Lessons from lipid biomarkers preserved in methane-seep carbonates from the early Permian of Western Australia

• Published in: Chemical geology Vol. 668; p. 122343
• Main Authors: Wang, Siyu, Birgel, Daniel, Krake, Nicola, Shen, Chuanbo, Haig, David W., Peckmann, Jörn
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20-11-2024
• Subjects: Anaerobic oxidation of methane; Early Permian; Lipid biomarkers; Organic sulfur compounds; Seep carbonates; Western Australia; Western Australia; Anaerobic oxidation of methane; Organic sulfur compounds; Seep carbonates; Early Permian; Lipid biomarkers
• DOI: 10.1016/j.chemgeo.2024.122343
• ISSN: 0009-2541

Sulfate-driven anaerobic oxidation of methane (SD-AOM) is the key biogeochemical process at marine seeps, seafloor environments sustaining lush chemosynthesis-based life. While an extensive molecular record of SD-AOM has been established for Cenozoic and Mesozoic seeps, to date only one reported case of SD-AOM exists for the Paleozoic. To get new insight into the dominant biogeochemical processes at Paleozoic seeps, a detailed lipid biomarker study was conducted on post-glacial early Permian seep carbonates from Western Australia. The encountered biomarker inventory comprises two diagnostic isoprenoid hydrocarbons with low δ13C values: mixed phytane and crocetane (−124 to −110‰) and 2,6,10,15,19-pentamethylicosane (PMI; −128 to −102‰), compounds known to be produced by anaerobic methane-oxidizing archaea (ANME). Other known biomarkers of ANME like glycerol dibiphytanyl glycerol tetraethers (GDGTs) and sn2-hydroxyarchaeol are not preserved in the Permian seep deposits despite the low to moderate thermal maturity of the Paleozoic limestones. Still, degradation products of these compounds including biphytanes and phytane, respectively, yield δ13C values (biphytanes: −117 to −111‰) typical of ANME lipids. The combined phytane/crocetane peaks show similar 13C depletion as other ANME lipids, suggesting a derivation of the precursor lipids of phytane from ANME. Among the detected lipids, biomarkers of sulfate-reducing bacteria, the syntrophic partners of ANME in SD-AOM, include the 13C-depleted terminally branched fatty acids iso- and anteiso-C15:0 and -C17:0 as well as iso- and anteiso-alkanes with 15 and 17 carbons (δ13C values: −97 to −63‰), the latter representing probable degradation products of fatty acid and bacterial mono- and diether precursors. ANME-derived lipids (phytane and PMI) are recognized as organic sulfur compounds (OSCs) in the free hydrocarbon fraction, comprising thiolanes, thianes, and thiophenes. The ANME-derived OSCs are accompanied by sulfurized alkanes with 16 and 18 carbons (δ13C values: −83 to −79‰), tentatively interpreted to derive from unsaturated glycerol ester or ether lipids synthesized by seep-dwelling sulfate-reducing bacteria, while a derivation of these compounds from sulfide-oxidizing bacteria can neither be substantiated nor excluded. We suggest that OSCs formed in the shallow sedimentary subsurface during early diagenesis, reflecting fast entombment and preservation in authigenic carbonates. Rapid OSC formation was probably caused by (1) the presence of excess hydrogen sulfide, which derived from SD-AOM and (2) the scarcity of reactive iron. The studied Permian seep limestones of Western Australia expand our knowledge of the biogeochemical processes at Paleozoic seeps and provide a unique example of how early sulfurization of organic compounds may aid the preservation of biomarkers.
•13C-depleted biomarkers of SD-AOM consortia are found in Permian seep carbonates.•Excess hydrogen sulfide from SD-AOM may have induced the formation of organic sulfur compounds (OSCs).•Bacterial OSCs correspond to lipids synthesized by SD-AOM consortia.•OSCs represent the products of early diagenesis in shallow post-glacial sediments.