Radium isotopes as tracers of hydrothermal inputs and neutrally buoyant plume dynamics in the deep ocean

Radium (Ra) isotopes are enriched in hydrothermal fluids, have a wide range of half-lives and are minimally impacted by scavenging removal processes; therefore, they have the ability to provide key information on the fate of trace elements in both near- and far- field hydrothermal plumes. To expand our understanding of Ra isotopes and their usefulness as tracers of hydrothermal activity, we measured Ra isotopes in both high and low temperature vent fluids, and in neutrally buoyant plumes. The time scales of plume transport were derived from Ra isotopes in neutrally buoyant plumes emanating from 15°S on the East Pacific Rise (EPR) and the TAG vent field on the Mid Atlantic Ridge (MAR) relative to their source ratio in high-temperature vent fluids. In near-field (< 100 km from vent) neutrally buoyant plumes, the short-lived Ra isotopes (223Ra: t1/2 = 11.4 d, 224Ra: t1/2 = 3.66 d) suggest plume ages of ~ 1 month and ~ 5 d over the EPR and MAR, respectively. Farther afield (> 100 km from vent), the distribution of the longer-lived 228Ra (t1/2 = 5.75 y) is indicative of hydrothermal plumes with different points of origin along the EPR. Radium-228 derived plume ages were used to calculate the residence time of hydrothermal dissolved Fe, which was in the range of 9-20 y (with an upper limit of 50 y depending on model assumptions). These are shorter than typical deep ocean Fe residence times, a result of scavenging removal of Fe from the neutrally buoyant plume during transport. This study highlights the utility of Ra isotopes in providing time-scales of hydrothermally derived trace element cycling in the deep ocean.