The isoprenoid lycopane occurs in the membranes of cultured archaea, but in environmental samples it has traditionally been interpreted to derive from phytoplankton. In the deposits of the Muschelkalk Sea, a Triassic marginal sea, lycopane contents increased during a freshening trend at the Anisian-Ladinian boundary. Based on peak abundances of lycopane at pycnoclines, we hypothesize an episodic occurrence of haloclines in the Muschelkalk Sea, with freshwater runoff mostly affecting surface waters. Freshwater dissolved inorganic carbon is typically 13C-depleted. An up-section trend of decreasing δ13C values of the biomarkers of phytoplankton (phytane) and heterotrophic consumers (2,6,10,14,18-pentamethylicosane; regular PMI) from −33‰ to −41‰ is therefore in accord with freshening. Even lower δ13C values of lycopane and the co-occurring archaeal lipid biphytane (average: −48‰ and −47‰, respectively) agree with archaea as source organisms of lycopane in the Muschelkalk Sea. We put forward a novel application of a common isoprenoid biomarker, linking the occurrence of 13C-depleted lycopane to marine freshening events.
Evers et al. (2026) studied this question.