New Constraints on Archean–Paleoproterozoic Carbonate Chemistry and pCO2

Abstract:

Very few constraints exist on Archean and Proterozoic seawater chemistry, leaving huge uncertainties on the boundary conditions for the evolution of life and a habitable environment. Ancient carbonate chemistry, which is intimately related to oceanic pH and atmospheric pCO₂, remains particularly uncertain, despite its importance for understanding environments and temperatures on early Earth. Using a new application of high-precision calcium isotope measurements, we present data from the Tumbiana Formation (2.7 Ga, Western Australia), the Campbellrand Platform (2.6 Ga, South Africa) and the Pethei Group (1.9 Ga, Northwest Territories, Canada) that allow us to place constraints on carbonate chemistry both before and after the Great Oxidation Event. By analogy with calcium isotope behavior in sulfate minerals (Blättler and Higgins, 2014) and Mono Lake (Nielsen and DePaolo, 2013), we infer a lower limit on the ratio of calcium ions to carbonate alkalinity during deposition of these three sedimentary sequences. These data rule out the soda ocean hypothesis (Kempe and Degens, 1985) and make further predictions about the role of CO₂ in solving the faint young Sun problem.