The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the Three Gorges area, South China
The Ediacaran radiogenic Sr isotope excursion in the Doushantuo Formation in the Three Gorges area, South China
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DOI：
10.1016/j.precamres.2009.10.006
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发表时间：
2010
影响因子：
3.8
通讯作者：
Y. Sawaki;T. Ohno;Miyuki Tahata;T. Komiya;T. Hirata;S. Maruyama;B. Windley;Jian Han;D. Shu
中科院分区：
文献类型：
--
作者：
Y. Sawaki;T. Ohno;Miyuki Tahata;T. Komiya;T. Hirata;S. Maruyama;B. Windley;Jian Han;D. Shu
埃迪卡拉时期是生命进化最重要的时期之一。然而，保存完好的埃迪卡拉岩石露头的稀缺仍然在解码周围地表环境变化和生物进化方面留下了模糊性。华南埃迪卡拉系地层几乎连续出露，以碳酸盐岩为主，次之黑色页岩和砂岩，化石较多，适合研究埃迪卡拉系环境和生物变化。我们在三峡地区的四个地点对陡山沱组进行了钻探，获得了连续、新鲜的样品，没有表面蚀变和氧化。我们分析了新鲜碳酸盐岩的 87Sr/86Sr 和 88Sr/86Sr 比值，这些比值是根据显微观察和 Sr 含量、Mn/Sr 和 Rb/Sr 比值以及 δ18O 值的地球化学特征，使用多收集器电感耦合等离子体质谱仪 (MC-ICP-MS) 进行选择的。钻探样品的 87Sr/86Sr 比率的化学地层学显示埃迪卡拉时期平滑的曲线和两个大的正移。 δ13C、δ18O 和 87Sr/86Sr 值的详细化学地层学与 Mn 和 Fe 含量的结合使我们能够解码埃迪卡拉纪地表环境变化及其原因。 87Sr/86Sr 的第一次大正偏移与负δ13C 和正δ18O 偏移一起发生。较高的 87Sr/86Sr 值表明大陆风化作用增强，而 δ18O 正偏移表明全球变冷。全球变冷导致的全球退化加剧了暴露的海洋有机沉积物的氧化腐烂和大陆风化。营养物质的加速流入促进了初级生产力，导致溶解有机碳（DOC）的氧化，而来自大陆的硫酸盐流入量增加导致硫酸盐活性还原，导致大量 DOC 的再矿化，这两者都导致了 δ13C 负异常。 580Ma Gaskiers冰川作用解释了正87Sr/86Sr、负δ13C和正δ18O偏移之间的密切相关性。 87Sr/86Sr 的第二次大正移首先伴随着 δ13C 正偏移，并持续到 Shuram δ13C 负偏移。 δ13C 和 87Sr/86Sr 值的正相关性与由于大陆碰撞形成跨冈瓦纳山脉而增强的大陆风化速率一致，并且由于大陆风化的增强以及随之而来的海水中更高的营养物含量而具有较高的初级活动。 Mn和Fe含量的增加意味着由于海洋中流动的有氧呼吸更加活跃和还原物质的氧化，海水氧含量逐渐下降。在 Shuram 偏移中，较高的 87Sr/86Sr 值以及 Mn 和 Fe 含量从增加到减少的转变伴随着大的负 δ13C 偏移。较高的 87Sr/86Sr 值是大陆风化增强的第一个令人信服的证据，这是由于硫酸盐流入量增加导致更活跃的硫酸盐还原导致 DOC 再矿化而导致大的 δ13C 异常的原因。偏移早期和中期较高的Mn和Fe含量表明由于DOC的氧化衰变导致海水氧含量下降，而在后期Mn和Fe含量下降与海洋氧化作用一致。加斯基尔斯冰川作用后埃迪卡拉生物群的出现和最新埃迪卡拉生物群的繁荣伴随着更多放射源的形成……
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 Fm at four sites in the Three Gorges area to obtain continuous, fresh samples without surface alteration and oxidation. We analyzed87Sr/86Sr and88Sr/86Sr 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 δ18O values, with a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS). The chemostratigraphy of the87Sr/86Sr ratios of the drilled samples displays a smooth curve and two large positive shifts during Ediacaran time. The combination of the detailed chemostratigraphies of δ13C, δ18O and87Sr/86Sr 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 of87Sr/86Sr occurred together with negative δ13C and positive δ18O excursions. The higher87Sr/86Sr values indicate an enhancement of continental weathering, whereas the positive δ18O 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 δ13C anomaly. The 580Ma Gaskiers glaciation accounts for the close correlation among the positive87Sr/86Sr, negative δ13C and positive δ18O excursions. The second large positive shift of87Sr/86Sr firstly accompanied a positive δ13C excursion, and continued through the Shuram δ13C negative excursion. The positive correlation of δ13C and87Sr/86Sr 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, higher87Sr/86Sr values and a transition from increase to decrease in Mn and Fe contents were accompanied by the large negative δ13C excursion. The higher87Sr/86Sr values are the first compelling evidence for enhanced continental weathering, which was responsible for the large δ13C 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 …