Organic petrography, biomarkers, and stable isotope (δ13C, δD, δ15N, δ18O) compositions of liptinite-rich coals

Abstract

Liptinite-rich coals were evaluated using organic petrography, biomarkers, and stable isotopes to investigate their origin and paleoenvironmental significance, particularly, to explore fractionation characteristics of stable isotopes (δ13C, δD, δ15N, and δ18O) between bulk coal, extractable organic matter (EOM), and extracted coal residue (ER). The samples were collected from Cenozoic coal seams from the Yunnan and Liaoning Provinces in China. The samples are characterized by the enrichment of different types of liptinite. The late Pliocene sample YNP (liptinite = 73.5%) is dominated by sporinite and bituminite, whereas the late Pliocene sample YND (liptinite = 65.5%) is characterized by isolated resinite particles and amorphous resinite. The liptinite in the Eocene sample SB (46.0%) is represented by blocky resinite with homogeneous morphology. The differences in distribution and morphology of liptinite are due to the different coal-forming plants and depositional environments, as indicated by the biomarker compositions. Biomarker results indicated that the sample YNP was formed mainly by Pinaceae and angiosperms under oxidizing conditions with bacterial/fungal degradation, whereas the sample YND was derived from the woody parts of gymnosperms with lower contributions of angiosperms under reducing conditions with low microbial activity. The sample SB predominantly originated from Cupressaceae/Pinaceae under reducing conditions with a lack of bacterial/fungal degradation.The fractionations of δ13C, δD, δ15N, and δ18O between bulk coal, EOM, and ER in the liptinite-rich coals are different. The δ13C values of bulk coal, EOM, and ER are mostly controlled by precursor paleovegetation and isotopic composition of CO2. The lower δ13C values of samples YNP and YND from the late Pliocene compared to that of the Eocene sample SB resulted from the change of palaeoconditions (e.g., δ13C of atmospheric CO2, cooling, and decrease of CO2 concentration). The δ13C values of EOM in the samples YNP and YND are about 3‰ lower than δ13C values of bulk coal/ER, whereas δ13C fractionation between EOM and bulk coal/ER is small (< 0.8‰) in the sample SB, probably due to the very limited microbial/fungal degradation. The δD values of EOM are 50 to 90‰ lower than that of bulk coal/ER within the same sample, reflecting different isotopic compositions of monomeric compounds compared to the macromolecular matrix of kerogen most probably due to differences in H-isotope fractionation during biosynthesis. The δ15N of bulk coal and ER show limited fractionation (< 0.5‰) and are mainly controlled by precursor paleovegetation and microbial induced degradation processes. The studied liptinite-rich coals yield higher δ18O values than those detected in humic coals. The differences of δ18O values (bulk coal, EOM, ER) between the samples YNP/YND and the corresponding δ18O values of the sample SB indicate that this isotope parameter is mostly controlled by the δ18O value of source water. The weaker fractionation of oxygen isotope between EOM and bulk coal/ER compared to δ13C and δD may be attributed to the fact that oxygen-containing compounds within EOM did not experience a cleavage or any change of the O-containing bonds. The calculated δ18O and δD values of source rain waters fall in the range of modern rain waters at different sites.