Lipid taphonomy in non-skeletal carbonates: implications for geobiology and palaeoenvironmental reconstruction using organic biomarkers

Marine and freshwater non-skeletal carbonates – including, tufa, ooids, grapestones, peloids, concretions and methane seep carbonates – are important geochemical archives for past environments. Non-skeletal carbonates accrete under appropriate physical, chemical and biological conditions, rather than by active metabolic processes. Relatively few studies have explored the potential molecular information within associated organic matter. Biomass decay and alteration into simpler molecules is relevant for the search for ancient and extraterrestrial life and the assessment of taphonomic pathways that favour molecular preservation is crucial for separating biogenic and syngenetic signals from non-biogenic or signals from more recent contamination(1). Here, lipid inventories from a range of non-skeletal carbonates of different types, ages and environmental settings are compared to assess the occurrence and variation of lipid biomarkers and evaluate the most diagnostic information. Intercrystalline lipids – occurring within cracks and pore spaces of carbonates - were extracted from powdered carbonates by organic solvent extraction. Following solvent extraction, mineral-bound intracrystalline lipids were targeted by acid dissolution of solvent extracted residues and extracting resulting solutions using liquid-liquid extraction. Extracts from both pools were subjected to acid methanolysis and silylation and analyzed by gas chromatography-mass spectrometry.

Intercrystalline free lipids were characterized by high concentrations of unsaturated fatty acids: C18:1ω9 typically represented greater than 30% of total lipids detected by GC-MS, followed by C18:2ω6, C20:1ω9 and C16:1ω7. Given the co-occurrence of phytosterols ergosta-5,22E-dien-3β-ol, stigmasta-5,22E-dien-3β-ol, campest-5-en-3β-ol and stigmast-5-en-3β-ol and hopanoic acids, intercrystalline organic matter is mainly of microalgal and cyanobacterial autotrophic origin(2). Since unsaturated fatty acids are extremely labile, typically decaying in days to weeks(3), the signal from this free lipid pool is almost certainly derived from living or very recently living endolithic biomass(4). In contrast, unsaturated lipids were minor/negligible components of mineral-bound intracrystalline lipid extracts. Saturated fatty acids dominated bound lipids, particularly with an increased relative abundance of bacterial methyl-branched fatty acids and long chain C20-C30 fatty acids. These distributions reflect the molecular signature of heterotrophic decomposition of organic matter during early diagenesis and resulting selective preservation of saturated long chain fatty acids. Furthermore, the preponderance of fatty acids in carbonate-bound lipids over sterols, alkanols and alkanes (despite their occurrent in free lipids), indicates a bias towards fatty acid preservation in carbonates, possibly due to inherent higher absorption energies of carboxylic acid functional groups with carbonate mineral phases(5). Given the occurrence of endolithic microbes and high porosity and permeability of non-skeletal carbonates, the risk of modern environmental and biological contamination is high for freely extractable lipids. There was a remarkable consistency in mineral-bound lipid distributions between carbonates of diverse formation mechanism and ages, which suggests the original molecular fingerprint from autotroph biofilms is not well preserved. More detailed isotopic and lipidomic analysis of intracrystalline mineral-bound lipids may reveal information for geobiological and palaeoenvironmental research questions.

References:

1. Finkel et al (2025). Astrobiology 25:611–32
2. O’Reilly et al. (2014) Estuar Coast Shelf Sci. 136:157–71
3. Sun et al. (1997) Geochim Cosmochim Acta. 61:341–55.
4. Dance (2015) Proc Natl Acad Sci U S A. 112:2296.
5. Claesson et al. (2024) Phys Chem Chem Phys. 26:2780–805.