Molecular properties of gases of ODP Leg 204 sites

The recognition of finely disseminated gas hydrate in deep marine sediments heavily depends on various indirect techniques because this mineral quickly decomposes upon recovery from in situ pressure and temperature conditions. Here, we discuss molecular properties of closely spaced gas voids (formed as a result of core recovery) and gas hydrates from an area of relatively low gas flux at the flanks of the southern Hydrate Ridge offshore Oregon (ODP Sites 1244, 1245 and 1247). Within the gas hydrate occurrence zone (GHOZ), the concentration of ethane (C2) and propane (C3) in adjacent gas voids shows large variability. Sampled gas hydrates are enriched in C2 relative to void gases but do not contain C3. We suggest that the observed variations in the composition of void gases is a result of molecular fractionation during crystallization of structure I gas hydrate that contains C2 but excludes C3 from its crystal lattice. This hypothesis is used to identify discrete intervals of finely disseminated gas hydrate in cored sediments. Variations in gas composition help better constrain gas hydrate distribution near the top of the GHOZ along with variations in pore water chemistry and core temperature. Sediments near the base of the gas hydrate stability zone are relatively enriched in C2+ hydrocarbon gases. Complex and poorly understood geological and geochemical processes in these deeper sediments make the identification of gas hydrate based on molecular properties of void gases more ambiguous. The proposed technique appears to be a useful tool to better understand the distribution of gas hydrate in marine sediments and ultimately the role of gas hydrate in the global carbon cycle.

深海沉积物中细分散型天然气水合物的识别高度依赖各类间接探测技术，因该矿物脱离原位温压环境后会快速分解。本文针对俄勒冈州近海南部水合物脊的大洋钻探计划（Ocean Drilling Program，ODP）1244、1245及1247站位低气通量区域，探讨了岩心提取过程中形成的紧密分布气体孔隙与天然气水合物的分子特征。在天然气水合物赋存带（Gas Hydrate Occurrence Zone，GHOZ）内，相邻气体孔隙中的乙烷（C2）与丙烷（C3）浓度存在显著差异。采集得到的天然气水合物相较于孔隙气体更富集C2，但未检出C3。我们认为，观测到的孔隙气体组分差异，是含C2但将C3排除在晶格外的I型结构天然气水合物结晶过程中发生分子分馏的结果。该假说被用于识别岩心沉积物中离散的细分散型天然气水合物层段。结合孔隙水化学特征与岩心温度变化，气体组分差异可进一步约束GHOZ顶部附近的天然气水合物分布。天然气水合物稳定带底部附近的沉积物相对富集C2及以上烃类气体。这些深层沉积物中复杂且机制尚未明确的地质与地球化学过程，使得基于孔隙气体分子特征识别天然气水合物的结果更具歧义。本文提出的技术可为明晰海洋沉积物中天然气水合物的分布，乃至阐明天然气水合物在全球碳循环中的作用提供有效工具。