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A new method in physical limnology based on the radioactive tracer tritium and its stable daughter product, 'He, is examined. The 3He produced by the in situ decay of tritium can be used to calculate an effective water mass age. These ages can then be used to estimate gas exchange rates, gas renewal epilimnion.Physical mixing mechanisms in lakes have been qualitatively explained for many years. However, the quantitative treatments lag considerably due to lack of adequate methods. While a nomograph for determining gas exchange parameters, turnover characteristics, and eddy diffusivities would be an enormous advance, we are hardly in a position to compile one. Our present understanding of limnological mixing modes and time scales is just too limited.To contribute to an eventual compilation of such a nomograph, we have developed a new tool for physical limnology. It shows promise for explaining some of the least understood processes, such as the rate of

We develop a new model bomb-3H budget for the North Atlantic between 1950 and 1986. The model, which calculates the atmospheric and continental 3H delivery as well as the advective 3H transports from the South Atlantic and Arctic, agrees within about 10% with the 3H inventories computed from the 1972 GEOSECS and the 1981-1983 Transient Tracers in the Ocean (TTO) observations. The decay-corrected 3H inventory for the North Atlantic increased by about 430-/0 between the GEOSECS and TTO programs (1972 to 1981), and the inflow of high 3H polar water from the Arctic into the North Atlantic is found to be crucial for correctly simulating this increase. Key aspects of the model that differ from previous studies include the treatment of vapor/rain isotopic equilibrium, the continental vapor flux, and the downward flux of water vapor into the ocean. The sensitivity of the atmospheric 3H delivery to model parameters and to seasonal and interannual variability are explored. 1. INTRODUCTION The usefulness of ocean tritium data, particularly in the diagnosis of quantitative circulation problems, is limited by how well the deposition history of bomb-tritium is known for the ocean [Memery and Wunsch, 1990]. Using precipitation tritium data and a simple air-sea exchange model, Weiss et al. [1979] and Weiss and Roether [1980] developed a tritium delivery function that has become the generally accepted model for the 3H flux boundary condition to the world ocean. Many questions remain, however, about both the physical mechanisms involved in [Koster et al., 1989] and the magnitude and distribution of bomb-tritium deposition to the ocean [Memery and Wunsch, 1990]. Based on new data and model results, we develop an improved model for predicting the atmospheric and advective tritium delivery to the North Atlantic Ocean. We explore the sensitivity of the 3H model to individual parameters and uncertainties in the input data. We then compare the model predicted tritium inventories for the North Atlantic basin with observational data from the 1972 GEOSECS program and 1981-1983 Transient Tracers in the Ocean (TTO) program. The atmospheric delivery rate of tritium, in the form of tritiated water (HTO), to the ocean (Datm) can be represented by three components, a precipitation flux, a downward HTO vapor flux, and a back-flux due to reevaporation of HTO from the ocean surface [Weiss and Roether, 1980]' D•.tm --PCp--[-VdownCv-Vup!CS ß (1) P, Vdown, and Vup represent precipitation and the downward and upward fluxes of water vapor across the air-sea interface 1Now at Paper number 93JC00917. 0148-0227/93/93JC-00917505.00 respectively, and Cp, Cv, and Cs are the tritium concentrations in precipitation, water vapor, and surface water. All of the model and observed tritium concentrations in the paper are expressed in Tritium Units (1 TU equals 1 T atom per lx10 ls H atoms) and have been decay corrected to a uniform date, January 1, 1981, expressed as TU81N. c• is the isotopic equilibrium factor for HTO/H20 in liquid-vapor exchange and has a value of...