The concept of ``age'' is used to mean the elapsed time since a water mass was last at the ocean surface in direct contact with the atmosphere. When in contact with the atmosphere the water is ventilated, that is it gets renewed with respect to atmospheric gas concentrations. If the age and distance from the region of ventilation are known, and a pathway is assumed, then the velocity can be calculated. The age information can be derived from radioactive decay as for tritium, or from the temporally changing atmospheric concentrations as for the CFCs.
Measurement of tritium and its daughter product 
He on the
same sample can be used to calculate an age [e.g. Jenkins and
Clarke, 1976]. The tritium/
He age in years (ç) is calculated
according to:
where [
H] and [
He] are the concentrations
of tritium and 
He in tritium units. The 
He concentration is
reset to near zero in the surface waters, although 
He concentrations
above solubility equilibrium have been observed in regions of ice cover
or upwelling [e.g. Fuchs et al., 1987; Schlosser et
al., 1990]. The 
He in the oceans is derived from two sources:
radiogenic from decay of tritium, and primordial from ridge
systems. The concentrations of 
He in the deep waters of the
North Atlantic are considerably lower than those in the Pacific
[ Jenkins and Clarke, 1976]. Recently Doney and Jenkins
[1994] have had success correcting for the primordial 
He signal
using correlations with silica concentrations. In the western
North Atlantic, they found the primordial effect on the
tritium/
He derived velocity of the DWBC is small. An advantage
of using the tritium/
He age is that it is independent of the
initial tritium concentration of a water mass [ Schlosser and
Smethie, 1995].
The CFC age is calculated using an atmospheric mixing ratio (þ*) [cf. Weiss et al., 1985; Wallace and Moore, 1985; Fine et al., 1988] as follows:
where C
, and C
are measured seawater concentrations of two
CFC compounds, F
and F
are their solubility functions,
is the potential temperature, and S is the salinity. The
calculated atmospheric mixing ratio can then be compared with past
atmospheric ratios (e.g., Figure 2) to determine a date of
equilibration or age. The age will be a true age if: the water
mass was in equilibrium with the atmosphere at the time of
formation, mixing occurs only with CFC-free water, and the CFC
concentration ratio is unaltered during the process of formation
and flow over the sills.
Tritium/
He ages can be applied to oceanographic processes
occurring on time scales of several months up to several decades.
The tritium/
He age is limited by the measurement precision of
He/
He and tritium concentrations [ Schlosser and
Smethie, 1995]. The CFC ages are mostly limited to those years
when the atmospheric ratio of two compounds has been increasing at
different rates. The F11/F12 ratio can be applied to processes
that occurred during the period of the 1950s to the mid-1970s, the
F113/F11 ratio for the mid-1970s to the present, and the CCl
/F11
ratio for the 1950s (limited by F11) to the present. Thus, as
these transient tracers are transported equatorward in the DWBC,
they serve as a marker for waters formed in the high latitudes, and
they carry a built in time clock.