科研之友
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摘要
Cap Carbonates overlie the Marinoan Snowball Earth-related glacial diamictite, and possibly record the drastic surface environmental change and biological evolution after the Snowball Earth. We conducted on-land drilling from the Liantuo Formation, through the Nantuo, to the lower Doushantuo Formation in the Three Gorges area of South China to collect fresh, continuous samples in the Three Gorges area. We obtained high-resolution chemostratigraphies of delta C-13 and delta O-18 values of carbonates from the topmost part of the Nantuo Formation to the Cap Carbonate, in order to decode the detailed surface environmental change in the shallow marine setting. The delta C-13 chemostratigraphy possesses some unique characteristics: (1) stable delta C-13 values as a whole, but ubiquitous low delta C-13 anomalies through the Cap Carbonate, (2) increase of the delta C-13 values from -3 to +5 parts per thousand across the C2/C3 boundary, (3) no delta C-13 anomaly between the C1 and C2 boundary, and (4) presence of an anomalous high delta C-13 value (+2.3 parts per thousand) and a faint positive correlation between delta C-13 and delta O-18 values in the C1 unit. Evidence of quite low delta C-13 anomalies (with a nadir of -41 parts per thousand), ubiquitous negative delta C-13 anomalies through the Cap Carbonate, and a high delta C-13 anomaly accompanied with a faint positive correlation between delta C-13 and delta O-18 values in the C1 unit supports decomposition and formation of methane hydrate during Cap Carbonate formation. The drastic increase of delta C-13 values from the upper C2 to C3 units indicates enhancement of primary productivity and organic carbon burial, possibly due to high continental fluxes after the Snowball Earth event, evidenced by high Sr isotope values. The increase is restricted to the proximal side of the inner shelf in South China, and the timing of the increase of delta C-13 values of carbonates is earlier at Three Gorges area than any other area, suggesting that the enhancement of primary productivity started in the proximal environment because of higher continental influxes. The increase in oxygen contents of seawater due to the enhanced primary productivity possibly resulted in the emergence of multicellular animals soon after Cap Carbonate deposition.
