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

We use 39 Ar in conjunction with CFCs, natural radiocarbon, and the cyclostationary tracers PO 4 *, 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 39 Ar 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, Γ, the TTD width, Δ, 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 Γ and f 1 reflect the distribution of the major water masses. At the deepest locations in the North Atlantic Γ ≃ 300 −100 +300 a, while at the deepest locations in the South Atlantic Γ ≃ 500 −100 +200 a. The Pacific is nearly homogeneous below 2000 m with Γ ≃ 1300 −50 +200 a in the North Pacific and Γ ≃ 900 −100 +200 a in the deep South Pacific. The Southern Ocean locations have little vertical structure, with Γ ranging from 300 to 450 a with an uncertainty of about −40 +150 a. The importance of diffusion compared to advection as quantified by Δ/Γ 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 39 Ar is to reduce f 1 and to correspondingly increase Γ by about a century. The additional constraint provided by 39 Ar reduces the entropic uncertainties of f 1 by roughly 50% on average.