THE EVOLUTION OF THE MARINE CARBONATE FACTORY (Invited Presentation)

Formation of calcium carbonate (CaCO₃) is the primary pathway by which carbon is returned from the ocean-atmosphere system to the solid Earth. On long timescales, waxing and waning of the carbonate reservoir reflects changes in carbon inputs to Earth’s surface environments from CO₂ degassing and carbon removal tied to weathering. The removal of carbon dissolved in the oceans by precipitation of carbonate minerals—the marine carbonate factory—plays a critical role in shaping marine biogeochemical cycling and contributes to modulation of atmospheric CO₂ concentrations and climate. A paucity of empirical constraints on the scale and pace of this process has led to widely divergent views on how the marine carbonate factory has changed through time. Here, we use a recently developed geochemical system—stable strontium isotopes—to provide novel perspectives on the evolution of the marine carbonate factory and carbonate mineral saturation states. We report the stable strontium isotope ratios (δ^88/86Sr) for Precambrian calcites spanning from 3.0 to 0.65 Ga and present a new view of the evolution of the marine carbonate factory and seawater carbonate saturation state. We suggest that other processes—such as porewater production of authigenic carbonates—must have represented a major carbon burial sink through much of this interval. Further, we provide evidence indicating that the rise of the skeletal carbonate factory significantly decreased marine carbonate saturation states.