Key Laboratory of Petroleum Geochemistry, Research Institute of Petroleum Exploration and Development, China National Petroleum Corporation, Beijing, 100083, China.
Institute of Biology and Nordic Center for Earth Evolution (NordCEE), University of Southern Denmark, Campusvej 55, 5230, Odense M, Denmark.
Sci Rep. 2018 Sep 6;8(1):13324. doi: 10.1038/s41598-018-31378-6.
The Xiamaling Formation in the North China Block contains a well-preserved 1400 Ma sedimentary sequence with a low degree of thermal maturity. Previous studies have confirmed the dynamic and complex nature of this evolving marine setting, including the existence of an oxygen-minimum zone, using multi-proxy approaches, including iron speciation, trace metal dynamics, and organic geochemistry. Here, we investigate the prevailing redox conditions during diagenesis via the biomarkers of rearranged hopanes from the finely laminated sediments of the organic-rich black shales in Units 2 and 3 of the Xiamaling Formation. We find that rearranged hopanes are prominent in the biomarker composition of the oxygen-minimum zone sediment, which is completely different from that of the sediment in the overlying anoxic strata. Since the transition process from hopanes to rearranged hopanes requires oxygen via oxidation at the C-l6 alkyl position of 17α(H)-hopanes, we infer that dissolved oxygen led to the transformation of hopane precursors into rearranged hopanes during the early stages of diagenesis. The use of hopanoid hydrocarbons as biomarkers of marine redox conditions has rarely been previously reported, and the hydrocarbon signatures point towards oxic bottom waters during the deposition of Unit 3 of the Xiamaling Formation, which is consistent with the earlier oxygen-minimum zone environmental interpretation of this Unit.
华北板块下马岭组含有保存完好的 1400Ma 沉积序列，热成熟度较低。先前的研究已经通过铁形态、微量元素动态和有机地球化学等多指标方法证实了这种演化海洋环境的动态和复杂性，包括存在缺氧带。在这里，我们通过富含有机物的黑色页岩中精细层理沉积物的重排藿烷生物标志物，研究了下马岭组 2 号和 3 号单元成岩过程中的主要氧化还原条件。我们发现，重排藿烷在缺氧带沉积物的生物标志物组成中非常突出，这与上覆缺氧地层的沉积物完全不同。由于重排藿烷从藿烷转化为重排藿烷需要通过 17α(H)-藿烷 C-l6 烷基位置的氧化来提供氧气，因此我们推断，在成岩作用的早期阶段，溶解氧导致了藿烷前体转化为重排藿烷。利用藿烷类碳氢化合物作为海洋氧化还原条件的生物标志物以前很少有报道，而这些碳氢化合物的特征表明，在下马岭组 3 号单元的沉积过程中，底层水是含氧的，这与该单元早期的缺氧带环境解释是一致的。
