湖南会同寒武纪早期有机碳同位素地层学研究

Organic Carbon Isotope Stratigraphy of the Early Cambrian Huitong Section in Hunan Province, Southeastern Yangtze, China

寒武纪早期是地球海洋环境与生命演化的关键时期,但目前扬子东南缘深水相区的早寒武纪地层尚缺乏系统、精确的地层对比工作。该文选取湖南省怀化地区会同钻孔剖面(深水相区)的留茶坡组硅质岩、小烟溪组黑色页岩为研究对象,进行了高分辨的有机碳同位素(δ^(13)C_(org))地层对比,结果在会同剖面自下而上识别出四个正漂移(P1、P2、P3和P4)与两个负漂移(N1和N2),结合其他剖面的生物化石记录和锆石U-Pb年龄资料,将会同剖面有机碳同位素与湖南其他剖面,以及和云南和三峡等地浅水相区剖面的有机碳、无机碳同位素曲线进行对比,认为扬子东南缘埃迪卡拉系-寒武系界线在湖南深水相区可放置于留茶坡组上部较大的有机碳同位素负漂移(Basal Cambrian Carbon isotope Excursion,BACE)出现的位置,但由于钻孔深度不够,所以该负漂移未在会同剖面获得,而P1、P2和P3分别对应于寒武系的ZHUCE(ZHUjiaqing Carbon isotope Excursion,第二阶)、CARE(Cambrian Arthropod Radiation isotope Excursion,第三阶)和MICE(MIngxinsi Carbon Isotope Excursion,第四阶)正漂移,N1和N2分别对应于寒武系的SHICE(SHIyantou Carbon isotopeExcursion,第二阶)和AECE(Archaeocyathid Extinction Carbon isotope Excursion,第四阶)负漂移,因此会同剖面留茶坡组顶部至小烟溪组底部属于寒武系第二阶,小烟溪组下部属于寒武系第三阶,而小烟溪组中-上部属于寒武系第四阶,而顶部是否达到第四阶顶部尚无法确认。碳同位素的负漂移可能是海侵时期上升流水体将底层富含12C还原水体带至浅水地区所致,并分别与埃迪卡拉动物群、小壳化石动物群和古杯动物的灭绝密切相关;而在生物繁盛时期,海洋初级生产力升高,有机质埋藏增加,导致碳同位素的正漂移。

The early Cambrian is one of the critical periods during Earth evolution involving significant evolution of marineenvironment and metazoans. However, the early Cambrian strata in the deep-water setting along the southeastern margin of the YangtzePlatform are still lack of systematically and accurately stratigraphic correlations. Here we conduct a high-resolution organic carbonisotope chemostratigraphy of the deep-water chert in the Liuchapo Formation and black shales in the Xiaoyanxi Formation, which were collected from drill cores in the Huitong section of Hunan Province. Results indicate four positive δ13Corg excursions (termed P1, P2, P3and P4 ) and two negative δ13Corg excursions (termed N1 and N2 ) in ascending order. Combined with the fossil records and zircon U-Pbdating data, we correlate the δ13Corg curve of the Huitong section with the δ13Corg and δ13Ccarb curves of other sections in Hunan andshallow-water areas including Yunnan and Three Gorges. The correlation implies that the Ediacaran-Cambrian boundary was placed atthe negative δ13Corg excursion (Basal Cambrian Carbon isotope Excursion, BACE) in the upper Liuchapo Formation of Hunan Province.This negative δ13Corg excursion (BACE), however, has not been identified at the Huitong section, owning to the limited drill core depth.The three positive δ13Corg excursions P1, P2 and P3 can be correlated with ZHUCE (ZHUjiaqing Carbon isotope Excursion, Stage 2),CARE (Cambrian Arthropod Radiation isotope Excursion, Stage 3) and MICE (Mingxinsi Carbon Isotope Excursion, Stage 4) δ13Corgexcursions, respectively. In contrast, the two negative δ13Corg excursions N1 and N2 are likely correlated with SHICE (SHIyantou Carbonisotope Excursion, Stage 2) and AECE (Archaeocyathid Extinction Carbon isotope Excursion, Stage 4) δ13Corg excursions, respectively.Therefore, the upper Liuchapo and the basal Xiaoyanxi formations probably belong to the Cambrian Stage 2. The lower XiaoyanxiFormation belongs to the Cambrian Stage 3 and the middle-upper Xiaoyanxi Formation belongs to the Cambrian Stage 4. However, it isstill unclear whether the top of the Xiaoyanxi Formation covers the end of the Cambrian Stage 4. The negative δ13Corg excursionsprobably result from the transportation of 12C-enriched anoxic bottom water through upwelling from deep to shallow water, and might be closely associated with the mass extinction of Ediacaran fauna,small shelly fossils andarchaeocyathids.In contrast,the flourish of Cambrian fauna could increasethe primary productivity and associated rate of organic matter burial,which lead to the positiveδ13Corg excursions.