Patterns of isotopic fractionation in biogeochemical processes are reviewed and it is suggested that isotopic fractionations will be small when substrates are large. If so, isotopic compositions of biomarkers will reflect those of their biosynthetic precursors. This prediction is tested by consideration of results of analyses of geoporphyrins and geolipids from the Greenhorn Formation (Cretaceous, Western Interior Seaway of North America) and the Messel Shale (Eocene, lacustrine, southern Germany). It is shown (i) that isotopic compositions of porphyrins that are related to a common source, but which have been altered structurally, cluster tightly and (ii) that isotopic differences between geolipids and porphyrins related to a common source are equal to those observed in modern biosynthetic products. Both of these observations are consistent with preservation of biologically controlled isotopic compositions during diagenesis. Isotopic compositions of individual compounds can thus be interpreted in terms of biogeochemical processes in ancient depositional environments. In the Cretaceous samples, isotopic compositions of n-alkanes are covariant with those of total organic carbon, while delta values for pristane and phytane are covariant with those of porphyrins. In this unit representing an open marine environment, the preserved acyclic polyisoprenoids apparently derive mainly from primary material, while the extractable, n-alkanes derive mainly from lower levels of the food chain. In the Messel Shale, isotopic compositions of individual biomarkers range from -20.9 to -73.4% vs PDB. Isotopic compositions of specific compounds can be interpreted in terms of origin from methylotrophic, chemautotrophic, and chemolithotrophic microorganisms as well as from primary producers that lived in the water column and sediments of this ancient lake.

中文翻译：

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特定化合物同位素分析：重建古代生物地球化学过程的新工具

回顾了生物地球化学过程中同位素分馏的模式，并建议当底物大时同位素分馏会很小。如果是这样，生物标志物的同位素组成将反映其生物合成前体的同位素组成。该预测通过考虑来自 Greenhorn 地层（白垩纪，北美西部内陆海道）和 Messel 页岩（始新世，湖泊，德国南部）的地卟啉和地脂的分析结果进行测试。结果表明 (i) 与共同来源相关但结构发生改变的卟啉的同位素组成紧密聚集，以及 (ii) 与共同来源相关的土石脂和卟啉之间的同位素差异与在现代生物合成产品。这两个观察结果都与成岩过程中生物控制同位素组成的保存一致。因此，可以根据古代沉积环境中的生物地球化学过程来解释单个化合物的同位素组成。在白垩纪样品中，正构烷烃的同位素组成与总有机碳的同位素组成是协变的，而原始烷烃和植烷的 δ 值与卟啉的那些协变。在这个代表开放海洋环境的单元中，保存的无环聚异戊二烯显然主要来自初级材料，而可提取的正烷烃主要来自食物链的较低层次。在 Messel 页岩中，与 PDB 相比，单个生物标志物的同位素组成范围为 -20.9 至 -73.4%。