Timescales of hydrothermal scavenging in the South Pacific Ocean from ²³⁴Th, ²³⁰Th, and ²²⁸Th

Hydrothermal activity in the deep ocean generates plumes of metal-rich particles capable of removing certain trace elements from seawater by adsorption and sedimentation. This removal process, known as scavenging, can be probed using the insoluble radiogenic isotopes of thorium (Th), which are produced at a known rate in the water column via the decay of soluble uranium (²³⁴Th, ²³⁰Th) and radium (²²⁸Th) isotopes. We present dissolved and particulate measurements of these three thorium isotopes in a hydrothermal plume observed in the southeast Pacific Ocean on the GEOTRACES GP16 section. Since their half-lives vary from days (²³⁴Th) to years (²²⁸Th) to tens of thousands of years (²³⁰Th), the combination of their signals can be used to understand scavenging processes occurring on a wide range of timescales. Scavenging is a multi-step process involving adsorption and desorption onto particles, followed by particle aggregation, sinking, and eventual sedimentation. We use thorium isotopes to study how hydrothermal activity affects these steps. The rate constants for net adsorption of ²³⁴Th determined here are comparable to previous estimates from hydrothermal plumes in the Atlantic and North Pacific Oceans. The partitioning of ²³⁴Th and ²³⁰Th between large and small particles is more similar in the hydrothermal plume than above it, indicating faster aggregation of particles within the hydrothermal plume at stations nearby the East Pacific Rise than in waters outside the plume. In addition to rapid scavenging and aggregation near the ridge axis, we also infer continuous off-axis scavenging from observations and modeling of ²²⁸Th/²²⁸Ra activity ratios. The degree of depletion of the three thorium isotopes increases in order of half-life, with total ²³⁴Th activity close to that of its parent ²³⁸U, but ²³⁰Th showing nearly 70% depletion compared to expected values from reversible scavenging. By modeling the variations in depletion for the different isotopes, we show that much of the ²³⁰Th removal is inherited from scavenging events happening long before the most recent hydrothermal inputs.