华南埃迪卡拉纪陡山沱海洋中无机碳同位素组成变化

VARIATIONS OF THE CARBONATE CARBON ISOTOPE IN EDIACARAN DOUSHANTUO OCEAN OF SOUTH CHINA

报道了贵州江口桃映深水相剖面陡山沱组碳酸盐岩δ13C值的变化特征,结合其他已报道的数据,分析了华南扬子地区埃迪卡拉纪陡山沱海洋不同沉积环境,包括盆地相、斜坡相、台地相、台地边缘相碳酸盐岩中δ13C变化趋势及绝对值的异同,发现浅水区剖面记录的δ13C漂移次数多于深水区剖面,且不同相区δ13C值也有差异。δ13C值的差异与埃迪卡拉纪陡山沱海洋的阶段性演化密切相关。基于不同相区的δ13C值变化,埃迪卡拉纪陡山沱海洋的演化历史可分为3个时期:1)陡山沱组1段盖帽白云岩沉积期华南扬子地区很可能为一个开阔台地,白云岩中δ13C值可能继承于幔源CO2的碳同位素特征,深水区和浅水区碳酸盐中δ13C值无显著差异;2)陡山沱组2段和陡山沱组3段下部沉积时期盆地深水区中δ13C值显著低于浅水区,且深水区δ13C值与陡山沱组1段时期无显著差异,浅水区的δ13C值则显著升高;3)陡山沱组3段上部和陡山沱组4段沉积时期陡山沱盆地中δ13C值均显著下降,且不同沉积环境中的δ13C值差异度降低。盖帽后埃迪卡拉纪陡山沱海洋阶段性演化主要与不同时期深水区DOC库的逐步氧化有关。

Carbon isotope compositions in marine carbonate rocks have long been used for paleoceanographic reconstruction.In this paper,we present new and review previously published carbonate δ13C data of the Ediacaran Doushantuo Formation in South China.The combined datasets show that both δ13C values and their variations in four sedimentary facies（i.e.,platform,platform margin,slope,and basin） are significantly different,and pronounced δ13C shifts occur more frequently in shallow-water than in deep-water sections,which is here interpreted as being closely related to the stepwise evolution of the Ediacaran Doushantuo ocean of South China.The evolutionary history of the Ediacaran Doushantuo ocean could be divided into three stages according to the δ13C data： the first stage corresponds to the duration of cap dolostone deposition（i.e.,member I of Doushantuo Formation） during which the Yangtze block was probably an open-platform,carbonate δ13C values were possibly inherited from the mantle-sourced CO2 gas,and they do not show significant difference between shallow-and deep-water environments of the Doushantuo ocean.The second stage encompasses the member II and lower member III of the Doushantuo Formation,during which δ13C values from basinal facies are significantly lower than those from platform facies.Unlike the great increase in platform facies,δ13C values for deep-water carbonates do not show any significant difference compared with the previous stage.The third stage occurs in the upper member III and member IV of Doushantuo Formation.All δ13C values from different sedimentary facies decrease relative to those in the second stage,resulting in the relatively less differentiated δ13C values between shallow-and deep-water environments.The stepwise carbon isotope evolution of the Ediacaran Doushantuo ocean after the cap dolostone deposition can probably be attributed to the progressive remineralization of a large dissolved organic carbon（DOC） reservoir via bacterial sulfate reduction.