Development and Application of Th-228 for Tracking Deep Sea Mining Collector Plume Dispersal

Deep-sea mining poses significant challenges for understanding and managing the impacts of sediment redistribution and disturbance to benthic environments. This study demonstrates how the application of short-lived radioisotopes, specifically Th-228, can fill critical gaps in resolving annual to decadal-scale processes relevant to mining operations. Commonly used radioisotopes are on the monthly scale (Th-234) and the decadal to centennial scale (Pb-210). There is a need for a radioisotope proxy able to better resolve annual to decadal-scale processes, which are the timescales on which the mining industry is planning to operate. This study applies Th-228 to satisfy this resolution gap, which has a half-life of 1.9 years and can be dated to nearly 10 years, to provide a timescale appropriate proxy to determine the lasting spatial distribution of benthic mining plumes on the annual to decadal scale.  In 2021, multicores were collected for baseline measurements from the NORI-D lease area in the Clarion Clipperton Zone. During August-September of 2022, pre-collector test samples were obtained, and in  November-December of 2022, post-collector test samples were obtained. One year later (2023), additional post-disturbance samples were collected. Four cores were chosen and assigned to categories based on model projections and seafloor observations. The site's distance increased as they moved farther from the “track”. Sites were located within the collector path, 0.2 km East, 0.5 km East, 2 km East, and the control site, 4 km Northwest. This presentation will provide information about the development, calibration, and application of this Th-228 radioisotope tool in the context of sediment accumulation rates, sediment resuspension/deposition, and bioturbation rates/depths. A more detailed resolution of annual to decadal processes will strengthen the development of regulatory thresholds and design of adaptive management frameworks for deep-sea mining.