华南陡山沱组上部碳同位素负偏的磁性地层学约束

The Shuram negative carbon excursion (SE; the negative carbon excursion of the Upper Doushantuo Formation), recorded from globally distributed carbonate sequences from the Ediacaran period, is the largest amplitude inorganic carbon isotope anomaly ever discovered. Due to the lack of chronologic constraints, the global synchroneity of the SE is obscure and thus many hypotheses were proposed to decipher the excursion's origin. Recently, new magnetostratigraphic results had been gained from the SE sections of Australia and North America. Since magnetic field reversals are charterised by its global synchroneity, it is proposed to constraint the synchroneity of the SE by use of magnetostratigraphy. Carbon isotope of the Doushantuo Formation has been intensively observed around the Huangling Anticline, South China. But the published paleomagnetic data are still not enough for manetostratigraphy. Fortunately, the proposer has got some good data from the original paleomagnetic studies on the Doushantuo Formation around the Huangling Anticline. Based on this, high-resolution magnetostratigraphic research will be performed on the upper Doushantuo Formation around the Huangling Anticline. This will provide constraints on the synchroneity of the SE, by comparing with the results from Australia and North America.

埃迪卡拉纪Shuram碳同位素负偏是一次显著的全球范围碳同位素扰动，华南陡山沱组上部碳同位素负偏被认为可与其对比，但由于缺少年代学数据的约束，这一负偏的全球等时性尚无定论，其成因也因此存在多种假说。最近，澳大利亚和北美Shuram碳同位素负偏层段的磁性地层学研究取得了新进展。地磁场倒转具有全球等时性的特点，因而可以通过磁性地层学研究来约束这一碳同位素负偏的全球等时性。华南黄陵隆起周缘陡山沱组的碳同位素地层研究程度很高，但已有的古地磁数据尚不足以建立磁性地层格架。申请人在黄陵隆起周缘陡山沱组已开展了初步的古地磁研究，从部分样品中获得了可靠的结果。基于此，本项目拟选择华南黄陵隆起周缘陡山沱组上部碳同位素负偏层段开展高分辨率磁性地层学研究，通过与澳大利亚和北美结果的对比，约束该碳同位素负偏的全球等时性。

陡山沱组上部碳同位素负偏/Shuram负偏是地质历史时期最为显著的一次碳酸盐岩碳同位素负偏，可能记录了地球表生环境的剧烈变化，但其发生的时间、持续时长及全球同时性尚存争议。基于澳大利亚和北美Shuram负偏层段的磁性磁层学新进展和地磁场倒转的全球等时性，项目拟通过华南陡山沱组碳同位素负偏层段的磁性地层学研究约束其全球等时性。本项目在黄陵隆起周缘的5个剖面开展了古地磁学和岩石磁学研究，并在其中1个剖面进行了旋回地层学分析，取得的主要研究进展和成果包括：（1）从多个剖面陡山沱组获得了初步古地磁结果。（2）利用旋回地层学研究，精确约束了陡山沱组上部碳同位素负偏/Shuram负偏的持续时间为6.1 ± 0.2 Myr，结合最新的近似结束时间567.3 ± 3.0 Ma（Re-Os年龄），约束其起始时间为573.4 ± 3.0 Ma；华南多个剖面的陡三段和陡四段之间存在至少1 Myr的沉积间断，陡四段所记录的碳同位素负偏实为post-Shuram负偏，阿曼、澳大利亚和美国西部的死谷地区可能也存在类似现象；在Shuram负偏的全球等时框架下，揭示出华南和全球各剖面δ13C曲线的形状和变化幅值存在显著差异，可能受局部沉积环境中的同位素分馏和/或成岩作用影响所致。