Unique C-ring methylated hopene biomarkers in a Paleocene lake

Hopanoids are bacterial lipids that are ubiquitous on the surface of the Earth and in the sedimentary record. Hopanoids with an extra methyl group located in the A-ring (at the C-2 or C-3 position) of the carbon skeleton are well known and these 2- or 3-methylhopanoids have been used to infer specific biogeochemical or environmental conditions in the geological past, including the Paleogene. However, hopanoids with additional methylations outside the A-ring and/or more than one additional methylation in their pentacyclic ring system are rare. This paper reports the occurrence of hop-17(21)-enes methylated in the C-ring (tentatively assigned to the C-12 position) and dimethylated hop-17(21)-enes with methylations at both C-3 (A-ring) and C-12 (C-ring) in sediments from the Boltysh Core, Ukraine, that represents an early Paleocene (∼65 Myr) lake system. These (di)methyl hop-17(21)-enes are 1–2 orders of magnitude more abundant compared to regular hopanes in some samples. The occurrence of these unique hopanoids suggests abnormal environmental conditions during their deposition, likely attributable to volatile ecosystems in the aftermath of the Cretaceous - Paleogene boundary extinction and/or the environmental stressors associated with the lower C29n hyperthermal (65.3 Myr) that is recorded in this section.