An upgraded carbon-based method to estimate the anthropogenic fraction of dissolved CO₂ in the Atlantic Ocean

Abstract. An upgrade of classical methods to calculate the anthropogenic carbon (C_ant) signal based on estimates of the preformed dissolved inorganic carbon (C_T) is proposed and applied to modern Atlantic sections. The main progress has been the use of subsurface layer data (100–200 m) to reconstruct water mass formation conditions and obtain better estimates of preformed properties. This practice also eliminates the need for arbitrary zero-C_ant references that are usually based on properties independent of the carbon system, like the CFC content. The long-term variability of preformed total alkalinity (A_T) has been considered and the temporal variability of the air-sea CO₂ disequilibrium (ΔC_dis) included in the formulation. The change of ΔC_dis with time has shown to have non-negligible biases on C_ant estimates, producing a 4 μmol kg average decrease. The proposed φC_T method produces substantial differences in the C_ant inventories of the Southern Ocean and Nordic Seas (~18% of the total inventory for the Atlantic) compared with recent C_ant inventories. The overall calculated Atlantic C_ant inventory referenced to 1994 is 55±13 Pg C, which reconciles the estimates obtained from classical C_T-based C_ant calculation methods, like the ΔC*, and newly introduced approaches like the TrOCA or the TTD methods.