Xu Huiyuan, Liu Quanyou, Li Zhiquan, Hou Dujie, Han Xu, Li Peng, Li Pengpeng, Zhu Biqing
State Key Laboratory of Shale Oil and Gas Enrichment Mechanisms and Effective Development, Beijing 100083, China.
School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China.
ACS Omega. 2023 Feb 2;8(6):5820-5835. doi: 10.1021/acsomega.2c07382. eCollection 2023 Feb 14.
Organic-rich shales and mudstones have long been investigated regarding the control of source, environment, climate, etc. on the enrichment of organic carbon. However, little is documented about how autotrophy and heterotrophy influence organic carbon cycling/export. Here, we show molecular and carbon isotopic compositional changes of the shale or mudstone source rocks from the Chang 3 to 7 members of the Yanchang Formation. The Chang 7 member source rocks have higher quality (480-500 mg/g) and total organic carbon (TOC) (15.3% on average) than other member source rocks; the sterane/hopane ratio and the δC of organic carbon and kerogen decrease from the Chang 3 to 7 members, but Δδ ([average δC of -C + -C] - [average δC of pristane + phytane]) increases, and no aryl isoprenoids and C aromatic carotenoids (e.g., isorenieratane) were observed. These low maturity biomarker features suggest that there were no water stratification, photic zone euxinia (PZE), and no obvious change in the organic matter source, and the water column is generally anoxic. A comparison of the δC of Pr and Ph with the δC of the -C17 and -C18 alkanes reveals a shift in the mode of carbon cycling/export (autotrophy versus heterotrophy) in the Yanchang Formation and that there was dominant heterotrophic bacterial activity or bacterial biomass in the Chang 7 member. The TOC spike in the Chang 7 member may result from boosted carbon cycling/export that improves organic carbon preservation than other members. Possible external forcings on the shift are abundant hydrothermal- or volcanic-derived metal salts as electron acceptors in the palaeowater, which is a reasonable explanation for enhanced heterotrophic bacterial activity. This finding improves our understanding of heterotrophic bacterial activity control on organic matter (OM) preservation and may be a significant supplement for understanding the ecological or environmental forcings in the Yanchang Formation, Ordos Basin.
长期以来，富含有机质的页岩和泥岩一直是关于源岩、环境、气候等对有机碳富集的控制方面的研究对象。然而，关于自养和异养如何影响有机碳循环/输出的文献记载却很少。在此，我们展示了延长组3至7段页岩或泥岩源岩的分子和碳同位素组成变化。长7段源岩的质量（480 - 500毫克/克）和总有机碳（TOC）（平均15.3%）高于其他段源岩；从长3段到长7段，甾烷/藿烷比值以及有机碳和干酪根的δC值降低，但Δδ（[-C + -C的平均δC] - [姥鲛烷 + 植烷的平均δC]）增加，且未观察到芳基异戊二烯类和C芳香类胡萝卜素（如异菌绿素）。这些低成熟度生物标志物特征表明不存在水体分层、光合区缺氧（PZE），且有机质来源无明显变化，水柱总体缺氧。将Pr和Ph的δC与-C17和-C18烷烃的δC进行比较，揭示了延长组碳循环/输出模式（自养与异养）的转变，且长7段存在占主导地位的异养细菌活动或细菌生物量。长7段的TOC峰值可能是由于碳循环/输出增强，比其他段更有利于有机碳的保存。这种转变可能的外部驱动力是古水中大量来自热液或火山的金属盐作为电子受体，这是异养细菌活动增强的一个合理解释。这一发现增进了我们对异养细菌活动对有机质（OM）保存的控制的理解，可能是对鄂尔多斯盆地延长组生态或环境驱动力认识的一个重要补充。
