METHANOTROPHY, AUTHIGENIC CARBONATE, AND THE NEOPROTEROZOIC CARBON CYCLE

The Neoproterozoic carbonate sediments record some of the largest carbon isotope excursions in the geologic record. The cause of these oscillations has remained controversial, with many theories invoking large changes in organic carbon burial or large swings in atmospheric oxygen. More recently, Schrag et al. (2013) suggested that precipitation of authigenic carbonate minerals could drive changes in the d13C of the ocean, and of the primary marine carbonates that comprise the bulk of the d13c record. Authigenic precipitation is typically associated with sulfate and iron reduction, leading to isotopically depleted authigenic carbonates that have inherited the low d13C of remineralized organic carbon. In sulfate-poor sediments, low rates of methanotrophy may allow isotopically enriched CO2, the result of methanogenesis at depth, to diffuse into the highly alkaline zone associated with iron reduction. This leads to isotopically enriched authigenic carbonate. Using a diffusion-reaction model, we show that oscillations in the sulfate level of seawater can lead to rapid changes in the isotopic composition of authigenic carbonates, leading to potentially large, rapid changes in the d13C of seawater. Low sulfate concentrations in the Neoproterozoic ocean may help explain why this period was particularly sensitive to such oscillations.