In addition to temperature and salinity, we examined the evolution of the tritium/He and CFC distributions in the central Greenland Sea. Tracer data were collected during all JOHAN HJORT cruises and on several other cruises between 1972 and 1989. During the JOHAN HJORT cruises water samples for analysis of CFCs and tritium/He were collected from 5 L Niskin bottles. The Niskin bottles were designed for low CFC background.
Tritium is the radioactive isotope of hydrogen and decays with a half-life of 12.43 years to (Unterweger et al. [1980]). Most of the tritium presently observed in the environment has been produced during the atmospheric nuclear weapon tests in the 1950s and early 1960s. Atmospheric concentrations reached maximum values during the mid 1960s and have been decreasing since. Tritium enters the ocean via precipitation, water vapor exchange, and river runoff as tritiated water (HTO, e.g., Weiss and Roether [1980]). Tritium concentrations are reported in TU, where 1TU means a tritium to hydrogen ratio of 1E-18. In the TU81 notation, the tritium concentrations are decay corrected to the date of January 1, 1981.
He-3 and He-4 in the ocean originate from the atmosphere, the mantle, the crust, and tritium decay (for separation of the different components, see e.g., Schlosser [1992]). Usually, He-3 data are reported in the delta-notation, i.e., the percent deviation of the He-3/He-4 ratio of a sample from that of air (1.384E-6, Clarke et al. [1976]).
Simultaneous tritium and He-3 measurements allow us to calculate the tritium/He-3 age
where Tau is the tritium/He-3 age, T1/2 the half life of tritium and [3He] and [3H] are the tritiogenic He-3 and tritium concentrations, respectively (in TU). Samples for He isotope and tritium analysis were collected in 40ml copper tubes sealed by stainless steel pinch-off clamps. Additional tritium samples were stored in 1L glass bottles. Tritium samples were degassed on shore using a high vacuum extraction system and stored in special glass bulbs with low He permeability for ingrowth of tritiogenic He-3. He-3 grown in from tritium decay over a period of typically six months was measured on a commercial VG 5400 mass spectrometer with a specially designed inlet system. He samples were extracted using the same extraction system and measured on the same mass spectrometric system. Precision of the He-3 data was about +-0.2%, precision of the tritium data about +-2% or +-0.02TU. Tritium and He isotope measurement procedures used in the Lamont-Doherty Earth Observatory (L-DEO) noble gas laboratory are similar to those described in detail by Bayer et al. [1989].
Pre-industrial atmospheric concentrations of CFC11 and CFC12 were zero. Since the start of industrial CFC production in the 1930s, their atmospheric concentrations have steadily increased. Relative growth rates of CFC11 and CFC12 in the atmosphere were different until the mid 1970s, and have been practically identical since then. CFCs enter the ocean via gas exchange with the atmosphere. CFCs were measured on board using purge-and-trap gas chromatography with electron capture detection. A Varian 3300 gas chromatograph was used, together with a custom-built purge-and-trap apparatus similar to that described by Wallace et al. [1994] and more completely by Happel et al. [1996]. Calibration was achieved by injections of known amounts of a working standard with known concentrations of CFC11 and CFC12 (Happell and Wallace [in press]). All results are reported relative to the Scripps Institute of Oceanography (SIO) 1986 calibration scale (R. F. Weiss, personal communication). After subsampling the 5 L Niskin bottles, CFC samples were stored in 100ml or 50ml glass syringes under surface seawater prior to analysis. Accuracy of the analyses relative to the SIO calibration scale was about +-2%.