A Significant Change of the Carbon Cycle through the Pc/C Boundary to Early Cambrian: Evidence from the C-Isotopic Records of a Drilling Core in Three Gorges Area, S China

Large fluctuations of carbon isotopic-records for carbonate carbon (d13Ccarb) across the Precambrian/Cambrian (Pc/C) boundary indicate changes of the carbon cycle. We have proposed a representative d13Ccarb chemostratigraphy across the Pc/C boundary and recognized its large fluctuations in the Three Gorges area, South China (Ishikawa et al., 2008). Meanwhile, the d13C values for total organic carbon (d13Corg) have been rarely reported together with the d13Ccarb in this period. Hence, we report high-resolution d13Corg chemostratigraphy of drill core samples through the Pc/C boundary to Early Cambrian in the Three Gorges area, South China. By comparison between the d13Ccarb and d13Corg, we recognized two different terms. First term from the Pc/C boundary to early Nemakit-Daldynian (ND) is characterized by invariant d13Corg and large fluctuated d13Ccarb. Second term from middle ND to Atdabanian is distinctive in a parallel behavior of the d13Ccarb and d13Corg. The latter is consistent with the conventional, steady-state models of the carbon cycle. In contrast, the former is similar to the Neoproterozoic carbon cycle with two reactive pools of inorganic and organic carbon (Rothman et al., 2003). We consider the d13Corg of an unusually large oceanic reservoir of organic carbon would not be driven by the variation of input and output fluxes. We suggest the negative d13Ccarb anomaly across the Pc/C boundary resulted from the increased remineralization of this large reservoir. Furthermore, we identified a period of the diminution of this reservoir in the ND. We also estimated the fraction of burial organic carbon in the middle ND to Atdabanian, and found the organic carbon burial was enhanced in the late ND and then it increased from Tommotian to Atdabanian after reduction in basal-Tommotian. Hence, it implies the lowering pCO2 and the subsequent global cooling in the late ND. This possibly caused the global-scale regression in the basal-Tommotian (Ripperdan, 1994).