4-Methyl-24-Ethylcholestane in Upper Permian Fossil Conifer Wood from Paleo-Midlatitude of NE Pangea, Wutonggou Low-Order Cycle, Southern Bogda Mountains, NW
The use of C30 4-methylsteranes as an indicator of source organic matter relies on accurate identification of the compounds from their isomers and distribution of their precursors (i.e. sterols) in contributing organisms. C30 4-methylsteranes have been reported in many Mesozoic and pre-Mesozoic samples but not in Permian rocks. They may be produced from dinoflagellates or aquatic plants or methylotrophic bacteria. Significant amount of C30 4-methylsteranes was detected in preserved materials in two Upper Permian fossil conifer stems. The 4-methylsteranes have a 4-methyl-24-ethylcholestane structure, without 4α, 23, 24-trimethylcholestanes (also known as dinosteranes). The preservation of C30 4-methylsteranes with C29 steranes and absence of C27-28 steranes with rare hopanes suggest that the 4-methyl sterols produced by conifers may be an important potential precursor for 4-methylsteranes. Alternative sources include freshwater dinoflagellates or methylotrophic bacteria in the environments where the conifers lived, transported, and deposited. Significant amount of C30 4-methylsteranes in Permian fossil conifer in fluvial-lacustrine environments shows that regarding the 4-methylsteranes as an indicator of diagnostic marine input can lead to erroneous conclusions. The rarity of this biomarker in geological records makes it an effective proxy for nonmarine sequence stratigraphic correlation and oil-oil and oil-source rock correlation.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017