Gregg R. Brooks

As the deep-sea mining industry expands, it is crucial to develop sensitive indicators to measure the footprint of an ambient sediment plume from a collector vehicle (PCV). This study introduces a novel technique using Thorium-234, a short-lived radioisotope (half-life: 24.5 days), as a high-resolution indicator for tracking event-driven sedimentation, examining resuspended sediment particles' ability to re-scavenge thorium in the water column, and delineating the spatial extent of an ambient plume created by a PCV from the first full-scale collector test in over 40 years. Multicores were collected for baseline Thorium-234 activity from the NORI-D lease area in the Clarion Clipperton Zone (CCZ) in 2021 and were subsampled at 0.5-1.0 cm intervals. Pre-collector test samples were collected August-September, 2022 and Post-collector test samples were collected November-December of 2022. Thorium-234 activities were determined for each sediment subsample. Pre-collector surface (0-1 cm) excess Thorium-234 activities across all sites ranged from 0.31-1.9 dpm/g. Post-collector surface (0-1 cm) excess Thorium-234 activities across all sites ranged from 0.23-6.31 dpm/g. Our findings highlighted the highest Thorium concentrations 0.5 km away from the test site, which returned to background levels at 1 km away. This study provides insight into the spatial extent of the sedimentation footprint of fine-grained particles and can be used to verify plume models as well as provide a potential threshold for sedimentation.