Abstract
Abstract. This study evaluates the potential usefulness of the halogenated compounds HCFC-22, HCFC-141b, HCFC-142b, HFC-134a, HFC-125, HFC-23, PFC-14, and PFC-116 as oceanographic transient tracers to better constrain ocean ventilation processes. We do this mainly in terms of four aspects of the characteristics of the potential tracers: input function (including atmospheric history and historical surface saturation), seawater solubility, feasibility of measurement, and stability in seawater; of these, atmospheric history and seawater solubility have been investigated in previous work. For the latter two aspects, we collected seawater samples and modified an established analytical technique for the Medusa–Aqua system to simultaneously measure these compounds. HCFC-22, HCFC-141b, HCFC-142b, HFC-134a, and HFC-125 have been measured in depth profiles in the Mediterranean Sea for the first time and for reproducibility in the Baltic Sea. We found that the historical surface saturation of halogenated transient tracers in the Mediterranean Sea is estimated to have been nearly constant at 94 % based on historical data. Of the investigated compounds, HCFC-142b, HCFC-141b, and HFC-134a are found to currently be the most promising transient tracers in the ocean. The compounds that have the greatest potential as future tracers are PFC-14 and PFC-116, mainly hampered by the low solubility in seawater that creates challenging analytical conditions, i.e., low concentrations. HCFC-22 is found to be likely unstable in warm seawater, which compromises the potential as an oceanic transient tracer, although it is possibly useful in colder water. For the compounds HFC-125 and HFC-23, we were not able to fully evaluate their potential as tracers due to inconclusive results, especially regarding their solubility and stability in seawater, but also with regard to potential analytical challenges. On the other hand, HFC-125, HFC-23, and HCFC-22 might not need to be considered because there are alternative tracers with similar input histories that are better suited as transient tracers.
Highlights
Ocean ventilation is defined as the process that transports perturbations such as the concentration of trace gases from the surface mixed layer into the ocean interior
The atmospheric concentrations of CFC-12 and CFC-11 have decreased since the early 2000s and the early 1990s, respectively (Bullister, 2015), which has reduced their usefulness as oceanographic transient tracers for recently ventilated water masses
The depth ranges of winter mixed layer (WML) (Fig. S8) and the saturation for CFC-12 and SF6 (Fig. 8) were determined by profiles of temperature, potential density, and the CFC-12 and SF6 concentration for every historical cruise in the Mediterranean Sea from 1987 to 2018 (Schneider et al, 2014; Li and Tanhua, 2020)
Summary
Ocean ventilation is defined as the process that transports perturbations such as the concentration of trace gases from the surface mixed layer into the ocean interior. Ocean ventilation and mixing processes play significant roles in climate They largely control the accumulated uptake of anthropogenic carbon (Cant) at middle and high latitudes as well as the deep ocean’s oxygen supply. Tanhua: Medusa–Aqua system a transient racer in the true meaning of the word They have been used as oceanic transient tracers to study oceanic processes such as ventilation, mixing, and circulation. The atmospheric concentrations of CFC-12 and CFC-11 have decreased since the early 2000s and the early 1990s, respectively (Bullister, 2015), which has reduced their usefulness as oceanographic transient tracers for recently ventilated water masses. SF6 has been added to the suite of commonly measured oceanic transient tracers (Tanhua et al, 2004; Bullister et al, 2006) as it is an inert gas whose atmospheric abundance is increasing. Since a combination of multiple transient tracers is needed to constrain ocean ventilation (Stöven and Tanhua, 2014; Holzer et al, 2018), it is useful to explore novel transient tracers with monotonically changing input functions for a better understanding of ventilation and mixing processes in the ocean
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