well-defined variation in 87Sr/86Sr; and two peaks in the variation of δ34S(sulfate). Newly acquired isotopic data in Australia enable correlation with Canada: the Gillen Member of the Bitter Springs Formation, estimated to be about 840 Ma, is correlated with the upper Shaler Supergroup; the Sturtian glacials, about 700 Ma, with the Rapitan glacials; and the Marinoan glacials, about 600 Ma, with the Ice Brook glacials. We recognize only these two major glaciations, and possibly a third minor glaciation, at 570 Ma. Columns in Poland, Namibia, Iran, and Siberia, and possibly Oman and Mongolia provide correlation by δ13C, and in Svalbard, Siberia, Oman, and Mali by 87Sr/86Sr. The inter-glacial (700-600 Ma) peak of δ34S(sulfide) enables correlation among Australia, Namibia, and China. Two icehouse states were preceded by massive sequestering of CO2 and accompanied by catastrophic declines in biological productivity. During and immediately after the older, Sturtian, glaciation, the deeper parts of the ocean were anoxic and contained sufficient ferrous iron to sequester very large amounts of sulfur derived from bacterial reduction of sulfate; the evidence suggests that the resultant huge shift in the sulfur isotropic composition was global and accompanied by the reduction of as much as half the sulfate in the anoxic parts of the oceans. The effects of the Pan-African orogeny include mountain building and a high rate of sedimentation, which resulted in the burial of large amounts of organic matter and concomitant oxygenation of the hydrosphere and atmosphere. This reinforced a trend in oxygenation that began before the second, Marinoan, glaciation. A second huge sulfur isotope anomaly accompanies the tectonism, and has been explained previously as possibly resulting from the desiccation and flushing of evolving ocean basins. This may be linked to a remarkable carbon isotope anomaly immediately preceding the Cambrian: the anomaly could be due to release of methane from oceanic clathrates de-stabilized by combined sea level fall and global warming resulting from volcanic release of CO2.

Original languageEnglish
Pages (from-to)371-433
Number of pages63
JournalPrecambrian Research
Volume100
Issue number1-3
DOIs
Publication statusPublished - Mar 2000

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