Carbon conservation in continental subduction zones revealed by zinc isotopes in ultrahigh-pressure marbles

Transfer of carbon from Earth’s surface into the interior via subduction regulates atmospheric CO2 concentrations over geologic time. The transfer efficiency of carbon in continental subduction zones, however, is less known than that in oceanic subduction zones. Zinc isotopes are a sensitive proxy of carbonate dissolution, with C-bearing fluids preferentially incorporating heavy zinc. We investigate zinc isotopes in ultrahigh-pressure marbles from the Dabie-Sulu orogenic belt formed via continental subduction. Coupled declines of zinc concentrations and isotope ratios in calcite marbles compared with their sedimentary protoliths provide direct evidence for dissolution, but the dissolution fraction of calcite carbonates, as a whole, is almost negligible. For dolomite marbles, the elevated zinc concentrations and decreased zinc isotope ratios indicate that dissolved carbonates underwent strong refixation within the slab. These findings collectively demonstrate that carbonates exhibit conservation behaviors during continental crust subduction and consequent exhumation. The vast majority (>98%) of carbon in continental crust is ultimately stored in the exhumed slabs, indicating that the carbon budget in continental subduction systems is nearly balanced and has limited impact on the global carbon flux. Instead, oceanic subduction zones play an essential role in carbon transfer between Earth’s exterior and interior.