Gregg R. Brooks

In this paper, we present results that have important implications for understanding the sedimentation process and for evaluating the carbon cycle in deep-sea sediment of the southern Gulf of Mexico (sGM) in the current context of global climate change. Here we show results about short- and long-term biodiffusion rates obtained from natural (
Th,
Pb,
Pb,
Ra) and anthropogenic (
Pu) radionuclides in eleven sediment cores collected in the sGM: i) the outer continental shelf (2 cores), ii) the continental slope (5 cores), and iii) the abyssal plain (4 cores). The short- and long-term biodiffusion rates (D
and D
) ranged from 95 to 4135 cm
 kyr
and 54 to 549 cm
 kyr
, respectively. The D
(100 d time scale) values were less than one order of magnitude higher than the D
(100 yrs time scale), possibly because of the oligotrophic condition of the surface ocean in the sGM to limit the biological activity in sediments. Changes in redox conditions were confirmed by the enrichment of Fe and Mn in the sediment profiles. These redox changes are likely responsible for increased Ra mobilization in the sediments and, consequently, for changes in the accumulation of unsupported
Pb. This process acts as an autochthonous source of
Pb
within the sediment layers and explains the disequilibrium observed between
Pb and
Pb in subsurface sections of some sediment cores. Numerical simulation of
Pu profiles using an advection-diffusion equation in non-steady-state conditions fit well (R
 > 0.86) with measured
Pu profiles in four of six cores. The biodiffusion rates obtained from the Pu model were comparable to those obtained from
Pb, but different than the short-term biodiffusion rates elucidated with
Th. The accumulation rates from Pu deconvolutions were similar to data reported previously by
C dating.

Deep-sea mining for polymetallic nodules is currently exploratory, but commercial-scale operations require indicators of environmental change to support regulatory thresholds and inform adaptive management. In the Clarion-Clipperton Zone, where background sedimentation rates are low, seafloor imagery has validated mining plume deposition but cannot resolve repeated sedimentation as nodules become buried. Thorium-234 (
Th), a naturally occurring radionuclide with a 24.1-day half-life and strong particle reactivity, serves as a high-resolution geochemical tracer. Here we apply sedimentary
Th to identify the spatial extent of plume deposition following the NORI-D mining test. Excess
Th (
Th
) activity was low at baseline but elevated after mining and declined to background within 1-2 km of the directly mined area. Results suggest that mining plumes scavenge and redistribute
Th
, establishing a geochemical benchmark for plume extent and an operational tool for tracing recent sedimentation under future commercial-scale mining scenarios.

The Gulf of Mexico (GoM) is a unique ecosystem due to its physical characteristics, being influenced by the Mississippi River in the north and the Loop Current from the south, resulting in a gradient of organic to carbonate sediment composition from north to south. The continental slope of the northern and southwestern portions of the GoM are generally well studied; however, less is known about the southeastern GoM along the slope of Cuba. To fill this knowledge gap, sediment cores were collected in 2017 at nine stations (974–1580 m depth) to determine abiotic controls on the deep-sea benthic macrofauna community. Oceanographic data indicated a stratified water column typical of an oligotrophic ocean and no evidence of hypoxia. Sediment texture and composition indicated a west-east gradient likely determined by downslope transport of terrigenous material in the eastern part with a high proportion of carbonate in the west. Heavy metals (Cu, Hg, Pb, and Zn) at concentrations known to cause adverse benthic effects were present in the east near the city of Havana, with the macrofauna community showing characteristics indicative of environmental stress. Overall, this region supported a diverse community of macrofauna families of low abundance, typically only 1–2 animals, and high variability among replicates within stations. Rarefaction curves revealed higher biodiversity per number of individuals in the samples from Cuba compared to those from the nGoM at similar depths, though more samples would be needed to better reveal the true diversity. The major factors influencing macrofauna communities in the continental slope off northwestern Cuba are most likely the lack of organic-rich sediment and low sediment deposition rates, both of which can be attributed to the strong currents and lack of major terrigenous input, along with the regular natural disturbances which prevents domination.
•Along the Cuban slope, there was a total of 275 macrofauna collected from 65 families and 9 phyla in 25 cores.•Evidence of toxic levels of heavy metals and adverse community impacts were observed from the station offshore of Havana.•Southeast GoM has low abundance, but higher diversity and evenness compared to northern GoM samples from comparable depths.•Highly heterogenous habitat along Cuban slope resulting in large variation in macrofauna communities.

Microplastics have accumulated in the environment since plastic production began, with present-day observations that range from marine trenches to mountains. However, research on microplastics has only recently begun so it is unclear how they have changed over time in many oceanic regions. Our study addressed this gap by quantifying the temporal and spatial dynamics of microplastics in two deep-water regions of the Gulf of Mexico (GOM). We isolated agglutinated foraminifera from sediment cores and assessed microplastics that were incorporated into their tests. Our results indicated that microplastics were incorporated by agglutinated foraminifera after plastic production began. Microplastics were higher at deep-water sites and closer to the Mississippi River. This study confirms the presence of microplastic incorporation into agglutinated foraminifera tests and investigates microplastics in deep-water sediments in the GOM. Additional work is needed to fully identify the distribution of microplastics across the GOM and other oceanic basins.

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.

The Deep Water Horizon (DWH) oil spill occurred in the Northern Gulf of Mexico (NGoM) in 2010. Over 700 million liters of oil spilled into the NGoM in the 87 days the wellhead was actively leaking. Samples were taken from sites annually to semi-annually from 2010-2023 to provide a spatial and temporal benthic assessment for the NGoM. Reference conditions provide an ecological snapshot of the marine environment and allow for quantitative assessment of impact and response in the case of future oil spills. Benthic foraminifera, which are single-celled, testate organisms that inhabit the seafloor have proven to be excellent indicators and records of ecological change. Stable carbon isotopes in benthic foraminifera shells (tests) have also proven to be effective indicators of petroleum carbon incorporation into the benthic system. Seafloor sediment cores were most recently collected in 2023, at specific time series sites in the NGoM, as a part of the Scientist-At-Sea program. The sediment cores were subsampled at 2 mm increments. Calcareous foraminifera species, Cibicidoides pachyderma and C. wuellerstorfi, were isolated for stable isotope (oxygen and carbon) analysis using a stable isotope ratio mass spectrometer (SIRMS). Stable isotopes from benthic foraminifera from two of the time-series sites have been measured continually from 2010 to 2017. This study will provide a comparison of the latest benthic foraminifera stable carbon isotopes profiles with previous collections to determine long-term recovery of the system, gain insight into natural variability, and establish long-term preservation of the DWH signal in fossil (downcore) benthic foraminifera tests. These records will continue to aid in the understanding of natural seafloor carbon cycling, and also in the event of future pollution events such as oil spills.

Scientific advancements in isotope geochemistry and sedimentary geochronology have allowed for high-resolution sampling of radioisotope activities in marine sediments. Given the recent advances in deep sea mining, the ability to understand the nature of radioisotope scavenging on resuspended sediments could provide essential insights into the impacts it may have on benthic environments. Sediment plumes from collector vehicles used in deep sea mining often spread their footprint over the seafloor and sensitive indicators such as radioisotopes are critical to their further investigation. This pilot study attempts to replicate the natural process of radioactive isotope scavenging of resuspended sediments in a laboratory setting. Radioisotopes, such as (super 210) Pb and (super 234) Th, are valuable tools for evaluating human and natural impacts recorded in the sediment record. Radioisotopes adsorb to sediment particles and remain until they are deposited on the ocean floor and begin to decay. The known decay rate of (super 234) Th and (super 210) Pb allows for the calculation of a date of deposition and also for the determination of the spatial extent of the resuspended sediment in the surface sediments. A tank replicating the deep sea sedimentary environment was constructed for this study and filled with 4.5kg of sediment. This sediment was known to have been deposited over 100 years ago, thereby no longer showing any excess radioisotope activity. Roughly 100 liters of seawater flowed through the tank at one liter per minute as the sediments were resuspended every twenty minutes for thirty-second intervals. Gamma spectrometric analysis of post-experiment samples revealed that (super 234) Th adsorbed to the resuspended sediment during the experiment and supported the constant rate of supply age model. Results from the radioisotope (super 210) Pb, however, were inconclusive and require further testing. This pilot study only scratches the surface of the questions about the implications of resuspended radioisotope adsorption in deep sea settings. Radioisotope scavenging has high utility for determination of the spatial extent of ambient sediment plumes caused by deep sea mining. Further research into methods of artificial resuspension and adsorption are critical to fully understanding this process.

In the context of climate regulation and anthropogenic waste detoxification (e.g. oil spills), estimates of deep ocean sedimentation and carbon sequestration are of the utmost importance. Radiocarbon (14C) is a common radioisotope that can be used to establish millennial scale sediment accumulation rates. The objectives of this study were to: 1) establish ages for co-occurring total organic carbon (TOC) and planktic foraminifera (carbonate) in the northeastern Gulf of Mexico (GoM), 2) use these ages to estimate accumulation rates independently, 3) identify any evidence of redistribution, and 4) examine any offset between TOC and carbonate 14C ages as a tool to potentially identify selective TOC transport. Sediment samples were collected in May 2018 from the RV Point Sur using an Ocean Instruments MC-800 multi corer. Radiocarbon measurements of both planktic foraminifera and TOC subsamples were made at the National Ocean Science Accelerator Mass Spectrometry Facility (NOSAMS). Radiocarbon ages, calibrated using the OxCal 4.4, ranged from recent to 6407 BP. Linear (LAR: 4–24 cm/kyr) and mass accumulation rates (MAR: 1.5–11.5 g/cm2/kyr) were generally consistent with those reported by other recent studies in the GoM. At two sites, C14 ages decreased from the surface to the second sampling increment which was consistent with sediment redistribution. The TOC-carbonate offsets, which are indicative of lateral advection and organic matter aging, were lower than those found in the majority of other regions, which was consistent with less lateral transport or a more oligotrophic setting. The magnitude in radiocarbon age offsets with depth could potentially be used as a relative aging or transport assessment tool in areas with little resuspension. •Total organic carbon and planktic foraminifera 14C ages established for sites in the Gulf of Mexico.•Total organic carbon and carbonate 14C age offsets are indicative of advection and aging.•14C age offsets with depth could be used as relative aging or transport assessment tool.

The majority of observations and discrete samplings for Karenia brevis blooms and related impacts are focused in the neritic zone since blooms occur throughout the water column. Far less sampling in the benthic zone has been conducted, thus the role of the benthos in bloom initiation and other key dynamics remains unknown. Recent evidence that similar species, including Karenia mikimotoi, produce resting stages has led to an increased interest in understanding the life cycle of K. brevis and thus the role that the benthic environment plays on initiation and termination of bloom events. This collaborative effort involves working directly with the Florida Fish and Wildlife Conservation Commission-Fish and Wildlife Research Institute's Harmful Algal Bloom (FWRI-HAB) group to investigate K brevis bloom and benthic coupling with the goals of 1) providing environmental context (sedimentology) of the benthos; 2 ) establishing baselines of K brevis bloom impacts/conditions in benthic environments (benthic foraminifera assessments); and 3) evaluating the historical context and flux rates (short lived radioisotopes). These collective goals are critical for laying the groundwork for a cyst or benthos monitoring program to aid in forecasting bloom dynamics, especially initiation and termination. Samples were collected by FWRI and collaborators on directed and routine FWRI-HAB sampling efforts, and analyses were performed at Eckerd College Galbraith Marine Science Laboratory which include the examination of surficial sediments, sediment trap material, and sediment cores. These efforts are critical to the advancement and enhancement of bloom modeling and prediction capabilities.

This study investigates benthic impacts of the Piney Point discharge into Tampa Bay by characterizing the benthic environment and identifying associated changes. A time series of water quality measurements and sediment samples were collected along two shore normal transects into Tampa Bay from Port Manatee and Bishops Harbor during the discharge, and thereafter on a monthly basis. In situ measurements showed a pH decrease in close proximity to the discharge during the initial collection as compared to subsequent collections. Sedimentology was used to characterize the benthic environment to identify any changes in sediment source(s), deposition patterns and rates. Results indicate a variety of sediment types and the influence of anthropogenic activities (dredging/ shipping traffic) along the Port Manatee transect. Benthic foraminifera assemblages and indices, utilized as a bioindicators of ecological quality, indicate that sites near Port Manatee have low abundance (<30 individuals) and few to no living individuals. Sites further from Port Manatee have increased abundance, but are dominated by Ammonia spp. (low O2 and stress tolerant). Sediment and fish samples were analyzed for short-lived radioisotopes to determine if activities were elevated due to the discharge and subsequently to trace the spatial distribution and incorporation into the ecosystem (fish). Sediment samples have elevated activities in Port Manatee and Bishops Harbor that may be attributed to natural and/or anthropogenic influences including, but not limited to, groundwater discharge and input of phosphate-rich water/sediments, such as the Piney Point discharge. Radioisotope baselines will determine the relative contribution of the Piney Point discharge toward the elevated activities and its potential as a tracer of the discharge. Integration with previous benthic studies (baselines), and collaborator data provide a comprehensive understanding of the event, its potential impacts, and evolution over time.