RAPID SULFUR ISOTOPE SHIFTS IN MESOPROTEROZOIC SUCCESSIONS AS RECORDED IN CARBONATE-ASSOCIATED SULFATE: IMPLICATIONS FOR SULFATE AVAILABILITY IN THE PRECAMBRIAN OCEAN

Throughout the Phanerozoic, the balance between oxidation and reduction of sulfur at the earth’s surface is expressed in broad temporal shifts in the sulfur isotope composition of seawater sulfate as recorded in abundant gypsum deposits. In the near absence of gypsum in pre-Phanerozoic sediments, trace amounts of carbonate-associated sulfate (CAS) in abiotic limestones and dolostones can be used to constrain the sulfur isotope composition of seawater and thus the redox conditions of the Precambrian ocean-atmosphere system.

Stratigraphically controlled CAS isotope measurements from carbonates of the Newland Formation (lower Belt Supergroup, Montana) range from +11.4 to +27.3 permil across 400 meters of section in the Helena Embayment and overlap with roughly coeval barite data thought to reflect seawater sulfate. This comparatively short-term variation and the analogous isotopic shifts observed in pyrite from shales of the lower Belt Supergroup suggest that sulfate reservoirs in the Belt basin and perhaps the world ocean were dynamic and limited relative to Phanerozoic seawater. Similarly, sulfur isotope data cycle between +3.6 to +15.1 permil over 400 meters of the Helena Formation (middle Belt Supergroup). These rapid isotopic shifts in the Belt basin are similar to the 10-12 permil variation observed over 300 meters of carbonate and gypsum from the Bylot Supergroup, northern Baffin Island. Collectively, these data are consistent with the sulfate-limited Mesoproterozoic ocean suggested by the paucity of gypsum deposits and the small isotopic offsets often observed between coeval sulfate and sulfide. Our ongoing study of the Mescal Limestone, Arizona, and the Paradise Creek Formation, Australia, is testing this hypothesis. Beyond the isotopic implications, present efforts include the relationships between CAS concentrations and paleoenvironmental conditions.