Paper No. 76-12
Presentation Time: 10:45 AM
PROTEROZOIC OXYGENATIONS AND CARBON ISOTOPE EXCURSIONS DRIVEN BY COUPLED SURFACE-INTERIOR CARBON CYCLING (Invited Presentation)
The Neoproterozoic hosts some of Earth’s most dramatic steps in atmospheric oxygenation and attendant carbon isotope excursions, most notably the Neoproterozoic Oxygenation Event marked by a broad positive carbon isotope excursion followed by the Shuram negative carbon isotope excursion. It remains difficult to explain these phenomena within the traditional framework of the carbon cycle. Here, we present a novel carbon cycle model that accounts for the behavior of inorganic carbonate and graphitized organic carbon in the mantle. Specifically, high pressure experiments demonstrate that inorganic carbonate is more easily removed from subducted slabs at subarc conditions compared to graphitized organic carbon. In the model, we prescribe extreme weathering events, which significantly increase the burial flux of marine carbonate and organic carbon, thereby increasing atmospheric oxygen. An important feature of this model is that we keep forg constant at 0.20, therefore when the burial flux of carbonate and organic carbon increase, atmospheric oxygen increases with no expression in carbon isotopes of marine carbonates. Shortly after being subducted, subducted carbonates are liberated from the slab in fluids and melt, releasing CO2 at arc volcanoes. Conversely, graphitized organic carbon largely remains in the descending slab. This shifts global volcanic CO2 emissions to higher δ13C, resulting in an increase in δ13Ccarb. The δ13C of global CO2 emissions remains elevated until organic C is subducted deep into the mantle, entrained in upwelling mantle plumes, and ultimately emitted as CO2 at ocean island volcanoes. By the time the initial spike in subducted organic C is released at ocean island volcanoes several hundred million years later, the flux of carbonate released at arc volcanoes has decayed due its rapid response time to decreases in C subduction fluxes caused by decreased weathering. This sequence results in decreased δ13C of global CO2 emissions and the rapid transition from a positive carbon isotope excursion to an extreme negative carbon isotope excursion. While other possibilities exist, we demonstrate that the model presented here can reproduce the sequence of events observed in the Neoproterozoic, highlighting the importance of considering relationships between surface and interior processes when investigating the coevolution of life and geology on ancient Earth.