Lipid Biomarkers and Their Stable Carbon Isotopes in Ancient Seep Carbonates from SW Taiwan, China  

作　　者：GUAN Hongxiang XU Lanfang WANG Qinxian CHEN Duofu WU Nengyou MAO Shengyi 

机构地区：[1]Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences [2]Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology [3]University of Chinese Academy of Sciences [4]Shanghai Engineering Research Center of Hadal Science and Technology, College of Marine Sciences, Shanghai Ocean University

出　　处：《Acta Geologica Sinica(English Edition)》2019年第1期167-174,共8页地质学报（英文版）

基　　金：supported by Qingdao National Laboratory for Marine Science and Technology(No.QNLM2016ORP0210);the NSF of China(No.41473080 and 41673029)

摘　　要：Four massive brecciated, chimney-like, and slender pipe network carbonate samples(JA-4, JA-5, JX-8 and BG-12) were collected from southwestern Taiwan, which were suggested to have formed as a result of anaerobic oxidization of methane(AOM). Considering that the environmental conditions of the carbonates precipitation and the sources of carbon and organic matter need to be further declared, molecular fossils and compound-specific carbon isotopic investigations of the carbonates were conducted in this study. According to lipid biomarkers of 2,6,10,15,19-pentamethyleicosane(PMI) and squalane diagnostic to methanotrophic archaea, as well as the extremely low δ^(13)C values(as low as -1^(13).4‰) detected in samples JA-4, JA-5 and JX-8, these carbonates were revealed to be a result of AOM. Based on the varied δ^(13)C values of characteristic archaea biomarkers in specific samples, biogenic methane was proposed to be responsible for the formation of samples JA-4 and JA-5, whereas a mixed carbon source of ^(13)C-depleted methane and ^(13)C-enriched residual CO_2 from methanogenesis was suggested for the carbonate of JX-8 due to the co-occurrence of a highly positive δ^(13) C_(carb) value(+8‰) and a moderate ^(13)C depletion of PMI. The low content of AOM-related biomarkers and the absence of indicators for ANME-2 suggested that these carbonates were formed in weak seep settings. By comparison, no typical lipid biomarkers for methanotrophic archaea was detected in carbonate BG-12. The short-chain and long-chain n-alkanes accounted for 30% and 45% of all hydrocarbons, respectively, with a CPI value of 1.2, suggesting that the n-alkanes were derived from both marine organisms and terrestrial inputs. A low thermal maturity could be revealed by the incomplete equilibrium value of the C^(31)αβ 22S/(22S+22R) ratio(0.5), and the carbonate BG-12 was probably deposited in a suboxic condition indicated by a value of Pr/Ph ratio(2.5).Four massive brecciated, chimney-like, and slender pipe network carbonate samples(JA-4, JA-5, JX-8 and BG-12) were collected from southwestern Taiwan, which were suggested to have formed as a result of anaerobic oxidization of methane(AOM). Considering that the environmental conditions of the carbonates precipitation and the sources of carbon and organic matter need to be further declared, molecular fossils and compound-specific carbon isotopic investigations of the carbonates were conducted in this study. According to lipid biomarkers of 2,6,10,15,19-pentamethyleicosane(PMI) and squalane diagnostic to methanotrophic archaea, as well as the extremely low δ^(13)C values(as low as -1^(13).4‰) detected in samples JA-4, JA-5 and JX-8, these carbonates were revealed to be a result of AOM. Based on the varied δ^(13)C values of characteristic archaea biomarkers in specific samples, biogenic methane was proposed to be responsible for the formation of samples JA-4 and JA-5, whereas a mixed carbon source of ^(13)C-depleted methane and ^(13)C-enriched residual CO_2 from methanogenesis was suggested for the carbonate of JX-8 due to the co-occurrence of a highly positive δ^(13) C_(carb) value(+8‰) and a moderate ^(13)C depletion of PMI. The low content of AOM-related biomarkers and the absence of indicators for ANME-2 suggested that these carbonates were formed in weak seep settings. By comparison, no typical lipid biomarkers for methanotrophic archaea was detected in carbonate BG-12. The short-chain and long-chain n-alkanes accounted for 30% and 45% of all hydrocarbons, respectively, with a CPI value of 1.2, suggesting that the n-alkanes were derived from both marine organisms and terrestrial inputs. A low thermal maturity could be revealed by the incomplete equilibrium value of the C^(31)αβ 22S/(22S+22R) ratio(0.5), and the carbonate BG-12 was probably deposited in a suboxic condition indicated by a value of Pr/Ph ratio(2.5).

关 键 词：METHANE SEEPS lipid biomarkers ANAEROBIC oxidation of METHANE weak SEEP SETTINGS 

分 类 号：P618.13[天文地球—矿床学]