OCEANIC STRATIFICATION AND MIXING IN THE AFTERMATH OF A NEOPROTEROZOIC GLACIATION: CARBON AND SULFUR ISOTOPE EVIDENCE FROM THE ZHAMOKETI CAP DOLOSTONE IN NORTHWEST CHINA

The Tereeken diamictite and overlying Zhamoketi cap dolostone in the Quruqtagh area, eastern Chinese Tianshan of northwest China, represent a diamictite-carbonate pair deposited in the aftermath of a Neoproterozoic ice age (<727 but >615 Ma) possibly equivalent to the Marinoan glaciation. We analyzed stable isotopes (δ¹³C, δ¹⁸O, δ³⁴S) and elemental concentrations (Ca, Mg, S, Sr, Fe, and Mn) of the ~10-m-thick Zhamoketi cap dolostone. The δ¹³C values reveals that organic and carbonate carbon isotope compositions are relatively stable, averaging at –28.2‰ and –4.6‰, respectively. The lower 2.5 m is characterized by high abundances of CAS (carbonate associated sulfate) with δ³⁴S_CAS compositions varying between +9‰ and +15‰. In the overlying interval, CAS abundance decreases rapidly while δ³⁴S_CAS values raise ~5‰ within an interval of peloidal dolomicrosparite. Sulfur isotopic compositions of pyrite extracted from the cap dolostone are largely indistinguishable from those of CAS, but direct comparison shows that pyrite is typically enriched in ³⁴S relative to trace sulfate. Hypotheses to explain the observations must account for both the highly positive δ³⁴S for pyrites and the apparent inverse sulfur isotope fractionation between sulfate and sulfide. We propose that CAS and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin. In this model, CAS was derived from shallow, oxidized surface waters with moderate sulfate concentration and depleted in ³⁴S due to the post-glacial influx of sulfur from continent weathering. In contrast, sulfate available for the formation of pyrite (either in pore fluids or the water column) was derived from anoxic bottom water with low sulfate concentration and enriched in δ³⁴S due to long-term bacterial sulfate reduction during the ice age. The rapid shift in CAS abundance and sulfur isotope compositions within the cap dolostone is interpreted to reflect the mixing of deep and shallow water reservoirs.