The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the Three Gorges area, South China

摘要

The Ediacaran period was one of the most important times for the evolution of life. However, the scarcity of well-preserved outcrops of Ediacaran rocks still leaves ambiguity in decoding ambient surface environmental changes and biological evolution. The Ediacaran strata in South China are almost continuously exposed, comprise mainly carbonate rocks with subordinate black shales and sandstones, and they contain many fossils, suitable for study of environmental and biological changes in the Ediacaran. We conducted drilling through the Doushantuo I'm at four sites in the Three Gorges area to obtain continuous, fresh samples without surface alteration and oxidation. We analyzed Sr-87/Sr-86 and Sr-88/Sr-86 ratios of the fresh carbonate rocks, selected on the basis of microscopic observations and the geochemical signatures of Sr contents, Mn/Sr and Rb/Sr ratios, and delta O-18 values, with a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS). The chemostratigraphy of the Sr-87/Sr-86 ratios of the drilled samples displays a smooth curve and two large positive shifts during Ediacaran time. The combination of the detailed chemostratigraphies of delta C-13, delta O-18 and Sr-87/Sr-86 values and Mn and Fe contents enables us to decode the surface environmental changes and their causes in the Ediacaran. The first large positive excursion of Sr-87/Sr-86 occurred together with negative delta C-13 and positive delta O-18 excursions. The higher Sr-87/Sr-86 values indicate an enhancement of continental weathering, whereas the positive delta O-18 excursion suggests global cooling. Global regression due to global cooling enhanced the oxidative decay of exposed marine organic sediments and continental weathering. Accelerated influx of nutrients promoted primary productivity, resulting in oxidation of dissolved organic carbon (DOC), whereas active sulfate reduction due to a higher sulfate influx from the continents caused remineralization of the large DOC, both of which caused a negative delta C-13 anomaly. The 580 Ma Gaskiers glaciation accounts for the close correlation among the positive Sr-87/Sr-86, negative delta C-13 and positive delta O-18 excursions. The second large positive shift of Sr-87/Sr-86 firstly accompanied a positive delta C-13 excursion, and continued through the Shuram delta C-13 negative excursion. The positive correlation of delta C-13 and Sr-87/Sr-86 values is consistent with an enhanced continental weathering rate due to continental collisions that built Trans-Gondwana mountain chains, and with a higher primary activity due to the enhancement of continental weathering and consequent higher nutrient contents in seawater. The accompanied increase in Mn and Fe contents implies a gradual decline of the seawater oxygen content due to more active aerobic respiration and oxidation of reductive materials flowing in the oceans. In the Shuram excursion, higher Sr-87/Sr-86 values and a transition from increase to decrease in Mn and Fe contents were accompanied by the large negative delta C-13 excursion. The higher Sr-87/Sr-86 values are the first compelling evidence for enhanced continental weathering, which was responsible for the large delta C-13 anomaly through the remineralization of the DOC by more active sulfate reduction due to a higher sulfate influx. Higher Mn and Fe contents in the early and middle stages of the excursion suggest a decline in the oxygen content of seawater due to oxidative decay of the DOC, whereas in the late stages the decrease in Mn and Fe contents is consistent with oceanic oxygenation. The emergence of Ediacara biota after the Gaskiers glaciation and the prosperity of the latest Ediacaran is concomitant with the formation of more radiogenic seawater with high Sr-87/Sr-86 values, suggesting that enhanced continental weathering, and the consequent higher influx of nutrients, played an important role in biological evolution.