SULFUR ISOTOPE EVIDENCE FOR RAPID CEMENTATION OF THE NEOPROTEROZOIC ZHAMOKETI CAP CARBONATE IN THE QURUQTAGH AREA OF TIANSHAN

Cap carbonates ubiquitously overlie Marinoan-age diamictites and they show distinct sedimentary and geochemical features. In the Quruqtagh area of eastern Chinese Tianshan, three Neoproterozoic glaciations have been recognized; these are represented by, in ascending order, the Bayisi, Tereeken, and Hankalchough diamictites. The Tereeken diamictite shows unambiguous glaciation features, and is capped by a ~10 m thick carbonate of the Zhamoketi Formation. Sedimentary structures, carbon isotope signatures, as well as available geochronometric constraints are suggestive of a Marinoan age for the Tereeken glaciation. In this study, we analyzed sulfur isotopes of carbonate associated sulfate (CAS) and disseminated pyrite extracted from the Zhamoketi cap dolostone. d³⁴S_sulfate values range from 10‰ to 20‰ (VCDT). d³⁴S_sulfate values are similar, ranging from 11‰ to 18‰. D³⁴S (d³⁴S_sulfate – d³⁴S_pyrite) values are less than 8‰, and mostly less than 5‰. Various tests exclude the possibility that d³⁴S_sulfate values are contaminated by pyrite oxidation during laboratory preparation or weathering, nor could sulfate be the product of post-depositional oxidation of pyrite, because sulfate is much more abundance than pyrite. Thus, the small difference between sulfate and pyrite appears a real signal. However, this small difference does not appear to be related to ocean-wide sulfate limitation, because the Zhamoketi cap dolostone contains 80-800 ppm sulfate. CAS concentration shows a decreasing stratigraphic trend, suggesting that sulfate was removed from the ocean during the deposition of the Zhamoketi cap carbonate. We suggest that the fast sedimentation rate of the cap carbonate and fast removal of sulfate from the Quruqtagh ocean may have partially contributed to the little difference between d³⁴S_sulfate and d³⁴S_pyrite values of the Zhamoketi cap carbonate. In addition, the rapid rain of fine-grained carbonate and the virtually instantaneous cementation of the cap carbonate shut off all connections between pore water and sea water in the glacial aftermath. Thus, CAS was mostly derived from sea water sulfate, but pyrite from pore water sulfate through quantitative sulfate reduction.