ALBERT

Description: We present consistent annual mean atmospheric histories and growth rates for the mainly anthropogenic halogenated compounds HCFC-22, HCFC-141b, HCFC-142b, HFC-134a, HFC-125, HFC-23, PFC-14 and PFC-116, which are all potentially useful oceanic transient tracers (tracers of water transport within the ocean), for the Northern and Southern Hemisphere with the aim of providing input histories of these compounds for the equilibrium between the atmosphere and surface ocean. We use observations of these halogenated compounds made by the Advanced Global Atmospheric Gases Experiment (AGAGE), the Scripps Institution of Oceanography (SIO), the Commonwealth Scientific and Industrial Research Organization (CSIRO), the National Oceanic and Atmospheric Administration (NOAA) and the University of East Anglia (UEA). Prior to the direct observational record, we use archived air measurements, firn air measurements and published model calculations to estimate the atmospheric mole fraction histories. The results show that the atmospheric mole fractions for each species, except HCFC-141b and HCFC-142b, have been increasing since they were initially produced. Recently, the atmospheric growth rates have been decreasing for the HCFCs (HCFC-22, HCFC-141b and HCFC-142b), increasing for the HFCs (HFC-134a, HFC-125, HFC-23) and stable with little fluctuation for the PFCs (PFC-14 and PFC-116) investigated here. The atmospheric histories (source functions) and natural background mole fractions show that HCFC-22, HCFC-141b, HCFC-142b, HFC-134a, HFC-125 and HFC-23 have the potential to be oceanic transient tracers for the next few decades only because of the recently imposed bans on production and consumption. When the atmospheric histories of the compounds are not monotonically changing, the equilibrium atmospheric mole fraction (and ultimately the age associated with that mole fraction) calculated from their concentration in the ocean is not unique, reducing their potential as transient tracers. Moreover, HFCs have potential to be oceanic transient tracers for a longer period in the future than HCFCs as the growth rates of HFCs are increasing and those of HCFCs are decreasing in the background atmosphere. PFC-14 and PFC-116, however, have the potential to be tracers for longer periods into the future due to their extremely long lifetimes, steady atmospheric growth rates and no explicit ban on their emissions. In this work, we also derive solubility functions for HCFC-22, HCFC-141b, HCFC-142b, HFC-134a, HFC-125, HFC-23, PFC-14 and PFC-116 in water and seawater to facilitate their use as oceanic transient tracers. These functions are based on the Clark–Glew–Weiss (CGW) water solubility function fit and salting-out coefficients estimated by the poly-parameter linear free-energy relationships (pp-LFERs). Here we also provide three methods of seawater solubility estimation for more compounds. Even though our intention is for application in oceanic research, the work described in this paper is potentially useful for tracer studies in a wide range of natural waters, including freshwater and saline lakes, and, for the more stable compounds, groundwaters.

Description: The Mediterranean Sea is a small region of the global ocean but with a very active overturning circulation that allows surface perturbations to be transported to the interior ocean. Understanding of ventilation is important for understanding and predicting climate change and its impact on ocean ecosystems. To quantify changes of deep ventilation, we investigated the spatiotemporal variability of transient tracers (i.e. CFC-12 and SF6) observations combined with temporal evolution of hydrographic and oxygen observations in the Mediterranean Sea from 13 cruises conducted during 1987-2018, with emphasize on the update from 2011 to 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 despite the prevailing ventilation in the Adriatic Deep Water between 2011 and 2016, which could be a result of the weakened Adriatic source intensity. The EMDW has been a mixture of the older Southern Aegean Sea dense waters formed during the Eastern Mediterranean Transient (EMT) event, and the more recent ventilated deep-water of the Adriatic origin. In the western Mediterranean basin, we found uplifting of old WMDW being replaced by the new deep-water from the Western Mediterranean Transition (WMT) event and uplifting of the new WMDW toward the Alboran Sea. The temporal variability revealed enhanced ventilation after the WMT event but slightly weakened ventilation after 2016, which could be a result of combined influences from the eastern (for the weakened Adriatic source intensity) and western (for the weakened influence from the WMT event) Mediterranean Sea. Additionally, the Mediterranean Sea is characterized by a Tracer Minimum Zone (TMZ) at mid-depth of the water column attributed to the rapid deep ventilation so that the TMZ is the slowest ventilated layer. This zone of weak ventilation stretches across the whole Mediterranean Sea from the Levantine basin into the western basin.

Description: 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.

Description: 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.