Composition of lipids from coal deposits of the Far East: Relations to vegetation and climate change during the Cenozoic
Abstract Twenty-one samples of sub-bituminous coal and 1 sample of coaly shale have been collected from different coal seams, representing the Cenozoic coal basins widely distributed in the region of Primorye and Sakhalin (Russia), and NE China (Hunchun deposit). The seams belong to lower Miocene-upper Oligocene (Sineutesovsky), Oligocene-upper Eocene (Pavlovsky), upper Eocene (Ust-Davydovsky), lower Eocene (Uglovsky), and upper Paleocene (Kivdinsky) local chronostratigraphic units. Terpenoid biomarker compositions indicate the contribution of conifers and angiosperms to peat formation. The results are in agreement with paleobotanical data from sedimentary successions closely associated with the seams, highlighting abundant conifers of the taxodioid Cupressaceae and Pinaceae families, as well as Betula, Alnus, Fagus, and Ulmus during Cenozoic in the Far East. In contrast to previous findings about changes in peat-forming vegetation in central Europe, the terpenoid biomarker ratios revealed increased contributions of angiosperms during the Oligocene and gymnosperm domination during Eocene and Miocene. The different evolution may be related to differences in temperature and precipitation distribution, as well as in geomorphology and landscape evolution. A positive relationship between the di-/(di- + tri-)-terpenoid biomarker ratios and δ13C of Oligocene samples indicates the role of varying gymnosperm/angiosperm influences on the δ13C of coal. Differences in vegetation and climate are most probably responsible for the different trends in δ13C of terpenoid biomarkers over the Cenozoic, beside their general variation coupled to the evolution in δ13C of atmospheric CO2. Lower δ2H values of plant-derived lipids during the Oligocene were most probably caused by lower temperatures during coal formation. This conclusion is supported by the results of recent paleobotanical studies, yielding lower mean annual temperatures (MAT) during the Oligocene but comparable values for mean annual precipitation (MAP) during the Eocene and Oligocene in the Primorye region. Pimarane-type diterpenoids are significantly depleted in 2H. The results are in agreement with previous studies identifying different sources of hydrogen and/or different biosynthetic pathways during their formation.