Bulk Carbon Isotope Excursions of the Cenomanian through Turonian of Mid-Cretaceous in Southern Tibet
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摘要: 过去大量关于白垩纪中期Cenomanian-Turonian時期的古海洋及其界线時期缺氧事件研究的高分辨率碳氧同位素的实际材料绝大多数都来自深海钻探(DSDP)、大洋钻探(ODP)和欧美的一些较深水盆地.通过对西藏南部地区(位处特提斯洋东南)同一层位高分辨率碳同位素分析, 目的在于为Cenomanian-Turonian古海洋及全球事件的碳同位素响应提供西藏地区的对比材料, 并试图就浅海相的变化情况进行探讨.西藏定日和岗巴地区的高分辨率碳同位素分析结果显示, δ13C值在Cenomanian中晚期稳定, Cenomanian-Turonian界线时期快速高幅正偏, Turonian期持续负偏, 这种长期变化趋势在特提斯甚至全球可以对比.深入研究还发现, Turonian中后期存在2个较大幅度的负偏凹陷区, 与来自欧洲的相关资料在偏移时间和幅度方面惊人相似.对比分析表明, 岗巴地区δ13C值长期变化和短期波动幅度及步调与定日及全球其他地区存在一定差别, 幅度差异可能与样品制备和成岩改造有关, 步调不一致则可能受生物地层定位的影响.研究区近海-半远洋环境Cenomanian-Turonian界线时期未出现典型富有机质黑色页岩, 说明δ13C值极度正偏与黑色页岩的出现并非一一对应.Abstract: In the past, huge high-resolution data of carbon and oxygen isotopic values on paleoceanography of the Cenomanian-Turonian and on anoxic event of its boundary passage were derived from DSDP, ODP and deeper basins in Euramerica, but few and rough data relevant came from southern Tibet, China. The new carbon isotope high-resolution data of the Cenomanian-Turonian in this paper aim to provide a correlation among Euramerica, DSDP, ODP and southeastern Tethys Ocean (southern Tibet), and try to characterize the carbon isotopic curve in shallower lithofacies background. Carbon isotopic values obtained from the Gongzha Section of Tingri and the Zongshan Section of Gamba, in southern Tibet, indicate a long-term tendency of Cenomanian through Turonian similar to those from Tethys even globe. The carbon isotopic values seldom change in the middle-late Cenomanian. An abrupt and high-amplitude positive excursion occurs near the passage bed of Cenomanian and Turonian, then goes negative in gradual in whole Turonian. At the same time, two negative excursion troughs are recognized at both the middle H. helvetica zone and the boundary of the H. helvetica and M. sigali zones by a moderate positive area between, which is the same as in Europe and ODP in time and pace and is in harmony with the global carbon cycle and climate changes. However, the much alterable excursions and paces of the δ13C value at Gamba are different from those at Tingri and other locations in the world. The former difference may be the result of diagenetic overprint, and the latter may be mistaken biozone positions. According to the potential correlation tool of carbon isotopes, it is suggested that the horizon of the Cenomanian-Turonian boundary may need to be promoted upward ca. 40 m. Lack of typical organic black shale at the Cenomanian-Turonian boundary in shallow sea area including southern and northern Tibetan Tethys Himalayas indicates the high δ13C values in the oceanic sediments are not the unique result of the oceanic anoxic event but a mixed response of global carbon reservoir.
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Key words:
- carbon isotope /
- Cenomanian /
- Turonian /
- Cretaceous /
- Tethys Himalayas /
- Tibet
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图 1 西藏南部构造分带和剖面位置图
1.冈底斯弧; 2.日喀则弧前盆地; 3.雅鲁藏布江缝合带; 4.特提斯喜马拉雅; 5.结晶喜马拉雅; 6.构造分带的主要断裂; 7.剖面位置.A.特提斯喜马拉雅南亚带; B.特提斯喜马拉雅北亚带
Fig. 1. Tectonic zonation of South Tibet with locations of cross-sections
图 2 西藏定日贡扎剖面Cenomanian-Turonian阶地层格架及碳氧同位素偏移图
@.Coniacian阶; *.D. primitiva带; a.微晶灰岩; b.有孔虫微晶灰岩
Fig. 2. Stratigraphic framework and δ13C and δ18O value variations of the Cenomanian-Turonian at the Gongzha Section, Tingri
图 3 西藏岗巴宗山剖面Cenomanian-Turonian阶地层格架及碳氧同位素偏移
@.Coniacian阶; *.D. primitiva带; 细虚线为生物地层大致划分的CTB; 粗虚线为碳同位素推荐的CTB
Fig. 3. Stratigraphic framework and δ13C and δ18O value variations of the Cenomanian-Turonian at the Zongshan Section, Gamba
图 4 Cenomanian-Turonian期全球碳同位素偏移平滑曲线对比
最左侧综合曲线为时间比例尺的综合曲线, 其他为厚度比例尺的平滑曲线.粗点线为Turonian中期、Turonian中-晚期界线碳同位素负偏事件对比线.岗巴剖面虚线是依据碳同位素偏移曲线推荐的CTB.δ13C的单位为10-3PDB
Fig. 4. Global correlation of the smoothed δ13C value curves in the Cenomanian-Turonian
表 1 西藏定日贡扎剖面Cenomanian-Turonian期碳氧同位素比值
Table 1. Carbon and oxygen isotopic values in the Cenomanian-Turonian at the Gongzha Section, Tingri
表 2 西藏岗巴宗山剖面Cenomanian-Turonian期碳氧同位素比值
Table 2. Carbon and oxygen isotopic values in the Cenomanian-Turonian at the Zongshan Section, Gamba
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