中国西北准噶尔盆地南部二叠系芦草沟组白云岩碳、氧同位素特征及其地质意义

Gang Liu , Guo-Zhi Wang , Na Li , Hong-Ling He , Nan-Nan Lu , Qing Lei , Zi-Ang Wang
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摘要

古白云岩中异常的正δC 值非常罕见。二叠系卢草沟组湖相沉积的深灰色霰粒岩是准噶尔盆地的重要源岩,厚度从10厘米到150厘米不等的白云岩往往夹杂在霰粒岩中。本文在岩相切片研究的基础上,系统分析了白云岩的碳氧同位素,探讨了碳同位素值异常偏高的原因及其对重建古环境和古气候的意义。结果表明,卢草沟组白云岩碳同位素值异常偏高,δC值介于+3.2‰PDB和+19.6‰PDB之间,平均为+9.7‰PDB。δO值介于-17.4‰ PDB和-1.7‰ PDB之间,平均值为-8.1‰ PDB。从卢草沟组的下部到上部,碳同位素值逐渐增大,且越来越正,近岸沉积的白云岩的碳同位素比远岸沉积的白云岩的碳同位素更正。白云岩碳同位素的异常正值主要是由微生物甲烷化作用引起的,蒸发也有一定的作用。微生物主要分布在氧化还原界面。蒸发控制着沉积水的盐度和氧化还原界面的波动。近岸和近海沉积白云岩碳同位素的正偏差差异可能与沉积过程中氧化还原界面的位置有关。这些断面的岩相特征和碳氧同位素特征共同反映了古湖从水文开放环境逐渐演变为水文封闭环境,以及古气候可能从相对温暖过渡到干旱状态,这可能是对二叠纪全球气候变化的地球化学响应。
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Characteristics and geological significance of carbon and oxygen isotopes of the Permian Lucaogou Formation dolomite in the southern Junggar Basin, northwestern China
Anomalously positive δ13C values in ancient dolomites are very rare. Dark gray argillaceous rocks of the lacustrine sediments of the Permian Lucaogou Formation are important source rocks in the Junggar Basin, and dolomites of varying thicknesses from 10 cm to 150 cm are often interspersed in argillaceous rocks. Based on the study of petrographic sections, this paper systematically analyzes the carbon and oxygen isotopes of dolomite and discusses the causes of abnormally high carbon isotope values and their significance in reconstructing paleoenvironment and paleoclimate. The results show that carbon isotope values are abnormally high in the dolomite of Lucaogou Formation, and the δ13C value is between +3.2 ‰ PDB and +19.6‰ PDB, with an average of +9.7‰ PDB. The δ18O values range from -17.4‰ PDB to -1.7‰ PDB, with an average of -8.1‰ PDB. From the lower part to the upper part of the Lucaogou Formation, the carbon isotope value gradually increases and becomes increasingly positive, and the carbon isotope of the dolomite deposited near the shore is more positive than that of the dolomite deposited far from the shore. The anomalously positive δ13C of the dolomite is mainly caused by microbial methanogenesis, with some contribution from evaporation. Microorganisms are mainly distributed at the redox interface. Evaporation controls the salinity and fluctuation of the redox interface in sedimentary water. The positive deviation difference in carbon isotopes between nearshore and offshore sedimentary dolomites may be related to the location of the redox interface during deposition. Together, the petrographic features and carbon and oxygen isotope signatures of the sections reflect the gradual evolution of the paleolake from a hydrologically open environment to a hydrologically closed one and the possible transition of the paleoclimate from a relatively warm to an arid condition, which is possibly a geochemical response to global climate change in the Permian period.
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