The abundance distributions of more than 40 elements in the No. 502 Ordovician-Silurian (O/S) bounda-ry section at Fenxiang, Yichang have been studied by RNAA and INAA. The results show that in the bounda-ry bed, ther...The abundance distributions of more than 40 elements in the No. 502 Ordovician-Silurian (O/S) bounda-ry section at Fenxiang, Yichang have been studied by RNAA and INAA. The results show that in the bounda-ry bed, there is a distinctive Ir anomaly because the Ir concentration abruptly increases to 0.64 ppb.Furthermore, the Ir is positively correlated in abundance variation with some siderophile and sulphophile ele-ments. In the same bed of the O/S boundary section at Huanghuachang, Yichang, there is also a δ^(13)C excur-sion. These geochemical signs support the hypothesis that the terminal Ordovician mass extinction was proba-bly related to extraterrestrial event, and provide new evidence for defining the O/S boundary between theHirnantia-Kinnella and G. persculptus Zones.展开更多
Anomalously positive δ^(13)C 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 do...Anomalously positive δ^(13)C 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 δ^(13)C value is between+3.2‰PDB and+19.6‰PDB,with an average of+9.7‰PDB.The δ^(18)O 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 δ^(13)C 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.展开更多
文摘The abundance distributions of more than 40 elements in the No. 502 Ordovician-Silurian (O/S) bounda-ry section at Fenxiang, Yichang have been studied by RNAA and INAA. The results show that in the bounda-ry bed, there is a distinctive Ir anomaly because the Ir concentration abruptly increases to 0.64 ppb.Furthermore, the Ir is positively correlated in abundance variation with some siderophile and sulphophile ele-ments. In the same bed of the O/S boundary section at Huanghuachang, Yichang, there is also a δ^(13)C excur-sion. These geochemical signs support the hypothesis that the terminal Ordovician mass extinction was proba-bly related to extraterrestrial event, and provide new evidence for defining the O/S boundary between theHirnantia-Kinnella and G. persculptus Zones.
基金the National Science and Technology Major Project, China (Tight Reservoir Development Mechanism and Its Relationship with OilGas Reservoir Forming) (project no.: 2016ZX05002002-007)the Fundamental Research Funds for Sinopec Shengli Oilfield Co., China for support。
文摘Anomalously positive δ^(13)C 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 δ^(13)C value is between+3.2‰PDB and+19.6‰PDB,with an average of+9.7‰PDB.The δ^(18)O 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 δ^(13)C 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.
