The Ocean's Global 39Ar Distribution Estimated With an Ocean Circulation Inverse Model

Ar-39 with its 269-year half-life has great potential for constraining ocean ventilation and transport. Here we estimate the distribution of Ar-39 using a steady ocean circulation inverse model. Our estimates match available Ar-39 measurements to within an absolute error of similar to 9% modern argon without major biases. We find that Ar-39 traces out the world ocean's ventilation pathways and that the Ar-39 age Gamma(Ar) and the ideal mean age have broadly similar large-scale patterns. At the surface, Ar-39 is close to saturated except at high latitudes. Undersaturation imparts a finite Ar-39 age to surface waters relative to the atmosphere, with peak values exceeding 100 years in Antarctic waters. This reservoir age is propagated into the interior with Antarctic Bottom Water, elevating Gamma(Ar) by similar to 50 years in the deep Pacific and Indian oceans. Our estimates identify the large-scale gradients and uncertainty patterns of Ar-39, thus providing guidance for future measurements.