Constraints on Meso- to Neoproterozoic seawater from ancient evaporite deposits

Marine sulfate (SO₄ ²⁻) is intimately connected to the global carbon and oxygen cycles through its important role as an electron acceptor for the microbial respiration of organic carbon. The biogeochemical feedbacks within the sulfur, carbon, and oxygen cycles may have changed through time, reflecting changes in the concentration of sulfate in the oceans. Unfortunately, there is much uncertainty about the size of the marine sulfate reservoir throughout Earth history. In particular, conflicting estimates for marine sulfate exist during the latest Neoproterozoic, an interval of time associated with striking changes in Earth system evolution and oxidation: published interpretations of fluid inclusion chemistry place sulfate greater than 16 mmol/kg, whereas other interpretations of carbonate-associated sulfate data suggest concentrations less than 2 mmol/kg. Calcium isotope ratios in evaporite successions provide an independent method for deriving semi-quantitative constraints on sulfate concentrations, as well as other properties of seawater chemistry. Here, the calcium isotope behavior of bedded sulfate evaporites from ∼1050 Ma (Baffin and Bylot Islands, Nunavut, Canada), ∼830 Ma (Officer Basin, Western Australia), and ∼545 Ma (South Oman Salt Basin, Sultanate of Oman) are examined. In combination with other geological observations, the results suggest relatively low, millimolal-level sulfate in the latest Mesoproterozoic and a more specific range of 6–10 mmol/kg sulfate during the latest Neoproterozoic. These new constraints suggest that previous interpretations of sulfate concentrations and seawater chemistry need to be revised, opening up new possible solution spaces for the major ion composition of seawater.