How Do Subduction Zones Regulate the Carbon Cycle?

10.1 Carbon Distribution on Earth The core, mantle, and crust contain more than 99% of Earth’s carbon stocks. The remaining 1% is in the fluid Earth, split between the biosphere, atmosphere, and oceans. But this distribution must be considered as a snapshot in time, not a fixed property of the Earth system. Continuous exchange of carbon between fluid (ocean, atmosphere, and biosphere) and solid Earth (mainly mantle and crust) has modified the size of the fluid and solid carbon reservoirs over geological time, regulating atmospheric composition and climate. The subduction zone, where converging tectonic plates sink below one another or collide, is the main pathway for this exchange. It will be the focus of this chapter. Geologists believe that a long-term shift in regime of subduction carbon cycling is underway. Following an ecological innovation – the evolution of open-ocean calcifiers (e.g. coccolithophores and foraminifera) in the Mesozoic, marine regression and other changes – it is thought that the accumulation of carbonates on the seafloor (pelagic) has increased over the Cenozoic to reach about 50–60% of the global rate today (Table 10.1). Most of the carbonate that has accumulated over the last 100 Myr has not subducted yet (Table 10.1) and should do so sometime in the coming hundreds of millions of years. But when this will happen is unknown because there is no direct link between the precipitation of carbon on the seafloor and the birth of a subduction zone. Irrespective of when it happens, because the fates of shelf and deep-sea carbon materials differ, it has been proposed that intensification of deep-ocean carbonate deposition may eventually affect the prevailing regime of geological carbon cycling. To understand the link between oceanic carbon deposition centers and modes of longterm carbon cycling, we need to consider the fate of sedimentary carbon. Shelf and oceanic island carbon mostly escapes subduction and is accreted to continents during continental subduction and collision. While a fraction of pelagic carbon can also be thrusted within accretionary wedges and accreted, most is bound to be subducted, dissolved, or molten at various depths (Figure 10.1) within the sinking plate, before being released in the forearc, arc, or back-arc regions, or mechanically incorporated deeper into the mantle. The contrasted fate distinguishes two principal modes of tectonic carbon cycling: the shallow accretionary carbon cycle and the relatively deeper subduction zone carbon cycle (Figure 10.1). What is not clear yet is how fast those cycles operate and how they interact.