Work

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

Year: 2024

Type: article

Abstract: 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 δ 13 C 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 sn 2-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 δ 13 C values (biphytanes: −117 to −111‰) typical of ANME lipids. The combined phytane/crocetane peaks show similar 13 C 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 13 C-depleted terminally branched fatty acids iso- and anteiso- C 15:0 and -C 17:0 as well as iso - and anteiso -alkanes with 15 and 17 carbons (δ 13 C 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 (δ 13 C 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.

Source: Chemical Geology

DOI: 10.1016/j.chemgeo.2024.122343

Authors: Siyu Wang, Daniel Birgel, Nicola Krake, Chuanbo Shen, David W. Haig, Jörn Peckmann

Institutions: Universität Hamburg, China University of Geosciences, The University of Western Australia, Oceans Institute

Language: English

Topic: Paleontology and Stratigraphy of Fossils

Subfield: Paleontology

Field: Earth and Planetary Sciences

Domain: Physical Sciences

Open Access status: hybrid

Estimated APC paid: $3,420

Funders: Natural Science Foundation of Hubei Province, China Scholarship Council