CALCITIZED EVAPORITES AND THE EVOLUTION OF EARTH’S EARLY BIOSPHERE

Mineral assemblages within evaporite deposits provide critical constraint on the chemistry of parent waters. Evaporite deposits have thus been critical in placing constraints on both the chemical composition of ancient seawater and the long-term geochemical evolution of the Phanerozoic oceans (cf. Horita et al., 2002). Our understanding of the evolution of Proterozoic oceans, however, is severely limited by the paucity of marine evaporite deposits. Sulfate evaporites, for instance, appear in the aftermath of the Great Oxygenation Event, but remain severely limited until the late Mesoproterozoic, where laterally extensive, marine evaporites occur in the ~1.1 Ga Society Cliffs Formation, Bylot Supergroup, and the similarly aged Grenville Supergroup. The limited record of gypsum deposition during the Precambrian likely reflects a combination of globally low atmospheric oxygen levels, low concentrations of marine sulfate, extensive biological sulfate reduction in shallow marine environments, and the poor preservation potential of highly soluble evaporite phases.

By the late Mesoproterozic, however, biospheric oxygen appears to have been sufficient to provide sustained input of sulfate to shallow marine evaporative environments. Many shallow marine environments, however, contain geochemical evidence that suggests sulfate depleted conditions. Here we explore the composition of distinct, white limestone beds within peritidal strata of the ~1.1 Ga Atar Group, Mauritania, that contain nodules, ptygmatic folding, microkarstic surfaces, and dissolution breccias reminiscent of sulfate evaporites. Positive identification of the Atar Formation facies as calcitized evaporites would aid in the evaluation of marine sulfate levels and would help constrain geochemistry and processes of diagenesis within shallow marine depositional environments. We present petrographic data to diagnose relict features and textures indicative of gypsum or halite precursors. We also provide an assessment of elemental and isotopic compositions to help constrain environments of deposition and diagenesis. Interpretation of these data sets will be used to reconstruct depositional and diagenetic fluids and explore the behavior of sulfate in shallow marine environments during this critical period of Earth history.