Monoaromatic, Diaromatic, Triaromatic, and Tetraaromatic Hopanes in Kukersite Shale and Their Stable Carbon Isotopic Composition

详细信息    查看全文

• 作者：Jing Liao ; Hong Lu ; Guoying Sheng ; Ping鈥檃n Peng ; Chang Samuel Hsu
• 刊名：Energy & Fuels
• 出版年：2015
• 出版时间：June 18, 2015
• 年：2015
• 卷：29
• 期：6
• 页码：3573-3583
• 全文大小：674K
• ISSN：1520-5029

文摘

Abundant monoaromatic, diaromatic, triaromatic, and tetraaromatic hopanes as well as benzohopanes occurred in the Ordovician Estonia Kukersite shale. They were present in sufficient concentrations for reliable stable carbon isotopic composition measurements. There was almost no difference between steroids and monoaromatic steranes, but ¹³C enrichments of 2鈥扳€?鈥?were observed in aromatized hopanes, compared with hopanoids. Regular hopanes (C₂₇鈥揅₃₅) generally are proposed to be sourced from the bacteriohopanetetraols (C₃₅), while aromatic hopanes were derived from the diplopterol and/or diploptenes (both C₃₀). Hopanoids were aromatized via a progressive pathway from the D-ring to the A-ring with increasing aromaticity, with successive losses of four angular methyl groups on the A-, B-, C-, and D-rings, as well as one of the gem-dimethyl groups at C-4 (i.e., C-23 and C-24). Thus, different hopanoid precursors with their own 未¹³C values as well as the successive losses of up to five carbon atoms in the aromatization pathway help to explain the ca. 2鈥扳€?鈥?¹³C isotopic enrichment in the aromatized hopanes, relative to the nonaromatized hopanoids in the Kukersite shale. In contrast, the angular methyl group at C-18 in the steroid nucleus is not removed but migrated to C-17 in the formation of the C-ring monoaromatic steranes. Moreover, more rearrangement (75%) and less aromatization (25%), mainly for the formation of the C-ring aromatic steranes, occurred in steroid compounds. Thus, the 未¹³C values of the C-ring monoaromatic steranes exhibit no significant difference from those of the diasterenes in Kukersite shale. These monoaromatic, diaromatic, triaromatic, and tetraaromatic hopanes, as well as benzohopanes, in the Estonia Kukersite shale were the microbially mediated aromatization products of hopanoids in the early diagenetic stage. Their precursors of diplopterol or diploptene and abundant methylhopanes found in this study strongly indicate that cyanobacteria that previously had been suggested to have colonized with Gloeocapsomorpha prisca, was probably responsible for the microbially mediated aromatization.