A global perturbation to the sulfur cycle during the Toarcian Oceanic Anoxic Event

Earth and Planetary Science Letters 312 (2011) 484–496

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Earth and Planetary Science Letters

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A global perturbation to the sulfur cycle during the Toarcian Oceanic Anoxic Event

Benjamin C. Gill a,b,⁎, Timothy W. Lyons b, Hugh C. Jenkyns c

a Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, MA, 02138, USAb Department of Earth Sciences, University of California, Riverside, 900 University Avenue, Riverside, CA, 92521, USAc Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK

⁎ Corresponding author. Department of Earth and Plaversity, 20 Oxford Street, Cambridge, MA, 02138, USA.

E-mail address: [email protected] (B.C. Gill).

0012-821X/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.epsl.2011.10.030

a b s t r a c t
a r t i c l e i n f o
Article history:Received 28 April 2011Received in revised form 8 October 2011Accepted 16 October 2011Available online 24 November 2011

Editor: G. Henderson

Keywords:euxiniapyrite burialseawater sulfatesulfur cyclesulfur isotopesToarcian Oceanic Anoxic Event

The Mesozoic Era was punctuated by intervals of widespread anoxia within the ocean, termed oceanic anoxicevents or OAEs. The chemostratigraphy of these intervals also contains evidence of transient perturbations tomany biogeochemically important elemental cycles. Here we present high-resolution sulfur isotope datafrom three stratigraphic sections spanning the Toarcian Oceanic Anoxic Event (T-OAE) of the Early Jurassic.All sections show a similar increase in the sulfur isotope ratio of sulfate parallel to an overall positive excur-sion in carbon isotopes during the OAE interval. Based on forward box modeling, the sulfate-S isotope excur-sion can be generated by transiently increasing the burial rate of pyrite in marine sediments likely depositedunder euxinic (i.e., anoxic and sulfidic) conditions in the water column. In addition, modeling shows thatprolonged recovery of the δ34S of seawater sulfate—at least 8 Ma after the initial rise associated with theOAE—was due to the relatively long residence time of sulfate in the Jurassic ocean; estimates from our model-ing put the Toarcian marine sulfate concentrations at 4 to 8 mM. The similarity of the sulfur isotope recordsfrom the North European epicontinental (or epeiric) sea and Tethyan continental margin suggests that localmodification of the marine sulfur isotope signal was minimal: a point explored with isotope mixing models.Importantly, our results indicate that the sulfur isotope excursion reflects a globally significant perturbationin the sulfur cycle and that pyrite burial in the North European Epeiric Seaway alone cannot account for theexcursion. This study, along with recent work from other Phanerozoic intervals of widespread marine oxygendeficiency, confirms that the sulfur cycle can be perturbed significantly by enhanced pyrite burial duringperiods of prolonged oceanic anoxia/euxinia.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The Toarcian Ocean Anoxic Event (T-OAE) of the Early Jurassic wasa time of severe environmental turbulence marked by the widespreaddeposition of organic-rich sediments and increased rates of extinctionin the paleontological record (Aberhan and Baumiller, 2003; BucefaloPalliani et al., 2002; Caswell et al., 2009; Dera et al., 2010; Harries andLittle, 1999; Jenkyns, 1985, 1988; Little and Benton, 1995; McElwainet al., 2005). It is thought that, during this interval, a cascade of envi-ronmental changes and subsequent feedbacks drove a global rise intemperature (Bailey et al., 2003; Jenkyns, 2003, 2010; Kemp et al.,2005; McArthur et al., 2000; McElwain et al., 2005; Rosales et al.,2004a; Svensen et al., 2007), an enhanced hydrological cycle and sub-sequent increase in continental weathering (Cohen et al., 2004), oce-anic anoxia and euxinia (Bowden et al., 2006; Farrimond et al., 1989,1994; Pancost et al., 2004; Raiswell and Berner, 1985; Raiswell et al.,1993; Schouten et al., 2000; van Breugel et al., 2006) and ocean acid-ification (Hermoso et al., 2009a, 2009b; Hesselbo et al., 2000; Suan

netary Sciences, Harvard Uni-

l rights reserved.

et al., 2008). Furthermore, the carbon isotope record associated withthe T-OAE shows large positive and negative excursions, suggestingperturbations to the carbon cycle caused by globally increased ratesof organic matter deposition as well as introduction of isotopicallylight carbon into the ocean–atmosphere system (Hermoso et al.,2009a, 2009b; Hesselbo et al., 2000, 2007; Jenkyns and Clayton,1986, 1997; Jenkyns et al., 2001, 2002; Kemp et al., 2005; Küspert,1982; Rosales et al., 2001; Sabatino et al., 2009; Sælen et al., 2000).

However, interpretations of the T-OAE record have been conten-tious. In particular, the global extent of the event has been questioned(McArthur et al., 2008; van de Schootbrugge et al., 2005; Wignallet al., 2005). Much of this debate derives from the fact that most ofthe studied geochemical records of the T-OAE come from formerepeiric or epicontinental seaways that represent a relatively restrict-ed geographic area, namely, present-day northern Europe. This distri-bution of key outcrops has led many workers to question the globalsignificance of the geochemical signals expressed in the epeiric sedi-mentary successions (McArthur et al., 2008; Newton et al., 2011;van de Schootbrugge et al., 2005).

To date, there have been relatively few systematic investigationsof the sulfur cycle during the T-OAE interval (Brumsack, 1991; Ebliet al., 1998), despite geochemical evidence for widespread euxinia

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TETHYSOCEAN

TETHYS OCEANPANTHALASSIC OCEAN PANGEA

MS

PANGEA

PANGEA

D

Y

Paleo-TethysOcean

Fig. 1. Paleogeographic maps for the Early Jurassic and the North European and North-west Tethyan regions specifically, after Baudin et al. (1990), Scotese (2001) and van deSchootbrugge et al. (2005). Locations of our sample sites are noted as black circles:Yorkshire, England (Y); Dotternhausen, Germany (D) and Monte Sorgenza, Italy (MS).

485B.C. Gill et al. / Earth and Planetary Science Letters 312 (2011) 484–496

and pyritic deposition (Bowden et al., 2006; Farrimond et al., 1989,1994; Pancost et al., 2004; Raiswell and Berner, 1985; Raiswellet al., 1993; Schouten et al., 2000; van Breugel et al., 2006). However,the recent data of Newton et al. (2011) show a positive shift inthe sulfate-sulfur isotope record across the T-OAE interval. Significantdifferences between their two study locations led these authorsto conclude that sulfate-S isotope trends in epeiric settings were sub-stantially modified from the open-ocean sulfate pool and that EarlyJurassic marine sulfate concentrations were much lower than previ-ously considered with estimates as low as 1 mM. Here, we presentadditional sulfate-S isotope data from across the Toarcian OAE, fromNorth European epeiric and marginal Tethyan carbonate-platformsettings, to explore further the dynamics of the sulfur cycle duringthis interval and its implications for conditions in the global ocean.

2. Background

2.1. The sulfur biogeochemical cycle

The sulfur isotope composition of seawater sulfate reflects thebalance of fluxes of sulfur into and out of the ocean and their isotopiccompositions. The major inputs to the ocean are from the weatheringof sulfur minerals on continents and volcanic and hydrothermal emis-sions (as reviewed by Bottrell and Newton, 2006). Both these fluxessupply sulfur to the ocean with similar isotopic compositions (δ34S)in the range of 0 to +8‰ (Holser, 1988). The two major fluxes ofsulfur leaving the ocean are the burial of gypsum and pyrite in sedi-ments. Gypsum precipitation carries only a small negative isotope ef-fect of 1–2‰, and thus its burial does not greatly impact the isotopiccomposition of seawater (Raab and Spiro, 1991), although it canaffect the concentration of marine sulfate (e.g., Wortmann andChernyavsky, 2007).

Pyrite burial—the end-product of microbial sulfur metabolismsthat favor the lighter sulfur isotope, 32S and iron sulfide precipita-tion—by contrast imparts an appreciable fractionation on the marinereservoir of sulfate. The primary metabolism tied to pyrite formationis microbial sulfate reduction, which occurs when this ion is con-verted to sulfide by prokaryotic organisms in anoxic sediments andeuxinic water columns. The product sulfide then reacts with reducediron (Fe2+) to form pyrite (Berner, 1970, 1984). The net isotope frac-tionations achieved by microbial sulfate reduction alone were initiallythought to be as much as ~47‰ (Canfield, 2001; Canfield andThamdrup, 1994; Chambers and Trudinger, 1979; Detmers et al.,2001; Habicht and Canfield, 1997; Harrison and Thode, 1958; Kaplanand Rittenberg, 1964; Kemp and Thode, 1968), with larger fraction-ations only possible with additional oxidative microbial cycling(Canfield and Thamdrup, 1994; Habicht and Canfield, 1996, 1997,2001; Habicht et al., 1998). However, a recent incubation studyshowed that microbial sulfate reduction alone may produce largerfractionations, up to 70‰ (Canfield et al., 2010), which is consis-tent with environmental observations and recent models of themetabolism (Brunner and Bernasconi, 2005; Goldhaber and Kaplan,1980; Wortmann et al., 2001).

The overall mass of the marine sulfate reservoir dictates its sen-sitivity to isotopic change. The modern marine sulfate reservoiris uniform in concentration (28 mM) and isotopic composition(21‰), which reflects the long residence time (τ) of sulfate in themodern ocean of ~13–20 Ma (Berner and Berner, 1996 and refer-ences therein). However, in the geological past, the marine reser-voir of sulfate likely varied considerably (Adams et al., 2010; Gillet al., 2007, 2011; Hurtgen et al., 2009; Kah et al., 2004). Estimatesfor marine sulfate concentrations in the Jurassic vary: with datafrom fluid inclusions in Jurassic halite suggesting a range of 5 to19 mM (Horita et al., 2002), whereas estimates calculated fromrate of change in the Jurassic sulfate sulfur isotope record suggestlower concentrations, b5 mM (Newton et al., 2011).

All the above-mentioned processes control the secular variabilityin the sulfur isotope composition of marine sulfate, and thus recon-struction of the δ34S record of sulfate can be used to track changesin the input and removal pathways of sulfur to and from the ocean.For example, more positive marine sulfate δ34S values could reflectenhanced pyrite burial, while δ34S values closer to 0‰ indicate agreater relative importance of the input fluxes.

3. Materials and methods

3.1. Samples

The secular isotope record of marine sulfate-S can be trackedthrough analysis of sulfur isotope proxies shown to capture and pre-serve the isotopic composition of seawater sulfate. Carbonate-associated sulfate (CAS)—sulfate substituted into the crystal latticesof carbonate minerals—has emerged as a popular seawater proxy(Burdett et al., 1989; Kampschulte and Strauss, 2004; Kampschulteet al., 2001). Here we utilize the CAS from calcitic belemnite guardsand whole-rock carbonate samples to reconstruct the marine sulfatecurve across the Toarcian OAE.

Samples of belemnite calcite were collected from well-studiedLower Jurassic stratigraphic successions on the Yorkshire coast, north-east England (Cleveland Basin), and a quarry located in Dotternhausen,Germany (Southern German Basin) (Fig. 1A and B). The Yorkshirebelemnite samples were collected from several shorter stratigraphicsections exposed on either side of the Peak Fault, a syn-sedimentaryfeature that controlled sediment deposition and facies in the ClevelandBasin (Hesselbo and Jenkyns, 1995). Additionally, Jenkyns et al.(2002) measured some of the Yorkshire belemnites presented herefor their carbon and oxygen isotope ratios. Both the Yorkshire andDotternhausen sections were sites of organic-rich deposition during

486 B.C. Gill et al. / Earth and Planetary Science Letters 312 (2011) 484–496

the T-OAE andwere located within an extensivemarine area, the NorthEuropean Epeiric Seaway, positioned north of the Tethyan regionduring the Early Jurassic (Fig. 1A and B). Calcite from belemnite guards,due to the stability of their low-magnesium calcite mineralogy, isthought to preserve ancient marine chemistry reliably and has beenused extensively in paleoceanography. Specifically, past studies haveemphasized their carbon, oxygen, strontium and sulfur isotope ratios,aswell as their elemental compositions (Jones et al., 1994; Kampschulteand Strauss, 2004; McArthur et al., 2000; Newton et al., 2011; Rosaleset al., 2001, 2004a, 2004b; van de Schootbrugge et al., 2005).

Whole-rock carbonate samples were collected from an outcroplocated at Monte Sorgenza in southern Italy. The shallow-watercarbonates from this stratigraphic section span the Pliensbachianthrough Bajocian stages of the Jurassic and represent sedimentdeposited on the Campania–Lucania carbonate platform located onthe continental margin of the Tethys Ocean during the Early Jurassic(Fig. 1). The section contains a variety of clean, shallow-water carbon-ate lithologies, including skeletal packstones, oncolitic packstonesand ooid grainstones, as described by Woodfine et al. (2008) andreferences therein. Woodfine et al. (2008) also investigated this sec-tion for its carbon isotope stratigraphy, and our material comesfrom their sample set. Although whole-rock carbonate samples arethought to be less ideal for geochemical analysis, they can often becollected at higher stratigraphic resolution than can fossil material.Bulk rock carbonates may be more susceptible to diagenesis, butalteration potentially can be identified, as for belemnite calcite, bypetrographic and geochemical means (δ18O and Mn, Sr and Feconcentrations).

3.2. Methods

Samples of belemnite calcite were initially prepared by removingthe outermost portion of the belemnite guard and any rock matrix at-tached to the outside of the fossils. This preparation was particularlyimportant for the extraction of CAS because the enclosing rock andouter portions of these fossils commonly contain pyrite, which canpotentially be oxidized during the CAS procedure, leading to contam-ination of the extracted sulfate (Marenco et al., 2008; Mazumdaret al., 2008). The whole-rock carbonate samples were prepared byremoving weathered surfaces with a water-cooled saw and thencrushed in a ball mill to fine powders. Individual belemnites providedapproximately 2 to 10 g of the powder; our whole rock samples con-sisted of 10 to 30 g.

3.2.1. Carbonate-associated sulfate (δ34SCAS)The sample powders were immersed in a 10% NaCl solution and

agitated periodically over 24 h to remove gypsum and/or anhydritethat might have been present in the sample. The purpose of thisleach was specifically to target sulfate minerals derived from theoxidation of metal sulfides, principally pyrite; the inclusion of NaClincreases their solubility. The resulting solution was carefully dec-anted after this treatment.

The samples were then rinsed in deionized water to ensure the re-moval of sulfate liberated by NaCl solution. This step was repeatedand, after each rinse, the overlying solution was carefully decantedand discarded. The samples were then immersed in a 4% hypochloritesolution and agitated periodically over 48 h to remove any organicallybound sulfur. Two more deionized water rinses and subsequentdecants followed this treatment. The sample was then dissolved in4 N HCl, and the resulting solutions and residues were centrifugedand vacuum-filtered (45 μm) to isolate the soluble portions of eachsample.

An aliquot was taken from the supernatant liquid of the digestedsamples for elemental analysis; this procedure is described below.Saturated BaCl2 solution (1.2 M) was added to the remaining solutionto precipitate sulfate as BaSO4. The samples were left at room

temperature for at least three days to ensure complete precipitation.The BaSO4 was separated from the remaining solution via filtration(45 μm) and allowed to dry. The individual BaSO4 precipitates werethen homogenized with an agate mortar and pestle and loaded intotin capsules with excess V2O5 and analyzed for their 34S/32S ratiosat either the University of Indiana–Bloomington, using a FinniganMAT 252 gas-source isotope-ratio mass spectrometer (IRMS), or atthe University of California–Riverside, using a Thermo Delta V gas-source IRMS, both fitted with an elemental analyzer for on-line sam-ple combustion and analysis. All sulfur isotope compositions arereported in standard delta notation as per mil (‰) deviations fromVienna Canyon Diablo Troilite (V-CDT):

δ34Ssample ¼

"34=32Ssample�34=32Sstandard

− 1

#ð1Þ

Replicate analyses of samples and international standards (IAEASO-5, IAEA SO-6, and NBS 127) were equal to or better than 0.2‰.

3.2.2. Carbonate carbon and oxygen isotopesPowders of our belemnite samples were reacted with orthopho-

sphoric acid at 90 °C, and the resulting CO2 was analyzed on-linewith a VG Isogas Series II Prism IRMS at Oxford University. Carbonand oxygen compositions are reported in standard delta notation asper mil (‰) deviations from Pee Dee Belemnite (VPDB). Reproduc-ibility for the in-house standard (Carrara Marble) was better than0.1‰ for δ13Ccarb and 0.15‰ for δ18Ocarb.

3.2.3. Elemental compositionsAliquots of each sample digest were analyzed for their Ca, Mg, Fe,

Mn, Sr and S contents using an Agilent 7400 Quadrapole ICP-MS inthe Biogeochemistry Laboratory of the Department of Earth Sciences,University of California–Riverside. This ICP-MS contains an octapolecollision cell, which diminishes mass interferences for the elementsof interest. The collision cell gases used were helium for calcium anal-ysis, hydrogen for iron, and xenon for SO4 (measured as total S). Rep-licate analyses of samples were within 5% for all the elements, andmeasurements of in-house carbonate standards were also within 5%or better of their known values.

4. Results

4.1. Preservation of geochemical signals

A key consideration in any study using carbonates as proxiesfor ancient seawater is whether the geochemical data generated arerepresentative of ancient ocean chemistry. In order to assess whetherour δ34SCAS (and δ13Ccarb) data preserve primary marine signals, wecompared these isotopic ratios to additional geochemical parametersto screen for possible diagenetic alteration (δ18Ocarb, [Mn], [Fe], [Sr],[Mn]/[Sr], etc.), an approach that has been applied extensively inpast studies of Jurassic belemnite calcite (Jones et al., 1994; McArthuret al., 2000; Rosales et al., 2001; Sælen et al., 1996).

Cross-plots of seawater proxy data versus indicators of diagenesisare often used to determine whether carbonate rocks have experi-enced post-depositional resetting (Jacobsen and Kaufman, 1999;Lohmann, 1988). Linear or asymptotic trends between parameters,such as δ18Ocarb and δ13Ccarb, can indicate mixing between primarymarine and diagenetic end-members and denote partial resetting ofthe system (Banner and Hanson, 1990; Derry, 2010; Jacobsen andKaufman, 1999; Lohmann, 1988). Additionally, samples with valuesthat lie outside the main population of data can potentially indicatediagenetic effects. This type of alteration may be more relevant toour belemnite data because the fossils were hosted in fine-grained,

hughjenkyns

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organic-rich, siliciclastic sediment and would operate as individualentities during diagenesis.

Cross-plots of the geochemical data from our three sections areshown in Supplementary Figures S1 and S2. Belemnite and whole-rock samples are plotted on separate plots; Figure S1 displays the bel-emnite data from Yorkshire and Dotternhausen, whereas Figure S2shows the whole-rock data from Monte Sorgenza. Potential finger-prints of alteration include low [CAS], in concert with high [Mn],[Fe], [Mn]/[Sr] and/or δ34S, which might be expected for diagenesisunder reducing conditions. Overall, none of our datasets shows signif-icant correlation (linear or asymptotic), suggesting that the bulk ofthe data preserve primary marine δ34SCAS values. Several individualbelemnite data points do plot outside the main data clusters, withlow δ18Ocarb or high [Fe], [Mn] and [Mn]/[Sr] values that suggest

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Fig. 2. Chemostratigraphic plots of δ13C and δ34SCAS of belemnite calcite from the stratigraph(1998). Carbon isotope data, illustrating a broad positive excursion interrupted by a negat(dark gray circles), Jenkyns et al. (2002) (black circles) and this study (white circles). Ammet al. (2011): white and gray squares respectively. Samples that may be diagenetically alteretion of Figure S2 for explanation).

alteration. However, many of these potentially suspect samples stillhave δ34S values similar to stratigraphically adjacent, apparentlyunaltered belemnite calcite, suggesting diagenetic resetting ofδ18Ocarb, Fe, Mn and/or Sr systems but preservation of the sulfur iso-tope signal (Gill et al., 2008). Consequently, we have taken a reason-ably conservative approach, and any potentially altered samples havebeen indicated but not omitted from the illustrated stratigraphicplots.

Another concern with the use of CAS is contamination from sulfatederived from the oxidation of pyrite and other metal sulfides duringdissolution of the carbonate by hydrochloric acid (Marenco et al.,2008;Mazumdar et al., 2008). Althoughwe removed the outer portionof our belemnites with the intent of eliminating pyrite, it was impos-sible to ensure complete removal. Fortunately, the solutions produced

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ic succession from the Yorkshire coast. Stratigraphic section from Hesselbo and Jenkynsive excursion, are from Sælen et al. (1996) (light gray circles), McArthur et al. (2000)onite zones based on Howarth (1980a, b). δ34SCAS data are from this study and Newtond or contain chemical extraction artifacts are ghosted in the plot (see text and the cap-


by the hydrochloric acid digestion had low concentrations of Fe(b25 ppb), as ferric iron is a efficient oxidizer of pyrite—even overmo-lecular oxygen—at low pH values (Mazumdar et al., 2008).

Nonetheless, in order to identify possible contamination of pyritesulfur, we have cross-plotted our [CAS] and δ34S data, as well asboth [CAS] and δ34SCAS versus [Fe]. Correlations among these param-eters could imply a mixing of primary CAS with contaminating pyrite-derived sulfate (Marenco et al., 2008; Mazumdar et al., 2008). Thewhole-rock carbonate samples from Monte Sorgenza had no pyritesulfur detectable via the standard chromium reduction methoddescribed in Canfield et al. (1986) and therefore cross-plots are notshown. The belemnite samples were too small to extract for pyritevia Cr reduction. Fortunately, however, the cross-plots of δ34SCAS,[CAS] and [Fe] lack significant correlation (Figure S1, A, H and I),although some outlying data suggest potential inclusion of sulfatederived from the oxidation of pyrite. These samples are also designat-ed with ghosted points on the stratigraphic plots.

The last line of evidence for the primary nature of our data isthe consistency of observed trends among the chemostratigraphicprofiles. That said, the Yorkshire section does show significant varia-tion between many adjacent sample horizons (see discussion belowfor explanations for this variation). In light of the evaluations de-scribed above, the majority of our sulfur isotope data are consideredrelatively unaffected by diagenetic and preparation artifacts andtherefore track the secular trends of Jurassic seawater.

δ13Ccarb (‰VPDB

falc

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Fig. 3. Chemostratigraphic plots of δ13C and δ34SCAS of belemnite calcite from the stratigrapgrave Yorkshire, United Kingdom. Ammonite zones based on Howarth (1980b); stratigraph

4.2. Chemostratigraphic trends

The sulfate-S isotope profiles from all three sections show a posi-tive excursion parallel to a previously documented positive carbonisotope excursion (Figs. 2, 3, 4 and 5). Additionally, our δ34SCAS profilefor Yorkshire agrees well with the CAS data recently reported fromthis succession by Newton et al. (2011). The composite Yorkshire sec-tion is shown in Fig. 2, and the δ13Ccarb and δ34SCAS data from ourstudy are plotted alongside other belemnite data published fromthis succession, together illustrating a subdued negative excursionwithin a much broader positive excursion (Jenkyns et al., 2002;McArthur et al., 2000; Newton et al., 2011; Sælen et al., 1996). Inthe lower part of the succession, in the Pliensbachian and earliestToarcian, δ34SCAS values range between +14‰ and +18‰ andshow some variability relative to adjacent sample horizons. In thelowest falciferum Zone of the Toarcian (exaratum Subzone), however,δ34SCAS increases systematically up-section to values of +24‰. Thisrise, while apparent in the composite section, can also be seen in itsentirety within single stratigraphic sections spanning the appropriateinterval; an example from the section at Port Mulgrave (Hesselbo andJenkyns, 1995) is shown in Fig. 3. The positive shift also occurs con-currently with a +4‰ rise in δ13Ccarb from +2‰ to +6‰. Althoughδ13Ccarb falls back to pre-excursion values of around +2‰ in theToarcian bifrons ammonite Zone, δ34SCAS remains at more positivevalues—between +19‰ and +23‰—for the remainder of the

δ34SCAS (‰VCDT))

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hic sections located at Hawsers Bottoms (the lowest two sample levels) and Port Mul-ic section after Hesselbo and Jenkyns (1995).

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Fig. 4. Chemostratigraphic plots of δ13C and δ34SCAS of belemnite calcite from Dotternhausen, Germany. Carbon isotope data are from van de Schootbrugge et al. (2005) (black cir-cles) and our data (gray circles). Ammonite zones based on Riegraf et al. (1984). Ghosted data points are samples that may contain some diagenetic alteration or chemical extrac-tion artifacts (see text and the caption of Figure S2 for explanation).


composite section, which extends into the lowermost Aalenian Stage(Fig. 2). As in the lower portion of the succession, δ34SCAS in the uppersection shows significant variability—up to 3‰—between adjacentstratigraphic horizons.

The Dotternhausen section is shown in Fig. 4 and, like the Yorkshiresection, our δ34SCAS and δ13Ccarb results from this study are plotted withadditional belemnite δ13Ccarb data from the literature (van de Schoot-brugge et al., 2005). δ34SCAS in the lower Toarcian tenuicostatum ammo-nite Zone ranges between +15‰ and +17‰. Carbonate-carbonisotope data from this portion of the section vary considerably, risingfrom +1‰ to +4‰ in the tenuicostatum zone. δ13Ccarb values then fallto ~+2.5‰ at the falciferum–tenuicostatum zonal boundary. A significantgap exists in our sample set in the lower falciferum ammonite Zone (ele-gantulum, exaratum and lower elegans Subzones of German biostratigra-phy) due to the scarcity of belemnites in this stratigraphic interval. Thelimited δ13Ccarb data from this interval show a rise from +2‰ to +5‰,with values reaching an acme in the upper elegans Subzone. Above thebelemnite gap, δ34SCAS shows more positive values ranging between+19‰ and+21‰. δ34SCAS stays at these elevated values for the remain-der of the section, which covers the higher portion of the Toarcian falci-ferum and part of the bifrons ammonite zones.

Bulk-rock δ34SCAS data from the Tethyan section exposed at MonteSorgenza are shown in Fig. 5 and are plotted alongside the δ13Ccarbdata of Woodfine et al. (2008). δ34SCAS values start between +16.8‰and +17.6‰ in the Pliensbachian (the lowermost 150 m of the sec-tion), then gradually rise to around +19.2‰ and fall back to around+17.2‰ in the earliest Toarcian. δ13Ccarb data vary between +1‰and +3‰ for most of the Pliensbachian and show a positive 2‰ ex-cursion across the Pliensbachian–Toarcian boundary. After this posi-tive excursion, δ13Ccarb decreases to define a negative excursion tovalues below 0‰ followed up section by another positive shift withvalues reaching +4.5‰. δ34SCAS values increase systematically from+17.2‰ to +23‰ at the stratigraphic interval of the second positiveδ13Ccarb excursion. After the positive excursion, the δ13Ccarb values

decrease up section to values around +2‰ but later show a 2‰ neg-ative shift around 275 m, which is inferred to be the Aalenian–Bajo-cian boundary based on carbon and strontium isotope stratigraphy(Woodfine et al., 2008). δ34SCAS remains at more positive values forthe rest of the section but gradually declines from +23‰ to +21‰.

5. Discussion

5.1. The Toarcian positive sulfur isotope excursion

The sulfur isotope records from the sampled stratigraphic sectionsshow similar trends: all three record a positive isotope shift of 5 to7‰ in the Early Toarcian. The shift can be further constrained fromthe results at Yorkshire and Dotternhausen to initiate in the earlyfalciferum ammonite Zone (exaratum Subzone in the Yorkshire sec-tion) and peak in the same ammonite zone. The similarities betweenthe isotope trends is encouraging because our sampled sections spanan area extending from the middle of the North European EpeiricSeaway to the continental margin of the Tethys Ocean. This observa-tion supports the suggestion that we are observing a global oceano-graphic phenomenon.

The parallel excursions in sulfate-S and carbonate-C isotopes seenin our sections are compatible with the hypothesis that coupledmechanism(s) drove these geochemical changes. The positive carbonisotope excursion has been attributed to a transient increase inthe burial rate of organic carbon during the OAE, as evidenced bythe global occurrence of organic-rich sediments during this interval(Al-Suwaidi et al., 2010; Caruthers et al., 2011; Jenkyns, 1988;Jenkyns et al., 2002). This burial has been tied to increases in primaryproduction and/or organic matter preservation due to increasedanoxia/euxinia (Jenkyns, 2010). Because organic carbon is enrichedin the lighter isotope, 12C, due to fractionation accompanying photo-synthesis, significantly increased burial of such material causes a

Toar

cian

-Aal

enia

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50

0

Plie

nsba

chia

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14 18 20 22 24 16

δ13Ccarb (‰VPDB)

Calcaria

Lithiotis

Calcari Oolitici

Calcari Maculati

Met

ers

20-2 4

?

UnitInte

rnat

iona

l Sta

ge

-4

Fig. 5. Chemostratigraphic plots of δ13C and δ34SCAS for whole-rock carbonate fromMonte Sorgenza, Italy. Carbon isotope data, illustrating a broad positive interrupted by a negativeexcursion, and stage placements are from Woodfine et al. (2008).


transient positive shift in the isotopic composition of inorganiccarbon in the ocean–atmosphere system (Scholle and Arthur, 1980).

An increase in the burial of pyrite sulfur would also be expectedduring the OAE, caused by stimulation of microbial sulfate reduction.Additionally, in locations where euxinic water columns formed, addi-tional pyrite burial would have been favored by syngenetic pyriteformed in the water column or at the sediment—water interface asmicron-scale framboids (Wilkin et al., 1996). The overall global in-crease in pyrite burial would have resulted in a positive shift in theisotopic composition of the residual seawater sulfate reservoir dueto isotope fractionations associated with microbial sulfate reduction.

It seems reasonable to conclude that parallel positive carbon andsulfur isotope excursions were driven by increases in the burialrates of organic carbon and pyrite sulfur under widespread euxinicconditions since organic-rich strata associated with the T-OAE arealso highly pyritic (Raiswell and Berner, 1985; Raiswell et al., 1988,1993; Röhl et al., 2001). Furthermore, iron speciation and/or organicbiomarker studies show that most of these sediments were depositedunder euxinic conditions (Bowden et al., 2006; Farrimond et al., 1989,1994; Pancost et al., 2004; Raiswell and Berner, 1985; Raiswell et al.,1993; Schouten et al., 2000; van Breugel et al., 2006). The samemech-anism has been invoked for similar patterns observed in the carbonand sulfur isotope records of the Cretaceous Cenomanian–TuronianOceanic Anoxic Event (OAE II) and the Cambrian Steptoean PositiveCarbon Isotope Excursion (SPICE), where positive sulfate-S isotopeexcursions were found to be coincident with well-documented posi-tive carbon isotope excursions (Adams et al., 2010; Gill et al., 2007,2011; Ohkouchi et al., 1999).

Although our data show an abrupt rise in δ34Ssulfate, none of oursections records a return to pre-excursion baseline. Based on a

compilation of published sulfate-S isotope data (Fig. 6), δ34Ssulfatedoes eventually return to values similar to those before the excursionlater in the Jurassic. However, the precise timing for the relaxation ofδ34Ssulfate is not well defined from the compiled data. In our most stra-tigraphically persistent post-OAE dataset from Monte Sorgenza, themore positive, post-OAE δ34Ssulfate values extend to stratigraphiclevels attributed to the Bajocian Stage based on carbon and strontiumisotope stratigraphy (Woodfine et al., 2008). Such a stratigraphicplacement is at odds with the majority of the published δ34Ssulfatedata (Kampschulte et al. 2004), which show lower values in the Aale-nian but with a higher value in the later Bajocian, similar to our po-tential Bajocian values (Fig. 6).

One explanation for this apparent discrepancy may be the assign-ment of the upper part of the Monte Sorgenza section to the baseof the Bajocian (Woodfine et al., 2008). If this horizon is older thansuggested, our dataset may not extend as far into the Middle Jurassic(see Fig. 6 for two alternative temporal placements of the datafrom Monte Sorgenza). However, our δ34SCAS profile from Yorkshire,which has strong biostratigraphic constraints, provides some limitson the timing of the relaxation: δ34Ssulfate values are still approxi-mately +21‰ at the base of the Aalenian. Due to the paucity ofdata from this interval, further speculation is not warranted anddoes not significantly bear on our interpretations. Further refinementof the Jurassic sulfate-S isotope record during this interval will hope-fully resolve the precise duration of the isotope excursion.

5.2. Modeling of the Toarcian Sulfur Cycle

To explore the dynamics of the Toarcian sulfur isotope recordin more detail, we constructed a forward box model of the Jurassic

Hett. Sinemurian Pliensbach. Toarcian Bajoc. Bath. Call. Oxfordian Kimm.Aalen. Tithonian

Lower Middle Upper

Jurassic

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199.6196.5 189.6 183 175.6 171.6 167.7 164.7 161.2 155.7 150.8 145.5

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Evaporite Data

CAS Data

Kampschulte and Strauss (2004)

Monte Sorgenza (This study)

Yorkshire Coast (This study)

Yorkshire Coast (Newton et al. 2011)

Lower Lias Salt, Spain (2)

Argos Salt, Canada (5)

Tabanos Formation, Argentina (3)

Carmel Formation, USA (8)

Tabilito Formation, USA (1)

Haynesville Formation, USA (6)

Auquilo Formation, Argentina (7)

Hith Anhydrite, Saudi Arabia (3)

Munder Formation, Germany (20)

Gotnia Formation, Iraq (2)

Adaiyah Formation, Iraq (1)

Butmah Formation, Iraq (1)

Alan Formation, Iraq (1)

Gypsum Spring Formation, USA (14)

δ34

Ssu

lfate

Fig. 6. Summary of the δ34Ssulfate record of the Jurassic Period. Evaporite data are shown as formation averages; the number of data points included in the average is noted inparentheses next to the formation name in the key. Ages of the stage boundaries are from Ogg et al. (2008). CAS data are from Kampschulte and Strauss (2004), Newton et al.(2011) and this study. Due to the possible uncertainty of their stratigraphic placement, CAS data from the upper portion of the Monte Sorgenza data are shown as light graydots connected with grey lines. Evaporite data from Claypool et al. (1980), Holser et al. (1988), Lo Forte et al. (2005), Müller et al. (1966), Thode and Monster (1970), Urtillaet al. (1992) and Valentine (1997). Bars on evaporite data represent estimates on the age of the deposit and absolute range of values reported from each unit.

Table 1Values of model parameters explored in the Jurassic sulfur cycle model. Values takenfrom Kurtz et al. (2003) and Horita et al. (2002).

Model parameter Pre- and post- T-OAE steady state OAE state

MOa 5 to 14.4 (3.5 to 10 mM SO4

−2) –

FWb 1.5 1.5δW +8‰ +8‰Fpyb 0.38 0.38 to 3.04ΔS −35‰ −35‰ to −50‰Fgypb,c 1.12 1.12

a Reservoir units are 1018mol.b Flux units are 1018mol per million years.c Note the Fgyp does not directly appear in Eq. (1), but, however, does affect MO at

each time step of the model: the change in MO is equal to FW−Fpy−Fgyp.


sulfur cycle. Rather than using the isotope data to drive the model,boundary conditions for the sulfur cycle were initially set and laterperturbed during the model runs in order to recreate the observedisotopic profile.

The following expression for the change in the isotopic composi-tion of the ocean over a given time interval was used in the model(see Gill et al. (2011) and Kurtz et al. (2003) for additional details)

∂δO∂t ¼ FW δW−δOð Þ−FpyΔS

MOð2Þ

where M0 and δ0 are the amount of sulfate-S in the ocean reservoirand its isotopic composition, respectively. The input to the ocean,FW, represents the combined fluxes of sulfur delivered to the oceanvia weathering of the continents and magmatic processes. Thesefluxes were combined together in FW because their isotopic composi-tions—defined as δW in the model—are similar. The output fromthe ocean that fractionates sulfur isotopically is encompassed inthe term Fpy, representing the burial pyrite, and ΔS, the isotopic frac-tionation. Values explored for various parameters in the model areshown in Table 1.

Our model shows that the sulfate-S isotope excursion can bereplicated by transiently increasing the burial rate of pyrite sulfurby factors ranging from 2 to 8.5—depending on the model run condi-tions—for durations thought reasonable for the OAE (0.2 to 1.5 Ma;Kemp et al., 2005; McArthur et al., 2000; Sabatino et al., 2009; Suanet al., 2008). Fig. 7A shows the model's sensitivity to the transientchanges in pyrite burial of various magnitudes.

An additional factor that may have influenced the sulfur cycleduring the T-OAE was expansion of euxinic water columns. An in-crease in euxinic (syngenetic) pyrite formation would have changed

the global average value of ΔS. Syngenetic pyrite typically has amore negative δ34S value, and thus has a larger offset from the δ34Sof seawater sulfate, as compared to pyrite formed diageneticallywithin sediment, because microbial sulfate reduction in the euxinicenvironment has more open access to the marine sulfate pool(Gautier, 1986; Lyons, 1997; Lyons et al., 2003; Passier et al., 1999;Sageman and Lyons, 2004). Thus, under paleoceanographic condi-tions characteristic of OAEs, when the regional extent of euxinicconditions increased, the global average ΔS during pyrite burial likelyalso increased.

A change in ΔS during the T-OAE is supported by the δ34S of pyrite(−22.9±4.8‰ and −24±2.2‰) from two euxinic black shale unitsassociated with the T-OAE (Raiswell et al., 1993). Depending on thecorrelation to our sulfate-S isotope record, these values yield largeestimates for ΔS of 40 to 50‰. If the majority of the increase in pyriteburial during the T-OAE was syngenetically formed pyrite (i.e., pyrite

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Fig. 7. Representative sensitivity tests for the sulfur cycle model. Note that these sim-ulations are intended to demonstrate the effect of altering some of the parameterswithin the model and not to display all possible or even acceptable solutions. Inthese simulations, pyrite burial (Fpy) was transiently increased for a half millionyears at the 2 million year time point in each model run to simulate the effect of theT-OAE. A) Simulations showing the effect of varying the magnitude of the transient in-crease in pyrite burial (Fpy) during the T-OAE. Labels denote the factor of increase of Fpyfrom the pre-OAE steady-state rates. In these simulations, the initial marine sulfateconcentration was 7 mM. B) Simulations showing the effect of varying the ΔS of the in-crease in Fpy during the T-OAE. Both Fpy and ΔS were transiently increased 2 Ma intothe simulation; however, the increase in Fpy was kept constant at 8 times the pre-excursion rate. Labels denote the transient OAE value of ΔS. In these simulations, theinitial, steady-state marine concentration of sulfate was 7 mM. C) Simulations showingthe effect of varying the initial mass of the marine sulfate reservoir (M0) on δ34Ssulfate.Labels denote the initial concentrations of marine sulfate. Fpy was increased transientlyduring the OAE interval, but to the same magnitude (7 times the pre-excursion rate) ineach model run.

Table 2Estimates for pyrite sulfur burial within the North European Epeiric Seaway during theOAE.

Basin Estimatedbasin area(km2) a

Pyrite sulfurpyrite-S content b

(wt.%)

Average thicknessof OAE interval c

(m)

Pyrite sulfurburied d

(1015 mol)

Cleveland 68,750 3.63 (87) 30.21 6.23North German 110,625 3.49 (32) 8.31 2.65South German 66,875 2.85 (84) 5.5 0.87Paris 72,500 3.65 (6) 3.5 0.77Seaway total 10.5

a Estimated from the maps shown in Röhl and Schmid-Röhl (2005).b Pyrite sulfur data are from Brumsack (1991), Littke et al. (1991), Raiswell et al.

(1993), Frimmel et al. (2004), MacKenzie et al. (1980), McArthur et al. (2008). Aver-ages are weighted based on the stratigraphic interval they sample except those fromthe Paris Basin, which is the sparsest dataset. Values in parentheses are the numberof samples included in the average.

c Defined as the average thickness of the falciferum ammonite zone in each basin,which contains the rise in sulfate-S isotope values.

d Calculated by using the mass of sediment accumulation (basin area multiplied bythickness of the ammonite zone and rock density) multiplied by percent pyrite sulfur.


carrying a larger ΔS), a smaller overall increase in pyrite burial isneeded (Fig. 7B). However, increases in ΔS alone cannot generatethe observed excursion in δ34SCAS; increased pyrite burial must alsobe invoked.

Another factor that bears on the magnitude of the increase inpyrite burial needed to generate the isotope excursion is the size ofthe oceanic sulfate reservoir; the larger its size, the larger the increasein pyrite burial needed to shift the marine δ34S value. Fig. 7C depicts a

model sensitivity test wherein the initial mass of the marine sulfatereservoir was varied. As stated above, estimates for the Jurassic ma-rine sulfate concentration range from b2 to 19 mM. Based on ourmodel, we suggest that Early Jurassic sulfate concentrations were onthe lower end of this range: 4 to 8 mM. Otherwise, unreasonable in-creases in pyrite burial—greater than 8 times the pre-excursion rate—are needed to produce the observed positive excursion; burialrates higher than this would greatly exceed estimates for the deliveryof iron to the ocean available to form pyrite (Poulton and Raiswell,2002). Conversely, low concentrations of sulfate (b2 mM) yield aδ34S excursion (b8 Ma) that is unreasonably short based on the avail-able biostratigraphic constraints.

Our modeling exercise can shed light on the global-versus-localnature of the Toarcian OAE. The model provides estimates ofthe amount of pyrite-S burial needed to generate the δ34SCAS excur-sion. Among the various simulations, approximately 5×1017mol ofpyrite-S or greater are need to generate the sulfate-S isotope excur-sion. These estimates can be compared to calculations for the amountof pyrite-S buried within the North European Epeiric Seaway duringthe OAE. Table 2 shows these estimates for each of the various basinsin the epeiric seaway that were loci of organic carbon and pyrite-richdeposition. Based on our calculations, the amount of pyrite buriedover this interval in the seaway accounts for, at most, 2 to 4% of thepyrite burial need to drive the sulfate-S isotope excursion. Therefore,major additional sites of pyrite burial outside the seaway are neededto explain the observed sulfate-S isotope data. This result indicatesthat enhanced pyritic deposition associated with the T-OAE musthave been a geographically widespread phenomenon.

5.3. Epeiric sea modification of δ34Ssulfate

The overall similarity between the δ34SCAS stratigraphy of oursections is in contrast to the findings of Newton et al. (2011), who ob-served significant differences between the δ34SCAS epeiric record fromYorkshire and their Tethyan results from Tibet. These differences arereflected in the absolute δ34SCAS values between supposedly correla-tive stratigraphic levels and the magnitude of the observed δ34SCASexcursion; the excursion in Yorkshire is +5 to +7‰ compared to+16‰ in the Tibetan section (Fig. 6).

Newton et al. (2011) attribute the differences between their epei-ric and Tethyan δ34SCAS records to hydrographic isolation of the NorthEuropean Seaway from the open ocean. In particular, the lower valuesof δ34SCAS and dampened excursion seen in the epeiric record weresuggested to be products of enhanced riverine sulfate input into theNorth European Seaway.


To explore the limits of possible modification of an epeiric sulfatereservoir due to variable riverine input, we have created the two-component mixing models shown in Fig. 8. Both models show theeffect of mixing marine and riverine components with given concen-trations and sulfur isotope compositions. In both, the riverine end-member is defined as having 100 μM sulfate with a δ34S of +8‰,values that are typical of modern riverine sulfate (Berner and Berner1996). Differing the concentration of the sulfate in the marine end-member creates the displayed arrays; this concentration is reportedon each line of the array.

In the model depicted in Fig. 8A, the marine component has anisotopic composition of +17‰, which is the average δ34S observedbefore the excursion in our Tethyan section. The gray region showsthe field of pre-excursion δ34SCAS values observed within the NorthEuropean Epeiric Seaway from both Newton et al. (2011) and ourdataset. This array reveals that the isotopic composition of the epeiricsea can be modified significantly (>4‰) at very high proportions ofriverine water over seawater (>70%) and with low concentrationsof marine sulfate (b1 mM) in the marine component. We suggestthat mixing of marine and riverine sulfate and locally high rates ofpyrite burial could explain the small high-order, bed-to-bed 'scatter'

A

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125 μM250 μM

1 mM2.5 mM

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Pre-Excursion EpicontinentalValues

125 μM250 μM

500 μM 1 mM2.5 mM

5 mM

Fig. 8. Two-component mixing models to demonstrate the effect of varying the per-centage of riverine and marine water within the North European Epeiric Seaway.A) Mixing of a marine component with an isotopic composition of +17‰ with a river-ine component with an isotopic composition of +8‰ and a concentration of 100 μM.Lines showmixing curves with varying sulfate concentration in the marine component.B) Mixing of a marine component with an isotopic composition of +38‰ with a river-ine component with an isotopic composition of +8‰ and a concentration of 100 μM.Lines showmixing curves with varying sulfate concentration in the marine component.For reference, the surface water of the modern Black Sea, the largest stratified anoxic/euxinic basin today, has a salinity that is roughly half that of the open ocean (Özsoy andÜnlüata, 1997). The Baltic, an inland sea commonly used as an analog for ancient epei-ric seaways, has a salinity of 20–40% of the open ocean depending on location withinthe sea (Samuelsson, 1996).

observed in the epeiric δ34SCAS records and the differences in absolutevalues seen between our North European and Tethyan records.

Fig. 8B depicts a marine end-member with a sulfate-S isotopevalue of +38‰, the post-excursion value seen in the record fromTibet reported by Newton et al. (2011). The gray band shows thepost-excursion values observed within the North European EpeiricSeaway from both our work and that of Newton et al. (2011). Ourmixing model suggests that, if the Tibetan record represents globalocean δ34Ssulfate and the epeiric record resulted from mixing riverineand marine sulfate, such modification would only occur at extremelyhigh proportions of river water to seawater (>90%) and/or with ex-tremely low concentrations of marine sulfate (b1 mM). Such concen-trations are lower than those suggested by Newton et al. (2011) andcontrast with estimates (5–19 mM) derived from Jurassic halite fluidinclusions (Horita et al., 2002) and our box model (4–8 mM). Thus,modification of the δ34S of marine sulfate by riverine input to thedegree suggested by Newton et al. (2011) seems unlikely.

Given that the differences between the Tibetan and European re-cords were unlikely to have been caused by riverine dilution of themarine signal, we have to look to other explanations. A noteworthypoint is that there are few definitive age constraints for the Tibetansection, making precise correlation to the European Toarcian recorddifficult. The section contains only one biostratigraphic diagnostic ho-rizon that yielded nannoplankton of early mid-Aalenian age (Wignallet al., 2006). However, the lower portion of the succession containslithiotid bivalves and lituloid foraminifera that indicate the Pliensba-chian–Toarcian interval (Fraser et al., 2004; Wignall et al., 2006).These biostratigraphic data indicate that, minimally, the Tibetansection contains portions of the Toarcian and Aalenian. Particularlynoteworthy is the fact that the δ13Corg signatures of the Tibetan sec-tion are nowhere lower than −27‰, whereas marine organic matterdeposited during the T-OAE invariably has values lower than thisat the core of the negative excursion, extending down to ~−33‰(Al-Suwaidi et al., 2010; Caruthers et al., 2011; Hesselbo et al.,2000; Jenkyns et al., 2002; Kafousia et al., 2011; Kemp et al., 2005;Küspert, 1982; Sabatino et al., 2009). Hence, the bio- and chemostra-tigraphic constraints do not allow definitive correlation of the Tibetanδ34SCAS shift to that seen in the Early Toarcian in Europe; the possibil-ity remains that the Tibetan excursion is stratigraphically higher.

Another resolution may lie with the integrity of the Newton et al.(2011) CAS data. The Tibetan δ34SCAS values—especially those thatdefine the excursion—are much higher than those reported fromother locations from the Pliensbachian–Aalenian interval (FigureS3). This opens the possibility that they may represent a modifiedgeochemical signal. Newton et al. (2011) rightfully excluded CASdata that had anomalously low sulfate oxygen values (δ18OCAS),which suggest these sulfates were the products of pyrite oxidationeither during the burial and/or exhumation history of the rock or asa result of the chemical extraction of the CAS. However, it is alsopossible that CAS can be altered via incorporation of sulfate enrichedin 34S- (and 18O-) during the precipitation of carbonate during dia-genesis. This sulfate could have originally been derived from seawateror from a different fluid during burial. In the first case, the local porewater sulfate pool would have evolved isotopically—via microbialcycling of the sulfate—to more positive δ34S and δ18O values duringdiagenesis. In fact, the Tibetan CAS data with the most positive δ34Svalues also carry high δ18O values (up to +17‰ as compared to thedataset mean of +13.4‰), which suggest that alteration may haveoccurred in these samples.

Given the uncertainties regarding the correlation and the integrityof the sulfur isotope signal in the Tibetan succession, the jury is stillout with respect to the relationship between the data of Newton etal. and the other Jurassic records presented here. More data with bet-ter biostratigraphic context from the eastern Tethys and other global-ly distributed locations will ultimately resolve the global versus localnature of the δ34Ssulfate records during this portion of the Jurassic.


6. Conclusions

The ToarcianOceanic Anoxic Eventwas a severe environmental per-turbation that affected the biosphere, including the cycling of severalmajor elements and their isotopes. Our examination of the sulfate-S iso-tope records from T-OAE shows that the sulfur cycle was no exception.Sulfate sulfur isotope shifts of +5‰ to +7‰ co-occur with the overallpositive carbon isotope excursion attributed to this event. In light of theparallel behavior for the carbon and sulfate-S isotope during the excur-sions and our modeling of the Toarcian sulfur cycle, we conclude thatthe sulfur excursion was caused by a transient increase in pyrite burialduring the T-OAE. Spatial differences in local sulfur cycling, changes inisotopic offset between sulfate and pyrite (ΔS), the size of the marinesulfate reservoir and the magnitude of the change in pyrite burial alllikely played a role in the observed sulfur isotope records of the event.Additionally, local modification of the global isotope excursion wasminimal in epeiric-sea settings. Importantly, it appears that pyrite de-position within the North European Epeiric Seaway alone was insuffi-cient to drive the sulfate-S isotope excursion and enhanced burial ofpyrite must have been a more widespread phenomenon during the T-OAE, consistent with ocean-scale anoxia.

An understanding of the dynamics of the sulfur cycle during the T-OAE is key to interpreting the overarching mechanisms that underliethe event because it is fundamentally linked—directly and indirectly—to other important element cycles. A recent model of the sulfur cycleduring OAE-II of the Cretaceous, for example, suggests that marinesulfate levels regulated the recycling of phosphorus during thisevent, through remineralization of organic matter by microbial sul-fate reduction and other sulfur metabolisms (Adams et al., 2010). Ef-ficient phosphorus recycling is thought to be vital to maintaining highlevels of primary productivity and anoxic conditions during OAEs(Ingall and Jahnke, 1994; Mort et al., 2007; Van Cappellen and Ingall,1994). Iodine/calcium ratios also decreased dramatically during theToarcian OAE due to drawdown of the marine iodine inventory andreduction of iodate to iodide accompanying the spread of anoxic con-ditions (Lu et al., 2010). Another example is molybdenum, a redox-sensitive element whose reactivity is dependent on sulfide levels.The spread of euxinic conditions in the ocean can deplete the marineinventory of this biologically important micronutrient: a model thathas been proposed for the Toarcian and other intervals of prolongedoceanic anoxia (Algeo, 2004; Anbar and Knoll 2002; Gill et al., 2011;Hetzel et al., 2009; Pearce et al., 2008; Scott et al., 2008). Throughthese connections, the sulfur cycle is intimately tied to many of thekey mechanisms behind the generation, persistence and terminationof these paleoceanographic phenomena.

Supplementary materials related to this article can be found on-line at doi: 10.1016/j.epsl.2011.10.030.

Acknowledgments

Our thanks go to Steven Studley and Jon Fong (University ofIndiana-Bloomington) and Steve Bates, Bill Gilhooly and JeremyOwens for aid in sulfur isotope analyses, Norman Charnley for helpwith carbon and oxygen isotope analyses and Mike Formolo andManfred Jäger for fieldwork assistance. This work was funded by theNational Science Foundation (EAR-0719911). We also thank JeremyOwens and David Johnston for helpful discussions. Early versionsof this manuscript were improved by the thoughtful comments ofJessica Creveling, Ben Kotrc and Dave Johnston. Finally, we wouldlike to thank editor Gideon Henderson and three anonymous re-viewers whose comments greatly improved the manuscript.

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A global perturbation to the sulfur cycle during the Toarcian Oceanic Anoxic Event

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