CARBON RECYCLING AT SUBDUCTION ZONES

The seminal paper by Isacks, Oliver & Sykes (1968), “Seismology and the New Global Tectonics,” defined subduction zones with new tools in seismology, and predicted new lines of inquiry would emerge as a consequence. One such field is geochemical recycling, which emerged from the recognition that subduction zones are not just sites of plate convergence but regions where the interior and exterior of the Earth chemically exchange. Material from the surface of the earth sinks into the mantle, and some of this material returns to the surface via fluid generation in the downgoing plate, mantle wedge melting and arc volcanism. Such recycling has been illuminated with geochemical tracers, most notably trace metals and their isotopes. Critical volatile species (C-H-S) have been difficult to trace during subduction. Here we explore carbon cycling through subduction zones, which presents challenges due to its heterogeneous distribution on the seafloor as carbonate and organic carbon, and due to its deep exsolution into a gas phase during arc magmatism.

The subduction of sedimentary carbonate is rare today, due to the small fraction of subducting seafloor that has been above the calcite compensation depth. Recent in situ measurements of volcanic gas CO₂ and S, combined with decades of campaign measurements, has revealed higher CO₂/S gases emitted from volcanoes where carbonate sediment subducts, notably Central America, Columbia and New Zealand. Such gas signals represent among the first strong evidence for carbonate recycling at subduction zones. Seafloor sediments approach subduction zones with small amounts (generally < 1 wt%) of organic carbon (OC), but this small concentration nonetheless constitutes a significant flux over geological time with respect to the size, isotopic composition and electron balance of the exosphere. Sites with the greatest concentration of OC include those that receive terrigenous turbidites (e.g., Bengal Fan, Gulf of Alaska, etc.). We are using the mass balance of carbonate and OC to predict the isotopic composition of carbon subducting beneath each volcano along arc segments. Some regions will be dominated by subducting OC, and thus light carbon isotopes relative to the mantle. New carbon isotope data on volcanic gases will provide a new test for recycling of organic carbon at subduction zones.