CONSTRAINING PERTUBATIONS TO THE SULFUR CYCLE IN THE AFTERMATH OF SNOWBALL EARTH: GEOCHEMISTRY OF THE NEOPROTEROZOIC MAIEBERG FORMATION, NAMIBIA
In the locality studied, the Maieberg Formation contains approximately 30 meters of cap dolostone (Keilberg Member) followed by 95 meters of limestone and then 80 meters of dolostone. The transition from cap dolostone to limestone attends a sharp increase in siliciclastic input and Fe3+ phases, negative shifts in both d13C and d18O, and a 16 per mil decrease in d34Ssulfate from 31 to 15 per mil. d34Ssulfate then increases gradually through the limestone before rising abruptly to >40 per mil in the overlying dolostone. The rise in d34Ssulfate through the limestone correlates with a rise in Fe2+.
We speculate that the large shifts in d34Ssulfate reflect a complex interplay between riverine inputs, oceanic upwelling and the post-glacial transgression of an ocean with low sulfate concentrations enriched in 34S. The enriched d34Ssulfate values in both the cap and upper dolostones are consistent with the snowball Earth hypothesis in that they appear to reflect nearly complete reduction of sulfate in an anoxic global ocean. We interpret the abrupt decrease in d34Ssulfate, together with increases in siliciclastics and Fe3+ phases, as recording a dramatic increase in 34S-depleted continental run-off to an ocean with low sulfate concentrations.