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Maoming oil shales are typical low-altitude lacustrine sediments that were deposited during the late Paleogene. The hydrocarbon composition and compound-specific stable carbon isotopic composition (δ 13 C) of organic matter in the profile samples of the oil shales have been analyzed. The results show that algae and aquatic plants are major parent sources of the organic matter in the oil shales associated with a small portion of terrestrial higher plant input. The δ 13 C composition of the bulk organic matter and the  n -alkanes varies greatly on the profile from −26.9 to −15.8‰ and −31.7 to −16.2‰, respectively. While a good positive correlation among the δ 13 C composition of individual  n -alkanes implies that these  n -alkanes were originated from the similar source input. The δ 13 C composition of  n -alkanes on the profile displays a positive excursion trend from the bottom to the top, and this excursion was likely related to the general decreasing trend of the partial pressure of atmospheric CO 2  ( p CO 2 ) during the late Paleogene. The δ 13 C composition of the C 30 -4-methyl steranes ranges from −11.9 to −6.3‰, which is suggestive of  Dinoflagellates -related source input. Coincidently, the high abundance C 33 -botryococcanes were detected in the samples on the top section of the profile and display an extremely positive carbon isotopic composition of −4.5 to −8.4‰, suggesting that the lower partial pressure of atmospheric CO 2  had triggered a bicarbonate consumption mechanism for  Botryococcus braunii B . Therefore, the δ 13 C composition of  n -alkanes and C 33 -botryococcanes and their profile variation suggest that a general declining process associated with fluctuation in the partial pressure of atmospheric CO 2  is likely the major reason for the rapid climatic changes toward the end of the late Paleogene.

The Paleoclimate Significance of the δ13C Composition of Individual Hydrocarbon Compounds in the Maoming Oil Shales, China