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Changes in marine productivity and redox conditions during the end-Ordovician (Hirnantian) glaciation and Ordovician–Silurian transition were investigated through Mo-isotope and major- and trace-element analyses of the Wangjiawan (Hubei Province) and Nanbazi (Guizhou Province) sections from the Yangtze Platform of South China. Katian shales of the Wufeng Formation, which yield the graptolites Dicellograptus complanatus , Dicellograptus complexus , and Paraorthograptus pacificus , were deposited under euxinic conditions at both localities, as shown by high Mo EF , U EF , and δ 98 Mo values. A major sea-level regression during the Hirnantian glaciation resulted in shallowing and a shift toward better-oxygenated conditions within the Yangtze Sea, as well as deposition of thin-bedded siliceous sediments, calcirudite debris flows, and limestone turbidites of the Kuanyinchiao Formation, the base of which correlates with the first phase of the end-Ordovician mass extinction. The termination of the Hirnantian glaciation at the top of the Kuanyinchiao Formation was associated with a major sea-level transgression, a rapid expansion of euxinia in the Yangtze Sea (as documented by a return of high Mo EF , U EF , and δ 98 Mo values), and the second phase of the mass extinction, during which the cool-adapted Hirnantian Fauna went extinct. The long-term cooling trend of the Middle and Late Ordovician, which culminated in the Hirnantian glaciation, was driven by enhanced burial of organic carbon, as documented by δ 13 C carb , δ 13 C org , and δ 34 S py records. Increased organic carbon burial was linked to high rates of marine productivity, as shown by high TOC and biogenic Ba concentrations especially at the deeper, less-restricted Wangjiawan locale, producing the HICE (Hirnantian Isotopic Curve Excursion) δ 13 C carb excursion. The locus of organic carbon burial during the Hirnantian crisis shifted to deeper-water environments that were located outside the study region. The relatively rapid onset and termination of the Hirnantian glaciation were probably due to crossing of tipping points in the Late Ordovician climatic–oceanic system. [ABSTRACT FROM AUTHOR]

Redox-sensitive element (RSE) compositions in marine sedimentary archives have been important in improving our understanding of redox history, especially in the Meso-Neoproterozoic ocean, where early biological innovations took place before the Cambrian Explosion. However, despite increasing availability of RSE data that represent this long time interval, temporal comparisons are difficult, and thus provide poor constraints on the dramatic changes in atmosphere-ocean chemical settings. Here we analyzed carbonate rocks and black shales from the North China and Yangtze platforms (eastern China) for trace elements and isotopic compositions (δ13C, δ34S and 87Sr/86Sr). These data were combined with previously published RSE data to allow establishment of successive elemental chemostratigraphies from sequences of both black shale and carbonate RSE archives, thus yielding direct insights into the role of marine RSE cycling. Generally low Th/RSE ratios in relatively pure carbonate rocks (Al < 0.35%, Th < 0.5 μg/g) and black shales likely indicate enriched RSEs in oxidized shallow conditions, consistent with a more oxidized Neoproterozoic ocean or confined water body. The chemostratigraphy of RSEs not only shows enrichment in Neoproterozoic marine sediments, but temporally variable trends in the Meso-Neoproterozoic. A mass balance model based on modern marine budgets demonstrates that even a small oxidation event within the dominantly anoxic setting could be an important factor for increasing dissolved RSE reservoirs, and that observed heterogeneous elemental records may represent the amplified results of a fluctuating palaeoredox (oxic-anoxic) structure with pulsed marine oxidation. Meso-Neoproterozoic marine redox heterogeneity in eastern China suggests that increases in free oxygen were potentially related to: 1) supercontinent cycles, 2) enhanced weathering inputs with high 87Sr/86Sr ratios (> 0.705 at ca. <1.4–1.1 Ga), and 3) previously reported biological processes as emphasized by biological C-S cycles. Thus, pulsed oxic conditions may have provided sequential opportunities for biological innovation. [Display omitted] • Meso-Neoproterozoic sediments record the marine redox heterogeneity. • A oxidation event within anoxic oceans account for the greatest RSE fluctuation. • Pulsed oxic conditions promote biological innovation in ancient oceans. [ABSTRACT FROM AUTHOR]

Rocks of Darriwilian and Sandbian age (Middle–Late Ordovician) record the extraordinary peak of the Great Ordovician Biodiversification Event (GOBE) and significant global environmental changes. To track paleo-environmental changes through this critical time, we have analyzed iron speciation, trace elements and total organic carbon in samples of the Miaopo Formation from an intra-shelf basin in South China. The results reveal moderate primary productivity and possible fluctuations in benthic redox. Upwelling during marine transgression is thought to have increased the availability of nutrients in surface waters, sustaining the biological pump. Enhanced export of organic matter would have consumed dissolved oxygen at depth, driving the system toward anoxia. However, it is also possible for deep-water renewal events to transfer more dissolved oxygen into the bottom water. Those proxies suggests high primary productivity and varying sea level (global and local), not benthic anoxia, as the driver of accumulated "black shale". Thus, the local marine ecosystem thrived in this oxic intra-shelf basin, challenging the preexisting paradox that the high-diversity community flourished in an oxygen-depleted environment. • Ba xs and TOC suggest moderate-high primary productivity yet no major increase. • Enhanced anoxia occurred in porewaters rather than in the water column. • High productivity and sea level rise prompted the organic-rich shale to form. • Our study solve previous debates on O 2 -depleted waters and thrived community. [ABSTRACT FROM AUTHOR]