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

Abstract

We construct a time scale for the 840–544 Ma Neoproterozoic interval from isotopic variation of δ 13C carbonate and δ 13C organic, 87Sr/ 86Sr, and δ 34S sulfate in seawater measured from reference columns in Canada and Australia. We distinguish 18 features (Z–I) in the δ 13C carbonate and δ 13C organic curves: two intervals of well-defined variation in 87Sr/ 86Sr; and two peaks in the variation of δ 34S 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 δ 13C, and in Svalbard, Siberia, Oman, and Mali by 87Sr/ 86Sr. The inter-glacial (700–600 Ma) peak of δ 34S sulfide enables correlation among Australia, Namibia, and China. These correlations allow calibration of the resultant stratigraphy against time using the best available dates from a number of regions; we make simple linear interpolations between those dates. While recognizing that this can be no more than a rough approximation of the true ages away from the calibration points, the resultant age estimates have the merit of suggesting numerous tests of our stratigraphic scheme. The Neoproterozoic part of the 87Sr/ 86Sr curve resembles in range the Phanerozoic part, but of δ 13C and δ 34S sulfate do not: the 20.5‰ amplitude of δ 13C and 26.5‰ of δ 34S greatly exceed the Phanerozoic 7.5 and 17‰, reflecting radically reduced net carbon and sulfur fluxes in younger times. The earliest Phanerozoic explosion of organisms with carbonate skeletons and the proliferation of bioturbating organisms are coincident with the onset of a C-cycle with isotopic fluctuations damped in both frequency and amplitude. The Neoproterozoic glaciations at 700 and 600 Ma are marked by negative δ 13C carbonate and correspondingly depleted δ 13C organic and lower 87Sr/ 86Sr. The minimum 87Sr/ 86Sr at 840 Ma reflects the input to an ephemeral epeiric sea of excess 86Sr from extensive mafic volcanics in Australia and possibly Canada. The onset of higher values at 600 Ma corresponds to the early Pan-African and Cadomian amalgamation, and internal deformation, uplift, and erosion in Antarctica–Australia. The wealth of new data and the time framework they suggest allow us to build on the work of earlier authors and make a fresh attempt at explaining some of the major features of Neoproterozoic history, particularly in the interval from 700 Ma to the base of the Cambrian. Quantitative modeling is beyond the scope of our study, but we offer explanations for some of the isotopic features we have documented. Two icehouse states were preceded by massive sequestering of CO 2 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 CO 2.