Improved constraints on transit time distributions from argon 39: A maximum entropy approach

We use Ar-39 in conjunction with CFCs, natural radiocarbon, and the cyclostationary tracers PO4*, temperature, and salinity to estimate the ocean's transit time distributions (TTDs). A maximum entropy method is employed to deconvolve the tracer data for the TTDs. The constraint provided by the Ar-39 data allows us to estimate TTDs even in the deep Pacific where CFCs have not yet penetrated. From the TTDs, we calculate the ideal mean age, Gamma, the TTD width, Delta, and the mass fraction of water with transit times less than a century, f(1). We also quantify the entropic uncertainties due to the nonuniqueness of the deconvolutions. In the Atlantic, the patterns of Gamma and f(1) reflect the distribution of the major water masses. At the deepest locations in the North Atlantic Gamma similar or equal to 300(-100)(+300) a, while at the deepest locations in the South Atlantic Gamma similar or equal to 500(-100)(+200) a. The Pacific is nearly homogeneous below 2000 m with Gamma similar or equal to 1300(-50)(+200) a in the North Pacific and Gamma similar or equal to 900(-100)(+200) a in the deep South Pacific. The Southern Ocean locations have little vertical structure, with Gamma ranging from 300 to 450 a with an uncertainty of about (+150)(-40) a. The importance of diffusion compared to advection as quantified by Delta/Gamma has most probable values ranging from 0.2 to 3 but with large entropic uncertainty bounds ranging from 0.2 to 9. For the majority of locations analyzed, the effect of Ar-39 is to reduce f(1) and to correspondingly increase G by about a century. The additional constraint provided by Ar-39 reduces the entropic uncertainties of f(1) by roughly 50% on average.