The interpretation of transient tracer observations depends on dif•cult to obtain information on the evolution in time of the tracer boundary conditions and interior distributions. Recent studies have attempted to circumvent this problem by making use of a derived quantity, age, based on the simultaneous distribution of two complementary tracers, such as tritium and its daughter, helium 3. The age is defined with reference to the surface such that the boundary condition takes on a constant value of zero. We use a two-dimensional model to explore the circumstances under which such a combination of conservation equations for two complementary tracers can lead to a cancellation of the time derivative terms. An interesting aspect of this approach is that mixing can serve as a source or sink of tracer based age. We define an idealized "ventilation age tracer" that is conservative with respect to mixing, and we explore how its behavior compares with that of the tracer-based ages over a range of advective and diffusive parameters. in steady state, is that the tracer-derived age equations include mixing terms that do not conserve age. This difference does not constitute a problem in using the tracerderived age equation to obtain constraints on the flow and mixing fields. It is of concern, however, if one is actually trying to obtain the ventilation time, for example to find the average oxygen utilization rate of a water parcel over some time interval. Thus Jenkins [1977, 1980, 1982, 1987], in his various studies of OUR, and Weiss et al. [1985] in their study of North Atlantic Deep Water transport, have had to make some very restrictive assumptions about the effect of mixing in order to make use of the concept of age dating. For 9377

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