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

中文翻译：

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西澳大利亚二叠纪早期甲烷渗碳酸盐中保存的脂质生物标志物的教训


硫酸盐驱动的甲烷厌氧氧化（SD-AOM）是海洋渗漏和海底环境中维持茂盛的化学合成生命的关键生物地球化学过程。虽然新生代和中生代渗漏的 SD-AOM 的广泛分子记录已经建立，但迄今为止仅存在一例古生代 SD-AOM 的报道案例。为了对古生代渗漏的主要生物地球化学过程有新的了解，对西澳大利亚的冰后早二叠世渗漏碳酸盐进行了详细的脂质生物标志物研究。遇到的生物标志物清单包括两种具有低δ13C值的诊断类异戊二烯碳氢化合物：混合植烷和十字十六烷（-124至-110‰）和2,6,10,15,19-五甲基二十烷（PMI；-128至-102‰）化合物已知由厌氧甲烷氧化古菌（ANME）产生。尽管古生代石灰岩的热成熟度较低至中等，但二叠纪渗漏沉积物中并未保留其他已知的 ANME 生物标志物，如甘油二​​联植烷基甘油四醚 (GDGT) 和 2-羟基古菌醇。尽管如此，这些化合物（包括联植烷和植烷）的降解产物分别产生 ANME 脂质典型的 δ13C 值（联植烷：-117 至 -111‰）。植烷/十字十六烷的组合峰显示出与其他 ANME 脂质类似的 C 消耗，表明植烷的前体脂质源自 ANME。在检测到的脂质中，硫酸盐还原菌的生物标志物（SD-AOM 中 ANME 的互养伙伴）包括 C 耗尽的末端支链脂肪酸、C 和 -C 以及具有 15 和 17 个碳的 - 和 - 烷烃 (δ13C)值：-97 至 -63‰），后者代表脂肪酸和细菌单醚和二醚前体的可能降解产物。 ANME 衍生的脂质（植烷和 PMI）被认为是游离烃馏分中的有机硫化合物 (OSC)，包括硫杂环戊烷、噻烷和噻吩。 ANME 衍生的 OSC 伴随着 16 和 18 个碳的硫化烷烃（δ13C 值：-83 至 -79‰），初步解释为源自渗透居住的硫酸盐还原菌合成的不饱和甘油酯或醚脂质，而既不能证实也不能排除这些化合物源自硫化物氧化细菌。我们认为，OSCs 在早期成岩作用期间形成于浅层沉积地下，反映了自生碳酸盐的快速埋藏和保存。 OSC 的快速形成可能是由于 (1) 存在过量的硫化氢（源自 SD-AOM）和 (2) 活性铁的缺乏。对西澳大利亚二叠纪渗流石灰岩的研究扩展了我们对古生代渗流生物地球化学过程的了解，并提供了一个独特的例子，说明有机化合物的早期硫化如何有助于生物标志物的保存。