whole rock and discrete pyrite geochemistry as...

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Geochimica et Cosmochimica Acta xxx (2017) xxx–xxx

Whole rock and discrete pyrite geochemistry ascomplementary tracers of ancient ocean chemistry: An

example from the Neoproterozoic Doushantuo Formation, China

Daniel D. Gregory a,⇑, Timothy W. Lyons a, Ross R. Large b, Ganqing Jiang c,Aleksandr S. Stepanov b, Charles W. Diamond a, Maria C. Figueroa a, Paul Olin b

aUniversity of California, Riverside, CA, USAbCODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Australia

cUniversity of Nevada, Las Vegas, NV, USA

Received 25 November 2016; accepted in revised form 29 May 2017; available online xxxx

Abstract

The trace element content of pyrite is a recently developed proxy for metal abundance in paleo-oceans. Previous studieshave shown that the results broadly match those of whole rock studies through geologic time. However, no detailed study hasevaluated the more traditional proxies for ocean chemistry for comparison to pyrite trace element data from the same sam-ples. In this study we compare pyrite trace element data from 14 samples from the Wuhe section of the Ediacaran-ageDoushantuo Formation, south China, measured by laser ablation inductively coupled plasma mass spectrometry with newand existing whole rock trace element concentrations; total organic carbon; Fe mineral speciation; S isotope ratios; and pyritetextural relationships. This approach allows for comparison of data for individual trace elements within the broader environ-mental context defined by the other chemical parameters. The results for discrete pyrite analyses show that several chalcophileand siderophile elements (Ag, Sb, Se, Pb, Cd, Te, Bi, Mo, Ni, and Au) vary among the samples with patterns that mirror thoseof the independent whole rock data. A comparison with existing databases for sedimentary and hydrothermal pyrite allows usto discriminate between signatures of changing ocean conditions and those of known hydrothermal sources. In the case of theWuhe samples, the observed patterns for trace element variation point to primary marine controls rather than higher temper-ature processes. Specifically, our new data are consistent with previous arguments for pulses of redox sensitive trace elementsinterpreted to be due to marine oxygenation against a backdrop of mostly O2-poor conditions in the Ediacaran ocean—withimportant implications for the availability of bioessential elements. The agreement between the pyrite and whole rock datasupports the use of trace element content of pyrite as a tracer of ocean chemistry in ways that complement existingapproaches, while also opening additional windows of opportunity. For example, unlike the potential vulnerability of wholerock data to secondary alteration, the pyrite record may survive greenschist facies metamorphism. Furthermore, early-formedpyrite can be identified through textural relationships as a proxy of primary marine chemistry even in the presence ofhydrothermal overprints on whole rock chemistry via secondary fluids. Finally, pyrite analyses may allow for the possibilityof more quantitative interpretations of the ancient ocean once the elemental partitioning between the mineral and host fluidsare better constrained. Collectively, these advances can greatly increase the number of basins that may be investigated forearly ocean chemistry, especially those of Precambrian age.� 2017 Elsevier Ltd. All rights reserved.

Keywords: Pyrite; Trace elements; LA-ICPMS; Doushantuo; Oxygenation; Neoproterozoic

http://dx.doi.org/10.1016/j.gca.2017.05.042

0016-7037/� 2017 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: University of California, Riverside, Department of Earth Sciences, Geology Building 900 University Ave, USA.E-mail address: [email protected] (D.D. Gregory).

Please cite this article in press as: Gregory D. D., et al. Whole rock and discrete pyrite geochemistry as complementary tracers of ancientocean chemistry: An example from the Neoproterozoic Doushantuo Formation, China. Geochim. Cosmochim. Acta (2017), http://dx.doi.org/10.1016/j.gca.2017.05.042


mailto:[email protected]




2 D.D. Gregory et al. /Geochimica et Cosmochimica Acta xxx (2017) xxx–xxx

1. INTRODUCTION

Many previous studies have relied on Fe speciation,total organic carbon (TOC), trace element content, and iso-topic signatures of sedimentary rocks to investigate pastocean conditions and atmospheric oxygen levels (reviewedin Tribovillard et al., 2006; Lyons et al., 2014).

Recently, a new tool has been added: in-situ analysis ofsiderophile and chalcophile trace element contents in pyriteusing laser ablation inductively coupled plasma mass spec-trometry (LA-ICPMS; Gregory et al., 2014, 2015a,b; Largeet al., 2014, 2015b). These results are encouraging for fourmain reasons:

(1) Some of the existing whole rock techniques requireeuxinic depositional settings (e.g., Mo). As such,the number of basins suitable for analysis is limited,and extra analyses must be completed to ensure thata given shale sample under investigation is indeedeuxinic. Since pyrite forms in the majority of car-bonaceous sediments, whether deposited under oxicor anoxic conditions, this technique may not berestricted to euxinic basins. Importantly, the exclu-sively diagenetic pyrite of oxic conditions may limitinterpretations to diagenetic controls, althoughlong-term, first-order trends for seawater could bepreserved in pyrite that forms early, near thesediment-water interface. Pyrite formed syngeneti-cally in a euxinic water column may better capturethe chemistry of the open ocean.

(2) Metal remobilization is a major concern for wholerock techniques, but early diagenetic pyrite can bepreserved in the cores of larger pyrite grains up togreenschist facies (Large et al., 2007). Thus, accessto these unaltered pyrite cores greatly increases thenumber of basins that can be used for investigatingpaleo-oceanographic and atmospheric conditions,particularly in studies of the Precambrian ocean.

(3) LA-ICPMS pyrite analyses allow the relatively easymeasurement of several elements that are often belowdetection limits or require complicated additionalsteps for whole rock methods. As such, a morediverse suite of elements can be considered usingthe pyrite technique (e.g., Cd, Se, and Ag) becausethe in-situ nature of the analyses, compared to wholerock data, are less sensitive to dilution via high ratesof detrital or biogenic sedimentation. In other words,we can focus on data that are significantly concen-trated in pyrite relative to the surrounding sediments.

(4) Large pyrite trace element datasets are available forboth sedimentary pyrite (Large et al., 2014;Gregory et al., 2015a) and hydrothermal pyrite(e.g., Maslennikov et al., 2009; Revan et al., 2014;Steadman et al., 2015; Belousov et al., 2016; Gaddet al., 2016; Gregory et al., 2016; and Mukherjeeand Large, 2017). These different pyrite varieties havemarkedly different trace element abundances andthus can be used to determine whether trace elementenrichments are likely to be partly or wholly attribu-table to hydrothermal processes.

Please cite this article in press as: Gregory D. D., et al. Whole rock andocean chemistry: An example from the Neoproterozoic Doushantuo Forg/10.1016/j.gca.2017.05.042

There has been extensive interest in the use of pyritechemistry to understand changes in the chemical conditionsof the water column and/or pore waters present when pyriteformed. Early studies utilized sequential extraction tech-niques (Huerta-Diaz and Morse, 1992); however, thesemethods are prone to extracting trace elements from con-stituents other than just the pyrite. Later studies utilizedmechanical extraction of pyrite and analysis of just the pyr-ite fraction (Tribovillard et al., 2008; Berner et al., 2013;Pisarzowska et al., 2014). These studies yielded a numberof important observations, such as a link between enrich-ments in elements that form oxyanions in euxinic settings(Berner et al., 2013)—similar to some whole rock enrich-ment patterns (Mo, U, V, Re) in euxinic basins(Tribovillard et al., 2006). The studies also identified mech-anisms that can lead to Mo enrichment in pyrite formedvery early in the sediments in non-euxinic settings(Tribovillard et al., 2008). The effort required to extractthe pyrite mechanically and the difficulty in separating thedifferent generations of pyrite hampered the widespreadimplementation of this otherwise useful approach. Thesedifficulties have been overcome by the use of LA-ICPMS.

Importantly, all of these studies (Tribovillard et al.,2008; Berner et al., 2013; Pisarzowska et al., 2014), as wellas others (e.g., Gregory et al., 2014), have indicated that thetrace element content of pyrite is related to the elementalcompositions of the pore fluid or ocean waters in which itformed. This conclusion has been reinforced by an investi-gation of pyrite from hydrothermal systems that alsoincluded data from fluid inclusions and hydrothermal fluidsfrom the same system and showed that the trace elementcontent of pyrite tracks that of the fluids in which it formed(Tardani et al., 2017).

Since the application of LA-ICPMS to the analysis ofsedimentary pyrite, the pyrite approach has largely centeredon the generation of an extensive trace element datasetacross geologic time (Large et al., 2014, 2015a,b; Gregoryet al., 2015a). In a first-order sense, these data match exist-ing whole rock results spanning the same interval of time.For example, pyrite concentrations broadly match thewhole rock data for Mo in euxinic shales (Scott et al.,2008) and Ni/Fe ratios in banded iron formations(Konhauser et al., 2009) over the same time interval. How-ever, the samples from these different studies were often notfrom the same basins, thus masking important differencestied to local conditions, age disparities, and stratigraphicscales of sample resolution.

Gregory et al. (2015b) showed that Mo in whole rockbroadly matches Mo in pyrite from the Hamersley Basin,but there are very few studies that actually investigatedthe degree to which the pyrite trace element chemistrydirectly matches the results of whole rock proxies in thesame sample, and virtually none has explored this relation-ship in detail. Furthermore, no study has explored trace ele-ment content in pyrite within a framework of otherpaleoredox proxies, such as size distributions for fram-boidal pyrite, Fe speciation, S isotope analyses, and traceelement/TOC ratios for bulk data. This study aims to fillthat gap. Here we report 116 in-situ LA-ICPMS trace ele-ment analyses for pyrite from 14 samples of the well-

discrete pyrite geochemistry as complementary tracers of ancientormation, China. Geochim. Cosmochim. Acta (2017), http://dx.doi.



D.D. Gregory et al. /Geochimica et Cosmochimica Acta xxx (2017) xxx–xxx 3

studied Wuhe section of the Doushantuo Formation, southChina (Sahoo et al., 2016). Beyond our goal to refine thepyrite trace element proxy, our data will contribute to amore complete understanding of the changes in oceanchemistry and their relationships to Neoproterozoic bio-spheric oxygenation. These oceanic oxygenation eventsmay have stimulated evolutionary and ecological innova-tion among complex life in those waters.

1.1. The importance of the Doushantuo for the understanding

of ocean conditions at the end of the Proterozoic

Although the samples used in this study were chosen toprovide a broadly relevant proof of concept—they also bearspecific significance for an important chapter in Earth his-tory. Given this added value, it is important that we providesufficient background. The Doushantuo Formation isamong the most intensively studied rock units of its age.This attention is not without good reason since theDoushantuo Formation is exceptionally well preservedand has yielded many key insights into the post ‘snowballEarth’ and later Ediacaran world—a critical window in timethat saw dramatic changes in Earth’s climate, reorganiza-tions of biogeochemical cycling, and the rise of metazoanlife to major ecosystem significance. The geographic distri-bution of the Doushantuo Formation is also very large, pre-serving relatively continuous deposition over an area greaterthan 1.6 million km2. This fact, along with the many dozensof sections that have been described in stream valleys, roadcuts, and drill core, has allowed for important progressgained through integrated investigations of sedimentologyand geochemistry along lateral transects and depth gradi-ents throughout the basin. The results of these studies haveinformed our understanding of the recovery from globalglaciation, the origin of the largest negative carbon isotopeexcursion in Earth’s history, and the dynamics of atmo-spheric O2 leading up to the Precambrian/Cambrian bound-ary (e.g., Condon et al., 2005; McFadden et al., 2008;Kendall et al., 2015; Sahoo et al., 2016).

In addition to its wealth of environmental information,the Doushantuo Formation also hosts an exceptional pale-ontological record. From rich diversities of acritarchs andalgae to the putative embryos of some of the earliest meta-zoans, the Doushantuo Formation has provided multiplesnapshots of a critical chapter in the history of biologicalevolution. An important step in advancing our understand-ing of the drivers of major evolutionary developmentdemands that we link this exceptional fossil record withthe co-evolution of Ediacaran ocean chemistry to addressthe possibility of causal links. Establishing such relation-ships are among the challenges facing all Precambrian geol-ogy, and the most convincing results require integration ofa large number of studies from many formations on manycontinents. Nevertheless, there is no doubt that theDoushantuo Formation will continue to play a central rolein our understanding of the late Proterozoic, and the pyriteanalytical technique highlighted in this study are increasingour confidence in these interpretations.

Recently, the degree to which the Doushantuo Forma-tion may reflect the composition of the global ocean has


been questioned (Miller et al., 2017). Using sections fromnorthwestern Canada, Miller et al. (2017) argue that theglobal ocean was predominantly ferruginous during thetime of Doushantuo deposition and that the metal enrich-ments observed by Sahoo et al. (2016) are products of localconditions in south China. However, while it is importantto have data from several different locations to argue forglobal implications, it is not clear that the samples fromnorthwestern Canada come closer to capturing a global sig-nature compared to those from south China. Of the threesections reported by Miller et al. (2017), two are depositedin a shelf environment, which is more likely to have beendeposited in local sub-basins with chemistry that differedfrom the global ocean. In fact, significantly different condi-tions appear to have been present at the two shelf sites:Sheepbed Creek and Stoneknife Creek. The former wasdeposited in a setting, which, based on iron speciation, var-ied from ferruginous to oxic, while the latter was depositedunder ferruginous to euxinic conditions.

The third section, Rackla, is from a slope facies and themost likely of the three to mirror global ocean conditions.Unfortunately, the sediments from this section that imme-diately overly the cap carbonate, and are thus most likelyto be stratigraphically equivalent to the metal enrichmentidentified at the base of Doushantuo Member II (Sahooet al., 2012, 2016), are believed to have been altered(Miller et al., 2017). Therefore, the data from the Racklasection may not be reliable for determining the presenceor absence of a redox sensitive trace element anomaly insediments equivalent to the base of Doushantuo MemberII, and the section was not thick enough to have sampledan interval equivalent to the base of Doushantuo MemberIII. Furthermore, the samples above the altered zone haveinconclusive Fe-speciation results, with most analyses yield-ing FeHR/FeT ratios equal to or slightly below 0.38 (Milleret al., 2017)—making them ambiguously anoxic at best.Thus, the data from the Rackla section, the most likely tocapture the global ocean, do not show conclusive evidencefor deposition under ferruginous conditions. The data fromour study and Sahoo et al. (2016), in contrast, provide whatmay be an exceptional window to Ediacaran ocean chem-istry because of the persistence of local euxinia. The onlyway to conclusively demonstrate which section, if either,captures global oceanic conditions is to analyze additionalsections of equivalent age. Our study nevertheless attemptsto address the critical criticism that the Doushantuo pat-terns of enrichment are most likely secondary and thusunrelated to primary trends in global ocean chemistry.

2. REGIONAL GEOLOGY

A fully developed geologic context is required to maxi-mize the impact of this and any analogous or follow-upstudy. The South China Craton is host to a thick successionof well-studied Neoproterozoic strata (Fig. 1A) that thickensignificantly from northwest on the shelf to southeast in thebasin. At the base of the sequence, Tonian-age siliciclasticswere deposited on an active rift margin during the laterstages of the breakup of Rodinia (Jiang et al., 2011). Abovethis, the diamictites of the Chang’an Formation and its




Fig. 1. (A) Location of the Wuhe section and interpreted depositional environments of the Ediacaran stratigraphy in southern China. (B)Interpreted depositional setting of the Wuhe section. (C) Simplified stratigraphy of the Wuhe section and correlation with the Jiulongwansection. All figures are from Sahoo et al. (2016).


equivalents and the younger Nantuo Formation record theSturtian and Marinoan glaciations, respectively. The twodiamictites range in thickness from 0 to 10 m on the shelfto greater than 2 km in the slope and basin. A shale unit,the Datangpo Formation and its equivalents, separatesthe diamictites and shows a similar stratigraphic pat-tern—being either absent or thin (<10 m) in shelf settingsand thick (>100 m) toward the basin. The rift-to-drift tran-sition was likely completed during the Cryogenian, afterwhich the setting may have changed to a passive marginthroughout the remainder of the Neoproterozoic (Jianget al., 2003). With the development of a carbonate platformwith marginal shoal complexes and shelf lagoon, theDoushantuo Formation shows significant lateral faciesheterogeneity.

Deposition of the Doushantuo Formation began duringthe terminal deglaciation of the Marinoan ‘snowball Earth’event, and recent paleomagnetic data indicate that theSouth China Craton was situated at �23.5� N latitude atthe time (Zhang et al., 2015). Although the DoushantuoFormation varies laterally in both thickness and facies,the stratigraphic framework that was originally establishedin the Yangtze Gorges area, where it was divided into fourmembers, has been extended to describe all other sections.Member I is a �5 m ‘cap’ carbonate, which is presentthroughout the basin and varies little in thickness, even inthe inferred deep-water portions of the basin (Jiang et al.,2011). This consistency has been interpreted as reflectingdeposition during the late stages of glacial retreat whenthe South China Craton was largely free of glaciers (e.g.,


Jiang et al., 2006). The cap carbonate rests directly on thediamictite unit and represents the most significant markerbed for the Doushantuo Formation throughout the region.Member I transitions gradationally into Member II, whichgenerally consists of some combination of organic-richshale and interbedded carbonate, though lithologies varylaterally on a regional scale.

Shortly following deposition of the cap carbonate andits overlying shale, a carbonate shoal complex formed alongthe eastern edge of the platform. Lagoonal facies dominatethe remainder of the Doushantuo Formation north of theYangjiaping area, including in the Yangtze Gorges (Jianget al., 2011). However, the Wuhe section, the focus of thisstudy, was deposited basinward of any barrier complex thatmight have restricted exchange in shallower settings(Fig. 1B) and as such is believed to have shared a strongconnection with the open ocean. Member III of theDoushantuo Formation varies dramatically in lithologyand thickness regionally and therefore lacks beds suitablefor correlation over long distances. Conversely, MemberIV serves as the second significant regional marker for theDoushantuo Formation, distinguished by high TOC blackshales. This TOC enrichment may be related to depositionduring a transgressive event (e.g., Jiang et al., 2007, 2011).

The duration of Doushantuo deposition has been wellconstrained by ash beds near the top and bottom of the for-mation.An ash layer interbeddedwithin the cap carbonate atthe base has an age of 635.2 ± 0.6 Ma (Condon et al., 2005),and one near the Doushantuo/Dengying boundary has anage of 551.1 ± 0.7 Ma (Condon et al., 2005; Zhang et al.,





2005). Recent studies, however, suggested that the 551.1± 0.7 Ma age may be younger than the Doushantuo-Dengying boundary, and the duration of the DoushantuoFormation may instead have spanned from ca. 635 to560 Ma (An et al., 2015). Despite being relatively thin forsuch a long time period, only two disconformable surfaceshave been shown to be regionally significant, one withinMember III and the other near the top of the formation.

3. LOCAL GEOLOGY

TheWuhe section (26�45093,600N, 108�2500.500E) includes,from the bottom to top, the Doushantuo Formation(120 m), Dengying Formation (12 m), Luichapo Formation(40 m), and a small portion of the basal Niutitang Forma-tion (Sahoo et al., 2016). The Doushantuo Formation, thefocus of this study, has been subdivided into four differentintervals (Members I-IV; Jiang et al., 2011; Fig. 1C). Mem-ber I, a regional marker bed, is a 2.4 m thick cap carbonatethat lies immediately on top of the glacial diamictite of theNantuo Formation (Jiang et al., 2006). Members II andIII comprise sequences of carbonaceous shale and subordi-nate micritic or microcrystalline dolostone. These units areseparated by a dolostone breccia interval that can be tenta-tively correlated across the region (Jiang et al., 2007, 2011).At Wuhe, Members II and III are approximately 50 m and43 m thick, respectively (Sahoo et al., 2016). The uppermostpart of the Doushantuo Formation is defined as MemberIV—a black shale with distinctive phosphatic and pyritenodules. This unit is an important marker horizon thatcan be used to correlate among sections in the YangtzeGorges area (Sahoo et al., 2016).

4. PREVIOUS WORK ON THE WUHE SECTION

High-resolution studies of pyrite morphology (Wanget al., 2012), Fe speciation, redox sensitive trace element(RSTE), and S isotope analyses (Sahoo et al., 2016) werepreviously undertaken for the Wuhe section. The pyriteframboids were found to be <10 mm in diameter(n = 5274, mean 5.4 ± 0.6 mm; Wang et al., 2012), whichsuggests formation in a sulfidic (euxinic) water column(Wilkin et al., 1996). Iron speciation analyses tell a similarstory. Sahoo et al. (2016) used ratios of highly reactive Feto total Fe (FeHR/FeT) of >0.38 (0.72 ± 0.19) and FeT/Al > 0.5 (0.52 ± 0.21) to argue that the Wuhe stratigraphywas deposited under anoxic conditions (see Lyons andSevermann, 2006; Poulton and Canfield, 2011, for discus-sions about the Fe methods). Further, the FePY/FeHR inthe section is 0.87 ± 0.13, which is indicative of euxinic(anoxic and sulfidic) water column conditions (Poultonand Canfield, 2011; Sahoo et al., 2016).

Within this sequence of euxinic Doushantuo stratigra-phy, three distinct zones of RSTE (Mo, V, Cr, Re, andU) enrichment were identified in whole rock data, and theseare coupled with d34S decreases for associated pyrite (Sahooet al., 2016). These anomalies occur at the bases of Mem-bers II, III, and IV and are separated by intervals with pos-itive d34S signatures and roughly crustal levels of the RSTE.The enriched intervals were interpreted to reflect periods of


relatively high dissolved trace metal availability in seawaterlinked by Sahoo et al. (2016) to ocean-scale oxygenationevents. The assumption is that increased oxygenationwould have decreased the global extent of anoxic/euxinicseafloor, thus decreasing the sink for the RSTE and increas-ing the marine inventory. The net result was greater enrich-ment of these elements in local areas, such as the Wuhelocation, where euxinia was maintained despite the inter-preted marine oxygenation events in the broader ocean.Local euxinic areas in an oxic world can reflect regions ofhigh productivity. The S isotope data support the argumentfor oxygenation events. Specifically, negative shifts in d34Sin the high RSTE intervals indicate a larger sulfate reservoirin the face of lower global pyrite burial under more oxicmarine conditions. Conversely, the positive d34S signaturesin the low RSTE intervals suggest a lower sulfate reservoirduring the periods of relatively low marine oxygenation(Sahoo et al., 2016). The comprehensive suite of data avail-able for the Wuhe section makes it an ideal site to test thereliability and sensitivity of the pyrite trace element proxy.

5. METHODS AND MATERIALS

Splits of 14 of the samples used by Sahoo et al. (2016)were obtained from Members II, III, and IV of theDoushantuo Formation and the Luichapo Formation.The samples were split such that one set was trimmed andmounted in 2.5 cm epoxy disks for LA-ICPMS analysis,and the other set was powdered in a ball mill for bulk chem-ical procedures. The pyrite analyzed in these samples arelargely fine grained (10–25 mm) and anhedral, along witha few analyses of distinct framboids (Fig. 2). Electronmicroscope back-scattered electron images were producedusing a NovaNanoSEM 450 in CFAMM at the Universityof California, Riverside. Because the work of Sahoo et al.(2016) did not report data for as many trace elements aswere investigated in our pyrite study, splits of the samplesanalyzed for pyrite were reanalyzed for whole rock compo-sitions in order to explore the correlation between the wholerock and pyrite data for a larger suite of elements.

5.1. Whole rock trace element analysis

The samples were powdered using a SPEX mixer/millwith a silicon nitride ball mill. Approximately 100 mg ofeach powdered sample were ashed in a muffle furnace at550 �C for 12 h. The samples were digested in a 4:1 mixof concentrated nitric and hydrofluoric acid followed bydigestion in a 1:3 mix of concentrated nitric and hydrochlo-ric acid. Total trace element concentrations (Mo, Ni, Co,Mn, As, Pb, Sb, Zn, and Cu) were determined using an Agi-lent 7900 ICPMS at the University of California, Riverside.A blank solution was prepared for every batch of ten sam-ples, and the SDO-1 standard (USGS Devonian Ohio Shalestandard) was extracted and analyzed in parallel. Theblanks returned values below detection for every element,and the standard was within the range expected for everyelement of interest except Sb, which was 0.3 ppm over theupper range of certified values, and Zn, which was6.1 ppm over. Detection limits for major elements are typ-




Fig. 2. SEM back-scattered electron images of pyrite texture. A is from the sample taken at 13.7 m and B is from the sample taken at 3.7 m.Crystals with a euhedral form were avoided, whereas anhedral and large (>10 mm) framboids were analyzed.


ically better than 0.001–0.01% and for the trace elementsreported are better than 1 ppm.

5.2. LA-ICPMS

Pyrite grains and matrix spots were analyzed by LA-ICPMS at the ARC Centre of Excellence in Ore Deposits(CODES) at the University of Tasmania using a 193 nmsolid-state laser (UP193ss, NewWave Research) coupledto an Agilent 7700s quadrupole mass-spectrometer. Thelaser microprobe was furnished with constant-geometryLaurin Technic ablation chamber S-155 designed to takeup to twenty 2.5 cm diameter laser mounts.

A 10 or 15 lm beam size was used, depending on the sizeof the grain. The ablation was carried out in an atmosphereof pure He that was introduced into the ablation cell at arate of 0.8 l/min immediately past the ablation point. Toimprove the efficiency of aerosol transport, the He carriergas was mixed with Ar (0.85 l/min). Ablation was con-ducted with a laser pulse rate of 5 Hz using a beam fluenceof approximately 2 J/cm2. The ICPMS was adjusted toensure maximum sensitivity on isotopes between 80 and240 amu. The production of doubly charged and molecularoxide species was maintained at levels below 0.2%. Eachanalysis consisted of a 30 s laser-off period to measurebackground and a 40 s laser-on period for the analysis.Approximately ten spots were measured on pyrite and fivespots on the pyrite-free matrix for each sample. Calibrationstandards STDGL2b2 (Danyushevsky et al., 2011), GSD-1G (Guillong et al., 2005), and stoichiometric Peruvian pyr-ite (Gilbert et al., 2014a,b) were measured hourly togetherwith the unknowns. The Peruvian pyrite standard was ana-lyzed using the same spot size as the unknown pyrite inorder to account for Fe-S fractionation during the ablation(Gilbert et al., 2014a).

Sedimentary pyrite can contain abundant inclusions ofsilicate minerals, and hence typical LA-ICPMS analysesrepresent mixtures of pyrite and silicate matrix. To accountfor the two-component mixing in the LA-ICPMS analysesof sedimentary pyrite, the data were processed using analgorithm based on the linear regression of chalcophileand siderophile elements relative to S. Several steps wereincluded in the data processing. First, the ablation signal


was split into five segments of equal duration, each of whichwas calculated individually using equivalent time segmentsfor the calibration standards. The next step transformed themeasured counts to concentration units by the method ofLongerich et al. (1996), which quantifies the concentrationof an element in an unknown using known concentrationsin the standards analyzed at the beginning and end of eachanalytical run and the known concentration of one elementin the unknown (internal standard).

The composition of the mixtures of pyrite with matrixcan be highly variable, and since no element remains con-stant, the stoichiometric Fe content of pyrite was used tocalculate preliminary compositions. These preliminarycompositions were then normalized to 100% total of sidero-phile elements, chalcophile elements, and oxides of litho-phile elements. Linear regression equations were thencalculated relative to S for chalcophile and siderophile ele-mental data from the segmented pyrite and matrix analyses.The regression fits for the data were visually inspected, andanalyses with outliers were excluded. Occasionally, the pyr-ite data had linear relationships that did not pass throughthe matrix compositions. For these rare cases, matrix datawere excluded, and only pyrite analyses were used for thelinear regression. The final concentrations for trace ele-ments in pyrite were calculated by the regression equationsusing the calculated value of the S content that brought thesum of the chalcophile and siderophile elements to 100%.The calculated values were then compared to the detectionlimits estimated from noise for the gas background.

Data were removed if greater than 20% matrix was pre-sent (based on concentration of oxides of major and minorlithophile elements) or if Cu, Zn, Pb, and As were above1%; these high values would represent large inclusions ofother sulfide species, which would also impact the measure-ments of other elements.

6. RESULTS

6.1. LA-ICPMS analysis of pyrite

Geometric means and multiplicative standard deviations(MSD) for the LA-ICPMS analyses of pyrite for all ele-ments of interest (Pb, Ag, Sb, Bi, Se, Cd, Te, As, Cu, Zn,





Tl, Mo, Ni, Co, Mn, and Au) are given in Tables 1 and 2,respectively. The geometric mean and MSD are usedinstead of arithmetic mean and standard deviation becausethe trace element contents of sedimentary pyrite tend toshow, or approach, a log normal distribution for most ele-ments (Gregory et al., 2015a). When analyses were belowdetection limits, a value of half that limit was used to calcu-late the geometric mean and multiplicative standard devia-tion. Enrichment factors presented in Table 3 are calculatedas the ratio between the median for the trace element con-tent in pyrite at a given depth and the median for that ele-ment for the entire Doushantuo Formation pyrite dataset(e.g., EFpyNi3.7m = median(Ni3.7m)/median(Nitotal)). Halfthe detection limits were used for calculating the medianvalue when results were below detection. Elements withEF higher than 10 are considered strongly enriched, whilethose with EF between 5 and 10 are referred to as moder-ately enriched, and those between 2 and 5 are consideredweakly enriched. The results of the individual analyses areprovided in the electronic appendix. For comparison, wholerock data for elements of interest that were above detectionlimits are given in Table 4.

6.1.1. Chalcophile elements

The results for the LA-ICPMS concentrations for chal-cophile elements (Pb, Ag, Sb, Bi, Se, Cd, Te, and As) in pyr-ite are presented in Fig. 3. Selenium, Ag, Cd, Sb, Te, Tl, Pb,and Bi are generally enriched in samples at 3.7, 64.5, and116.3 m—the same intervals that contain whole rockenrichments in redox sensitive trace elements (Sahooet al., 2016). This relationship was particularly clear forAg (EFAg = 21.6, 63.2, and 126 for 3.7, 64.5, and116.3 m, respectively), Cd (EFCd = 3.08, 31.1, and 6.50for 3.7, 64.5, and 116.3 m, respectively), Sb (EFSb = 5.07,7.55, and 5.46 for 3.7, 64.5, and 116.3 m, respectively),and Pb (EFPb = 3.15, 10.8, and 5.39 for 3.7, 64.5, and116.3 m, respectively). Three of these elements showedadditional weak anomalies at other stratigraphic levels(EFAg = 3.37 at 6.8 m; EFCd = 2.19 and 2.17 at 56.3 and118.3 m, respectively; EFSb = 2.88 and 3.17 at 6.8 and81.8 m, respectively).

In the case of Te, the analyses are generally below detec-tion limits other than in the three enriched samples (3.7,64.5, and 116.3 m) and the overlying samples (6.8, 81.9,and 118.3 m). Selenium and Tl were enriched in two ofthe three stratigraphic levels showing whole rock enrich-ments in redox sensitive trace elements (Sahoo et al.,2016) (EFSe = 5.31 and 14.2 at 64.5 and 116.3 m, respec-tively; EFTl = 7.91 and 2.46 at 3.7 and 64.5 m, respectively).These elements also show weaker enrichments at otherstratigraphic levels (EFSe = 3.86 at 102.4 m andEFTl = 2.23 at 81.9 m).

Bismuth also shows moderate to strong enrichments inthe three intervals with elevated whole rock values(EFBi = 6.60, 36.2, and 8.93 at 3.7, 64.5, and 116.3 m,respectively); however, it is also strongly enriched(EFBi = 17.1) at 139.5 m and is the only chalcophile ele-ment measured showing strong enrichment outside of thethree horizons. Copper shows weak enrichments duringeach of the intervals with whole rock RSTE enrichments


(EFCu = 3.14, 2.87, and 3.36 at 3.7, 64.5, and 116.3 m,respectively) and slightly weaker enrichments in samplesclosely following those intervals (EFCu = 2.14 and 2.58 at6.8 and 81.9 m). An additional sample with a weak enrich-ment (EFCu = 2.53) occurs at 139.5 m.

Zinc is only moderately enriched (EFZn = 5.06) at 3.7 mand strongly enriched (EFZn = 12.3) at 64.5 m, but showsno enrichment at 116.3 m. There are also a moderateenrichment (EFZn = 7.48) at 139.5 m and weak enrichmentsof 2.77 and 4.16 at 13.7 and 81.9 m, respectively. Arsenic isweakly enriched in two of the intervals marked by wholerock enrichments (EFAs = 4.28 and 3.15 at 3.7 and116.3 m, respectively). It is also weakly to moderatelyenriched in strata overlying the intervals of whole rockenrichment (EFAs = 6.03, 2.28, and 2.41 at 6.8, 81.9, and118.3 m).

6.1.2. Siderophile elements

The siderophile elements analyzed by LA-ICPMS (Mo,Ni, Co, Mn, and Au) are presented in Fig. 4. Similar tothe chalcophile elements, many of the siderophile elementsinvestigated are weakly to strongly enriched in samples at3.7, 64.5, and 116.3 m. This relationship is most clearly evi-dent for Mo, which is strongly enriched at 3.7 and 116.3 m(EFMo = 23.9 and 14.7, respectively) and moderatelyenriched (EFMo = 6.29) at 64.5 m. Molybdenum also showsa strong enrichment immediately after the 3.7 m sample(EFMo = 12.2 at 6.8 m) and a weak enrichment followingthe 6.8 m sample (EFMo = 3.94 at 13.7 m). Although theymay not be considered Mo enrichments as defined here,the EFMo following the later two events show subtle enrich-ment with EFMo of 1.41 and 1.74 at 81.9 m and 118.3 m,respectively.

Manganese is moderately enriched at 64.5 m(EFMn = 6.72) and 116.3 m (EFMn = 5.53), as well asweakly enriched (EFMn = 3.92) at 3.7 m. However, Mn alsoshows a strong enrichment (EFMn = 10.9) at 81.9 m, whichis the first sample after the 64.5 m event, and a weakerenrichment (EFMn = 3.73) at 139.5 m. Similar to Mn, Auand Ni are both enriched (4.74 and 2.06, respectively) inthe 81.9 m sample following the 64.5 m event. Gold isweakly enriched at 3.7 m (EFMn = 2.61) and 13.7 m(EFMn = 3.03). Nickel is enriched in the samples from3.7 m (EFNi = 1.78), 64.5 m (EFNi = 1.84), and 116.3 m(EFNi = 2.29). In contrast, Co contents tend to vary little,with only one significant enrichment (EFCo = 3.64 at13.7 m), which does not correspond with any of the otherelements investigated.

6.2. Whole rock data

The EF for whole rock samples were calculated usingthe same method as EF for pyrite data (2 to 5 = weakenrichment, 5 to 10 = moderate enrichment, and>10 = strong enrichment). The same sample depths previ-ously interpreted to reflect oxygenation events show enrich-ments in Mo (EFMo = 8.0 at 3.7, 1.8 at 64.5 m, and 16.3 at116.3 m), consistent with the results of Sahoo et al. (2016).Similarly, Co shows a moderate enrichment (EFCo = 4.8) at3.7 m and a strong enrichment (EFCo = 19.3) at 116.8 m.




Table 1Geometric mean of trace element content of pyrite samples at different depths of the Wuhe section.

Depth (m) 139.5 118.3 116.8 110.7 102.4 95.6 81.9 64.5 56.3 50.1 32.4 13.7 6.8 3.7n 10 9 7 7 5 8 9 10 8 8 10 7 10 8Dominantpyrite texture

Anhedral,�15–25 mm

Anhedral,�15 mm


Anhedral,�20 mm

Anhedral,�15 mm

Anhedral,�20 mm

Framboidal,10–15 mm

Anhedral,�15 mm

Anhedral,�15 mm

Anhedral,�20 mm

Anhedral,�25 mm

Anhedral,<15 mm



Mo (ppm) 1.7 50.5 118 9.0 19.6 4.7 40.9 182 3.5 6.1 0.8 114 218 690

Ni (ppm) 307 211 763 436 313 268 784 675 172 214 261 441 570 731

Co (ppm) 89 46 115 92 54 46 47 69 45 70 78 231 139 67

Mn (ppm) 228 275 514 10 76 18 3990 2260 81 17 9 97 155 1320

Au (ppm) 0.037 0.043 0.078 0.064 0.031 0.034 0.211 0.036 0.035 0.045 0.034 0.142 0.080 0.125

As (ppm) 109 710 969 424 304 210 738 226 308 108 77 225 1860 1620

Se (ppm) 14 50 593 37 158 35 49 198 11 75 23 49 29 67

Cd (ppm) 0.27 1.29 3.68 0.75 0.23 0.36 0.72 23.4 1.38 0.17 0.27 1.16 0.26 2.42

Te (ppm) 0.28 0.32 4.33 0.51 0.35 0.26 0.72 3.06 0.35 0.42 0.34 2.75 2.17 2.05

Pb (ppm) 225 62 628 86 55 13 261 1910 83 24 22 289 456 624

Ag (ppm) 2.1 0.4 261 8.6 1.1 0.3 3.6 197 1.3 0.2 0.3 3.3 10.8 69.0

Sb (ppm) 13.0 2.9 125 16.5 16.8 6.4 68.1 148 8.4 5.6 12.9 15.8 71.0 111

Bi (ppm) 14.2 0.46 8.71 0.45 0.65 0.08 0.66 32.8 0.73 0.14 0.18 1.60 4.46 3.99

Cu (ppm) 772 95 1650 436 214 57.8 873 1190 114 86.5 51.1 357 775 1250

Zn (ppm) 44.6 14.7 12.9 2.91 27.7 4.09 100 298 8.01 9.41 3.34 58.4 32.3 164

Tl (ppm) 1.81 2.68 1.81 1.37 1.49 0.49 3.72 4.25 0.3 0.52 0.49 1.92 1.06 16.7

Numbers in bold correspond with periods of oxygenation as interpreted by traditional whole rock methods (Sahoo et al., 2016) and italicized numbers reflect samples where half or more of theanalyses were below detection limits for that element. When some analyses were below detection limits half of the detection limit was used to calculate geometric means.

8D.D

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Table 2Multiplicative standard deviation for trace element content of pyrite at different depths of the Wuhe section.

Depth (m) 139.5 118.3 116.8 110.7 102.4 95.6 81.9 64.5 56.3 50.1 32.4 13.7 6.8 3.7n 10 9 7 7 5 8 9 10 8 8 10 7 10 8Dominantpyrite texture


Anhedral,�15 mm


Anhedral,�20 mm

Anhedral,�15 mm

Anhedral,�20 mm

Framboidal,10–15 mm

Anhedral,�15 mm

Anhedral,�15 mm

Anhedral,�20 mm

Anhedral,�25 mm

Anhedral,<15 mm



Mo 3.17 1.98 22.4 4.27 1.55 3.31 1.36 1.56 1.87 4.65 3.22 1.52 2.96 1.73

Ni 1.49 1.84 1.93 1.70 1.63 1.73 1.15 1.33 1.56 1.44 1.43 1.51 2.60 1.80

Co 1.48 1.82 2.77 2.25 1.72 1.71 1.26 1.77 1.77 1.51 1.77 1.47 4.34 4.47

Mn 23.3 4.31 14.9 3.92 12.6 8.90 1.30 1.33 12.2 12.3 5.42 16.5 3.27 1.84

Au 1.60 1.11 2.71 2.66 1.21 1.25 1.59 1.59 1.23 1.13 1.50 1.12 2.29 3.31

As 2.36 1.24 1.38 2.46 1.89 1.94 1.67 1.22 2.11 2.26 1.96 1.77 1.85 2.71

Se 1.34 3.73 1.50 3.53 1.49 1.69 1.51 1.26 1.60 1.53 1.41 2.06 1.72 2.69

Cd 2.20 2.61 2.82 2.77 1.40 2.05 1.39 1.64 2.88 3.35 2.05 1.69 2.38 1.92

Te 1.43 1.12 3.86 1.11 3.30 1.27 2.13 1.52 1.23 1.12 1.50 1.12 1.58 3.00

Pb 1.70 2.60 4.37 2.25 1.94 2.94 1.11 1.44 2.42 1.92 1.60 1.46 3.17 1.29

Ag 3.13 4.03 7.57 3.55 1.81 2.32 1.28 1.44 2.66 3.42 1.81 1.76 1.37 1.33

Sb 1.94 3.60 2.78 4.95 2.65 3.17 1.76 1.32 2.75 2.22 1.41 1.76 2.35 1.72

Bi 2.79 5.34 1.93 3.26 2.48 3.26 2.08 1.58 2.30 3.31 1.53 1.74 3.34 3.45

Cu 4.08 2.83 2.93 4.69 2.43 3.17 1.17 1.79 1.90 1.96 1.79 1.54 2.23 2.39

Zn 12.1 3.22 3.31 2.06 2.92 3.40 1.17 1.69 3.36 9.55 2.48 2.52 2.48 2.67

Tl 1.98 2.51 2.21 5.60 2.09 3.31 1.22 1.61 6.17 2.60 1.93 1.66 3.20 3.16

Numbers in bold correspond with periods of oxygenation as interpreted by traditional whole rock methods (Sahoo et al., 2016) and italicized numbers reflect samples where half or more of theanalyses were below detection limits for that element. When some analyses were below detection limits half of the detection limit was used to calculate multiplicative standard deviations.

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Tab

le3

Enrichmentfactors

oftraceelem

ents

inpyritefrom

theWuhesection.

Depth

(m)

Mo

(ppm)

Ni

(ppm)

Co

(ppm)

Mn

(ppm)

Au

(ppm)

As

(ppm)

Se

(ppm)

Cd

(ppm)

Te

(ppm)

Pb

(ppm)

Ag

(ppm)

Sb

(ppm)

Bi

(ppm)

Cu

(ppm)

Zn

(ppm)

Tl

(ppm)

139.5

0.05

0.85

1.41

3.73

0.94

0.23

0.36

0.33

0.62

1.31

0.68

0.54

17.1

2.53

7.48

1.25

118.3

1.74

0.47

0.55

0.34

0.98

2.41

1.00

2.17

0.71

0.24

0.08

0.11

0.52

0.20

0.70

1.04

116.3

14.7

2.29

1.25

5.53

1.72

3.15

14.2

6.50

7.26

5.39

126

5.46

8.93

3.36

0.71

0.96

110.7

0.33

1.08

1.59

0.05

0.89

0.81

0.76

1.25

1.07

0.30

1.90

0.34

0.36

0.71

0.09

0.37

102.4

0.90

0.95

0.67

0.22

0.72

0.74

3.86

0.25

0.47

0.29

0.35

0.58

0.58

0.90

0.66

0.84

95.6

0.18

0.82

0.66

0.10

0.81

0.67

1.02

0.43

0.59

0.09

0.10

0.32

0.09

0.18

0.12

0.38

81.9

1.41

2.06

0.74

10.9

4.74

2.28

1.11

1.00

1.66

1.38

1.08

3.17

0.72

2.58

4.16

2.23

64.5

6.29

1.84

0.93

6.72

0.71

0.76

5.31

31.1

5.80

10.83

63.2

7.55

36.2

2.87

12.3

2.46

56.3

0.11

0.45

0.61

0.15

0.80

1.19

0.25

2.19

0.79

0.42

0.37

0.38

0.71

0.32

0.27

0.23

50.1

0.13

0.64

0.87

0.46

1.03

0.36

1.84

0.18

0.95

0.12

0.09

0.22

0.13

0.21

0.15

0.26

32.4

0.03

0.70

1.16

0.06

0.89

0.29

0.63

0.37

0.85

0.10

0.10

0.63

0.15

0.14

0.09

0.38

13.7

3.94

1.07

3.64

0.81

3.03

0.62

0.98

1.12

5.83

1.38

1.06

1.04

1.57

0.94

2.77

1.27

6.8

12.2

1.30

1.04

0.49

1.99

6.03

0.71

0.25

4.98

1.96

3.37

2.88

4.79

2.14

1.16

0.60

3.7

23.9

1.78

1.27

3.92

2.61

4.28

1.23

3.08

3.57

3.15

21.6

5.07

6.60

3.14

5.06

7.91

Numbersin

bold

correspondwithperiodsofoxygenationas

interpretedbytrad

itional

whole

rock

methods(Sah

ooet

al.,20

16).



However, Co is not enriched at 64.5 m. Of the other ele-ments, only Mn has a moderate enrichment (13.7 m); theother elements show low enrichments. The weak enrich-ments of Ni, Mn, As, Pb, Sb, Zn, and Cu occur at variousstratigraphic intervals not previously identified as oxygena-tion events.

7. DISCUSSION

7.1. Pyrite texture

The pyrite grains analyzed are interpreted to haveformed largely in the water column based on the generallyspherical shape of the pyrite and the relatively small size ofthe majority of framboids (Wang et al., 2012; Fig. 2). Theanhedral grains likely reflect minor pyrite overgrowth onindividual framboids, and larger anhedral grains likely rep-resent minor overgrowth of pyrite on framboid clustersbecause the outlines of smaller, spherical crystals can stillbe identified in many of these grains (Fig. 2B). The minorpyrite overgrowths that obscure original framboid textureare common (Wacey et al., 2015) due to the favorable reac-tion kinetics for pyrite to nucleate on earlier pyrite grains(Rickard, 2012).

Berner et al. (2013) and Pisarzowska et al. (2014) arguedthat some transition elements (specifically Ni, Co, Cu, andZn) are dominantly enriched in later formed diagenetic pyr-ite. Therefore, it is possible that the enrichment of these ele-ments is due to later pyrite overgrowths that contain moreof these elements. However, using the arguments of theseauthors we should expect the siderophile elements to showhigher concentrations in the samples with larger pyriteovergrowths. This relationship is not observed at Wuhe.The sample dominated by framboidal pyrite with no over-growths (81.9 m) is still weakly enriched in Ni, Cu, andZn and strongly enriched in Mn, whereas several sampleswith pyrite overgrowths are not enriched in these elements.

7.2. Comparison of pyrite trace element proxy to other

geochemical proxies

The geochemical proxies used by Sahoo et al. (2016) toinvestigate the Wuhe section include Fe speciation (wetchemical procedures specific to certain minerals and min-eral fractions—oxides, carbonate-bound, magnetite, andpyrite); d34S signature of pyrite; pyrite morphology; andwhole rock analyses of V, Mo, Re, and TOC (Fig. 5).The authors used these data to argue that the depositionof the Wuhe section was under mostly euxinic conditions.Three distinct periods of widespread, possibly global oxy-genation were inferred based on intervals with spikes inwhole rock trace element abundance, despite the persistenceof local euxinia at Wuhe. One oxygenation event was iden-tified at the beginning of the deposition of Member II(Sahoo et al., 2012; Hohl et al., 2017), one at the beginningof deposition of Member III, and one during the depositionof Member IV. Sahoo et al. (2016) reasoned that sedimen-tary rocks deposited under euxinic conditions should con-sistently capture trace elements in concentrationsproportional to their marine inventories (e.g., Algeo and




Table 4Whole rock composition from splits of samples used to make laser mounts.

Depth (m) Mo (ppm) Ni (ppm) Co (ppm) Mn (ppm) As (ppm) Pb (ppm) Sb (ppm) Zn (ppm) Cu (ppm)

139.5 0.8 5.4 3.7 80.9 2.6 9.0 0.4 9.2 17.5118.3 41.4 32.1 11.3 181 31.6 23.5 3.0 54.4 35.4116.8 113 76.3 467 134 31.1 25.8 6.0 31.8 90.1

110.7 14.1 46.1 6.2 589 32.5 18.4 9.0 39.4 65.7102.4 8.1 71.0 9.7 164 53.9 15.0 3.9 192 11795.6 4.1 62.2 20.7 396 39.8 20.9 4.7 180 66.681.9 2.1 48.3 71.5 437 35.5 28.9 3.7 54.9 67.664.5 12.6 87.0 17.3 116 19.2 53.6 7.4 48.6 71.1

56.3 2.1 26.4 37.9 232 33.2 21.9 1.9 29.4 36.450.1 4.2 41.5 7.5 390 31.2 10.5 2.0 115 49.132.4 1.6 33.7 46.5 509 17.7 12.6 5.5 29.7 40.213.7 5.8 33.5 27.6 1570 14.9 30.3 0.9 75.6 53.76.8 16.1 41.5 66.8 115 83.1 32.5 2.9 56.9 50.13.7 55.7 79.1 117 110 90.1 37.6 4.7 45.4 81.3

Numbers in bold correspond with periods of oxygenation as interpreted by traditional whole rock methods (Sahoo et al., 2016).


Lyons, 2006) and that rising and falling enrichments shouldreflect decreases and increases, respectively, in the globalextent of anoxic/euxinic seafloor. In other words, the globalmetal inventory should be high under widely oxic condi-tions, with enrichments that are correspondingly large inlocally euxinic settings (Reinhard et al., 2013; Owenset al., 2016).

A wider range of elements can be used for pyrite analy-ses because chalcophile and siderophile elements are prefer-entially incorporated into pyrite (Large et al., 2014), and weshould expect them to be drawn down under widespreadeuxinic conditions. In other words, as the area of euxinicbottom waters increases, trace elements are drawn downin seawater, and enrichments in coeval pyrite decrease cor-respondingly. Conversely, as the area of the seafloor that iseuxinic decreases, likely in response to overall increases inocean-atmosphere oxygenation, the inventories increase,and trace element enrichments in pyrite formed in locallysulfidic settings also increase. Additionally, increased oxy-genation of the atmosphere would enhance weathering onland, which could in turn increase the trace element dis-charge into the oceans.

All of the chalcophile elements investigated by micro-analysis of pyrite (As, Se, Cd, Te, Pb, Ag, Sb, Bi, Cu, Zn,and Tl) show enrichments at some of the same levels asthe metal enrichments of Sahoo et al. (2016; highlightedin light grey/blue in Fig. 3), and most are enriched at allthose depths. In some cases, the pyrite enrichments are sev-eral orders of magnitude beyond the pyrite baseline valuesfrom the same section. These observations favor the use oftrace elements in sedimentary pyrite to determine, in a first-order sense, the trace element abundance of early oceans.

Bismuth, and to a lesser extent Zn and Cu, exhibit addi-tional enrichments at 139.5 m (EFBi = 17.1, EFZn = 7.48,EFCu = 2.53); however, the lack of enrichment in the otherelements suggests that those depths should not be inter-preted as reflecting a general change in trace element inven-tory (such as would be expected for an oxygenation event).Examples like these reinforce the need for simultaneousanalysis of a large suite of elements when interpreting pyrite


LA-ICPMS data. For example, the 3.7 m sample had sixweak, four moderate, and two strong enrichments; the64.5 m sample had two weak, four moderate, and fivestrong enrichments; and the 116.3 m sample had two weak,six moderate, and three strong enrichments. These observa-tions suggest that those intervals are enriched in trace ele-ments due to large-scale changes in trace elementinventory, but highlight the possibility of other, likely localcontrols on overall enrichment.

An added benefit of the pyrite proxy is that it seems tobetter capture (and/or preserve) the enrichment patternsin some cases. For example, the interval at 64.5 m haslow whole rock Mo content compared to the other ele-ments, but the pyrite peak is more obvious (moderateenrichment EFMo = 6.29). Overall, we interpret our enrich-ment patterns in pyrite to be consistent with the idea ofincreased marine oxygenation at the time of deposition ofthese intervals (Sahoo et al., 2016), which would haveresulted in reduced anoxic/euxinic sinks in the ocean andperhaps an increase in the riverine flux via enhanced weath-ering of sulfides on land. The importance of this latter effectis not well known because sulfide minerals weather rapidlyeven under very low atmospheric O2. The degree to whichpatterns of continental weathering scale to rising and fallingO2 (beyond low threshold levels of O2) is fodder for futureresearch.

Regardless of our interpretations, the overall agreementbetween the pyrite and whole rock data is striking. Recallthat in many cases, such as for Mo (Chappaz et al.,2014), the majority of the elemental abundance in a givenwhole rock sample is not contained in pyrite. As such, thecorrespondence between the two datasets suggests insteadthat the whole rock results are tracking other host relation-ships, such as organic matter, and that the pyrite isresponding to the same control—most likely the ambientavailability of the specific trace element in the surroundingwater column and pore fluids.

The siderophile elements (Mo, Ni, Mn, and Au), exceptCo, show similar enrichments in the zones of whole rockRSTE enrichment; however, they are also likely to have




Fig. 3. Variation of chalcophile elements within the Wuhe section. The blue line connects the geometric means of the different samplesanalyzed from the section and diamonds show the concentration of individual LA-ICPMS analyses. When analyses returned below detectionlimit values, half the detection limit was used to calculate the geometric mean. Hollow squares indicate analyses that were below detectionlimits, and values representing half of the detection limits are plotted. Light blue background areas highlight stratigraphic levels that containoxygenation events.


additional enrichments elsewhere in the stratigraphy. Thesepatterns may reflect extended or delayed peak enrichmentrelative to the chalcophile elements (e.g., Ni increases from675 ppm at 64.5 m to 784 ppm at 81.9 m) before they returnto the background levels that mark the long interveningperiods assumed to reflect more widespread oceanic anox-ia/euxinia. This relationship is most evident in the enrich-ment in Doushantuo Member III (81.9 m), where pyrite is


more enriched in Mn (EFMn = 10.85), Ni (EFNi = 20.6),and Au (EFAu = 4.74) compared to the preceding zonemarked by enrichments in the other elements and assumedto reflect an oceanic oxygenation event. However, due to alimit in sample density at the interval the reason for thesesiderophile trace element enrichments remains unclear. Inany case, because of the possibility that siderophile ele-ments, other than Mo, are enriched at levels other than




Fig. 4. Variation of siderophile elements within the Wuhe section. The blue line connects the geometric means of the different samplesanalyzed from the section and diamonds show the concentration of individual LA-ICPMS analyses. When analyses returned below detectionlimit values, half the detection limit was used to calculate the geometric mean. Hollow squares indicate analyses that were below detectionlimits, and values representing half of the detection limits are plotted. Light blue background areas highlight stratigraphic levels that containoxygenation events.


those that contain RSTE enrichments in whole rock, thesiderophile elements other than Mo must be used with cau-tion when interpreting relative paleo-ocean trace elementabundances.

Sedimentary pyrite analyses tend to show a wide varietyof compositions from multiple grains within a single lasermount (Gregory et al., 2014, 2015a; Large et al., 2014,2015a,b)—generally spanning 1–2 orders of magnitude.While large, this variation is much smaller than the overfour orders of magnitude of variation commonly observedin hydrothermal systems (Reich et al., 2013; Deditiuset al., 2014; Tardani et al., 2017). Generally, variation isdue to the complicated pathways by which trace elementsare captured and held within the pyrite mineral; local,potentially evolving conditions in the setting where the pyr-ite forms; and possible nugget effects attributable to micro-inclusions within pyrite. Nevertheless, geometric mean andmedian values for trace element compositions generallyagree among pyrite samples from the same stratigraphiccolumn when deposited under similar conditions (e.g., Cari-aco Basin; Gregory et al., 2010). General agreement is alsoseen among different basins of the same age (Large et al.,2015b). These consistencies confirm the utility of pyritechemistry as a proxy for the trace element composition ofseawater. Nevertheless, while useful to identify first-ordertrace element inventory trends, any effort to quantify sea-water inventories precisely from pyrite enrichment patternsshould be undertaken with caution.

7.3. Correlation of pyrite analyses to whole rock data

Pyrite compositions tend to be enriched in Ag, Cd, Sb,Te, Pb, Bi, and Se and to a lesser extent Mn, Ni, Cu, Zn,


Au, and Tl at the same intervals as the whole rock analysesare enriched in Mo, V, U, and Re, and thus mutual first-order scaling with the ambient inventory is assumed. Therelationship is, however, complicated by the likelihood thatwhole rock data reflect a wider array of host relationships.For example, as mentioned briefly above, much if not mostof the Mo in black shales appears to be associated withmatrix material, perhaps organic matter, rather than pyrite(Chappaz et al., 2014), and the whole rock data are thus notcontrolled exclusively or even predominantly by the pyrite.This complication is evident in plots of geometric means forpyrite versus whole rock data normalized to TOC content(trace element composition from this study, TOC fromSahoo et al., 2016).

As an additional complication of the whole rockapproaches, trace element data are commonly normalizedto TOC contents to compensate for the strong covariationthat is often observed between trace element and organicconcentrations. In other words, the element might be scav-enged by organic substrates. At the same time, the localenvironmental factors favorable to metal enrichment(anoxia and sulfide buildup) would be impacted by localTOC content. As such, the observed variations may at leastpartly reflect trends in organic content rather than temporaldifferences in the marine inventory. The amount of detritalinput, as well as carbonate content and authigenic and bio-genic silica, can also affect the trace element concentrations.To compensate, the trace elements are often normalized toAl to account for changes in detrital flux (Tribovillardet al., 2006). These challenges can be avoided via the pyriteproxy technique. To date, Mo is the only paleo-redox proxyin the previously available whole rock data that is alsoknown to be significantly enriched in pyrite and easily mea-




Fig. 5. Pyrite morphology, FePy/FeHR (pyrite Fe/highly reactive Fe), Mo, V, Re whole rock geochemistry, and d34SPy (all from Sahoo et al.,2016) compared to Mo content of pyrite. If FePy/FeHR = 0.7–0.8, then sediment was deposited under euxinic conditions (Poulton andCanfield, 2011). Note the areas with elevated Mo in pyrite coincide with zones of interpreted oxygenation events based on the study of Sahooet al. (2016) highlighted in light blue.


sured with LA-ICPMS, and a moderate correlation wasobtained in our data between Mo normalized to TOCand Mo content in pyrite (R2 = 0.71; Fig. 6). This suggeststhat the enrichments of Mo in pyrite, though Mo is not pre-dominantly held in pyrite, are tracking relative abundancein seawater inventory.

The pyrite data in our study shows enrichments in manymore elements (Ag, Cd, Sb, Te, Pb, Bi, and Se and to a les-ser extent Mn, Ni, Cu, Zn, Au, and Tl) beyond those avail-able from the whole rock data. To test whether enrichmentsmay have been present in whole rock samples for elementsthat were overlooked in previous studies, we calculated cor-relation coefficients between trace elements in pyrite andtrace elements in whole rock for a broader range of data(normalized to TOC, Al and un-normalized). Other thanMo, only As in pyrite had a weak correlation to As in wholerock normalized to TOC (R2 = 0.53). No other significantcorrelations were obtained between trace elements in pyriteand trace elements in whole rock normalized to TOC (forNi, Cd, Co, Pb, Se, Mn, Cu, Zn, and Sb, R2 = 0.26, 0.06,0.07, 0.08, 0.09, 0.01, 0.03, 0.001, and 0.001, respectively).Correlations were no better when trace elements were nor-malized to Al, with no significant correlations for Mo, As,Ni, Cd, Pb, Se, Mn, Cu, Zn, and Sb (R2 = 0.02, 0.08, 0.01,0.03, 0.07, 0.005, 0.07, 0.001, 0.005, and 0.003, respectively).


Trace element content of pyrite versus trace element con-tent of un-normalized whole rock had better correlation(R2 = 0.17 for Mo, 0.70 for As, 0.36 for Ni, 0.53 for Cd,0.73 for Pb, 0.02 for Se, 0.02 for Mn, 0.11 for Cu, 0.03for Zn, and 0.19 for Sb). Better correlation can be obtainedfor some elements by separating the data into 2 groups,intervals of the first and second element enrichment zonesand immediately overlying stratigraphy (3.7 m, 6.8 m,13.7 m, 64.5 m, and 81.9 m) as group one and all other sam-ples as group two. By splitting the dataset correlation forMo, Cd, and Sb improve significantly (group oneR2 = 0.998 and group two R2 = 0.98 for Mo; group oneR2 = 0.997 and group two R2 = 0.69 for Cd; group oneR2 = 0.96 and group two R2 = 0.36 for Sb). This may sug-gest that these two groups represent samples that have pyr-ite as a significant host of these elements (group 1) andthose that do not (group 2).

Generally speaking the elements are preferentially incor-porated into pyrite; however, pyrite makes up too small apercentage of the rock to manifest significantly in the wholerock concentrations of trace elements. Though trace ele-ments in bulk samples and pyrite can both be used to iden-tify important, first-order changes in ocean trace elementinventories, the pyrite and whole rock data show differentsensitivities. This difference highlights the novel value of




Fig. 6. Selected correlation plots of geometric means of pyrite Mo, Se, and Pb content versus whole rock Mo/TOC, Se/TOC, and Pb/TOCrespectively; pyrite Mo, Se, and Pb content versus whole rock Mo/Al, Se/Al, and Pb/Al respectively; and pyrite Mo, Se, and Pb content versuswhole rock Mo, Se, and Pb respectively.


Please cite this article in press as: Gregory D. D., et al. Whole rock and discrete pyrite geochemistry as complementary tracers of ancientocean chemistry: An example from the Neoproterozoic Doushantuo Formation, China. Geochim. Cosmochim. Acta (2017), http://dx.doi.org/10.1016/j.gca.2017.05.042



Table 5Criteria for sedimentary pyrite, distal SEDEX pyrite, and orogenic gold related pyrite and corresponding ratios of pyrite reported in this study.

Depth Co/Ni Zn/Ni Cu/Ni As/Ni As/Au Ag/Au Sb/Au Bi/Au Tl/Co

Sedimentary pyrite range (Gregory et al., 2015a,b) 0.01–2 0.01–10 0.01–2 0.1–10 >200 >2 >100 >1Distal SEDEX pyrite TE range (py1) (Mukherjee and Large, 2017) 0.85 28 3.2 27 11Distal SEDEX pyrite TE range (py1) (Mukherjee and Large, 2017) 0.67 33 7.9 74 45Victory-Defiance orogenic gold pyrite trace element range (Gregory et al., 2016) 0.84 <0.01 0.01 BD BDEast Repulse orogenic gold pyrite trace element range (Gregory et al., 2016) 0.50 <0.01 0.01 143 1.4WH09-139.5 0.29 0.15 2.51 0.36 2970 56.6 351 386 0.02WH09-118.3 0.22 0.07 0.45 3.36 16,600 8.6 68 10.7 0.06WH09-116.8 0.15 0.02 2.16 1.27 12,500 3350 1600 112 0.02WH09-110.7 0.21 0.01 1.00 0.97 6590 134 257 7.1 0.01WH09-102.4 0.17 0.09 0.68 0.97 9760 35.2 541 20.9 0.03WH09-95.6 0.17 0.01 0.22 0.78 6110 8.5 186 2.4 0.01WH09-81.9 0.06 0.13 1.11 0.94 3500 16.9 323 3.2 0.08WH09-64.5 0.10 0.44 1.76 0.33 6330 5520 4140 919 0.06WH09-56.3 0.26 0.05 0.66 1.79 8930 38.1 244 21.0 0.01WH09-50.1 0.33 0.04 0.40 0.50 2410 5.3 125 3.1 0.01WH09-32.4 0.30 0.01 0.20 0.29 2250 9.3 378 5.3 0.01WH09-13.7 0.52 0.13 0.81 0.51 1580 23.5 111 11.2 0.01WH09-6.8 0.24 0.06 1.36 3.26 23,300 135 891 56.0 0.01WH09-3.7 0.09 0.22 1.71 2.21 12,900 550 885 31.8 0.25

WH09 is the prefix for that Wuhe samples and the following number represents there stratigraphic depth. Trace element ratios are mass ratios.

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the pyrite technique and its complementary and overlap-ping relationship with the whole rock approach.

7.4. Could the periods of high metal enrichment be due to

hydrothermal input?

Perhaps the greatest specific value of the pyrite approachis its capacity to see through secondary enrichments fromhydrothermal overprints. Typically, hydrothermal pyritecan be identified by coarser, more euhedral textures andcan easily be avoided during data collection. However, thisis not always the case for discrete analyses and never thecase for whole rock approaches. Distal sedimentaryexhalative-style (SEDEX) pyrite mineralization is especiallydifficult to discriminate from pyrite formed in sedimentswithout hydrothermal input. Both can have similar fram-boidal textures, which can be found in SEDEX mineraliza-tion due to the abundance of iron introduced into the basinby the hydrothermal fluids (Gadd et al., 2016).

To assess whether the trace elements in pyrite in thisstudy are likely to have had a hydrothermal origin we usedthe geochemical classification system proposed by Gregoryet al. (2015a) and compared our results to distal SEDEXstyle mineralization trace element content (Mukherjee andLarge, 2017) and orogenic gold-related pyrite trace elementcontent (Gregory et al., 2016). Table 5 contains eight differ-ent ranges of trace element ratios proposed by Gregoryet al. (2015a) to identify sedimentary pyrite (Co/Ni, Zn/Ni, Cu/Ni, As/Ni, As/Au, Ag/Au, Sb/Au, and Bi/Au)and one trace element ratio (Tl/Co) used to identify distalSEDEX mineralization (Mukherjee and Large, 2017). Withone exception, all of our trace element ratios satisfy the cri-teria of Gregory et al. (2015a) for sedimentary pyrite. Theexception was the Sb/Au ratio in the sample from118.3 m. In this sample, the Au content was below detectionlimits, and a value of half the detection limit was used forthe calculation. It is probable that this exception is an arti-fact of an incorrect estimate of Au content rather than evi-dence for a hydrothermal component.

Distal SEDEX mineralization, the most likely source ofhydrothermal input that cannot be identified texturally, isidentified by high Zn/Ni and Tl/Co ratios (28 and 33 forZn/Ni; 11 and 45 for Tl/Co; Mukherjee and Large, 2017).Our data again show no evidence of hydrothermal input,with Zn/Ni < 0.44 and Tl/Co < 0.25 for geometric meansof all samples—in other words, values that are two ordersof magnitude lower than those identified by Mukherjeeand Large (2017).

To further investigate the possibility of hydrothermalmetal input, we compare geometric mean ratios for pyritefrom the Wuhe section with those from orogenic golddeposits from the St. Ives Au district, Australia (Table 5;Gregory et al., 2016). While the Co/Ni ratio of the ore-related pyrite are similar to our data, the Cu/Ni and As/Ni ratios of the ore pyrite are far below (1–2 orders of mag-nitude) compared to those from pyrite of the Wuhe section.Similarly, the As/Au and Ag/Au ratios at Wuhe are wellabove those for pyrite from the East Repulse deposit (datafrom Victory-Defiance, the other deposit at St. Ives, were


below detection for all these elements). Again, the dataseem to suggest that hydrothermal processes played littleor no role in the observed trends from the Wuhe sectionof the Doushantuo Formation.

While the current knowledge suggests no hydrothermalinput was present at Wuhe, investigators employing thistechnique should continue to review the economic geologyliterature to ensure that future studies do not identifyhydrothermal fluid compositions that have the same pyritetrace element ratios as sedimentary pyrite.

8. CONCLUSIONS AND FUTURE

RECOMMENDATIONS

Trace element data for pyrite from the Wuhe section ofsouth China show enrichments in several elements in theintervals marked by whole rock enrichment of differenttrace elements (except Mo which is enriched in both) sug-gesting increased marine oxygenation on a broad scale.Further, the inclusion of pyrite data comes with severaladvantages. First, pyrite chemistry is less likely than wholerock analyses to be affected by metamorphic fluid remobi-lization, expanding the number of basins that can be inves-tigated. Second, the low detection limits of LA-ICPMS andthe natural enrichment of several elements in pyrite relativeto the whole rock composition allow for more elements tobe investigated compared to traditional whole rock tech-niques. These additional elements, with further investiga-tion, may carry specific utility beyond identifying globalredox structure. These may include biological relevancefor particular elements seen best in pyrite (e.g. Ni formethanogenesis) or specific redox implications carried bycertain elements. Third, existing databases for sedimentarypyrite and hydrothermal pyrite show that hydrothermalpyrite has distinct trace element signatures relative to sedi-mentary pyrite, and thus pyrite chemistry can be used todetermine whether trace element patterns reflect changesin ocean chemistry or the presence of cryptic hydrothermalinputs. Finally, the pyrite proxy may, in a first-order sense,be less dependent on local redox conditions and specificallythe requirement for euxinia, which is essential in whole rockevaluations of Mo, for example.

Despite these advantages, pyrite chemistry alone isinsufficient for determinations of important factors inpaleo-ocean chemistry, such as the presence of euxinia orextent of the sulfur reservoir, among others. Therefore,we suggest that at the basin scale, in order to obtain themost complete understanding of ancient ocean conditions,pyrite chemistry should be combined with traditional wholerock techniques, including iron speciation as an indepen-dent measure of local redox conditions. Finally, our newpyrite data support the work of Sahoo et al. (2016) andspecifically the argument for three trace element enrichmentevents, possibly related to oxygenation, during the Edi-acaran Period.

Future work should focus on studies of pyrite from abasin that has experienced different degrees of metamor-phism to confirm that the trace element content of pyritecan be preserved up to greenschist facies—and perhaps





beyond. While the work by Large et al. (2007) suggests asmuch, this important inference has not been confirmed ina single basin with variable degrees of metamorphism. Suc-cessful confirmation will allow the investigation of paleo-ceanographic conditions in metamorphosed sedimentarybasins by LA-ICPMS analysis of pyrite and thus willgreatly increase the number of basins and sections avail-able, especially for Precambrian reconstructions. Further,at this time the amount of variation we should expect whencomparing pyrite formed in the sediments versus (syn-genetic) formation in the water column is not known.

More generally, the fidelity of paleoceanographicrecords in samples dominated by diagenetic pyrite remainsimportant to study, as pore waters can deviate rapidly anddramatically from the conditions in the overlying waters.We can imagine, however, that first-order relationships(e.g., Archean vs. Proterozoic vs. Phanerozoic or majoranoxic or oxic events) could be captured. At the same time,diagenetic pyrite carries particular value as it can speakuniquely to pore water trace element availability that varieswith increasing burial depth and, more generally, frominventories in the overlying water. It is these pore watersupplies that can impact the trace metal-dependent deepbiosphere. Finally, we still know very little about variationsin the trace element compositions of pyrite formed underdifferent coeval depositional and diagenetic conditions. Aswe move forward, studies of pyrite framboids from diversemodern depositional environments with differing water col-umn redox conditions will be essential.

ACKNOWLEDGMENTS

The NASA Astrobiology Institute and the NSF FESD andEarth-Life Transitions programs provided financial support forthis research (TWL and DDG). Analytical work was also fundedby an Australian Research Council grant to Ross Large. We wouldlike to thank Martin Reich and two anonymous reviewers for valu-able comments on the manuscript. We would further like to thankSelina Wu for useful comments on several different versions of themanuscript and Steve Bates and Andy Robinson for expert help inthe analysis of the whole rock samples. We further thank IlkuenLee at CFAMM for expert aid in taking the SEM images.

APPENDIX A. SUPPLEMENTARY MATERIAL

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.gca.2017.05.042.

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whole rock and discrete pyrite geochemistry as...

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