Physical characterization of core samples recovered from Gulf of Mexico

Seventy whole rounds from conventional cores obtained during drilling to 300 mbsf at Atwater Valley and Keathley Canyon in the Gulf of Mexico in April and May 2005 were tested to determine geophysical and geomechanical parameters (liquid and plastic limit, porosity, specific surface, pH, sediment electrical conductivity, P- and S-wave velocities and undrained shear strength). Available data from a pressure core are included as well. Results show that the sediments are high specific surface plastic clays, and exhibit pronounced time-dependent stiffness recovery. Strains during coring disturb specimens, yet, the water content retains the effective stress history and permits gaining stiffness and strength information from conventional cores. Remolding is exacerbated when gas expands upon decompression; the limited pressure core data available show the advantages of preserving the pore fluid pressure during core recovery and testing. Valuable parameters for sediment characterization and engineering analysis are extracted from the data using pre-existing soil models.     

Distinctiveness of the mesopelagic fish fauna in the Gulf of Mexico

We quantify the similarity of the Gulf of Mexico mesopelagic fish fauna to that in adjacent oceanic regions, the Venezuelan and Colombian Basins of the Caribbean Sea and the North and South Sargasso Seas. The South Sargasso and Colombian are the least similar of the areas in terms of their faunal composition, and the Venezuelan and Colombian Basins are the most similar. The Gulf fauna lies somewhere in between, and is a composite of that in the Sargasso and Caribbean Seas. The Gulf of Mexico displays the greatest abundance, biomass and richness (S=140 species), and is an intermediate in evenness (J=0.66) and percent endemism (7.1%). Our findings support the view that the centrally located Gulf is an oceanic ecotone between the Atlantic Tropical and Subtropical faunal regions.     

Data assimilation considerations for improved ocean predictability during the Gulf of Mexico Grand Lagrangian Deployment (GLAD)

Ocean prediction systems rely on an array of assumptions to optimize their data assimilation schemes. Many of these remain untested, especially at smaller scales, because sufficiently dense observations are very rare. A set of 295 drifters deployed in July 2012 in the north-eastern Gulf of Mexico provides a unique opportunity to test these systems down to scales previously unobtainable. In this study, background error covariance assumptions in the 3DVar assimilation process are perturbed to understand the effect on the solution relative to the withheld dense drifter data. Results show that the amplitude of the background error covariance is an important factor as expected, and a proposed new formulation provides added skill. In addition, the background error covariance time correlation is important to allow satellite observations to affect the results over a period longer than one daily assimilation cycle. The results show the new background error covariance formulations provide more accurate placement of frontal positions, directions of currents and velocity magnitudes. These conclusions have implications for the implementation of 3DVar systems as well as the analysis interval of 4DVar systems.     

Organic matter in deepwater sediments of the Northern Gulf of Mexico and its relationship to the distribution of benthic organisms

Sediment samples were selected from 28 sites across the deep (212–3527 m) northern Gulf of Mexico (GOM) as part of the Deep Gulf of Mexico Benthos (DGoMB) program, and analyzed for geochemical parameters related to organic carbon (OC) distribution and characteristics. The results of this study indicate that the OC content of sediments in the deep northern GOM is controlled by several factors; including water depth, overlying water productivity, sediment carbonate content, sediment oxygen exposure time, OC sources, and regional influences. The best correlation between sediment OC content, on a CaCO₃-free basis, and other parameters examined was an inverse correlation of OC with water depth. The OC/SA ratio had a wide range of values and, along with variable sources of sedimentary OC, indicated that the organism-available concentration of metabolizable organic matter may not be simply related to sedimentary OC content. This was perhaps reflected in the observation that benthic macrofaunal and meiofaunal biomass abundances were well correlated with sedimentary OC, but the abundance of bacteria in sediments was not.     

Sediment community oxygen consumption in the deep Gulf of Mexico

Sediment community oxygen consumption (SCOC) has been measured from the continental shelf out to the Sigsbee Abyssal Plain in the NE Gulf of Mexico (GoM). SCOC rates on the continental shelf were an order of magnitude higher than those on the adjacent continental slope (450–2750 m depth) and two orders of magnitude higher than those on the abyssal plain at depths of 3.4–3.65 km. Oxygen penetration depth into the sediment was inversely correlated with SCOC measured within incubation chambers, but rates of SCOC calculated from either the gradient of the [O₂] profiles or the total oxygen penetration depth were generally lower than those derived from chamber incubations. SCOC rates seaward of the continental shelf were lower than at equivalent depths on most continental margins where similar studies have been conducted, and this is presumed to be related to the relatively low rates of pelagic production in the GoM. The SCOC, however, was considerably higher than the input of organic detritus from the surface-water plankton estimated from surface-water pigment concentrations, suggesting that a significant fraction of the organic matter nourishing the deep GoM biota is imported laterally down slope from the continental margin.     

The distribution of phenylalkanes in the modern sediment associated with gas hydrate from the Gulf of Mexico

Phenylalkanes with carbon numbers between 16 and 19, characterized by the main carbon-18, have been identified in the modern sediments collected from gas hydrate area from the Gulf of Mexico. The structure of phenylalkanes with four isomers for every carbon number was determined by means of their mass spectra and previous studies. The distribution of the series characterized by a low molecular mass was similar to the distribution of n-alkane, alkylcyclohexanes and alkylbenzenes in each sample. There were differences in the distribution of the phenylalkane series between the S - 1, S - 4, S - 7, S - 9 samples and the S - 8, S - 10 and S - 11 samples. The phenylalkanes might be derived from Archaea associated with anaerobic oxidation of methane （AOM） processes in S - 1, S -4, S -7 and S -9 samples according to their distribution resembled with the distribution of the extract from a type of Archaea. The distribution of alkylcyclohexanes and alkylbenzenes in S - 1, S - 4, S - 7 and S - 9 sample was found to be similar to each other. The odd-over-even predominance of alkylcyclohexanes was seen as the input of some bac- terial.     

Foldbelts with early salt withdrawal and diapirism: Physical model and examples from the northern Gulf of Mexico and the Flinders Ranges, Australia

A physical experiment shows that shortening applied to existing diapirs and minibasins produces anomalous structural styles that are unlike those of more typical foldbelts. Strong minibasins remain largely undeformed while weak diapirs localize contractional strain. Short diapirs form the cores to folds and thrusted folds, whereas tall diapirs are squeezed and often welded, commonly leading to the extrusion of allochthonous material. Key features of the model are observed in real examples. In the northern Gulf of Mexico passive margin, minibasins were originally separated by a polygonal pattern of deep salt ridges, with diapirs located at ridge intersections. Gravity spreading resulted in squeezed diapirs (and associated allochthonous salt) connected by variably oriented contractional, extensional, and strike-slip structures. In the Flinders Ranges convergent-margin foldbelt of South Australia, preexisting diapirs were squeezed, welded, and thrusted, with anticlines plunging away in multiple directions, so that minibasins are surrounded by highly variable structures. A different geometry is observed in La Popa Basin, Mexico, where squeezing of a linear salt wall produced a vertical weld with diapirs at the terminations, rather than the culmination. In all areas, foldbelt geometries are strongly influenced by the preestablished salt-minibasin architecture.     

墨西哥湾天然气水合物矿产资源预测模型及其应用

墨西哥湾是世界上研究天然气水合物较深入的海区,调查资料丰富,已在50多处采集到天然气水合物样品,具备建立天然气水合物矿产资源预测模型的条件。选择34处已知天然气水合物矿点和34处已知无矿点作为训练区,建立矿点存在与否的预测模型。模型的相关系数值表明墨西哥湾天然气水合物与盐底辟关系密切。将该模型应用于整个墨西哥湾北部,初步获得了天然气水合物存在可能性概率分布图;概率大于0.7的预测单元包含已知矿点中的30个,利用该阈值圈定了墨西哥湾天然气水合物潜在资源分布区。     

Distributions of carbohydrate species in the Gulf of Mexico

In order to study the role of polysaccharides in the cycling of marine organic matter and transparent exopolymeric particles (TEP), the concentrations of total carbohydrates (p-TCHO), total uronic acids (URA) and total acid polysaccharides (APS) in suspended and sinking particles, as well as carbohydrates in the filter-passing “dissolved” phase (d-TCHO), were measured in vertical profiles along a N–S transect in the Gulf of Mexico, across a cold core (CCR) and a warm core (WCR) ring (eddy) during both July 2000 and May 2001. The concentrations of d-TCHO in 2000 ranged from 4 to 22 μM C, with a subsurface maximum, which was located slightly above the depth of chl a maximum, amounting to, on average, 34% of DOC in the CCR, and 13% in the WCR. The concentration of particulate carbohydrates (p-TCHO) in different size fractions (0.7–10, 10–53, and >53 μm) ranged from 0.04 to 1.1, 0.005 to 0.40, and 0.006 to 0.26 μM C, respectively, indicating that carbohydrates are mostly concentrated in small particles (0.7–10 μm). URA and APS were similarly concentrated in small particles, in which, on average, URA accounted for 87% and 57% of total URA, and APS for 92% and 88% of total APS in 2000 and 2001, respectively. URA accounted for 3–9% of carbohydrates in suspended particles, suggesting that URA are a minor component of the p-TCHO pool. Due to its surface-reactive nature, URA could play a major role in the coagulation of particles and macromolecules despite its relatively low abundance. While, on average, p-TCHO and total APS were more enriched in suspended particles than in sinking particles in both 2000 and 2001, the opposite was true for URA in both years. The greater contents of URA that are present in settling particles compared to suspended particles could indicate a mass flow in the direction of sinking particles, either caused by coagulation, by bacterial reworking of particulate and colloidal organic matter, or by their more refractory nature.     

墨西哥湾陆坡区冷泉碳酸盐岩的烷烃分布特征

对墨西哥湾北部水深2200m的Alaminos Canyon645区块（AC645区）和水深540m的Green Canyon185区块（GC185区）冷泉碳酸盐岩的有机质进行了研究,结果显示深水和浅水区冷泉碳酸盐岩的有机质丰度和烷烃组成差别较大,下陆坡深水AC区样品有机质含量低,正构烷烃、萜烷、甾烷各组分的含量相对较高,正构烷烃以低碳数占绝对优势,并含有种类丰富的三环萜烷和五环三萜烷,且以17a（H）,21B（H）-藿烷为主峰碳,反映了深水区冷泉碳酸盐岩的有机质来源于细菌和低等藻类,有少量的深部油气藏有机质的渗漏输入,并受微弱的微生物作用改造。上陆坡浅水GC区样品有机质含量较高,其中正构烷烃含量较低,并且以难以分开的复杂混合物（UCM）为主,同样含有种类丰富的以17α（H）,21β（H）-降藿烷为主峰碳的三环萜烷和五环三萜烷,表明有机质主要来源于深部油气藏渗漏的有机质,并遭受了强烈的微生物降解。本文系统地研究了冷泉碳酸盐岩中正构烷烃、萜烷、甾烷的组成和分布特征,并探讨了有机质来源和冷泉渗漏的关系。     

Sequence stratigraphy of the Miocene section,southern Gulf of Mexico

This study focuses on the interpretation of stratigraphic sequences through the integration of biostratigraphic, well log and 3D seismic data. Sequence analysis is used to identify significant surfaces, systems tracts, and sequences for the Miocene succession.The depositional systems in this area are dominantly represented by submarine fans deposited on the slope and the basin floor. The main depositional elements that characterize these depositional settings are channel systems (channel-fills, channel-levee systems), frontal splays, frontal splay complexes, lobes of debrites and mass-transport complexes.Five genetic sequences were identified and eleven stratigraphic surfaces interpreted and correlated through the study area. The Oligocene-lower Miocene, lower Miocene and middle Miocene sequences were deposited in bathyal water depths, whereas the upper Miocene sequences (Tortonian and Messinian) were deposited in bathyal and outer neritic water depths. The bulk of the Miocene succession, from the older to younger deposits consists of mass-transport deposits (Oligocene-lower Miocene); mass transport deposits and turbidite deposits (lower Miocene); debrite deposits and turbidite deposits (middle Miocene); and debrite deposits, turbidite deposits and pelagic and hemipelagic sediments (upper Miocene). Cycles of sedimentation are delineated by regionally extensive maximum flooding surfaces within condensed sections of hemipelagic mudstone which represent starved basin floors. These condensed sections are markers for regional correlation, and the maximum flooding surfaces, which they include, are the key surfaces for the construction of the Miocene stratigraphic framework. The falling-stage system tract forms the bulk of the Miocene sequences. Individual sequence geometry and thickness were controlled largely by salt evacuation and large-scale sedimentation patterns. For the upper Miocene, the older sequence (Tortonian) includes sandy deposits, whereas the overlying younger sequence (Messinian) includes sandy facies at the base and muddy facies at the top; this trend reflects the change from slope to shelf settings.     

Remotely sensed sea-surface chlorophyll and POC flux at Deep Gulf of Mexico Benthos sampling stations

Biweekly composite averages of the standing stock of sea-surface chlorophyll (SSC) were derived from SeaWiFS satellite ocean-color data at 44 benthic sampling stations occupied along the continental slope and rise by the Deep Gulf of Mexico Benthos (DGoMB) program. At the 22 DGoMB sites north of 26°N and west of 91°W in the NW Gulf of Mexico, annual average SSC was 0.19 mg m⁻³, ranging at most locations from annual highs of about 0.3 mg m⁻³ in November–February to lows of about 0.1 mg m⁻³ in May–August. Comparison of three years of SeaWiFS data (January 1998–December 2000) showed little inter-annual variation at these NW Gulf stations. In contrast, at the 22 NE Gulf sites north of 26°N and east of 91°W, SSC averaged 2.8 times higher than in the NW Gulf, showing also strong inter-annual variation. Maxima in the NE region occurred in November–February and also during summers. The summer maxima were associated with Mississippi River water transported offshore to the east and southward by anticyclonic eddies in the NE Gulf. The apparent increases in SSC in June–August at NE Gulf stations reached average monthly concentrations >50% greater than in November–February. Based on a primary productivity model and a vertical flux model, the calculated export of particulate organic carbon (POC flux reaching the seafloor) was estimated as 18 mg C m⁻² day⁻¹ at the 22 NE Gulf stations, and 9 mg C m⁻² day⁻¹ at the 22 NW Gulf stations. These estimates are comparable to fluxes measured by benthic lander by others in the DGoMB program, which may drive the differences in west versus east bathymetric zonation and community structure of macrobenthos that were sampled with large box corers by others in the DGoMB program.     

Radiocarbon-derived sedimentation rates in the Gulf of Mexico

Sedimentation rates were determined for the northern Gulf of Mexico margin sediments at water depths ranging from 770 to 3560 m, using radiocarbon determinations of organic matter. Resulting sedimentation rates ranged from 3 to 15 cm/kyr, decreasing with increasing water depth. These rates agree with long-term sedimentation rates estimated previously using stratigraphic methods, and with estimates of sediment delivery rates by the Mississippi River to the northern Gulf of Mexico, but are generally higher by 1–2 orders of magnitude than those estimated by ²¹⁰Pb_xs methods. Near-surface slope sediments from 2737 m water depth in the Mississippi River fan were much older than the rest. They had minimum ¹⁴C ages of 16–27 kyr and δ¹³C values ranging from −24‰ to −26.5‰, indicating a terrestrial origin of organic matter. The sediments from this site were thus likely deposited by episodic mass wasting of slope sediment through the canyon, delineating the previously suggested main pathway of sediment and clay movement to abyssal Gulf sediments.     

Allochthonous salt, structure and stratigraphy of the north-eastern Gulf of Mexico. Part I: Stratigraphy

Major sequence boundaries associated with eustatic sea level changes are correlated to the general stratigraphy of the north-eastern Gulf of Mexico. The details of a Middle Cretaceous Flooding Surface, marking a major break in sedimentation, are documented. The sequence stratigraphic work provides an example of the ‘stratigraphic signature of the Neogene’. Three major episodes of sediment accumulation are represented by: (1) Late Jurassic (150.5 Ma) to Middle Cretaceous (94 Ma) aggradation and progradation of sediments with significant sediment accumulation in the present shelf and slope areas; (2) an extended period of starved sedimentation during 94-30 Ma corresponding to Middle Cretaceous flooding events (93.5 and 91.5 Ma) and the lack of sediment supply; and (3) since Late Oligocene time, unusually rapid sedimentation rates that characterize the deep water study area. These patterns of sediment accumulation directly affect the formation of allochthonous salt in the study area.     

The generation and preservation of multiple hurricane beds in the northern Gulf of Mexico

Cores collected from Mississippi Sound and the inner shelf of the northeast Gulf of Mexico have been examined using ²¹⁰Pb and ¹³⁷Cs geochronology, X-radiography, granulometry, and a multi-sensor core logger. The results indicate that widespread event layers were probably produced by an unnamed hurricane in 1947 and by Hurricane Camille in 1969. Physical and biological post-depositional processes have reworked the event layers, producing regional discontinuities and localized truncation, and resulting in an imperfect and biased record of sedimentary processes during the storms. The oceanographic and sedimentological processes that produced these event beds have been simulated using a suite of numerical models: (1) a parametric cyclone wind model; (2) the SWAN third-generation wave model; (3) the ADCIRC 2D finite-element hydrodynamic model; (4) the Princeton Ocean Model; (5) a coupled wave–current bottom boundary layer-sedimentation model; and (6) a model for bed preservation potential as a function of burial rate and bioturbation rate. Simulated cores from the Mississippi Sound region are consistent with the observed stratigraphy and geochronology on both the landward and seaward sides of the barrier islands.     

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.     

基于随机森林方法反演墨西哥湾海表盐度

盐度是表征物理和生物地球化学过程的重要参数之一,光学遥感可满足较高分辨率的监测需要并避免射频干扰问题,为沿海水域的海表盐度研究提供可行的途径。本文基于MODIS-Aqua的412 nm、443 nm、488 nm、555 nm和667 nm波段的遥感反射率(Rrs412、Rrs443、Rrs488、Rrs555、Rrs667)、海表温度以及实测的海表盐度数据构建随机森林模型,基于模型结果分析墨西哥湾海表盐度时空异质性及海表盐度与影响因子(海表温度和遥感反射率)之间的相关关系。研究结果表明：(1)随机森林模型能较准确地反演墨西哥湾海表盐度,其均方根误差为0.335,决定系数为0.931;(2)湾区海表盐度空间分布呈近岸−河口低、离岸高,环状向内增值的态势,其变化受河流流量、风力以及环流的影响;(3)海表温度与海表盐度存在较强的相关性,海表温度对海表盐度的反演影响显著;(4)海表温度、遥感反射率与海表盐度的相关性呈现空间异质性。     

Patterns of summer vertical and horizontal currents in coastal waters of the northern Gulf of Aqaba, Red Sea

This study was focused on analysis of the horizontal and vertical current components with correspondence to tide variation, heat flux, seiches and relative backscatter intensity in coastal waters of the northern Gulf of Aqaba during the summers of 2001–2004. Spectrum analysis has shown eight distinguishable peaks of the tide measurements. In addition to semidiurnal and diurnal barotropic tides, signal periods of 8.13, 5.89–6.32 and 4.06 h were related to shallow water compound and overtides of the principal solar and lunar constituent and to seiches (second and third mode) generated in the Red Sea, whereas seiches of the first mode might enhance the semidiurnal tide signal. The shortest periods of 1.02–1.05, 0.50 and 0.36 h were related to seiches (first, second and third mode, respectively) generated in the Gulf of Aqaba. The spectrum analysis and cross-correlation tests of the horizontal and vertical current components and relative backscatter intensity suggested that the vertical motion at the diurnal period was either due to migration of zooplankton or to water convection. Both of which were of the same order as vertical motion induced by horizontal motions of water particles projected parallel on the bottom gradient line. On the other hand, semidiurnal and diurnal periods detected in the vertical currents were caused by the strong signal in the cross-shore current component, first mode seiches generated from the Red Sea and tide. This could be considered as an indicator of the dominant force influencing the currents, such as winds, thermocline depth, seiches and tidal forcing. A permanent convection during summer is likely to occur in the shallow coastal waters due to relatively high inclined bottom, high evaporation and blowing of dry air, as well as the observed dominance of the downward vertical currents in coastal waters.