上扬子地区奥陶纪—志留纪转折期古气候演化与有机碳同位素变化成因  

Paleoclimate evolution and origin of organic carbon isotope variations during the Ordovician-Silurian transition in the Upper Yangtze area

作　　者：杜梧槚 高平 蔡益栋[1] 刘若冰[4] 曹刚杉 DU Wujia;GAO Ping;CAI Yidong;LIU Ruobing;CAO Gangshan(School of Energy Resources,China University of Geosciences(Beijing),Beijing 100083,China;State Key Laboratory of Organic Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China;University of Chinese Academy of Sciences,Beijing 100049,China;Sinopec Exploration Company,Chengdu 610041,China)

机构地区：[1]中国地质大学(北京)能源学院,北京100083 [2]中国科学院广州地球化学研究所,有机地球化学国家重点实验室,广东广州510640 [3]中国科学院大学,北京100049 [4]中国石化勘探分公司,四川成都610041

出　　处：《沉积与特提斯地质》2024年第4期809-825,共17页Sedimentary Geology and Tethyan Geology

基　　金：国家自然科学基金项目(42272140,U19B6003-03-01)。

摘　　要：奥陶纪—志留纪是地球历史上的一个重要转折期,古气候发生了剧烈变化,其中的晚奥陶世赫南特阶碳同位素漂移事件(HICE)广为人知,但关于古气候变化和碳同位素漂移的原因及其相互关联尚不明晰。以上扬子焦石坝地区JY4井五峰—龙马溪组黑色页岩为研究对象,利用有机碳含量、有机碳同位素(δ^(13)C_(org))、主量元素和微量元素分析,计算出不同页岩段的化学蚀变指数(CIA),结合生物地层学,发现WF2—WF4段对应的古气候条件由温暖潮湿逐渐转变为寒冷干燥,LM1—LM4段仍为寒冷干燥的古气候,但化学风化作用向上呈现出不断增强的趋势,LM5段开始逐渐过渡至温暖潮湿的古气候环境,LM6—LM7及其以上段则出现了一次明显的转变为寒冷干燥古气候条件的幕式波动。元素地球化学指标表明,WF2—WF4段页岩的沉积环境逐渐由氧化转变为还原,但观音桥段海水中氧含量迅速上升;LM1—LM3段页岩沉积于极度缺氧甚至硫化的沉积环境中,向上LM4段氧含量逐渐增加,大体上逐渐由缺氧向贫氧、次贫氧以及富氧条件过渡。观音桥段δ^(13)C_(org)异常“正漂”可能主要与有机碳埋藏及其氧化溶解有关,化学风化作用导致的营养物质输入增强也具有一定的作用,而龙马溪组黑色页岩段δ^(13)C_(org)异常“负漂”的出现则可能与大规模海侵导致^(12)C重新回到海洋碳库有关。The Ordovician-Silurian transition is an important period in Earth’s history,marked by drastic changes in paleoclimate and the well-known Hirnantian isotope carbon excursion(HICE)in the Late Ordovician.However,the causes of paleoclimate changes and carbon isotope excursions,as well as their correlations,are still unclear.Based on the analysis of the total organic carbon(TOC)contents,organic carbon isotopes(δ^(13)_(Corg)),as well as major and trace elements,the chemical index of alteration(CIA)values of the Wufeng-Longmaxi Formation black shales of Well JY4 in Jiaoshiba area in the upper Yangtze region were calculated.Combined with biostratigraphy,it is found that the paleoclimate conditions of WF2-WF4 members gradually changed from warm and humid to cold and arid.The paleoclimate of LM1-LM4 members remained cold and arid,with an increasing trend upward in chemical weathering.The LM5 Member marked a gradual transition back to warm and humid climate environment,while the paleocimate conditions of LM6-LM7 and their above members showed an episodic fluctuation to cold and arid.The elemental geochemical proxies indicate that the sedimentary environment of the WF2-WF4 members shifted from oxic to anoxic conditions,with a rapid increase in oxygen content in Guanyinqiao Formation.The shales of LM1-LM3 members were deposited in an extremely anoxic and sulfidic environment,with oxygen content gradually increasing upward in the LM4 Member,transitioning from anoxic to suboxic,hypoxic,and finally oxidized conditions.The"positive drift"ofδ^(13)_(Corg)in Guanyinqiao Formation is likely related to the burial and oxidation of organic carbon,with enhanced nutrient input from chemical weathering also playing a role.The subsequent"negative drift"in the black shale of Longmaxi Formation may be associated with the return of 12C to the marine carbon pool due to large-scale transgression.

关 键 词：五峰—龙马溪组 古气候 碳同位素 古环境 地球化学 

分 类 号：P532[天文地球—古生物学与地层学]