DeSoto Canyon Research Articles

Constraining the Spatial Extent of Marine Oil Snow Sedimentation and Flocculent Accumulation Following the Deepwater Horizon Event Using an Excess 210Pb Flux Approach.

Following the Deepwater Horizon (DWH) event in 2010, there were several lines of evidence indicating the presence of marine oil snow sedimentation and flocculent accumulation (MOSSFA). A significant amount of marine oil snow formed in the water column of the northern Gulf of Mexico (nGoM), settled rapidly, and ultimately accumulated in the sediments of the nGoM. This study utilized a commonly used radioisotope tracer (excess 210Pb, 210Pbxs) from 32 sediment cores collected from 2010 to 2013 to characterize the spatial extent of MOSSFA on the seafloor. Relative to pre-DWH conditions, an increase in 210Pbxs flux occurred in two distinct regions: (1) in the western portion of the study area on an east-northeast to west-southwest axis, stretching 230 km southwest and 140 km northeast of the DWH wellhead, and (2) in the eastern portion of the study area on a 70 km northeast to southwest axis near the DeSoto Canyon. The total sedimentary spatial extent of MOSSFA, as calculated by increased 210Pbxs flux after 2010, ranged from 12 805 to 35 425 km2. 210Pbxs flux provides a valuable tool for documenting the spatial extent of MOSSFA following DWH and will continue to aid in the determination of advective transport and ultimate depocenters of MOSSFA material.

Near-inertial motions in the DeSoto Canyon during Hurricane Georges

Hurricane Georges passed directly over an array of 13 moorings deployed in the DeSoto Canyon in the northern Gulf of Mexico on 27–28 September 1998. Current velocity data from the mooring array were analyzed together with a primitive-equation model simulation with realistic hurricane forcing, to characterize the generation and propagation of the hurricane-generated near-inertial waves. The model successfully reproduces the observed mean (sub-inertial) and near-inertial motions. The upper ocean response is strongly impacted by the canyon ‘wall’: a strong jet is formed along the slope, and the near-inertial motions on the shelf are rapidly suppressed. The model results moreover suggest that strong near-inertial waves in the mixed layer are mostly trapped in an energy flux recirculating gyre around the canyon. This gyre retains the near-inertial energy in the canyon region and enhances the transfer of near-inertial energy below the mixed layer. Additional simulations with idealized topographies show that the presence of a steep slope rather than the canyon is fundamental for the generation of this recirculating gyre. The near-inertial wave energy budget shows that during the study period the wind generated an input of 6.79×10–2Wm–2 of which about 1/3, or 2.43×10–2Wm–2, was transferred below the mixed layer. The horizontal energy flux into and out of the canyon region, in contrast, was relatively weak.

Submesoscale currents in the northern Gulf of Mexico: Deep phenomena and dispersion over the continental slope

This study examines the mesoscale and submesoscale circulations along the continental slope in the northern Gulf of Mexico at depths greater than 1000m. The investigation is performed using a regional model run at two horizontal grid resolutions, 5km and 1.6km, over a 3year period, from January 2010 to December 2012.Ageostrophic submesoscale eddies and vorticity filaments populate the continental slope, and they are stronger and more abundant in the simulation at higher resolution, as to be expected. They are formed from horizontal shear layers at the edges of highly intermittent, bottom-intensified, along-slope boundary currents and in the cores of those currents where they are confined to steep slopes. Two different flow regimes are identified. The first applies to the De Soto Canyon that is characterized by weak mean currents and, in the high-resolution run, by intense but few submesoscale eddies that form near preferentially along the Florida continental slope. The second is found in the remainder of the domain, where the mean currents are stronger and the circulation is highly variable in both space and time, and the vorticity field is populated, in the high-resolution case, by numerous vorticity filaments and short-lived eddies.Lagrangian tracers are deployed at different times along the continental shelf below 1000m depth to quantify the impact of the submesoscale currents on transport and mixing. The modeled absolute dispersion is, on average, independent of horizontal resolution, while mixing, quantified by finite-size Lyapunov exponents and vertical relative dispersion, increases when submesoscale processes are present. Dispersion in the De Soto Canyon is smaller than in the rest of the model domain and less affected by resolution. This is further confirmed comparing the evolution of passive dye fields deployed in De Soto Canyon near the Macondo Prospect, where the Deepwater Horizon rig exploded in 2010, and at the largest known natural hydrocarbon seep in the northern Gulf, known as GC600, located a few hundred kilometers to the west of the rig wellhead.

Delineation of post-KPg carbonate slope deposits as a sedimentary record of the Paleogene linkage of De Soto Canyon and Suwannee Strait, northern Gulf of Mexico

Seismicity generated from the Chicxulub impact has been postulated as the cause for the dramatic alteration of basin margin morphology and catastrophic movement of sediments in the Gulf of Mexico (GOM). Immediately following the impact, the formation of the Cretaceous-Paleogene boundary deposit (KPBD) was widespread and caused considerable erosion as portions of the Florida Escarpment collapsed, inducing sediment failure on the northern Florida Platform and formation of the ancestral De Soto Canyon. Overlying the more prominent KPBD existed a distinct, younger, post-Cretaceous/Paleogene carbonate slope deposit (CSD) confined within the De Soto Canyon bathymetric feature. Presence of this discrete unit provided insight into the post-impact history of De Soto Canyon and its long-duration connection to the Suwannee Strait, which linked the GOM with the Atlantic Ocean for almost 40 ma. We have postulated that the bathymetric low of the De Soto Canyon acted as a conduit for west to east sediment movement from nearby carbonate-dominated shorelines into the canyon in episodic sediment transport events from the Danian to the earliest Miocene. Closure of the Suwannee Strait, due to sediment infilling, terminated deposition of the De Soto Canyon CSD. This was followed by major siliciclastic influx as the paleo-Tennessee drainage system began to enter the Mississippi Canyon area.

Shelf break exchange events near the De Soto Canyon

Observations of currents, temperature, sea-surface height, sea-surface temperature and ocean color, derived from moorings, surface and deep drifters, hydrographic surveys, and satellites, are used to characterize shelf-slope exchange events near the apex of the De Soto Canyon in the northeast Gulf of Mexico. During the winter of 2012–2013, shelf-break time series showed a number of events where cold shelf water extruded over the slope. These events were largely consistent with slope eddies of both signs influencing shelf break currents. Larger-scale circulations, derived from the Loop Current and a separating Loop Current eddy, strongly influenced circulation over the De Soto slope during summer 2012, with flow patterns consistent with potential vorticity conservation over shoaling topography. Statistical investigation into shelf-slope exchange using large numbers of surface drifters indicated that export from the shelf is larger than vice-versa, and is more uniformly distributed along the shelf break. Import onto the shelf appears to favor a region just east of the Mississippi Delta, which is also consistent with the observed onshore transport of surface oil from the Deepwater Horizon disaster.

Improved Surface Velocity and Trajectory Estimates in the Gulf of Mexico from Blended Satellite Altimetry and Drifter Data

AbstractThis study investigates the results of blending altimetry-based surface currents in the Gulf of Mexico with available drifter observations. Here, subsets of trajectories obtained from the near-simultaneous deployment of about 300 Coastal Ocean Dynamics Experiment (CODE) surface drifters provide both input and control data. The fidelity of surface velocity fields are measured in the Lagrangian frame by a skill score that compares the separation between observed and hindcast trajectories to the observed absolute dispersion. Trajectories estimated from altimetry-based velocities provide satisfactory average results (skill score > 0.4) in large (~100 km) open-ocean structures. However, the distribution of skill score values within these structures is quite variable. In the DeSoto Canyon and on the shelf where smaller-scale structures are present, the overall altimeter skill score is typically reduced to less than 0.2. After 3 days, the dataset-averaged distance between hindcast and drifter trajectories, , is about 45 km—only slightly less than the average dispersion of the observations, km. Blending information from a subset of drifters via a variational method leads to significant improvements in all dynamical regimes. Skill scores typically increase to 0.8 with reduced to less than half of . Blending available drifter information with altimetry data restores velocity field variability at scales not directly sampled by the altimeter and introduces ageostrophic components that cannot be described by simple Ekman superposition. The proposed method provides a means to improve the fidelity of near-real-time synoptic estimates of ocean surface velocity fields by combining altimetric data with modest numbers of in situ drifter observations.

Sedimentation Pulse in the NE Gulf of Mexico following the 2010 DWH Blowout.

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.

A simulation analysis of the plankton fate of the Deepwater Horizon oil spills

A two-dimensional (2-D) ecosystem model, set within the De Soto Canyon ecotone of the Northern Gulf of Mexico (NGOM) and driven by 3-D flow fields from decoupled water and air circulation models, explores the daily food web and sedimentary consequences, as well as potential public health implications, of oil-, nutrient-, and overfishing-induced transitions of dominant particle transports to the sea floor: from changing vectors of copepod fecal pellets to those of marine snow over the annual period of 2010–2011. Recent spilled petrochemicals are found to minimally impact already decimated zooplankton populations on the West Florida shelf (WFS). They facilitate instead formation of marine snow macroaggregates. These recent oil effects just exacerbate other results of prior overfishing in the absence of major eutrophication along the eastern side of this ecotone. East of the De Soto Canyon ecotone, overly optimistic removals of piscivore fish stocks over the last half-century had already caused a trophic cascade, with ~95% of WFS marine pelagic herbivore losses occurring by 2010, before the Deepwater Horizon [DWH] blowout. By contrast, west of De Soto Canyon, zooplankton on the eutrophic Louisiana shelf were less impacted by overfishing, retaining order of magnitude more stocks of the same genera of copepods. Yet, during descent of modeled marine snow to the ~1200-m isobath of the upper slope, ~98% of the particulate import to the benthos is now mainly clay minerals of Mississippi River origin along the ecotone. The lithogenic particles are scavenged by aggregates from the whole water column, not only biotic plankton from just the near-surface euphotic zone. The model results here replicate concurrent time series of: 1) annual sediment accumulations, measured at the sea floor; 2) bimonthly onshore nutrient supplies, due to upwelling, forced remotely by the Loop Current; and 3) near-surface weekly phytoplankton changes, seen by satellite. Because of such minimal grazing stresses, after the most recent DWH oil reductions to ~50% of the remaining few WFS copepods, their decreased herbivory amounts to only an 8% fecal pellet contribution to the model's particle fluxes, with 92% settling as marine snow to the sea bottom of the continental slope in 2010–2011 By contrast, 12% of the biotic particle exports, entrained within marine snow, result from increased loadings of Mississippi River nutrients via opened flood gates, while 50% of the ungrazed sinking phytoplankton are fueled by decadal anomalies of increased nutrient supplies from greater upwelling by the Loop Current. Finally, of the exiting phytodetritus embedded within marine snow from the 2010–2011 water columns, 38% are due to uptake of autocthonous nutrients in the slope waters. But, explicit upward fluxes of nutrient-poor, petrochemical dissolved organic carbon [DOC] substrates for use by aphotic chemolithoautotrophic bacterial bioremediators, responding to DWH oil releases and external dissolved nutrients, are now ignored in the present model. These model results also relate downstream, wind-borne trajectories of evaded potential marine aerosolized toxins to both human asthma episodes and total mercury amounts in coastal soils, without explicit sea-air exchanges of initial aerosols. Thus, future simulation analyses, with instead extant numerical descriptions of breaking wave exports of oil, harmful algal bloom [HAB], and Hg aerosol poisons, must fully couple onshore 3-D aerial imports of this suite of marine toxins to direct causal factors of adjacent human health impacts, constrained by known surrogate asthma hospitalization rates. They must next deconvolve net multiyear oil and top-down NGOM indirect losses of herbivores, within now unbalanced marine food webs of an overfishing-induced trophic cascade, from other concurrent anthropogenic forcings, due to in situ mercury, pesticide, and radionuclide poisonings, within linked habitats of both water and air, subject to continued climate and biotic changes of the southeastern U.S. seaboard.

Hydrocarbons in Deep-Sea Sediments following the 2010 Deepwater Horizon Blowout in the Northeast Gulf of Mexico

The Deepwater Horizon (DWH) spill released 4.9 million barrels of oil into the Gulf of Mexico (GoM) over 87 days. Sediment and water sampling efforts were concentrated SW of the DWH and in coastal areas. Here we present geochemistry data from sediment cores collected in the aftermath of the DWH event from 1000 – 1500 m water depth in the DeSoto Canyon, NE of the DWH wellhead. Cores were analyzed at high-resolution (at 2 mm and 5 mm intervals) in order to evaluate the concentration, composition and input of hydrocarbons to the seafloor. Specifically, we analyzed total organic carbon (TOC), aliphatic, polycyclic aromatic hydrocarbon (PAHs), and biomarker (hopanes, steranes, diasteranes) compounds to elucidate possible sources and transport pathways for deposition of hydrocarbons. Results showed higher hydrocarbon concentrations during 2010-2011 compared to years prior to 2010. Hydrocarbon inputs in 2010-2011 were composed of a mixture of sources including terrestrial, planktonic, and weathered oil. Our results suggest that after the DWH event, both soluble and highly insoluble hydrocarbons were deposited at enhanced rates in the deep-sea. We proposed two distinct transport pathways of hydrocarbon deposition: 1) sinking of oil-particle aggregates (hydrocarbon-contaminated marine snow and/or suspended particulate material), and 2) advective transport and direct contact of the deep plume with the continental slope surface sediments between 1000-1200 m. Our findings underline the complexity of the depositional event observed in the aftermath of the DWH event in terms of multiple sources, variable concentrations, and spatial (depth-related) variability in the DeSoto Canyon, NE of the DWH wellhead.

Nonlocal impacts of the Loop Current on cross‐slope near‐bottom flow in the northeastern Gulf of Mexico

AbstractCross‐slope near‐bottom motions near De Soto Canyon in the northeastern Gulf of Mexico are analyzed from a multidecadal ocean model simulation to characterize upwelling and downwelling, important mechanisms for exchange between the deep ocean and shelf in the vicinity of the 2010 BP Macondo well oil spill. Across the continental slope, large‐scale depression and offshore movement of isopycnals (downwelling) occur more frequently when the Loop Current impinges upon the West Florida Shelf slope farther south. Upwelling and onshore movement of isopycnals occurs with roughly the same likelihood regardless of Loop Current impingement on the slope. The remote influence of Loop Current on the De Soto Canyon region downwelling is a consequence of a high‐pressure anomaly that extends along the continental slope emanating from the location of Loop Current impact.

An analytic solution for barotropic flow along a variable slope topography

AbstractAn analytic solution is derived for the generic oceanographic situation of a barotropic current flowing along sloping topography. It is shown that the shallow water equations can be reduced to a heat‐like equation in which βeffect is balanced by Ekman dissipation. For constant topography, the system is found to admit a well‐known similarity solution and this solution is generalized to the case of variable topography. Several properties of the solution are explored, and an example is given for flow along the northern Gulf of Mexico slope, between the De Soto Canyon and the Mississippi Canyon. This “Topographic β‐plume” solution may serve as a model for further research concerning the influence exerted by geophysical boundary layers on the interior flow via their structure and stability.

Deep crustal structure of the northeastern Gulf of Mexico: Implications for rift evolution and seafloor spreading

AbstractWe image deep crustal structure using marine seismic refraction data recorded by a linear array of ocean‐bottom seismometers in the Gulf of Mexico Basin Opening project (GUMBO Line 3) in order to provide new constraints on the nature of continental and oceanic crust in the northeastern Gulf of Mexico. GUMBO Line 3 extends ~524 km from the continental shelf offshore Pensacola, Florida, across the De Soto Canyon and into the central Gulf basin. Travel times from long offset, wide angle reflections and refractions resolve compressional seismic velocities and layer boundaries for sediment, crystalline crust, and upper mantle. We compare our results with coincident multichannel seismic reflection data. Our velocity model recovers shallow seismic velocities (~2.0–4.5 km/s) that we interpret as evaporites and clastic sediments. A Cretaceous carbonate platform is interpreted beneath the De Soto Canyon with seismic velocities >5.0 km/s. Crystalline continental crust thins seaward along GUMBO Line 3 from 23–10 km across the De Soto Canyon. High seismic velocity lower crust (>7.2 km/s) is interpreted as extensive syn‐rift magmatism and possibly mafic underplating, common features at volcanic rift margins with high mantle potential temperatures. In the central Gulf basin we interpret thick oceanic crust (>8 km) emplaced at a slow full‐spreading rate (~24 mm/yr). We suggest a sustained thermal anomaly during slow seafloor‐spreading conditions led to voluminous basalt flows from a spreading ridge that overprinted seafloor magnetic anomalies in the northeastern Gulf of Mexico.

Why no red tide was observed on the West Florida Continental Shelf in 2010

Over the past two decades, the two most anomalous years for water properties on the west Florida continental shelf were 1998 and 2010. In both instances, the shelf was ventilated by relatively cold, nutrient-rich waters of deep ocean origin, which reset the background state underlying shelf ecology. The ventilation in both of these years derived from prolonged interactions of the Gulf of Mexico Loop Current with the shelf slope near the Dry Tortugas located on the southwest corner of the shelf. By contacting relatively shallow isobaths, the boundary current interactions there set the entire shelf into motion, facilitating upwelling across the shelf break, even to DeSoto Canyon some 500km away, and then across the shelf to the near shore. Such prolonged and intense upwelling of nutrient-rich water in 2010 contrasted with the more typically occurring locally wind driven upwelling conditions, whereby waters upwelled at the near shore are from the inner shelf, versus the deep ocean. Thus not all upwelling scenarios have similar consequences. Whereas the typical wind driven upwelling scenario is necessary for Karenia brevis red tide blooms to manifest along the coastline, the rarer, deep ocean induced upwelling scenario (as occurred in 1998 and particularly in 2010) acts to suppress K. brevis red tides because of the elevated inorganic nutrient conditions that they facilitate. Hence, minimal cell counts above background were observed in 1998, and no cell counts above background were observed in 2010. We conclude that the lack of red tide along the west coast of Florida in 2010 was due to anomalously large and protracted upwelling of nutrient-rich waters of deep ocean origin caused by Loop Current and eddy interactions with the shelf slope.

Habitat, movements and environmental preferences of dusky sharks, Carcharhinus obscurus, in the northern Gulf of Mexico

The dusky shark (Carcharhinus obscurus) is the largest member of the genus Carcharhinus and inhabits coastal and pelagic ecosystems circumglobally in temperate, subtropical and tropical marine waters. In the western North Atlantic Ocean (WNA), dusky sharks are overfished and considered vulnerable by the International Union for the Conservation of Nature. As a result, retention of dusky sharks in commercial and recreational fisheries off the east coast of the United States (US) and in the northern Gulf of Mexico is prohibited. Despite the concerns regarding the status of dusky sharks in the WNA, little is known about their habitat utilization. During the summers of 2008–2009, pop-up satellite archival tags were attached to ten dusky sharks (one male, nine females) at a location where they have been observed to aggregate in the north central Gulf of Mexico southwest of the Mississippi River Delta to examine their movement patterns and habitat utilization. All tags successfully transmitted data with deployment durations ranging from 6 to 124 days. Tag data revealed shark movements in excess of 200 km from initial tagging locations, with sharks primarily utilizing offshore waters associated with the continental shelf edge from Desoto Canyon to the Texas/Mexican border. While most sharks remained in US waters, one individual moved from the northern Gulf of Mexico into the Bay of Campeche off the coast of Mexico. Sharks spent 87 % of their time between 20 and 125 m and 83 % of their time in waters between 23 and 30 °C. Since dusky sharks are among the most vulnerable shark species to fishing mortality, there is a recovery plan in place for US waters; however, since they have been shown to make long-distance migrations, a multi-national management plan within the WNA may be needed to ensure the successful recovery of this population.

Meiofauna and Trace Metals From Sediment Collections in Florida After the Deepwater Horizon Oil Spill

Sediment from the Florida Gulf continental shelf was collected from 18 sites during October and November 2010 for meiofauna and trace-metals analysis. Collections were obtained using a Shipek® grab on the National Oceanic and Atmospheric Administration ship Pisces and spanned from the head of the DeSoto Canyon to off the southern end of the Florida peninsula approximately following the 100–200-m contour. Mean abundance of the dominant meiofaunal groups (nematodes, copepods, and polychaetes) was unchanged when compared with 2007–2009 data. Nematodes and copepods correlated positively with each other, and negatively with latitude and longitude, suggesting that there were higher densities in southern Florida. These results contrast with those from 2007–2009 in that previously nematodes had no correlation with latitude or longitude in Florida. Nickel (Ni) and vanadium (V) concentrations were higher in the western Florida locations and correlated positively with increasing depth. No relationship was found between Ni, V, and meiofauna densities.

Bryde's whale calls recorded in the Gulf of Mexico

Bryde's whale calls recorded in the Gulf of Mexico

Standing stocks and body size of deep-sea macrofauna: Predicting the baseline of 2010 Deepwater Horizon oil spill in the northern Gulf of Mexico

A composite database encompassing 6 benthic surveys from years 1983 to 2003 was constructed to evaluate the distribution of macrofaunal biomass in the deep Gulf of Mexico (GoM) prior to the Deepwater Horizon oil spill. Predictive models based on optimal scaling of ocean color data and high resolution bathymetry were employed to map the benthic biomass in the vicinity of spill site because no previous sampling had been conducted at that exact location. The predicted biomass declines with water and mixed layer depth and is an increasing function of surface primary production and temporal variation of sea surface temperature. The decline of animal size with depth, however, was a function of a shift of dominant abundance from large to small taxa. At a local scale, high benthic biomass in the N GoM was associated with the enhanced productivity by the nutrient-laden Mississippi River outflows, offshore transport of the river plumes, and upwelling along the northern edge of the Loop Current. The apparent biomass enhancement at the Mississippi and De Soto Canyon and deep sediment fan was presumably related to lateral down-slope advection of organic carbon from the surrounding continental margin. Except for the Campeche Bank, the meager biomass of the Mexican margin may reflect the characteristic low-productivity Caribbean water that enters the GoM through Yucatan Strait. Benthic biomass in the N GoM was not statistically different between comprehensive surveys in the years 1983–1985 and 2000–2002. The stock assessment and biomass predictions from 669 cores at 170 locations throughout the deep GoM provide an important baseline of the sediment-dwelling fauna that may be subjected to immediate or long-term impacts from the oil spill or from climate change.

The dynamics of the Mississippi River plume: Impact of topography, wind and offshore forcing on the fate of plume waters

[1] High-resolution numerical simulations of the northern Gulf of Mexico region using the Hybrid Coordinate Ocean Model (HYCOM) were employed to investigate the dynamical processes controlling the fate of the Mississippi River plume, in particular the conditions that favor cross-marginal transport. The study focuses on the effects of topography, wind-driven and eddy-driven circulation on the offshore removal of plume waters. A realistically forced simulation (nested in a data-assimilative regional Gulf of Mexico HYCOM model) reveals that the offshore removal is a frequent plume pathway. Eastward wind-driven currents promote large freshwater transport toward the shelf break and the DeSoto Canyon, where eddies with diameters ranging from 50 to 130 km interact with the buoyant plume and effectively entrain the riverine waters. Our estimates show that the offshore removal by eddies can be as large as the wind-driven shelf transport. The proximity of eddies to the shelf break is a sufficient condition for offshore removal, and shelf-to-offshore interaction is facilitated by the steep bottom topography near the delta. Strong eddy-plume interactions were observed when the Loop Current System impinged against the shelf break, causing the formation of coherent, narrow low-salinity bands that extended toward the gulf interior. The offshore pathways depend on the position of the eddies near the shelf edge, their life span and the formation of eddy pairs that generate coherent cross-shelf flows. This study elucidates the dynamics that initiate a unique cross-marginal removal mechanism of riverine low-salinity, nutrient-rich waters, allowing their export along connectivity pathways, induced by a large-scale current system.

Mississippi River discharge over the last ~560,000 years — Indications from X-ray fluorescence core-scanning

The long term history of terrigenous flux to the Gulf of Mexico via the Mississippi River is hardly known. We here present geochemical and sedimentological data to approximate the varying Mississippi River sediment influx into the northeastern Gulf of Mexico (GoM) over the last six glacial-interglacial cycles (MIS 1 to 14). Our study is based on the IMAGES sediment core MD02-2576 that was recovered from the DeSoto Canyon and is located ~ 200 km south to the recent Mississippi River delta and ~ 150 km east of the recent coastline of Florida. Concentrations of siliciclastic elements in bulk sediment samples were estimated from XRF scanning and calibrated by single bulk XRF-analyses. Elemental ratios of the sedimentary record correspond to ratios from the Mississippi River catchment rather than to the core close Alabama and Mobile River catchments. The siliciclastic major element potassium (K) with enhanced surface concentrations in the northwestern Mississippi River catchment shows varying occurrence downcore and here serves as a proxy for Mississippi River sediment discharge variability. Changes in sedimentation rate and magnetic susceptibility further support the variations in Mississippi River influx. Our data were compared with Mississippi River terrestrial archives in the form of loess and terrace deposits that back up our interpretations of enhanced glacial phase Mississippi River influx triggered by strengthened fluvial river runoff and changing fluvial and ice sheet dynamics. Mississippi River influx was at a maximum during glacial MIS 2/3, MIS 8 and MIS 10. Late glacial MIS 6 deviates from this pattern being a period of reduced Mississippi River influx at the core location, probably due to a westward shift of the Mississippi River delta.

GeoStreamer – increasing the signal-to-noise ratio using a dual-sensor towed streamer

<P align=justify>Seismic exploration today and in the future will require looking for deeper reservoirs, under more complex overburden and within increasingly complex stratigraphic traps. To be able to image these reservoirs well, the bandwidth of the seismic data must be extended at both low and high frequencies and an improvement in signal-to-noise ratio is also required. A new dual-sensor streamer was launched in 2007 and has since been deployed in 2D mode in several basins worldwide. The first 3D dual-sensor streamer survey at the DeSoto Canyon in the Gulf of Mexico was completed in early 2009. The combined dual-sensor data from the GeoStreamer are superior to the data recorded using a conventional towed streamer in two ways. Elimination of receiver ghosts provides a much clearer, higher resolution image and, due to the ability to tow deeper, the stronger low reflection signal frequencies and the weaker swell noise result in greater penetration. A gain of 10−15 dB in signal-to-noise ratio is achievable for the low frequencies when the streamer is towed at a depth of 15−30 m.</P>