부산대학교 도서관

The variation and its level of hydrogen isotopic composition of individual n-alkanes in sediments from different latitudes and climatic environments from immature to post-mature are still unclear. In response to this scientific problem, anhydrous closed pyrolyses were carried out on herbaceous peats from different latitude and climatic environments and the n-alkanes and their hydrogen isotopic compositions in the original samples and pyrolysis products were studied. The results showed that there were significant differences in the distribution and hydrogen isotopic composition of the n-alkanes between different latitudes and climatic environments, and it was found that thermal maturity had an important influence on the distribution and hydrogen isotopic composition of the n-alkanes. The results revealed that n-alkanes pyrolyzed by peat in the low-latitude tropical region generally have lower carbon preference index (CPI) and higher average chain length (ACL) values compared to those in the high-latitude cold region and the ACL value decreased with thermal evolution. Therefore, it should be prudent to use the CPI value and the ACL value to study the maturity of the organic matter and the paleoclimate environment, respectively. It was also found that the distribution pattern of the δD value of n-alkanes between the original and pyrolysis samples was similar from the pre-oil generation to the primary oil generation stages (200–350 °C), and was significantly different during the post-oil generation stage (400 and 450 °C). We observed that, at the 450 °C pyrolysis temperature point, the maximum difference in average n-alkane δD values and δD values of individual n-alkanes between the pyrolysis and original samples was 108 and 165‰, respectively. The D enrichment of pyrolysis n-alkanes relative to the original sample should be derived from isotopic thermodynamic fractionation. The study also found that n-alkanes pyrolyzed by peat from the high-latitude cold region have a light hydrogen isotopic composition compared to those from the low-latitude tropic region and the difference in average δD values of pyrolysis n-alkanes between the two peats becomes smaller with increasing thermal maturity; namely, the difference was 67‰ in the pre-oil generation stage, 44‰ in the incipient oil generation to main oil stages, and very small in the post-oil generation stage. The relationships between the δD values of different carbon number n-alkanes pyrolyzed by peats from the high-latitude cold and low-latitude tropical regions and the relationship between the average δD and ACL values of n-alkanes were established. These provide sedimentary n-alkane hydrogen isotope reference data to identify the ancient climatic environment, organic matter sources, and their maturity levels.