Decarbonation efficiency in subduction zones: Implications for warm Cretaceous climates

Subduction zones play a fundamental role in the geochemical cycle of carbon, and related arc volcanism is believed to exert primary control on atmospheric CO2 concentrations over geological time. Arc volcanism may have been particularly important in the most recent Greenhouse of the late Cretaceous, where it has been hypothesized that the subduction of the carbonate-rich Tethys contributed to overall higher volcanic CO2 outgassing rates and thus a warmer climate. To test this hypothesis, the decarbonation efficiencies of modern subduction zones were calculated through a geochemical database that compared subaerial arc CO2 fluxes with the subducting crust and sediment geochemistry. The modern data are used to postulate a CO2 recycling and degassing scenario for arc volcanism related to the closure of the Tethys. Our analysis indicates that the thermal structure of subduction zones controls the extent and depth of slab decarbonation, while the sediment geochemistry (e.g. the amount of carbonate sediment) may be of secondary importance. The calculated decarbonation efficiency of modern arcs ranges from 18 to 70%. Our calculations support recent models predicting carbon recycling through infiltration-driven decarbonation, and limited by water availability at sub-arc depths. This analysis allows us to make inferences about the potential volcanic CO2 flux from subduction of the Tethys during the Cretaceous, suggesting between an 8 and 222% increase over modern CO2 outgassing. We suggest that the primary reason for the increase in CO2 outgassing in the Cretaceous is contamination of arc magmas by platform carbonates in the overlying crust and increased decarbonation efficiency.