Carbon isotope chemostratigraphy of a Precambrian/Cambrian boundary section in the Three Gorge area, South China: Prominent global-scale isotope excursions … (original) (raw)

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 δ 13 C curve of the Early Cambrian has been poorly established mainly due to the lack of a continuous stratigraphic record. Here we report high-resolution δ 13 C 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 Fm.), through a lowermost Early Cambrian muddy limestone (Yanjiahe Fm.) 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 δ 13 C carb increases moderately from 0 to + 2‰ (positive excursion 1: P1), and then drops dramatically down to −7‰ (negative excursion 1: N1). Subsequently, the δ 13 C carb increases continuously up to about + 5‰ at the upper part of the Nemakit-Daldynian stage. After this positive excursion, δ 13 C carb sharply decreases down to about −9‰ (N2) just below the basal Tommotian unconformity. These continuous patterns of the δ 13 C shift are irrespective of lithotype, suggesting a primary origin of the record. Moreover, the obtained δ 13 C 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 δ 13 C profile best represents the global change in seawater chemistry. The minimum δ 13 C of the N1 (−7‰) is slightly lower than carbon input from the mantle, thus implying an enhanced flux of 13 C-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 δ 13 C rise up to +5‰ (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 δ 13 C (−9‰) 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.