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The terminal Ediacaran Dengying Formation (c. 551.1-538.8 Ma) in South China is one of two successions where Ediacara-type macrofossils are preserved in carbonate facies along with skeletal fossils and bilaterian animal traces. Given the remarkable thickness of carbonate-bearing strata deposited in less than 12.3 million years, the Dengying Formation holds the potential for construction of a relatively continuous chemostratigraphic profile for the terminal Ediacaran Period. In this study, a detailed sedimentological and chemostratigraphic (delta 13C(carb), delta O-18(carb), delta C-13(org), delta S-34(pyrite), and Sr-87/Sr-86) investigation was conducted on the Dengying Formation at the Gaojiashan section, Ningqiang County of southern Shaanxi Province, South China. Sedimentological results reveal an overall shallow-marine depositional environment. Carbonate breccia, void-filling botryoidal precipitates and aragonite crystal fans are common in the Algal Dolomite Member of the Dengying Formation, suggesting that peritidal facies were repeatedly karstified. The timing of karstification was likely early, probably soon after the deposition of the dolomite sediments. The presence of authigenic aragonite cements suggests high alkalinity in the terminal Ediacaran ocean. Geochemical analysis of micro-drilled samples shows that distinct compositions are registered in different carbonate phases, which should be considered when constructing chemostratigraphic profiles representative of true temporal variations in seawater chemistry. Integrated chemostratigraphic data suggest enhanced burial of organic carbon and pyrite, and the occurrence of extensive marine anoxia (at least in the Gaojiashan Member). Rapid basinal subsidence and carbonate accumulation during a time of elevated seawater alkalinity and increased rates of pyrite burial may have facilitated the evolutionary innovation of early biomineralizing metazoans. ; UMD Geology Department; NASA Astrobiology Institute at UW-Madison; ET-HOME (Evolution and Tracers of the Habitability of Mars and Earth) Astrobiology Research Consortium in Belgium; Analytical, Environmental and Geo-Chemistry research group at VUB; American Association of Petroleum Geologists (AAPG) Grants-In-Aid Program Marilyn Atwater Memorial Grant; Explorers Club Washington Group; open research grant of the State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences [193107]; NASA Exobiology grantNational Aeronautics & Space Administration (NASA) [NNX12AR91G, 80NSSC18K1086]; NSF Sedimentary Geology and Palaeontology grant [EAR0844270, EAR1528553]; Young Scientists Fund of Shaanxi Province [2015KJXX-26] ; This study was started when the first author HC was a Ph.D. graduate student at the University of Maryland. It was progressively improved during HC's first post-doctoral position at the NASA Astrobiology Institute, University of Wisconsin-Madison and HC's second post-doctoral position at Vrije Universiteit Brussel, Belgium. HC would like to thank the UMD Geology Department, the NASA Astrobiology Institute at UW-Madison, the ET-HOME (Evolution and Tracers of the Habitability of Mars and Earth) Astrobiology Research Consortium in Belgium and the Analytical, Environmental and Geo-Chemistry research group at VUB for support.; This research is funded by the American Association of Petroleum Geologists (AAPG) Grants-In-Aid Program Marilyn Atwater Memorial Grant to HC, the Explorers Club Washington Group grant to HC, the open research grant (193107) of the State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences to HC, the NASA Exobiology grant (NNX12AR91G to AJK and 80NSSC18K1086 to SX), the NSF Sedimentary Geology and Palaeontology grant (EAR0844270 to AJK; EAR1528553 to SX) and the Young Scientists Fund of Shaanxi Province (No. 2015KJXX-26) to YC. ; Public domain authored by a U.S. government employee

Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). ; The terminal Ediacaran Period witnessed the decline of the Ediacara biota (which may have included many stemgroup animals). To test whether oceanic anoxia might have played a role in this evolutionary event, we measured U isotope compositions (d238U) in sedimentary carbonates from the Dengying Formation of South China to obtain new constraints on the extent of global redox change during the terminal Ediacaran. We found the most negative carbonate d238U values yet reported (−0.95 per mil), which were reproduced in two widely spaced coeval sections spanning the terminal Ediacaran Period (551 to 541 million years ago). Mass balance modeling indicates an episode of extensive oceanic anoxia, during which anoxia covered >21% of the seafloor and most U entering the oceans was removed into sediments below anoxic waters. The results suggest that an expansion of oceanic anoxia and temporal-spatial redox heterogeneity, independent of other environmental and ecological factors, may have contributed to the decline of the Ediacara biota and may have also stimulated animal motility. ; NASA Exobiology Program || (no. NNX13AJ71G) NSF Frontiers in Earth System Dynamics program || (award EAR-1338810) NASA grant || (no. NNX15AL27G) Natural Sciences and Engineering Research Council of Canada Discovery Grant || (RGPIN-435930). American Association of Petroleum Geologists Grants-In-Aid Program Explorers Club Washington Group Exploration Field Research Grant Carnegie Institution for Science