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

We construct a time scale for the 840-544 Ma Neoproterozoic interval from i sotopic variation of delta(13)C(carbonate) and delta(13)C(organic) Sr-87/Sr -86, and delta(34)S(sulfate) in seawater measured from reference columns in Canada and Australia. We distinguish IS features (Z-I) in the delta(13)C(c arbonate) and delta(13)C(organic) curves. two intervals of well-defined var iation in Sr-87/Sr-86; and two peaks in the variation of delta(34)S(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 glacia ls, about 700 Ma, with the Rapitan glacials; and the Marinoan glacials, abo ut 600 Ma, with the Ice Brook glacials. We recognize only these two major g laciations, and possibly a third minor glaciation, at 570 Ma. Columns in Po land, Namibia, Iran, and Siberia, and possibly Oman and Mongolia provide co rrelation by delta(13)C, and in Svalbard, Siberia, Oman, and Mall by 87Sr/8 6Sr. The inter-glacial (700-600 Ma) peak of delta(34)S(sulfide) enables cor relation among Australia, Namibia, and China. These correlations allow cali bration of the resultant stratigraphy against time using the best available dates from a number of regions; we make simple linear interpolations betwe en those dates. While recognizing that this can be no more than a rough app roximation of the true ages away from the calibration points, the resultant age estimates have the merit of suggesting numerous tests of our stratigra phic scheme. The Neoproterozoic part of the 87Sr/86Sr curve resembles in ra nge the Phanerozoic part, but those of delta(13)C and delta(34)S(sulfate) d o not: the 20.5 parts per thousand amplitude of delta(13)C and 26.5 parts p er thousand of delta(34)S greatly exceed the Phanerozoic 7.5 and 17 parts p er thousand, reflecting radically reduced net carbon and sulfur fluxes in y ounger times. The earliest Phanerozoic explosion of organisms with carbonat e skeletons and the proliferation of bioturbating organisms are coincident with the onset of a C-cycle with isotopic fluctuations damped in both frequ ency and amplitude. The Neoproterozoic glaciations at 700 and 600 Ma are ma rked by negative delta(13)C(carbonate) and correspondingly depleted delta(1 3)C(organic) and lower 87Sr/86Sr. The minimum Sr-87/Sr-86 at 840 Ma reflect s the input to an ephemeral epeiric sea of excess Sr-86 from extensive mafi c 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 wea lth of new data and the time framework they suggest allow us to build on th e work of earlier authors and make a fresh attempt at explaining some of th e major features of Neoproterozoic history, particularly in the interval fr om 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 feat ures we have documented. Two icehouse states were preceded by massive seque stering of CO, and accompanied by catastrophic declines in biological produ ctivity. During and immediately after the older,Sturtian. glaciation, the deeper parts of the ocean were anoxic and contained suffici ent ferrous iron to sequester very large amounts of sulfur derived From bac terial 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 hi gh rate of sedimentation, which resulted in the burial of large amounts of organic matter and concomitant oxygenation of the hydrosphere and atmospher e. This reinforced a trend in oxygenation that began before the second, Mar inoan, glaciation. A second huge sulfur isotope anomaly accompanies the tec tonism, and has been explained previously as possibly resulting from the de siccation and hushing of evolving ocean basins. This may be linked to a rem arkable carbon isotope anomaly immediately preceding the Cambrian: the anom aly could be due to release of methane from oceanic clathrates de-stabilize d by combined sea level fall and global warming resulting from volcanic rel ease of CO2. (C) 2000 Elsevier Science B.V. All rights reserved.