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Science Journals See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/325884509 Extensive marine anoxia during the terminal Ediacaran Period Article in Science Advances · June 2018 DOI: 10.1126/sciadv.aan8983 CITATIONS READS 4 683 9 authors, including: Feifei Zhang Shuhai Xiao Yale University Virginia Polytechnic Institute and State University 44 PUBLICATIONS 102 CITATIONS 267 PUBLICATIONS 9,078 CITATIONS SEE PROFILE SEE PROFILE Brian Kendall Stephen Romaniello University of Waterloo Arizona State University 51 PUBLICATIONS 2,234 CITATIONS 55 PUBLICATIONS 518 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Ediacaran Taphonomy View project High-resolution chemostratigraphic global correlations View project All content following this page was uploaded by Feifei Zhang on 21 June 2018. The user has requested enhancement of the downloaded file. SCIENCE ADVANCES | RESEARCH ARTICLE ECOLOGY Copyright © 2018 The Authors, some Extensive marine anoxia during the terminal rights reserved; exclusive licensee Ediacaran Period American Association for the Advancement 1 2 3 1 4 5 of Science. No claim to Feifei Zhang *, Shuhai Xiao , Brian Kendall , Stephen J. Romaniello , Huan Cui , Mike Meyer , original U.S. Government 1 6 1,7 Geoffrey J. Gilleaudeau , Alan J. Kaufman , Ariel D. Anbar Works. Distributed under a Creative The terminal Ediacaran Period witnessed the decline of the Ediacara biota (which may have included many stem- Commons Attribution group animals). To test whether oceanic anoxia might have played a role in this evolutionary event, we measured NonCommercial 238 U isotope compositions (d U) in sedimentary carbonates from the Dengying Formation of South China to ob- License 4.0 (CC BY-NC). tain 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 co- eval 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. Downloaded from INTRODUCTION boundary (3, 5). This temporal mismatch and a protracted decline of Macroscopic and morphologically complex multicellular eukaryotes, the biota have prompted some to favor a biotic replacement model as including possible representatives of stem-group animal phyla, diversified an explanation for the decline of the Ediacara biota (3, 5). This model in the second half of the Ediacaran Period starting ~570 million years suggests that Ediacaran organisms were progressively replaced by http://advances.sciencemag.org/ (Ma) ago (1). The fossil record of these eukaryotes is sometimes referred newly evolved bilaterian animals, which may have directly or indirect- to as the Ediacara biota (2). The Ediacara biota reached their maximum ly modulated the availability of resources, including both substrates taxonomic and morphological diversity about 560 Ma ago, then subse- and nutrients (3). quently declined in the terminal Ediacaran Period (~550 to 541 Ma Resolving this debate requires the integration of paleontological ago), and almost completely disappeared at the Ediacaran-Cambrian and geochemical data. At a global scale, this integration is challeng- transition about 541 Ma ago (2–4). The causes of the decline and even- ing because precise stratigraphic correlation of Ediacaran strata is tual disappearance of the Ediacara biota remain a subject of intensive difficult (17) and proxies for global ocean redox conditions are few. debate (3, 5, 6). Sperling et al. (18) assembled a global compilation of Fe speciation Changes in ocean redox conditions are often implicated as a driver of data, but the temporal resolution of this compilation is dependent thedeclineoftheEdiacarabiota(3, 7–9). Given the physiological require- on interregional correlation and is insufficient for the reconstruction ments of many Ediacaran organisms, particularly macroscopic diffusion- of short-term oceanic redox dynamics in the terminal Ediacaran Pe- on June 20, 2018 dependent stem-group animals (10), O2 availability is expected to have an riod. In contrast, other studies have focused on local redox proxies impact on the distribution of the Ediacara biota both locally and globally from fossiliferous successions so that geochemical and paleontolog- (11–14). It has been shown that Ediacaran organisms were locally ical data were collected from the same successions, avoiding the chal- restricted to oxygenated environments under highly dynamic redox lenges of interregional correlation. For example, Darroch et al. (5), conditions (13). Planavsky et al. (12) suggested that generally low global Wood et al. (13), and Tostevin et al. (6) applied Fe speciation and Ce O2 availability until ~750 Ma ago was a key factor delaying the rise of anomalies to carbonates and siliciclastic rocks of the Nama Group to multicellular animals. Canfield et al. (11) specifically linked the initial understand factors controlling the distribution and decline of the diversification of the Ediacara biota at ~570 Ma with a deep ocean oxy- Ediacara biota in the terminal Ediacaran Period. Darroch et al. (5) genation event (15, 16). However, the possible relationship between presented Fe speciation data from a single terminal Ediacaran sec- ocean redox evolution and the decline of the Ediacara biota has not tion that records lower genus richness than older Ediacaran assem- been clearly demonstrated. blages. Their data show that these sediments that record declining Previous studies reported geochemical evidence for oceanic an- diversity of the Ediacara biota were deposited in oxic environments, oxia at the Ediacaran-Cambrian boundary (7, 8). However, current and they concluded that oxygen stress probably did not play a role in fossil evidence points to a significant decline in biodiversity in the ter- the decline of the Ediacara biota. However, their data are also con- minal Ediacaran Period, up to 10 Ma ago before the Ediacaran-Cambrian sistent with the possibility that terminal Ediacaran organisms were challenged by a global expansion of anoxia and were restricted to oxic refugia in oceans with highly heterogeneous redox conditions. 1School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, Wood et al. (13) and Tostevin et al. (6) have shown that terminal USA. 2Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA. 3Depart- ment of Earth and Environmental Sciences, University of Waterloo, Waterloo, Ontario Ediacaran organisms are not found in anoxic environments and that N2L 3G1, Canada. 4Department of Geoscience and NASA Astrobiology Institute, Univer- their distribution is strongly controlled by oxygen availability. Ter- sity of Wisconsin-Madison, Madison, WI 53706, USA. 5Carnegie Institution for Science, minal Ediacaran organisms may have been globally challenged by an Washington, DC 20005, USA. 6Geology Department and Earth System Science Inter- 7 expansion of oceanic anoxia and locally relegated to oxic refugia. disciplinary Center, University of Maryland, College Park, MD 20742, USA. School of Mo- 238 235 d238 lecular Sciences, Arizona State University, Tempe, AZ 85287, USA. Uranium isotopes ( U/ U, denoted as U) in carbonates *Corresponding author. Email: [email protected] maybeuniquelysuitedtodifferentiateglobalfromlocalperturbations Zhang et al., Sci. Adv. 2018; 4: eaan8983 20 June 2018 1of11 SCIENCE ADVANCES | RESEARCH ARTICLE in ocean redox chemistry (19–21) and therefore to test the degree to this simplified calculation, all types of sinks other than anoxic/euxinic which changing global ocean redox conditions may have shaped sinks are lumped into a single “other” sink (see the Supplementary D the evolutionary trajectory of the Ediacara biota. To meet this chal- Materials for other sinks and other). Because the fraction of U removed lenge and to specifically address the question of whether the decline into anoxic/euxinic sedimentary sinks is intimately coupled to the pro- d238 of the Ediacara biota was driven by an episode of expanded marine portion of anoxic/euxinic versus oxic bottom waters, Useawater can anoxia, we analyzed d238U in carbonates from the Dengying Formation be used to calculate the area of anoxic/euxinic seafloor using Eq. 4, (South China; ca. 551 to 541 Ma ago) that also contains terminal derived from the U isotope mass balance and assuming a first-order Ediacaran macrofossils characteristic of the Nama assemblage. The in- relationship between U burial rate into each sink and the global tegration of d238U data and paleontological data from the same suite seawater U reservoir (see the Supplementary Materials for detailed of rocks allows us to reconstruct global ocean redox conditions at a calculation and parameters used in the calculation) time when the Ediacara biota began to decline. 238 238 d Useawater ¼ d Uinput THE URANIUM ISOTOPE PROXY Aanoxic⋅kanoxic⋅Danoxic þ Asuboxic⋅ksuboxic⋅Dsuboxic þ Aoxic⋅koxic⋅Doxic Variations in global ocean d238U may reflect changes
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