Tracer of ocean circulation
Since the history of CFC concentrations in the atmosphere is relatively well known, they have provided an important constraint on ocean circulation. CFCs dissolve in seawater at the ocean surface and are subsequently transported into the ocean interior. Because CFCs are inert, their concentration in the ocean interior reflects simply the convolution of their atmospheric time evolution and ocean circulation and mixing.
The entry of CFCs into the ocean makes them extremely useful as transient tracers to estimate rates and pathways of ocean circulation and mixing processes.[68][69] However, due to production restrictions of CFCs in the 1980s, atmospheric concentrations of CFC-11 and CFC-12 has stopped increasing, and the CFC-11 to CFC-12 ratio in the atmosphere have been steadily decreasing, making water dating of water masses more problematic.[69] Incidentally, production and release of sulfur hexafluoride (SF6) have rapidly increased in the atmosphere since the 1970s.[69] Similar to CFCs, SF6 is also an inert gas and is not affected by oceanic chemical or biological activities.[70] Thus, using CFCs in concert with SF6 as a tracer resolves the water dating issues due to decreased CFC concentrations.
Using CFCs or SF6 as a tracer of ocean circulation allows for the derivation of rates for ocean processes due to the time-dependent source function. The elapsed time since a subsurface water mass was last in contact with the atmosphere is the tracer-derived age.[71] Estimates of age can be derived based on the partial pressure of an individual compound and the ratio of the partial pressure of CFCs to each other (or SF6).[71]
The age of a water parcel can be estimated by the CFC partial pressure (pCFC) age or SF6 partial pressure (pSF6) age. The pCFC age of a water sample is defined as:
where [CFC] is the measured CFC concentration (pmol kg−1) and F is the solubility of CFC gas in seawater as a function of temperature and salinity.[72] The CFC partial pressure is expressed in units of 10–12 atmospheres or parts-per-trillion (ppt).[73] The solubility measurements of CFC-11 and CFC-12 have been previously measured by Warner and Weiss[73] Additionally, the solubility measurement of CFC-113 was measured by Bu and Warner[74] and SF6 by Wanninkhof et al.[75] and Bullister et al.[76] Theses authors mentioned above have expressed the solubility (F) at a total pressure of 1 atm as: where F = solubility expressed in either mol l−1 or mol kg−1 atm−1, T = absolute temperature, S = salinity in parts per thousand (ppt), a1, a2, a3, b1, b2, and b3 are constants to be determined from the least squares fit to the solubility measurements.[74]
It can be noted that the solubility of CFCs increase with decreasing temperature at approximately 1% per degree Celsius.[71]
Once the partial pressure of the CFC (or SF6) is derived, it is then compared to the atmospheric time histories for CFC-11, CFC-12, or SF6 in which the pCFC directly corresponds to the year with the same. The difference between the corresponding date and the collection date of the seawater sample is the average age for the water parcel.[71] The age of a parcel of water can also be calculated using the ratio of two CFC partial pressures or the ratio of the SF6 partial pressure to a CFC partial pressure.[71]
