Low-molecular-weight hydrocarbons in sediments of Broken Ridge and Ninetyeast Ridge

Core samples taken during Leg 121 drilling aboard the JOIDES Resolution in the central Indian Ocean were analyzed for their low-molecular-weight hydrocarbon contents. Forty-three samples from the Broken Ridge and 39 samples from the Ninetyeast Ridge drill sites, deep-frozen on board immediately after recovery, were studied by a dynamic headspace technique (hydrogen-stripping/thermovaporization). Light hydrocarbons (saturated and olefinic) with two to four carbon atoms, and toluene as a selected aromatic compound, were identified. Total C2-C4 saturated hydrocarbon yields vary considerably from virtually zero in a Paleogene calcareous ooze from Hole 757B to nearly 600 nanogram/gram of dry-weight sediment (parts per billion) in a Cretaceous claystone from Hole 758A. An increase of light-hydrocarbon yields with depth, and hence with sediment temperature, was observed from Hole 758A samples down to a depth of about 500 meters below seafloor. Despite extreme data scatter due to lithological changes over this depth interval, this increased yield indicates the onset of temperature-controlled hydrocarbon formation reactions.
Toluene contents are also extremely variable (generally between 10 and 100 ppb) and reach more than 300 ppb in two samples of tuffaceous lithology (Sections 121-755A-17R-4 and 121-758A-48R-4). As for the saturated hydrocarbons, there was also an increase of toluene yields with increasing depth in Hole 758A.

针对中央印度洋海域乔迪斯·决心号（JOIDES Resolution）第121航次钻探获取的岩心样品，本研究开展了低分子量烃类组分分析。本次研究对象包括取自破碎海岭（Broken Ridge）的43个样品，以及九十东海岭（Ninetyeast Ridge）钻探站位的39个样品，所有样品在回收后即刻于船上进行深冻保存，采用动态顶空技术（dynamic headspace technique，即氢吹扫/热蒸发法（hydrogen-stripping/thermovaporization））进行分析。本次分析鉴定的组分包括C2-C4轻质烃类（饱和烃与烯烃），以及选定的芳香族化合物甲苯。C2-C4饱和烃总产率差异显著：757B钻孔古近纪钙质软泥样品中的烃产率几乎为零，而758A钻孔白垩纪黏土岩样品中的烃产率可高达近600纳克/克干沉积物（即十亿分之一浓度，ppb）。对758A钻孔的研究显示，在海底以下约500米的深度区间内，轻质烃产率随深度增加（对应沉积物温度升高）而呈上升趋势。尽管该深度区间内因岩性变化导致数据离散度极高，但产率的上升趋势表明，温度调控的烃类生成反应已进入启动阶段。
甲苯含量同样存在显著差异，整体分布区间为10~100 ppb，在两个凝灰质岩性样品（121-755A-17R-4与121-758A-48R-4岩心段）中，甲苯含量可超过300 ppb。与饱和烃的变化规律一致，758A钻孔中的甲苯产率同样随深度增加而升高。