Irreversible change of the oceanic carbon cycle in the earliest Cambrian: High-resolution organic and inorganic carbon chemostratigraphy in the Three Gorges area, South China
Irreversible change of the oceanic carbon cycle in the earliest Cambrian: High-resolution organic and inorganic carbon chemostratigraphy in the Three Gorges area, South China
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DOI：
10.1016/j.precamres.2011.10.004
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发表时间：
2013-02
影响因子：
3.8
通讯作者：
T. Ishikawa;Y. Ueno;D. Shu;Yong Li;Jian Han;Junfeng Guo;N. Yoshida;T. Komiya
中科院分区：
文献类型：
--
作者：
T. Ishikawa;Y. Ueno;D. Shu;Yong Li;Jian Han;Junfeng Guo;N. Yoshida;T. Komiya
新元古代末期及其向寒武纪的过渡经历了重大的演化和地球化学变化。前寒武系/寒武系界线(Pc/C界线)无机碳(δ13Ccarb)碳同位素比值的大幅波动表明当时大洋碳循环发生了重大变化。本文首次对三峡地区中国南部Pc/C界线上的岩心样品进行了高分辨率δ~(13)Corg化学地层学研究，并根据δ~(13)Corg与δ~(13)Ccarb的关系讨论了碳循环。我们发现，在δ~(13)C跨越Pc/C边界的显著负漂移过程中，δ~(13)Corg和δ~(13)C碳值的变化是解耦的，这表明存在一个巨大的有机碳库。此前，大型OCP被推断为在元古界末期建立，并在埃迪卡拉纪中期舒拉姆游程结束时消失。相反，我们的研究表明，大型OCP一直持续到Nemakit-Daldyian早期。另一方面，在内马基特-达尔迪期至阿达巴期中期，δ~(13)C和δ~(13)Corg平行变化，这与显生界通常观察到的情况相同。为了定量评估这些情景，我们进行了数值模拟，计算了δ13Ccarb的时间演化、无机和有机碳库的质量以及它们之间的通量。结果表明，如果去除有机碳的速率常数比今天低得多，那么观察到的δ13C碳漂移和稳定的δ13Corg可以通过提高初级生产力和大OCP的再矿化而产生。脱钩的负δ13C碳漂移的结束归因于海洋中无机碳和有机碳的去除率的增加。有机碳埋藏通量的增强消除了大型有机碳化合物的存在，从而在Nemakit-Daldyian早期建立了现代碳循环。推测Nemakit-Daldyian中期有机碳和无机碳埋藏率的快速增加可能是由浮游后生动物的出现和第一骨骼动物的出现造成的。生物创新可能已经不可逆转地改变了恰好在Pc/C边界之后的海洋碳循环。
The terminal Neoproterozoic and its transition into the Cambrian witnessed major evolutionary and geochemical changes. Large fluctuations in the carbon isotope ratio for inorganic carbon (δ13Ccarb) across the Precambrian/Cambrian boundary (Pc/C boundary) indicate a significant change in the oceanic carbon cycle at that time. This work presents the first high-resolution δ13Corgchemostratigraphy of drill core samples across the Pc/C boundary in the Three Gorges area, South China, and discusses carbon cycles based on the relation between δ13Corgand δ13Ccarb. We found that changes of δ13Corgand δ13Ccarbvalues were decoupled during the significant negative excursion of δ13Ccarbacross the Pc/C boundary, suggesting the presence of a huge organic carbon pool (OCP). Previously, the large OCP was inferred to have been established in the terminal Proterozoic and to have disappeared at the end of the mid-Ediacaran Shuram excursion. In contrast, our study suggests that the large OCP lasted until the early Nemakit-Daldynian stage. On the other hand, in the middle Nemakit-Daldynian to Atdabanian, the δ13Ccarband δ13Corgchanged in parallel as observed normally in the Phanerozoic. In order to evaluate these scenarios quantitatively, we performed numerical simulations and calculated the time evolution of the δ13Ccarb, masses of the inorganic and organic carbon pools and the fluxes between them. As a result, we show that the observed negative δ13Ccarbexcursion with steady δ13Corgcan be produced by enhanced primary productivity and remineralization of the large OCP, if the rate constant for removal of organic carbon was considerably lower than that of today. The end of the decoupled negative δ13Ccarbexcursion is attributable to an increase in the removal rates of inorganic and organic carbon from the ocean. The enhanced flux of organic carbon burials eliminated the large OCP, so that the modern-style carbon cycle was established by the early Nemakit-Daldynian stage. The inferred rapid increases of both organic and inorganic carbon burial rates in the middle Nemakit-Daldynian stage were likely caused by the appearance of planktonic metazoans with guts making fecal pellets and by the first skeletal animals. The biological innovations may have changed the oceanic carbon cycle irreversibly just after the Pc/C boundary.