Abstract:

The late Eocene to the early Oligocene (about 33.5 Ma) climate transition determined by the marine sediment studies was a global event based on marine sediments during the Paleogene, while there is few researches on the terrestrial sediment records of this climate transition. The Youganwo Formation oil shales from Maoming basin of Guangdong Province, deposited during the late Eocene to the early Oligocene, have been studied in terms of organic matter abundance, hydrocarbon components and the carbon isotopic composition of individual hydrocarbon compounds, and aiming to study the response of terrestrial organic matter to the EoceneOligocene transition (EOT) at the low latitude region. The results show that the δ13C composition of nalkanes in the shallower buried sample display a significant positive excursion ranged from -19.1‰ and -25.9‰ with an average of -22.2‰. By contrast, the δ13C composition of nalkanes in the deeper buried sample is much negative and varied from -23.7‰ and -30.2‰ with averaged at -26.3‰. The difference between the δ13C compositions of corresponding nalkanes in these two samples vary from 1.5‰ to 8.1‰ with an average deviation of 4.6‰. This significant shift in the δ13C composition of nalkanes signals that the terrestrial organism(s) at the low latitude land response to the paleoclimate changes which may closely related to the decrease in the concentration of atmospheric CO2 and the ocean temperature during EOT. Therefore, the discrepancy in the δ13C composition of nalkanes may suggest that there was an significant shift or increase from C3 plant to C4 plant at the low altitude continental region of Maoming basin, which resulted in a remarkable change in the δ13C composition of sedimentary organic matter; while it is also possible that the positive excursion of organic matter might be due to the decrease in the concentration of atmospheric CO2 that gives rise to the δ13C composition of atmospheric CO2. Finally, this primary study suggests that more detailed research on the Maoming oil shale could provide some critical information to a better understanding of low latitude continental paleoclimate changes during EOT.

Key words: Eocene Oligocene transition, low altitude terrestrial organic matter, carbon isotopic composition of individual hydrocarbon compounds, Maoming oil shale