Abstract
3β-Methylhopanes, methylated at the C-3 position of the A-ring, have been detected ubiquitously in sedimentary rocks with carbon numbers of C28 and C30–C36. These compounds have been attributed to aerobic methanotrophs due to their strongly 13C-depleted compositions. Here we report 3β-methylhopanes with carbon numbers up to C45 in source rocks from the first member of the Upper Cretaceous Qingshankou Formation, Songliao Basin, northeast China. The extended homologues up to C45 might be derived from long-chain alkyl bacteriohopanetetrols by consecutive enzymatic additions of a C5 sugar. Extremely 13C-depleted compositions, combined with a high 3β-methylhopane index, suggest an intense CH4 cycle induced by blooms of aerobic methanotrophs in the Songliao paleo-lake.
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