Evaluation of anthropogenic carbon in the Nordic Seas using observed relationships of N, P and C versus CFCs

Several methods to compute the anthropogenic component of total dissolved inorganic carbon (CTanthro) in the ocean have been reported, all in some way deducing (a) the effect by the natural processes, and (b) the background concentration in the pre-industrial scenario. In this work we present a method of calculating CTanthro using nutrient and CFC data, which takes advantage of the linear relationships found between nitrate (N), phosphate (P) and CFC-11 in the Nordic Seas sub-surface waters. The basis of the method is that older water has lower CFC-11 concentration and also has been exposed to more sinking organic matter that has decayed, resulting in the slopes of P versus CFC-11 and N versus CFC-11 being close to the classic Redfield ratio of 1:16. Combining this with the slope in total alkalinity (AT) versus CFC-11 to correct for the dissolution of metal carbonates gives us the possibility to deduce the concentration of anthropogenic CT in the Nordic Seas. This further allowed us to compute the inventory of anthropogenic CT below 250 m in the Nordic Seas in spring 2002, to ∼1.2 Gt C.