Deep carbon cycle constrained by carbonate solubility

Abstract Earth’s deep carbon cycle affects atmospheric CO 2 , climate, and habitability. Owing to the extreme solubility of CaCO 3 , aqueous fluids released from the subducting slab could extract all carbon from the slab. However, recycling efficiency is estimated at only around 40%. Data from carbonate inclusions, petrology, and Mg isotope systematics indicate Ca 2+ in carbonates is replaced by Mg 2+ and other cations during subduction. Here we determined the solubility of dolomite [CaMg(CO 3 ) 2 ] and rhodochrosite (MnCO 3 ), and put an upper limit on that of magnesite (MgCO 3 ) under subduction zone conditions. Solubility decreases at least two orders of magnitude as carbonates become Mg-rich. This decreased solubility, coupled with heterogeneity of carbon and water subduction, may explain discrepancies in carbon recycling estimates. Over a range of slab settings, we find aqueous dissolution responsible for mobilizing 10 to 92% of slab carbon. Globally, aqueous fluids mobilise $${35}_{-17}^{+20}$$ 35 − 17 + 20 % ( $${27}_{-13}^{+16}$$ 27 − 13 + 16 Mt/yr) of subducted carbon from subducting slabs.