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扬子东南缘早寒武世氮同位素特征及其古海洋环境意义

Early Cambrian Nitrogen Isotopic Characteristics and Its Significance for the Paleomarine Environment in the Southeastern Margin of Yangtze

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[目的]早寒武世生物大爆发通常被认为与硝酸盐浓度增加有关,然而最新研究表明该时期硝酸盐浓度无明显变化.为查明硝酸盐对生物大爆发的影响,重建了这一时期古海洋环境与氮循环特征.[方法]以扬子东南缘斜坡区钻孔岩心(ZK0202井)为研究对象,通过对主量元素、微量元素、稀土元素、碳—氮同位素等地球化学指标进行实验分析.[结果]早寒武幸运阶—第二阶中期(>526 Ma)古海洋以深化变层的分层海洋为特征,同期固氮和反硝化作用都有发育,以固氮作用为主.第二阶晚期—第三阶早期(ca.526～518 Ma)古海洋以强烈缺氧硫化为特征,固氮作用是最重要的氮循环途径.第三阶中晚期(<518 Ma)古海洋以次氧化—氧化为主,固氮作用依然发育.氧化还原状态的演化可能与海平面变化及热液作用引起的高初级生产力有关,而持续的固氮作用显示了同期硝酸盐浓度维持在较低水平.[结论]硝酸盐的多寡或许不是生物大爆发的主控因素,海洋氧化和丰富的食物链底层生物可能是生物大爆发的重要诱因.研究进一步提升了华南扬子东南缘早寒武世斜坡区古海洋环境与氮循环的研究程度,为正确认识该时期生物—环境协同演化机制提供了新的参考.
[Objective]The early Cambrian explosion is thought to be related to the increase of nitrate concentra-tion;however,a recent study suggested that no significant change occurred in the nitrate concentration during this period.To identify the influence of nitrate on the biological explosion,this study reconstructed the characteristics of the paleomarine environment and nitrogen cycle during this period.[Methods]The borehole core(well ZK0202)in the slope area of the southeast margin of the Yangtze was used as the anatomical research object,and the major ele-ments,trace elements,rare earth elements,carbon-nitrogen isotopes,and other indices were analyzed.[Results]The early Cambrian Fortunian-middle Age 2(>526 Ma)paleomarine environment was characterized by a stratified ocean with a deepened chemocline,and both nitrogen fixation and denitrification developed during the same period,with nitrogen fixation being the dominant effect.In the late Age 2-early Age 3(ca.526-518 Ma),the paleomarine en-vironment was characterized by strong anoxic-euxinic behavior,and nitrogen fixation was the most important nitrogen cycle pathway.The middle and late Age 3(<518 Ma)paleomarine environment was dominated by suboxic-oxic be-havior,and nitrogen fixation was still developed.The evolution of the redox state may be related to the high primary productivity induced by hydrothermal action,and the persistent nitrogen fixation shows that the nitrate concentration was maintained at a low level during the same period.[Conclusions]Therefore,the abundance of nitrate may not be the main controlling factor for the biological explosion,and ocean oxidation and the abundance of organisms at the base of the food chain may be important triggers.The above research results further enhance the degree of research on the paleomarine environment and nitrogen cycle in the early Cambrian slope area of the southeast margin of the South China Yangtze,providing new references for the correct understanding of the environment-biological co-evolution in this period.

early Cambrianredoxnitrogen cyclebiological explosion

宋颖、刘宇、向庭弘、周小琳、曹光耀、张璐

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成都理工大学沉积地质研究院,成都 610059

成都理工大学沉积与生物地球化学国际研究中心,成都 610059

成都理工大学油气藏地质及开发工程全国重点实验室,成都 610059

重庆市地质矿产勘查开发局607地质队,重庆 401121

中国地质调查局成都地质调查中心,成都 610081

自然资源部沉积盆地与油气资源重点实验室,成都 610081

中国地质调查局沉积地质研究中心,成都 610081

深海圈层与地球系统教育部前沿科学中心,海底科学与探测技术教育部重点实验室,中国海洋大学海洋地球科学学院,山东 青岛 266100

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早寒武世 氧化还原 氮循环 生物大爆发

2024

10.14027/j.issn.1000-0550.2023.130

沉积学报
中国矿物岩石地球化学学会沉积学专业委员会 中国地质学会沉积地质专业委员会 中国科学院地质与地球物理研究所兰州油气资源研究中心

沉积学报

CSTPCD北大核心
影响因子：1.54
ISSN：1000-0550
年,卷(期)：2024.42(6)