College of Earth Science and Engineering, Shandong University of Science and Technology, 579 Qianwangang Road, Huangdao District, Qingdao, 266590, Shandong Province, People's Republic of China.
Chengdu Institute of Geology and Mineral Resources, China Geological Survey, Chengdu, 610081, Sichuan Province, People's Republic of China.
Sci Rep. 2022 Aug 1;12(1):13186. doi: 10.1038/s41598-022-17105-2.
The Ordovician-Silurian transition was a critical period in geological history, during which profound changes in climatic, biotic, and oceanic conditions occurred. To explore the provenance, palaeoclimate, and palaeoredox conditions in the Sichuan Basin during the Late Ordovician-early Silurian interval, we conducted mineralogical, geochemical, and isotopic analyses of three formations (Wufeng, Guanyinqiao and Longmaxi formations) in the Xindi No. 2 well. The ternary and bivariate diagrams indicate that the provenance is mainly felsic igneous rocks and originated mainly from a collisional setting, presumably due to an active continental margin. The chemical index of alteration (CIA) values in the lower Wufeng and Longmaxi formations are relatively high (67.48-73.57), indicating a warm and humid climate. In contrast, the CIA values declined rapidly (58.30-64.66) during the late Katian to early Hirnantian, which had a fluctuating cold and dry climate and was interrupted by a transient warm and humid climate. The palaeoredox indices (Mo concentrations and Mo/U, U/Th, V/Cr, Ni/Co, and V/V + Ni values) during the Late Ordovician-early Silurian indicate two cycles of water column euxinia. The first cycle occurred during Wufeng Formation deposition, with bottom waters evolving from oxic-suboxic to suboxic-anoxic. Most samples show relatively low redox-sensitive trace element concentrations during the Guanyinqiao Formation, pointing to oxic-suboxic conditions. The second cycle, during the late Hirnantian, transitioned from oxic to euxinic water conditions. Our δC data are comparable to previously reported records and exhibit a strong correlation between the Hirnantian isotopic carbon excursion (HICE), climate change, and redox conditions. We suggest that the variations in the δC values are related to two elements: (1) increased photosynthetic activity under oxic water conditions, and (2) increased carbonate weathering exposed by the glacio-eustatic sea- level. In addition, the high δC values might indicate a more shelf-proximal setting during Xindi No. 2 well deposition. The δC isotopic data effectively constrain the timing of the Late Ordovician mass extinction (LOME) and the evolution of the temporal changes in the climatic and ocean redox conditions, suggesting an apparent stratigraphic coincidence between climate and redox fluctuations and two-phase extinctions, which implies a strong causal relationship. The LOME was systematically driven by the combination of cooler glacial temperatures, glacio-eustatic sea-level fluctuations, and anoxic water conditions that caused the two pulses of extinction in the Yangtze shelf sea.
奥陶纪-志留纪过渡时期是地质历史上的一个关键时期，在此期间，气候、生物和海洋条件发生了深刻的变化。为了探究四川盆地晚奥陶世-早志留世期间的物源、古气候和古氧化还原条件，我们对新地 2 号井的三个地层（五峰组、观音桥组和龙马溪组）进行了矿物学、地球化学和同位素分析。三元和二元图解表明，物源主要为长英质火成岩，主要来源于碰撞环境，可能是由于活动大陆边缘。下五峰组和龙马溪组的化学蚀变指数（CIA）值相对较高（67.48-73.57），表明气候温暖湿润。相比之下，在大灭绝晚期至早赫南特期，CIA 值迅速下降（58.30-64.66），气候波动不定，寒冷干燥，期间短暂出现温暖湿润气候。晚奥陶世-早志留世的古氧化还原指标（Mo 浓度和 Mo/U、U/Th、V/Cr、Ni/Co 和 V/V+Ni 值）表明，水柱缺氧事件发生了两个循环。第一个循环发生在五峰组沉积期间，底层水从氧化-亚缺氧演变为亚缺氧-缺氧。观音桥组的大多数样品显示相对较低的氧化还原敏感微量元素浓度，表明氧化-亚缺氧条件。第二个循环发生在晚赫南特期，从氧化转变为缺氧水条件。我们的δC 数据与以前的报告记录相当，并显示出赫南特同位素碳激变（HICE）、气候变化和氧化还原条件之间的强烈相关性。我们认为，δC 值的变化与两个因素有关：（1）在含氧水条件下光合作用活性的增加，以及（2）冰川海平面变化暴露的碳酸盐风化作用的增加。此外，高 δC 值可能表明新地 2 号井沉积期间更靠近陆架的位置。δC 同位素数据有效地约束了晚奥陶世大灭绝（LOME）的时间，并揭示了气候和海洋氧化还原条件随时间变化的演化，表明气候和氧化还原波动与两个阶段灭绝之间存在明显的地层一致性，这意味着两者之间存在强烈的因果关系。LOME 是由较冷的冰川温度、冰川海平面波动和缺氧水条件的综合作用驱动的，这导致了扬子陆架海的两次灭绝脉冲。
