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Accompanying the Late Permian mass extinction (LPME), the sedimentary environment underwent significant changes in both marine and non-marine settings. However, researchers have not yet fully understood these changes. As an ever-present candidate for the Global Stratotype Section and Point (GSSP) for the Permian–Triassic boundary (PTB), the Shangsi section has preserved trails of geological events and environmental information from the eastern Paleo-Tethys Ocean. In this study, we describe the microfacies of PTB layers at Shangsi through micropetrological and geochemical analyses. Calcite replacement in siliceous radiolarians is observed across the layers between Beds 24 and 26, suggesting that the δ13Ccarb composition of siliceous limestones in the upper Permian (Clarkina yini conodont zone) was strongly affected by diagenesis. After deeply excavating samples in the Bed 27b, ferruginous microspheres were still found, indicating they are not industrial fly ash. The foraminifer fauna in the lower part of Bed 28 was likely the result of the Lilliput effect, indicating deteriorated environmental conditions following the mass extinction. A dramatic increase in terrestrial material input in Bed 29 probably represents an increase in the weathering rate during the earliest Triassic. Multiple volcanic ash beds in the Shangsi section coincide with the deterioration of the deep-water environment. These observations suggest that microfacies may help elucidate the detailed pattern of high-resolution isotope changes across the PTB in the Shangsi section.
Diatom fossils offer reliable means of evaluating biotic, climatic, and ecological conditions of past environments. Here we present new insights in reconstructing the mid-Miocene paleoenvironment of the Ashfall Fossil Beds in Nebraska, USA. Extensive excavation of megafauna and flora at the Ashfall site has provided a well-preserved glimpse into the Clarendonian Land Mammal age of the North American Plains. From sedimentary evidence the environment at the site has been described as a small waterhole in a seasonally wet depression located in a stream bed. Here we report a diatom fossil assemblage containing 38 species spanning 30 genera found in the ash and underlying sand formations at the Ashfall site. We also provide insights into environmental conditions prior to and during the eruption and deposition of the ash, including water chemistry. Diatom assemblages from the underlying sand and the ash deposits indicate drastically different depositional environments. We find that the diatom fossils robustly separate across the sand-ash transition and propose two possible explanations for the differences observed in the assemblages. Either the diatom assemblages across the sand-ash transition represent a dry to wet season preservation, or the ash from the eruption changed the water chemistry and the diatom assemblage changed in response to changing water conditions. It is also possible both scenarios have some influence on diatom composition.
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