墨西哥湾GC238区冷泉碳酸盐岩的微结构与石化微生物特征

采于墨西哥湾GC238海底天然气渗漏区浅表层的冷泉碳酸盐岩呈结核状产出,由方解石微晶和胶结物及少量的黄铁矿构成。胶结物由直径为0.1～0.5 m的方解石化的球体、卵形体、棒状体组成,充填于方解石晶体之间。冷泉碳酸盐岩结核下表面发育有由方解石化的球体、卵形体、棒状体组成的薄层,其中的一些球状集合体（约5 m）断面显示发育有核和外壳的层圈结构。黄铁矿呈草莓状,也具有相似的层圈结构。这种层状结构与活体古细菌被硫酸盐还原细菌包裹的层圈结构相似。样品中所保存的球体、卵形体、棒状体及其所组成的层圈结构可能是石化的甲烷氧化古细菌和硫酸盐还原细菌。     

南海北部冷泉碳酸盐岩和石化微生物细菌及地质意义

通过对2002年广州海洋地质调查局在南海北部海域拖网采集的碳酸盐岩样品进行岩石、矿物、微量元素和同位素特征的研究,证实在南海水合物远景区发育有冷泉碳酸盐岩。冷泉碳酸盐岩形态类似于烟囱,主要由方解石、伊利石、石英、黄铁矿和一种未知的矿物组成。保存了很可能是石化的甲烷氧化和硫酸盐还原细菌。碳酸盐岩矿物的δ^13CPDB为-51.24‰～-51．757‰。碳酸盐岩矿物的页岩标准化稀土元素配分模式具无Ce和Eu异常、微弱的轻稀土亏损和中稀土富集特征,表明形成于还原的缺氧环境,显示了冷泉碳酸盐岩的特征,指示在南海北部水合物远景分布区内冷泉活动的近海底可能发育有水合物。     

海底天然气渗漏系统微生物作用及冷泉碳酸盐岩的特征

海底天然气渗漏系统是全球海洋环境中广泛分布的自然现象。部分渗漏天然气通过细菌作用转变为二氧化碳,同时海水硫酸盐被还原为硫化氢, 与孔隙水中的钙和铁结合而沉淀冷泉碳酸盐岩。冷泉碳酸盐岩的常见矿物有微晶方解石、文石、白云石和黄铁矿。冷泉碳酸盐岩常发育一些特殊的组构, 如黄铁矿环带结核、溶蚀面、平底晶洞、凝块和向下生长的叠层石组构等。碳酸盐岩特别负的δ13C值指示碳来源于生物成因的甲烷, 而18O富集可能与天然气水合物的分解有关。冷泉碳酸盐岩中黄铁矿的δ34S值低于海水的, 这指示硫来源于微生物还原的海水硫。冷泉碳酸盐岩中的生物标志化合物及其极负的δ13C值指示微生物的生命代谢活动。     

南海北部深海甲烷冷泉自生碳酸盐岩显微结构特征与流体活动关系初探

对2002年"海洋四号"在南海北部深水冷泉活动区水深约3000m海底获得的1块自生碳酸盐岩展开了岩石学、地球化学和显微结构的分析研究。结果表明,该HD-83站位岩石样品为有多孔不规则块体,属于与扩散流有关的冷泉自生碳酸盐岩。其自生碳酸盐矿物成分为镁方解石或文石,含量达全岩的17%。全岩碳同位素值δ13C为-61.403‰,氧同位素值δ18O为3.5606‰,属于甲烷冷泉喷口成因自生碳酸盐岩,甲烷来源以微生物成因为主。该岩石显微结构有泡孔状、充填通道、不规则裂隙、虫孔边缘加厚4种主要类型。通过对比,推知这些显微结构可能与甲烷分解排气释放或流体逃逸通道有关。本研究注意到在岩石的裂隙或孔道中菌丝体普遍存在,可能属于菌席的组成份子,依靠通道内流体进行化能合成。     

南海北部冷泉碳酸盐岩研究进展及其对大洋钻探选址的指导意义

冷泉系统在大陆边缘海域中广泛发育,我国的南海北部海域因其独特的地质条件成为了研究冷泉系统的天然实验室。在冷泉系统中,由于甲烷厌氧氧化作用的存在,极大地促进了碳酸盐岩等自生特征矿物的形成。而自生碳酸盐岩继承了冷泉环境中的地球化学特征,是记录冷泉活动信息的优良载体。近十几年以来,国内外科学家对南海北部冷泉碳酸盐岩进行了大量的研究,取得了一系列的成果,但有关南海北部碳酸盐岩最新研究成果的整体认识和未来的研究方向仍缺乏相应的汇总分析。因此,本文对近年来南海北部冷泉碳酸盐岩的相关研究成果进行了综述。目前研究主要关注冷泉碳酸盐岩的成岩过程及其环境影响,包括流体来源、氧化还原环境、元素循环等,而有关冷泉活动对全球变暖和碳循环影响的研究较少;同时大多研究基于单一期次的冷泉活动,有关地质历史时期的冷泉活动鲜有报道。本文在对已有研究基础上进行综述,认为未来需要在南海北部合适的区域进行大洋钻探,重建地质历史时期以来的冷泉活动,探讨其对全球变暖和碳循环的影响。基于前期的工作成果,笔者认为琼东南海域和东沙海域冷泉区是对地质历史时期冷泉活动进行深入研究的理想区域。     

黑海西北部冷泉碳酸盐岩的沉积岩石学特征及氧化还原条件的稀土元素地球化学示踪

黑海西北部罗马尼亚大陆架(水深120m)和乌克兰陆坡(水深190m)发育冷泉碳酸盐岩结壳。XRD测试表明此结壳主要由高镁方解石和文石组成。结壳中与渗漏系统微生物活动有关的凝块和葡萄状文石等特殊的沉积组构非常发育。冷泉碳酸盐岩酸可溶部分(碳酸盐岩相矿物)的稀土元素含量很低(0.068×10-6~2.817×10-6),稀土元素页岩标准化配分模式显示罗马尼亚大陆架冷泉碳酸盐岩具有明显的Ce负异常,乌克兰陆坡冷泉碳酸盐岩具Ce的正异常,表明它们是分别在氧化和还原环境中沉积的。稀土元素和V、Cd和U等微量元素的含量在泥晶中最高,亮晶中最低,可能反映成岩过程对元素含量有控制作用。     

南海北部海马冷泉区表层沉积物的AOM生物标志化合物特征及意义

选取采自南海天然气水合物赋存区海马冷泉,管状蠕虫区（ROV06站位）和贻贝区（HM101站位）的2个表层沉积物柱状样品,提取其中的生物标志化合物,对其种类和稳定碳同位素进行了测定,用以探讨海底表层沉积物中的有机质来源、微生物种群分布及其对冷泉渗漏活动的响应特征. 两个站位的沉积物中均发现了大量与甲烷厌氧氧化古菌（ANME）有关的生物标志物,如2,6,11,15?四甲基十六烷（crocetane）、2,6,10,15,19?五甲基二十烷（PMI）等类异戊二烯烃,古醇（archaeol）、sn2?羟基古醇（sn2?OH?Ar）等,以及来源于硫酸盐还原菌（SRB）的异构/反异构脂肪酸iso?C₁₅和ai?C₁₅等. 这些生物标志物均具有极低的碳同位素特征（古菌生标δ13C值低至-126‰,硫酸盐还原菌生标δ13C值低至?89‰）,表明沉积物中发生了甲烷厌氧氧化作用（AOM）. ROV06和HM101站位沉积物中均检测到了crocetane,大多数sn2?羟基古醇/古醇大于1,同时ai?C₁₅/iso?C₁₅脂肪酸比值小于2,这说明两个站位沉积物中的甲烷厌氧氧化古菌主要以ANME?2/DSS为主,指示甲烷渗漏强度较强. ROV06站位的表层沉积物含有crocetane,但sn2?羟基古醇/古醇小于1,且ai?C_15/iso?C₁₅脂肪酸比值大于2.1,指示了ANME?1/DSS和ANME?2/DSS混合存在的种群特征,说明ROV06站位顶部甲烷渗漏强度有减小的趋势. 根据古菌种群ANME?2化合物对甲烷的碳同位素分馏（Δ:-50‰）及古菌生物标志物（PMI、古醇、sn2?羟基古醇）的平均δ13C值,计算得到甲烷δ13C值（-58‰~-53‰）,显示甲烷为热成因和生物成因混合气. 虽然ROV06和HM101站位的甲烷具有相近的δ13C值,但ROV06站位的SRB生物标志物比HM101站位要更加亏损13C（Δδ13C:18‰）,这可能与管状蠕虫的共生菌（硫氧化菌）吸收硫化物并释放出硫酸盐有关,因为其不断释放出的硫酸盐很可能极大地增强了甲烷厌氧氧化作用,使沉积物中含有更多13C亏损的无机碳.      

南海北部冷泉碳酸盐岩中系列藿烷酸的检出及意义

对南海东沙海域九龙甲烷礁冷泉碳酸盐岩的藿烷酸系列化合物进行了研究, 结果显示冷泉碳酸盐岩中含有17β(H), 21β(H)-30-藿烷酸至17β(H), 21β(H)-33-藿烷酸系列化合物. 其中site1样品TVG3-C2(ANME-2古菌种群为主)中的17β(H), 21β(H)-32-藿烷酸化合物亏损13C(-69.8‰), 且其同位素值较接近iso-/aiso-C_15:0脂肪酸(-75.2‰~-90.0‰), 说明其母质摄入了甲烷来源的碳.尽管site2(TVG13-C3和TVG14-C2)和site3(TVG8-C5)自生碳酸盐岩中藿烷酸化合物都表现出了非渗漏特征(-30.7‰~-40.3‰, ANME-1古菌种群为主), 但这些样品中藿烷酸的δ13C值同样较接近硫酸盐还原菌来源的iso-/anteiso-C_15:0的δ13C值(-32.5‰~-49.8‰).由此说明大多数存在于以ANME-1种群为主的渗漏环境中的硫酸盐还原菌并没有参与到甲烷厌氧氧化作用中, 而导致藿烷酸化合物和硫酸盐还原菌来源的生物标志物并不明显亏损13C, 并且δ13C值相近.      

冷泉流体沉积碳酸盐岩的地质地球化学特征

冷泉流体是指来自海底沉积界面之下的低温流体以喷涌和渗漏方式注入盆地, 并产生系列的物理和化学及生物作用, 这种作用及产物称为冷泉?它是继洋中脊以盆下源中高温流体的热泉被发现和研究之后的又一个新的盆地流体沉积领域?日前研究较多的是以水? 碳氢化合物 (天然气和石油) ? 硫化氢? 细粒沉积物为主要成分, 温度与海水相近的流体, 广泛发育于活动和被动大陆边缘斜坡海底?冷泉流体沉积体系发育高密度的化学自养生物群, 以碳酸盐岩和天然气水合物为主, 有少量的硫化物和硫酸盐等?冷泉碳酸盐岩的产状有丘? 结核? 硬底? 烟囱? 胶结物和小脉等, 以化学自养生物碎屑和多期次的自生碳酸盐胶结物组成的生物丘最为常见, 它在物质来源? 形成环境? 形成作用等方面与传统来源于海水碳的碳酸盐岩建隆不同, 用术语 C h e r m o h e r m 表示, 以区别于传统海水碳酸盐岩建隆术语b i o h e r m s ? l i t h o h e r m s ? p s e u d o b i o h e r m s 和 b i o s t r o m e s ?地层中石化的化学自养生物丘常是含有大量底栖生物化石的碳酸盐岩建隆产于深水相沉积地层中, 在沉积环境和相分析上出现纵向和横向的不连续, 甚至出现反常现象?矿物以镁方解石? 白云石和文石为主, 与传统的碳酸盐岩相似, 在地球化学组成上最大的区别是冷泉流体沉积碳酸盐岩的碳来源于冷泉体系中的细菌生物成因碳, 具有特别负的碳同位素值?冷泉在海底主要沿构造带和高渗透地层呈线性群, 或围绕泥火山或盐底劈顶部呈圆形或不规则状冷泉群分布,或以海底地形低凹处和峡谷转向处呈孤立冷泉形式产出?冷泉流体以沉积建造流体为主?上覆快速堆积? 成岩压实和胶结作用? 构造挤压和变形作用? 深部的后生作用和成岩作用? 海底沉积物中的天然气水合物分解作用是建造流体向上运移进入海底成为冷泉的驱动力?冷泉碳酸盐岩的沉积作用主要有胶结作用? 充填作用和生物化学沉积作用?冷泉流体中的碳主要是以甲烷为主的碳氢化合物形式存在, 经微生物作用转变为 C O₂ ,最终形成冷泉碳酸盐岩?     

意大利亚平宁地区第三纪中新世“Calcaria Lucina”冷泉碳酸盐岩中结晶扇的沉积岩石学及地球化学特征

结晶扇是冷泉碳酸盐岩的一种标志性沉积构造,主要由文石或重晶石等矿物呈放射状聚集形成。意大利亚平宁地区的第三纪中新统广泛发育冷泉碳酸盐岩,其中"Calcari a Lucina"冷泉碳酸盐岩中发育有典型的结晶扇。结晶扇主要由放射状文石聚集形成,含少量的方解石,具有明显的生长纹层。文石纹层的δ13CPDB为-37.9‰~-53.5‰,δ18OPDB为+2.4‰~+6.0‰,相邻文石纹层之间的δ13CPDB和δ18OPDB值差别明显,可能是与碳和氧的来源及流体渗漏速度有关。全岩稀土元素总量很低,w(REE)为0.23×10-6,页岩标准化模式具有明显的Ce负异常,显示结晶扇形成于氧化环境。     

Pyritization of Iron in Sediments from the Continental Slope of the Northern Gulf of Mexico

In sediments from the continental slope of the Northern Gulf of Mexico, generally,the degree of iron pyritization (DOP) is low (<0.1) and dissolved sulfide is belowdetection limits (5 M), whereas dissolved Fe is typically about 50 to100 M. Therefore, the dissolution of kinetically reactive iron minerals generallydominates over the rate of sulfide production in sediments throughout this region.However, in sediments where hydrocarbons have been added via seepage from thesubsurface, dissolved-Fe is undetectable, DOP can approach 1, and high concentrationsof dissolved sulfide (up to 11 mM) are commonly present. Even though thesesediments have high total reduced sulfide (TRS) concentrations (typically 150 to370 mol gdw^-1), their average C/S ratio is about 4 times that of normal marine sediments reflecting the major input of hydrocarbons. DOP is significantly (20%) higher when calculated using reactive-Fe extracted by citrate dithionite than by cold 1N HCl. This difference is primarily due to the greater extraction efficiency of the cold HCl method for silicate-Fe. TRS tends to rise to a maximum, and remains close to constant even at high (mM) dissolved sulfide concentrations. These TRS concentrations, therefore, represent the size of the ``kinetically' reactive-Fe pool during early diagenesis.     

An alternative Pangea reconstruction for Middle America with the Chortis Block in the Gulf of Mexico: tectonic implications

Understanding Pangea breakup requires a robust reconstruction, and this article focuses on the Middle America sector of the supercontinent. Although most Pangean reconstructions locate the Yucatan Block along the southern USA, the Chortis Block is generally placed off southern Mexico (Pacific model), undergoing sinistral relative motion during the Mesozoic and Cenozoic. However, the Pacific model is inconsistent with the absence of a Cenozoic fault linking the Cayman transforms and the Middle America Trench. We present an alternative Pangean reconstruction, where both the Yucatan and Chortis Blocks are placed in the future Gulf of Mexico, moving Mexico westwards along the Mojave–Sonora megashear to accommodate overlap with South America. Subsequent Mesozoic and Cenozoic evolution is inferred to have occurred in two stages: (i) Jurassic clockwise rotation along the Mojave–Sonora and West Florida megashears, followed by (ii) Cenozoic anticlockwise rotation along the Sierra Madre Oriental and East Yucatan megashears. The first stage is linked to the breakup of Pangea where the Gulf of Mexico formed as a pull-apart basin. The second stage is related to the evolution of the Caribbean where the Chortis and Yucatan Blocks rotated into the trailing side of the Caribbean Plate (pirate model). The new reconstruction is consistent with major parameters, such as (i) gravity, magnetic, and palaeomagnetic data; (ii) the westward continuation of the Cayman transform faults through the Chiapas foldbelt and along the N–S front of the Sierra Madre Oriental foldbelt; (iii) the 27–19 Ma removal of the southern Mexican forearc; (iv) offset of the Cretaceous volcanic arc (Guerrero-Suina); (v) the deflection of the Laramide orogen (Sierra Madre Oriental–Zongolica–Colon); and (vi) the continuity of Cretaceous platformal carbonates containing Caribbean fauna across Middle America. In this latter context, the Motagua high-pressure belt is interpreted as a Cretaceous extrusion zone into the upper plate above a subduction zone rather than as an oceanic suture.     

Total mercury and methylmercury in sediments near offshore drilling sites in the Gulf of Mexico

Concentrations of total Hg in sediments near six drilling sites in the Gulf of Mexico were elevated well above average background values of 40–80 ng/g. The excess Hg was associated with barite from discharged drilling mud. In contrast with total Hg, concentrations of methylmercury (MeHg) in these sediments did not vary significantly at nearfield (<100 m) versus farfield (>3 km) distances from the drilling sites. Observed variability in concentrations of MeHg were related to local differences in redox state in the top 10 cm of sediment. Low to non-detectable concentrations of MeHg were found in nearfield sediments that were anoxic, highly reducing and contained abundant H₂S. At most drilling sites, nearfield samples with high concentrations of total Hg (>200 ng/g) had similar or lower amounts of MeHg than found at background (farfield) stations. Higher values of MeHg were found in a few nearfield sediments at one site where concentrations of TOC were higher and where sediments were anoxic and moderately reducing. Overall, results from this study support the conclusion that elevated concentrations of MeHg in sediments around drilling sites are not a common phenomenon in the Gulf of Mexico.     

Oil seepage and carbonate formation: A case study from the southern Gulf of Mexico

Oil seeps from the southern Gulf of Mexico can be regarded as natural laboratories where the effect of crude oil seepage on chemosynthesis‐based communities and carbonate precipitation can be studied. During R/V Meteor cruise 114 the seep sites UNAM (Universidad Nacional Autónoma de México) Ridge, Mictlan Knoll and Tsanyao Yang Knoll (Bay of Campeche, southern Gulf of Mexico) were investigated and sampled for authigenic carbonate deposits containing large amounts of liquid oil and solid asphalt. The δ¹³C values of individual carbonate phases including: (i) microcrystalline matrix aragonite and calcite; (ii) grey, cryptocrystalline to microcrystalline aragonite; and (iii) clear, fibrous aragonite cement, are between ?30‰ and ?20‰, agreeing with oil as the primary carbon source. Raman spectra reveal that residual heavy oils from all sites are immature and most likely originate from the same reservoir. Geochemical batch modelling using the software code PHREEQC demonstrates how sulphate‐driven oxidation of oil‐derived low‐molecular to high‐molecular weight hydrocarbons affects carbonate saturation state, and shows that the oxidation state of carbon in hydrocarbon compounds and oxidation rates of hydrocarbons control carbonate saturation and precipitation at oil seeps. Phase‐specific trace and rare earth element contents of microcrystalline aragonite and calcite, grey cryptocrystalline aragonite and clear aragonite were determined, revealing enrichment in light rare earth elements for grey aragonite. By comparing trace element patterns of carbonates with those of associated oils, it becomes apparent that liquid hydrocarbons constitute an additional source of trace metals to sedimentary pore waters. This work not only demonstrates that the microbial degradation of oil at seeps may result in the precipitation of carbonate minerals, it also elucidates that trace metal inventories of seep carbonates archive diagnostic elemental patterns, which can be assigned to the presence of heavy hydrocarbons in interstitial pore waters.     

Study on System of Faults in the Gulf of Mexico and Adjacent Region based on Gravity Data

In the Gulf of Mexico and adjacent landmasses,faults are very complex,and their distribution is closely related to plate tectonics,ocean-land boundary,and former structure.The plane position of the faults can be identified by the maximum characteristic of the vertical derivative of the normalized vertical derivative of the total horizontal derivative(NVDR-THDR)of the Bouguer gravity anomaly.The apparent depth of the faults is inverted by the Bouguer gravity anomaly curvature property.Based on tectonic evolutionary processes and the plane distribution and apparent depth characteristics of the faults,a complete fault system for the Gulf of Mexico and adjacent areas has been established,including 102 faults.The apparent depths of 33 first-class faults are 16-20 km and for 69 second-class faults are 12-16 km.The F_1-2and F_1-3subduction fault zones are two caused by the subduction of the Cocos Plate into the old Yucatan and Chorti landmasses;F_1-11and F_1-12fault zones extend westward to the coast of Guatemala and do not extend into the continent;F_1-17and F_1-20faults,which control the boundary of the oceanic crust,do not extend southward into the continent.The fault system,which radiates in a"fan-shaped"structure as a whole,unfolds to the northeast.Faults of different nature and sizes are distributed in the Cocos Plate subduction zone,Continental,Gulf of Mexico,Yucatan old landmass and Caribbean Plate in NW,NNW,NS,NE and NEE directions.In the Gulf of Mexico region,the fault system is a comprehensive reflection of former tectonic movements,such as plate movement,drift of old landmasses and expansion of oceanic crusts.The first-class faults control the plate and ocean-continental boundaries.The second-class faults are subordinate to the first-class faults or related to the distribution of different sedimentary layers.     

A 1200-year proxy record of hurricanes and fires from the Gulf of Mexico coast: Testing the hypothesis of hurricane-fire interactions

We present here the first high-resolution pollen record of vegetation response to interactions of hurricane and fire disturbances over the past 1200 yr from a small lake in Alabama on the Gulf of Mexico coast. The paleotempestological record inferred from the overwash sand layers suggests that the Alabama coast was directly struck by Saffir-Simpson category 4 or 5 hurricanes twice during the last 1200 yr, around 1170 and 860 cal yr BP, suggesting an annual landfall probability of 0.17% for these intense hurricanes. The charcoal data suggest that intense fires occurred after each of these hurricanes. The pollen data suggest that populations of halophytic plants (Chenopodiaceae) and heliophytic shrubs (Myrica) expanded after the hurricane strikes, probably due to saltwater intrusion into the marshes and soil salinization caused by overwash processes. Populations of pines (Pinus sp.) decreased significantly after each intense hurricane and the ensuing intense fire, suggesting that repeated hurricane-fire interactions resulted in high tree mortality and probably impeded recruitment and recovery. Our data support the hypothesis that the likelihood and intensity of fire increased significantly after a major hurricane, producing responses by vegetation that are more complex and unpredictable than if the disturbance agents were acting singly and independently.     

盐构造与深水重力流的相互作用及响应

在发育盐构造的被动大陆边缘盆地中,明确盐构造活动与深水重力流事件的沉积耦合特征是该类盆地相对优质储层和甜点区预测、评价的基础。通过综合钻井、岩心分析和三维地震精细解释,对墨西哥湾Sureste盆地中新统深水重力流沉积与盐构造之间的作用方式及响应进行研究,并探讨不同作用方式的成因及油气地质意义。研究表明,Sureste盆地陆坡区发育一系列与盐构造活动相关的微盆、地形坡折、局部凸起和弯曲通道等地貌单元;在这种复杂陆坡地貌背景下,早期和同沉积期盐构造既可以疏导或限制重力流,形成导向、限定、局限和侧向迁移等多种作用类型,也可以被重力流侵蚀形成不整合,后期盐构造活动对深水重力流沉积的改造表现为截切和变形两种样式;不同作用方式和匹配关系受重力流的性质、侵蚀能力与盐构造的规模、数量、走向以及活动时间和强度的控制,局限、侵蚀和截切对应了盐构造与深水重力流沉积相互作用的三个端元;早期和同沉积期盐构造活动影响深水碎屑岩储层的质量和分布,后期盐构造活动对构造、构造—岩性圈闭的形成至关重要。