Decarbonation efficiency in modern subduction zones and the source of high Cretaceous pCO2

Arc volcanism at subduction zones plays a fundamental role in the long-term carbon cycle. In its simplest form, the long term carbon cycle is comprised of four major processes: CO₂ is supplied to the atmosphere by volcanic degassing in arcs, CO₂ is removed from the atmosphere through silicate weathering reactions, carbon is deposited in the oceans as calcium carbonate, and returned to the mantle or the atmosphere via subduction and subduction-related volcanoes, respectively. It has been hypothesized that arc-volcanic carbon output may have been particularly important in the Cretaceous Greenhouse, when subduction of the carbonate-rich Tethys ocean crust could have led to enhanced CO₂ outgassing rates and thus an overall warmer climate. Previous studies have suggested, however, that the devolatilization of subducting sediments and altered oceanic crust is critically dependent on the thermal structure of the downgoing slab [Kerrick and Connolly, 2001]: most subducting sediments are devolatilized in the forearc and do not contribute to subarc volcanism. We make a first attempt to compile a global modern inventory of 'decarbonation efficiencies' of subduction zones and apply our findings to the likely subduction zones in the Tethys. Similar to previous studies, our analysis indicates that the thermal structure of subduction zones controls the extent and depth of slab decarbonation, while the sediment geochemistry may be of secondary importance. The calculated decarbonation efficiency of modern arcs ranges from 18-70%. We highlight the importance of the composition and thickness of the overlying crust, and the importance of crustal contamination of magmas by platform carbonates in the Andes and Lesser Antilles. We suggest that over geological time, the proportion of subduction under thick continents (e.g. Andes) versus subduction under thin continents or ocean crust (e.g. Central America) may be critical for modulating long term CO₂ outgassing from volcanoes. This analysis allows us to make inferences about the potential volcanic CO₂ flux from subduction of the Tethys during the Cretaceous, suggesting between a 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, the subduction of highly 'decarbonation-efficient' crust, in addition to the increased subduction of carbonate-bearing sediments. A number of uncertainties remain, however, particularly regarding the quantification of accurate carbon outputs from arc volcanoes (during both eruptive and non-eruptive periods), and the importance of altered oceanic crust as a carbon source, which will require focused future studies.