米仓山-汉南隆起牛蹄塘组页岩稳定碳同位素组成及其意义

Composition of carbon isotope of Niutitang Formation in Micangshan-Hannan Uplift and its significances

关键词:  米仓山-汉南隆起  牛蹄塘组  页岩  早寒武世  碳同位素  古海洋环境

Key words:  Micangshan-Hannan uplift  Niutitang Formation  shale  Early Cambrian  carbon isotope  paleo-oceanographical environment

摘要:

利用牛蹄塘组暗色泥页岩样品进行有机碳同位素(δ¹³C_org)、无机碳同位素(δ¹³C_carb)、氧同位素(δ¹⁸O)、总有机碳(TOC)、微量元素测定,分析米仓山-汉南隆起区早寒武世古海洋环境及其与页岩有机质生产的关系。基于原始海洋巨大溶解有机碳库(DOC)和海洋分层理论,结合不同古海洋微生物种属碳分馏特征,描述米仓山-汉南隆起区早寒武世古海洋环境和有机质生产模式。结果表明:研究区牛蹄塘组页岩中获取的碳氧同位素具有良好的原始性,对研究区古海洋环境具有良好的指示意义;δ¹³C_org明显富集轻碳同位素(¹²C)显示强烈的负异常,分布在-34.032‰～-29.537‰,属于L型无定形有机质,且更富集轻碳同位素;δ¹³C_carb明显富集重碳同位素(¹³C),分布在-1.21‰～3.64‰;氧同位素δ¹⁸O分布在-8.94‰～-3.5‰;研究区南北部碳氧同位素空间分布具有差异性,与南部西河乡剖面相比,北部宁强铁锁关和两河村剖面的碳同位素具较高的δ¹³C_org,偏正的δ¹³C_carb和高的δ¹³Ccarb-org,且δ¹³C_carb与δ¹³C_org呈明显的解耦关系而与δ¹³Ccarb-org线性相关;牛蹄塘组页岩碳氧同位素组成取决于海侵过程中藻类、化能自养、甲烷细菌、甲烷氧化菌等特殊的古海洋微生物种属分布或活动强度与上升洋流、海洋分层的有机结合。

Abstract:

Based on the shale samples from Niutitang Formation, the organic carbon isotope(δ¹³C_org), carbonate carbon isotope(δ¹³C_carb), oxygen isotope(δ¹⁸O), total organic carbon (TOC), and trace elements in shale were determined to analyze the paleo-oceanographical environment of Early Cambrian in Micangshan-Hannan Uplift and its effect on organic matter production. According to the theories of dissolved organic carbon (DOC) in original ocean and ocean stratification, and combined with the carbon isotope fractionation characteristics of different palaeo-marine microbial species, the paleo-oceanographical environment and organic matter production of the Early Cambrian in the Micangshan-Hanan Uplift were described. The results show that the carbon-oxygen isotopes in Niutitang shale keep original and have a good indication of the paleo-oceanographical environment. The organic matter in Niutitang shale enriched light carbon isotope(¹²C) and the values of δ¹³C_org range from -34.032‰ to -29.537‰, which are much lower than others. It reflects the amorphous organic matter (AOM) with Type-L in Niutitang shale. The carbonate enriched heavy carbon isotope(¹³C) and the values of δ¹³C_carb ranges from -1.21‰ to 3.64‰. The oxygen isotope ranges from -8.94‰ to -3.5‰. The space distribution of carbon-oxygen isotope is different in Micangshan-Hannan Uplift. Compared to the carbon isotope in the southern Xihexiang, the northern δ¹³C_carb is partial positive and the difference between δ¹³C_carb and δ¹³C_org(δ¹³Ccarb-org) is bigger,and the δ¹³C_carb has a good correlation with δ¹³Ccarb-org but not with δ¹³C_org. The composition of carbon-oxygen isotope of Niutitang shale depends on theorganic combination of ascending current, ocean stratification and special microorganism in paleo-ocean as algae, methane bacteria, methane-oxidizing bacteria and chemoautotrophic bacteria.