Using Excess 3He to Estimate Southern Ocean Upwelling Time Scales

Using a recently compiled global marine data set of dissolved helium isotopes and helium and neon concentrations, we make an estimate of the inventory of hydrothermal 3He in the Southern Ocean to be 4.9 ± 0.6 × 104 moles. Under the assumption that the bulk of the hydrothermally sourced 3He is upwelled there, we use recent estimates of the global hydrothermal 3He flux to determine an e‐folding residence time of 99 ± 18 years, depending on assumptions of water mass and upwelling boundaries. Our estimate is within the broad range of values obtained from recent Southern Ocean circulation models. Plain Language Summary Seafloor hydrothermal systems inject significant amounts of dissolved iron, but it remains unknown what fraction of that trace element survives in situ chemical scavenging to upwell in the Southern Ocean to arrive at the sea surface. Since iron is a necessary trace‐level nutrient for marine biological productivity, knowledge of the time it takes for deep water to rise to the surface is important. Along with dissolved iron, helium‐3, a rare, inert isotope, is also injected into the deep water. Since the global flux of this isotope has been independently determined by large‐scale ocean models, we can estimate the upwelling time scale by dividing it into the inventory of this isotope in the Southern Ocean. Key Points The Southern Ocean contains an inventory of hydrothermally‐sourced 3He If we divide this inventory by the global hydrothermal flux that must support this inventory, we obtain an estimate of an upwelling time scale for deep waters in the Southern Ocean