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 oxidi...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).展开更多
文摘异戊二烯类甘油二烷基甘油四醚脂类化合物(isoprenoid Glycerol dialkyl glycerol tetraethers,isoGDGTs)在海洋奇古菌(Thaumarchaeota)中广泛存在,其结构对温度变化的敏感性使其成为广受欢迎的古气候与古温度重建材料。在北冰洋和西太平洋气候以及黑潮等多种因素的影响下,冲绳海槽中部成为研究全新世以来古海洋和古气候变化的天然实验室。本文通过研究C14柱状沉积物的GDGTs组成、含量变化特征及其延伸的86个碳原子的四醚指标(tetraether index of tetraethers consisting of 86 carbons,TEX86H),分析冲绳海槽中部的GDGTs来源,并定量计算C14柱状沉积物记录的海洋表面温度(seasurfacetemperature,SST),从而探讨8.2ka以来冲绳海槽中部古温度变化的驱动机制。通过甲烷指数和支链/异戊二烯类指标等,我们确认isoGDGTs主要来自于氨氧化古菌,适用于古温度重建。距今8.2ka以来,TEX86H SST的变化范围是21.6~27.2℃。冲绳海槽中部SST主要受到西热带太平洋、低纬度冬季日晒量的影响;TEX86H指标记录的温度上升趋势与东亚夏季风强度的减弱不一致。7.4—6.6ka冷事件广泛存在于冲绳海槽的SST记录中,但只在TEX86H数据中显示较大幅度的降低(~5℃),我们推测可能受到Kikai-Akahoya火山灰(~7.3ka)的影响。
基金supported by Qingdao National Laboratory for Marine Science and Technology(No.QNLM2016ORP0210)the NSF of China(No.41473080 and 41673029)
文摘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).
