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

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.