Molecular and isotopic compositions of hydrocarbons at DSDP Site 76-533

In an investigation of gas hydrates in deep ocean sediments, gas samples from Deep Sea Drilling Project Site 533 on the Blake Outer Ridge in the northwest Atlantic were obtained for molecular and isotopic analyses. Gas samples were collected from the first successful deployment of a pressure core barrel (PCB) in a hydrate region. The pressure decline curves from two of the four PCB retrievals at in situ pressures suggested the presence of small amounts of gas hydrates. Compositional and isotopic measurements of gases from several points along the pressure decline curve indicated that (1) biogenic methane (d13C = -68 per mil; C1/C2 = 5000) was the dominant gas (>90%); (2) little fractionation in the C1/C2 ratio or the C carbon isotopic composition occurred as gas hydrates decomposed during pressure decline experiments; (3) the percent of C3, i-C4, and CO2 degassed increased as the pressure declined, indicating that these molecules may help stabilize the hydrate structure; (4) excess nitrogen was present during initial degassing; and (5) C1/C2 ratios and isotopic ratios of C gases were similar to those obtained from conventional core sampling. The PCB gas also contained trace amounts of saturated, acyclic, cyclic, and aromatic C5-C14 hydrocarbons, as well as alkenes and tetrahydrothiophenes. Gas from a decomposed specimen of gas hydrate had similar molecular and isotopic ratios to the PCB gas (d13C of -68 per mil for methane and a C1/C2 ratio of about 6000). Regular trends in the d13C of methane (about -95 to -60 per mil) and C1/C2 ratios (about 25000 to 2000) were observed with depth. Capillary gas chromatography (GC) and total scanning fluorescence measurements of extracted organic material were characteristic of hydrocarbons dominated by a marine source, though significant amounts of perylene were also present.

在深海沉积物中天然气水合物的研究中，研究人员从西北大西洋Blake外脊的深海钻探项目（Deep Sea Drilling Project）533号站位获取了气体样本，用于分子与同位素分析。这些气体样本来自水合物区域首次成功部署压力岩心筒（pressure core barrel，PCB）的采集成果。四个PCB回收样本中，有两个在原位压力下的压力下降曲线表明存在少量天然气水合物。沿压力下降曲线多个点位的气体组成及同位素测量结果显示：（1）生物成因甲烷（δ¹³C=-68‰；C1/C2=5000）为优势气体（占比>90%）；（2）在压力下降实验中，天然气水合物分解过程中C1/C2比值或碳同位素组成几乎未发生分馏；（3）随着压力降低，C3、i-C4及CO₂的脱气百分比升高，提示这些分子可能有助于稳定水合物结构；（4）初始脱气阶段存在过量氮；（5）C1/C2比值及C类气体同位素比值与常规岩心采样所得结果相似。PCB气体中还含有微量饱和、无环、环状及芳香族C5-C14烃类，以及烯烃与四氢噻吩。从分解的天然气水合物样本中提取的气体，其分子与同位素比值与PCB气体相近（甲烷δ¹³C=-68‰，C1/C2约为6000）。甲烷δ¹³C值（约-95至-60‰）与C1/C2比值（约25000至2000）随深度变化呈现规律性趋势。对提取有机物的毛细管气相色谱（capillary gas chromatography，GC）及全扫描荧光测量结果表明，烃类以海洋来源为主，但也存在大量苝。