| Summary: | The accurate estimation of the physical-chemical properties in the water masses at their time of formation is of foremost relevance in the context of anthropogenic CO2 (Cant) quantification. The tracer concentrations of the sub-surface layer (100-200 m) are thought to represent best water mass formation conditions compared to surface properties. The sub-surface data are proposed as a recommended alternative to be used in the absence of direct late wintertime surface observations for the pu+rpose of parameterizing preformed property concentrations. The scarcity of surface carbon system measurements is often the case in the high latitudes, where water mass formation typically occurs. Year-through, it is observed that the physical and biological processes affect the stability of the sub-surface layer by at least one order of magnitude less than the surface layer. In addition, it is found that during late wintertime the surface and sub-surface tracer concentrations are more alike. Alternatively, the water mass thermohaline variability of the 100-200 meter sub-surface layer encloses and represents all water masses outcropped in the Atlantic Ocean. Consequently, a revision of the classical back-calculation techniques for determining Cant is carried out. A new set of preformed total alkalinity (ATº) and air-sea CO2 disequilibrium (ΔCdis) parameterizations obtained from sub-surface data in modern Atlantic cruises is introduced. The change of ATº and ΔCdis over time is also examined and quantified. The temporal variability of ΔCdis has a non-negligible effect on Cant estimates, producing an average decrease of 4 μmol·kg-1. Compared with recent Cant inventories the results obtained with the revised back-calculation methodology show substantial differences in the Southern Ocean and Nordic Seas. These regions represent ~18% in terms of the total inventory. The estimated overall Atlantic Cant inventory referenced to 1994 is 55 ±13 Pg C. Peer reviewed
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