Paleoenvironmental implications of molecular and carbon isotopic distributions of n-alkanes in the hominoid-bearing upper Miocene sediments from the Zhaotong Basin, southwestern China

摘要

It is crucial to understand the relationship between hominoid primates and their living environment. Located at the southeastern margin of the Tibetan Plateau, the Shuitangba (STB) outcrop from the Zhaotong Basin is famous for bearing the remains of the youngest known ape fossil in the terminal Miocene (similar to 6.2 Ma). In this study, the molecular and carbon isotope compositions of n-alkanes, together with the loss on ignition and the bulk carbon isotope composition of total organic matter, were investigated in the STB outcrop to infer the paleoenvironmental conditions in this uppermost Miocene sedimentary sequence, with the emphasis on the habitat of the STB hominoid. In the hominoid-bearing peaty clay layer and the surrounding lignite layers, the n-alkane molecular and carbon isotope compositions support a primary contribution from terrestrial higher plants. In contrast, n-alkanes in the upper shallow lake sediments are derived from multiple origins, mainly from aquatic plants and bacteria. Both carbon preference index (CPI) and average chain length (ACL) vary closely with the lithology, but the CPI record shows the larger amplitude of change than the ACL and varies sharply during the lithological boundary. It is notable that the changes of the molecular and carbon isotope compositions of long-chain n-alkanes do not parallel with the abrupt increase of Poaceae pollen percentage during the STB hominoid appearing stage. Such an inconsistency between herb pollen percentage and n-alkane ratios may result from the different sources. Our results clearly support that these organic matter-based ratios have the potential to record the paleoenvironmental changes in ancient lacustrine sediments.