Mo isotope and trace element patterns of Lower Cambrian black shales in South China: Multi-proxy constraints on the paleoenvironment
Mo isotope and trace element patterns of Lower Cambrian black shales in South China: Multi-proxy constraints on the paleoenvironment
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DOI：
10.1016/j.chemgeo.2012.05.016
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发表时间：
2012-07-15
期刊：
影响因子：
3.9
通讯作者：
Escher, Peter
中科院分区：
文献类型：
--
作者：
Xu, Lingang;Lehmann, Bernd;Escher, Peter
前寒武纪/寒武纪边界标志着地球历史上的一个关键时期。海洋的古环境变化被认为是海洋生物进化（“寒武纪大爆发”）等一些重大事件的可能解释。在此，我们报告了华南扬子地台早寒武世定泰剖面（贵州省）黑色页岩和三个同沉积多金属镍钼（PGE-Au）硫化物矿床（贵州大竹流水和马鹿河以及湖南三岔）的微量元素、铁形态和钼同位素特征的显着变化范围。根据（同位素）地球化学特征，我们将定泰剖面分为三个层段：层段1（0-1 m）具有极端的金属和TOC富集和重的δ（98）/Mo-95值。该单元具有纯环境的所有特征，然后是具有可变 delta(98)/Mo-95 成分的区间 2 (1-11 m)。氧化还原敏感元素含量及其比例升高，以及相对较高的 Fe-HR/Fe-T 比例（“活性铁”）表明存在缺氧条件。上覆层段3（11~33 m）为含氧至缺氧环境，最下部δ(98)/Mo-95值一般较轻，最上部δ(98)/Mo-95值波动较大，氧化还原敏感元素含量相对较低。我们提出了一个部分封闭的海洋盆地模型，该模型具有周期性补给以及不断变化的氧化还原条件，以应对 delta(98)/Mo-95 值的显着变化。轻δ(98)/Mo-95值表明有限的海水环流以局部大陆输入为主，周期性地由具有重δ(98)/Mo-95的淡水海水补充。全球黑色页岩Mo同位素数据的汇编表明，Mo同位素变化具有一致的时间趋势，反映了海洋的氧演化。与现代常生沉积物相比，下寒武统黑色页岩的 delta(98)/Mo-95 值相对较轻，表明早寒武世存在更广泛的缺氧海洋环境。 (C) 2012 Elsevier B.V. 保留所有权利。
The Precambrian/Cambrian boundary marks a critical interval in Earth history. Paleoenvironmental changes of the oceans have been suggested as a possible explanation for some significant events such as the marine biological evolution ("Cambrian explosion"). Here we report a remarkable variation range in trace element, iron speciation and molybdenum isotope signatures of black shales from the Early Cambrian Dingtai profile (Guizhou province) and three synsedimentary polymetallic Ni-Mo-(PGE-Au) sulfide ore deposits (Dazhuliushui and Maluhe in Guizhou province, and Sancha in Hunan province) on the Yangtze Platform, South China. Based on the (isotope) geochemical features, we classify the Dingtai profile into three intervals: interval 1 (0-1 m) has extreme metal and TOC enrichment and heavy delta(98)/Mo-95 values. This unit has all features of a euxinic environment, and is then followed by interval 2 (1-11 m) with variable delta(98)/Mo-95 composition. Elevated contents of redox-sensitive elements and their ratios, and relatively high Fe-HR/Fe-T ratios ("reactive iron") suggest anoxic conditions. The overlying interval 3 (11-33 m) reveals oxic to suboxic conditions with generally light delta(98)/Mo-95 values in the lowermost part and fluctuant delta(98)/Mo-95 values in the uppermost part, and relatively low redox-sensitive element contents. We propose a model of a partly closed ocean basin with periodic replenishment together with changing redox conditions for the remarkable variability of delta(98)/Mo-95 values. Light delta(98)/Mo-95 values indicate restricted seawater circulation dominated by local continental input, periodically replenished by fresh seawater with heavy delta(98)/Mo-95.A global compilation of Mo isotope data from black shales indicates that the Mo isotopic variations have a consistent temporal trend, reflecting the oxygen evolution of the oceans. Relatively light delta(98)/Mo-95 values of the Lower Cambrian black shales compared to those of modern euxinic sediments are indicative of more widespread anoxic marine environments during the Early Cambrian. (C) 2012 Elsevier B.V. All rights reserved.