Louisiana Continental Slope Research Articles

Sediments as a Source of Iron, Manganese, Cobalt and Nickel to Continental Shelf Waters (Louisiana, Gulf of Mexico)

Continental shelf sediments are a key source of trace metals to the ocean. In this study, we investigate the impact of sedimentary processes on water column concentrations of iron (Fe), manganese (Mn), cobalt (Co), and nickel (Ni) at five stations on the Louisiana continental shelf and slope, Gulf of Mexico. The highest trace metal concentrations were observed close to the seafloor at the most nearshore shelf station (water depth of 16 m), with most of the metals present in particulate form. This enrichment in the bottom water is likely the combined effect of input of trace metals in suspended matter from the Mississippi/Atchafalaya Rivers and, for Mn, Co, and Ni, benthic release from the shelf sediments. While particulate matter was the dominant carrier of Fe and Mn in bottom waters in the shelf and slope regions, Co and Ni were nearly exclusively present in dissolved form. Hence, lateral transport of Co and Ni in shelf waters is decoupled from that of Fe and Mn. Concentrations of particulate and dissolved trace metals in the water column generally decreased from the shelf to the slope, while those in the sediment increased. This suggests an increased retention of metals deposited on the sediment with distance from the coast, linked to the decrease in organic matter input and associated reductive sediment processes. The offshore decline in sediment trace metal mobilization is likely typical for river-dominated continental margins where most organic matter is deposited close to the coast.

Submarine channel evolution linked to rising salt domes, Gulf of Mexico, USA

An examination of halokinetics and channel evolution together in a deepwater system provides an opportunity to investigate how submarine channel morphology is locally affected by rising salt domes. The study area is located in the northern Gulf of Mexico (GOM), directly off the Louisiana continental slope in a prominent salt dome region. The influence of salt growth on submarine channel evolution is relatively understudied, particularly in the GOM. Utilizing high-resolution 3D seismic and well data and seismic geomorphology techniques, a long-lived (~3 Myr) Plio-Pleistocene submarine channel system has been investigated to show a relationship between variable phases of salt motion and planform morphology of preserved submarine channels.Our data suggest that local salt motion acts as a driver for submarine channel evolution. During the late Pliocene, when salt moved upward at a relatively fast rate, channels show distinct entrenchment with narrow channel belts and overall less sinuosity. When salt motion slowed down at the beginning of the Pleistocene, channels aggraded rapidly with preserved levees, and moved toward an equilibrium state with the expansion of channel belt widths. As our results indicate, the rate of salt diapirism exerted a first-order control on channel location and morphology and distribution of reservoir-prone units. This study cautions against readily invoking allogenic factors (e.g., sea level and climate) in explaining changes in submarine channel behavior and associated fan sedimentation, particularly in regions with salt tectonics.

Side-scan sonar imaging of hydrocarbon seeps on the Louisiana continental slope

Submarine hydrocarbon seeps are widespread in the northwestern Gulf of Mexico and give rise to unusual seafloor features that are commonly poorly defined because water depth limits the resolution afforded by sea-surface seismic profiles. Detailed seep images are possible with side-scan sonars because such devices use beam-formed high-frequency sound to give high spatial resolution. Furthermore, side-scan sonars are useful for mapping seeps because their sound waves respond to changes in near-seafloor sediment properties caused by seep activity. To examine seep sonar signatures, we used a broad-swath, sea-surface side-scan sonar to survey three areas of the Louisiana continental slope where seeps are known to occur. In sonar mosaics from upper-slope study sites, seep features are the main anomalous seafloor response and display high backscatter of sonar waves. The mosaics show fault traces, fault scarps, authigenic carbonate mounds, irregular patches of seep-affected seafloor, mud mounds and mud volcanoes, craters, and sediment flows. Most of these features are located along faults that control seepage. Cores from seep-affected areas imply that enhanced sonar backscatter results from a combination of the perturbation of seafloor sediment physical properties and the introduction of seep-produced foreign material, including gas, oil, carbonate nodules and fragments, and chemosynthetic organism debris. In the lower slope survey area, tectonic activity and resultant mass sediment movements make identification of seeps more difficult. In this area, many sonar backscatter anomalies appear to result from sediment flows and mass movements on the edges and bottoms of intrasalt basins. Basin walls appear as highly dissected, commonly showing crenulations and grooves, which were probably created by sediment failure and erosion by downslope sediment transport. Seeps are commonly difficult to discern because of the abundance of other anomalous features, but seem to be located mainly on basin walls and summits of salt highs.

Links between chemosynthetic production and mobile predators on the Louisiana continental slope: stable carbon isotopes of specific fatty acids

The Gulf of Mexico supports chemoautotrophic communities associated with hydrocarbon seeps. The chemoautotrophic symbiont-containing metazoans are dominated by mussels and tube worms that harbor bacteria which utilize reduced carbon (CH 4) and sulfur (H 2S) compounds as an energy source. In the deep sea, nutritional input from photosynthetic production is scarce, and chemoautotrophic production may be a significant source of nutrients to mobile, benthic predators. Large and significant differences exist between the stable carbon isotope signatures of chemosynthetic and photosynthetic primary production. This isotopic difference makes it possible to determine the importance of each type of primary production to heterotrophs because heterotrophs incorporate the isotope signature of their food. Here, we examine the carbon isotope signatures of specific fatty acids in heterotrophic predators caught both within and approximately 2 km from known chemosynthetic communities. Heterotroph fatty acid signatures were compared to those observed in chemoautotrophic symbiont-containing fauna in order to evaluate degree of usage. Most organisms had at least a 6‰ range in their fatty acids. This reflected patterns of de novo synthesis, with essential and precursor fatty acids being 13C enriched (in heterotrophs) and highly unsaturated fatty acids being 13C depleted. The fatty acid δ 13C values show that heterotrophs had a wide range in their utilization of chemosynthetic production. Bathynomus giganteus (giant isopod), captured off-site, did not utilize chemosynthetic production. Its nonessential fatty acids ranged from −26.3‰ to −18.8‰ (δ 13C), reflecting kinetic isotope effects during de novo synthesis. Essential omega-3 fatty acids, which are directly incorporated from diet, originated from photosynthetic production (δ 13C=−16.2‰). Sclerasterias cf. tanneri (starfish), captured on-site, relied predominantly on chemosynthetic production. Its fatty acid δ 13C values ranged from −30.5‰ to −42.2‰, reflecting chemosynthetic production. Other predators such as Eptatretus sp. (hagfish) and Rochina crassa (spider crab) derived variable percentages of their fatty acid pool from chemosynthetic production, estimated to be 38% and 5%, respectively. Bulk analysis of δ 15N and δ 34S show chemosynthetic production usage patterns generally consistent with the specific fatty acid δ 13C data.

Geophysical signatures of mud mounds at hydrocarbon seeps on the Louisiana continental slope, northern Gulf of Mexico

Mud mounds are common features at cold seeps in many areas of the world. They are abundant in the northern Gulf of Mexico, where they occur at the numerous hydrocarbon seeps on the continental margin. Although the mounds are widely known, their geophysical signatures and geology have not been widely studied in detail. In this study, we examined mounds in two areas of the upper Louisiana slope using geophysical data of several types: 11–12 kHz side-scan sonar, 3.5 kHz echo-sounder profiles, near-bottom 2–12 kHz chirp sonar profiles, and three-dimensional (3D) multichannel seismic data. In side-scan images the mud mounds appear as ovoid high backscatter spots associated with larger, irregular high backscatter zones along faults and fault complexes. Three different sonar responses were noted: (1) a more-or-less uniform appearance, (2) a central higher backscatter spot surrounded with a lower return halo (bulls-eye), and (3) a more-or-less uniform high backscatter area with an acoustic dead-spot near its center. Near-bottom chirp sonar records indicate high lateral variability in seafloor reflection characteristics, implying that the long-range acoustic signature of a given mound results from the integration of seafloor characteristics. In these records, anomalous seafloor echo types include strong seafloor reflectors (hardbottom) or buried hardbottom, acoustic wipe-out (attenuation), and acoustic turbidity (prolonged echoes). Cores from acoustically anomalous seafloor areas indicate that a variety of seep-related material has been introduced into the near-seafloor sediments. The prevalence of strong bottom return signatures implies that the formation of hard grounds or crusts is an integral part of hydrocarbon seep evolution and suggests that much of the ovoid high backscatter signatures may result from such features. Multichannel seismic reflection data from several of the active mounds show columnar subsurface chaotic reflection zones and strong reflections in sediments directly underneath. Seafloor seismic amplitude maps from these mounds are either generally uniform, with a positive or negative anomaly, or display a ring of higher reflectivity around the mound periphery. Positive or negative anomalies may depend on the degree of seafloor lithification and sediment gas charging. The ring feature is thought to result from the preferential concentration of seep-debris in the mound periphery relative to the center. Some mounds, which show no seismic reflection anomaly from the seafloor, appear inactive. Altogether, observations from this study are consistent with the mud mounds being formed by sedimentary volcanism. The data imply they are Holocene features that have a life-cycle that includes hardbottom formation, which progressively crusts over the fluid vent, ending its activity.

Long-term history of chemoautotrophic clam-dominated faunas of petroleum seeps in the Northwestern Gulf of Mexico

Chemoautotrophic clam-dominated assemblages are commonly associated with petroleum seepage on the continental slope of the Gulf of Mexico. We examine the persistence and resilence of these communities by evaluating downcore trends in abundance, biomass, and trophodynamics in communities from four separate petroleum seep sites on the Louisiana continental slope. Some petroleum seep sites retained optimal habitat for some species continuously over geologically-relevant periods of time. More commonly, however, habitat optimality varied substantially over time scales of hundreds of years. Thus, one important characteristic of these sites was the degree of persistence of the chemoautotrophic biota. A fauna typically was persistent over a time span of a few hundred years, but was typically not persistent over a longer time span. The mechanisms producing local extinction remain unclear, however temporal variations in juvenile survivorship seem to be substantially larger than temporal variations in larval settlement, to the extent that the heavily taphonomically-biased record of juvenile individuals permits such a conclusion. When local extinctions occurred in the chemoautotrophic biota, the biota was replaced by a normal slope biota or a mixture of a normal slope biota and the juveniles of chemoautotrophic species that failed to survive to adulthood. Thus, the only faunal transitions were between specific chemoautotrophic faunas and the non-chemoautotrophic fauna. Not one distinctive faunal transition between two chemoautotrophic faunas was observed. Accordingly, each discrete chemoautotrophic fauna was resilient over long time scales; time scales of geological importance.

The generation of internal waves on the continental shelf by Hurricane Andrew

Observed currents, temperature, and salinity from moored instruments on the Louisiana continental slope and shelf reveal multiple baroclinic oscillations during Hurricane Andrew in August 1992. These measurements are supplemented by numerical models in order to identify possible internal wave generation mechanisms. The Princeton Ocean Model is run with realistic topography, stratification, and wind forcing to extend the observations to Mississippi Canyon and other areas on the shelf. A two‐layer isopycnal model is used with idealized topography and spatially uniform winds to isolate internal waves generated in and around the canyon. The combination of the observations and the results from the numerical models indicates several possible mechanisms for generating long internal waves: (1) near‐inertial internal waves were generated across the slope and shelf by dislocation of the thermocline by the wind stress; (2) interaction of inertial flow with topography generated internal waves along the shelf break, which bifurcated into landward and seaward propagating phases; (3) downwelling along the coast depressed the thermocline; after downwelling relaxes, an internal wave front propagates as a Kelvin wave; and (4) Poincaré waves generated within Mississippi Canyon propagate seaward while being advected westward over the continental slope. These processes interact to produce a three‐dimensional internal wave field, which was only partly captured by the observations.

Mapping Turbidite System Pathways on the Louisiana Continental Slope with GLORIA and Multibeam Bathymetry: ABSTRACTS

Mapping Turbidite System Pathways on the Louisiana Continental Slope with GLORIA and Multibeam Bathymetry: ABSTRACTS

Composition and morphogenesis of the tubes of vestimentiferan worms

Abstract The structure and chitin-protein content of the tubes of different vestimentiferan species are compared. Riftia pachyptila and Tevnia jerichonana were collected at a hydrothermal vent site at 13°N on the East Pacific Rise; Ridgeia piscesae was from a hydrothermal vent site on the Juan de Fuca Ridge and Lamellibrachia sp. came from a cold seep on the Louisiana continental slope in the Gulf of Mexico. The tube ultrastructure shows chitin microfibrils organized in parallel bundles with various orientations. The chitin content varies considerably between species and is unrelated to seep or vent origin, whereas the protein content is much higher in the cold seep species than the hydrothermal vent species. All the species have specialized chitin secreting glands, as described for R. pachyptila . Based on this investigation, a tentative model of tube morphogenesis under cell control is proposed.

Mud Volcanoes in the Gulf of Mexico: A Mechanism for Mixing Sediments of Different Ages in Slope Environments: ABSTRACT

ABSTRACT The extrusion of older, unconsolidated sediments onto the seafloor during the Quaternary is one explanation for frequent occurrences of displaced fossil microfaunas encountered at depth in wells drilled on the flanks of salt diapirs in the continental slope environment. Samples from mud volcanoes on the Louisiana continental slope were collected in 1992 and 1993 for foraminiferal studies by using the Johnson Sea-Link I & II manned submersibles. Age dating by use of planktonic foraminifera has shown that sediments ranging in age from middle Miocene to Pleistocene are being deposited on the modern seafloor by active mud vents/volcanoes. These sediments, when buried by modern pelagic or more rapidly deposited terrigenous clastics, would be anomalous when encountered in coreholes or exploratory wells. Vents, as shown by seismic sections, are associated with faults which provide the connection to older subsurface fine-grained sediments that are: 1) mobilized by gas and fluids, 2) carried to the surface, and 3) ejected onto the seafloor to form mud volcanoes and fine-grained sediment flows. The mud volcanoes are in water depths ranging from 300 to 690 meters located in Garden Banks Block 382, Green Canyon Blocks 143 and 272, and Mississippi Canyon Block 929. This study is part of an ongoing investigation of the geology around cold vents which support chemosynthetic faunal communities in the Gulf of Mexico. These results have important implications for age dating subsurface sediments encountered in wells and coreholes on the continental slope.

Three generations of mud volcanoes on the Louisiana continental slope

Three mud volcanoes were identified in a small area of the upper continental slope of the Gulf of Mexico. One is actively accreting by small, intermittent eruptions of gas and fluid mud. Eruptions occur from a pool of gas- and oil-saturated muds that upwell and spill over the central crater edge and down the flanks of the coneshaped buildup. A second structure, a possible sand volcano, is in the last stages of activity, with only slight evidence of gas-saturated sediments. The third structure is a dormant mud volcano with furrowed sides and scattered authigenic carbonate plates and rubble but no venting of gas or fluids.

Fossil Foraminifera from four active mud volcanoes in the Gulf of Mexico

Samples were collected for foraminiferal studies by the Johnson Sea-Link I and II manned submersibles on the Louisiana continental slope. This paper documents that the mud, extruded onto the sea floor from depth by four mud volcanoes, ranges in age from Miocene to Pleistocene based on studies of the planktonic foraminiferal fauna. The vents are in water depths ranging from 300 to 690 m located in Garden Banks Block 382, Green Canyon Blocks 143 and 272, and Mississippi Canyon Block 929. Two mud volcanoes in GB 382 and MC 929 also have rich fossil foraminiferal microfaunas. We suggest that the extrusion of fossil sediments onto the sea floor during the Quaternary is a reasonable explanation for frequent occurrences of displaced fossil microfaunas encountered at depth in wells drilling on the flanks of salt diapirs in the slope environment. Results of this study have important implications for age dating subsurface sediments in bathyal locations.

Taphonomic Rates of Molluscan Shells Placed in Autochthonous Assemblages on the Louisiana Continental Slope

A mixed assemblage of lucinid and mussel shells were placed in mesh bags and left at a site of autochthonous death assemblage formation in a petroleum seep community on the Louisiana upper continental slope for a period of 3 yr. Upon recovery, the shells were assessed for taphonomic alteration and compared to a control assemblage of unaltered shells. The data verify a basic assumption of taphofacies analysis; that evidence of taphonomic processes preserved with the assemblage does in fact document the primary taphonomic processes that biased the assemblage from the original assemblage of living preservable organisms. Significant variability in taphonomic rates existed between shells from locations 10 m apart, as is typical of autochthonous assemblages, so that small-scale variability in the taphonomic process was important

Pliocene to Recent Infilling and Subsidence of Intraslope Basins Offshore Louisiana

Intraslope basins on the lower eastern Louisiana continental slope are modern analogs of large oil- and gas-charged basins infilled on the upper Louisiana slope and buried landward beneath the shelf. High-resolution, gridded multibeam bathym-etry and single-channel seismic reflection profiles are used to (1) characterize the morphology and shallow stratigraphy of 46 intraslope basins within the outer slope region; and (2) look for surficial evidence among the basins of slope channelways that fed sediment flows to the Mississippi Fan during the Pliocene-Pleistocene. Computer algorithms, traditionally employed for automatically mapping river networks in gridded land topography, are used to extract morphologic measurements of the intraslope basins from the gridded bathymetry The basins are found to average approximately 15 km in length, 10 km in width, and 200 m in depth and occur over a map area of about 50 km2. These dimensions, however, can range over an order of magnitude. The intraslope basins examined exhibit (1) distinctive plots of basin area vs. relief (i.e., hypsometric curves) and (2) different near-surface (

Drifting buoy observations of a loop current anticyclonic eddy

The Loop Current penetrated deep into the Gulf of Mexico in early 1989. After several eddy formation and reattachment cycles, a southwestward propagating anticyclonic eddy was formed during the summer. The Loop Current and subsequent eddy produced strong currents over widespread areas on the Louisiana continental slope, prompting a series of current measurement programs. Because of the lack of satellite infrared coverage in the Gulf of Mexico during the summer, the trajectories of 53 ARGOS tracked drifting buoys deployed in the Gulf of Mexico in 1989 were assembled to determine the synoptic history of the Loop Current and anticyclonic eddy during this event. Ten of the most critical summertime buoy trajectories are discussed here. In addition, the trajectories of three of the buoys simultaneously deployed at different radii in the eddy were chosen for analysis with a kinematic feature model. The model assumes the looping buoy motion is generated by an elliptical orbit around a translating eddy center. The buoy trajectories were used to determine time series of the feature model parameters, including eddy center, shape and circulation characteristics. The time series results indicate that the eddy actually propagated in a series of short sprints separated by longer stalls. During the sprints, the eddy propagated as an elliptical but otherwise symmetric solid body. During the stalls, significant asymmetries developed. The asymmetries are consistent with attachment to the Loop Current during the first stall, the effects of topographic Rossby wave dispersion during the second, and the effects of planetary Rossby wave dispersion during the third. The results provide both a framework for the interpretation of in situ observations and a detailed evolutionary history for numerical modelers.

Mud Volcanoes of the Louisiana Continental Slope

Mud Volcanoes of the Louisiana Continental Slope

Taphonomic Signature of Petroleum Seep Assemblages on the Louisiana Upper Continental Slope: Recognition of Autochthonous Shell Beds in the Fossil Record

Chemoautotrophically-based benthic communities on the Louisiana continental slope are currently producing the only significant localized, autochthonous shell accumulation in the northern Gulf of Mexico shelf and slope region. These sites are well below storm wave base and are not subject to anthropogenic disturbance. Five distinctive biofacies are associated with petroleum seepage, dominated respectively by vestimentiferan tubeworms, lucinid, thyasirid and vesicomyid clams and mytilid mussels. The taphonomy of petroleum seeps includes dissolution as the most pervasive mode of shell alteration throughout all the biofacies

Time averaging and temporal persistence in chemoautotrophic molluscan-dominated death assemblages on the Louisiana continental slope

Petroleum seeps on the Louisiana continental slope produce luxurious communities based on chemoautotrophic symbiotic bacteria. Petroleum seeps are typical cold seep communities and generate classic autochthonous death assemblages. Fossil cold seep communities are well-known, common and widespread. Most fossil analogues are dominated by lucinids, but petroleum seeps are not. A combination of sample collection and in situ experimentation has been used to determine rates of taphonomy and time averaging in petroleum seep assemblages.Cores were obtained and sectioned from seep sites in Green Canyon lease blocks 272, 234, and 184 and Garden Banks lease block 386. Lucinids and thyasirids were selected for dating time-since-death to determine the importance of time averaging in these assemblages. Dating was accomplished by measuring the free amino acid content of the shells. Time-since-death became progressively older with depth; accordingly little time averaging had occurred in these autochthonous assemblages. Lucind and mussel shells were placed on the sea floor and recovered 3 yr later. Comparison of each species to the controls left on a laboratory shelf for 3 yr shows that taphonomic alteration was rapid. Mussels were more severely altered than lucinids. Mussels were more heavily dissolved, had more altered edges, were more prone to fragmentation and exhibited greater weight loss than lucinids. The rapid taphonomic loss of the mussels suggests that the preponderance of lucinids in the fossil record is an artifact of preservation. Taphofacies analysis suggests the same; thus verifying an important assumption of taphofacies analysis that taphonomic signatures record biases in preservation by identifying inter-species differences in the rates of important taphonomic processes. Significant variability in taphonomic rates exists between shells from locations 10 m apart indicating significant local variation in the taphonomic process. Local variability in taphonomic and community attributes is characteristic of many autochthonous assemblages.

Sea floor responses to hydrocarbon seeps, Louisiana continental slope

Observations and samples from research submersible dives confirm that brines, crude oil, fluid mud, and gases are common seep products. Through this mechanism a unique interplay of geochemical, geologic, and biological processes resulting in unusual sea floor features ranging from carbonate-rich nodular sediments to mounds with tens of meters relief. Stable carbon isotopes occluded in the carbonates provide a permanent imprint that links these authigenic carbonates to by-products of microbial breakdown of crude oil and gas. Recent DSV ALVIN dives confirm that hydrocarbon seeps and their accompanying chemosynthetic communities and authigenic carbonate mounds occur over the entire depth range of the slope.

Gulf of Mexico Hydrocarbon Seep Communities V. Biofacies and Shell Orientation of Autochthonous Shell Beds below Storm Wave Base

Clam and mussel assemblages associated with petroleum seepage on the Louisiana continental slope form the only substantial shell accumulations below storm wave base on the northwestern Gulf of Mexico shelf and slope. Four distinct biofacies are present at the seeps, dominated respectively by mussels, lucinid clams, vesicomyid clams and tubeworms. Each primary seep site is typically composed of a series of not necessarily contiguous, autochthonous beds dominated by one biofacies. Mussels and tubeworms often co-occur, but neither normally co-occur with lucinid or vesicomyid clams. Lucinid and vesicomyid clam beds have the best chance of preservation. Despite essentially undisturbed accumulation in quiet water below storm wave base, concavity ratios rarely differ from 1:1 and frequency of articulation may be low. Dominantly concave-up valves previously reported in quiet water may result from man's fishing activities. Significant variability in shell orientation, frequency of articulation and concavity ratio between adjacent samples indicates that many individual stratigraphically-equivalent samples should be used in any taphofacies analysis of assemblages formed in low-energy environments. Lucinid beds which form below the sediment surface and vesicomyid beds which form on the sediment surface differed significantly in shell orientation and articulation frequency. Assemblages forming below storm wave base in low-energy environmentsmore » may comprise a wide variety of taphofacies depending upon whether formation occurs primarily beneath the sediment surface or on the sediment surface, despite contemporaneous formation under similar environmental conditions.« less