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Geochimica et Cosmochimica Acta 240 (2018) 220–235

Nitrogen isotope constraints on the early Ediacaran oceanredox structure

Xinqiang Wang a,b,⇑, Ganqing Jiang c,⇑, Xiaoying Shi a,b,⇑, Yongbo Peng d

Deborah C. Morales c

aSchool of Earth Sciences & Resources, China University of Geosciences, Beijing 100083, ChinabState Key Laboratory of Biogeology & Environmental Geology, Beijing 100083, ChinacDepartment of Geoscience, University of Nevada, Las Vegas, NV 89154-4010, USA

dDepartment of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA

Received 28 December 2017; accepted in revised form 21 August 2018; Available online 29 August 2018

Abstract

Nitrogen (N) is a macronutrient essential to all living organisms and its availability in Precambrian oceans may haveplayed an important role in the early evolution and diversification of eukaryotes. It has been hypothesized that the billion-year evolutionary stasis of eukaryotes during the mid-Proterozoic (ca. 1.8–0.8 Ga) was linked with N limitation. As a corol-lary, the rapid diversification of eukaryotes during the Ediacaran implies the lift of this barrier. Indeed, recent nitrogen isotope(d15N) studies suggested a stable oceanic nitrate (NO�

3 ) pool and perhaps oxygenated mid-depth oceans since ca. 750 Ma. Thisinference, however, contrasts with the iron, sulfur, and trace element geochemical data that suggested much later ocean oxy-genation during the Ediacaran or Paleozoic. To better understand the relationship between nitrogen isotope and other redoxproxy data, we have conducted a nitrogen isotope study on the organic-rich black shales of the basal Doushantuo Formation(ca. 635–632 Ma) in multiple sections across a shelf-to-basin transect, from which iron, sulfur and trace element geochemicaldata are available. The results show exclusively positive d15N values in all sections across the basin, with most values withinthe range of +3‰ to +12‰ and a modal value of +5 ± 1‰. The positive d15N values from the basal Doushantuo black shalesare comparable with those of the modern ocean (+3‰ to +14‰; modal value of +5‰) and most likely resulted from an aer-obic nitrogen cycle with partial water column denitrification in the presence of a stable nitrate pool. In general, it is difficult tomaintain a stable oceanic NO�

3 reservoir in strongly stratified oceans. Therefore, the new d15N data, in combination with othergeochemical data, provide evidence for an oxygenated mid-depth ocean during the early Ediacaran. The co-occurrence of aer-obic N isotope signal and euxinia in some samples implies that the early Ediacaran ocean in South China could be periodicallystratified well below the photic zone, with a chemocline at least down to the mid-depth (e.g., >1000 m) or close to the watercolumn/sediment interface.� 2018 Elsevier Ltd. All rights reserved.

Keywords: South China; Doushantuo Formation; Black shale; Aerobic nitrogen cycle; Ocean oxygenation

https://doi.org/10.1016/j.gca.2018.08.034

0016-7037/� 2018 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors at: School of Earth Sciences &Resources, China University of Geosciences, Beijing 100083, China(X. Wang and X. Shi) and Department of Geoscience, Universityof Nevada, Las Vegas, NV 89154-4010, USA (G. Jiang).

E-mail addresses: wxqiang@cugb.edu.cn (X. Wang), ganqing.jiang@unlv.edu (G. Jiang), shixyb@cugb.edu.cn (X. Shi).

1. INTRODUCTION

Nitrogen (N) is an essential element for the synthesis ofnucleic acids and proteins that are fundamental compo-nents of all organisms (e.g. Falkowski, 1997; Tyrrell,1999; Canfield et al., 2010; Stueken et al., 2016). The avail-

X. Wang et al. /Geochimica et Cosmochimica Acta 240 (2018) 220–235 221

ability of dissolved N (mostly ammonium NH4+ and nitrate

NO3�) in the ocean may have played a significant role in

shaping the evolutionary trajectory of life in Earth history,particularly for eukaryotes because they are incapable ofdirectly transferring atmospheric dinitrogen (N2) tobioavailable N through N fixation as some prokaryotesdo (Anbar and Knoll, 2002). Although the first appearanceof eukaryotes may be traced back to the Paleoproterozoicor even late Archean, their evolution during the mid-Proterozoic (1.8–0.8 Ga) was remarkably slow (e.g.,Knoll, 2014 and references therein). This billion-year evolu-tionary stasis was thought to have a connection with N lim-itation, possibly due to inefficient N fixation in the surfaceocean under low trace metal (especially Mo) concentrations(e.g., Anbar and Knoll, 2002; but see Godfrey et al., 2013,for a different view) and/or fast removal of oceanic N inanoxic and euxinic environments that were much morewidespread than in the modern ocean (e.g., Scott et al.,2008; Poulton and Canfield, 2011; Partin et al., 2013;Reinhard et al., 2013; Large et al., 2014; Lyons et al.,2014; Planavsky et al., 2014; Cole et al., 2016; Tang et al.,2016). Recent N isotope (d15N) studies lend support to thishypothesis. The d15N data from the Mesoproterozoic BeltSupergroup in the United States (Stueken, 2013) and theBangemall Supergroup and Roper Group in Australia(Koehler et al., 2017) show strong depth-related d15N gra-dient from onshore to offshore sections, implying the sepa-ration of an aerobic N cycle in oxic shallow water from ananaerobic N cycle in anoxic deep water. Such spatially vari-able d15N suggests a limited oceanic nitrate pool that wasonly available in onshore environments (Stueken, 2013;Stueken et al., 2016; Koehler et al., 2017).

Following the long-term evolutionary stasis, the lateNeoproterozoic, especially the Ediacaran Period (ca. 635–541 Ma), witnessed rapid rise and diversification of eukary-otes (e.g., Erwin et al., 2011; Yuan et al., 2011; Liu et al.,2014; Riedman et al., 2014; Droser and Gehling, 2015; Yeet al., 2015; Xiao et al., 2016; Brocks et al., 2017). This bio-logical innovation implies mitigation of the N limitationbarrier and requires a large nitrate pool in the ocean to sup-port structurally complex ecosystems (e.g., Kamp et al.,2015; Ader et al., 2016; Olson et al., 2016; Stueken et al.,2016). Recent nitrogen isotope studies provide support tothis prediction. The d15N data from multiple late Neopro-terozoic successions show positive d15N values with a modeof ca. +4‰ (Ader et al., 2014; Johnson et al., 2017), whichare comparable with those of the modern ocean (Sigmanet al., 2009; Tesdal et al., 2013; Algeo et al., 2014) and implya stable oceanic nitrate pool (and oxygenated ocean) sinceca. 750 Ma (Ader et al., 2014, 2016).

The persistency of a stable nitrate pool and ocean oxy-genation since 750 Ma, however, contrasts with many othergeochemical data. Iron speciation data from late Neopro-terozoic successions indicate that the ocean was dominatedby anoxic/ferruginous conditions (e.g., Canfield et al., 2008;Li et al., 2010, 2012a; Johnston et al., 2013; Fan et al., 2014;Sperling et al., 2015; Huang et al., 2017), with perhapslocally oxygenated deep waters in some late Ediacaranbasins (Canfield et al., 2007). Redox sensitive trace element(RSTE) data (especially Mo, V and U) and sulfur (d34S),

molybdenum (d98Mo) and uranium (d238U) isotope dataall suggest an overall anoxic late Neoproterozoic oceanwith multiple short-lived (and likely progressive) ocean oxy-genation events (e.g., McFadden et al., 2008; Sahoo et al.,2012, 2016; Och and Shields-Zhou, 2012; Kendall et al.,2015; Lau et al., 2017; Gregory et al., 2017; Shi et al.,2018). In this latter interpretation, pervasive ocean oxy-genation did not happen until the late Ediacaran (e.g.,Fike et al., 2006; McFadden et al., 2008; Scott et al.,2008; Partin et al., 2013; Lyons et al., 2014), early Cam-brian (e.g., Wen et al., 2011; Chen et al., 2015; Wanget al., 2015; Li et al., 2017a) or even much later in lateDevonian (Dahl et al., 2010; Sperling et al., 2015; Lentonet al., 2016; Wallace et al., 2017).

The apparent discrepancy between the unidirectionalocean oxygenation since ca. 750 Ma implied by the d15Ndata and the much later, stepwise oxygenation suggestedby other geochemical data requires a comparison of thesegeochemical redox proxies in the same stratigraphic con-text. In general, d15N values of organic matter record theisotope signature of the surface ocean within the photiczone and cannot provide quantitative information aboutbottom-water redox conditions. However, in a stronglystratified ocean, as has been proposed for the Ediacaran,it is difficult to maintain a stable nitrate pool due to the lossof nitrate through fast denitrification and anammox (Aderet al., 2014, 2016). In addition, because denitrifiers outcom-pete sulfate reducers, nitrate and euxinia rarely occur at thesame stratigraphic interval (e.g., Canfield, 2006; Lam andKuypers, 2011; Boyle et al., 2013), unless euxinia happenswell below the photic zone in a stratified ocean or basin(Stueken et al., 2016). Both iron speciation and RSTEenrichment data could reflect bottom-water redox condi-tions (e.g., Poulton and Canfield, 2011; Tribovillard et al.,2006), with the magnitude of RSTE enrichments in euxinicsediments potentially recording the size of the RSTE reser-voirs and thus the overall ocean redox state (e.g., Scottet al., 2008; Sahoo et al., 2012; Robbins et al., 2016). Thecombination of these redox data in multiple sections withina correlatable stratigraphic framework would provideimportant constraints on the redox structure of individualsedimentary basins and the global ocean.

In this paper, we report d15N and d13Corg (organic car-bon isotope) data of the basal Doushantuo Formationblack shales (ca. 635–632 Ma) from multiple sections acrossa shelf-to-basin transect of the Ediacaran Yangtze Platform(Fig. 1). This interval is particularly suitable for evaluatingthe relationship between d15N and other redox proxiesbecause of its widespread distribution across the basin(Jiang et al., 2011) and the availability of iron speciation,d34S, and RSTE data from many sections (McFaddenet al., 2008; Jiang et al., 2010; Li et al., 2010; Sahooet al., 2012; Gregory et al., 2017; Jin et al., 2018). Previousstudies have reported a few d15N data from this interval(Ader et al., 2014; Kikumoto et al., 2014; Wang et al.,2017), but the data resolution was too low to evaluate thespatial d15N variability across the basin and the relationshipbetween d15N and other redox proxy data. Integration ofnew and existing d15N data with available iron speciation,sulfur isotope and RSTE data provides a unique

Fig. 1. (A) Global reconstruction at ca. 635 Ma showing the location of South China (Zhang et al., 2013); (B) Paleogeographic map of theearliest Ediacaran in South China (modified from Jiang et al., 2011). (C) A shelf-to-basin transect from north to south of the EdiacaranYangtze Platform (modified from Jiang et al., 2010). Red stars in B and C indicate location of the study sections. Blue stars refer to three othersections used for correlation. Note that the Lantian section, which is possibly a lower slope environment, is not shown in C. (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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opportunity for better understanding the basin-scale Ncycle, the stability of the marine nitrate pool, and theirimplications for the early Ediacaran ocean redox structure.

2. GEOLOGICAL SETTING AND SAMPLING

SECTIONS

The Ediacaran Yangtze Platform in South China wasinferred to have evolved from the Nanhua rift basinbetween the Yangtze Block and Cathaysia Block at ca.820 Ma (e.g. Wang and Li, 2003; Jiang et al., 2011). Recentpaleomagnetic data show that South China was located inthe middle latitude (ca. 40�N) in the Northern Hemisphereat the Cryogenian-Ediacaran transition, with a possibleconnection to the India Block (Fig. 1A; Zhang et al.,2013). Thereafter, it moved southward and reached aroundthe equator during the early Cambrian (Zhang et al., 2015).Integrated paleomagnetic data show that South China wasnot attached to any other major continents during the Edi-acaran and it was possibly an independent entity with con-nection to the global ocean (Zhang et al., 2015; Sahoo et al.,2016).

The Ediacaran Doushantuo Formation (ca. 635–551 Ma) on the Yangtze Platform overlies the glacialdiamictite of the Nantuo Formation that is correlated withthe Marinoan glaciation (Zhang et al., 2008). Facies analy-ses indicate that during the early Ediacaran, the YangtzePlatform deepened towards the southeast (Fig. 1B; Jianget al., 2011). The Doushantuo Formation is commonlydivided into four members. Member I refers to the 3–7-m-thick cap carbonate that was dated at 635.2 ± 0.6 Ma(Condon et al., 2005). Member II consists of organic-richblack shales with carbonate interbeds. A TIMS U-Pb ageof 632.5 ± 0.5 Ma was obtained from a tuffaceous bedwithin the basal Member II black shales, �5 m above thetop of the cap carbonate (Condon et al., 2005). MemberIII is dominated by carbonates, with subordinate phospho-rites and shales. Member IV is composed of black shales,the top of which was dated at 551.1 ± 0.7 Ma (Condonet al., 2005; Zhang et al., 2005). The four-member subdivi-sion of the Doushantuo Formation was based on thelithologies seen in the Yangtze Gorges area and their corre-lation across the basin is not always straightforward due tofacies changes and stratigraphic truncations (Jiang et al.,

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2011; An et al., 2015; Cui et al., 2015; Zhou et al., 2017).However, the basal Member II black shales, which arethe focus of this study, are widespread and correlatable,serving as a marker interval across the basin (Jiang et al.,2011).

Fresh samples were collected from the basal Member IIblack shale interval in five well-preserved outcrop sections(Fig. 1B and C), including the outer shelf Zhongling section(29�5701000N, 110�4001300E), the upper slope Siduping (28�5500500N, 110�2605500E) and Taoying (27�5000100N, 109�0100400E) sections, the lower slope Wuhe section (26�4505600N, 108�2500100E) and the basinal Yuanjia section(27�2902300N, 110�1403700E). We specifically chose the Mem-ber II black shales as the target because the earliest modern-like RSTE enrichments were reported from this interval(Sahoo et al., 2012, 2016; Jin et al., 2018), which have beeninterpreted as evidence for pervasive ocean oxygenation.Particularly, we analyzed the same samples from the sec-tions where iron speciation, sulfur isotope, and trace ele-mental concentration data are available (Sahoo et al.,2012; Jin et al., 2018).

3. ANALYTICAL METHODS

Organic carbon isotope (d13Corg) analyses were per-formed at the China University of Geosciences (Wuhan)(CUGW) and the University of Nevada Las Vegas(UNLV), following previously published procedures(Jiang et al., 2012; Luo et al., 2014). Sample powders weredecarbonated with 6 M hydrochloric acid (HCl) or usingacid fumigation (Harris et al., 2001). The carbonate-freeresidue was then rinsed with deionized water repeatedlyuntil the pH reached near neutral. After drying in oven, iso-tope values are measured using an elemental analyzer (EA)coupled with a conflow interface that automatically trans-fers carbon dioxide gas into a Finnigan Delta Plus massspectrometer (UNLV) or a Mat-253 mass spectrometer(CUGW). Isotope values are reported in per mil relativeto VPDB. Analytical reproducibility monitored by acetani-lide (Costech Analytical Technologies; d13Corg = �30.20‰;d15N = 0.29‰; TOC = 70.09%; TN = 10.36%) at UNLVwas better than 0.20‰ for d13Corg. Reproducibilitymonitored by GBW04407 (China national standards;d13Corg = �22.4‰) and duplicate analyses at CUGW wasbetter than 0.20‰ for d13Corg.

Nitrogen isotope composition was analyzed at the Oxy-Anion Stable Isotope Consortium (OASIC) at the Louisi-ana State University (LSU). Aliquots of powdered samplesbetween 20 and 50 mg were decarbonated in silver capsulesfor 12 h through acid fumigation (Harris et al., 2001) with12 M HCl. The silver capsules with carbonate-free residuewere then neutralized using a stepwise washing process:each capsule was placed in a weighing dish (20 mL) filledwith DI water for 6 h and then dried at 70 �C for 4 h. Thiswash and dry process was repeated up to 3 times until pHtests gave a near neutral value (�6.0). Afterwards, sampleswere dried and wrapped in tin capsules prior to analysis.Nitrogen isotopes were analyzed using Vario MicrocubeElemental Analyzer (EA) connected to an Isoprime 100 iso-tope ratio mass spectrometer (IRMS) in the OASIC lab at

LSU. Results are reported as standard d notation as per mil(‰) deviations from atmospheric N2 (‰, Air). Uncertain-ties determined by duplicates of acetanilide are better than0.3‰ for d15N and 0.08% for total nitrogen (TN). Measure-ments of C and N concentrations of procedural blanks (sil-ver + tin capsules) were below detection limits, suggestingthat contamination from capsules and acid did not con-tribute much to our results. For comparison, a subset ofsamples (n = 18) without acid treatment are also analyzedfor d15N.

Potassium contents (K2O) were measured using a hand-held energy dispersive XRF spectrometer (HHXRF). Ref-erence material (GBW07107) with known values wasmeasured after every five samples. The analytical uncer-tainty for K2O was �10%.

4. RESULTS

The C and N isotope compositions, total organic carbon(TOC), total nitrogen (TN) and potassium contents areshown in Figs. 2 and 3 and in supplementary Table S1.Among these, the d13Corg data from the Zhongling andYuanjia sections were previously reported in Jiang et al.(2010). In Table S1, we also include available data fromthe Yangtze Gorges area (Kikumoto et al., 2014), theYangjiaping section (Ader et al., 2014), and the Lantiansection (Wang et al., 2017). The iron speciation, Mo con-centration, pyrite sulfur isotope (d34Spyrite) data (Sahooet al., 2012; Jin et al., 2018) and their relationship withd15N are provided in Fig. 4 and Supplementary Table S2.Comparison of d15N of untreated and acid-treated samplesis provided in Fig. 5 and Supplementary Table S3.

In the shelf margin Zhongling section (Fig. 2A), thed15N values vary from +0.3‰ to +7.4‰, with an averageof +4.1‰. Relatively high values clustering at around+4.5‰ are observed in the lower part of the analyzed inter-val, corresponding to a negative excursion in d13Corg. Lowd15N (+0.1‰ to +2.5‰) and high d13Corg (�27.6‰ to�28.7‰) values are present in the upper part of this section.

In the upper slope Siduping section (Fig. 2B), the d15Nvalues are variable in the first 2 m of Member II, with val-ues ranging from +4.1‰ to +7.5‰ and an average of+5.8‰. A gradual decrease of d15N from +7.1‰ to+4.6‰ is present in the upper part of the section before aminor rise to +5.8‰. The d13Corg values from the Sidupingsection fall in a narrow range between �34‰ and �32‰and show a general increase upsection (Jiang et al., 2010;Wang et al., 2016).

The d15N of the upper slope Taoying section showssome variability (+4‰ to 12.4‰) in the lower part(Fig. 2C), where strong Mo enrichments were reported(Fig. 3A). Positive d15N values continue upsection,independent of changes in Mo concentration, FeHR/FeT,FeP/FeHR, and d34Spyrite (Fig. 4A). The average d15N valuein this section is +5.5‰, similar to that of the Siduping sec-tion. However, the d13Corg values in the Taoying section are1–2‰ lower than those of the Siduping section. Mostd13Corg values are between �36‰ and �34‰, with a meanof �35.2‰.

Fig. 2. Variations of d15N, d13Corg, TN, TOC and C/N in Zhongling (A), Siduping (B), Taoying (C), Wuhe (D) and Yuanjia (E) sections. Thed13Corg data in the Zhongling (A) and Yuanjia (E) sections are from Jiang et al. (2010). Vertical dash lines in the d15N panels indicate theaverage d15N (+5‰) of the modern ocean nitrate pool.

224 X. Wang et al. /Geochimica et Cosmochimica Acta 240 (2018) 220–235

Fig. 3. Crossplots of (A) d13Corg and TOC, (B) TOC and TN, (C) TN and K2O, (D) TOC and C/N, (E) d15N and TN, (F) d15N and C/N.

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Compared with the other sections, the d15N values arerelatively stable throughout the Wuhe section (Fig. 2D).Most values are within a narrow range of +3‰ to+4.5‰ and have an average of +3.6‰, 1–2‰ lower thanthose from the Taoying and Siduping sections. Thisinvariant trend in d15N contrasts with the abrupt decreasein Mo concentrations from 172 lg/g in the lower part tonear crustal value in the upper part (Fig. 4B). Again, thed15N does not change with FeHR/FeT, FeP/FeHR, ord34Spyrite (Fig. 4B). The d13Corg values of this sectionshow a negative shift from �32.9‰ to �35.6‰ in thefirst 4 m of Member II, followed by a slight increase to�34.1‰ at the top of the section.

In the basinal Yuanjia section, the d15N starts withstable values around +3‰ and reaches high values up to+7.7‰ in the middle part (Fig. 2E). The last two sampleshave unusually high (+11.1‰) and low (+1.8‰) d15N val-ues. Similar to the Taoying and Wuhe sections, the d15Nvariation is independent of change in Mo concentration,FeHR/FeT, FeP/FeHR, or d34Spyrite (Fig. 4C). The d13Corg

displays a similar trend with that of d15N, with low butstable values (around �33‰) in the lower part and highbut variable values (from �32.8‰ to �27.4‰) in the upperpart of the section.

In all sections, the molar C/N ratios are dominated byvalues between 10 and 80 (Fig. 2), consistent with those

Fig. 4. The profiles of d15N, Mo content, Mo/TOC, Fe speciation and d34Spyrites from the Taoying (A), Wuhe (B), and Yuanjia (C) sections.Based on the iron speciation data, the data points are marked as oxic (FeHR/FeT < 0.22), oxic? (0.22 < FeHR/FeT < 0.38), ferrugenious(FeHR/FeT > 0.38), euxinic (FeP/FeHR > 0.7) and undefined (without Fe speciation data). The Mo, Mo/TOC, Fe speciation and d34Spyritesdata are from Sahoo et al. (2012). For the Yuanjia section (C), the Fe speciation data of Jin et al. (2018) are added (marked by thinner-linedcircles). Vertical dash lines in the d15N panels indicate the average d15N (+5‰) of modern ocean nitrate pool. Vertical dash lines in theMo and d34Spyrites panels indicate the average upper crust Mo concentration and the average d34S value of coeval carbonate associated sulfate(Li et al., 2010), respectively.

226 X. Wang et al. /Geochimica et Cosmochimica Acta 240 (2018) 220–235

reported from this period in other sections of South China(Kikumoto et al., 2014; Wang et al., 2017) and globally(Ader et al., 2014). Most untreated samples have d15N val-ues 1–3‰ higher than those of the acid-treated samples, butthere is no direct co-variation (Fig. 5).

5. DISCUSSION

5.1. Effects of acid treatment on d15N

The 1–3‰ difference in d15N between untreated andacid-treated samples (Fig. 5) raises concerns about theeffects of acidification on C and N isotope data. Many stud-ies have demonstrated that acid treatment and rinsing ofsediment samples would lead to changes in C, N concentra-tions and isotopes (e.g., Ryba and Burgess, 2002; Kennedyet al., 2005; Brodie et al., 2011; Meisel and Struck, 2011;Schlacher and Connolly, 2014). These studies mostly

focused on immature sediment or biological (e.g., shell,bones) samples in which some labile organic compoundsmay be lost during acidification and rinsing. In the rocksamples such as the ones analyzed in this study, the matureorganic matter may not be as reactive as fresh organic mat-ter. The d15N difference between untreated and acid-treatedrock samples is most likely caused by the loss of some clay-bound N and carbonate-associated N during acidificationand rinsing. Since clay-bound N could be acquired duringdiagenesis and metamorphism (see Section 5.2), acid-treated samples may better record the d15N of rock samples,as some researchers have suggested (e.g., Garvin et al.,2009; Meisel and Struck, 2011; Cremonese et al., 2013;Stueken, 2013; Stueken et al., 2015, 2017; Koehler et al.,2017; Luo et al., 2018). In addition, Ader et al. (2014) alsoshowed that there is no significant difference in d15Nbetween decarbonated and untreated samples from theDoushantuo Formation. Nonetheless, we recognize the

Fig. 5. Crosspot of the d15N values of untreated and acid-treatedsamples.

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need for a more carefully designed, comparative study ofuntreated and acid-treated rock samples in the future, par-ticularly for the EA-IRMS method (Ader et al., 2016). Con-sidering that up to 2‰ differences in d15N betweenuntreated and acid-treated samples are common (Rybaand Burgess, 2002; Kennedy et al., 2005; Brodie et al.,2011; Meisel and Struck, 2011; Schlacher and Connolly,2014), in the following sections, spatial and temporald15N variations smaller than 2‰ will not be overly inter-preted as paleoceanographic signals.

5.2. Diagenetic evaluation

The d13Corg data from the study sections are within thetypical range (�38‰ to �30‰) of the Ediacaran organic-rich black shales (McFadden et al., 2008; Jiang et al.,2010, 2012; Johnston et al., 2013; Wang et al., 2016; Liet al., 2017b). The lack of significant correlation betweend13Corg and TOC (Fig. 3A) suggests that the degradationof organic matter during diagenesis had limited effect ond13Corg. Nitrogen in sediments is present in two forms,organic-bound nitrogen and clay-bound nitrogen (mainlyas NH4

+) (see reviews in Ader et al., 2016; Stueken et al.,2016). Clay-bound N sourced from terrestrial input mayaffect the bulk d15N values in low-TOC sediments, but thehigh TOC contents and the strong positive correlationbetween TOC and TN in our samples (Fig. 3B) suggest thatthe nitrogen in our samples is mainly from organic matter.This inference is supported by the lack of correlationbetween TN and K2O (Fig. 3C).

During early diagenesis, degradation of some labilecompounds would release organic-bound nitrogen asNH4

+, which can be trapped by clays or oxidized to otherN forms (e.g., N2, N2O, NO�

2 or NO�3 , depending on the

O2 concentration in porewater). The isotope effect associ-ated with this process could be markedly different betweenanoxic and oxic environments. In oxic diagenetic environ-ments, degradation of labile organic compounds may

increase the bulk d15N by up to 4‰, due to the isotopicfractionation during NH4

+ release and partial oxidation ofNH4

+ in porewater (e.g., Altabet et al., 1999; Freudenthalet al., 2001; Lehman et al., 2002; Prokopenko et al.,2006). In anoxic diagenetic environments, anaerobic oxida-tion of organic matter only causes small (�1–2‰) N iso-tope change (Altabet et al., 1999; Freudenthal et al., 2001;Lehman et al., 2002; Thunell et al., 2004; Prokopenkoet al., 2006; Robinson et al., 2012), even in cases when morethan half of the organic nitrogen was converted to clay-bound NH4

+ (e.g., Muller, 1977). Iron speciation data fromthe study interval show spatial and temporal variations inbottom-water redox conditions (Sahoo et al., 2012, 2016;Och et al., 2016; Jin et al., 2018), but the lack of co-variation between d15N and FeHR/FeT or FeP/FeHR

(Fig. 4) suggests that the general d15N trend in these sec-tions was not significantly modified by changes in redoxconditions. However, it is likely that the few data pointsaway from the d15N trend in each section (Figs. 2 and 4)may have resulted from the degradation of labile organiccompounds under changing redox conditions.

During burial diagenesis, thermal maturation of organicmatter would release NH4

+, which can be absorbed by clayminerals in a closed system or sometimes migrates out ofthe system. However, the alteration of d15N through ther-mal maturation is commonly insignificant because thed15N of released NH4

+ is close to that of organic matterand bulk sediments (see review in Ader et al., 2016). As bur-ial depth and temperature increase, thermal denitrogena-tion associated with metamorphism may increase the d15Nof sediments through preferential removal of 14N. Thedegree of d15N change through metamorphism dependson the metamorphic grade of rocks, commonly within 1–2‰ for the greenschist facies, 3–4‰ for amphibolite facies,and up to 6–10‰ for upper amphibolite phases (e.g.,Bebout and Fogel, 1992; Mingram and Brauer, 2001; Jia,2006; Thomazo and Papineau, 2013; Ader et al., 2016;Stueken et al., 2017). Metamorphic alteration on the d15Nof our samples, if any, would be minor considering thatthe Ediacaran-early Cambrian rocks in South China arebelow the greenschist facies (Wang et al., 1993).

In summary, although elevated C/N ratios (Fig. 2) ofour samples relative to the Redfield ratio (C/N � 7.0)and the strong positive correlation between TOC andC/N (Fig. 3D) may imply partial loss of N during diagene-sis and metamorphism, the lack of correlation betweend15N and TN or C/N (Fig. 3E and F) suggests that the lossof N may not have substantially altered the d15N trend ofthe measured sections.

5.3. The N cycle in the early Ediacaran Yangtze platform

The new C and N isotope data from five sections, incombination with those previously published data fromthree other sections (Three Gorges core section, Yangjiap-ing section and Lantian section) (Ader et al., 2014;Kikumoto et al., 2014; Wang et al., 2017), show spatialvariations in both d13Corg and d15N (Fig. 6). In the lower4 m of the measured sections, most d13Corg values fallbetween �32‰ and �36‰, except for a few scattered high

Fig. 6. The d13Corg and d15N data of the basal Member II black shales of the Doushantuo Formation across the early Ediacaran YangtzePlatform. Data points are distinguished by their paleogeographic location. Data of the Zhongling, Taoying, Siduping, Wuhe, and Yuanjiasections are from this study (see Fig. 2). Data source of the other three sections: Three Gorges (Kikumoto et al., 2014); Yangjiaping (Aderet al., 2014); Lantian (Wang et al., 2017). Vertical dash lines in the d15N panel indicate the average d15N (+5‰) of the modern ocean nitratepool.

Fig. 7. Histogram of the d15N data from the basal Member IIblack shales of the Doushantuo Formation in South China. Thed15N data are compiled from the five sections of this study (Fig. 2)and three other sections previously published (Ader et al., 2014;Kikumoto et al., 2014; Wang et al., 2017). The mode of d15Ndistributions (+5 ± 1‰) is similar to that of the modern ocean.

228 X. Wang et al. /Geochimica et Cosmochimica Acta 240 (2018) 220–235

values that may be caused by incomplete decarbonationduring analyses and/or imprecise stratigraphic correlation(e.g., the same thickness does not mean the same amountof time). There is an up to 4‰ difference in d13Corg amongsections that does not follow a depth gradient. The lowerd13Corg in the Taoying (upper slope) and Wuhe (lowerslope) sections suggests chemoautrophic and/or methan-otrophic biomass contribution to TOC in high productivityregions of the basin. The d13Corg difference increases upsec-tion, which has been ascribed to the development of a larged13C gradient (Jiang et al., 2007; Hohl et al., 2015; Wanget al., 2016; Furuyama et al., 2017).

Most d15N values fall between +3‰ and +8‰, exceptfor a few scattered low (<2‰) and high (>9‰) values(Fig. 6). There are significant spatial variations in d15Nbetween sections, but there is no persistent depth-relatedgradient. The most important feature is that the d15N fromall sections is exclusively positive, with a mode of +5 ± 1‰(Fig. 7). Such a d15N distribution contrasts with those ofthe Mesoproterozoic basins that show spatially separatedaerobic and anaerobic N cycles from onshore to offshoreenvironments (e.g., Stueken, 2013; Koehler et al., 2017).

The positive d15N from the Member II black shalesacross the entire Yangtze Platform requires substantial lossof isotopically light nitrogen (14N) in the bioavailable Npool. If nitrate (NO3

�) was the main source of bioavailableN for assimilation, like in the modern ocean, denitrification

and/or anammox are the major processes for the loss of14N. If NH4

+ was the dominant N source for assimilation,potential processes in the N cycle for the loss of 14N mayinclude NH3 volatilization, partial assimilation of NH4

+,

δ15N(‰)

FeP

/FeH

R

0

0.2

0.4

0.6

0.8

1

1.2

0 2 4 6 8 10 12 14

TYWHYJ

δ15N(‰)

FeH

R/F

eT

0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12 14

TYWHYJ

0.7

0.38

0.22

Fig. 8. Crossplots of d15N and Fe speciation data. Horizontal dashlines indicate the thresholds to distinguish anoxic vs. oxic condi-tions (FeHR/FeT > 0.38, anoxic; FeHR/FeT < 0.22, oxic) and fer-rugineous vs. euxinic environments (FeP/FeHR > 0.7, euxinic)(Poulton and Canfield, 2011). Vertical dash line represents theaverage d15N of the modern ocean nitrate pool. Fe speciation dataare from Sahoo et al. (2012).

X. Wang et al. /Geochimica et Cosmochimica Acta 240 (2018) 220–235 229

partial nitrification followed by denitrification and/oranammox (e.g. Ader et al., 2016; Garvin et al., 2009;Godfrey and Falkowski, 2009; Papineau et al., 2009;Thomazo et al., 2011; Stueken, 2013; Godfrey et al., 2013;Stueken et al., 2015; Zerkle et al., 2017). NH3 volatilizationreleases isotopically light N (14N), with isotopic fractiona-tion up to 40–50‰ (Li et al., 2012b; Stueken et al., 2015),resulting in 15N enrichment in residual NH4

+. This process,however, happens only locally in restricted basins with sus-tained high alkalinity (Stueken et al., 2015), which is diffi-cult to be maintained in sedimentary basins like theYangtze platform that had open-ocean connection (Jianget al., 2011; Zhang et al., 2015). Partial assimilation ofNH4

+ by photoautotrophs preferentially takes 14N, resultingin progressive 15N-enrichment in the residual NH4

+ pool(e.g. Papineau et al., 2009; Higgins et al., 2012; Stuekenet al., 2016). This process happens in NH4

+-replete environ-ments with a stable NH4

+ pool and implies an anoxic photiczone. Positive d15N profiles resulting from partial NH4

+

assimilation commonly show temporally fluctuating andspatially variable negative and positive d15N values(Stueken, 2013; Papineau et al., 2009), which are not seenin the Member II black shales (Figs. 2 and 6). Partial nitri-fication of NH4

+ followed by complete denitrification couldresult in 15N-enriched NH4

+, which, if utilized by photosyn-thetic organisms, would produce organic matter with posi-tive d15N (Thomazo et al., 2011; Godfrey et al., 2013). Thisprocess has been invoked to interpret the positive d15Nshifts in predominately anoxic and stratified basins of thelate Archean and Paleoproterozoic ocean (Thomazoet al., 2011; Godfrey et al., 2013). It is, however, rarely doc-umented in modern ocean environments except in somelocal basins with strong seasonal oxygen fluctuations(Hadas et al., 2009; Granger et al., 2011; Morales et al.,2014). Even in the modern Black Sea, nitrification rapidlyproceeds to completion under suboxic conditions near thechemocline (Fuchsman et al., 2008). Therefore, partialnitrification-complete denitrification may have happenedin water bodies with strong transient variation in oxygenconcentrations, or in the shelf and upwelling regions ofthe late Archean and Paleoproterozoic ocean where a stableNH4

+ supply from the deeper, anoxic ocean was available,resulting in short-term positive d15N shift and spatially-restricted high d15N. It is difficult to interpret the persis-tently high d15N in our measured sections across the basin,which is independent of temporally fluctuating redoxchanges (Fig. 4).

The d15N distribution from the Member II black shales(Figs. 6 and 7) is similar to that of the modern ocean (e.g.,Sigman et al., 2009; Tesdal et al., 2013; Algeo et al., 2014).In the modern ocean, fixed N (ammonia) is quantitativelytransferred to nitrate (NO3

�) without significant isotopefractionation. Portion of the dissolved N pool in the oceanis lost through denitrification (NO3

� ?NO2� ? N2) and less

commonly anammox (NH4+ + NO2

� ?N2 + 2H2O) in theoxygen minimum zone (OMZ) or in sediments (Sigmanet al., 2009). Since denitrification and anammox release iso-topically light N2 that has d

15N values 20–30‰ lower thanits parent NO3

� (Sigman et al., 2009; Stueken et al., 2016),the residual nitrate pool in the ocean becomes

15N-enriched. Organic matter formed by photosyntheticorganisms that take N from the nitrate pool would havepositive d15N (average d15N � +5‰ in the modern ocean).The range of d15N values (+3‰ to +12‰) and the mode of+5‰ from the Doushantuo Member II black shales (Figs. 6and 7) are comparable with those of the modern ocean(ranging from +3‰ to +14‰ with a modal value of+5‰; Sigman et al., 2009; Tesdal et al., 2013; Algeoet al., 2014), implying an oxygenated photic zone with astable nitrate pool as the N source for photosynthesis.

5.4. Implications for the early Ediacaran ocean redox

structure

High enrichments of RSTEs (particularly Mo, V and U)and low pyrite sulfur isotope (d34Spyrite) data from the samestratigraphic interval were interpreted as evidence ofpervasive ocean oxygenation and expansion of the RSTE

230 X. Wang et al. /Geochimica et Cosmochimica Acta 240 (2018) 220–235

reservoirs during the early Ediacaran (Sahoo et al., 2012,2016; Gregory et al., 2017). This interpretation has beenchallenged by the fact that high RSTE enrichments compa-rable with those of the modern euxinic shales are onlyfound in three deep-water sections on the Yangtze platform(Fig. 4; Sahoo et al., 2012) and they are absent in other sec-tions in South China and globally (Och et al., 2016; Milleret al., 2017). On the contrary, the Fe-based redox proxydata suggest a strongly stratified early Ediacaran ocean,possibly topped with a thin oxic layer (e.g., Li et al.,2010; Sperling et al., 2015; Huang et al., 2017).

The two contrasting ocean redox structures would resultin different N behavior that can be tested and/or evaluatedby the d15N data. The modern ocean-like positive d15N dis-tribution and the lack of negative d15N values across theentire Yangtze Platform imply a stable nitrate pool thatcould be maintained only if the ocean was well oxygenated(Ader et al., 2014, 2016). In strongly stratified basins, suchas the modern Black Sea, denitrification (and anammox) atthe chemocline leads to rapid nitrate loss and enhanced Nfixation, lowering the d15N of dissolved N in the water col-umn (Fulton et al., 2012). Consequently, the nitrate concen-tration in the Black Sea is less than 3.5 lM (Fuchsmanet al., 2008), which is an order of magnitude lower than thatof the open ocean (�30 lM; Lam and Kuypers, 2011). Sim-ilarly, in some Mesoproterozoic basins, nitrate was dis-tributed only in the nearshore area, due to a shallowchemocline located probably within the photic zone in off-shore environments (Stueken, 2013; Koehler et al., 2017).During some Mesozoic oceanic anoxic events (OAEs) whenanoxic water impinged on the photic zone, N fixation andNH4

+ assimilation were the dominant processes maintainingprimary productivity owing to the depletion of the nitratepool (Jenkyns et al., 2007; Junium and Arthur, 2007;Higgins et al., 2012). Even in a dynamically maintained

Fig. 9. Schematic diagram showing the N cycle and ocean redox structpositive d15N distribution, in combination with Fe speciation, RSTE enricstable nitrate pool. During the later stage, expansion of local anoxic envdepletion of RSTEs, which in turn slowed down primary production anddrop of Mo concentrations to crustal values but virtually invariant d15Nproductivity, redox, N cycle and seawater Mo. Solid lines: positive feedb

stratified ocean with a relatively deep chemocline (but mostof the deep ocean is still anoxic), nitrate could be availableonly locally in the upwelling regions (Ader et al., 2016).Therefore, the positive d15N across the shelf-to-basin tran-sect suggests that the early Ediacaran ocean was likely welloxygenated. If the early Ediacaran ocean was anoxic andstratified, as many have suggested (e.g., Li et al., 2010;Johnston et al., 2013; Sperling et al., 2015; Huang et al.,2017), the chemocline could be, at least episodically, downto the mid-depth (e.g., >1000 m), based on the presence ofoxic water column in the basinal sections (Fig. 4) that werepaleobathymetrically estimated as greater than 1200 m(Jiang et al., 2010).

A significant number of positive d15N values from thestudy sections are preserved in euxinic black shales (Figs. 4and 8) indicated by Fe speciation data (Sahoo et al., 2012;Jin et al., 2018). Because denitrifiers outcompete sulfatereducers, persistent sulfidic conditions can only happenwhen nitrate is exhausted (e.g., Canfield, 2006; Lam andKuypers, 2011; Boyle et al., 2013). The co-occurrence ofpositive d15N values and euxinia in some of the analyzedsamples in the slope and basin sections implies that sulfidicconditions must have been localized and limited to bottomwaters far below the photic zone (Fig. 9). This redox struc-ture is consistent with the iron speciation and trace element(particularly, Mo) concentration data that show significanttemporal and spatial variations (Fig. 4; Sahoo et al., 2012;Jin et al., 2018). It also explains the spatial variations ind34Spyrite between the Taoying and other sections. In theTaoying section, relatively high d34Spyrite values are mostlyfrom oxic/suboxic shales (Fig. 4), in which sulfate reductionmay have happened in sulfate-limited porewater.

An interesting phenomenon is that positive d15N valuescontinue upward in spite of a sharp decrease of RSTEs inthe upper part of the analyzed interval (Fig. 4). The

ure during the early Ediacaran in South China. The modern-likehment, and d34Spyrites data, suggests a well oxygenated ocean with aironments due to increased productivity may have resulted in thehelped maintain the stable nitrate pool. This is evidenced by the fastupsection (Fig. 2). Inset indicates the feedback between primary

ack; dashed lines: negative feedback.

X. Wang et al. /Geochimica et Cosmochimica Acta 240 (2018) 220–235 231

decrease of RSTEs was ascribed to the shrink of the RSTEreservoirs in response to the expansion of anoxic conditionsafter a brief oxygenation event (Sahoo et al., 2016). The dis-crepancy between the decrease of RSTEs and virtuallyinvariable positive d15N values may be explained by theincrease of bottom-water anoxia well below the photiczone, which could have efficiently lowered the RSTEs butstill maintained a stable nitrate pool. In addition, the low-ering of RSTEs may have led to nutrient deficiency, slowingdown primary productivity and nitrogen demand that mayhave helped maintain the stability of the nitrate pool. Thisphenomenon, however, reinforces the limitation of usingd15N data to interpret the deep-ocean oxygenation: positived15N values across sedimentary basins may imply oxicwater columns below the photic zone but are difficult togive quantitative information about the bottom-waterredox conditions.

6. CONCLUSIONS

High resolution d15N and d13Corg data were reportedfrom multiple sections of the basal Doushantuo Formationblack shales across a shelf-to-basin transect of the earlyEdiacaran Yangtze Platform. The exclusively positived15N values of +3‰ to +12‰ with a mode of +5 ± 1‰from the basal Doushantuo shales are comparable withthose of the modern ocean (+3‰ to +14‰ with a modeof +5‰) and are interpreted as recording the isotope signa-ture of a stable nitrate pool that extended at least downbelow the photic zone. In combination with iron speciation,sulfur isotope, and trace element concentration data avail-able from the same stratigraphic interval, it implies thatthe early Ediacaran ocean, if it was stratified, would havethe chemocline well below the photic zone, possibly closeto the seawater/sediment interface, instead of a stronglystratified ocean with a thin oxic surface layer. A stablenitrate pool in the ocean may have helped lift the N limita-tion barrier and accelerate the evolution and diversificationof eukaryotes.

ACKNOWLEDGMENTS

This research is funded by National Natural Science Founda-tion of China (41402026) and the Fundamental Research Fundsfor the Central Universities (2652017228). We thank JunhuaHuang, Biao Chang and Haoming Wei for their kind helps fororganic carbon isotope and potassium content analyses. Thanksare given to Prof. Timothy W. Lyons for discussions on thenitrogen isotope data and to Dr. Swapan Sahoo for the earlierjoint field work in some of the sections (Wuhe and Taoying).We are grateful to Prof. Thomas Algeo and three anonymousreviewers for their constructive comments that helped improvethe manuscript.

APPENDIX A. SUPPLEMENTARY MATERIAL

Supplementary data associated with this article can befound, in the online version, at https://doi.org/10.1016/j.gca.2018.08.034.

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Associate editor: Jack J. Middelburg