The distribution of molecular fossils derived from dinoflagellates in Paleogene lacustrine sediments (Bohai Bay Basin, China)

Steroids with unconventional side chains in ancient sediments and petroleum have been increasingly used as age diagnostic markers. Triaromatic dinosteroids and dinosteranes occur widely throughout the Eocene–Oligocene Shahejie Formation in the southeastern Bohai Bay Basin, China, consistent with its abundant fossil dinoflagellate assemblages. In addition, 24-norcholestane biomarkers, which commonly appear in marine oils, have been found in this lacustrine formation. Although a firm link has not been established between 24-norcholestane and specific modern taxa, both diatoms and dinoflagellates are shown to be likely sources of 24-norcholestanes. Although the Shahejie strata lack intact diatom frustules, they do exhibit a good relationship between 24-norcholestane, other molecular fossils of dinoflagellates and dinocysts. Dinoflagellates are important precursor taxa of 24-norcholestanes and the various species identified in Eocene–Oligocene lacustrine shales and mudstones of the Shahejie Formation are all potential sources of the 24-norcholestanes.