Hayes J M, Freeman K H, Popp B N, Hoham C H
Department of Chemistry, Indiana University, Bloomington 47405, USA.
Org Geochem. 1990;16(4-6):1115-28. doi: 10.1016/0146-6380(90)90147-r.
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.
回顾了生物地球化学过程中的同位素分馏模式，并提出当底物量大时同位素分馏将很小。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。通过考虑对来自绿角组（白垩纪，北美西部内陆海道）和梅塞尔页岩（始新世，湖泊，德国南部）的地质卟啉和地质脂质的分析结果来检验这一预测。结果表明：（i）与共同来源相关但结构已改变的卟啉的同位素组成紧密聚集；（ii）与共同来源相关的地质脂质和卟啉之间的同位素差异与现代生物合成产物中观察到的差异相等。这两个观察结果都与成岩作用期间生物控制的同位素组成的保存一致。因此，单个化合物的同位素组成可以根据古代沉积环境中的生物地球化学过程来解释。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协变，而姥鲛烷和植烷的δ值与卟啉的δ值协变。在这个代表开放海洋环境的单元中，保存下来的无环多异戊二烯显然主要来自初级物质，而可提取的正构烷烃主要来自食物链的较低层级。在梅塞尔页岩中，单个生物标志物的同位素组成相对于PDB范围为-20.9%至-73.4%。特定化合物的同位素组成可以根据甲基营养型、化学自养型和化能无机营养型微生物以及生活在这个古代湖泊水柱和沉积物中的初级生产者的来源来解释。
