Gregg R. Brooks

Deep-sea offshore northwestern Cuba is the less studied zone of the Gulf of Mexico (GoM). Our study aimed to set an environmental baseline and investigate a potential west-east gradient of sediment properties and nematode diversity along the northwestern Cuba. Sediments were collected by multicorer at nine sites in the insular slope between 974 and 1682 m depth. Sediment texture and composition showed a west-east gradient caused likely by narrowing of shelf width and increasing of downslope transport of terrigenous material. We found clear signatures of heavy metal pollution likely derived from Havana City but also from open-sky mining (Castellanos mining complex) and port dredging operations (Mariel). Nematode assemblages were dominated by the deep-sea genera: Acantholaimus, Metadasynemella, Desmodorella, Cervonema, Daptonema, Halalaimus, and Pselionema. α-diversity was about 20–50 genera and γ-diversity of about 100 genera. The β-diversity was substantial likely because small-scale patchiness of resources and heavy metals stress that increased assemblage variability. Individual weight decreased with depth indicating food-supply limitation in the deep sea; also, heavy metals were negatively correlated with weight suggesting deleterious effects on the growing. Our findings indicate that heavy metal pollution has reached deep-sea sediments and corroborate the long-range effects of anthropogenic activities on deep sea. Taxonomic diversity and biological traits (life strategy and trophic composition) constituted a powerful bioindicator of benthic health and benchmark for future potential disturbances in the region. Northwestern Cuba slope is still an understudied portion of the GoM and more research is needed to fully understand its biodiversity and biogeochemical patterns.

Tropical islands such as St. John in the U.S. Virgin Islands are naturally susceptible to terrigenous (land-based) sediment erosion due to their high-relief slopes, fast weathering rates, and intense precipitation events. Nearshore ecosystems that exist near these islands tend to thrive in static conditions, and are especially stressed by increases in terrigenous input. In the last few decades, island development and population have increased dramatically in some areas of St. John. We conducted a detailed characterization of watersheds and their sediments from ‘source to sink’ in eastern St. John. To accomplish this we combined field observations and sampling with a digital elevation model. Our research was focused on several morphologically similar embayments in eastern St. John; three impacted by anthropogenic development (Coral Harbor, Johnson Bay, and Sanders Bay) and an adjacent, virtually undeveloped bay within the Virgin Islands National Park and Virgin Islands Coral Reef National Monument (Otter Creek). We found a large disparity in upslope watershed size between Otter Creek and Coral Harbor: Otter Creek (0.09 km²) is ∼73× smaller than Coral Harbor (6.54 km²). As expected, watersheds transport terrigenous volcaniclastic sediments directly to the marine environment where shallow-water marine carbonates precipitate. Terrigenous volcaniclastic sediments persist furthest from the source in the basin of the largest watershed with the most development (Coral Harbor), and decay closest to the source in the basin of the smallest watershed with the least development (Otter Creek). Due to large disparities in watershed size, further research is required in order to determine the relative contribution of development on the distribution of terrigenous sediments.

The objective of this study was to investigate the impacts of the Deepwater Horizon (DWH) oil discharge at the seafloor as recorded in bottom sediments of the DeSoto Canyon region in the northeastern Gulf of Mexico. Through a close coupling of sedimentological, geochemical, and biological approaches, multiple independent lines of evidence from 11 sites sampled in November/December 2010 revealed that the upper ~1 cm depth interval is distinct from underlying sediments and results indicate that particles originated at the sea surface. Consistent dissimilarities in grain size over the surficial ~1 cm of sediments correspond to excess ²³⁴Th depths, which indicates a lack of vertical mixing (bioturbation), suggesting the entire layer was deposited within a 4–5 month period. Further, a time series from four deep-sea sites sampled up to three additional times over the following two years revealed that excess ²³⁴Th depths, accumulation rates, and ²³⁴Th inventories decreased rapidly, within a few to several months after initial coring. The interpretation of a rapid sedimentation pulse is corroborated by stratification in solid phase Mn, which is linked to diagenesis and redox change, and the dramatic decrease in benthic formanifera density that was recorded in surficial sediments. Results are consistent with a brief depositional pulse that was also reported in previous studies of sediments, and marine snow formation in surface waters closer to the wellhead during the summer and fall of 2010. Although sediment input from the Mississippi River and advective transport may influence sedimentation on the seafloor in the DeSoto Canyon region, we conclude based on multidisciplinary evidence that the sedimentation pulse in late 2010 is the product of marine snow formation and is likely linked to the DWH discharge.

Over 100 sediment cores were collected from US Virgin Island coastal marine and salt pond environments to document the record of sediment delivery to the coastal system on annual to millennial time scales, and the extent to which human activities have influenced sedimentation patterns. Cores were analyzed for sedimentology, geochronology (²¹⁰Pb, ¹³⁷Cs, ⁷Be and ¹⁴C), and high-resolution elemental composition (scanning x-ray fluorescence (XRF) and laser ablation inductively coupled mass spectrometry (LA–ICP–MS)). The centennial–millennial record reveals the natural transition from terrestrial to marine ~6–4 kya in response to the Holocene sea-level rise. Coastal salt pond cores record initial pond development as coral growth between island headlands cut off and isolated existing marine embayments ~2 kya. The decadal–centennial record is dominated by human activities, primarily road construction, manifested as a ~10× increase in sediment accumulation rate where development is heavy, a ~2.5× increase where development is moderate, and no detectable increase in undeveloped areas. Annual millimeter- to centimeter-scale laminae in salt pond cores represent individual depositional ‘events’. Light-colored, Ca- and Sr-rich sands represent marine overwash deposits that in some cases can be tied to tropical cyclones. Dark-colored Al-, Fe-, Ti-, Si-rich sandy muds represent island runoff when rainfall rates exceed a threshold of ~1.2 cm/day. Organic-rich layers represent microbial mat growth between depositional ‘events’. Marine overwash and rainfall/runoff layers fluctuate on an annual–decadal scale during the historical period, but show much more variability over the previous ~1400 years, suggesting sediment source(s), depositional processes, and/or driving mechanisms have not remained constant during at least the late Holocene.

Analyses of sediment cores and surface samples collected in Coral Bay, St. John, USVI, reveal four sedimentary facies. The bottom three reflect the natural transgressive evolution of the bay during the Holocene sea-level rise. The surficial facies represents a dramatic increase in terrigenous sediment input since the 1950's. This surficial layer is defined by a decrease in grain size, increase in organic content, increase in terrigenous constituents, and subtle decrease in calcium carbonate content compared to underlying sediments. Based upon 210Pb and 14C dating, accumulation rates have increased by roughly one order-of--magnitude since the 1950's as a result of this increase in terrigenous sediment input. The surficial sediment layer likely represents an anthropogenic signal reflecting the dramatic increase in island development over the past few decades. The surficial “impacted” layer is most pronounced (i.e., exhibits the greatest deviation from underlying sediments) adjacent to the most hea...

Tampa Bay, a large, microtidal, clastic-filled estuary incised into Tertiary carbonate strata, is the largest estuary on Florida’s west coast. A total of 250 surface sediment samples and 17 cores were collected in Tampa Bay in order to determine the patterns and controlling factors governing the recent infilling and modern sediment distribution, and to examine the results in terms of current models of estuarine sedimentation and development. Surficial sediments in Tampa Bay consist of three facies types, each occurring in a distinct zone: modern terrigenous clastic muds occurring in the upper bay and around the bay periphery; relict, reworked-fluvial, quartz-rich sands occupying the open portion of the middle bay; and modern carbonate-rich, marine-derived sands and gravels occupying the lower bay. Factors controlling sediment distribution include: sediment source and supply rate; bathymetry, which is a function of the antecedent topography; and the winnowing effect of wind-generated waves that prohibits modern accumulation in the shallow middle bay. These factors also play a major role in the recent infilling history of Tampa Bay, which has progressed in four stages during the Holocene sea-level rise. Recently developed models of estuarine sedimentation are based primarily on mesotidal to macrotidal estuaries in terrigenous clastic settings in which sedimentation patterns and infilling history are a result of the relative contribution of marine and fluvial processes. Tampa Bay differs in that it was originally incised into carbonate strata, and neither fluvial or marine processes are interpreted to be major contributors to modern sediment distribution. Tampa Bay, therefore, provides an example of an unusual estuary type, which should be considered in future modeling efforts. *** DIRECT SUPPORT *** A01BY083 00004