Gregg R. Brooks

Sediment cores were collected in the Gulf of Mexico (GoM) to investigate the sediment record of synthetic based drilling mud (SBM) and evaluate the scope of the Deepwater Horizon (DwH) oil spill. Synthetic drilling mud containing barium sulfate is used at oil drilling sites to control the temperature and pressure inside the well during drilling activities. On April 20, 2010, the DwH oil spill triggered the release of over 4 million barrels of oil in the ocean. Attempts were made to stop the flow of oil from the well, including pumping synthetic barium sulfate (barite) drilling mud into the well. This failed to stop or reduce the oil flow and nearly 30,000 barrels of drilling mud were dispersed onto the seafloor. Due to the unique characteristics and uses of SBMs, they may be used as tracers in deep-sea sediments to investigate the environmental implications of barium from oil drilling and assist with the evaluation of the implications of barium increases in benthic environments. The objectives of this research are as follows: (I) Determine if there is variability of barium in sediments in northeast the GoM, (II) If yes, determine if this can be attributed to oil drilling activities and/or other anthropogenic activities. Sediment cores were analyzed by X-ray Fluorescence (XRF) spectroscopy to determine the elemental composition of sediment samples and evaluate variability in Ba concentrations. Short lived radioisotope analyses ( (super 210) Pb) were conducted to provide age control over the past approximately 100 years to determine timing of Ba input. Preliminary findings indicate that sediment surrounding the DwH site exhibit increased concentrations of Ba at the surface, likely due to the DwH event. Samples collected from areas of active drilling exhibit spikes in Ba concentrations at various intensities possibly indicating drilling activities from surrounding wells. This research can be used to evaluate the effectiveness of regulations on synthetic drilling mud put in place to prevent contamination and toxicity to benthic communities, as well as provide a potentially more persistent tracer of oil spill events. This will help further determine the spatial extent of the DwH event on the seafloor as well as its preservation in the sedimentary record.

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.