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Geochimica et Cosmochimica Acta 239 (2018) 330–345

Zn-Sr isotope records of the Ediacaran Doushantuo Formationin South China: diagenesis assessment and implications

Yiwen Lv a, Sheng-Ao Liu a,⇑, Huaichun Wu b, Simon V. Hohl c, Shouming Chen d

Shuguang Li a

aState Key Laboratory of Geological Processes and Mineral Resources, and School of Earth Science and Resources, China University

of Geosciences, Beijing 100083, ChinabSchool of Marine Sciences, China University of Geosciences, Beijing 100083, China

cState Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210023, Chinad Institute of Geology, Chinese Academy of Geological Science, Beijing, China

Received 3 July 2017; accepted in revised form 2 August 2018; Available online 11 August 2018

Abstract

Recent studies show that zinc isotopes could provide valuable clues to environmental change and biogeochemical cycle ofthe past oceans. This study reports a modified procedure for leaching the carbonate fractions in sedimentary rocks, a thor-ough evaluation of diagenetic effects, and systematic variations of Zn and Sr isotope ratios in lower part of the Ediacaranstratigraphic unit deposited in the aftermath of the Marinoan glaciation in South China. The influence of post-depositional diagenesis on Zn isotope compositions of the studied samples is assessed by comparing d66Zn to other geochem-ical indexes (87Sr/86Sr, d13C, d18O and Mn/Sr ratios). In the five studied cap carbonate sections (Member I of the DoushantuoFormation), dolostones from four sections have d66Zn values positively correlated with d18O values and negatively correlatedwith 87Sr/86Sr ratios (0.7081–0.7204). These correlations suggest that these cap dolostone samples have been modified by post-depositional diagenesis. The light d66Zn value (�0.02‰) suggests that initial Zn isotope ratios of cap dolostones could havebeen reset by hydrothermal fluids with relatively high Zn concentration and low d66Zn values.

By contrast, carbonates from Member II of the Doushantuo Formation above cap dolostones are relatively pristine basedon their low 87Sr/86Sr ratios (0.7079–0.7086) being indistinct from the proposed early Ediacaran seawater 87Sr/86Sr values.Chemical and isotopic variations in these samples are interpreted to reflect primary signals that record paleo-environmental changes of the early Ediacaran ocean. A rapid increase of d66Zn from �0.3‰ to 1.1‰ occurs in the middlepart of Member II, accompanying by relatively invariant 87Sr/86Sr ratios that imply insignificant changes in input from con-tinental weathering. Considering the limited change in atmospheric oxygen during this period, the rapid d66Zn raise indicatesan increase in buried organic matters, which is consistent with the coupled positive shift of d13Ccarb, as well as the fossilrecords found in the same strata. These results provide insights into Zn cycling in the post-Marinoan oceans and facilitatethe application of Zn isotopes in carbonates as a proxy for the fate of marine organic matter.� 2018 Elsevier Ltd. All rights reserved.

Keywords: Zinc isotopes; Strontium isotopes; Diagenesis; Cap dolostones; South China

https://doi.org/10.1016/j.gca.2018.08.003

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

⇑ Corresponding author.E-mail address: lsa@cugb.edu.cn (S.-A. Liu).

1. INTRODUCTION

Zinc is an essential micronutrient for organism andplays a key role in controlling the primary productivity in

Fig. 1. Zinc isotope compositions of marine carbonates reported inprevious studies and this paper. Data sources: Neoproterozoicdolostones, John et al. (2017) and this study; Phanerozoic deep-seacarbonates, Pichat et al. (2003); Others, Little et al. (2016).

Y. Lv et al. /Geochimica et Cosmochimica Acta 239 (2018) 330–345 331

modern seawater (Morel and Price, 2003). The concentra-tion patterns of Zn in oceans mirror those of essentialmacronutrients like phosphorus and nitrogen, which aredepleted in the surface seawater and enriched in deepoceans (Bruland et al., 1978; Vance et al., 2017). The uniquepattern is typically ascribed to the utilization of Zn by pri-mary producers in the photic zone and the regeneration indeep seawater (Moore et al., 2013).

Analytical advances in the determination of Zn isotopecomposition of seawater have promoted the understandingof bio-geochemical cycling of Zn in the modern ocean(Marechal et al., 2000; Vance et al., 2008, 2016; John andConway, 2014; Little et al., 2014, 2016; Zhao et al., 2014).The deep seawater has a homogenous Zn isotope composi-tion (0.50 ± 0.15‰; d66ZnJMC 3-0749L), in contrast to thelarge Zn isotope variation (up to �1‰) of surface seawaterwhich is probably controlled by biological processes(Conway and John, 2014; John and Conway, 2014; Zhaoet al., 2014; Samanta et al., 2016). Intracellular Zn in phy-toplankton is as much as 0.8‰ lighter than the dissolvedphase in culture experiments, domonstrating that biologicalassimilation preferentially takes up the lighter Zn isotopes(John and Conway, 2014; Kobberich and Vance, 2017).Biological scavenging of heavy Zn onto organic matter isanother vital process to control the Zn isotope compositionof surface seawater (John and Conway, 2014). Riverine Znwith d66Zn of about 0.3‰ is the main input to the ocean,which is lighter than the seawater average (�0.5‰; Littleet al., 2014). Carbonates and Fe-Mn oxides, the majorsinks, have heavier Zn isotope compositions compared withseawater (Marechal et al., 2000; Pichat et al., 2003), whileorganic carbon-rich and sulfide-rich sediments are isotopi-cally lighter than seawater (Little et al., 2016; Vanceet al., 2016). The established Zn isotope budget of the mod-ern ocean is the foundation for utilization of Zn isotopes totrace paleoceanographic changes and to reconstruct ancientbiogeochemical cycles of Zn (e.g., John et al., 2017; Liuet al., 2017; Wang et al., 2018). For example, Liu et al.(2017) reported a rapid negative shift of Zn isotope ratiosright before the mass extinction at the Permian-Triassicboundary, which was interpreted as a result of rapid andmassive input of silicate Zn linked to intensive volcanism.

Understanding the fractionation of Zn isotopes duringmarine carbonate precipitation is important for applyingthe Zn isotope proxy in paleo-environmental reconstruc-tion. Relevant work includes theoretical modelling (Fujjiet al., 2014) and experimental studies (Veeramani et al.,2015; Dong and Wasylenki, 2016). Experiment resultsshowed that heavy Zn isotopes are preferentially incorpo-rated into carbonates during precipitation and adsorptionprocesses (Veeramani et al., 2015; Dong and Wasylenki,2016). While deep-sea carbonates are on average 0.4‰heavier than seawater, they display a large Zn isotope vari-ation ranging from 0.32‰ to 1.34‰ (Fig. 1; Pichat et al.,2003). Neoproterozoic cap dolostones from Australia havea wide range of d66Zn values from 0.14‰ to 0.98‰ (aver-age = 0.42‰), most of which overlap the range of clasticsediments (0.2–0.4‰) and are lower than Phanerozic car-bonates (�0.9‰) and modern seawater (Fig. 1;Kunzmann et al., 2013; John et al., 2017). These low

d66Zn values have recently been interpreted to record theincreased Zn sequestration in sulfide phases following theMarinoan glaciation (John et al., 2017). Commonly, zincisotope analysis of carbonates is achieved via leaching tech-nique aiming to seperate carbonate from detrital minereals.However, the influence of leaching techniques and diagene-sis on carbonate Zn isotope signatures is not yet well con-strained. Especially for dolostones, the effect of acidstrength on the recovery of Sr and other trace metals awaitsfurther investigation (Liu et al., 2013a). Further, it is neces-sary to evaluate the possible contributions of siliciclasticcomponents and diagenetic carbonate phases. Sequentialleaching is an approach to obtain reliable information ofisotopic composition in carbonates. Previously, a two-stepleaching procedure has been designed to extract the carbon-ate fraction of sediments for Zn isotope analysis (Pichatet al., 2003; John et al., 2017; Liu et al., 2017). Here, webuild on an approach by including other parameters suchas C-, O-, Sr-isotopes and Mn/Sr ratios with the purposeof assessing diagenesis influence on carbonate Zn isotopevalues. Such work is foundamental for the application ofZn isotope proxy in constraining paleoceanographicconditions.

332 Y. Lv et al. /Geochimica et Cosmochimica Acta 239 (2018) 330–345

In this study, a series of sequential leaching steps havebeen designed and tested based on the previous work ofLiu et al. (2013a) to extract the presumably primary Znand Sr isotope ratios in carbonates. Following the estab-lished method, we conducted a combined study of C-, O-,Sr-isotopes and Mn/Sr ratios on cap dolostones directlyoverlying the Cryogenian Nantuo diamictite and the Chen-jiayuanzi section from the lower Doushantuo Formation inSouth China. The Doushantuo Formation is fossiliferousand records the early evolution of multicellular organisms(Xiao et al., 1998). All of the obtained data are used toidentify the primary Zn isotope signatures and to recon-struct paleoceanographic conditions in the aftermath ofthe Marinoan glaciation.

2. GEOLOGICAL SETTING AND SAMPLE

DESCRIPTION

The Doushantuo Formation in South China, depositedca. 635–551 Ma ago, has been widely studied to understandthe biological evolution and accompanied geochemicalchanges in the ocean after the Marinoan glaciation (Jianget al., 2011). Marine sediments of the Doushantuo Forma-tion are thought to have been deposited in a passive conti-nental margin on the southeastern side of the YangtzeBlock (Jiang et al., 2007). The Doushantuo Formation cov-ers most of the Ediacaran Period and has a well-establishedage of 635 ± 0.6 Ma at the base and 551.1 ± 0.7 Ma at thetop based on precise U-Pb dating (Condon et al., 2005).

The base of the Doushantuo Formation (Member I)consists of 3–8-m-thick cap dolostones that are in sharpcontact with the underlying diamictites of the Nantuo For-mation (Jiang et al., 2006a). The locations of the five stud-ied cap dolostone sections (i.e. Tongren (TR), Sidouping(SDP), Wuhe (WH), Duoding (DD) and Chenjiayuanzi

Fig. 2. (A) Locations of cap carbonates in South China and (B) depositishales in South China (modified after Jiang et al., 2011). In Fig. 2B, the upthe lower panel represents conditions during deposition of strata overlyingTongren, SDP = Sidouping, CJYZ = Chenjiayuanzi.

(CJYZ)) are distributed along a platform-to-basin transect(Fig. 2a). Of these, the TR, SDP and WH sections wereformed in slope environments, whereas the DD and CJYZsections were formed in shallow-water shelf environments(Fig. 2b; Jiang et al., 2011). Jiang et al. (2006a) reporteda detailed description of sedimentary structures for theWH, DD and SDP sections, and thus only a brief descrip-tion is summarized below. The base layer (C1) consists of astrongly brecciated dolomite with secondary calcite cemen-tation and tepee-like structures; the overlying unit (C2) con-sists of a laminated, less disrupted dolomite; the top unit(C3) is usually a silty lime-mudstone of variable thickness.The C3 unit was not developed in the WH section.

The CJYZ section was chosen here to study the earlyEdiacaran era. It is only 600 m away from the Tianjiayuanzisection and is the candidate for the Global Stratotype Sec-tion and Point (GSSP) for the base of the Ediacaran System.The studied section starts with cap dolostones overlying theCryogenian Nantuo diamictite. Above cap dolostones, thelithology of CJYZ section gradually transits from dolo-stones into mudstones and muddy dolostones in the middleof Member II of the Doushantuo Formation (Liu et al.,2014b). The basal Member II black shales were depositedduring the latest stage of postglacial transgression or sea-level highstand. The depositional environment is describedas an open shelf during the time of cap dolostone depositionand changes transitionally into an intrashelf basin or lagoon(Fig. 2; Jiang et al., 2011; Liu et al., 2014b). Here we studiedthe carbonate part from the lower part (45 m-thick; MemberI and part of Member II) of the 180 m-thick DoushantuoFormation in the CJYZ section. Stratigraphic correlationof the Doushantuo Formations between CJYZ and othersections has been reported by Liu et al. (2014b). Based onstratigraphic correlation, the cap dolostones are reasonablyassigned an age of ca. 635 Ma.

onal environments of cap dolostones and overlying carbonates andper panel represents deposition of the cap dolostone and shales andthe Doushantuo Formation. DD = Duoding, WH =Wuhe, TR =

Y. Lv et al. /Geochimica et Cosmochimica Acta 239 (2018) 330–345 333

There are three prominent negative d13Ccarb anomaliesidentified in the Doushantuo Formation (Jiang et al.,2007). Our samples cover the first negative d13Ccarb anom-aly (EN1) in the Member I and subsequent positive d13Ccarb

excursion (EP1) in the Member II of the Doushantuo For-mation. The basal part of the CJYZ section is 2 m thick capdolostones (Member I of the Doushantuo Formation) withnegative d13Ccarb values (�0.1‰ to ��3.5‰; EN1). Thedolostones below 8 m from Member II of the DoushantuoFormation have d13Ccarb values increasing from +1‰ to+8‰, while the dolostones above 8 m have relatively steadyd13Ccarb values around +5‰ (EP1; Jiang et al., 2011; Liuet al., 2014b). Sahoo et al. (2012, 2016) reported the absenceof redox change during the deposition of the EN1 and EP1intervals, except an oxygenation event at ca. 635 Marecorded in shales overlying cap dolostones. Large numbersof microfossils including acanthomorphic acritarchs werepreserved in Member II of the Doushantuo Formation,and a detailed biostratigraphic framework has beenreported in Liu et al. (2013b, 2014b). In the CJYZ section,acanthomorphic acritarchs start firstly at the horizon at 9.2m above the base of the Doushantuo Formation, andincrease significantly at about 40 m.

3. METHODS

3.1. Sequential leaching procedure

In order to eliminate the altered part as much as possible,presumably unaltered areas of dolomicrites (DM) weremicro-drilled avoiding sheet-crack cements such as calciteand siliceous veins and pyrite laminas. To obtain primaryelemental and isotopic information, 50 mg 200-mesh car-bonate sample powder was placed into centrifuge vials andtreated with a 13-step leaching procedure. This leaching pro-cedure is modified from a method previously utilized foreffectively leaching Sr in carbonates by Liu et al. (2013a).Three cap dolostone samples (WH-18, DD-10 and CJ0.1)from the WH, DD and CJYZ sections were selected toestablish the leaching procedure and to check for the relia-bility of leaching data. These samples have Si, Fe and Mgcontents covering most of the range for samples analyzedin this study (Table S1). Leaching is in the following order:2 steps of 1 mol�L�1 ammonium acetate (N1 andN2; remov-ing adsorbed Zn on mineral surfaces and/or exchangeableions in clays), 2 steps of 0.27 vol.% acetic acid (S1 and S2;removing secondary calcite), 2 steps of 1 vol.% acetic acid(S3 and S4), 5 steps of 5 vol.% acetic acid (S5–S9), and 2steps of 10 vol.% acetic acid (S10 and S11). More detailsof the target phase for different reagents will be discussedin Section 5.1. At each step, samples were treated with thesupersonic treatment for 10 min, heating at 65 �C for 20min and centrifugation at 3600 rpm for 5 min. Leachatewas collected by filtration through 0.22 lm cellulose acetatesyringe filters. 87Sr/86Sr ratios and elemental concentrationsof supernatants were analyzed for each step, but steps S5–S10 were combined for Zn isotope analysis because Zn inleachate from each of these steps (<100 ng) was not enoughfor Zn isotope analysis. The data reliability obtained fromthis method will be discussed in detail in Section 5.1.

Following the 13-step leaching procedure, all dolostonesamples were leached with steps S9–S11 for element concen-tration and Sr isotope analysis and steps S3–S10 for Zn iso-tope analysis. Zinc obtained in steps S3–S10 was �300 ngfor each sample. To remove potential organic material insolution before elemental and isotopic analysis, the liquidfraction was evaporated to dryness and dissolved in concen-trated nitric acid three times. All experiments were per-formed in a clean room under a laminar flow hood (class100). The total procedural blank was 8 ng for Zn and 5ng for Sr.

3.2. Element concentration analysis

Major and trace elemental concentrations of each leach-ing step and bulk rock samples were obtained using an Agi-

lent 7900 inductively coupled plasma-mass spectrometry(ICP-MS) at the Mineral Laser Microprobe Analysis Lab-oratory at the China University of Geosciences, Beijing(CUGB). The analytical setup used a Scott type quartzspray chamber and a 100 ll/min nebulizer. Aspiration timewas 120 s with 25 samples/peak and 95 total scans. Sampleswere prepared in 5% (g/g) HNO3 by diluting stock solutionsto relatively low concentrations (1,000,000 � dilution) formajor elements (Mg, Ca and Al) and to higher concentra-tions (1000 � dilution) for trace elements (Sr, Zn, Cr andV). Element concentrations were determined by externalcalibration against in-house standards and internal correc-tion for mass drift using Rh (0.0125 lg/ml) as an internaldrift monitor. Background corrections were performed bysubtraction of the raw intensities of aspirated 5% HNO3.The international igneous rock standards (BCR-2 and W-2a) were used to monitor accuracy and precision of analy-sis. Analytical precision was better than ± 5% (RSD) forSr, Zn, Cr, V and Al and better than ±10% for Mg and Ca.

3.3. Zn and Sr isotope analysis

Zinc and Sr isotope analysis was carried out on aThermo-Finnigan Neptune plus multi-collector inductivelycoupled plasma mass-spectrometry (MC-ICPMS) equippedwith a Cetac ASX-110 automatic sampler and a PFATeflon self-aspirating micronebulizer system at the IsotopeGeochemistry Laboratory of CUGB. Details of the Znpurification procedure and isotope ratio analysis are out-lined in Liu et al. (2014c) and Lv et al. (2016), and arebriefly described below. Zinc separation was achieved byone column purification using 2 ml of Bio-Rad strong anionresin AG-MP-1M. For isotope analysis, samples werediluted to �200 ppb Zn solution in 3% (g/g) HNO3. Thesample–standard bracketing method was used for mass biascorrection. The whole-procedural analysis of Zn isotopeswas obtained from repeated analysis of leachates from thesame sample and the reproducibility is better than±0.10‰ (2SD). Zinc isotope compositions are reported inthe d-notation as per mil deviation from standard referencematerial (JMC 3-0749L): d66,68Zn (‰) = [(66,68Zn/64Zn)sam-

ple/(66,68Zn/64Zn)JMC3-0749] � 1. The international rock

standards COQ-1 (carbonatite) and BHVO-2 (basalt) wererepeatedly analyzed and yielded d66Zn values of 0.27 ±

Tab

le1

Al/Mg(g/g),

87Sr/86Sran

dd6

6Zn(‰

)ratiosobtained

intheleachingexperim

ent.

Step

DD-10

CJ0.1

WH-18

DD-10

CJ0.1

WH-18

DD-10

CJ0.1

WH-18

Al/Mg

87Sr/86Sr

d66Zn

d68Zn

Zn(ng)

d66Zn

d68Zn

d66Zn

d68Zn

N1

0.01

90.00

60.39

70.71

059

0.71

600

0.71

552

0.17

0.35

870.30

0.64

n.a

n.a

N2

1.11

50.02

70.85

10.71

306

0.71

038

0.71

517

0.46

0.96

660.07

0.15

n.a

n.a

S1

0.01

30.00

10.00

10.71

260

0.70

953

0.71

462

0.63

1.23

211

0.96

1.94

0.78

1.59

S2

0.07

60.01

90.01

10.71

277

0.70

840

0.71

369

0.55

1.15

256

0.59

1.20

0.60

1.20

S3

0.03

80.00

70.01

30.71

291

0.70

799

0.71

340

0.81

1.63

301

0.52

1.03

0.58

1.12

S4

0.01

70.00

70.02

20.71

256

0.70

750

0.71

317

0.74

1.50

246

0.62

1.23

0.70

1.48

S5

0.00

60.01

00.01

60.71

176

0.70

755

0.71

214

S6

0.01

10.00

50.01

40.71

125

0.70

739

0.71

126

S7

0.00

50.00

40.01

20.71

093

0.70

731

0.71

066

0.70

*1.43

*40

2*0.49

*0.93

*0.80

*1.61

*

S8

0.01

10.01

00.01

90.71

050

0.70720

0.71

000

0.75±0.06(1sd)

1.47±0.05(1sd)

0.54±0.07(1sd)

1.06±0.15(1sd)

0.69±0.11(1sd)

1.40±0.25(1sd)

S9

0.01

80.00

50.02

80.71039

0.70

727

0.70

979

S10

0.01

70.00

90.01

90.71

073

0.70

733

0.70944

S11

0.02

80.01

90.04

30.71

086

0.70

755

0.71

300

Note:87Sr/86Srratioin

bold

typeisthelowestva

lueobtained

ineach

step

ofleachingexperim

ent.Zincisotopicratioswithasterisk

(*)aretheva

lues

ofcombined

steps5–

10.Thenumbersin

italic

typearetheav

erag

ean

dstan

darddeviation(1sd)ofZnisotopic

ratiosofsteps3–

10.Zn(ng)

forallstepsisfrom

sample

DD-10.

334 Y. Lv et al. /Geochimica et Cosmochimica Acta 239 (2018) 330–345

0.04‰ (2SD) and 0.28 ± 0.04‰ (2SD), respectively, consis-tent with the recommended values (Chen et al., 2013; Liuet al., 2016; Wang et al., 2017).

Strontium was separated from matrix elements usingcation exchange resins (4 ml Bio-Rad AG50W-X12), andSr isotope ratios were measured using the MC-ICPMS atCUGB. Mass fractionation was corrected assuming 88Sr/86-Sr 8.375209 and using the exponential law (Nier, 1938).Minor interferences of 87Rb on 87Sr were corrected using 85-Rb/87Rb = 2.59265. The 2r uncertainty of analytical repro-ducibility for Sr isotope ratio measurement is between0.0020% and 0.0030%. The 87Sr/86Sr ratios of the NBS-987 standard measured after the analysis of every ten sam-ples yielded an average value of 0.71028 ± 0.00002 (2r, n =15).

4. RESULTS

4.1. Sequential leaching results of carbonates

Trace element concentrations, Zn and Sr isotope com-positions of the leaching steps of three selected dolostonesamples (CJ0.1, DD-10 and WH-18) are reported in Table 1and plotted in Fig. 3. The leaching results obtained in thisstudy yield a very similar 87Sr/86Sr pattern to the study ofLiu et al. (2013a). In detail, 87Sr/86Sr ratios decrease withincreasing proportions of the leached fraction and reach alowest value in step S9 or S10, then increase suddenly instep 11. The first four steps (N1, N2, S1 and S2) have rela-tively high 87Sr/86Sr and Al/Mg ratios and variable d66Zn,especially the N1 and N2 leached by ammonium acetate(Fig. 3). Leachates from the following steps (S3, S4 andS5–S9) show steady and low Al/Mg ratios, and d66Zn val-ues of these steps are almost constant within the externalreproducibility (±0.1‰, 2SD).

4.2. Elemental and isotopic variations in cap dolostones and

Doushantuo Member II

Major elemental concentrations of bulk carbonates arelisted in Table S1 in the Supplementary materials. Elemen-tal ratios, Zn and Sr isotopes of the leachates are listed inTable 2. We used element ratios, measured in the same ali-quot, to describe the relative change of elemental contentsin different leaching steps. Zn isotope variations of capdolomites show different patterns in different transects(Fig. 4). The TR section shows relatively high d66Zn values(up to 1.1‰) in the upper part, similar to the patternobserved in Nuccaleena cap dolostone from Australia(John et al., 2017). By contrast, other cap dolomite sections(DD, WH and SD) have d66Zn values that are relatively low(0.0–0.6‰) in the upper part but are higher in the lowerpart (Fig. 4).

The d13Ccarb and d18Ocarb values of WH, DD and SDPsections have been reported previously in Jiang et al.(2006b). Despite a weak positive correlation (R2 = 0.39)between d13Ccarb and d18Ocarb in the TR cap dolomites,samples from other sections as well as the CJYZ transectdo not show a similar correlation (Fig. S1). The d13Ccarb

and d18Ocarb values of all cap dolomites studied are not cor-

Fig. 3. Sequential leaching results of three selected samples forelemental, Zn and Sr isotopic analysis. Data are reported Table 1.The leachates after step S2 (S3-S10) are taken to represent thecomposition of the primary carbonates. See text for interpretation.

Y. Lv et al. /Geochimica et Cosmochimica Acta 239 (2018) 330–345 335

related with stratigraphic height (Fig. S2). d18Ocarb valuesof most TR and SDP samples are below �6‰ with an aver-age of �9‰ and �8‰, respectively, lower than valuesobtained for DD, WH and CJYZ samples (�2‰, �7‰and �3‰, respectively). The two transects TR and SDPalso have the highest 87Sr/86Sr (0.71675 and 0.71196). Therange of 87Sr/86Sr ratios for all cap dolostones from theWH, SDP, DD and TR sections is from 0.7081 to 0.7204.The SDP transect has lowest d66Zn value of 0.14 ± 0.27‰(Fig. 5). The C, O and Zn isotope variations of DD transectare in the same range as the WH and CJYZ transects, whilethe obtained 87Sr/86Sr ratios are elevated in these samples(up to 0.71354). 87Sr/86Sr ratios at WH and CJYZ rangefrom 0.70727 to 0.70968.

The cap dolomites have homogeneous Al/Mg(g/g) andMg/Ca(mol/mol) ratios, and they are not correlated with both87Sr/86Sr and d66Zn (Table 2; Fig. 6a and b). Above capdolomites, samples of Member II of the Doushantuo For-mation at CJYZ have Mg/Ca(mol/mol) ratios (0.32–0.81)lower than that of cap dolomites (average = 0.88; Table 2).The Mn/Sr(g/g) ratios of the leachates are variable and someare as high as 76 (Fig. 6c). The Mn/Sr ratios of CJYZ sam-ples decrease up-section from a ratio of �13 but drop downto less than 1 above the height of 8 m.

5. DISCUSSION

In this section, we first evaluate the reliability of dataobtained from the leaching technique. Then, we assess the

influence of post-depositional diagenesis on obtained Znisotope ratios of carbonates by considering co-variationof d66Zn with other geochemical indices (87Sr/86Sr, d13Ccarb,d18Ocarb, and Mn/Sr) in the same samples. Finally, weattempt to reconstruct paleoceanographic changes in thepost-Marinoan ocean on the basis of RSEs concentration,Zn, Sr and C isotopes of the least altered samples.

5.1. Leach experiments to evaluate diagenetic overprint

The first two leaching steps (N1 and N2) using ammo-nium acetate have the highest 87Sr/86Sr ratios and likely rep-resent adsorbed Sr ions on mineral surfaces and/orexchangeable ions in clay phases (Liu et al., 2013a). Thesefractions have d66Zn values of 0.07–0.43‰, lower than those(0.52–0.96‰) obtained in the acetic acid leaching steps (S1–S10) (Table 1; Fig. 3). These low-d66Zn components are likelyrelated to preferential adsorption of Zn ontominerals includ-ing Fe-Mn oxides (D66Znsolution-surface = �0.2‰ to +0.3‰;Pokrovsky et al., 2005; Bryan et al., 2015). Zinc adsorptedonto clay phases could have considerable effects on 87Sr/86Srcompositions of leachates, although it may play a minor rolein Zn isotope compositions. Heavy Zn is preferentiallyadsorbed onto clays with Zn isotope fractionation varyingfrom 0.11‰ to 0.49‰ (Guinoiseau et al., 2016), which isinconsistent with the low-d66Zn values obtained in N1 andN2 steps. Although isotope fractionation of Zn may varywhen adsorbed to different components, the significantlylower d66Zn and higher 87Sr/86Sr and Al/Mg of steps N1and N2 indicate that these components should be eliminatedin order to obtain primary elemental and isotope signaturesof the carbonate fraction.

Fractions S1 and S2, which were leached with 0.27 vol.%acetic acid, have high radiogenic Sr isotope ratios (Fig. 3).These fractions represent dominantly secondary calciteswith low Mg/Ca and high Rb/Sr ratios (Liu et al.,2013a). The Al/Mg ratio was measured to evaluate the rel-ative proportions of silicates and dolomites in this leach,assuming that Al and Mg are the major elements in silicateand dolomite, respectively. Al/Mg ratios were slightly highin the S2 fraction. Zinc isotope ratios of fraction S1 and S2were also more variable than the values obtained for frac-tions S3–S10 (Fig. 3). This difference is possibly related toisotopic variations in fluids from which secondary calcitesprecipitated. Overall, these two fractions should also beeliminated in order to obtain primary information of thecarbonate fraction.

Zinc isotope ratios of fractions S3–S10 from all threetest samples were much less variable that those obtainedin earlier leaching steps (N1, N2 and S1–S2). The S3–S10fractions have relatively constant d66Zn values with a smallstandard deviation of 0.06–0.11‰ (1 s.d.), and also havealmost constant Al/Mg ratios (Table 1 and Fig. 3). Wetherefore conclude that the Zn fractions of stage S3–S10are from the same (Mg-rich) phase, which could only bedolomite given that dolomite is the dominant phase in thestudied dolostones. A slight increase in Al/Mg and 87Sr/86-Sr ratios of step 11 could be the result of dissolution of sil-icate phase after most carbonates were dissolved.Consequently, we hypothesize that Zn isotope ratios of

Table 2C, O, Sr and Zn isotope compositions, Mg/Ca, Al/Mg and Mn/Sr ratios of all samples analyzed in this study.

Sample No. Depth (m) 87Sr/86Sr d13C (‰)a d18O (‰)a d66Zn (‰) 2sd d68Zn (‰) 2sd 68Zn/66Zn Al/Mg (g/g) Mg/Ca (mol/mol) Mn/Sr (g/g)

Tongren

TR-75 8.1 ndb �0.6 �9.7 0.36 0.02 0.73 0.03 1.99 0.023 0.86 9TR-73 7.9 0.715098 �0.9 �9.5 0.52 0.03 1.02 0.05 1.96 0.004 0.93 12Repeat 0.62 0.02 1.24 0.08 1.99TR-68 7.1 nd �1.9 �9.8 0.39 0.03 0.78 0.07 1.98 nd nd ndTR-67 7.0 nd �1.8 �8.7 0.28 0.02 0.58 0.05 2.06 nd nd ndTR-61 6.3 nd �2.3 �9.3 0.31 0.04 0.62 0.04 1.97 nd nd ndTR-58 5.4 0.717284 �0.9 �8.9 0.31 0.04 0.61 0.04 1.95 0.006 0.94 13TR-52 4.9 0.718780 �3.1 �11.6 0.30 0.03 0.59 0.03 1.98 0.010 0.99 6TR-47 4.3 nd �2.9 �10.5 0.28 0.04 0.56 0.03 1.97 0.010 0.93 21TR-40 3.9 nd �2.5 �11.0 0.32 0.01 0.63 0.02 1.98 0.007 0.91 15TR-31 3.1 0.718138 �1.2 �10.3 0.35 0.01 0.68 0.02 1.97 0.008 0.99 9TR-30 2.9 0.717123 �1.6 �10.4 0.34 0.05 0.68 0.06 1.97 0.006 0.99 9TR-23 1.8 0.716299 �3.3 �8.4 0.47 0.04 0.94 0.02 1.98 0.007 0.94 4TR-20 1.5 0.715820 �3.0 �7.6 0.51 0.02 1.00 0.01 1.96 0.007 0.96 4Repeat 0.44 0.09 0.87 0.03 1.96TR-16 1.0 0.715143 �3.3 �9.9 0.30 0.03 0.58 0.07 1.97 0.010 0.93 6TR-14 0.8 0.715065 �3.3 �8.7 0.40 0.01 0.78 0.01 1.97 nd nd ndTR-13 0.7 0.715712 �3.4 �8.8 0.37 0.04 0.73 0.10 1.98 0.008 0.95 7TR-10 0.5 0.720434 �2.4 �10.6 0.36 0.03 0.70 0.02 1.96 0.013 0.94 11

Duoding

DD-3 0.15 0.710479 �0.8 �6.3 0.21 0.05 0.42 0.22 2.02 0.022 0.80 22DD-7 0.85 0.709593 0.5 �1.6 0.44 0.04 0.88 0.07 1.99 0.007 0.87 6DD-10 1.45 0.710394 0.3 �2.1 0.70 0.08 1.43 0.18 2.04 0.008 0.87 42DD-14 2.80 0.710395 0.7 �4.9 0.56 0.07 1.12 0.16 2.00 0.004 0.89 40DD-17 4.90 0.710541 2.2 �2.2 0.57 0.03 1.15 0.08 2.02 0.004 0.92 11Repeat 0.62 0.05 1.27 0.03 2.05DD-9 1.25 0.711042 0.9 �0.4 0.60 0.07 1.21 0.16 2.02 0.006 0.87 25DD-15 3.50 0.713003 0.9 �3.7 0.33 0.02 0.69 0.13 2.07 0.016 0.86 48Repeat 0.38 0.03 0.77 0.01 2.00DD-16 4.20 0.713537 1.6 1.0 0.41 0.02 0.86 0.03 2.07 0.023 0.90 34

Wuhe

WH-18 0.05 0.709441 �1.2 �6.3 0.56 0.03 1.13 0.07 2.02 0.027 0.78 19WH-19 0.10 0.709008 �4.2 �3.1 0.51 0.16 1.05 0.27 2.05 0.006 0.83 17WH-24 0.65 0.708295 �1.9 �15.1 1.03 0.04 2.06 0.06 2.00 0.003 0.89 3WH-25 1.00 0.708114 �4.3 �14.3 0.67 0.05 1.34 0.08 2.02 0.009 0.82 12WH-27 1.50 0.708229 �3.1 �4.2 0.42 0.11 0.84 0.22 1.99 0.010 0.83 5WH-28 1.75 0.708694 �3.0 �5.5 0.47 0.03 0.95 0.01 2.03 0.009 0.83 6WH-29 2.00 0.709681 �2.8 �5.5 0.17 0.06 0.36 0.07 2.05 0.013 0.80 9repeat 0.26 0.08 0.56 0.11 2.11

(continued on next page)

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Table 2 (continued)

Sample No. Depth (m) 87Sr/86Sr d13C (‰)a d18O (‰)a d66Zn (‰) 2sd d68Zn (‰) 2sd 68Zn/66Zn Al/Mg (g/g) Mg/Ca (mol/mol) Mn/Sr (g/g)

Sidouping

SDP-3 0.80 0.712446 �5.3 �12.4 �0.02 0.08 �0.03 0.11 2.02 nd nd ndSDP-4 1.10 0.711970 �2.5 �7.6 0.25 0.12 0.51 0.31 2.07 0.004 0.93 20SDP-5 1.30 0.712022 �2.3 �8.3 0.30 0.05 0.62 0.03 2.04 nd nd ndSDP-8 2.05 0.712722 �2.9 �7.5 0.01 0.05 0.01 0.05 1.90 0.003 0.94 35SDP-9 2.35 0.712007 �2.8 �7.8 0.09 0.05 0.19 0.06 2.11 0.004 0.97 38SDP-12 3.15 0.711892 �3.0 �7.5 0.06 0.08 0.12 0.04 2.12 0.006 0.92 26SDP-14 3.65 0.709355 �3.1 �6.8 0.38 0.04 0.79 0.20 2.10 0.002 1.00 32SDP-15 3.85 0.711564 �3.4 �7.5 0.21 0.03 0.43 0.06 2.02 0.008 0.91 76SDP-16 4.00 0.712415 �3.8 �7.8 0.07 0.05 0.15 0.05 2.09 nd nd ndSDP-17 4.15 0.713220 �3.9 �7.9 0.04 0.05 0.08 0.05 2.08 0.022 0.83 41

Chenjiayuanzi

CJ0.1 0.10 0.707273 �2.7 �5.3 0.61 0.03 1.21 0.03 2.00 0.055 0.68 12.5CJ0.25 0.25 0.708713 �3.5 �6.9 0.65 0.01 1.29 0.04 1.99 0.041 0.65 6.5Repeat 0.72 0.02 1.43 0.01 1.99CJ0.7 0.7 0.707362 �3.1 �3.2 0.48 0.03 0.96 0.02 1.99 0.026 0.78 12.1CJ2.1 2.1 0.707580 2.0 �2.3 0.71 0.02 1.42 0.10 1.99 0.005 0.32 7.2CJ2.8 2.8 1.0 �3.5 0.64 0.04 1.26 0.09 1.96 nd nd ndCJ3.2 3.2 1.0 �2.0 0.55 0.00 1.08 0.01 1.96 nd nd ndRepeat 0.49 0.01 0.97 0.07 1.99CJ3.7 3.7 2.0 �4.0 0.55 0.06 1.11 0.01 2.00 0.059 0.43 6CJ4.1 4.1 0.0 �6.1 0.54 0.02 1.07 0.02 1.99 0.021 0.57 5.7CJ8.0 8.0 0.707889 6.0 �2.2 0.53 0.02 1.06 0.06 1.97 0.030 0.46 0.3CJ9.1 9.1 0.707932 5.9 �2.4 0.36 0.05 0.71 0.11 1.99 0.033 0.65 0.6CJ9.45 9.45 6.2 �3.8 0.53 0.02 1.05 0.06 1.97 nd nd ndCJ11.0 11.0 0.708577 5.8 �3.2 0.62 0.02 1.22 0.06 1.98 0.022 0.60 0.6CJ11.25 11.3 0.708189 4.1 �4.5 0.39 0.06 0.78 0.05 1.97 0.022 0.57 0.5CJ11.8 11.8 3.9 �4.4 0.30 0.02 0.62 0.02 2.03 nd nd ndCJ17.0 17.0 0.707970 3.3 �1.6 0.48 0.02 0.96 0.02 2.00 0.007 0.72 0.5CJ22.3 22.3 3.7 �3.1 0.51 0.02 1.02 0.04 1.99 0.016 0.65 0.3CJ25.8 25.8 6.8 �3.7 0.37 0.06 0.73 0.05 1.97 nd nd ndCJ26.2 26.2 0.708157 7.5 �3.2 0.40 0.05 0.80 0.01 1.98 0.007 0.68 0.3CJ29 29.0 6.3 �4.1 0.57 0.04 1.13 0.01 1.99 nd nd ndCJ33 33.0 3.4 �4.3 0.62 0.02 1.23 0.05 1.98 nd nd ndCJ33.35 33.3 0.708079 4.0 �3.4 0.63 0.01 1.26 0.01 1.99 0.006 0.73 0.4Repeat 0.60 0.06 1.20 0.15 2.00CJ34.6 34.6 0.707897 4.9 �1.3 0.85 0.07 1.68 0.07 1.97 0.051 0.81 0.5CJ35.3 35.3 3.7 �1.7 0.60 0.03 1.19 0.02 1.98 nd nd ndCJ37.3 37.3 0.707906 4.1 �1.9 0.69 0.00 1.37 0.03 1.99 0.006 0.39 0.2CJ39 39.0 0.707965 5.4 �0.6 1.17 0.02 2.33 0.06 1.99 0.027 0.45 0.3CJ41 41.0 0.707932 5.7 �1.0 0.86 0.00 1.73 0.03 2.00 0.006 0.41 0.2

a C and O stable isotope data are reported in per mil relative to VPDB.b No data.

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Fig. 4. Zinc isotope profile of five studied sections. The gray bars represent the isotopic value of crustal Zn. The green area is the cap dolomiteoverlying the Nantuo diamictites. Data are reported in Table 2. See text for interpretation.

338 Y. Lv et al. /Geochimica et Cosmochimica Acta 239 (2018) 330–345

fractions S3–S10 should best represent Zn isotope composi-tions of the dolomite phases in the studied lithologies. Itshould also be noted that the d66Zn uncertainty (�0.1‰)generated during Zn isotope analysis of sequential leachingis markedly smaller than the d66Zn range observed in thestudied samples (up to 1.0‰; Fig. 4), indicating that thetemporal Zn isotope variation observed in these samplesis geologically significant.

5.2. Modeling of post-depositional diagenesis and its effect on

Zn isotope ratios

Post-depositional diagenesis and dolomitization arecommonly characterized by low d18Ocarb values, a positivecorrelation of low d13Ccarb with decreasing d18Ocarb values,as well as high 87Sr/86Sr and Mn/Sr ratios (Brand andVeizer, 1980, 1981; Jacobsen and Kaufman, 1999;Halverson et al., 2007). The C-, O-, Sr- and stable metal(e.g. Cr and Mg) isotope compositions of carbonate rocksare easily modified by fluid-rock interaction with diageneticfluids of meteoric water, continental basin brined water orhydrothermal fluid (Halverson et al., 2007; Huang et al.,2011; Hohl et al., 2015; Rodler et al., 2016; Chanda et al.,2017). The same process may also reset Zn isotope compo-sition of dolostones. Since Zn and Mn have smaller ionicradii than calcium and have large distribution coefficient,they are preferentially incorporated into carbonate fromdiagenetic fluid (Crocket et al., 1966; Pingitore Jr., 1978;Brand and Veizer, 1980; Swart, 2015). The diagenetic phasemay therefore be expected to have higher Zn and Mn con-centration and different Zn isotope composition comparedwith the primary phase. Thus, combining measurementsof Zn isotopes with C-, O-, Sr-isotopes and Mn/Sr ratios

are recommended to evaluate influence of diagenesis onthe primary d66Zn values of carbonates, which is necessaryprior to discussing their paleo-environment significance.

The models for fluid-rock interaction on the basis of C-,O-, Sr-isotopes and Mn/Sr ratios have been thoroughly dis-cussed in previous studies (Banner and Hanson, 1990;Jacobsen and Kaufman, 1999). Here we use the model forfluid-rock interaction in an open system described inJacobsen and Kaufman (1999) to assess how Zn isotopecompositions of sedimentary carbonates will be changedby fluids during meteoric diagenesis, dolomitization andmetamorphism. A description of the equations and param-eters is briefly listed below. The final Zn isotope composi-tion of the rock (di

r(g)) for a simple open-system varies asa function of the weight ratio of fluid to rock (g), the ratioof the initial Zn concentration in fluid (Ci

w0) to that in rock(Ci

r0), the initial Zn isotope composition in the fluid (diw0)

and the rock (dir0), the effective fluid–rock distribution coef-

ficient (Di), and water–rock fractionation factor (Di). Theseare described in the following equation:

dri ðgÞ ¼Cr0i

Cw0i

� �dr0i þ Di exp g

Di

� �� 1

h iDi þ dw0i� �

Cr0i

Cw0i

� �þ Di exp g

Di

� �� 1

h in o

The relationships of dir(g) values with the degree of

fluid–rock interaction (g) and other parameters (C-, O-,Sr-isotopes and Mn/Sr ratios) are illustrated in Fig. 5a–e.The effective fluid–rock distribution coefficient (Di) usedin this model is not constant. For fluids with relatively highZn concentration such as hydrothermal fluid (Ci

r0/Ciw0 =

0.5–4), the Zn isotope composition of carbonates could beeasily modified via fluid–rock interaction with low ratioof fluid to rock (g < 1; Fig. 5a). Zinc isotope ratios could

Fig. 5. Diagrams illustrating the modeling results for the correlations between Zn isotopic compositions and the weight ratio of fluid to rock(g) and other indicators (C, O and Sr isotopes): d66Zn versus g (a), d13Ccarb versus d

66Zn (b), 87Sr/86Sr versus d66Zn (c), d18Ocarb versus d66Zn

(d), and Mn/Sr(g/g) versus d66Zn (e). The final Zn isotope composition of the rock is modeled with different initial fluid composition (Ci

w0/Cir0

= 0.05, 0.5 and 5000) and different effective fluid–rock distribution coefficient (Di = 100/20). In this model, Zn isotope composition of the fluid(di

w0) is assumed to be the value of lithogenic Zn (�0.3‰; Chen et al., 2013; Wang et al., 2017). The concentration (Ciw0) varies largely from 1

ppt in river and up to several tens ppm in hydrothermal fluid (Von Damm et al., 1985; Little et al., 2014). The dir0 and Ci

r0 values of the primarycarbonates are assumed to be the average value (0.9‰ and 5–18 ppm) of modern deep-sea carbonates (Pichat et al., 2003; Little et al., 2014).The ideal distribution coefficient of Zn incorporated into calcite is similar to Mn (Pingitore Jr., 1978), whereas the effective distributioncoefficient Di value could have a large range especially for non-equilibrium conditions. Hence we settle the Di value from 20 to 100 for Zn and600 for Mn. Other parameters follow those in Jacobsen and Kaufman (1999), except initial fluid composition. Here the initial fluid is set tohave relatively high 87Sr/86Sr and low d18Ocarb as have been reported in South China (Jiang et al., 2006b; Hohl et al., 2017).

Y. Lv et al. /Geochimica et Cosmochimica Acta 239 (2018) 330–345 339

be decreased significantly even while d13Ccarb,87Sr/86Sr and

Mn/Sr ratios remain unchanged (Fig. 5b, c and e). In con-trast, a near-linear relationship is observed between d66Znand d18Ocarb (Fig. 5d). Oxygen isotopes are thus a betterindicator of hydrothermal influence on carbonate Zn iso-tope composition compared with carbon isotopes, 87Sr/86Srand Mn/Sr ratios. For fluids with low Zn concentrationslike river water and seawater (Ci

r0/Ciw0 = �5000), Zn iso-

tope ratios are commonly less sensitive to alteration than

C, O and Sr isotopes (Fig. 5b–d). Instead, a near-linear rela-tionship between d66Zn and Mn/Sr ratios is found for sam-ples with high ratios of Ci

r0/Ciw0 (Fig. 5e), which could be

used to monitor the influence of meteoric diagenesis onZn isotope ratios. For the studied samples from theDoushantuo Formation with large fluctuations in C andO isotope compositions, carbonates that preserve primarySr isotopic ratios and low Mn/Sr ratios are more reliableto record primary Zn isotope composition.

Fig. 6. Correlations between Zn isotopic ratios and 87Sr/86Sr (a),d18Ocarb (b), d13Ccarb (c) and Mn/Sr (g/g) (d) for leachate of capdolostones from different sections in this study. Data are reportedin Table 2. The dash line represents the trend of two end-membersmixing. The yellow line is the modelling of water–rock interaction.The initial fluid composition: 87Sr/86Sr = 0.712, d18Ocarb = �8‰,d66Zn = 0.0‰. Other parameters for modeling: Di = 100, Ci

w0/Cir0

= 4.

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5.3. Post-depositional diagenesis of cap dolostones

The 87Sr/86Sr ratios of cap dolostones from the WH,SDP, DD and TR sections (0.7081–0.7204) are higher thanthe values of coeval seawater obtained from the relatively

pristine cap dolostones in the CJYZ section (0.7073–0.7087), the Yanwutan section (0.7078) in South China,Araras carbonate (0.7074–0.7082) in Brazil and Nuccaleenacap dolostones (0.7072–0.7092) in South Australia(Nogueira et al., 2007; Liu et al., 2014a; Hohl et al., 2015;this study). The altered Sr isotope composition indicatesthat post-depositional diagenesis and dolomitization prob-ably have already overprinted primary 87Sr/86Sr values(Halverson et al., 2007). The same process might also influ-ence the Zn isotope composition of dolostones, thusexplaining the observed low d66Zn values (avg. 0.37‰) ofthe WH, SDP, DD and TR samples compared with theCJYZ samples (avg. 0.58‰; Fig. 1). The cap dolostoneswith low d66Zn also tend to have high 87Sr/86Sr and low d18-Ocarb (Fig. 6a and b). This could be the result of mixing oftwo end-members of the primary phase and the diageneticphase. The modelling for diagenesis in Section 4.2 is appliedto the studied samples (Fig. 6). The SDP samples are plot-ted in the trend of fluid-rock interaction with high Zn con-centration of initial fluid (Ci

r0/Ciw0 = �4; Fig. 6a and b).

The expected high Zn concentration of diagenetic fluidpoints to hydrothermal activities, which also accounts forthe observed low d66Zn. Primary d13Ccarb and d18Ocarb val-ues and Mn concentration of Neoproterozoic cap dolo-stones have previously been shown to be highly variable(Fig. 6c and d; Kennedy et al., 2001; Jiang et al., 2006b,2007), and thus d13Ccarb, d

18Ocarb values and Mn/Sr ratiosare not used to evaluate diagenesis influence on Zn isotopecomposition of the studied cap dolostones.

The negative d66Zn value of �0.02‰ in sample SDP-3 ismost likely due to hydrothermal modification. Hydrother-mally sourced Zn in the modern ocean has low d66Zn valueof down to �0.50‰, which is significantly lower than river-ine Zn ranging from +0.19‰ to +0.56‰ (Conway andJohn, 2014; Little et al., 2014). Thus, the lowest d66Zn value(�0.02‰) observed in some dolostone samples is far lowerthan riverine d66Zn value, which was likely caused by signif-icant modification by hydrothermal activities identified incap dolostones of South China (Huang et al., 2011). Post-depositional hydrothermal-activities likely contributed tothe extremely negative carbon isotope composition in capdolostones of South China (Jiulongwan and Huajipo sec-tions; Bristow et al., 2011). Isostatic rebound during thedeglaciation may have driven more magmatic activities indeep-crust, bringing hydrothermal fluids into the basin(Shields, 2005). It is unclear whether the hydrothermalactivities happened during the deposition of cap dolostonesor during late-stage diagenesis. We suspect, however, thatthe low d66Zn values are the results of enhanced hydrother-mal activity in the deep-basin during deposition, since mostof the low d66Zn values are found in the sections from theslope near the deep basin rather than the shelf (Fig. 4).

There is no strong correlation between d66Zn and Mg/Ca in our samples (Fig. 7a), indicating that Mg/Ca ratiosare unsuitable to monitor Zn isotopic variation duringdolomization. Potential dissolution of silicate phases duringcarbonate extraction could result in high Al/Mg ratios andlow measured d66Zn values. Our data do not show decreas-ing d66Zn with increasing Al/Mg (Fig. 7b), suggesting aninsignificant introduction of dissolved silicate-derived Zn.

Fig. 7. Diagrams illustrating the correlations of Zn and Sr isotopes with Mg/Ca(mol/mol) (a), and Al/Mg(g/g) ratios (b) Data are reported inTable 2.

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In summary, we conclude that cap dolostones from theWH, SDP, TR and DD sections are unlikely to haverecorded the primary Zn isotope signature of seawatersdue to modification by post-deposition diagenesis.Although all sections show a coupled positive d66Zn excur-sion near the base (Fig. 4), the strong possibility of diage-netic overprint make any further discussion challenging.Instead, these observations raise an important issue worthyof attention when using Zn isotope ratios to study paleo-environmental significance. For example, the positive corre-lation of Zn and O isotopes observed in Phanerozoic car-bonates and Nuccaleena cap dolostone could be relatedto diagenetic alteration (Pichat et al., 2003; John et al.,2017).

5.4. Elevated d66Zn in the early Ediacaran Ocean: coupling

to organic matter burial?

In contrast to the WH, SDP, TR and DD sections (capdolostones), samples from CJYZ including Member I andpart of Member II of the Doushantuo Formation appearto be pristine (Figs. 6 and 7). Samples from this sectionhave low Mn/Sr ratios (<1) and seem to have preserved ini-tial 87Sr/86Sr of seawater (average = 0.708) that is consis-tent with the inferred contemporaneous seawater value(�0.708; Kaufman et al., 1993; Hoffman et al., 1998;Halverson et al., 2007; Sawak et al., 2010; Cox et al.,2016). The CJYZ samples can thus provide constraints onchemical and isotopic changes of the early Ediacaran ocean

on the Yangtze platform. In the lower Doushantuo Forma-tion in CJYZ section, the obtained 87Sr/86Sr ratios vary sig-nificantly from 0.7073 to 0.7087 in cap dolostones (MemberI), but remain relatively constant at 0.7080 in Member II(Fig. 8a). The variable 87Sr/86Sr ratios in cap dolostonesimply a stratified water column or varying continentalweathering during the deglaciation (Shields, 2005; Liuet al, 2014a). The highest 87Sr/86Sr ratio (0.7087) signalsradiogenic strontium input from silicate weathering(Hoffman et al., 1998; Halverson et al., 2007). Above capdolostones, carbonates of Member II have relatively con-stant 87Sr/86Sr ratios, suggesting a stable sedimentary envi-ronment without significant change in input sources toinfluence the Zn-isotope composition of contemporaneousseawater (Fig. 8a).

Zinc and carbon isotope variations in Member II of theDoushantuo Formation in CJYZ section, 40 m above capcarbonate, can be divided into three stages (A, B and C;Fig. 8a). d13Ccarb values show three positive excursions instages A, B and C, whereas d66Zn display a prominent pos-itive shift from 0.3‰ to 1.2‰ in stage C, in contrast to aslightly negative d66Zn shift in stages A and B (Fig. 8a).We interpret the co-variation of d66Zn and d13Ccarb in stageC as a result of increased organic matter burial. The detailsare discussed below.

In modern oceans, the Zn isotope values of seawater maybe elevated with light Zn isotopes preferentially sequesteredin sulfide-minerals and/or organic-rich sediments (Littleet al., 2016; Vance et al., 2016). The redox-sensitive element

Fig. 8. (a) Variations of C, Sr and Zn isotopes with depth in the CJYZ section, which are divided into three stages (A, B, and C). Capdolostones are the samples in the dark grey shade at the bottom of the section, which record the first negative d13Ccarb values (EN1) during theEdiacaran era. The empty cycle denotes diagenetically modified samples with 87Sr/86Sr ratio larger than 0.7085. U–Pb age at the horizon ofcap dolostone is from Condon et al. (2005). (b) Plot of d13Ccarb aganist d

66Zn in CJYZ section. The d13Ccarb values of CJYZ section have beenreported in Liu et al. (2013b, 2014b).

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enrichments and sulfur isotope data of black shales demon-strated a relatively stable redox state of the ocean duringdeposition of the middle part of Member II of the Doushan-tuo Formation (Sahoo et al., 2012, 2016). Therefore, we pro-pose that the positive d66Zn shift may have been caused byincreased burial of organic matter enriched in isotopicallylight Zn. Carbon and zinc isotope compositions areexpected to be positively correlated when organic carbonturnover is the dominant factor, considering that organiccarbon reservoirs are enriched in light isotopes of both car-bon and zinc (Jiang et al., 2007; Little et al., 2016). The pos-itive correlation between d66Zn and d13Ccarb (R2 = 0.70;Fig. 8b) in stage C is consistent with this scenario. It is note-worthy that more specimens of acritarches were discoveredat the horizon with positive d66Zn and d13Ccarb shifts in theCJYZ section (Fig. 8a; Liu et al., 2013b, 2014b). Anincreased abundance of acitarch fossils was also reportedin the biostratigraphically comparable layer of the nearbyNiuping section (Figs. 2 and 8a; Liu et al., 2013b, 2014b).Although the increase of fossil quantity and taxa in each sec-tion could be local and related to several factors such as thepreservation of fossils and high primary productivity, thecoupling of elevated d66Zn and positive d13Ccarb excursionpoints to increased biological productivity. That is, the bur-ial of organic matters was likely enhanced during depositionof the middle part of Member II in the Doushantuo Forma-tion of the CJYZ section.

The d13Ccarb values are highly variable within 20 mabove cap carbonate in the Yangtze Gorges area duringthe Ediacaran era (Jiang et al., 2007). The variability of d13-

Ccarb may be the product of multiple processes, includingthe fluctuation of organic carbon burial, change of faciesand diagenetic alteration (Kaufman and Knoll, 1995;Jiang et al., 2007; Zhao and Zheng, 2010). The change inorganic carbon flux does not reconcile with the opposingsigns of the positive d13Ccarb and negative d66Zn excursionsin stages A and B. Based on our modelling results (see Sec-tion 5.2), Zn isotopes are less likely altered by fluid-rockinteractions compared to carbonate d13C values (Fig. 5b),which might explain the small Zn isotope variation butup to 2‰ positive shift of carbonate d13C. Hence, diage-netic fluids with high d13Ccarb and low d66Zn may resultin the variability of d13Ccarb and d66Zn at stages A and B.The presence of calcite in dolostone is another factor thatmay affect carbonate d13C values. Calcite could have d13-Ccarb lower than coexisting dolomite by 1.5 ± 0.5‰(Sheppard and Schwarcz, 1970). Based on our leachingresults, calcite could have slightly higher d66Zn than thatof dolomite (Fig. 3). Therefore, the d66Zn and d13Ccarb pat-terns in stages A and B could be the result of diageneticalteration and/or variable proportions of calcite indolostone.

6. CONCLUSION AND WIDER IMPLICATIONS

In this study we demonstrate that post-depositional dia-genesis could cause low d66Zn values in cap carbonate sec-tions. The influence of post-depositional diagenesis ondolostone Zn isotope composition is demonstrated in sam-ples from four sections studied, based on comparison with

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87Sr/86Sr, d13Ccarb, d18Ocarb and Mn/Sr ratios. The capdolostones with negative d66Zn values have potentially beenmodified by hydrothermal fluids with high Zn concentra-tion, confirming the previously identified hydrothermalactivities during the early Ediacaran on the Yangtze Plat-form (Bristow et al., 2011; Huang et al., 2011). John et al.(2017) explained the low d66Zn values of Neoproterozoiccap dolostones above the Marinoan glaciation as a conse-quence of reducing conditions during deposition. Our studyindicates that hydrothermal modification and diagenesis areother important processes of resulting in low d66Zn valuesin carbonates.

Several events of biological evolution, e.g., the diversifi-cation of Doushantuo-Pertatataka acritarchs, the evolutionof macrophagous eumetazoans, have been identified duringthe early Ediacaran Period between �634 Ma and �604 Ma(Xiao et al. 1998, Xiao and Knoll, 2000; Zhou et al., 2007;Yuan et al. 2011). However, the biostratigraphic record issometimes locally affected by the taphonomic conditionand palaeoenvironmental facies. The co-occurrence of pos-itive d66Zn and d13Ccarb excursions in lower part of the Edi-acaran stratigraphic unit may simply indicate increasedorganic matter burial, rather than a global evolutionaryevent. The increased burial of organic matter may implythat the oceanic environments had become habitable forprimary producers. Our study highlights the potentialapplication of Zn isotopes in carbonates to trace thechanges in organic matter burial of the past oceans in com-bination with carbon isotopes.

ACKNOWLEDGEMENTS

We would like to thank Zhang L. and Liu C. for assistance inthe lab. We are grateful to Prof. Jiang G.-Q. for stimulating discus-sions on an early version of this manuscript. Constructive com-ments from Silke Severmann, Huan Cui and two anonymousreviewer, and careful handling by the editor Silke Severmann aregreatly appreciated. This work is supported by the National Natu-ral Foundation of China (Nos. 41622303, 41422202, 41603010), the‘‘Strategic Priority Research Program” of the Chinese Academy ofSciences (Grant No. XDB18030603), the Fok Ying Tung Educa-tion Foundation (2-2-2016-01) and the Fundamental ResearchFunds (2-9-2015-299) of CUGB to LSA, and the financial supportfrom China Scholarship Council to LYW.

APPENDIX A. SUPPLEMENTARY MATERIAL

Supplementary data associated with this article can befound, in the online version, at https://doi.org/10.1016/j.gca.2018.08.003.

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Associate editor: Silke Severmann