科研之友
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摘要
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 (delta C-13(carb)) 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 delta C-13(org) chemostratigraphy 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 delta C-13(org) and delta C-13(carb). We found that changes of delta C-13(org) and delta C-13(carb) values were decoupled during the significant negative excursion of delta C-13(carb) across 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 delta C-13(carb) and delta C-13(org) changed 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 delta C-13(carb) masses of the inorganic and organic carbon pools and the fluxes between them. As a result, we show that the observed negative delta C-13(carb) excursion with steady delta C-13(org) can 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 delta C-13(carb) excursion 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.
