Contour 2 shows repair of your own ratios away from CFC-11/CFC-twelve and you can CFC-113/CFC-twelve from inside the remote United states sky. Brand new proportion regarding CFC-11/CFC-12 from inside the heavens improved dramatically throughout the later 1940s so you can in the 1977, and this off CFC-113/CFC-12 had a moderate raise for the eighties.
The slowing and turnover of CFC atmospheric mixing ratios in the 1990s has resulted in ambiguity in CFC-based apparent age. Sulphur hexafluoride (SFsix) has been developed as an alternative to CFCs for gas-solubility dating of water (Busenberg and Plummer, 1997). Significant production of SF6 began in the 1960s for use in high voltage electrical switches. SF6 is extremely stable, with an estimated atmospheric lifetime of 800 (Morris et al., 1995) to 3200 years (Ravishankara et al., 1993). SF6 can be analysed by GC-ECD techniques to a precision of 1-3% (Wanninkhof et al., 1991; Law et al., 1994), so, even though atmospheric mixing ratios are small (currently about 10 pptv), dating is possible from about 1970 to the present.
The earliest reported measurement of the atmospheric concentration of SF6 was 0.03 pptv in 1970 (Lovelock, 1971). SF6 is accumulating rapidly in the atmosphere with a current growth rate of more than 7% per year. The historical atmospheric mixing ratio of SF6 has been reconstructed from production records and atmospheric measurements (Ko et al., 1993; Elkins et al., 1996; Maiss et al., 1996; Geller et al., 1997; Levin and Hesshaimer, 1996), and retrieved from concentrations measured in seawater (Law et al., 1994) and in previously-dated groundwater (Busenberg and Plummer, unpub. data). Figure 1 shows a preliminary reconstruction of northern hemisphere SF6 mixing ratios. As atmospheric CFC concentrations fall, an even more sensitive dating tool will be the ratio of SF6 to, for example, CFC-12 (Figure 2). However, because of the low solubility of SF6 (Mroczek, 1997), apparent ages can be very sensitive to excess air, so this needs to be accurately determined.
Busenberg and Plummer (1997) used SF6 successfully to date shallow groundwater on the Delmarva Peninsula, Maryland, and water from springs in the Blue Ridge Mountains of Virginia. Although SF6 is mostly of anthropogenic origin, concentrations between one and two orders of magnitude higher than that possible for modern air-water equilibrium were measured in groundwater from volcanic rift zones in New Mexico and Idaho (Busenberg and Plummer, 1997). There is likely a natural, igneous source of SF6 that will complicate dating in some environments. SF6 has also been used as an artificial tracer (Wilson and Mackay, 1996; Schlatter et al., 1997) that is injected into groundwater or surface water. Clark et al. (1995b) found that wastewater discharged to the Hudson River at New York City was contaminated with SF6. Busenberg and Plummer (1997) found minor contamination of SF6 in Rio how to message someone on abdlmatch Grande water near an outfall from a sewage treatment plant. All other surface waters had SF6 concentrations near equilibrium with air.
Many techniques are present throughout recharge and in the groundwater environment that can affect CFC concentrations beyond those people set by sky-h2o harmony, and consequently affect interpretation off obvious age. Desk 2 listing initial techniques, their affect noticeable CFC years, and relative strengths.
Figure 3 Levels out of CFC-eleven, CFC-12, and CFC-113 in water charged between 1940 and you may 1998 from the temperatures out-of 5 so you can 25 °C at sea level.
magnitude of age errors one to originate from uncertainties from inside the demand heat. Such, in case your CFC-eleven attention is actually 600 pg kilogram -one in a particular liquid test one to charged at the fifteen°C, but cost temperatures was mistakenly believed getting 5°C, an obvious decades that is some several decades too-old would feel tasked, predicated on CFC-eleven. A suspicion in the recharge temperatures off ± 2°C causes many years concerns out of three years otherwise less to own h2o energized just before about 1990 (Busenberg ainsi que al., 1993). Over-estimation off demand temperatures results in apparent age too-young, and you will significantly less than-estimate offers ages that will be biased old. Uncertainty when you look at the cost temperature out-of ± 2°C contributes to uncertainty when you look at the obvious CFC chronilogical age of 12 months otherwise reduced for liquid billed before the middle-1970s. But not, while the heavens combo percentages peaked throughout the middle- and later-1990’s, CFC obvious decades turned most sensitive to concerns inside cost temperatures.