Spatial and temporal variability of a coarse-sand anomaly on a sandy inner shelf,northeastern Gulf of Mexico

A 10-km² area of anomalously uniform coarse sand and low relief, in 30 m water depth off Panama City, Florida, USA, exists as a palimpsest hardbottom within the prevailing sand-wave system. Side-scan sonar observations over a ten-year period document the morphologic response of the anomaly to the bottom—wave—current interaction regime. The anomaly and surrounding sand waves remained fixed during this period. Ripples form within the anomaly in response to seasonal storms and decay during quiescent periods. Hurricane response includes minor realignment of sand waves, formation of megaripples, and deposition of a thin mud layer.     

Movement of sediment on the Gulf of Mexico,continental slope and upper continental shelf

Abstract

Grain size, coarse fraction analyses, and depositional environment as interpreted from microfauna are related to the character of sparker reflections at the location of core holes drilled by Exxon, Chevron, Gulf, and Mobil on the continental slope of the northern Gulf of Mexico. Continuous sparker reflections are correlated with slowly deposited, evenly bedded sediments containing bathyal faunas. The coarse fraction is dominated by the tests of foraminifera. Discontinuous, discordant reflections and diffractions are correlated with sediments more rapidly emplaced in the bathyal environment of the continental slope by slumping and sliding from the continental shelf. Their coarse fraction is dominated by terrigenous sand grains. A large portion of the volume of continental slope sediments appears to consist of these “displaced”; sediments, including an area 3–24 km wide and 80 km long, southeast of Corpus Christi, Texas. Comparable processes of movement of sediments are interpreted on the continental shelf south of the Southwest Pass of the Mississippi River. Bathymetry in this area is characterized by a series of subaqueous “gullies”; radiating from the river mouth and leading to terraces at their southern extremities. Side‐scan sonar and PDR surveys show a rough bottom in these “gullies”; and terraces, as contrasted with a relatively smooth bottom elsewhere. The rough bottom is interpreted as indicative of slump and creep of the sediments from shallower water. Some foundation soil borings in this area south of Southwest Pass find a low‐strength material gradually increasing in strength with depth. Other borings find a “crust”; of anomalously strong material 8–15 m below the mudline. The microfauna recovered from the “crust”; has moved to its present position by slump or creep from shallower water along a pattern comparable to the gullies shown in the present‐day bathymetry.     

Gassy sediment occurrence and properties: Northern Gulf of Mexico

Free gas is ubiquitous at shallow sediment depths of the northern margin of the Gulf of Mexico. Gassy sediment patches are between 250 and 500 m in horizontal size. Often the gassy layers are within 100 m from the sea floor and are only a few meters thick. Both biogenic and thermogenic gas hydrates have been recovered. Stability values of temperature and pressure indicate that hydrates can exist in water depths less than 500 m. Gassy sediment geoacoustic parameter values are not well constrained because of a lack of concurrent measurements of acoustic properties and sediment gas content. For Gulf of Mexico gassy sediment, some reportedin situ values of sound speed are reduced by an order of magnitude below values for water saturated sediments. More commonly, sound speed is reduced from water saturated sediment values by only 15 to 50 percent.     

Dilute-acid trace-metal fraction in northeastern Gulf of Mexico sediments

Variations in dilute-acid (partial digest) metal concentrations in bottom sediments from 49 stations on the Mississippi, Alabama and Florida continental margin are investigated with the aid of ordination and other multivariate statistical techniques. Total iron, carbonate content, clay fraction and water depth correlate well with the overall leachable sediment metal values for pooled station replicates over four sampling periods. Individual metal concentrations also are correlated with these and other environmental parameters using stepwise multiple linear regression techniques. A weak positive association between Ba, Cu, Cr, Ni, Pb, and V burdens in the demersal fishSyacium papillosum and the dilute-acid leachable sediment metal TOC and carbonate concentrations is demonstrated.     

Hydrodynamic and sedimentary responses to two contrasting winter storms on the inner shelf of the northern Gulf of Mexico

Results are presented from the deployment of three bottom-mounted instrumentation systems in water depths of 6–9 m on the sandy inner shelf of Louisiana, USA. The 61-day deployment included nine cold front passages that were associated with large increases in wind speed. Two of the most energetic cold front passages were characterized by distinct meteorological, hydrodynamic, bottom boundary layer, and sedimentary responses and may potentially be treated as end-member types on a continuum of regional cold front passages. Arctic surges (AC storms) have a very weak pre-frontal phase followed by a fairly powerful post-frontal phase, when northeasterly winds dominate. Migrating cyclones (MC storms) are dominated by a strong low-pressure cell and have fairly strong southerly winds prior to the frontal passage, followed by strong northwesterly winds.

On the basis of measurements taken during this study, AC storms are expected to have a lower average significant wave height than MC storms and are dominated by short-period southerly waves subsequent to the frontal passage. Currents are weak and northerly during the pre-frontal phase, but become very strong and southwesterly following the passage. Sediment transport rate during AS storms was not as high as during MC storms, and the mean and overall direction tended to be southwesterly to westerly, with low-frequency flows producing easterly transport, and wind-wave flows producing southeasterly transport.

MC storms had the most energetic waves of any storm type, with peaks in significant wave height occurring during both the pre- and post-frontal phases. The wave field during MC storms tended to be more complex than during AS storms, with an energetic, northerly swell band gradually giving way to a southerly sea band as the post-frontal phase progressed. Currents during MC storms were moderate and northerly during the pre-frontal phase, but became much stronger and southeasterly during the post-frontal phase. Shear velocity was high during both the pre- and post-frontal phases of the storm, although sediment transport was highest following the frontal passage. Mean and overall sediment transport was directed southeasterly during MC storms, with low-frequency and wind-wave flows producing northerly transport. In summary, the data sets presented here are unique and offer insight into the morphosedimentary dynamics of mid-latitude, micro-tidal coasts during extratropical storms.     

Sea-floor sediment distribution in the Gulf of Mexico

Carbonate content, smear-slide analysis and diffuse reflectance spectrophotometry were utilized to determine modern sediment composition and distribution throughout the Gulf of Mexico. In all, 186 core top and grab samples distributed throughout the Gulf were analyzed. Reflectance spectra were taken from thick smear slides from the near ultraviolet, through the visible, and into the near infrared. The first derivatives of the percent reflectance data were subjected to factor analysis producing factors that grouped covarying first-derivative wavelengths. Factors were interpreted by comparison to first-derivative curves for known sediment components and minerals. Interpretation was aided by the mapping of both calcium carbonate content and smear-slide sediment classes. The most easily interpreted factor solution was produced by analyzing only the visible region of the spectrum and extracting seven factors which explained 98% of the cumulative variance. These factors, in order of their relative importance, are interpreted as (1) marl and calcareous clay, (2) glauconite, (3) kaolinite, (4) organic matter, (5) phosphorite, (6) hematite, and (7) goethite. Some factor maps are consistent with known sources of fluvial sediment input; for example, kaolinite is deposited off rivers draining the southeastern US. Other factor maps are related to the origin of the material in the factor, glauconite, for example, being confined to low sedimentation regions of the outer shelf. The most unusual observation concerns the distribution of hematite, which appears to be transported from the rivers of south Texas, primarily the Rio Grande, across the shelf then eastward downslope along the base of the Sigsbee Escarpment. This eastward transport seems to be explainable only by transport in bottom currents flowing along the base of the Sigsbee Escarpment.     

The 20th-century development and expansion of Louisiana shelf hypoxia,Gulf of Mexico

Since systematic measurements of Louisiana continental-shelf waters were initiated in 1985, hypoxia (oxygen content <2 mg L⁻¹) has increased considerably in an area termed the dead zone. Monitoring and modeling studies have concluded that the expansion of the Louisiana shelf dead zone is related to increased anthropogenically derived nutrient delivery from the Mississippi River drainage basin, physical and hydrographical changes of the Louisiana Shelf, and possibly coastal erosion of wetlands in southern Louisiana. In order to track the development and expansion of seasonal low-oxygen conditions on the Louisiana shelf prior to 1985, we used a specific low-oxygen foraminiferal faunal proxy, the PEB index, which has been shown statistically to represent the modern Louisiana hypoxia zone. We constructed a network of 13 PEB records with excess ²¹⁰Pb-derived chronologies to establish the development of low-oxygen and hypoxic conditions over a large portion of the modern dead zone for the last 100 years. The PEB index record indicates that areas of low-oxygen bottom water began to appear in the early 1910s in isolated hotspots near the Mississippi Delta and rapidly expanded across the entire Louisiana shelf beginning in the 1950s. Since ~1950, the percentage of PEB species has steadily increased over a large portion of the modern dead zone. By 1960, subsurface low-oxygen conditions were occurring seasonally over a large part of the geographic area now known as the dead zone. The long-term trends in the PEB index are consistent with the 20th-century observational and proxy data for low oxygen and hypoxia.     

Patterns and controls of surface sediment distribution: west-central Florida inner shelf

The west-central Florida inner shelf represents a transition between the quartz-dominated barrier-island system and the carbonate-dominated mid-outer shelf. Surface sediments exhibit a complex distribution pattern that can be attributed to multiple sediment sources and the ineffectiveness of physical processes for large-scale sediment redistribution. The west Florida shelf is the submerged extension of the Florida carbonate platform, consisting of a limestone karst surface veneered with a thin unconsolidated sediment cover. A total of 498 surface sediment samples were collected on the inner shelf and analyzed for texture and composition. Results show that sediment consists of a combination of fine quartz sand and coarse, biogenic carbonate sand and gravel, with variable but subordinate amounts of black, phosphorite-rich sand. The carbonate component consists primarily of molluskan fragments. The distribution is patchy and discontinuous with no discernible pattern, and the transition between sediment types is generally abrupt. Quartz-rich sediment dominates the inner 15 km north of the entrance into Tampa Bay, but south of the Bay is common only along the inner 3 km. Elsewhere, carbonate-rich sediment is the predominate sediment type, except where there is little sediment cover, in which cases black, phosphorite-rich sand dominates. Sediment sources are likely within, or around the periphery of the basin. Fine quartz sand is likely reworked from coastal units deposited during Pleistocene sea-level high stands. Carbonate sand and gravel is produced by marine organisms within the depositional basin. The black, phosphorite-rich sand likely originates from the bioerosion and reworking of the underlying strata that irregularly crop out within the study area. The distribution pattern contains elements of both storm- and tide-dominated siliciclastic shelves, but it is dictated primarily by the sediment source, similar to some carbonate systems. Other systems with similar sediment attributes include cool-water carbonate, sediment-starved, and mixed carbonate/siliciclastic systems. This study suggests a possible genetic link among the three systems.     

Interaction between diapirism and sediment loading at the shelf-slope boundary,northwest Gulf of Mexico

During the last low stand of sea level, rivers and streams drained across the present northwestern Gulf of Mexico continental shelf depositing sediments in several shallow-water deltas near the present shelf-slope boundary. The weight of these wedges of prograded sediments triggered or augmented both subsidence of local depositional basins and upward movement of diapiric material around the basin edges. A depositional basin off the southwestern Louisiana coast records migration of the basinal axis during late Pleistocene and Holocene time indicating relative growth of diapirs along the basin margin throughout the most recent geological record.     

Gulf of Mexico sediment sources and sediment transport trends from magnetic susceptibility measurements of surface samples

The magnetic properties from 200 trigger core-top and Van Veen grab sediment samples recovered from throughout the Gulf of Mexico have been analyzed and used to characterize sediment source and flow pattern distributions. Magnetic parameters included are anhysteretic remanent magnetism (ARM) and magnetic susceptibility (MS) measurements. Results from these measurements are compared to previously determined calcium carbonate percentages, and clay and hematite influx trajectories into the Gulf of Mexico for the same samples reported by Balsam and Beeson [Balsam, W.L. and Beeson, J.P., 2003. Sea-floor sediment distribution in the Gulf of Mexico, Deep-Sea Res. I, 50, 1421–1444.]. The ARM results give an estimate of magnetic grain size distributions, and by analogy, grain size distributions in general, whereas MS patterns show high detrital sediment accumulation zones within the Gulf. The dominant influx of modern high susceptibility sediment into the Gulf of Mexico appears to originate from the Red River, flow into Atchafalaya River Basin and out into the Gulf from Atchafalaya Bay, with significant additional contributions from the Mississippi River through the Southwest Pass of the Mississippi River Delta. This material then moves across the continental shelf and down through the Mississippi Canyon into the deep Gulf where it is redistributed at depths > 3600 m. The eastern shelf margins in the Gulf, offshore from Alabama and Florida, are accumulating calcite- or quartz-rich medium to fine-grained sediment that has a very low or diamagnetic MS signature. From the Louisiana to Texas Gulf coast margins, MS is moderate to high, suggesting a river influx of magnetic constituents from the volcanic fields in New Mexico, and from igneous and metamorphic sources in the Mississippi Basin. Offshore from western Mexico, the MS is high to moderate, but the Yucatan Shelf margin is characterized by low to diamagnetic MS values due to sediment dominated by calcite sands and oozes, a trend that continues to the east onto the West Florida Shelf. Additional measurements of samples collected in association with sites characterized by hydrocarbon seepage exhibit anomalously low MS values. The samples from the lower shelf and slope areas are typified by iron reduction by bacterial organisms in these samples. These results produce anomalous localized lows in the MS trends observed.     

Distribution of riverine sediment chemistry on the shelf,slope and rise of the Gulf of Papua

The tectonically active islands of the Indo-Pacific Archipelago deliver much sediment to the ocean margins. In the Gulf of Papua on the south coast of Papua New Guinea (PNG), the chemical composition of surface sediment from grab samples indicates that Fly River muds are dispersed to the north and east, where they are joined by sediment plumes from the other large rivers along the south coast of PNG. This is the likely source of terrestrial sediment on the Papuan Plateau and the northern Coral Sea Abyssal Plain. The sediment is transported through submarine troughs and canyons offshore, far to the east of the riverine inputs. Immediately south and 30–50 km offshore from the Fly and Purari deltas is a platform of algal and reef carbonate materials, containing little or no terrestrial surface sediment.     

The effect of a submarine canyon on the river sediment dispersal and inner shelf sediment movements in southern Taiwan

This study examines the influence of a submarine canyon on the dispersal of sediments discharged by a nearby river and on the sediment movement on the inner shelf. The study area includes the head region of the Kao-ping Submarine Canyon whose landward terminus is located approximately 1 km seaward from the mouth of the Kao-ping River in southern Taiwan. Within the study area 143 surficial sediment samples were taken from the seafloor. Six hydrographic surveys along the axis of the submarine canyon were also conducted over the span of 1 yr. Three different approaches were used in the analysis of grain-size distribution pattern. They include (1) a combination of ‘filtering’ and the empirical orthogonal (eigen) function (EOF) analysis technique, (2) the McLaren Model, and (3) the ‘transport vector’ technique. The results of the three methods not only agree with one another, they also complement one another. This study reveals that the Kao-ping Submarine Canyon is relatively a stratified and statically stable environment. The hydrographic characteristics of the canyon display seasonal variability controlled primarily by the temperature field and the effluent of the Kao-ping River. The hydrographic condition and the bottom topography in the canyon suggest the propagation of internal tides during the flood season (summer) of the Kao-ping River. The submarine canyon acts as a trap and conduit for mud exchange between the Kao-ping River and offshore. Near the head of the canyon there is a region of sediment transport convergence. This region is also characterized by high mud abundance on the seafloor that coincides with the presence of high suspended sediment concentration (SSC) spots in the bottom nepheloid layer. Outside the submarine canyon on the shelf where the evidence of wave reworking is strong, the northwestward alongshore transport dominates over the southeastward transport, which is a common theme on the west coast in southern Taiwan.     

Cenozoic evolution of sediment accumulation in deltaic and shore-zone depositional systems, Northern Gulf of Mexico Basin

Paleogeographic and volumetric lithofacies mapping of 18 Cenozoic genetic sequences within the Northern Gulf of Mexico Basin quantifies the proportional sequestering of sediment within wave-dominated shore-zone vs. deltaic systems through time. Three long-term depositional phases are revealed by plots, based on paleogeographic and sediment isochore maps, of total shore-zone system area and volume to total delta system area and volume (SZ/D). (1) SZ/D area and volume ratios are highly variable in Paleocene through Eocene sequences. However, typical volume ratios for major genetic sequences (Upper, Middle, and Lower Wilcox; Queen City (QC), and Yegua) range between 0.2 and 0.6. Minor sequences (Sparta (SP), Jackson (JS)), which record very low rates and volumes of sediment accumulation, have the greatest variability in their ratios. (2) Oligocene and Miocene sequences display consistently high proportions of shore-zone sediment. SZ/D area ratios range from 0.6 to 1.0, and volume ratios cluster between 0.4 and 0.8. (3) A substantial late Neogene decrease in SZ/D ratios is presaged in the late Miocene sequence. Pliocene and Pleistocene sequences are uniformly characterized by very low ratios of <0.2. Consistently high Oligocene–Miocene ratios reflect a post-Eocene period of strong E–W climate gradient across the Northern Gulf margin. Shore-zone volume displays no correlation to overall rate of sediment supply. The late Neogene decrease in proportional shore-zone system importance corresponds to development of the West Antarctic and Northern Hemisphere ice sheets and related increase in amplitude and frequency of glacioeustatic sea level cycling.     

Wet season fine sediment dynamics on the inner shelf of the Great Barrier Reef

Fine sediment dynamics were recorded in February 2007 in coastal waters of the Great Barrier Reef during a moderate flood of the Tully River. An estuarine circulation prevailed on the inner continental shelf with a surface seaward velocity peaking at 0.1 m s⁻¹ and a near-bottom landward flow peaking at 0.05 m s⁻¹. Much of the riverine mud originating from eroded soils was exported onto a 10 km wide coastal strip during the rising stage of the river flood in the first flush. In coastal waters, suspended sediment concentration peaked at 0.2 kg m⁻³ near the surface and 0.4 kg m⁻³ at 10 m depth during calm weather, and 0.5 kg m⁻³ near the surface and 2 kg m⁻³ at 10 m depth during strong winds when bottom sediment was resuspended. Diurnal irradiance at 4 m depth was almost zero for 10 days. The sedimentation rate averaged 254 (±33) g m⁻² d⁻¹ over the 28-day study period, and concentrations of dissolved and particulate nutrients originating from the river were high. The observed low irradiance would have prevented coral photosynthesis, while the sedimentation rate would have been lethal to some juvenile corals. The mud may ultimately be minnowed out over long periods, however, flushing of the mud occurs at time scales much longer than the flood event and the mud is likely to affect coral physiology for significant periods after the flood has subsided. The data show the need to better control erosion on farmed land for the conservation of coral reefs on the inner shelf of the Great Barrier Reef.     

Overview of the recent sediment hydrocarbon geochemistry of Atlantic and Gulf Coast outer continental shelf environments

An overview of the hydrocarbon geochemistry of recent marine sediments from the U.S. Atlantic and Gulf of Mexico Outer Continental Shelf (OCS) regions is presented. Hydrocarbon levels along the Atlantic OCS are fairly uniform, ranging between <0·1 and 20 ppm, with the higher values occurring in areas of fine-grained sediment accumulation. Elevated hydrocarbon concentrations appear to be associated with anthropogenic inputs of silt/clay-sized particles to OCS sediments via particulate resuspension and transport, as evidenced by an increase in contributions from the unresolved complex mixture (UCM) feature in gas chromatograms. Compositional characteristics of hydrocarbons in the high-carbonate sediments of the Eastern Gulf OCS indicate a primarily marine origin. Proceeding north and west along the shelf, sedimentary hydrocarbons assume a more terrestrial and/or anthropogenic character in response to a greater input of silt/clays from the Mississippi River. Highest hydrocarbon concentrations in this region (up to 70 ppm) are found in the shallow nearshore areas west of the Mississippi River discharge and in the vicinity of Galveston Bay along the South Texas OCS.In the North Atlantic, Eastern Gulf, South Texas Gulf and coastal Louisiana sediments a linear relationship exists between total hydrocarbon and total organic carbon concentrations, indicating that each area consists of a geochemical ‘province’ defined by a source input or depositional regime specific to the region. The use of trace parameter/bulk parameter ratios (such as total hydrocarbon or individual PAH concentrations/total organic carbon content) in defining such provinces, and thus in serving as a basis for evaluating variations in source inputs as part of future surveys, is demonstrated. This approach views the hydrocarbon content and composition of Atlantic and Gulf coast OCS sediments as points on a spectrum defined by input types (sources) and deposition patterns (sedimentation and erosion), which conceptually link all the regions into a unified system.     

Acoustic wave attenuation in the gas hydrate-bearing sediments of Well GC955H,Gulf of Mexico

A better understanding of wave attenuation in hydrate-bearing sediments is necessary for the improved geophysical quantification of marine gas hydrates. Here we compare the attenuation behavior of hydrate-saturated vs water-saturated sediments at site GC955H, in the Gulf of Mexico, which was surveyed during the JIP Leg II expedition. We compute the P-wave attenuation of the gas hydrate bearing sediments using the median frequency shift method on the monopole waveforms. The results show that P-wave attenuation due to low saturation (<?0.4) in hydrate-filled fractures of fine-grained sediment is comparable to that of the water-filled fracture case. On the contrary, P-wave attenuation due to high saturation (>?0.4) in the hydrate-filled pores of coarse-grained sediments can be up to as much as three times more than that of the water-saturated case. The correlation analysis shows that the P-wave attenuation increases with the increasing gas hydrate saturation for the highly saturated gas hydrate-bearing sand interval while the correlation of the P-wave attenuation and hydrate saturation is weak for low saturated gas hydrate-bearing shale interval. The results show that P-wave attenuation is more likely to be used as a geophysical proxy for gas hydrate quantification of highly concentrated coarse-grained sediment rather than for that of fine-grained sediment. To examine the P-wave behavior in sand, we use the improved LCAM model, which accounts for physical factors such as grain boundary roughness and squirt flow to explain the observed differences in P-wave attenuation between hydrate and water-saturated coarse-grained sediment. Our results provide further geophysical evidences for P-wave behavior in the gas hydrate-bearing sediments in the field.     

Sorted bedforms on the inner shelf off northeastern New Zealand: spatiotemporal relationships and potential paleo-environmental implications

Sorted bedforms are heterogeneous shelf seabed features found ubiquitously on the inner shelf of New Zealand and around the world. In this study we examine the shallow stratigraphy of sorted bedforms using diver-collected short cores together with the textural analysis of the associated surface sediments in the Tairua-Pauanui embayment on the northeast coast of the North Island of New Zealand. Combining sonar and textural analysis together with the local oceanographic conditions provides new insight into the interpretation of sorted bedform features. In this regional case study, sorted bedforms are found to have a stratigraphic signature characterized by alternating fine and coarse sequences that does not reflect alternating calm (low-energy) and storm (high-energy) cycles. Instead, the core sequences suggest the signature of a heterogeneous inner shelf sedimentary facies developed from morphodynamic feedback mechanisms operating at the scale of the bottom boundary layer. The resulting sedimentary sequence (alternating coarse and fine units) found throughout this study site is the result of contemporaneous sorting processes.     

Speculations on the geological causes of backscatter variation on GLORIA sonographs from the Mississippi and De Soto fans,Gulf of Mexico

Published analyses of 61 piston cores, bottom photographs, and dredge samples provide ground truth for 6.5 kHz GLORIA side-scan sonar records of the Mississippi and De Soto fans. GLORIA sound appears to have penetrated through up to 4 m of foraminiferal ooze and terrigenous mud to reach sandy sediments. Possible primary geological causes of high backscatter include slump structures at various scales (1->1,000 cm), possible debris flow fabrics (roughness 1–100 cm) in sandy (5–21%) sediments, and thin ironstone crusts with a roughness of tens of centimeters.