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

Oceanic transient tracers have been concerned for more than four decades due to their ability in visualizing and quantifying ocean ventilation and understanding the effects of changing climate. They trace pathways climate anomalies follow as they enter and move through the ocean and provide us with valuable time information. When such time information is interpreted depending on input function (time changing concentrations), they are chronological transient tracers, such as dichlorodifluoromethane (CFC-12) and sulfur hexafluoride (SF6). During the past ~15 years, the non-monotonous change of atmospheric history of CFC-12 limited its ability as an oceanic transient tracer for recently ventilated water masses, but it still works for deep waters. Therefore, we took the Mediterranean Sea as an example and investigated the recent changes in deep ventilation based on long-term observations of CFC-12 and SF6 in the first manuscript. Since a combination of multiple transient tracers can better interpret ocean ventilation, we looked for and evaluated potential novel transient tracers: hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) in the second and third manuscripts. The specific findings are described below. In the first study, highly variable deep ventilation in the Mediterranean Sea in time and space are reported based on a combination of observations of traditional chronological transient tracers, hydrographic properties and apparent oxygen utilization from 13 cruises conducted during 1987-2018. Spatially, both the Eastern and Western Mediterranean Deep Water (EMDW and WMDW) show a general west-to-east gradient of increasing salinity and potential temperature but decreasing oxygen and transient tracer concentrations. Temporally, stagnant and weak ventilation is found in most areas of the EMDW during the last decade in spite of prevailing ventilation in the Adriatic Deep Water between 2011 and 2016, which could be a result of the weakened Adriatic source intensity. In the Western Mediterranean Sea, enhanced ventilation after the Western Mediterranean Transition (WMT) event is observed, and slightly weakened ventilation after 2016 could be a combined influence from the Eastern (for the weakened Adriatic source intensity) and the Western (for the weakened influence from the WMT event) Mediterranean Sea. In the second and third studies, we explored and evaluated potential novel chronological transient tracers: chlorodifluoromethane (HCFC-22), 1,1-dichloro-1-fluoroethane (HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC-142b), 1,1,1,2-tetrafluoroethane (HFC-134a), pentafluoroethane (HFC-125), fluoroform (HFC-23), carbon tetrafluoride (PFC-14, CF4) and hexafluoroethane (PFC-116) from four aspects: input function (including atmospheric history and historical surface saturation), seawater solubility, feasibility of measurement and stability in seawater. By comprehensive analysis and evaluation, the most promising oceanic transient tracers are HCFC-142b and HCFC-141b currently since they fulfil essential requirements by virtue of well-documented atmospheric history, established seawater solubility, feasible measurements and inertness in seawater. However, they will likely only work for the next few years/decades considering the restrictions on their production and consumption imposed by the Montreal Protocol and their (future) decreasing atmospheric mole fractions. The compounds that have the greatest potential as oceanic transient tracers in the future are PFC-14 and PFC-116 because of their high stability in seawater, the long and well-document atmospheric concentration histories and well-constructed seawater solubility functions. The challenge is how to measure them accurately due to their low solubility. For HFC-134a, we are not able to fully evaluate its potential as a tracer due to the inconclusive results, especially on its solubility and stability in seawater, but also with regard to potential analytical challenges. HFC-125, HFC-23, and HCFC-22 can no longer be considered because there are alternative tracers with similar input functions that are better suited as oceanic transient tracers. In total, this work helps us understand ocean ventilation in the Mediterranean Sea in the past ~30 years (with an emphasis on the recent changes) from the perspective of the traditional chronological transient tracers, as well as explored and evaluated the potential novel chronological transient tracers in the ocean. The outcome sets the base for further investigation of these alternative tracers in order to better interpreting ventilation in the global ocean and understanding the effects of climate change.

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