Abstract C27–C29 steranes are common constituents of crude oils and hydrocarbon source rocks and are usually dominated by either C27 or C29 compounds. In the current study, however, steranes dominated by C28 components were detected in eight lacustrine source rock samples in the first member of the Shahejie Formation in the Qikou Sag, Bohai Bay Basin of Eastern China. Ratios of C28/(C27 + C28 + C29) ααα 20R steranes in four samples can exceeded 50%. Furthermore, C27 triaromatic steroids, presumably originating from C28 steranes, were also observed at high levels, relative to C26 and C28 homologues. Stable carbon isotope compositions of a C28 ααα 20R sterane and C27 20R triaromatic steroid were found to be similar, with δ13C at −37‰ and −36‰ respectively, much lower than values for pristane (by ∼7‰) and total organic carbon (by ∼10‰). The abnormally low δ13C values of the steroids suggest that C28 ααα 20R sterane was biosynthesized by algae blooming in the water column during phases of overturn and mixing, such as in autumn, during which recycled CO2 and nutrients welling up from hypolimnion to the epilimnion are assimilated, contributing to a high algal productivity and low δ13C in algal organic matter.

中文翻译：

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渤海湾盆地岐口凹陷湖相烃源岩中C28甾烷来源的稳定碳同位素证据

摘要 C27-C29 甾烷是原油和烃源岩的常见成分，通常以 C27 或 C29 化合物为主。而本次研究在渤海湾盆地歧口凹陷沙河街组一段的8个湖相烃源岩样品中检测到了以C28组分为主的甾烷。四个样品中 C28/(C27 + C28 + C29) ααα 20R 甾烷的比例可以超过 50%。此外，相对于 C26 和 C28 同系物，还观察到高水平的 C27 三芳族类固醇，推测来自 C28 甾烷。发现 C28 ααα 20R 甾烷和 C27 20R 三芳族甾体的稳定碳同位素组成相似，δ13C 分别为 -37‰ 和 -36‰，远低于原始烷烃（约 7‰）和总有机碳的值（约 10‰）。