Observations of CFCs and SF6 as ocean tracers

An advantage of using chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6) as tracers of ocean circulation is that the time-dependent source functions permit calculation of rates for ocean processes. These compounds are also sensitive indicators highlighting interior ocean regions where surface-derived anomalies can be transported on timescales of decades. Significant applications for CFCs have been for the deep limb of the Atlantic meridional overturning circulation, upper ocean ventilation, and biogeochemical rates, including apparent oxygen utilization rates and anthropogenic CO2 inventories. Although CFCs have started to decrease in the atmosphere, SF6 continues to increase. There are benefits to measuring both CFCs and SF6: A large global CFC data set exists; CFCs are still increasing in older waters; SF6 expands estimates of age; and calculations of anthropogenic CO2 inventory are enhanced. Thus, the outlook for using CFCs as tracers for oceanic processes, and in particular in concert with SF6, remains very positive.