Total organic carbon, delta13C, biomarker, gas compilations and hopanoids analysis from IODP Sites 307-U1317 and 307-U1318@en

During the Integrated Ocean Drilling Program (IODP) Expedition 307 for the first time a cold-water coral carbonate mound was drilled down through its base into the underlying sediments. In the current study, sample material from within and below Challenger Mound, located in the Belgica carbonate mound province in the Porcupine Basin offshore Ireland, was investigated for its distribution of microbial communities and gas composition using biogeochemical and geochemical approaches to elucidate the question on the initiation of carbonate mounds. Past and living microbial populations are lower in the mound section compared to the underlying sediments or sediments of an upslope reference site. A reason for this might be a reduced substrate feedstock, reflected by low total organic carbon (TOC) contents, in the once coral dominated mound sequence. In contrast, in the reference site a lithostratigraphic sequence with comparatively high TOC contents shows higher abundances of both past and present microbial communities, indicating favourable living conditions from time of sedimentation until today. Composition and isotopic values of gases below the mound base seem to point to a mixed gas of biogenic and thermogenic origin with a higher proportion of biogenic gas. Oil-derived hydrocarbons were not detected at the mound site. This suggests that at least in the investigated part of the mound base the upward flow of fossil hydrocarbons, being one hypothesis for the initiation of the formation of carbonate mounds, seems to be only of minor significance.综合大洋钻探计划（Integrated Ocean Drilling Program, IODP）307航次首次完成了冷水珊瑚碳酸盐岩丘（cold-water coral carbonate mound）的全穿透钻探——从丘体顶部钻穿基底直至下伏沉积物。本研究针对爱尔兰近海豪猪盆地（Porcupine Basin）贝尔吉卡碳酸盐岩丘省范围内的挑战者丘（Challenger Mound）内部及下方的样品材料，采用生物地球化学与地球化学方法分析其微生物群落分布与气体组分，以阐明碳酸盐岩丘的形成起始机制。相较于下伏沉积物或上坡对照站位的沉积物，丘体段的古代与现存微生物种群丰度均更低。该现象的成因可能为底物供给不足，这一特征可由曾以珊瑚为主导的丘体序列中较低的总有机碳（Total Organic Carbon, TOC）含量得到反映。与之形成鲜明对比的是，对照站位的岩石地层序列总有机碳含量相对较高，其古代与现存微生物种群的丰度均更高，表明自沉积形成至今，该站位始终具备适宜微生物生存的环境条件。丘体基底下方的气体组分与同位素值似乎指向生物成因与热成因混合气体，且生物成因气占比更高。研究未在丘体站位检出油源烃类，这表明至少在所研究的丘体基底区域，作为碳酸盐岩丘形成起始假说之一的化石烃类向上运移作用，其重要性似乎较为有限。