扬子地块南缘中—晚寒武世浅海多次短暂增氧及其诱因:来自碳酸盐岩铈异常及碳-锶同位素证据

Shallow marine oxidation pulses during the Middle-Late Cambrianin South China and their potential triggers: Evidences fromcarbonate Ce anomaly and C-Sr isotopes

中—晚寒武世(509~485 Ma)是地球历史上的一个动荡时期,期间发生了多次碳同位素漂移和生物灭绝事件,表明地球表层环境可能发生了剧烈变动。本文以湖南省永顺县王村剖面的清虚洞组、敖溪组和花桥组的碳酸盐岩地层为研究对象,通过碳、锶同位素地层学对比和稀土Ce异常的分析,揭示出扬子地块南缘在中晚寒武世发生了四次短暂的浅海短暂增氧(CeN/CeN<0.8),分别位于乌溜期(约509~504.5 Ma)、早鼓山期(约505 Ma)、古丈期(约500.5~497 Ma)和早排碧期(约497~496 Ma),其中排碧期早期和乌溜期内发生的增氧可能指示了全球表层海水的广泛增氧,而鼓山期早期和古丈期内发生的增氧可能仅局限于扬子地块南部边缘海。根据最新的生物地层学研究成果,浅海短暂增氧发生的时间与华南地区三叶虫和总体的生物多样性高峰基本对应,指示浅海氧气含量的上升可能促进了生物多样性的发展。海水δ13C和87Sr/86Sr值的变化趋势指示大陆风化增强向海洋输入大量营养物质,导致表层海洋的初级生产力升高,可能是浅海氧化程度相对升高的重要驱动因素。

The Middle-Late Cambrian(509~485 Ma)was an unsteady period in Earth history,during which several carbon isotope shifts and biological extinction events occurred,indicating that the Earth􀆳s environment may have experienced drastic changes.In this study,we report systematicδ13C,87Sr/86Sr and Ce anomaly data of carbonate from the Wangcun section in South China.Our results suggest that four transient shallow marine oxidation pulses(CeN/CeN<0.8)occurred in South China during the Middle-Late Cambrian,which located in Wuliuan(ca.509~504.5 Ma),Early Drumian(~505 Ma),Guzhangian(ca.500.5~497 Ma)and Early Paibi(ca.497~496 Ma),respectively.The oxidation pulses that occurred in Early Paibi and Wuliuan may be indicative of widespread oxidation of global surface seawater,while the other two pulses may represent local oxidation confined to the marginal seas in South China.When compared with the latest biostratigraphic findings,we suggest that the timing of the onset of the shallow marine oxidation pulses largely corresponds to the peak of trilobites and overall species diversity in South China,which indicates that shallow ocean oxygen rises may have contributed to the development of biodiversity animal radiations.The relative trends in seawaterδ13C and 87Sr/86Sr values indicate that enhanced continental weathering may have resulted in increases of nutritional matter input to the sea and thus increase of marine primary productivity,which may have been an important driver of the shallow marine oxidation pulses.