Excess 3He in the ocean surface layer

Abstract

The actual ocean‐atmosphere 3He solubility equilibrium is a dynamic one, and it corresponds to δ3He values that are about 0.2% lower than the static equilibrium values reported by Benson and Krause (1980). We estimate this shift theoretically by considering gas transfer between air bubbles and water, and we present evidence from laboratory and field observations. A compilation of ocean surface layer δ3He observations shows widespread occurrence of excess 3He (≤ 2%) relative to a dynamic equilibrium with atmospheric helium. Large excesses are found for deep surface layers under conditions of seasonal thermocline erosion (winter convection). A simulation of observed δ3He depth profiles in an area of deep winter convection west of the British Isles (by means of a simple one‐dimensional model) is generalized to estimate initial tritium‐ 3He ages at the point of formation of a subsurface water mass as a function of the seasonal‐maximum surface layer depth, Dmax. In situations of areally extended seasonal convection with Dmax < 600 m, the remaining initial‐age uncertainty is believed to amount to no more than about ± 0.3 years. Far larger uncertainties must be expected in situations of deep‐reaching chimney‐type convection. The observed occurrence of excess 3He also in summer observations is taken to reflect intermittent surface‐layer/thermocline exchange.