The mid-Proterozoic was a time of apparent prolonged biological and geochemical stability, preceeding the environmental turmoil and rapid biological innovations that characterized the Neoproterozoic. Despite an upswing in work on the mid-Proterozoic over the past decade, basic aspects of the carbon cycle and Earth's surface redox state during this time period remain poorly understood. To provide a new window into the mid-Proterozoic environmental evolution, we have investigated carbonates in the well-exposed Muskwa Assemblage located in NE British Columbia, Canada, for a combined stratigraphic and geochemical study. Rare Earth Elements and Yttrium (REE+Y) geochemistry was applied to carbonate rocks deposited over a marine paleoenvironmental depth gradient in order to characterize prevailing water column redox conditions. These data provide evidence for an extremely shallow chemocline with oxic surface waters being restricted to within storm-wave base, which is consistent with globally low oxygen in the ocean-atmosphere system. Furthermore, there is evidence for facies-dependent carbonate δ13C variability within the Muskwa Assemblage comparable in scale to typical stratigraphic variability recorded in carbonate strata of this age, highlighting the problems of using small carbon isotope excursions for chemostratigraphy.

中文翻译：

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加拿大不列颠哥伦比亚省中元古代Muskwa组合的浅水氧化还原条件

中元古代是生物和地球化学稳定性明显延长的时期，先于新元古代特征的环境动荡和快速生物创新。尽管在过去十年中元古代中期的工作有所增加，但在此期间碳循环和地球表面氧化还原状态的基本方面仍然知之甚少。为了提供了解中元古代环境演化的新窗口，我们对位于加拿大不列颠哥伦比亚省东北部的暴露良好的马斯克瓦组合中的碳酸盐进行了地层学和地球化学联合研究。将稀土元素和钇 (REE+Y) 地球化学应用于沉积在海洋古环境深度梯度上的碳酸盐岩，以表征主要的水柱氧化还原条件。这些数据为极浅的趋化层提供了证据，表层含氧水被限制在风暴波基地内，这与海洋-大气系统中的全球低氧是一致的。此外，有证据表明 Muskwa 组合内相依赖的碳酸盐 δ13C 变异性在规模上与该时代碳酸盐岩地层中记录的典型地层变异性相当，突出了使用小碳同位素偏移进行化学地层学的问题。