ISOTOPE COMPOSITIONS OF GASES IN SEDIMENTS FROM THE CHILE CONTINENTAL MARGIN 1
ISOTOPE COMPOSITIONS OF GASES IN SEDIMENTS FROM THE CHILE CONTINENTAL MARGIN 1
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智利大陆边缘沉积物中气体的同位素组成 1
DOI：
10.2973/odp.proc.sr.141.024.1995
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发表时间：
2006
期刊：
影响因子：
--
通讯作者：
B. M. Didyk
中科院分区：
文献类型：
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作者：
A. Waseda;B. M. Didyk
报告了ODP Leg 141站点859、860、861和863处气穴气体的分子和同位素测量结果。甲烷δ′3c值在-86‰~ -61‰之间。甲烷δD值在-249‰~ -163‰之间。所有样品的C]/(C2 + C3)比值均大于300。这些数据表明甲烷主要由细菌活动产生，主要的产甲烷途径是CO2还原。乙烷δ 1c值在-70‰~ -44%c之间。这些值是天然气中乙烷的最轻的碳同位素值之一，表明乙烷的来源也主要是细菌。在860站点海底以下200 m （mbsf）的深层地层中，乙烷δ 1c值在-60‰~ -45‰范围内变重，C]/C3比值随深度的增加而减小。这表明存在少量的产热碳氢化合物。有机质的低成熟程度表明，热生烃组分不是在原位产生的，而是从更成熟的地层中运移而来，可能是由这些地层中活跃的流体流动驱动的。智利前弧区边缘的地震剖面表明存在模拟底部反射器（BSRs）。bsr被认为标志着一层天然气水合物的基底（Shipley等人，1979），并与水合物的实际采收率有关（Shipley和Didyk， 1982）。水合物是冰状固体，含有大量气体，主要是甲烷（Kvenvolden和Barnard， 1983）。水合物层被认为与游离烃气体处于压力-温度平衡状态（Miller et al., 1991）。海洋钻探计划（ODP）在智利三重交界处（图1）附近的Leg 141首次有意穿透水合物层的底部。水合物层在三个位置（859、860和861）被穿透。虽然没有发现冻结的天然气水合物，但在这三个地点的沉积物中都含有丰富的甲烷。甲烷的碳同位素组成被广泛用于烃类气体的成因分类（Rosenfield and Silverman, 1959; Bernard, 1978; Schoell, 1980; Rice and Claypool, 1981）。它可以区分细菌和热源碳氢化合物。Schoell（1980）和Whiticar et al.（1986）还表明，甲烷的氢同位素组成与碳同位素组成结合，表征了细菌甲烷形成的不同途径。本文通过稳定碳、氢同位素组成和气体分子组成的分析，探讨了轻烃的成因特征。在智利三重交界处（图1），一个活跃的伸展脊和邻近的年轻海洋地壳正俯冲到南美洲大陆之下。在ODP Leg 141期间，在四个地点恢复了上新世至更新世的沉积序列。三个地点（859、860和861）沿着智利三重交界处以北约35公里的东西倾斜样带钻探，水深从1652米到2741米不等。站点863（2564米水深）于1995年在Lewis， s.d., Behrmann, j.h., Musgrave, r.j.和Cande， S.C.（编）基地钻探。项目：ODP， Sci。结果，141：大学站，得克萨斯州（海洋钻探计划）。2 J APEX研究中心，日本千叶261三滨区滨田1-2-1 . 3国家企业Petróleo，炼油厂Petróleos Concón s.a., Casilla 28-D，维纳德尔马，智利位于俯冲扩张轴正上方的海沟-斜坡盆地，作为859点的走向样带。方法通过透明塑料岩心衬垫，用空心冲孔钻取岩心内可见的气穴，将样品膨胀到20毫升真空样管（真空容器）中。碳氢化合物气体的分子组成在惠普5890a气相色谱仪上测定。对于同位素分析，单个气体组分由气相色谱仪分离，随后使用真空制备线在850°C的CuO上燃烧成CO2和H2O （Schoell, 1980）。燃烧产生的H2O在480°C下与密封玻璃管中的锌反应还原为H2 （Vennemann and O'Neil, 1993）。用VG Isotech Sira Series II质谱计测定了甲烷的稳定碳、氢同位素值。几个样品中有足够的乙烷可供碳同位素测量。碳的同位素比值采用PDB （Pee Dee Belemnite）标准，氢的同位素比值采用SMOW （standard Mean Ocean Water）标准：δRJ%c) = 1} × 1000′(1)，其中RJRb分别为c / c和D/H。甲烷δC的重现性为±0.15‰，乙烷δC的重现性为±0.3‰，甲烷δD的重现性为±3%。结果和讨论本研究分析的四个地点的真空容器气体样本按埋深列于表1。报道了本研究测定的甲烷和乙烷的δC值和甲烷的δD值，以及船载烃组成测定。在859,860和861地点，沿着东西倾斜样带钻探，经常出现气穴，并持续观察到高浓度的甲烷。在Site 863与Site 859的走向样带处钻探，气穴相对较少，大多数真空取样器样品中的甲烷浓度较低。只有两个样本含有足够的甲烷用于863站点的同位素分析。所有样品中的二氧化碳浓度均低于0.1% （1000ppm）。由于CO2浓度低，我们无法测量其碳同位素值。
Molecular and isotope measurements of gases from gas pockets at ODP Leg 141 Sites 859, 860, 861, and 863 are reported. The δ' 3 C values of methane range between -86‰ and -61 ‰. The δD values of methane range between -249‰ and -163‰. The C]/(C2 + C3) ratios of all samples are higher than 300. These data indicate methane is mainly produced by bacterial activity, and the primary methanogenic pathway is CO2 reduction. The δ 1 C values of ethane range between -70‰ and -44%c. These values are among the lightest carbon isotope values reported for ethane in natural gas, indicating the origin of ethane is also predominantly bacterial. In deeper strata below 200 m below seafloor (mbsf) at Site 860, the δ 1 C values of ethane become heavier from -60‰ to -45‰ and the C]/C3 ratios decrease with increasing depth. This indicates the presence of a small amount of thermogenic hydrocarbons. The low degree of maturation of the organic matter suggest the thermogenic hydrocarbon components are not generated in situ but have migrated from more mature strata, possibly driven by active fluid flow in these strata. INTRODUCTION Seismic sections across the Chile margin in the forearc region indicate the presence of bottom-simulating reflectors (BSRs). BSRs are thought to mark the base of a layer of gas hydrates (Shipley et al., 1979), and have been associated with actual recovery of hydrates (Shipley and Didyk, 1982). Hydrates are ice-like solids that contain large concentrations of gas, predominantly methane (Kvenvolden and Barnard, 1983). The hydrate layer is thought to be in pressure-temperature equilibrium with free hydrocarbon gases (Miller et al., 1991). Ocean Drilling Program (ODP) Leg 141 in the vicinity of the Chile Triple Junction (Fig. 1) has, for the first time, intentionally penetrated the base of the hydrate layer. The hydrate layer was penetrated at three sites (Sites 859, 860, and 861). Although no frozen gas hydrates were recovered, sediments contained abundant methane at all three sites. The carbon isotope composition of methane is widely used for the genetic classification of hydrocarbon gases (Rosenfield and Silverman, 1959; Bernard, 1978; Schoell, 1980; Rice and Claypool, 1981). It allows the distinction between bacterial and thermogenic hydrocarbons. Schoell (1980) and Whiticar et al. (1986) have shown additionally that the hydrogen isotope composition of methane in combination with the carbon isotope composition characterizes different pathways of bacterial methane formation. Here, we investigate the genetic characterization of light hydrocarbons by analyses of stable carbon and hydrogen isotope compositions in combination with molecular compositions of gases. GEOLOGIC SETTING At the Chile Triple Junction (Fig. 1) an active spreading ridge and adjacent young oceanic crust are being subducted beneath the continent of South America. During ODP Leg 141, Pliocene to Pleistocene sedimentary sequences were recovered at four sites. Three Sites (859, 860, and 861) were drilled along an east-west dip transect about 35 km north of the Chile Triple Junction at water depths ranging from 1652 to 2741 m. Site 863 (2564 m water depth) was drilled at the base Lewis, S.D., Behrmann, J.H., Musgrave, R.J., and Cande, S.C. (Eds.), 1995. Proc. ODP, Sci. Results, 141: College Station, TX (Ocean Drilling Program). 2 J APEX Research Center, 1-2-1 Hamada, Mihama-ku, Chiba 261, Japan. 3 Empresa Nacional del Petróleo, Refineria de Petróleos Concón S.A., Casilla 28-D, Vina del Mar, Chile. of a trench-slope basin directly above the subducted spreading axis as a strike transect with Site 859. METHODS Gas pockets visible in the core through clear plastic core liner were tapped with a hollow punch, and samples were expanded into 20-ml evacuated sample tubes (vacutainers) aboard JOIDES Resolution. The molecular compositions of hydrocarbon gases were determined on a Hewlett-Packard 5890a gas chromatograph. For the isotope analyses the individual gas components were separated by a gas chromatograph and subsequently combusted to CO2 and H2O over CuO at 850°C, using a vacuum preparation line (Schoell, 1980). The combustion produced H2O is reduced to H2 by reaction with zinc in sealed glass tubes at 480°C (Vennemann and O'Neil, 1993). The stable carbon and hydrogen isotope values of methane were measured using a VG Isotech Sira Series II mass spectrometer. Enough ethane was available from several samples for carbon isotope measurements. Isotope ratios are reported in the usual δ-notation relative to the PDB (Pee Dee Belemnite) standard for carbon and SMOW (Standard Mean Ocean Water) standard for hydrogen: δRJ%c) = 1} × 1000' (1) where RJRb is C/C and D/H, respectively. The reproducibility of isotope values is ±0.15‰ for δC of methane, ±0.3‰ for δC of ethane, and ±3%c for δD of methane. RESULTS AND DISCUSSION Vacutainer gas samples from four sites analyzed in this study are listed by depth of burial in Table 1. δC values of methane and ethane and δD values of methane determined in this study are reported, as well as the shipboard determination of hydrocarbon composition. Gas pockets frequently appeared and high concentrations of methane were consistently observed at Sites 859,860, and 861 drilled along the eastwest dip transect. At Site 863 drilled as the strike transect with Site 859, gas pockets were relatively rare, and concentrations of methane were low in most vacutainer samples. Only two samples had enough methane for isotope analyses at Site 863. Carbon dioxide concentrations are below 0.1% (1000 ppm) in all samples. We could not measure carbon isotope values of CO2 due to its low concentration.