Nordic seas transit time distributions and anthropogenic CO2

The distribution and inventory of anthropogenic carbon (DICant) in the Nordic seas are determined using the transit time distribution (TTD) approach. To constrain the shape of the TTDs in the Nordic seas, CO2 is introduced as an age tracer and used in combination with water age estimates determined from CFC-12 data. CO2 and CFC-12 tracer ages constitute a very powerful pair for constraining the shape of TTDs. The highest concentrations of DICant appear in the warm and well-ventilated Atlantic water that flows into the region from the south, and concentrations are typically lower moving west into the colder Arctic surface waters. The depth distribution of DICant reflects the extent of ventilation in the different areas. The Nordic seas DICant inventory for 2002 was constrained to between 0.9 and 1.4 Gt DICant, corresponding to ∼1% of the global ocean DICant inventory. The TTD-derived DICant estimates were compared with estimates derived using four other approaches, revealing significant differences with respect to the TTD-derived estimates, which can be related to issues with some of the underlying assumptions of these other approaches. Specifically, the Tracer combining Oxygen, inorganic Carbon and total Alkalinity (TrOCA) method appears to underestimate DICant in the Nordic seas, the ΔC* shortcut and the approach of Jutterström et al. (2008) appear to overestimate DICant at most depths in this area, and finally the approach of Tanhua et al. (2007) appears to underestimate Nordic seas DICant below 3000 m and overestimate it above 1000 m.