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.     

Predictors of species richness in the deep-benthic fauna of the northern Gulf of Mexico

Species richness in macrofauna and megafauna collected with box cores and trawls from 35 standard stations over a depth range of 175–3720 m in the northern Gulf of Mexico was examined in terms of two primary questions: (1) are observed patterns random? and (2) if not, what environmental factors might account for the patterns? A null model tested whether richness vs. depth distributions were random. Groups with species that had broad vertical depth ranges fit the null model better than groups with small ranges, but for almost all phyla a non-random pattern was indicated. With randomness as a proximal explanation ruled out, further examination of the relationship between richness and environmental factors was justified. A generalized linear model (GLM) showed that a suite of 18 factors categorized as food-related, habitat-related, pollution-related and location-related were significantly related to richness patterns, but that different mixes of factors applied to different phyla. No one factor accounted for any observed patterns. Thus, each taxonomic group needs to be examined individually, and no generally applicable explanation for the causes of richness patterns may exist. Nonetheless, mapping richness itself indicates valuable areas in the Mississippi Trough that must receive special consideration and possible protection.     

Occurrence of larval fishes in the surf zone of a northern Gulf of Mexico barrier island

Larval fishes were collected from the surf zone of Horn Island, Mississippi between March 1978 and April 1979. A standardized total of 39 435 larvae were taken from 222 collections in the inner and outer surf zone regions, representing fish in 69 taxa. Overall, considerably more larvae were collected in the outer surf zone (78·3%) than in the inner surf zone (21·7%). Engraulids, Chloroscombrus chrysurus and Symphurus spp. were the most abundant larvae taken from the outer surf zone while engraulids, Leiostomus xanthurus, Brevoortia patronus and Trinectes maculatus were the numerically dominant larvae in the inner surf zone. Seasonal peaks in abundance occurred at the outer surf zone stations during May and June and at the inner surf zone stations during December. Larval densities were significantly greater in night collections than in day collections.The occurrence of early larvae, late larvae and juveniles suggests that the surf zone habitat is important to several species of coastal marine fishes. Menticirrhus littoralis, Harengula jaguana and Trachinotus carolinus appear to most readily utilize the surf zone as a nursery area.     

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。     

High-resolution seismic characterization of the gas and gas hydrate system at Green Canyon 955, Gulf of Mexico,USA

The Pliocene and Pleistocene sediments at lease block Green Canyon 955 (GC955) in the Gulf of Mexico include sand-rich strata with high saturations of gas hydrate; these gas hydrate accumulations and the associated geology have been characterized over the past decade using conventional industry three-dimensional (3D) seismic data and dedicated logging-while-drilling (LWD) borehole data. To improve structural and stratigraphic characterization and to address questions of gas flow and reservoir properties, in 2013 the U.S. Geological Survey acquired high-resolution two-dimensional (2D) seismic data at GC955. Combined analysis of all available data improves our understanding of the geological evolution of the study area, which includes basin-scale migration of the Mississippi River sediment influx as well as local-scale shifting of sedimentary channels at GC955 in response to salt-driven uplift, structural deformation associated with the salt uplift, and upward gas migration from deeper sediments that charges the main gas hydrate reservoir and shallower strata. The 2D data confirm that the sand-rich reservoir is composed principally of sediments deposited in a proximal levee setting and that episodes of channel scour, interspersed with levee deposition, have resulted in an assemblage of many individual proximal levee deposit “pods” each with horizontal extent up to several hundred meters. Joint analysis of the 2D and 3D data reveals new detail of a complex fault network that controls the fluid-flow system; large east-west trending normal faults allow fluid flow through the reservoir-sealing fine-grained unit, and smaller north-south oriented faults provide focused fluid-flow pathways (chimneys) through the shallower sediments. This system has enabled the flow of gas from the main reservoir to the seafloor throughout the recent history at GC955, and its intricacies help explain the distributed occurrences of gas hydrate in the intervening strata.     

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.     

墨西哥湾西北陆坡天然气水合物资源评价

墨西哥湾西北陆坡天然气水合物发育广泛,水合物天然气资源量估计约(10～14)×1012m3。构造控制的混合成因(生物甲烷和热成因天然气)水合物含量高、资源密度大,经济潜力高;盆地生物成因甲烷水合物呈细分散状发育,沉积物中水合物含量低、资源密度小、经济潜力低。综合地质、技术和开采成本3方面因素,在目前全球几个工程程度较高的水合物发育区中,墨西哥湾水合物的经济潜力最大,其中工作程度最高的7个水合物发育区中,MC852/853区的经济潜力最高。     

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.     

Stakeholder perceptions of the northern Gulf of Mexico grouper and tilefish individual fishing quota program

The Gulf of Mexico Fishery Management Council determined that previous management of the grouper and tilefish fisheries in the northern Gulf of Mexico were not meeting management goals, and developed a catch shares program using individual fishing quotas (IFQs) beginning in January 2010 in order to more effectively manage these fisheries. An IFQ is a management method in which individual fishers and corporations are allocated the right to harvest a percentage of a fishery's total allowable catch, thus specifying how much of a particular species each fisher can harvest. This study makes use of a mail out survey to document the perceptions of fishers, seafood wholesalers, fisheries managers, and academics with an interest in the (northern) Gulf of Mexico grouper and tilefish IFQ program. While fishers, seafood dealers, fisheries managers, and academics all acknowledge that the IFQ program will create some problems, commercial fishers and dealers were far more skeptical of the alleged benefits of IFQs. Moreover, larger commercial operators were more inclined to agree with managers and academics that the IFQ program will produce several benefits for their operations and the fisheries. Some smaller operators believe that they will be driven to ignore the new rules or be forced out of business. In the future, the Gulf Council might do two things: put a bit more effort into making fishers aware of the potential benefits of IFQs, and develop alternatives with more flexibility perhaps working more closely with communities of fishers, who prize their independent way of life above all else.     

Microstructures of structure I and II gas hydrates from the Gulf of Mexico

Gas hydrate samples from various locations in the Gulf of Mexico (GOM) differ considerably in their microstructure. Distinct microstructure characteristics coincide with discrete crystallographic structures, gas compositions and calculated thermodynamic stabilities.     

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.     

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.     

Heat-flow regimes and the hydrate stability zone of a transient,thermogenic, fault-controlled hydrate system (Woolsey Mound northern Gulf of Mexico)

This study aims to constrain the base of the hydrates stability field in structurally complexsites using the case of Woolsey Mound, a fault-controlled, transient, thermogenic hydrates system, in Mississippi Canyon Block 118, northern Gulf of Mexico. We have computed the base of the hydrates stability field integrating results from a recent heat-flow survey, designed to investigate geothermal anomalies along fault zones which exhibit different fluid flux regimes. An advanced “compositional” simulator was used to model hydrate formation and dissociation at Woolsey Mound and addresses the following hypotheses:

• 1.Migrating thermogenic fluids alter thermal conditions of the Hydrate Stability Zone (HSZ), so heat-flow reflects fault activity;
• 2.Gas hydrate formation and dissociation vary temporally at active faults, temporarily sealing conduits for migration of thermogenic fluids;
• 3.High salinity and inclusion of thermogenic gases with higher molecular weight than methane produce opposite effects on the depth to the bottom of the hydrate stability zone.

Applications of results include identifying and quantifying hydrate deposits in shallow sediments using an interdisciplinary approach that includes multiple resolution seismic data evaluation, geological and geochemical groundtruthing and heat-flow analyses as a proxy for activity along faults.     

Time series video analysis of bubble release processes at natural hydrocarbon seeps in the Northern Gulf of Mexico

This research quantifies the rate and volume of oil and gas released from two natural seep sites in the Gulf of Mexico: lease blocks GC600 (1200 m depth) and MC118 (850 m depth). Our objectives were to determine variability in release rates and bubble size at five individual vents and to investigate the effects of tidal fluctuations on bubble release. Observations with autonomous video cameras captured the formation of individual bubbles as they were released through partially exposed deposits of gas hydrate. Image processing techniques determined bubble type (oily, gaseous, and mixed: oily and gaseous), size distribution, release rate, and temporal variations (observation intervals ranged from 3 h to 26 d). A semi-automatic bubble counting algorithm was developed to analyze bubble count and release rates from video data. This method is suitable for discrete vents with small bubble streams commonly seen at seeps and is adaptable to multiple in situ set-ups. Two vents at GC600 (Birthday Candles 1 and Birthday Candles 2) were analyzed. They released oily bubbles with an average diameter of 5.0 mm at a rate of 4.7 bubbles s⁻¹, and 1.3 bubbles s⁻¹, respectively. Approximately 1 km away, within the GC600 seep site, two more vents (Mega Plume 1 and Mega Plume 2) were analyzed. These vents released a mixture of oily and gaseous bubbles with an average diameter of 3.9 mm at a rate of 49 bubbles s⁻¹, and 81 bubbles s⁻¹, respectively. The fifth vent at MC118 (Rudyville) released gaseous bubbles with an average diameter of 3.0 mm at a rate of 127 bubbles s⁻¹. Pressure records at Mega Plume and Rudyville showed a diurnal tidal cycle (24.5 h). Rudyville was the only vent that demonstrated any positive correlation (ρ = 0.60) to the 24.5 h diurnal tidal cycle. However, these observations were not conclusive regarding tidal effects on bubble release.     

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.