Abstract

The cycling of carbon between the deep Earth and the atmosphere plays a significant role in modulating global climate. While carbon degassing at volcanic arcs are generally assumed to be the main contributor to atmospheric carbon, our research reveals that mid-ocean ridge degassing surpasses arc emissions before 50 Ma. Combining thermodynamic modelling of subducting carbon reservoirs with reconstructions of remobilised crustal carbon through the Phanerozoic, we show that volcanic arc emissions reduce to merely ~12% of mid-ocean ridge outflux before 120 Ma. This reflects the absence of deep-sea carbonate sediments entering subduction zones, leaving less voluminous metamorphic degassing of continental carbonate platforms next to convergent margins as the main mechanism for arc CO2 degassing. We find that the balance between oceanic volcanic outgassing and removal of carbon via subduction tracks major climate shifts, including the Late Palaeozoic, Late Cretaceous and Cenozoic icehouse climates.