墨西哥湾布什山冷泉碳酸盐岩沉积岩石学及地球化学

对墨西哥湾布什山采集的7块冷泉碳酸盐岩样品进行了沉积岩石学及地球化学研究,结果表明:冷泉碳酸盐岩以结壳的形式产出,结壳中双壳类和管状蠕虫等冷泉生物碎屑发育,结壳主要由泥晶、微晶和亮晶文石组成(含量89%～99%,平均94%)。碳酸盐岩13C中等亏损(δ13C最负值为-29.4‰,PDB),表明碳的来源主要是非甲烷的碳烃化合物,18O相对富集(δ18O为+2.4‰～+5.0‰,PDB),可能与原地水合物分解形成富集18O的孔隙水有关。碳酸盐矿物相(5%硝酸可溶部分)的稀土元素总量较低(0.40×10-6～20.85×10-6),稀土元素Ce异常特征和页岩标准化配分模式在不同样品、甚至同一样品结晶程度不同的碳酸盐矿物相(泥晶、微晶和亮晶)中变化显著,表明布什山冷泉碳酸盐岩形成过程中氧化还原条件复杂多变,控制氧化环境变化的主要因素可能是流体的渗漏速率。     

南海北部陆坡九龙甲烷礁冷泉碳酸盐岩沉积岩石学特征

本文对南海北部陆坡九龙甲烷礁采集的两个冷泉碳酸盐岩样品TVG-1和TVG-11进行了矿物学、岩石学和碳氧同位素分析研究。X射线衍射分析（XRD）结果表明,自生矿物主要为文石、白云石和高镁方解石等,碎屑矿物含量较少。TVG-11中存在大量的文石,而TVG-1只保留了已转化为方解石的文石假象,据此可以推断TVG-1的形成要早于TVG-11。扫描电镜（SEM）显微结构特征显示,自生矿物主要是通过细菌的厌氧氧化作用,直接从冷泉中沉淀出来的。碳、氧同位素测试结果显示,碳酸盐岩具有强δ^13C亏损和一定程度的δ^18O富集的特征,TVG-1和TVG-11的δ^13C值分别为-46．22％。和-52．88％,均低于-40%,表明碳源于微生物作用的甲烷厌氧氧化,是指示该海域存在冷泉的重要证据;TVG-1和TVG-11的δ^18O值分别为3．19‰和4．07‰,存在一定程度的δ^18O富集。碳氧同位素和矿物的显微结构特征都说明九龙甲烷礁碳酸盐岩是烃类渗漏微生物作用而形成的自生碳酸盐岩,可能是天然气水合物分解引起的沉积,显示该区存在水合物的可能性很大。     

墨西哥湾深水下陆坡区冷泉碳酸盐岩中的环烷烃及单环芳烃的特征与来源

研究了墨西哥湾水深约2 200 m的下陆坡Alaminos Canyon 645区块(AC 645区)冷泉碳酸盐岩中含量较高的烷基环戊烷、烷基环己烷、直链烷基苯及苯基烷烃化合物的组成和分布特征。烷基环戊烷和烷基环己烷以低碳数烃占绝对优势,主峰碳分别为C14和C15,目前发现的烷基环己烷主要来源于嗜热嗜酸的细菌或藻类直链脂肪酸经环化作用的产物。烷基苯碳数分布范围为C13—C23,以C13—C20的低碳数烷基苯为主,主峰碳为C16,具有明显偶碳优势,很可能来源于冷泉渗漏环境中的喜盐细菌。     

南海北部烟囱状冷泉碳酸盐岩的沉积环境分析

对南海北部陆坡具有圈层结构的烟囱状冷泉碳酸盐岩的岩石学、矿物学和地球化学特征进行了研究,结果表明:烟囱内外层具有明显不同的特征,内层碳酸盐含量比外层高,内层的碳酸盐矿物也更富含MgCO3,并且矿物颗粒较大,结晶程度较高;外层的铝、硅等陆源碎屑元素比内层含量高。内外层的碳、氧同位素也具有明显差别。对于研究的两个样品,内层的δ13 C分别为-40.80×10-3和-31.27×10-3,δ18 O分别为2.67×10-3和2.00×10-3,而外层的δ13 C分别为-31.26×10-3和-30.99×10-3,δ18 O分别为0.48×10-3和1.85×10-3。结合前人在该区域附近的研究结果认为,烟囱状碳酸盐岩具有内层比外层相对较负δ13C和较为富集δ18 O的特征。1mm间隔取样的δ13C和δ18 O剖面变化与烟囱状碳酸盐岩的圈层结构吻合较好,由外层至内层δ13C的变化范围为-27.50×10-3～-32.05×10-3,δ18 O的变化范围为0.78×10-3～2.34×10-3。外层6mm的δ13 C和δ18 O具有明显的负线性相关,相关系数r2为0.996 7。根据上述特征,讨论了渗漏甲烷流体的性质、扩散机制及对沉积环境的影响,推测具有圈层结构的烟囱状碳酸盐岩是由单一渗漏通道形成的,生长顺序有待进一步定年证实。烟囱外部6mm受海水影响较大。研究结果对于恢复南海北部陆坡甲烷渗漏系统、建立冷泉和热泉之间的联系具有一定意义。     

南海东沙海域冷泉渗漏区沉积物稀土元素地球化学特征

东沙海域是我国典型的冷泉活动区,该区域表层沉积物的稀土元素地球化学特征不仅受其源区控制,同时也可能会受到冷泉渗漏活动的影响。本文选取位于南海北部陆坡973-4站位的柱状样为研究对象,该站位位于21°54.3247’N、118°49.0818’E,水深为1666m,柱状样总长1375cm,采用电感耦合等离子质谱(ICP-MS)、X射线荧光光谱(XRF)等分析测试方法,测得样品的稀土元素以及部分微量和主量元素数据,并结合总碳(TC)、总有机碳(TOC)以及有孔虫质量分数等数据,探讨了冷泉泄漏对周围成岩环境及沉积物稀土元素地球化学特征的影响。结果发现,973-4站位柱状样在海底以下459~619cm深度范围内稀土元素含量整体异常减少,但所有样品稀土元素分布模式及相关稀土元素参数均十分一致。结合冷泉活动研究,表明该区域沉积物元素地球化学特征的异常与物源无关,是受自生碳酸盐岩的增加所影响。此外,通过分析δCe值随深度的变化以及自生碳酸盐岩和硫酸盐-甲烷转换带(SMTZ)的分布情况发现三者相关度很高,表明自生碳酸盐岩的增加很可能是该区域发生的冷泉渗漏导致的甲烷厌氧氧化作用(AOM)所产生的。     

南海东北部陆坡结核成因及对冷泉活动的指示

本文对2018年利用“海龙Ⅲ”ROV在南海东北部陆坡采集到的多金属结核样品进行了矿物学和地球化学分析。结果表明研究区结核以棒状为主,具有2层结构。结核外层的矿物学和元素组成特征与水成型多金属结核较为相似。结核内层多发育孔洞,以针铁矿为主,有黄铁矿残余,Fe、As元素含量高,Mn、REE等元素含量低。NH-2结核样品内部的碳酸盐组分的全岩δ¹³C为-30.91‰。所研究结核样品的早期形成过程与冷泉活动密切相关,结核生长后期冷泉活动停止或是减弱,在等深流作用下结核黄铁矿被氧化成针铁矿,之后锰铁氧化物继续在结核外部生长。     

海底甲烷缺氧氧化与冷泉碳酸盐岩沉淀动力学研究进展

海底缺氧带甲烷氧化作用是一个重要的甲烷生物地球化学过程,已被许多地球化学现象所证实。甲烷缺氧氧化有效地减少了渗漏到海水和大气中的甲烷通量,但目前仅有的数据还不能很好地限定甲烷缺氧氧化在全球甲烷循环和全球碳循环中的作用。甲烷缺氧氧化的机理还存在争议,很可能是一个"反甲烷生成"过程。在许多天然气渗漏发育区域,由于甲烷缺氧氧化作用引起环境碱度的增加而沉淀冷泉碳酸盐岩,在海底表层沉积物中形成块状碳酸盐岩结壳。但冷泉碳酸盐岩生成所需的物理化学和生物地球化学条件在很大程度上还不清楚。数值计算表明,孔隙水中溶解足够量的甲烷、冷泉渗漏强度适中、较小的生物扰动作用有利于冷泉碳酸盐岩的生成,而过高的沉积速率则抑制冷泉碳酸盐岩结壳的生成。因此,海底发育冷泉碳酸盐岩可以指示天然气渗漏系统的演化特征。     

墨西哥湾北部陆坡天然气水合物成藏系统

为了深入了解墨西哥湾北部陆坡天然气水合物赋存状况、水合物储层特征以及资源评价,借助天然气水合物成藏系统概念,在收集和整理国内外学者的研究成果的基础上提出了散失均衡体系。对该区天然气水合物气体供应、流体运移、聚集成藏及水合物散失均衡体系进行了综合分析,认为散失均衡体系应该作为天然气水合物成藏系统的重要组成部分予以重视。     

海底甲烷冷泉特征与冷泉生态系统的群落结构

海底冷泉及冷泉生态系统是现代海洋地质学和生物学研究的前沿领域,它提升了人类对深海资源的认识和利用,拓展了深海极端环境下生命的潜在界线。综述了海底甲烷冷泉的成因、冷泉喷发的类型及其特征,海底冷泉分布、形成的主要阶段(150—100Ma BP,42—28Ma BP和12Ma BP以来)以及指示海底冷泉系统的简要特征;综述了冷泉化能自养生态系统及其食物链构成和特征,专性种、潜在专性种和非专性种的生物组成及其特征,冷泉环境中有孔虫和轻小型底栖动物的主要变化;最后划分了我国近海的冷泉区(点)并简要介绍它们的分布和特征,为开展我国近海冷泉及其冷泉生物群落的调查和研究提供了基础资料。     

Large-scale environmental influences on the benthic macroinfauna of the southern Gulf of Mexico

The influence of large-scale natural disturbance from winter storms (‘northers’) and river runoff on the macrobenthic community structure of the southern Gulf of Mexico was investigated in both carbonate and transitional carbonate–terrigenous sedimentary environments. Samples of the infauna were obtained in three seasons from 13 stations from two 250 km transects along 80–170 and 20–50 m water depth. Samples after the northers season had the lowest total number of families and individuals, 114 and 2940, respectively, compared to the dry and rainy seasons with 129 and 132 families and 11580 and 15266 individuals, respectively. Spatial patterns of macroinfauna composition varied across and along the shelf as a response to sedimentary environments and depth. Coarser sediments from the carbonate area harboured the highest mean densities per station with 500–24,000 individuals m⁻² and 108–122 families in total, compared to the transitional sediment with 500–8200 individuals m⁻² and 56–74 families across the three seasons. Univariate and multivariate statistical techniques demonstrated that low densities and number of taxa were associated with winter storms, but storm influence was dependent on depth and sediment type. Multiple linear regression analysis and BIOENV analysis indicated that sediment mean grain size, percentage of clay and organic matter best explained the macroinfauna spatial patterns, although BIOENV indicated that depth has an overriding role. An increase in densities of opportunistic taxa (numerous polychaetes of small sizes) was observed four months after the ‘northers’ and this was more evident in the area of carbonate sediment. Additionally a combined disturbance from northers and river runoff is suspected to be responsible for a naturally impoverished macroinfauna community in the transitional sedimentary environment.     

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.     

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.     

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.     

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.     

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.     

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.     

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.