Trace sulfate in mid-Proterozoic carbonates and the sulfur isotope record of biospheric evolution

Concentrations of oceanic and atmospheric oxygen have varied over geologic time as a function of sulfur and carbon cycling at or near the Earth's surface. This balance is expressed in the sulfur isotope composition of seawater sulfate. Given the near absence of gypsum in pre-Phanerozoic sediments, trace amounts of carbonate-associated sulfate (CAS) within limestones or dolostones provide the best available constraints on the isotopic composition of sulfate in Precambrian seawater. Although absolute CAS concentrations, which range from those below detection to similar to 120 ppm sulfate in this study, may be compromised by diagenesis, the sulfur isotope compositions can be buffered sufficiently to retain primary values. Stratigraphically controlled delta S-34 measurements for CAS from three mid-Proterozoic carbonate successions (similar to 1.2 Ga Mescal Limestone, Apache Group, Arizona, USA; similar to 1.45-1.47 Ga Helena and Newland formations, Belt Supergroup, Montana, USA; and similar to 1.65 Ga Paradise Creek Formation, McNamara Group, NW Queensland, Australia) show large isotopic variability (+9.1 parts per thousand\ to + 18.9 parts per thousand, -1.1 parts per thousand to + 27.3 parts per thousand, and + 14.1 parts per thousand to +37.3 parts per thousand, respectively) over stratigraphic intervals of similar to 50 to 450 m. This rapid variability, ranging from scattered to highly systematic, and overall low CAS abundances can be linked to sulfate concentrations in the mid-Proterozoic ocean that were substantially lower than those of the Phanerozoic but higher than values inferred for the Archean. Results from the Belt Supergroup specifically corroborate previous arguments for seawater contributions to the basin. Limited sulfate availability that tracks the oxygenation history of the early atmosphere is also consistent with the possibility of extensive deep-ocean sulfate reduction, the scarcity of bedded gypsum, and the stratigraphic delta S-34 trends and S-34 enrichments commonly observed for iron sulfides of mid-Proterozoic age. Copyright (c) 2005 Elsevier Ltd.