Gregg R. Brooks

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.

The Deepwater Horizon oil spill (DWHOS) released an unprecedented amount of oil ( approximately 4.0 million barrels) at a depth of 1500 m, resulting in the formation of multiple hydrocarbon mixtures at the surface and within the water column (e.g., oil slicks, submerged plumes) due to distinct weathering processes in different water column compartments. Moreover, an unexpected and protracted sedimentation event was observed of oil-associated marine snow (MOS), and oil-mineral aggregates (OMAS) referred to as MOSSFA (marine oil snow sedimentation and flocculent accumulation). Although multiple studies have examined DWH oil residues across a broad range of environments, a comprehensive discussion of the role of the different hydrocarbon mixtures from the water column on sediment chemistry, oil-residues fate, and impacts is not available. Here, we present a wide range of chemical data and diagnostic ratios from sediment cores collected in the northern GoM to identify and quantify water column hydrocarbon mixtures in the sediment samples. Results indicate that larger amounts of deposited oil-residues came from the water column (e.g., OMAS), followed by the surface, and a small portion from the submerged plumes (53%, 46%, and 1% respectively, of the total amount of oil that remained in the environment after recovery efforts). These results demonstrate that most of the weathering of the spilled oil occurred before deposition and tight connectivity between the water column and the sedimentary environment. This water-sediment connectivity is critical for understanding the effect of other physical processes (e.g., resuspension) on redistribution of oil-residues on the seafloor. Furthermore, this approach can be used for forecasting the long-term fate of deposited oil-residues on the seafloor, critical during response and planning activities associated with oil spills at depth.

High-resolution bathymetry data is limited to less than 5% coverage of the wide, shallow West Florida Shelf. The Continental Shelf Characterization and Mapping Project (C-SCAMP) has collected over 1200km (super 2) of high-resolution multibeam bathymetry and backscatter data from 2015 to 2017, amounting to an additional 1%, and mapping efforts are ongoing. Complementary data sets including sediment analysis of Shipek grab samples and visual analysis of towed-underwater video from the Camera-Based Assessment Survey System (C-BASS) help to further identify seafloor characteristics and habitat assemblages in these areas. Multibeam data reveal three paleoshoreline complexes of similar character between 40m and 80m water depth. These paleo-peninsulas extend 30-40km oblique to regional contours. Each area includes a main ridge axis with smaller ridge complexes splitting off on the southern end, and a prominent ridge along the steeper western margin of the feature. Preserved features observed in bathymetry within these paleo-peninsulas include shorelines, dune complexes, shoals, tidal deltas, and spit formations. Preliminary analysis of sediment samples shows that higher backscatter on the shallower portions of these features corresponds with coarser-grained sediments. The high-relief ridges apparent in bathymetry are shown to be moderate- to high-relief hard bottom in towed-underwater video. The analysis of these different data types will result in detailed description of the geomorphology and benthic habitat characteristics, including relationships between depth, slope, rugosity, backscatter, and bottom types. These characteristics are influenced by paleoshoreline structures. Previously collected sub-surface data, as well as modern analogs, such as the west coast of Florida, Western Australia and other low-latitude, low-relief coasts provide insight into the geologic origin of these features.

The Caribbean has significant exposure to tsunamis from multiple sources, such as earthquakes, volcanoes, and landslides. Due to the limited historical record in the region, paleotsunami deposits provide important information about the size, location, and sources of these events. In turn, these data inform the public and policymakers about the tsunamigenic threat to their communities. A key challenge is that tsunami deposits are often poorly preserved. However, a good candidate for high preservation potential are coastal salt ponds commonly found on the perimeter of tropical islands. The US Virgin Islands has both high susceptibility to tsunamis and large, low lying salt ponds. The most prominent historical example of a tsunami in the US Virgin Islands is the 1867 event which caused widespread devastation throughout the region, including Puerto Rico. One of the hardest hit locations was Frederiksted, on the western end of St. Croix, US Virgin Islands with 7m runups that beached the USS Monongahela. Frederiksted is also in close proximity to a large coastal salt pond. We targeted this, and older, events by collecting a series of sediment cores at four sites in the salt pond during a summer 2017 field campaign. At each location we acquired a 3" aluminum core and a 4" acrylic companion core to core refusal, which most often occurred at a impenetrable horizon. Maximum core recovery was .79m and the average was .54m. Each 4" core was extruded in 1cm intervals and used to determine grain size, total carbon content, and age dating via radioisotope dating. The 3" core was scanned in a X-Ray CT Lab, split, described, and samples from key layers were targeted for detailed sedimentological analyses. The defining stratigraphic sequence is fine-grained muds interspersed with coarse-grained units that exhibit a fining-upwards trend and contained a variety of marine debris, which we infer to represent tsunami or tropical storm event deposits. However, each core did not exhibit the same stratigraphic sequence, suggesting that core location is highly important to accurately establishing the tsunami record. Further analyses will constrain age and stratigraphic control.

A suite of short push cores were collected in and around seagrass meadows located within Tomales Bay, CA and Coral Bay, St. John, U.S. Virgin Islands to determine regional differences in sedimentology between sediments in seagrass and sediment outside seagrass beds. Seagrasses are the least studied type of blue carbon, however preliminary results already indicate large carbon reservoirs below the beds in both the northern California estuary and Coral Bay. Push cores were analyzed for grain size, carbonate content, total organic matter, and microfossil assemblages. Microfossil assemblages were examined in order to determine changes in biological communities over time. Distinguishable patterns in sedimentology between seagrass and non-seagrass sediments may provide indications about surrounding water chemistry. Due to upwelling off the coast of California, the water of Tomales Bay is highly acidic. For this reason, it is expected that carbonate content will be higher in sediment from seagrass beds compared to outside the beds in Tomales Bay. In Coral Bay, however, there should be no difference in carbonate content between sites within seagrass beds and adjacent sediment because the water in that region has a higher pH and therefore will not be as highly impacted by the presence of seagrass beds.

Throughout history, significant portions of the native vegetation of many Caribbean islands were replaced by cropland. Even though most islands eventually underwent reforestation, sediment yields and deposition rates appear to be higher now than throughout the past millennia, and this suggests that coral reef systems are experiencing an unprecedented level of sediment-related stress. Given the present-day emphasis on erosion control projects to restore coral reefs of the US Caribbean, it is of utmost importance to develop a quantitative understanding of the effects of both land development and watershed restoration activities on sediment delivery at various spatio-temporal scales. Efforts to measure contemporary erosion, sediment delivery and deposition rates have been conducted on the island of St. John-USVI since 2009. Sediment yields under natural conditions from the small (<10 km2) watersheds in this dry sub-tropical setting are between 1 and 10 Mg km-2 yr-1. Current sediment yields are 2 - 50 times higher than background depending on unpaved road network abundance and characteristics. Our efforts indicate that a watershed restoration program implemented in 2010-2011 within the 13-km2 Coral Bay watershed resulted in the reduction of annual sediment delivery rates from 445 Mg yr-1 to 327 Mg yr-1. Marine sedimentation rates of terrigenous materials based on sediment trap data were 6 - 24 times greater below developed watersheds relative to undeveloped catchments and were consistent with spatial comparisons of modeled sediment yields. At sites located within reef systems, total and silt deposition rates during sampling periods with major storms exceeded rates shown to harm corals more frequently in developed areas. Terrigenous sedimentation rates during periods with equivalent storms were reduced following watershed restoration. These results suggest that targeted watershed restoration may be effective in reducing sedimentation where land development and sediments are considered a major threat to coral reefs.