Biomarker Evidence for Photic-Zone Euxinia and Oxygen-Bearing Geolipids in Miocene Sediments of the Valjevo-Mionica Basin.

This study investigates the occurrence of aromatic carotenoids and oxygen-bearing geolipids, namely γ-n-alkyl-γ-butyrolactones, δ-n-alkyl-δ-valerolactones, and γ-isoprenoid-alkyl-γ-butyrolactones, in Miocene sediments from the Valjevo-Mionica Basin (Serbia), from the 360-400 m depth interval. Soluble organic matter was analyzed by gas chromatography-mass spectrometry (GC-MS), and the biomarker data were integrated with previously published geological, mineralogical, and geochemical data to reconstruct the depositional environment. Aromatic carotenoid distributions provide evidence for photic-zone euxinia (PZE) and indicate changes in microbial community composition within the stratified lacustrine system. Isorenieratane and β-isorenieratane predominance in the 360-370 m interval is most consistent with strongly stratified PZE conditions favoring low-light-adapted green sulfur bacteria. In contrast, the 370-380 m interval records a greater contribution from purple sulfur bacteria, as indicated by abundant okenane, whereas the prevalence of renierapurpurane in the 380-400 m interval, together with elevated Li, Mg, and B concentrations and clay minerals, is compatible with a significant cyanobacterial contribution under alkaline, restricted-basin conditions. The occurrence of γ- and δ-lactones is consistent with anoxic, saline depositional environments characterized by limited bioturbation and active microbial transformation of organic matter, with subsequent cyclization and dehydration occurring during early diagenesis. Mass spectral fragmentation patterns indicate the presence of a previously undescribed compound (molecular mass ~568 Da) within the 370-390 m interval, for which intramolecular lactonization of a torularhodin-derived precursor is proposed as one plausible formation pathway. These findings expand the diversity of biomarkers associated with PZE and contribute to paleoenvironmental reconstructions of paleolakes, which represent sensitive archives of global paleoclimate change.