Carbon isotope chemostratigraphy of a Precambrian/Cambrian boundary section in the Three Gorge area, South China: Prominent global-scale isotope excursions just before the Cambrian Explosion

摘要

Carbon isotope chemostratigraphy has been used for worldwide correlation of Precambrian/Cambrian (Pc/C) boundary sections, and has elucidated significant change of the carbon cycle during the rapid diversification of skeletal metazoa (i.e. the Cambrian Explosion). Nevertheless, the standard delta C-13 curve of the Early Cambrian has been poorly established mainly due to the lack of a continuous stratigraphic record. Here we report high-resolution delta C-13 chemostratigraphy of a drill core sample across the Pc/C boundary in the Three Gorge area, South China. This section extends from an uppermost Ediacaran dolostone (Dengying Fin.), through a lowermost Early Cambrian muddy limestone (Yanjiahe Fitt.) to a middle Early Cambrian calcareous black shale (Shuijingtuo Fm.). As a result, we have identified two positive and two negative isotope excursions within this interval. Near the Pc/C boundary, the delta C-13(carb) increases moderately from 0 to +2 parts per thousand (positive excursion 1: P1), and then drops dramatically down to -7 parts per thousand (negative excursion 1: N1). Subsequently, the delta C-13(carb) increases continuously up to about +5 parts per thousand at the upper part of the Nemakit-Daldynian stage. After this positive excursion, delta C-13(carb) sharply decreases down to about -9 parts per thousand (N2) just below the basal Tommotian unconformity. These continuous patterns of the delta C-13 shift are irrespective of lithotype, suggesting a primary origin of the record. Moreover, the obtained delta C-13 profile, except for the sharp excursion N2, is comparable to records of other sections within and outside of the Yangtze Platform. Hence, we conclude that the general feature of our delta C-13 profile best represents the global change in seawater chemistry. The minimum delta C-13 of the NI (-7 parts per thousand) is slightly lower than carbon input from the mantle, thus implying an enhanced flux of C-13-depleted carbon just across the Pc/C boundary. Hence, the ocean at that time probably became anoxic, which may have affected the survival of sessile or benthic Ediacaran biota. The subsequent delta C-13 rise up to +5 parts per thousand (P2) indicates an increase of primary productivity or an enhanced rate of organic carbon burial, which should have resulted in lowering pCO(2) and following global cooling. This scenario accounts for the cause of the global-scale sea-level fall at the base of the Tommotian stage. The subsequent, very short-term, and exceptionally low delta C-13 (-9 parts per thousand) in N2 could have been associated with the release of methane from gas hydrates due to the sea-level fall. The inferred dramatic environmental changes (i.e., ocean anoxia, increasing productivity, global cooling and subsequent sea-level fall with methane release) appear to coincide with or occur just before the Cambrian Explosion. This may indicate synchronism between the environmental changes and rapid diversification of skeletal metazoa.