Calcitized Evaporites in the Precambrian: Deposition and Diagenesis in a Low Sulfate Ocean

One of the most profound byproducts of the Great Oxidation Event (GOE) was the onset of oxidative weathering on land and the riverine delivery of sulfate to marine systems. The marine sulfur system therefore plays a critical role in reconstructing the oxygenation of the early Earth. Although the exact nature of marine oxygenation in aftermath of GOE remains uncertain, examination and modeling of marine sulfate suggest that sulfate concentrations remained below 2 millimolar (mM) until at least the middle Mesoproterozoic, and may have only reached about 10 mM in the latest Neoproterozoic. Such a low concentration of marine sulfate had dramatic effects on the deposition and diagenesis of marine sulfate deposits. Prior to the Mesoproterozoic, precipitation of sulfate evaporites was inhibited by the low concentration of marine sulfate and potentially by elevated carbonate saturation. The first indisputable gypsum-anhydrite deposits occur in a variety of fluvial, playa, and marine sabkha environments at 2.1 Giga annum (Ga), although much of the rock record is represented by relatively few pseudomorphs and molds. The first laterally extensive, marine sulfate evaporite deposits are found in the late Mesoproterozoic, where up to 60 meters of bedded marine gypsum and anhydrite occur within the Grenville Supergroup, and gypsum beds up to several meters thick occur in the lower Bylot Supergroup. Together, these occurrences indicate that, by the mid-Mesoproterozoic, marine sulfate concentrations were sufficient to sustain the first widespread evaporite deposition.

Unfortunately, our understanding of the dynamics of marine sulfate reservoir growth is hampered by the sparse preservation of marine evaporites. Here I present a series of unusual limestone deposits in the 1.1 Ga Atar Group Formation, Toudeni Basin, Mauritania, that contain both macroscopic and microscopic features suggestive of a primary origin as sulfate evaporites. Carbon- and oxygen-isotope compositions indicate a history of diagenesis similar to that of surrounding marine limestones. Elemental concentrations, however, show retention of elevated sodium and potassium, suggesting retention of original hypersaline composition. Models of potential fluid composition indicate deposition under low sulfate, high salinity conditions, with diagenetic calcitization under low water–rock ratios from a combination of fresh, marine, and hypersaline marine fluids.