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(Δ13c-Δ18o-87Sr/86Sr-Δ15n) Study of the Doushantuo Formation In

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(Δ13c-Δ18o-87Sr/86Sr-Δ15n) Study of the Doushantuo Formation In 1 An integrated chemostratigraphic (δ13C-δ18O-87Sr/86Sr-δ15N) study of 2 the Doushantuo Formation in western Hubei Province, South China. 3 4 Zhongwu Lan a,b*, Yuji Sano b, Takuya Yahagi b, Kentaro Tanaka b, Kotaro Shirai b, Dominic 5 Papineau c,d, Yusuke Sawaki e, Takeshi Ohno f, Mariko Abe g, Hongwei Yang h, Hai Liu h, Tao 6 Jiang h, Teng Wang h 7 8 a State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese 9 Academy of Sciences, Beijing 100029, China 10 b Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba 277-8564, Japan 11 c London Centre for Nanotechnology, University College London, London WC1E 6BT, UK 12 d Department of Earth Sciences, and Center for Planetary Sciences, University College London, 13 London WC1E 6BT, UK 14 e Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1, Komaba, 15 Meguro-ku, Tokyo 153-8902, Japan 16 f Department of Chemistry, Faculty of Science, Gakushuin University, Mejiro, 1-5-1, Toshima-ku, 17 Tokyo 171-8588, Japan 18 g Department of Subsurface Geobiological Analysis and Research (D-SUGAR), Japan Agency for 19 Marine-Earth Science and Technology (JAMSTEC), 2-15, Natsushima-cho, Yokosuka, Kanagawa 20 237-0061, Japan 21 h Central South Institute of Metallurgical Geology, Yichang 443003, China 22 23 ABSTRACT 24 25 High-resolution isotopic analyses were conducted on core samples from the Ediacaran 26 Doushantuo Formation at the Wangjiapeng section, western Hubei Province in South China, 13 27 whereby two laterally traceable, negative δ Ccarb excursions (EN1 and EN3) were recognized. 28 The magnitude and duration of these excursions permit intra-basinal and inter-basinal correlation, 29 which indicates that they probably represent a global change in seawater composition. The 13 13 13 30 occurrence of decoupled δ Ccar-δ Corg with almost invariable δ Corg values at Wangjiapeng, 31 Zhongling, Yangjiaping sections is consistent with remineralization of a dissolved organic carbon 32 (DOC) pool by means of sulfate reduction, as recorded in EN3. The synchronous presence of EN3, 33 a shift to higher 87Sr/86Sr and decrease of Mn and Fe contents and δ15N values together points to a 34 glacial influence whereby oxygenation and remineralization of reduced carbon produced 35 13C-depleted DIC. Glaciations cause a decrease in sea level, which itself leads to increased 36 continental shelf area to be exposed to surface weathering, and ultimately to enhanced delivery of 37 radiogenic 87Sr. The increase of 87Sr/86Sr ratios, sulfate and phosphate are consequences of surface 38 runoff into oceanic environments and such perturbations induce biogeochemical changes. 39 40 KEY WORDS: Ediacaran chemostratigraphy; δ13C-δ18O-87Sr/86Sr-δ15N; Doushantuo Formation; 41 South China; Ediacaran glaciation; open marine environment 42 43 1. Introduction 44 45 The Ediacaran Period (c. 635-c. 541 Ma) has increasingly attracted worldwide 46 attention because it was during this time interval that glaciations occurred (Pu et al., 47 2016) and that new animal forms (e.g. bilaterians) and behaviors (e.g. 48 biomineralization and bioturbation) first emerged (McCall, 2006; Xiao et al., 1998, 49 2014; Muscente et al., 2015). This period witnessed intriguing and unprecedented 50 chemo-oceanographic fluctuations related to a significant oxygenation event and 51 widespread biogeochemical perturbations (Shields-Zhou and Och, 2011). The carbon 52 isotopic record of ancient seawater recorded in marine carbonates serves as a key 53 proxy for exploring these variations, especially the temporal and causal relationships 54 with biological and environmental factors, although they are often ambiguous partly 55 because of the lack of high resolution chemostratigraphic data. 56 Information on these biological and environmental events is well preserved in the 57 lower-middle Ediacaran Doushantuo Formation in South China (Xiao et al., 2007; Li 58 et al., 2010; Lu et al., 2013; Liu et al., 2014; Cui et al., 2015, 2016; Muscente et al., 59 2015; Xiao et al., 2016). The Doushantuo Formation is dominated by carbonate, 60 phosphorite, and black shale that show depositional facies varying from 61 shallow-marine shelf to deep-marine basin (Zhu et al., 2007). As such, the 62 Doushantuo Formation presents a natural sedimentological laboratory for 63 chemostratigraphic study. Three striking fluctuations in ocean composition occurred 64 during the deposition of the Doushantuo Formation as evidenced by three negative 13 65 δ Ccarb excursions, which may record global-scale events following the late 66 Cryogenian glaciation (Jiang et al., 2003; Wang et al., 2008; Tahata et al., 2013; Lu et 67 al., 2013; Zhu et al., 2013; Furuyama et al., 2016; Zhou et al., 2016). These are 1) the 13 68 negative δ Ccarb excursion (EN1) in the basal Ediacaran cap carbonates that locally 13 69 exhibits δ Ccarb values down to −48‰ indicative of methane release from a gas 70 hydrate destabilization event (Jiang et al., 2003; Wang et al., 2008), 2) the relatively 71 less negative δ13C excursion of ca. −5‰ (EN2) in middle Ediacaran strata (Zhou and 13 72 Xiao, 2007), and 3) the remarkable negative δ Ccarb excursion (EN3) with values as 73 low as −12‰ recorded globally in middle Ediacaran strata (McFadden et al., 2008). 13 74 These three negative δ Ccarb excursions have been frequently used for correlation of 75 Ediacaran stratigraphy as well as reconstruction of Ediacaran palaeoenvironments in 76 view of their different amplitudes and stratigraphic levels (Zhou and Xiao, 2007; Zhu 13 77 et al., 2007). However, the origin of these negative δ Ccarb excursions, particularly 78 EN2 and EN3, and their possible global correlation remain unclear, with various 79 models proposed such as involvement of a DOC reservoir or recycled/detrital organic 80 carbon (McFadden et al., 2008; Jiang et al., 2010, 2012; Johnston et al., 2012; 81 Ishikawa et al., 2013; Wang et al., 2016). In view of the potential diagenetic 82 overprinting and variations in the depth of chemocline between different sections, 83 more detailed work is required to give further constraints. 84 Carbon isotope stratigraphy combined with strontium and nitrogen isotopic 85 stratigraphy, sequence stratigraphy and redox sensitive elements such as Fe and Mn 86 has helped to clarify some problematic correlations of Ediacaran sedimentary 13 87 successions and the origin of negative δ Ccarb excursions as well as recover redox 88 conditions of ancient seawater, although in some sections correlations have been 89 difficult to establish due to spatially limited outcrops, poor constraints on 90 bathymetry-dependent facies variations, and low sampling resolution (Yang et al., 91 1999; Sawaki et al., 2010; Jiang et al., 2007, 2011; Zhu et al., 2013; Cremonese et al., 92 2013; Kikumoto et al., 2014; An et al., 2015; Cui et al., 2015; Zhou et al., 2017; Wang 93 et al., 2018). As a result, the origin and timing of both EN2 and EN3, and their 94 chronological order with respect to Ediacaran glaciation, remains ambiguous, which 95 has led to the proposal of two possible models for Ediacaran subdivisions and 96 correlation (Xiao et al., 2016). Further high resolution chemostratigraphic studies can 97 contribute to enhance our understanding of the occurrence, order and causal 98 relationships between chemo-oceanographic chemical variations and Ediacaran 99 glaciation during this key geological period. 100 In this study, drill core samples of the Doushantuo Formation are used to ensure 101 that the chemostratigraphy is complete, continuous, and studied at high-resolution as 102 well as to minimize the influence of late oxidation and surface weathering. Previously 13 13 15 87 86 103 published δ Ccarb, δ Corg, δ N and Sr/ Sr stratigraphy are compiled and directly 13 87 86 104 compared to new detailed chemostratigraphy of δ C and Sr/ Sr. With this approach, 13 105 the origin of Ediacaran negative δ Ccarb excursions and their chemostratigraphic 106 correlations are discussed. 107 108 2. Geological setting 109 110 The Ediacaran Yangtze platform in South China developed on a Neoproterozoic 111 rifted continental margin that is believed to have started along the southeastern side of 112 the Yangtze block at ca. 800 Ma (Wang and Li, 2003; Jiang et al., 2007). A passive 113 margin setting is assumed to accommodate the post-glacial Ediacaran carbonate and 114 siliciclastic rocks (Jiang et al., 2003a). Ediacaran sedimentary rocks are widely 115 exposed around the Yangtze block and preserve a key biostratigraphic and 116 chemostratigraphic record of the co-evolution of life and its environments in the 117 Doushantuo and Dengying formations (Figure 1A, B; Zhou and Xiao, 2007; Zhu et al., 118 2007; Chen et al., 2014; Liu et al., 2014; Xiao et al., 2014; Cui et al., 2015). 119 Detailed facies analyses and palaeogeographic reconstructions based on more 120 than twenty sections suggest that the Doushantuo Formation was deposited in three 121 platform facies belts, as represented by 1) proximal inner shelf peritidal mixed 122 carbonate and shale, 2) an intra-shelf lagoon containing mixed carbonate, phosphorite, 123 and shale, and 3) an outer shelf shoal complex with increasing water depth from 124 northwest to southeast (Figure 1C; Zhu et al., 2007; Jiang et al., 2011). The 125 Doushantuo Formation was deposited in two stages. The first stage and, in particular, 126 the couplet of cap carbonate and overlying black shale of the lower Doushantuo 127 Formation indicates an open shelf/ramp depositional environment after the Nantuo 128 glaciation. The second stage is characterized by its rimmed carbonate shelf with a 129 shelf-margin barrier separating the intra-shelf lagoon from open ocean settings (Jiang 130 et al., 2011). One of the most complete and continuous outcrops of the Doushantuo 131 Formation is exposed at the intra-shelf basin Jiulongwan section within the Yangtze 132 Gorges area (Liu and Sha, 1963; Sawaki et al., 2010).
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