Concentration of C2-C6 hydrocarbon gases in dry sediments of ODP Sites 180-1109 and 180-1115

Low molecular weight hydrocarbon (LMWH) distributions were examined in sediments from Sites 1109 and 1115 in the western Woodlark Basin using purge-trap thermal adsorption/desorption gas analysis. A number of different hydrocarbon components >C1, which were not detected during shipboard gas analysis, were detected at both sites using the purge-trap procedure. Concentrations of ethane, propane, and butane remained relatively low (<100 pmol/g) throughout Site 1109 and had no consistent trend with depth. In contrast, the longer-chain components increased in concentration with depth. Hexane concentrations rose to 716 pmol/g at the base of the site with a concomitant increase in both 2-methyl- and 3-methylpentane. At Site 1115, concentrations of ethane, propane, butane, and isobutylene + 1-butene remained low (<60 pmol/g) throughout the site and again had no consistent trend with depth. 2-Methylpentane, 3-methylpentane, and hexane concentrations had a subsurface maximum that coincided with sediments containing abundant plant-rich material. The LMWH downhole profiles plus low in situ temperatures suggest that the LMWH components were formed in situ by low-temperature biological processes. Purge-trap analysis has indicated the presence of some unexpected deep low-temperature bacterial reactions, which demonstrates that further analysis of LMWH may provide valuable information at future Ocean Drilling Program sites.本研究采用吹扫捕集热吸附/解吸气体分析方法，对伍德拉克海盆西部1109与1115站位沉积物中的低分子量烃（Low molecular weight hydrocarbon, LMWH）分布特征展开了调查。相较于船载气体分析，本次吹扫捕集实验在两个站位均检出了多种此前未被发现的C1以上烃类组分。1109站位中，乙烷、丙烷与丁烷的浓度始终维持在较低水平（<100 pmol/g），且随深度未呈现出一致的变化趋势；与之相反，长链烃组分的浓度则随深度增加而上升，该站位底部的己烷浓度升至716 pmol/g，同时2-甲基戊烷与3-甲基戊烷的浓度也同步升高。1115站位中，乙烷、丙烷、丁烷以及异丁烯+1-丁烯的浓度始终处于较低水平（<60 pmol/g），同样未随深度呈现出一致的变化趋势；2-甲基戊烷、3-甲基戊烷与己烷的浓度在地下出现峰值，该峰值与富含植物源有机质的沉积物层位相吻合。低分子量烃的井下剖面特征结合原位低温环境表明，该类烃组分是通过原位低温生物过程形成的。吹扫捕集分析揭示了部分此前未被发现的深部低温细菌反应过程，这表明针对低分子量烃的进一步分析可为未来大洋钻探计划（Ocean Drilling Program）站位的相关研究提供极具价值的信息。