Paleoceanographic changes of the Late Pliensbachian^EarlyToarcian interval: a possible link to the genesis of an

Oceanic Anoxic Event

T.R. Bailey a;�, Y. Rosenthal a, J.M. McArthur b, B. van de Schootbrugge a,M.F. Thirlwall c

a Institute of Marine and Coastal Sciences and Department of Geology, Rutgers University, 71 Dudley Road, New Brunswick, NJ, USAb Department of Geological Sciences, University College London, Gower Street, London, UK

c Department of Geology, Royal Holloway University of London, Egham, Surrey, UK

Received 17 August 2002; received in revised form 15 March 2003; accepted 5 May 2003

Abstract

Secular records of the elemental and isotopic composition of belemnite calcite were studied in Pliensbachian andToarcian sections from the Yorkshire coast, UK, and Southern Germany, to investigate oceanographic change duringan interval prior to and including the Toarcian Oceanic Anoxic Event (OAE). Records from Southern Germany arecorrelated to the UK stratigraphy using strontium isotope stratigraphy. The geochemical trends measured frombelemnite calcite are consistent between the two sections, and are interpreted in terms of temperature and salinity ofthe northwest European epi-continental sea. The data suggest that a dramatic environmental change coincided withthe Toarcian OAE. Belemnite Mg/Ca, Sr/Ca, and Na/Ca ratios increase by a factor of between 1.7 and 2 coincidentwith a 3x negative shift in N

18O from the mid-tenuicostatum zone until the lower falciferum zone of the UKammonite biostratigraphy (a period of V0.6^0.7 Myr). Taken at face value, the Mg/Ca and N

18O data argue for anabrupt warming of 6^7‡C and substantial freshening during this interval. Global warming accompanied by anaccelerated hydrological cycle and increased runoff is proposed to explain these changes. Prior to these events, datafrom lower in the Yorkshire section suggest a possible cooling accompanied by a shift to more saline waters duringthe period from the upper Pliensbachian margaritatus zone to the Toarcian lower tenuicostatum zone. This earlierevent may also have been important in causing density stratification in the northwest European epi-continental sea.= 2003 Elsevier Science B.V. All rights reserved.

Keywords: belemnites; Mg/Ca; oxygen isotopes; strontium isotope stratigraphy; paleotemperature; black shale

1. Introduction

Elevated rates of global extinction occurred

over an interval of approximately 4 Myr spanningthe Pliensbachian^Toarcian boundary in theEarly Jurassic [1^3], a period of eustatic sea levelrise [4]. In northwest Europe the latter stages ofthis extinction overlapped with the Early Toar-cian Oceanic Anoxic Event (OAE), de¢ned by aworldwide deposition of black shales representing

0012-821X / 03 / $ ^ see front matter = 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0012-821X(03)00278-4

* Corresponding author.E-mail address: bailey@imcs.rutgers.edu (T.R. Bailey).

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elevated burial rates of organic carbon [5]. Blackshale deposition has been attributed to enhancedproductivity and/or poorly oxygenated bottomwaters of global extent. The cause of the largeperturbation in the global carbon cycle in theEarly Toarcian remains unknown.Previous studies have shown that the isotopic

and trace element chemistry of well preserved be-lemnite calcite o¡ers the best window into thegeochemistry of the Jurassic oceans due to its re-sistance to post-depositional alteration by burial£uids [6^9]. Trace element contents of belemnitesfrom the Pliensbachian^Toarcian boundary sec-tion of the Yorkshire coast, UK, were ¢rst re-ported by McArthur et al. [10]. These authorssuggested that primary trace element signals arepreserved by belemnite calcite in this section, and

demonstrated an inverse correlation between N18O

and trace element data (Mg/Ca, Sr/Ca, and Na/Ca). As previous authors have suggested the useof belemnite Mg contents for Jurassic paleother-mometry [11,12] and it is well known that temper-ature is a primary control on the fractionation ofoxygen isotopes, this correlation led McArthur etal. [10] to postulate a temperature control on thevariation of Mg/Ca, Sr/Ca, and Na/Ca ratiosthrough time in this section.This study aims to build upon the initial work

of McArthur et al. [10]. The Pliensbachian^Toar-cian boundary sections studied here are exposedalong the Yorkshire coast, UK, between RobinHood’s Bay and Hawsker Bottoms, and at quar-ries at Dotternhausen and Asel¢ngen, SW Ger-many. These sections are dominated by mud-

Fig. 1. (A) Global lower Jurassic (Sinemurian to Toarcian) paleogeography. (B) Sinemurian to Toarcian paleogeography of thenorthwest European region, modi¢ed from [50]. Land areas are shown in gray.

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stones that were deposited in the Cleveland basin(Yorkshire) and South German basin within anepi-continental sea at V35‡N (Fig. 1). Thestudied sections have excellent lithostratigraphicand biostratigraphic frameworks [13,14]. We col-lected additional Mg/Ca, Sr/Ca, Na/Ca, and Mn/Ca data from samples used in the McArthur et al.[10] study, added Li/Ca data, and extended theserecords back to the mid-Pliensbachian. We com-pare the records from Yorkshire with data fromthe contemporaneous German section.

1.1. Tectonic, paleogeographic,and climatic setting

The Early Jurassic period witnessed the contin-ued breakup of Pangea that began with the em-placement of the Central Atlantic MagmaticProvince at the Triassic^Jurassic boundary. Theonset of the Karoo^Ferrar £ood basalt eruptions,dated at 183P 2 Ma [3], may be similarly linked tothe breakup of Southern Pangea. The onset ofmid-ocean ridge spreading in the proto-Atlanticseaway is estimated to have occurred between184 and 187 Myr [15]. The northwest Europeanregion lay to the east of the proto-Atlantic seawayand to the northwest of the Tethys ocean, andconsisted of a broad epi-continental shelf-seawith island land masses (Fig. 1). It thus had thepaleogeographic situation of an embayment,being surrounded by Pangea on three sides. Thenarrow northwest European seaway connectedthe northwest European epi-continental sea tothe waters of the high-latitude Paci¢c Ocean.Lower Jurassic climate is thought to have beenglobally 5^10‡C warmer than present, with vastarid areas at low and mid-latitudes in the interiorof Western Pangea, and high-latitude temperatezones. Monsoonal atmospheric circulation likelydominated the mid-latitude coasts of the Tethyanand Paci¢c oceans, causing high rainfall rates thatalso a¡ected the mid-latitude eastern arm of Pan-gea [16].

2. Methods

The excellent biostratigraphic control of the

studied sections enables the precise location ofsamples upon a scale of stratigraphic thickness,or position within an ammonite subzone. Samplelocations are reported as stratigraphic height rel-ative to a datum at the base of the falciferum zonein the UK ammonite biostratigraphic scheme.Sample heights from Germany are correlated totheir equivalent heights in the Yorkshire sectionas discussed in Section 3.Belemnite samples used in this study were ini-

tially collected for the studies of McArthur et al.[10] and Bailey [17]. The outer surfaces, apicalline, apex, and alveolus of belemnite rostra wereremoved using diamond cutting tools. Rostrafrom Yorkshire were fragmented to milligramsize and acid-cleaned. At least 10 fragmentsjudged to be the best preserved by visual assess-ment under the microscope were hand-picked andpowdered in an agate pestle and mortar to reduceintra-sample variation in trace element contentthought to be caused by ontogenetic e¡ects [Bai-ley, unpublished]. This method improved the pre-cision of repeated analyses of a single belemnitefrom V8% rsd (1c) for Mg/Ca and V5% rsd(1c) for Sr/Ca obtained from the analysis of in-dividual calcite fragments, to 6 2% rsd (1c) forMg/Ca and 6 0.5% rsd (1c) for Sr/Ca obtainedfrom the analysis of several individually preparedpowders. Powders were prepared from the Ger-man belemnites using a dental drill to avoid theapical line and edges, and to sample from di¡er-ent areas of the acid-cleaned rostra to homogenizeinternal variability in trace element content. Pow-ders were dissolved in 0.065 N HNO3, and dilutedwith 0.5 N HNO3 to V4 mM Ca concentrationfor analysis. Acids were prepared using ultrapureSEASTAR0 16 N HNO3 and ddH2O. Mg/Ca, Sr/Ca, Na/Ca, Li/Ca, and Mn/Ca were measured us-ing a Finnigan MAT Element sector ¢eld induc-tively coupled plasma mass spectrometer (ICP-MS) operated in low resolution (m/vm=300) fol-lowing the method outlined in Rosenthal et al.[18] and modi¢ed by Lear et al. [19]. During thestudy period, the analytical precision as deter-mined by replicate analyses of consistency stan-dards was better than 2% rsd (1c) both for Mg/Ca (6.1 mmol mol31), Sr/Ca (1.9 mmol mol31),and Mn/Ca (29 Wmol mol31), and better than

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4.5% rsd (1c) for Na/Ca (15.8 mmol mol31) andLi/Ca (43.4 Wmol mol31). The analytical precisionfor all elemental ratios is signi¢cantly less than thedegree of scatter caused by intra-sample and in-ter-sample natural variability.Trends in the ratio of element to calcium (El/

Ca) obtained from di¡erent preparations of thesame samples are reproducible between the studyof McArthur et al. [10] (analyzed by ICP-AES)and this study (analyzed by ICP-MS), with abso-lute values from McArthur et al. [10] showingsmall constant o¡sets from the data of this study,of +2 mmol/mol in Na/Ca and of 30.2 mmol/molin Sr/Ca. These constant o¡sets have been re-moved from the data of McArthur et al. [10]when plotted in Fig. 2. The observed trends aretherefore not artifacts of sample preparation oranalytical bias.The German samples were analyzed for 87Sr/

86Sr following the technique of Thirlwall [20].87Sr/86Sr values are reported relative to a valueof 0.710248 for the SRM987 standard. The pre-cision of our measurements based on replicateanalyses of SRM987 is P 6U1036 (2 S.D.,n=4).Samples for stable isotope analyses were re-

acted with 100% orthophosphoric acid at 90‡Cusing a Multi-prep peripheral device and analyzedwith an Optima mass spectrometer at RutgersUniversity. Repeat analysis of NBS19 yielded pre-cision better than P 0.06x for N

13C and betterthan P 0.08x for N

18O. As with the data ofMcArthur et al. [10], values are reported versusx PeeDee Belemnite (PDB).

3. Stratigraphic correlation of the German sectionto UK stratigraphy

Due to di¡erences between the ammonite bio-stratigraphic schemes over the interval of interest,correlation between the German and UK sectionswas performed by strontium isotope stratigraphy,which is based on the principle that stratigraphiclevels recording the same 87Sr/86Sr value are ofequivalent age [21,22]. Comparison of the 87Sr/86Sr values of six German samples with the recordof 87Sr/86Sr variation with stratigraphic height atYorkshire (Fig. 2) allowed correlative stratigraph-ic heights in the Yorkshire section to be assignedto these samples. This comparison was performedby splitting the Yorkshire 87Sr/86Sr record (Fig. 2)into four intervals of stratigraphic height and ap-plying a linear least-squares regression to eachinterval. The intervals are the same as those de-scribed in McArthur et al. [10] and are given inTable 1. The error of the linear least-squares re-gression ¢t (Table 1) combined with the analyticalerror of the measurement of 87Sr/86Sr from theGerman sample gives the ¢nal error on the corre-lative stratigraphic height at Yorkshire (Table 2).This ¢nal error is less than 3.1 m for the Dottern-hausen locality and less than 5 m for the Asel¢n-gen locality. Samples from Germany that werenot analyzed for 87Sr/86Sr were assigned a corre-lative stratigraphic height in the Yorkshire sectionby linear interpolation between the six sampleswith 87Sr/86Sr measurements (Table 2). The corre-lation of the samples from Asel¢ngen is less wellconstrained than for samples from Dotternhau-

Table 1Regression lines used to assign a correlative height in the Yorkshire section (H) to German samples analyzed for 87Sr/86Sr

Regression line Stratigraphicinterval

Equation of linearleast-squares regression line

Error onslope

Error onintercept

R2 n

(m) (S.E.) (S.E.)

A 340.04 to 326.34 87Sr/86Sr= 0.706998634.907945U1036 H

4.657822U1037 0.0000156 0.94 9

B 314.77 to 30.46 87Sr/86Sr= 0.7070934+1.465629U1036 H

2.315903U1037 0.0000019 0.70 19

C 0.15 to 7.32 87Sr/86Sr= 0.7070851+1.052613U1035 H

8.237057U1037 0.0000041 0.92 17

D 7.32 to 87.38 87Sr/86Sr= 0.7071667+8.161450U1037 H

3.722182U1038 0.0000020 0.90 56

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sen, as there are only two 87Sr/86Sr measurementsfrom Asel¢ngen and the analytical error on theoldest sample (AS9) is relatively high (Table 2).The similarity of the shapes of the N13C records

from both localities when plotted versus strati-graphic height in the Yorkshire section (Fig. 2)provides an independent indicator that the corre-lation by 87Sr/86Sr is reliable, particularly the co-incidence of the N13C peak in the lower falciferumzone.

4. Results

The most striking feature of the geochemicalrecords from the Yorkshire section is at 310 to5 m stratigraphic height, between the mid-tenui-costatum and lower falciferum zones (Fig. 2). Heresynchronous shifts to higher values for Mg/Ca,Sr/Ca, and Na/Ca, which are associated with anegative shift in N

18O, are seen. In contrast, theLi/Ca ratio shifts to lower values during this in-terval. These shifts span lithological boundariesand occur just prior to the OAE as de¢ned bythe black shale deposition of the lower falciferumzone (Fig. 2).The N

18O and El/Ca data from Germany areconsistent with the UK records when plotted ver-sus their equivalent stratigraphic height at York-shire, exhibiting covariation in Mg/Ca, Sr/Ca andNa/Ca, that is inversely correlated with N

18O val-ues (Fig. 2). The Li/Ca record from Germany hasa larger degree of scatter than the Yorkshire rec-ord and does not exhibit the trend seen in theYorkshire Li/Ca record. Mg/Ca, Sr/Ca, Na/Caand N

18O data from Germany follow the general

trend delineated by the Yorkshire records, show-ing low values for these El/Ca ratios coincidentwith higher N

18O in the tenuicostatum zone, andhigh values of these El/Ca ratios coincident withlower N

18O in the falciferum zone. Lower in thestratigraphy, in the margaritatus and spinatumzones, there is a discrepancy between the datafrom the di¡erent localities. At these levels(V320 to 340 m), data from Germany showEl/Ca and N

18O values similar to those character-izing the tenuicostatum zone above, whereas datafrom Yorkshire exhibit a shift to higher El/Ca andlower N18O.

5. Discussion

5.1. Reliability of geochemical proxies

For reasons given at length in McArthur et al.[10], the samples are regarded as recording origi-nal elemental compositions. In our new data, Mn/Ca values do not correlate with any other geo-chemical parameters, and all Mn/Ca values arebelow 100 Wmol/mol, the lower limit for samplesconsidered likely to have been altered under re-ducing conditions [22]. Diagenesis under oxidizingconditions is not thought to have taken place dueto the high organic matter content of the sections[7]. Excepting the Li/Ca data, the consistency be-tween the El/Ca and N

18O records from Germanyand Yorkshire for the period from the tenuicosta-tum to falciferum zones supports the notion thatthese data record a regional signal of the north-west European epi-continental sea for this inter-val. In addition, the low Mg/Ca values of the

Table 2Details of the strontium isotope correlation of German samples to the Yorkshire section

Sample Height in Germansection

87Sr/86Sr P 2 S.E. Regression lineused

Correlative height inYorkshire section

+ ¢nal error 3 ¢nal error

(m) (m) (m) (m)

DOT24 7.45 0.707174P 9 C 8.45 2.07 1.77DOT14 4.77 0.707148P 10 C 5.98 1.96 1.68DOT10 1.97 0.707098P 8 C 1.23 1.35 1.16DOT1 0.71 0.707087P 3 B 34.71 3.07 2.23AS26 0.00 0.707091P 9 B 34.74 3.09 3.09AS9 314.80 0.707224P 28 A 345.92 4.99 4.12

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Fig. 2. El/Ca ratios, N18O, N13C, and 87Sr/86Sr data from the Pliensbachian^Toarcian of Yorkshire (solid symbols) and Southwest Germany (open symbols). Dataare plotted relative to the Yorkshire stratigraphy, reported in Hesselbo and Jenkyns [13], as described in Section 3. The ammonite biostratigraphic scheme is givenby Howarth in [51], and the numerical timescale is from Palfy and Smith [3]. Ammonite subzones are shown only for the bifrons, falciferum and tenuicostatumzones (cra= crassum, ¢b= ¢bulatum, com= commune, fal = falciferum, exa= exaratum, sem= semicelatum, ten= tenuicostatum, cle = clevelandicum, pal = paltum). Thelower falciferum zone OAE is denoted by the gray band.

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tenuicostatum zone (V8 mmol/mol) are similar tothose measured in belemnites from the same bio-zone of the highly condensed Junction Bed inDorset, UK (Jenkyns, personal communication).In the spinatum zone, the German data are morelikely to represent a regional signal than theYorkshire data, as a spinatum zone to tenuicosta-tum zone minimum in Mg/Ca and correspondingmaximum in N

18O values also occurs in belemnitesof equivalent age from NE Spain [23], thoughexact correlation between the UK and Spanishbiostratigraphic schemes is uncertain. Data fromthe spinatum zone of Yorkshire are thereforelikely to represent a signal speci¢c only to theUK section.We collected records of Li/Ca, as a study of

recent biogenic carbonates (benthic foraminifera)shows that this ratio could respond in the oppo-site sense to Mg/Ca to temperature change [24].However, the divergence of Li/Ca data betweenthe two studied sections suggests that regionalLi/Ca signals are not recorded by these belem-nites. This may be expected, as the small size ofthe Li ion makes it more likely to be one of the¢rst elements to be lost from the calcite latticeupon exposure to the high temperatures of theburial environment.

5.2. Genera and species e¡ects

Studies of the chemistry of modern biogeniccarbonates show that trace metal partition coe⁄-cients into calcite may be species- and genus-spe-ci¢c. It is therefore possible that some componentof the geochemical trend across the tenuicostatumto falciferum zone boundary may be related to achange in the dominant belemnite subfamily(from Belemnitinae to Megatuethidinae). Multiplebelemnite genera are reported from the Yorkshiresequence at levels preceding and throughout thespinatum and tenuicostatum zones [25] and oursamples represent a random selection of the di¡er-ent genera and species at all stratigraphic levels. Itwas possible to identify some of the belemnitestaken from 0 to 25 m in the Yorkshire sectionto the genus level. Genus-speci¢c e¡ects arethought to be minimal for this interval, as theEl/Ca trends are delineated by a single genus

(Acrocoelites) (Fig. 3). Based on these data wesuggest that the di¡erence in El/Ca ratios betweentwo genera is on the same order as the scatter ofthe data (Fig. 3). In addition, species-speci¢c frac-tionation is not observed in stable isotope datafrom this section [8]. We therefore regard it asunlikely that the geochemical trends from the te-nuicostatum zone to the falciferum zone, which areobserved in sections from the UK, Germany andSpain, are caused solely by species- or genus-spe-ci¢c e¡ects.

5.3. Changes in environmental conditions

In the following discussion we ¢rst consider thelarge-amplitude geochemical shift from the mid-tenuicostatum zone to the lower falciferum zonethat occurred during the OAE. We then considerthe implications of the preceding overall excursionto low values of El/Ca and high N

18O during theupper Pliensbachian margaritatus and spinatumzones and Toarcian lower tenuicostatum zonethat is suggested by the Yorkshire data (‘pre-mar-garitatus values’ from Yorkshire cannot be veri-¢ed from the German section as it does not ex-tend this far back in time).We interpret the geochemical records obtained

from belemnite calcite primarily in terms ofchanges in environmental conditions (temperatureand salinity). The change by a factor of between1.7 and 2 in N

18O, Mg/Ca, Sr/Ca, and Na/Ca,across the tenuicostatum to falciferum zoneboundary began at the base of the semicelatumsubzone and ended approximately halfwaythrough the exaratum subzone (Fig. 2). Usingthe subzone durations of McArthur [10], whichare derived from the linear relationship betweenstratigraphic level and the rate of change of 87Sr/86Sr with time, this geochemical change occurredover a period ofV0.6^0.7 Myr. This large rate ofchange shows that these geochemical shifts areunlikely to represent a short-term change in‘whole ocean’ chemistry, since all these elementshave residence times longer than 1 Myr.The largest shift in the N

18O record is seenat the transition from the mid-tenuicostatumzone (30.5x) to the lower falciferum zone(33.5x). If the entire shift is attributed to tem-

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perature, the temperature change (vT) calculatedfrom the inorganic paleotemperature equation of[26] is 13‡C (Table 3). As this temperature changeis very large, it is likely that temperature was notthe sole cause of N18O variation and that a changein seawater N18O and/or salinity also occurred.The incorporation of Mg into calcite precipi-

tated from seawater is governed by the distribu-tion coe⁄cient KD, where Mg/Cacalcite =KDMg/Caseawater. Experimental studies have shown thatfor inorganic calcite the value of KD is dependentupon the temperature of precipitation, causing an

increase in Mg/Ca of 3% per ‡C temperature in-crease [27,28]. Values of KD are very di¡erent forbiologically mediated precipitation of calcite, butare consistently dependent upon temperature inall known studies. Field and culture studies sug-gest that the relationship between foraminiferalMg/Ca and temperature is best described by anexponential curve (Mg/Ca)calcite =BeAT , where Aspeci¢es the Mg/Ca dependence on temperature,T is the calci¢cation temperature and B is thespecies-speci¢c pre-exponential constant. Studiesof planktonic and benthic foraminifera suggest

Fig. 3. Plots of El/Ca data from Yorkshire versus stratigraphic height. Belemnites are identi¢ed to the genera level where possi-ble.

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that A ranges from 0.09 to 0.11, i.e. a 10P 1%increase in Mg/Ca per ‡C [19,29^34]. This esti-mate is also consistent with the calibration of os-tracodes [35]. It therefore seems that a Mg/Ca^temperature dependence of 10P 1% per ‡C maybe a robust property of most biogenic calcites.Hence it is very plausible that Mg/Ca in belem-nites had similar temperature dependence.It is not possible to calculate absolute paleo-

temperatures as belemnites are extinct and theelement to calcium ratios of contemporaneousseawater are unknown. However, it is possibleto estimate the likely magnitude of a temperaturechange (vT) by assuming that the temperaturesensitivity of Mg/Ca in belemnite calcite is withinthe range observed in modern biogenic calcites.For the Mg/Ca change from the mid-tenuicosta-tum zone to the lower falciferum zone (about8 mmol mol31) we estimate vT to be between6 and 7‡C, which corresponds to a rate of changeof V0.01‡C/kyr (Table 3). This vT estimate issubstantially less than that obtained from thechange in N

18O of belemnite calcite (V13‡C). Itis noteworthy that the Mg/Ca-derived vT estimateis una¡ected by the value chosen for the Mg/Caof Early Jurassic seawater, which impacts onlyvalues of absolute temperature. Consequently, ifthe Mg/Ca increase from the mid-tenuicostatumzone to lower falciferum zone re£ects only achange in temperature, a substantial salinity de-crease must be invoked to explain the residualnegative N18O shift ofV1.3x. Assuming a mod-ern vN

18O/vS ratio of 0.5x per 1 psu change insalinity known from the modern subtropical At-lantic Ocean [36], we estimate a salinity decreaseof about 2.5. If correct, the results would suggestthat a seawater warming of about 6^7‡C from the

mid-tenuicostatum zone to lower falciferum zonewas associated with large freshening. This conclu-sion would be consistent with previously pub-lished data suggesting a climatic optimum in thefalciferum zone [37], and low surface water salin-ity for the lower falciferum zone for both theYorkshire [8] and German sections [14]. The largefreshening is coincident with black shale deposi-tion in the lower falciferum zone. Therefore animportant factor in the development of the OAEmay have been a density strati¢cation of theupper water column. As the OAE is a globalevent [5] this raises the question as to whetherthe geochemical signals recorded on the northwestEuropean shelf are representative of the globalocean or regional change.It is widely assumed that the Lower Jurassic

87Sr/86Sr record obtained from samples from thenorthwest European shelf represents a global sig-nal [3,38^41] because the residence time of Sr inthe modern ocean is several orders of magnitudelonger than the oceanic mixing time. It is there-fore plausible that the mid-tenuicostatum zone tolower falciferum zone shift in N

18O and Mg/Care£ects global change. We consider it likely thatthe salinity change ofV2.5 interpreted from N

18Oand Mg/Ca data represents a response to globalchange that has been ampli¢ed by the e¡ects ofthe regional paleogeographic setting of the north-west European shelf. This wide shallow shelf sur-rounded by continents delivering a large fresh-water £ux as a result of a monsoon-dominatedclimate is likely to have had a water columnthat was predisposed towards vertical strati¢ca-tion. We therefore propose that an acceleratedhydrological cycle caused by global warming ledto reduced surface water salinity via elevated lev-

Table 3Possible temperature changes associated with the shift in Mg/Ca and N

18O from the mid-tenuicostatum zone to lower falciferumzone

Record Approximate shift from mid-tenuicostatum zone to lower falciferum zone Calibration Calculated vT(‡C)

Mg/Ca 8^16 mmol/mol Foraminifera [19,33] V6.8Ostracode [35] V6.8

N18O 30.5 to 33.5x (PDB) Belemnite [26] V13

vT based on N18O shift is calculated assuming no change in the N

18O of seawater, taken to be 31x SMOW for an ice-freeworld [49].

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els of continental runo¡, and resulted in the strat-i¢cation of shelf waters. The osmium isotopic ra-tio of seawater may provide independent evidencefor an accelerated hydrological cycle at the timeof the OAE, where it shows unusually radiogenicvalues indicative of intense continental weathering[42,43]. Similarly, the increasing 87Sr/86Sr trendbeginning in the tenuicostatum zone has been in-terpreted as re£ecting an increased £ux of radio-genic Sr from continental weathering [3], thoughthe shape of the 87Sr/86Sr record in Fig. 2 ispartly a result of varying sedimentation rates inthe Yorkshire section [10], and other interpreta-tions of the 87Sr/86Sr record have been proposed[40].

5.4. Geochemical changes during the interval fromthe upper Pliensbachian margaritatus zone tothe Toarcian lower tenuicostatum zone

The geochemical trend prior to the upperPliensbachian margaritatus zone in the Yorkshirerecord suggests an overall excursion to low valuesof Mg/Ca, Sr/Ca, and Na/Ca, and higher N

18Oduring the margaritatus, spinatum and lower te-nuicostatum zones (Fig. 2). The exact onset ofthis excursion is di⁄cult to pinpoint with thepresent dataset due to the relatively large uncer-tainty (V5 m) in the correlation of the oldestAsel¢ngen sample to the Yorkshire stratigraphy,and the lack of pre-margaritatus zone samplesfrom Germany.Assuming it represents a regional signal, we in-

terpret this margaritatus zone to lower tenuicosta-tum zone excursion as recording a period wherecool and relatively saline waters existed on thenorthwest European shelf. We speculate thatsuch an excursion occurred over a period ofV1^2 Myr prior to the Early Toarcian falciferumzone OAE. This timing suggests that the oceano-graphic changes responsible for this margaritatuszone to lower tenuicostatum zone excursion arelikely to have preconditioned the epi-continentalsea for the later OAE. We suggest that the exis-tence of saline waters on the shelf during the mar-garitatus, spinatum and lower tenuicostatum zonesmay have preconditioned shelf waters for the sub-sequent density strati¢cation that was triggered by

the large freshening across the tenuicostatum^fal-ciferum zone boundary.Published paleontological and isotopic geo-

chemical data support the hypothesis that ocean-ographic change occurred during the spinatumand tenuicostatum zones. Elevated levels of extinc-tion have been recognized globally over thePliensbachian^Toarcian boundary [1], and thetenuicostatum zone coincides exactly with an in-terval of benthic extinction recorded in the York-shire section [2]. N13C data obtained from belem-nite samples (Fig. 2) indicate that rates of globalorganic carbon burial began to increase aroundthe level of the Pliensbachian^Toarcian boundary,as suggested by data from other studies [44,45]. Inaddition, it has been suggested that the low 87Sr/86Sr values from the mid-margaritatus to tenuicos-tatum zones (Fig. 2) were driven down by theinput of non-radiogenic Sr from hydrothermalweathering at mid-ocean ridges [40]. The coinci-dence of a global extinction in benthic fauna, theonset of globally enhanced burial rates of organiccarbon, and lowered 87Sr/86Sr values lends weightto the hypothesis that global oceanographicchange caused an excursion in N

18O and Mg/Caduring the margaritatus, spinatum and lower te-nuicostatum zones.

6. Caveats to interpretation in terms oftemperature and salinity

The explanation of the N18O and Mg/Ca trendsin terms of temperature and salinity changes rep-resents the simplest interpretation of the data.However, we acknowledge that there are a num-ber of important caveats to this interpretation,which arise from a consideration of the Sr/Caand Na/Ca data.Unlike Mg/Ca, there are presently not enough

data addressing the temperature sensitivity of oth-er elements in modern biogenic calcites. Althoughexact temperature sensitivities are not known forany geochemical parameter in belemnite calcite,based on thermodynamic expectations and ourcurrent knowledge of the temperature dependenceof trace element and isotopic proxies in moderninorganic and biogenic calcites, it is unlikely that

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temperature sensitivities would be equal for Mg/Ca, Sr/Ca, and Na/Ca. The covariation of bothSr/Ca and Na/Ca with N

18O in our records istherefore problematic. Fig. 4 shows that in bothsections, during the interval of decreasing N

18Ovalues (semicelatum to falciferum subzones), Na/Ca data have a weaker correlation with N

18O thando Mg/Ca and Sr/Ca data. This is also the casefor the interval when N

18O shows little variation(stokesi to tenuicostatum subzones), where Na/Cashows no correlation with N

18O in the Germansection (Fig. 4). This suggests that the correlationobserved in the Yorkshire data for this intervalresults from the high El/Ca values of the spinatumzone that may be due to local or diagenetic ef-fects. We conclude from Fig. 4 that Na/Ca datado not show signi¢cant variation with N

18O andare therefore unlikely to re£ect temperature var-

iation, as expected by analogy to experiments onmodern biogenic calcites [46] [Lear, unpublished].Studies on modern planktonic foraminifera [30]and coccolithophorids [47] suggest a Sr/Ca tem-perature sensitivity that is approximately 10Ulower than the temperature sensitivity of Mg/Ca.If temperature was the sole cause of variation inthe records presented here, we would thereforeexpect the relative changes in Sr/Ca to beV10U lower than the relative change in Mg/Ca. As this is not the case, it is unlikely thatour belemnite Sr/Ca records re£ect temperaturealone. In modern mollusks, Sr/Ca appears to bein£uenced predominantly by physiological factors[48], and it is therefore possible that in belemnitesthe response of Sr/Ca to temperature is magni¢edby physiological e¡ects.An alternative explanation for the similar mag-

6

8

10

12

14

16

-5 -4 -3 -2 -1 0 1

R = 0.52

R = 0.02

1

1.5

2

2.5

-5 -4 -3 -2 -1 0 1

R = 0.64

R = 0.44

5

10

15

20

25

-5 -4 -3 -2 -1 0 1

R = 0.75

R = 0.40

6

8

10

12

14

16

-5 -4 -3 -2 -1 0 1

R = 0.67

R = 0.32

1

1.5

2

2.5

-5 -4 -3 -2 -1 0 1

R = 0.75

R = 0.71

5

10

15

20

25

-5 -4 -3 -2 -1 0 1

R = 0.83

R = 0.75

= Yorkshiresemicelatum to falciferum subzones

stokesi to tenuicostatum subzones

Mg

/Ca m

mo

l/m

ol

δ18O (‰ PDB)

Sr/

Ca m

mo

l/m

ol

δ18O (‰ PDB)

Na/C

a m

mo

l/m

ol

δ18O (‰ PDB)

Na/C

a m

mo

l/m

ol

δ18O (‰ PDB)

Sr/

Ca m

mo

l/m

ol

δ18O (‰ PDB)

Mg

/Ca m

mo

l/m

ol

δ18O (‰ PDB)

= Germany

Fig. 4. Cross-plots of N18O against El/Ca and their correlation coe⁄cients for the period from the semicelatum to falciferum sub-zones (during the negative shift in N

18O), and the period from the stokesi to tenuicostatum subzones (when there is little changein N

18O).

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T.R. Bailey et al. / Earth and Planetary Science Letters 212 (2003) 307^320 317

nitudes of the relative changes in Mg/Ca and Sr/Ca is secular variation in the chemistry of theambient water. Though with the present datathis possibility cannot be wholly excluded, it isdi⁄cult to envisage changes in seawater chemistryoccurring over time intervals of a few Myr with-out invoking sustained periods of isolation of thenorthwest European shelf waters from the globalocean.

7. Conclusions

Late Pliensbachian^Early Toarcian belemnitesfrom Yorkshire (UK) and Southwest Germanyrecord primary El/Ca (Mg/Ca, Sr/Ca, Na/Ca),N18O, and N

13C values that represent a regionalsignal of the northwest European epi-continentalsea. El/Ca changes during the Early Toarcian in-terval from the mid-tenuicostatum zone to lowerfalciferum zone occur too rapidly to be the resultof a change in whole ocean chemistry, and likelyrespond to environmental change. By inference toMg/Ca temperature dependencies of modern bio-genic calcites, belemnite Mg/Ca data indicate thata warming of 6^7‡C occurred during this interval.The concomitant N18O shift is of larger magnitudethan would be caused by a temperature increaseof 6^7‡C alone, which suggests that this warmingwas accompanied by a salinity decrease of V2.5.Earlier in the Pliensbachian, in the margarita-

tus, spinatum, and lower tenuicostatum zones, Mg/Ca and N

18O data indicate the presence of coolerand relatively more saline waters in the northwestEuropean epi-continental sea. Mg/Ca and N

18Odata therefore suggest a switch from cool and sa-line waters in the northwest European epi-conti-nental sea in the Late Pliensbachian, to warmerand fresher waters in the Early Toarcian. Thissequence of events likely led to a density strati¢-cation of the water column in the Early Toarcian,which perhaps represents the ¢nal trigger for thefalciferum zone OAE.We hypothesize that Early Toarcian global

warming and a resultant globally accelerated hy-drological cycle delivered nutrients and freshwaterto shallow shelf-seas worldwide by enhanced con-tinental weathering and runo¡. These global sig-

nals may have been enhanced in the studied sec-tions by the paleogeographic setting of thenorthwest European epi-continental sea. A con-tinual delivery of nutrients from continentalweathering, being a source external to the strati-¢ed shelf waters, could explain how high levels ofproductivity were sustained for the duration ofthe OAE (V0.5 Myr).

Acknowledgements

We would like to thank Manfred Ja«ger fromRohrbach Zement Industries for assistance insampling at the Dotternhausen quarry, DesmondDonovan for identifying belemnite species, PaulField for assistance with ICP-MS analyses, andJim Wright for stable isotope analyses. We thankPaul Falkowski, Carrie Lear, and Peter Doyle forhelpful discussions, and Mark Leckie and ananonymous reviewer for constructive commentswhich improved the manuscript. This work wasconducted as part of the EREUPT project fundedby NSF Biocomplexity Grant no. OCE-0084032to P.F.[BOYLE]

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