Sedimentary organic matter record of Early Cretaceous environmental changes in Western Liaoning Province, NE China

The abundance and composition of organic matter and the δ¹³C compositions of some specific individual compounds were analysed in sediments collected from an excavated profile of the Early Cretaceous Yi-xian Formation in Liaoning Province, NE China, in order to provide organic geochemical information on palaeoclimate and environmental changes. The distribution of n-alkanes varies steadily through the profile, with long-chain n-alkanes dominating in the lower and upper sections, and medium chain-length n-alkanes dominating in the middle section of the profile. The δ¹³C values of n-alkanes, hopanes and steranes show similar upward positive trends through the profile, suggesting that these compounds originated from the same source material and also reflect consistent changes in the environment of the sedimentary basin. The C₂₄ 17,21-secohopane was only detected in the lower and middle sections of the profile and has a more positive carbon isotopic composition by about 3.7–6.7‰ than that of associated n-alkanes, hopanes, steranes and C₃₀ 8,14-secohopane. This implies that the C₂₄ 17,21-secohopane had a different origin, possibly connected with the volcanic ash deposits and massive vertebrate remains. Reconstruction of the δ¹³C composition of atmospheric CO₂ based on plant-derived C₂₉ and C₃₁ n-alkanes shows a change from −10‰ to −3‰ from the bottom to the middle of the profile. This upward positive excursion through the profile is consistent with the worldwide variation of atmospheric CO₂ during the Early Cretaceous and indicates an upward warming trend, likely related to globally intensified volcanic activity.