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

Abstract

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