A high-resolution chemostratigraphy of post-Marinoan Cap Carbonate using drill core samples in the Three Gorges area, South China
A high-resolution chemostratigraphy of post-Marinoan Cap Carbonate using drill core samples in the Three Gorges area, South China
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DOI：
10.1016/j.gsf.2015.07.008
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发表时间：
2016-07
影响因子：
8.9
通讯作者：
H. Sato;Miyuki Tahata;Y. Sawaki;S. Maruyama;N. Yoshida;D. Shu;Jian Han;Yong Li;T. Komiya
中科院分区：
文献类型：
--
作者：
H. Sato;Miyuki Tahata;Y. Sawaki;S. Maruyama;N. Yoshida;D. Shu;Jian Han;Yong Li;T. Komiya
盖碳酸盐覆盖在与马里诺雪球地球相关的冰川混积岩之上，可能记录了雪球地球之后剧烈的地表环境变化和生物进化。我们从华南三峡地区的连沱组经南沱一直到陡山沱组下部进行了陆上钻探，在三峡地区采集了新鲜、连续的样品。我们获得了从南沱组顶部到盖碳酸盐岩碳酸盐岩δ13C和δ18O值的高分辨率化学地层图，以解码浅海环境中详细的地表环境变化。 δ13C 化学地层学具有一些独特的特征：(1) 总体上稳定的 δ13C 值，但通过碳酸盐岩帽普遍存在低 δ13C 异常，(2) C2/C3 边界上 δ13C 值从 -3 增加到 +5‰，(3) C1 和 C2 边界之间没有 δ13C 异常，以及 (4) 存在异常 高 δ13C 值（+2.3‰），并且 C1 单元中 δ13C 和 δ18O 值之间存在微弱的正相关性。有证据表明，相当低的 δ13C 异常（最低点为 -41‰），盖碳酸盐岩普遍存在负 δ13C 异常，以及高 δ13C 异常，同时在 C1 单元中 δ13C 和 δ18O 值之间存在微弱的正相关性。 C1 单元支持碳酸盐岩形成过程中甲烷水合物的分解和形成。 δ13C 值从上部 C2 到 C3 单元的急剧增加表明初级生产力和有机碳埋藏的增强，这可能是由于雪球地球事件后大陆通量高所致，高 Sr 同位素值证明了这一点。增加仅限于华南内陆架的近侧，并且三峡地区碳酸盐岩δ13C值增加的时间早于其他地区，表明由于大陆流入量较高，初级生产力的增强是从近端环境开始的。由于初级生产力的提高，海水中氧含量的增加可能导致碳酸盐岩沉积后不久多细胞动物的出现。
Cap Carbonates overlie the Marinoan Snowball Earth-related glacial diamictite, and possibly record the drastic surface environmental change and biological evolution after the Snowball Earth. We conducted on-land drilling from the Liantuo Formation, through the Nantuo, to the lower Doushantuo Formation in the Three Gorges area of South China to collect fresh, continuous samples in the Three Gorges area. We obtained high-resolution chemostratigraphies ofδ13C andδ18O values of carbonates from the topmost part of the Nantuo Formation to the Cap Carbonate, in order to decode the detailed surface environmental change in the shallow marine setting. Theδ13C chemostratigraphy possesses some unique characteristics: (1) stableδ13C values as a whole, but ubiquitous lowδ13C anomalies through the Cap Carbonate, (2) increase of theδ13C values from −3 to +5‰ across the C2/C3 boundary, (3) noδ13C anomaly between the C1 and C2 boundary, and (4) presence of an anomalous highδ13C value (+2.3‰) and a faint positive correlation betweenδ13C andδ18O values in the C1 unit.Evidence of quite lowδ13C anomalies (with a nadir of −41‰), ubiquitous negativeδ13C anomalies through the Cap Carbonate, and a highδ13C anomaly accompanied with a faint positive correlation betweenδ13C andδ18O values in the C1 unit supports decomposition and formation of methane hydrate during Cap Carbonate formation. The drastic increase ofδ13C values from the upper C2 to C3 units indicates enhancement of primary productivity and organic carbon burial, possibly due to high continental fluxes after the Snowball Earth event, evidenced by high Sr isotope values. The increase is restricted to the proximal side of the inner shelf in South China, and the timing of the increase ofδ13C values of carbonates is earlier at Three Gorges area than any other area, suggesting that the enhancement of primary productivity started in the proximal environment because of higher continental influxes. The increase in oxygen contents of seawater due to the enhanced primary productivity possibly resulted in the emergence of multicellular animals soon after Cap Carbonate deposition.