Changing ventilation of the Mediterranean Sea studies with a suite of novel halogenated transient tracers

The Mediterranean Sea is characterized by very active and variable deep and intermediate water ventilation that can be quantified by transient tracers, such as CFC-12 and SF₆. However, a combination of several transient tracers is needed to empirically constrain ocean ventilation with the transit time distribution (TTD) model. For this purpose, CFC-12 has been widely used as an oceanic transient tracer since the 1980s, SF₆ since the 1990s. However, since the atmospheric concentration of CFC-12 is declining, novel transient tracers are needed to characterize ventilation, in particular for recently ventilated waters.

Here we utilize transient tracer data from four recent campaigns in the Mediterranean to report on recent changes in ventilation, and report on observations of alternative tracers. We use these to evaluate a selection of potential alternative tracers (HCFCs, HFCs and PFCs) based on four aspects: atmospheric history, seawater solubility, stability and feasibility of measurement. Consistent annual mean atmospheric histories were reconstructed by combining archived air measurements, firn air measurements, published model calculations and observations. Seawater solubility functions were constructed based on freshwater solubility functions and salting-out coefficients estimated by poly-parameter linear free-energy relationships. The stability in seawater was estimated by analysis of their ocean partial lifetimes, seawater surface saturation and interior ocean concentrations compared to CFC-12 measurements. For measurements, we report on the new Medusa-Aqua system able to simultaneous measure selected CFCs, HCFCs, HFCs and PFCs in seawater. Seawater samples from three profiles were taken and measured for the first time for HCFCs and HFCs in the Mediterranean. By evaluation of selected HCFCs, HFCs and PFCs on the four aspects, HCFC-141b and PFCs were found to be the most promising and potential oceanic transient tracers, respectively.