Redox changes in the outer Yangtze Sea (South China) through the Hirnantian Glaciation and their implications for the end-Ordovician biocrisis

Spatiotemporal Evolution and Genesis of the Late Ordovician—Early Silurian Marine Euxinia in Northeastern Upper Yangtze Basin, South China

“…Sulfate inputs to the marine environment are mainly affected by continental weathering, hydrothermal activity, and volcanic activity (Poulton and Canfield, 2011). There was a significant negative correlation between CIA corr and δ 34 S py in Shaba section (r −0.54, p < 0.01, n 24; Figure 8F)shows that the input of terrigenous debris was the main factor affecting the change in δ 34 S py , and also explaining the reason for the increase in δ 34 S py value in the glacial period, that is, an insufficient supply of sulfate and the resulting low ε MSR (Li et al, 2021). The lower CIA corr during the glacial period reduced the sulfate input, thereby limiting the fractionation of sulfur isotopes in the MSR (Algeo et al, 2015), then by being coupled with a decrease in OM flux, resulted in an increase in the δ 34 S py value.…”

“…As mentioned above, the redox conditions of the water column in the study area during the O-S period, especially the distribution of euxinia, showed obvious spatiotemporal heterogeneity. Liu et al (2016) and Li et al (2019), Li et al (2021) explained this phenomenon in terms of tectonic movement, CIA intensity, and sulfate availability. On this basis, our study further explored the control mechanism of euxinia development using detailed geochemical data.…”

“…The latest research on marine redox conditions in this area indicates that the source weathering intensity and effectiveness of sulfate may be the main controlling factors for the development of anoxic water columns, especially euxinia (Zou et al, 2018;Li et al, 2019;Li et al, 2021). However, because of the lack of highresolution geochemical records, especially the lack of research evidence on paleo-ocean topography and external ocean current communication in the continental margin area, the controlling factors of redox conditions in the Upper Yangtze Basin water column need to be explored further.…”

Development Characteristics of Shale Lithofacies in the Longmaxi Formation and their Main Controlling Factors in the Changning Area, South Sichuan Basin, SW China

“…The Sichuan Basin, which is located on the southwestern margin of the Yangtze Platform, is a complex marine and terrestrial superposed basin with stratigraphical sequence of Presinian metamorphic rocks as the basement, marine carbonate rocks from the Sinian to Middle Triassic, and continental clastic rocks from the Late Triassic to Eocene (Wang et al, 2002). From the Late Ordovician to Early Silurian, the Yangtze Plate collided with Cathysia, which resulted with the uplift of the Yangtze Plate in the northwest and downward deformation in the southeastern margin, leading to a deep-water sedimentary environment formed in the Upper Yangtze Platform (referred to as the Yangtze Sea by Li et al, 2021) (Figure 1A). Bounded by the Chuanzhong uplift, Qianzhong uplift, and Xuefeng uplift and separated by the interior submarine high of the Hunan-Hubei Arch, the Yangtze Sea evolved from a shallow carbonate platform into a semi-isolated and siliciclastic-dominated cratonic basin from the Late Ordovician to Early Silurian (Li et al, 2021).…”

Geological evidences and mechanisms for oceanic anoxic events during the Early Paleozoic

“…晚凯迪期，表层海水初级生 产力较高，虽然"Boda warming"存在争议，但内陆棚深水环境因顺循环不畅而呈硫化状态、 外陆棚与斜坡等相带底水以铁化状态为主，偶见氧化状态 [34] . 进入赫南特冰期，内陆棚浅水 环境以及外陆棚水下高地周边受海平面下降影响，底水被氧化，而外陆棚与斜坡等相带则因 为冰期水循环加剧，营养供应增强，表层海水高生产力得以维持，底水保持缺氧状态 [35,117] . 晚赫南特期，大气 CO 2 浓度升高，温度回升，冰川消退，海平面上升，大陆边缘近海盆地出 现水体分层，表层海水的高生产力持续输出有机质至底水，在消耗尽溶解氧之后，形成底水 缺氧甚至厌氧，同时形成海侵型黑色页岩 [19,36] .…”