Belemnites of Valanginian, Hauterivian and Barremian age : Sr-isotope stratigraphy, composition ( 87 Sr/ 86 Sr, N 13 C, N 18 O, Na, Sr, Mg), and palaeo-oceanography J.M. McArthur a; , J. Mutterlose b , G.D. Price c , P.F. Rawson a , A. Ru¡ell d , M.F. Thirlwall e a Department of Earth Science, University College London, Gower Street, London WC1E 6BT, UK b Institut fu «r Geologie, Ruhr-Universita «t Bochum, Universita «tsstr. 150, D-44801 Bochum, Germany c Department of Geological Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK d School of Geography, Queen’s University of Belfast, Belfast BT7 1NN, UK e Department of Geology, Royal Holloway and Bedford New College, Egham Hill, Egham, Surrey TW20 0EX, UK Received 21 August 2002; received in revised form 31 July 2003; accepted 5 September 2003 Abstract We present new data on 87 Sr/ 86 Sr, N 13 C and N 18 O, and elemental compositions of belemnites from 85 m of Valanginian, Hauterivian and Barremian strata at Speeton, Yorkshire, eastern England. The 87 Sr/ 86 Sr data provide a global standard for 87 Sr/ 86 Sr isotopic dating, and correlation to the biostratigraphic schemes of NW Europe. Values of 87 Sr/ 86 Sr increase from 0.707380 ɔ 0.000003, at the base of the Hauterivian, to 0.707493 ɔ 0.000004 in the earliest Late Barremian Paracrioceras elegans ammonite Zone before decreasing thereafter towards an Aptian minimum. The downturn in the elegans Zone coincided with the onset of volcanism on the present Ontong Java Plateau. A linear interpretation of the 87 Sr/ 86 Sr profile shows that the relative durations of ammonite zones differ by a factor 9 18. The basal Hauterivian unconformably overlies Valanginian strata ; the discontinuity in 87 Sr/ 86 Sr across this surface represents a gap in sedimentation of 2.0 myr. In our belemnites (mostly of the genera Hibolites, Acroteuthis, and Aulacoteuthis) the absence of a correlation between N 18 O and N 13 C suggests that strong non-equilibrium fractionation has not affected the isotopic composition of the calcite. Our N 18 O values therefore approximate to a valid record of marine palaeo-temperatures. Specimens of the genus Hibolites have N 18 O values that are 0.4x more positive than those of co-occurring specimens of the genus Acroteuthis. This offset may be explained as resulting from small (0.4x) departures from equilibrium during precipitation of calcite, different depth habitats, or changing temperature in the Speeton sea in the time that elapsed between deposition of our individual belemnites. The averaged belemnite record of N 18 O through the section shows that seawater warmed from around 11‡C at the base of the Hauterivian to a maximum around 15‡C in the middle of the Hauterivian regale Zone, and returned to a cooler temperature of around 11‡C by the middle of the overlying inversum Zone, a temperature that persisted to the basal Barremian. Through the Barremian, temperature increased to a peak of 20‡C in the early Late Barremian elegans Zone then, in the same zone, precipitately and temporarily decreased to around 14‡C at about the time of onset of volcanism on the Ontong Java Plateau, before they returned to around 16‡C in the uppermost part of the section. In specimens of Aulacoteuthis and Acroteuthis, a good correlation between N 18 O and the content of Na, Sr, and Mg suggests that incorporation of these 0031-0182 / 03 / $ ^ see front matter ȣ 2003 Elsevier B.V. All rights reserved. doi :10.1016/S0031-0182(03)00638-2 * Corresponding author. E-mail address: [email protected](J.M. McArthur). Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2004) 253^272 www.elsevier.com/locate/palaeo
Transcript
Belemnites of Valanginian, Hauterivian and Barremian age:Sr-isotope stratigraphy, composition (87Sr/86Sr, N13C, N18O,
Na, Sr, Mg), and palaeo-oceanography
J.M. McArthur a;�, J. Mutterlose b, G.D. Price c, P.F. Rawson a, A. Ru¡ell d,M.F. Thirlwall e
a Department of Earth Science, University College London, Gower Street, London WC1E 6BT, UKb Institut fu«r Geologie, Ruhr-Universita«t Bochum, Universita«tsstr. 150, D-44801 Bochum, Germany
c Department of Geological Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UKd School of Geography, Queen’s University of Belfast, Belfast BT7 1NN, UK
e Department of Geology, Royal Holloway and Bedford New College, Egham Hill, Egham, Surrey TW20 0EX, UK
Received 21 August 2002; received in revised form 31 July 2003; accepted 5 September 2003
Abstract
We present new data on 87Sr/86Sr, N13C and N
18O, and elemental compositions of belemnites from 85 m ofValanginian, Hauterivian and Barremian strata at Speeton, Yorkshire, eastern England. The 87Sr/86Sr data provide aglobal standard for 87Sr/86Sr isotopic dating, and correlation to the biostratigraphic schemes of NW Europe. Valuesof 87Sr/86Sr increase from 0.707380; 0.000003, at the base of the Hauterivian, to 0.707493; 0.000004 in the earliestLate Barremian Paracrioceras elegans ammonite Zone before decreasing thereafter towards an Aptian minimum. Thedownturn in the elegans Zone coincided with the onset of volcanism on the present Ontong Java Plateau. A linearinterpretation of the 87Sr/86Sr profile shows that the relative durations of ammonite zones differ by a factor 9 18. Thebasal Hauterivian unconformably overlies Valanginian strata; the discontinuity in 87Sr/86Sr across this surfacerepresents a gap in sedimentation of 2.0 myr. In our belemnites (mostly of the genera Hibolites, Acroteuthis, andAulacoteuthis) the absence of a correlation between N
18O and N13C suggests that strong non-equilibrium fractionation
has not affected the isotopic composition of the calcite. Our N18O values therefore approximate to a valid record ofmarine palaeo-temperatures. Specimens of the genus Hibolites have N
18O values that are 0.4x more positive thanthose of co-occurring specimens of the genus Acroteuthis. This offset may be explained as resulting from small (0.4x)departures from equilibrium during precipitation of calcite, different depth habitats, or changing temperature in theSpeeton sea in the time that elapsed between deposition of our individual belemnites. The averaged belemnite recordof N
18O through the section shows that seawater warmed from around 11‡C at the base of the Hauterivian to amaximum around 15‡C in the middle of the Hauterivian regale Zone, and returned to a cooler temperature of around11‡C by the middle of the overlying inversum Zone, a temperature that persisted to the basal Barremian. Through theBarremian, temperature increased to a peak of 20‡C in the early Late Barremian elegans Zone then, in the same zone,precipitately and temporarily decreased to around 14‡C at about the time of onset of volcanism on the Ontong JavaPlateau, before they returned to around 16‡C in the uppermost part of the section. In specimens of Aulacoteuthis andAcroteuthis, a good correlation between N
18O and the content of Na, Sr, and Mg suggests that incorporation of these
0031-0182 / 03 / $ ^ see front matter G 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0031-0182(03)00638-2
trace elements in these genera is largely controlled by temperature. The dependency of concentration on temperatureranges from 7 to 20% per degree Celsius, if equilibrium fractionation of oxygen isotopic composition is assumed, sothe Mg, Na and Sr content of these genera may be used as palaeo-temperature proxies. The trace element content ofHibolites shows no relation to stable oxygen isotopic composition and so does not record palaeo-temperature.G 2003 Elsevier B.V. All rights reserved.
Interpreting Earth history for Early Cretaceoustimes has proven a challenging task, but one thatis of interest because of the change to ‘green-house’ conditions in the early-to-mid-Cretaceous(Lini et al., 1992), when the northern hemispherewas divided into Boreal and Tethyan bioticRealms. Of particular interest is the degree towhich the emplacement in Barremian/Aptiantimes of the World’s largest igneous province,the Ontong Java Plateau (OJP), contributed toenvironmental change. There is some evidencethat it did have an impact on climate, biotic di-versity and ocean-wide patterns of sedimentation(Larson and Erba, 1999; references therein).These authors acknowledge the di⁄culty of link-ing OJP volcanism to world events, but amongstthe evidence that does so is the onset of the de-cline of marine 87Sr/86Sr in the Late Barremian, atabout the time of onset of OJP volcanism (ibid).We have determined a precise record of 87Sr/86Srthrough much of Barremian time to test the linkbetween emplacement of the OJP and the 87Sr/86Sr record. We extend the record back throughHauterivian time, and a fragment of Valanginiantime, in order to provide a standard curve fordating and correlation with 87Sr/86Sr in this inter-val. Such a record will assist the integration ofBoreal and Tethyan histories which have beenhampered by marked £oral and faunal di¡erencesbetween the realms, particularly during latest Ju-rassic and earliest Cretaceous times.
Also of interest is the fact that palaeo-environ-mental interpretations, for this and other periods,often invoke palaeo-temperatures determinedfrom the oxygen isotopic composition of belem-nite calcite (Hudson and Anderson, 1989; Ander-son et al., 1994; Saelen et al., 1996; Ditch¢eld,
1997; Price and Sellwood, 1997; Podlaha et al.,1998; Price et al., 2000; van de Schootbrugge etal., 2000; Rosalesd et al., 2001; (Niebuhr andJoachimski, 2002). In view of this, we follow Pod-laha et al. (1998) in asking whether belemnite cal-cite forms in equilibrium, or in disequilibrium,with its ambient water, and so whether such in-terpretations are valid. We do so by examiningthe chemical and isotopic composition of EarlyCretaceous belemnites of both Boreal andTethyan a⁄nities that are proven to be well-pre-served, not least because they retain their originalvalues of 87Sr/86Sr. We show that our belemnitesprecipitated calcite under conditions that closelyapproximated isotopic equilibrium for oxygen,and that the Na, Sr, and Mg contents of belem-nites hold promise as palaeo-temperature proxies.
2. Geological setting
2.1. Stratigraphy and faunas
The Speeton Clay Formation is exposed at Fil-ey Bay, Speeton, in Yorkshire, northeastern Eng-land (Fig. 1). The formation (Fig. 2) comprisesabout 100 m of interbedded marine claystonesand calcareous mudrocks of which we sampled85 m. The sediments rest unconformably on theKimmeridge Clay Formation which is of Volgianage (Rawson et al., 1978). The stratigraphical suc-cession and biozonations shown in Fig. 2 are ac-companied by a plot of 87Sr/86Sr in belemnitesagainst their stratigraphic level in order to showin outline the distribution of our samples throughthe section. The sequence has a number of strati-graphic breaks and condensed intervals, the mostnotable of which is at the base of the Hauterivianpart of the section where the Upper Valanginian
Fig. 1. Upper ¢gure shows the present location and geology of the Speeton area. Lower ¢gure shows the palaeo-geography ofnorthwest Europe during Early Cretaceous times (adapted from Mutterlose, 1998). The locality at Speeton occupied a positionproximal to both the Tethyan and Boreal Realms, with in£ux of Hibolites (Tethyan) belemnites from the south as sea level rosein early Hauterivian times.
is absent (Fig. 2). Breaks and condensed intervalsare represented by phosphatic nodule beds, con-centrations of belemnites, and intensely glauco-nitic levels. The base of the Ryazanian (base ofBed E) is marked by a major transgression (Raw-son and Riley, 1982) which probably correspondsto a maximum £ooding surface (Haq et al., 1987;Ru¡ell, 1991). The early Hauterivian period wasmarked by a series of minor regressive episodes(Ru¡ell, 1991).
The strata at Speeton were divided into fourmajor units, beds A^D, labelled from the topdown by Lamplugh (1889) on the characteristics
of their belemnite fauna. The A beds (Aptian^Albian) and the C beds (Hauterivian) are domi-nated by characteristically Tethyan belemnites ofthe genera Neohibolites (A beds) and Hibolites(C beds). The B beds (Barremian; Rawson andMutterlose, 1983) and the D beds (upper Ryaza-nian ^ lowermost Hauterivian) are dominated byBoreal belemnites (e.g. Rawson, 1973; Mutter-lose, 1992) of the genera Acroteuthis (D beds),Oxyteuthis, Praeoxyteuthis, and Aulacoteuthis(B beds), with small Hibolites co-occurring inthe B beds. Overlap of forms is very limited:e.g. a few specimens of the Boreal genus Acroteu-
Fig. 2. Ammonite and nannofossil biostratigraphy, lithostratigraphy and outline log, stratigraphic levels above Bed E as datum(see text for more explanation), and outline 87Sr/86Sr record of the sequence at Speeton, Yorkshire, UK.
Table 1Isotopic and chemical data for belemnites from the Berriasian-to-Barremian strata of the Yorkshire coast, UK
Stage Sample No. Level Zone Bed No. Specimen type 87Sr/86Sr N13C N
18O Ca Mg Sr Na Fe Mn
(m) Means + 3 n (x) (x) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)
Barremian Top B 115.0 P. bidentatum Hibolites sp. 0.707428 1 0.16 30.1 39.9 1865 1105 1385 13 3Barremian 1681 84.9 P. denckmanni Bed 45 Oxyteuthis brunsvicensis 0.707478 5 5 2 0.45 30.17 38.8 1835 985 1260 141 9Barremian xxvi mitte 83.4 P. denckmanni Middle Bed 47 Aulacoteuthis descendens 0.707482 4 33 3 0.82 30.58 39.9 1110 1060 671 8 4
82.90 Base denkmanniBarremian 1372 82.0 P. elegans Middle Bed 49 Aulacoteuthis descendens 0.707484 3 6 4 2.29 30.84 39.2 1150 1025 662 4 4Barremian 1111 79.1 P. elegans Middle Bed LB1A Aulacoteuthis descendens 0.707493 2 2 2 1.66 30.47 39.9 911 1035 548 4 4Barremian 1116 78.5 P. elegans LB1A 0.4 m up from base Aulacoteuthis descendens 0.707482 1 0.64 31.76 39.1 1590 1135 1325 130 5Barremian 1157 78.5 P. elegans LB1A 0.4 m up from base Aulacoteuthis descendens 0.707487 1 1.31 30.36 40.0 1030 948 555 141 5Barremian 1130 78.5 P. elegans LB1A 0.4 m up from base Aulacoteuthis descendens 0.707488 1 0.60 31.63 38.9 1585 1230 996 10 3Barremian 1091 78.5 P. elegans LB1A 0.4 m up from base Aulacoteuthis descendens 0.707483 1 1 2 1.45 30.59 39.0 1195 1060 763 133 4Barremian LB1A large 78.2 P. elegans LB1A base Aulocateuthis sp. 0.707480 3 3 2 1.53 30.44 39.5 1140 1020 470 18 4Barremian LB1A small 78.2 P. elegans LB1A base Aulocateuthis sp. 0.707486 1 1.60 30.21 40.8 1390 1000 522 10 5Barremian LB1A base 78.2 P. elegans LB1A base 0.707484 1 0.40 30.33 39.7 1770 959 1170 10 16
78.20 Base elegansBarremian 2001/5 77.6 P. ¢ssicostatum LB1B, 1.7 m above base Bed LB1D Aulocoteuthis compressa
(Mutterlose)1.23 31.26 40.3 1150 1090 645 7 4
Barremian 2001/4 77.5 P. ¢ssicostatum LB1B, 1.7 m above base Bed LB1D Aulacoteuthis sp. 0.61 31.20 38.4 1245 1025 716 5 5Barremian 2001/3 77.5 P. ¢ssicostatum LB1B, 1.7 m above base Bed LB1D Aulocoteuthis descendens
Valanginian 2001/11 7.8 paratollia D4C, 1.40 m above base Acroteuthis (A) explanoides(Pav)
0.04 30.49 39.0 530 1270 848 16 5
Valanginian 2001/10 7.8 paratollia D4C, 1.35 m above base Acroteuthis (A) explanoides(Pav)
30.74 30.35 40.0 749 1060 913 77 6
Valanginian 2001/9 7.6 paratollia D4C, 1.20 m above base Acroteuthis (A) cf. explanoides 30.04 0.06 40.2 585 991 645 0 3Valanginian 2001/8 7.5 paratollia D4C, 1.10 m above base A. (A) subquadratoides (Swin) 30.20 30.10 39.8 967 1340 1110 16 4Valanginian 2001/7 6.9 paratollia D4C, 0.5 m above base Acroteuthis (A) explanoides
(Pav)30.71 30.63 38.0 1010 1200 745 99 11
Valanginian 2001/6 6.7 paratollia D4C, 0.3 m above base Acroteuthis (A) explanoides(Pav)
0.12 30.20 39.5 432 1220 777 58 10
Valanginian 2001/15 6.4 paratollia Base D4C A. (A) kemperi (Pickney) 30.53 30.59 40.0 594 1255 637 5 5Valanginian 2001/14 6.3 paratollia 0.15 m below base D4C Acroteuthis (A) cf. explanoides 30.14 0.00 39.8 632 1155 859 135 15Base Valanginian 6.10 Base paratolia Base D4DBerriasian SB2 3.1 P. albidum D6A. Middle Not known 0.707265 5 3 5 30.63 0.73 39.2 566 1050 690 7 7Berriasian SB1 1.5 P. albidum D7A. Middle Not known 0.707264 5 9 4 30.38 0.65 39.5 758 1140 721 7 7
Stratigraphic levels are in metres from the base of Bed E and are from Rawson and Mutterlose (1983) and Rawson (unpublished data). Analytical uncertainties giv-en for 87Sr/86Sr are the maximum and minimum deviations from the mean of n replicates, or ; 0.000015 for singlet analysis.
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this occur in the low and top C beds (Table 1;Hibolites beds; Rawson, 1973; Mutterlose et al.,1987; Mitchell, 1992). While Neohibolites is the¢rst belemnite with a truly cosmopolitan distribu-tion, the opportunistic taxon Hibolites periodi-cally migrated northwards during the late Valan-ginian and Hauterivian. This eurythermal genus(Mutterlose, 1988) underwent an endemic evolu-tion in the Boreal sea. Out of the Tethyan stock, agroup evolved with small guards that forms acontinuous lineage from the Endemoceras regaleammonite Zone (late early Hauterivian) to theLate Barremian (Mutterlose, 1988).
2.2. Palaeogeography
During the Early Cretaceous, Speeton was lo-cated on the southwestern margin of the SouthernNorth Sea basin at a palaeo-latitude of about 40^45‡N (Fig. 1; also, see Ziegler, 1982; Knox, 1991;Rawson, 1992). The area known now as north-west Europe formed the southernmost extensionof the Boreal^Arctic sea, with seaways extendingtowards the Tethys in the south (Ziegler, 1982;Mutterlose, 1992). As the area lay at the marginof the Boreal Realm (Fig. 1), it was in£uenced byin£uxes of nanno£ora and fauna from bothrealms (Rawson, 1973; Mutterlose, 1992). ForEarly Cretaceous times, the strong separation ofthe Boreal and Tethyan Realms has been ascribedto a physical division resulting from the disposi-tion of continents coupled to changes in sea leveland the deepening of connecting seaways (Fig. 1;Doyle, 1987; Haq et al., 1987; Ru¡ell, l991; Mut-terlose, 1992) and, alternatively, to pronounceddi¡erences in temperature between these provin-ces (e.g. Rawson, 1973; Stevens, 1973). Attemptsto resolve these con£icting views have, in part,resorted to classical interpretations of stableisotopic palaeo-temperatures (e.g. Price et al.,2000) and carbon isotopic considerations (Weis-sert and Lini, 1991), which suggest that seasonallycold ocean temperatures, and limited polar ice,existed during the Early Cretaceous (but see alsoBennet and Doyle, 1996). Such interpretationsrely on the assumption that biogenic calcite (in-cluding that of belemnites) was precipitated inequilibrium with seawater and that its isotopic
composition can be interpreted in terms of oceantemperature.
3. Samples, analytical methods and results
Our samples comprise 26 of the belemnites re-ported in Price et al. (2000) with 84 additionalbelemnites collected at Speeton by the authors.The stratigraphic levels of samples are given inTable 1, along with compositional data and iden-ti¢cations mostly to the genus level. No sampleswere obtained from the upper 80% of the speeto-nensis Zone as this unit is very poorly exposed.We sampled 85 m of section, spanning mostly theHauterivian to lower Barremian interval, but in-cluding small slices of Valanginian sediment at thebase of the section. This thickness of sedimentrepresents a depositional period of about 7 myr(timescale of Gradstein et al., 1995) at an overallsedimentation rate of 11 m per myr in the Hau-terivian/Barremian. Given that our sample posi-tioning is certain to no better than about 10 cm,and is sometimes much worse, our temporal res-olution in sampling is no better than ; severalthousand years.
Belemnites were prepared for analysis by usingdiamond cutting tools to remove the apex, exteri-ors, apical line, and alveolus. The remains werefragmented (sub-mm), cleaned in 1.2 M hydro-chloric acid, washed in ultra-pure water, anddried in a clean environment. Fragments werepicked under the binocular microscope to securethose judged to be best preserved, ground to apowder, and analysed for 87Sr/86Sr, N
18O, N13C,
Ca, Mg, Na, Sr, Ba, Fe, Mn, and Rb. The resultsare in Table 1.
For chemical analysis, subsamples were dis-solved in 1.2 M hydrochloric acid. Concentrationsof Rb were measured by furnace-AAS; other ele-ments were analysed with ICP-AES. The precisionof the analysis was better than ; 5%. For 87Sr/86Sr analysis, subsamples were dissolved in ultra-pure 6 M nitric acid, evaporated to dryness inorder to oxidise organic matter, and convertedto chloride salt by subsequent evaporation to dry-ness with ultra-pure 6 M hydrochloric acid. Sam-ples were then taken up in 2.5 M hydrochloric
acid and Sr was separated by standard methods ofion-exchange chromatography. Values of 87Sr/86Srwere determined with a VG-354 ¢ve-collectormass spectrometer using the multi-dynamic rou-tine SRSQ that includes corrections for isobaricinterference from 87Rb (Thirlwall, 1991). Datahave been normalised to a value of 0.1194 for86Sr/88Sr and a value of 0.710248 for NIST 987,which equals a value of 0.709175 for EN-1 in ourlaboratory. Based upon replicated analysis ofNIST 987, the precision of our measurements(2 S.E.M.) was ; 15U1036 (n=1), ; 11 (n=2),; 9 (n=3) and ; 8 (n=4). In practice, the uncer-tainty on replicate measurements from the sam-ple-picking stage was generally better than these¢gures. Mean 87Sr/86Sr are reported in Table 1,together with the maximum and minimum devia-tions from the mean of replicate analyses and thenumber, n, of replicates for each sample. Totalblanks were 6 0.2 ng of Sr and subsample con-tained around 5 Wg of Sr. Concentrations of Rbwere too low to require correction for radiogenic87Sr. Data for N13C and N
18O were provided by H.Erlenkeuser of Kiel University, with additionaldata being taken from Price et al. (2000) withcorrections. For isotopic data, analytical precisionwas 0.1x for both N
13C and N18O with respect to
repeat analysis of NBS-19. The results of thechemical and isotopic analyses are given in Ta-ble 1.
4. Discussion
4.1. Sample preservation
Cathodoluminescence, trace element composi-tion, examination in thin section and by SEM(Veizer, 1974, 1983; Saelen and Karstang, 1989;Jones et al., 1994; McArthur, 1994; Podlaha etal., 1998) and discordancy of 87Sr/86Sr are usefulfor identifying alteration in samples. In particular,di¡erent (sub)samples from a level that give thesame 87Sr/86Sr are likely to be recording an orig-inal 87Sr/86Sr value (Jones et al., 1994; McArthur,1994) and so the original compositions for otherparameters both elemental and isotopic. Likewise,a well-de¢ned trend in 87Sr/86Sr through a section
is indicative of good sample preservation (Fig. 1).Our samples meet these tests of preservation: theyare visually well-preserved, have isotopic and ele-ment compositions that replicate well, have lowconcentrations of Fe and Mn (Table 1), andhave concentrations of Na, Sr, and Mg, typicalof well-preserved belemnites (Saelen and Kar-stang, 1989; McArthur et al., 2000; van deSchootbrugge et al., 2000), so we take it thatour samples retain their original elemental andisotopic compositions.
4.2. Isotopic trends in 87Sr/86Sr
The 87Sr/86Sr of samples is plotted in Figs. 2and 3 against biostratigraphy and stratigraphiclevel. Samples from the gottschei Zone could notbe positioned accurately within the zone, so theyare plotted at its mid-point. The data group intothree segments. The 87Sr/86Sr value of the lower-most two samples (SB1, SB2 mean 0.707265) in-dicates a late Berriasian (Ryazanian) age of 138.3Ma +1.3/30.7 (numerical ages from McArthur etal., 2001). An interval without preserved marinefaunas separates the lowermost two samples fromSB3 (Table 1) which has an 87Sr/86Sr age that isearliest Valanginian (136.5 ; 0.4 Ma). Above thislevel, between 8.2 m (in D4A) and 11.5 m (top ofD2E), which includes the uppermost Paratolliabeds and the Polyptichites beds, 18 samples havean invariant 87Sr/86Sr of 0.707338; 0.000002(2 s.e., n=38), which gives an 87Sr/86Sr age of135.3 ; 0.3 Ma, suggesting a hiatus of 1.3 ; 0.5Ma, or an extremely condensed section, occurswithin beds D4A-C. This 87Sr/86Sr value corre-lates to the Valanginian Karakaschiceras biassa-lensis SZ (McArthur and Janssen, in preparation),and the invariance of 87Sr/86Sr between 8.2 and11.5 m constrains the duration of this intervalto 9 0.2 myr, since marine 87Sr/86Sr increasedby about +0.000020 per myr during Valanginiantime (Jones et al., 1994; McArthur et al., 2001;Price and Grocke, 2002).
A hiatus (Rawson, 1971; Fig. 2) separates theValanginian Bed D2E from the overlying Hauteri-vian Bed D2D (base of the amblygonium Zone).Across this boundary, an increase of 87Sr/86Sr in0.000042 (Fig. 3) represents a gap of some 2.0 myr
(McArthur et al., 2001). The base of the Hauteri-vian, at 11.5 m, has an 87Sr/86Sr of 0.707380;0.000003 and a rate of change of +0.000008 permetre. Upsection, the rate declines to e¡ectivelyzero within the gottschei Zone, steepens markedlythrough the uppermost Hauterivian marginatusZone and lowermost Barremian variabilis Zone,and £attens through the rarocinctum Zone (closeto 0.70748) and most of the ¢ssicostatum Zone.Thereafter, it increases through the upper third ofthe latter and the lower elegans Zone, and peaksin the middle elegans Zone (0.707493; 0.000004),after which it decreases. We have one samplefrom the topmost Barremian (P. bidentatumZone) at Speeton (Table 1) which has an 87Sr/86Sr of 0.707430; thus, 87Sr/86Sr continues to de-cline above the denckmanni Zone.
4.3. A link between 87Sr/86Sr and Ontong JavaVolcanism?
Larson and Erba (1999) attribute the decline in87Sr/86Sr from the Late Barremian to the late Ap-tian to the e¡ects of volcanism between about 125and 120 Ma that was associated with the emplace-ment of the basalts of the OJP. Our data showthat marine 87Sr/86Sr peaked in the mid-elegansammonite Zone which, according to the schemegiven in Bown (1998), is about one-third of ChronCM3 from its top. This is early in Boreal nanno-fossil Zone BC15, and is in the lower part ofTethyan nannofossil Zone CC6. This stratigraphicplacement has a numerical age of around 124^125Ma (on the revised timescale of Larson and Erba,1999), an age range that is coincident with thatgiven for the onset of OJP volcanism by Larsonand Erba (1999, especially their ¢gure 7) aftertheir reinterpretation of numerical ages and bio-stratigraphy of materials associated with the for-mation of the OJP. A 3.5 km thick sequence ofvolcanic lavas on Malaita gives a similar meanage of around 125; 2 (Tejada et al., 2002) afterrecalculation to FCT=28.15 Ma and Mnhb-1 of523.0 used in Larsen and Erba (1999), although itis not clear what part of the volcanic stratigraphyof the OJP they represent.
Citing Bralower et al. (1997), Larson and Erba(1999) state that the decline in 87Sr/86Sr started
Fig. 3. Detailed 87Sr/86Sr record of the sequence at Speeton,Yorkshire, UK. (a) Lower Barremian to Hauterivian,(b) Hauterivian, (c) lowest Hauterivian, Valanginian and Ber-riasian. Uncertainty on 87Sr/86Sr is better than ;0.000015for a single analysis. Data for 87Sr/86Sr are reported to NIST987 of 0.710248 and EN-1 of 0.709175. Data of Jones et al.(1994) are normalised to NIST 987 by addition of 0.000022.Note the hiatus at 11.5 m between Valanginian strata andthe overlying Hauterivian strata.
0.5^1 myr after the level of magnetic reversal M0.Our data show that the decline started in theupper part of M3, some 3 myr before that time,and that it coincides with the time of eruption ofthe ¢rst sea£oor basalts (so far known) on theOJP (Larson and Erba, 1999; Tejada et al.,2002). As hydrothermal circulation would havecommenced before the onset of sea£oor eruptionof basalt, i.e. as the basaltic source approachedthe sea£oor, our demonstration that the declinein 87Sr/86Sr started earlier than proposed by Lar-son and Erba (1999) ¢ts better with their proposalthan did their age estimated for the onset of de-cline in 87Sr/86Sr. Nevertheless, the magnitude andtiming of any hydrothermal £ux associated withthe rising OJP magmas are matters of speculation.Furthermore, the onset of an increased hydrother-mal in£ux to the oceans would be bu¡ered by theexisting mass of oceanic Sr, so 87Sr/86Sr would beunlikely to change before at least 1 myr hadpassed (Richter and Turekian, 1993). More inter-estingly, marine 87Sr/86Sr continued to decline forsome 3^5 myr after the cessation of the ¢rst phaseof OJP volcanism (Bralower et al., 1997) to aminimum around 115 Ma (the shortness of whichis possibly an artefact of age calibration). If theonset of OJP volcanism made marine 87Sr/86Srdecline, its cessation (or substantial diminution)would have arrested that decline. The fact thatit did not might suggest that OJP volcanism wasjust one global event of many (e.g. changes in
global MOR production), that were themselvesthe main driver for changes in marine 87Sr/86Srduring the Cretaceous.
4.4. Relative duration of ammonite biozones
The 87Sr/86Sr pro¢le through the Hauterivianand Barremian at Speeton does not change line-arly with stratigraphic height (Figs. 2 and 3): 87Sr/86Sr changes more rapidly through ammonitezones that are lithologically thin, than it doesthrough zones that are thick. For example, inthe three thickest zones (speetonensis, gottschei,¢ssicostatum) 87Sr/86Sr increases little, whilst theincrease is more rapid in the variabilis Zone, pos-sibly through the underlying marginatus Zone,and through the noricum^amblygonium zones, allof which are thin in comparison. These variablerates of change in 87Sr/86Sr with stratigraphic levelresult from changing sedimentation rates throughthe sequence (cf. McArthur et al., 2000). Stronti-um isotope pro¢les through Hauterivian strata ofthe Vocontian basin, southeastern France (van deSchootbrugge, in review; McArthur et al., unpub-lished data) show that marine 87Sr/86Sr increasedlinearly with stratigraphic position, and so withtime. We use this assumption of linearity to com-pute the zone thicknesses assuming a constantrate of sedimentation (and so rate of changewith time of 87Sr/86Sr). The results are shown inTable 2 and reveal that the recalculated relative
Table 2Relative durations of ammonite biozones for Hauterivian and early Barremian time, calculated assuming a linear rate of changeof 87Sr/86Sr in seawater through the interval (see text for justi¢cation)
Zone base 87Sr/86Sr of zone base and2 s.e. uncertainty
Thickness (m)
Measured to 87Sr/86Sr
elegans 0.707485 (3)¢ssicostatum 0.707476 (3) 15.8 5.6rarocinctum 0.707474 (3) 8.1 1.2variabilis and the base of the Barremian 0.707461 (3) 1.5 8.1marginatus 0.707454 (3) 2.2 4.4gottschei 0.707450 (3) 11.9 2.5speetonensis 0.707434 (3) 12.8 10inversum 0.707424 (2) 2.9 6.2regale 0.707388 (4) 10.7 22.5noricum^amblygonium and the base of the Hauterivian 0.707380 (4) 0.9 5
Measured thickness 66.7 m, change in 87Sr/86Sr 0.000105.
thicknesses, and so relative durations, of Hauteri-vian and Barremian ammonite zones di¡er by fac-tors up to 18. The measured thicknesses do notre£ect the relative durations of the zones. Missingstrata within the Valanginian sequence at Speetonpreclude the use of 87Sr/86Sr pro¢les in this way inthat interval.
4.5. Belemnite compositions
The N18O values of belemnites have been used
widely to deduce ocean palaeo-temperatures(Lowenstam and Epstein, 1954; Hudson and An-derson, 1989; Anderson et al., 1994; Saelen et al.,1996; Ditch¢eld, 1997; Price and Sellwood, 1997;Podlaha et al., 1998; Price et al., 2000; van deSchootbrugge et al., 2000; Rosales et al., 2001;Niebuhr and Joachimski, 2002), as have Mg/Ca,and Sr/Ca values (Berlin et al., 1967; Yasamanov,1981). If belemnite-N18O is a valid palaeo-climateproxy that re£ects both ice volume and temper-ature, and if the Sr/Ca and Mg/Ca values of be-lemnites are temperature dependent (Berlin et al.,1967; Yasamanov, 1981), as is the case for manymodern calcifying groups (foraminifera, ostra-cods, molluscs; Chave, 1954; Nu«rnberg et al.,1996; Rosenthal et al., 1997; Mashiotta et al.,1999; Lea et al., 1999; Lear et al., 2000; Elder-¢eld and Ganssen, 2000; Bailey et al., 2003), thecombination of both should allow us to test forthe existence of signi¢cant ice volume during theEarly Cretaceous (cf. the approach of Lear et al.,2000 using foraminifera). In view of this, it isworthwhile trying to establish which groups ofbelemnites, if any, do faithfully record palaeo-oceanographic conditions, and which do not. Tohelp this quest, we show elemental and isotopiccompositions of belemnites from Speeton as crossplots in Fig. 4 and against stratigraphic level inFigs. 5 and 6.
Fig. 4. Cross plots of element concentrations for Acroteuthis,Aulacoteuthis, and Hibolites. The co-variation of Na withMg is good in Aulacoteuthis (open circles), less good and ofa steeper slope in Hibolites (triangles) and poor in Acroteu-this (¢lled circles are Valanginian specimens, diamonds areHauterivian specimens). In Aulacoteuthis and Acroteuthis, Srco-varies closely with Na and Mg, but in Hibolites, it co-varies with neither.
18O values ofbelemnites to calculate palaeo-temperatures is be-set by problems viz. salinity e¡ects, biologicalfractionation, diagenetic alteration, and uncer-tainty regarding the isotopic composition of theocean, which are readily acknowledged by mostauthors. The problem introduced by diageneticalteration was highlighted by Longinelli (1969),Spaeth et al. (1971) and Veizer (1974); methodshave been developed to identify, and so avoid, it(Veizer, 1983; McArthur, 1994; Podlaha et al.,1998). The problem of ice volume is commonlyresolved in the Mesozoic by assuming an ice-freeworld and so an ocean composition of around31x (SMOW, which is 31.2% PDB): it wouldbe solved were we able to develop a robust Mg/Cathermometer as such a thermometer would be in-dependent of ice volume. Attempts to address thesalinity issue have sometimes appealed to palaeo-climate modelling (e.g. Price and Sellwood, 1997).The problem posed by the possible operation of
non-equilibrium fractionation during calci¢cation(e.g. McConnaughey, 1989a,b) has proven intract-able. The elemental and isotopic compositions ofmany calcite-secreting groups, e.g. molluscs(Mitchell et al., 1994), the rudist bivalve Torreites(but not other rudists ; Steuber, 1999) and bra-chiopods (to some degree; Carpenter and Loh-mann, 1995; Curry and Fallick, 2002), suggestthat biological fractionation occurs during calci¢-cation so, by analogy, it probably a¡ects belem-nite compositions and confounds some of theirrecords of ambient conditions. Some authorshave attempted to identify non-equilibrium frac-tionation by comparing di¡erent belemnite genera(e.g. Ditch¢eld, 1997; Price and Sellwood, 1997),or minimise it by using one species alone (Nie-buhr and Joachimski, 2002), whilst others notethat well-preserved biogenic carbonate from dif-ferent groups (ammonites, bivalves, belemnites)gives di¡erent palaeo-temperatures (Anderson etal., 1994). Given the above, the observation that
Fig. 5. Variation of element and isotopic compositions of belemnite calcite with stratigraphic height through the Valanginian,Hauterivian and Barremian sequence at Speeton, Yorkshire. Specimens of the belemnite genus Hibolites contain higher contentsof Mg and Na, and heavier values, by some 0.4x in N
18O, than do others. Of the trace element concentrations, those of Srchange least through the section. Symbols as in Fig. 4.
belemnite palaeo-temperature trends through timeare noisy (Podlaha et al., 1998) is no surprise,especially when the possible e¡ect on N
18O ofshort-term climate change is also considered(ibid).
In our section, specimens of Acroteuthis of earlyHauterivian and earliest Barremian age are iso-topically lighter in N
18O by about 0.4x (on aver-age) than are specimens of Hibolites of equivalentage (Figs. 5 and 6). The di¡erence in N
18O con-trasts with that between Hibolites (mean N
18O+0.6 ; 0.4; 2 s.d., n=14) and Acroteuthis (mean+1.2 ; 0.8; 2 s.d., n=5), from the Early Creta-ceous Tordenskjoldberget Member of the Kong-sYya Formation of Svalbard, that was recordedby Ditch¢eld (1997). Furthermore, specimens ofHibolites and Belemnopsis, of Late Jurassic age(Price and Sellwood, 1997), have values of N
18Othat di¡er by only 0.17x. The di¡erences (ifreal) might be due to di¡erences in metabolic frac-tionation, to slight di¡erences in the depth of hab-itat, given that 0.4^0.6x represents only about1.6^2‡C in temperature, to di¡ering salinity ineach area, or to short-term climate change, butit is di⁄cult to reconcile the opposite sense ofthe di¡erences between Acroteuthis and Hibolitesin Speeton and Svalbard with any of these alter-natives. Considering climate change, the co-occur-
rences of Hibolites and Acroteuthis at Speeton areseparated by between 20 and 50 cm in this slowlyaccumulating section (stratigraphic placement isnot available for Svalbard), so they are separatedin time by between 18 000 and 45 000 years. Thistime span is ample for climate, and so local oceantemperature, to change. Nevertheless, it requiresthat the Boreally derived (northern) genus Acro-teuthis preferred warmer water than did theTethyan-derived (southern) genus Hibolites.
Whatever the real reason for the di¡erencesnoted above, we note that they are small. Wealso note that between 53.4 and 55.5 m in thesection (Table 1), the mean N
18O of three speci-mens of P. jasiko¢ana (0.38x), four of Hibolites(0.2x) and two of Acroteuthis (0.10x) are veryclose (Table 1, Fig. 4), a fact that suggests theo¡sets seen lower in the section are caused, atleast in part, by real temperature di¡erences,rather than metabolic e¡ects. In summary, thefact that several genera of belemnites at Speetongive N
18O values that are within 0.4x of eachother suggests that the N
18O values of our belem-nites are recording at least the major trend in pa-laeo-temperature through our section, and may bere£ecting more subtle changes as well.
What are those trends? The N18O values of be-
lemnite calcite are around 30.2 ; 0.4x in theValanginian, where specimens are exclusively at-tributed to the genus Acroteuthis. At the Hauteri-vian/Valanginian boundary (at 11.5 m), which ismarked by a time gap of 2.0 myr (see earlier sec-tions), values jump to around +0.4x (for Acro-teuthis) and to around +0.8x (for Hibolites).They become more negative upsection to arounda level of about 19^22 m (middle of the regaleZone), where these genera record values of31.02x (Acroteuthis) and 30.27x (Hibolites)respectively. At 24 m (lower inversum Zone),specimens of Hibolites again record values around+0.3x. Between 11.5 and 24 m, 87Sr/86Sr in-creases by 0.000044, a change that represents aperiod of close to 2 myr. In belemnites of Barre-mian age, values become lighter upsection andreach 31.76x in the Barremian elegans Zone(ignoring one outlier at 65 m in the basal ¢ssico-statum Zone), immediately after which valuesbrie£y and abruptly lighten to around 30.2 to
Fig. 6. Variation of N18O with stratigraphic height in the lowerHauterivian. From the base of the Hauterivian at 11.5 m,N18O in both Acroteuthis and Hibolites becomes more nega-tive upsection, whilst Hibolites de¢nes a return to originalvalues of N
18O by 24 m after peaking around 19^22 m. Theinterval 11.5^24 m represents about 2 myr (based on thechange in 87Sr/86Sr) and the N
18O record shows a pronouncedwarming to 17‡C and return to cool temperature (10‡C) dur-ing that time period. Symbols as in Fig. 4.
30.6x. The implications of these data for pa-laeo-temperatures through the section are dis-cussed later.
Trace elements : the contents of Mg, Na, and Srin our belemnites show a large range but, in speci-mens of Acroteuthis and Aulacoteuthis, all threeelements correlate positively, albeit with varyingdegrees of closeness, a co-variance shown manytimes before for biogenic carbonate. In contrast,
in the genus Hibolites, no co-variance is seen be-tween Sr and Mg, a weak co-variance may bepresent between Na and Sr, whilst the co-variancebetween Mg and Na is su⁄ciently convincing tobe thought real. Concentrations of both Mg andNa are higher in specimens of Hibolites, which aremostly of Hauterivian age, than in other genera(Figs. 4 and 5) which are (mostly) of Valanginianor Barremian age. On the three trace elements,
Fig. 7. Relation of Mg to N18O and N
13C in the belemnite genera Aulacoteuthis, Acroteuthis, and Hibolites. In Aulacoteuthis, Mgconcentrations increase as N
18O becomes more negative, trends that are compatible with increasing temperature of ambient sea-water. This trend is more poorly de¢ned for Acroteuthis and absent in Hibolites. Assuming equilibrium compositions for calcite,the trends de¢ne temperature dependencies for Mg concentrations in Aulacoteuthis and Acroteuthis given in Table 3. Symbols asin Fig. 4.
concentrations of Sr show the least change withstratigraphic level.
Concentrations of Mg in modern biogenic cal-cite increase with increasing temperature by be-tween 5% and 10% per degree (Nu«rnberg et al.,1996; Rosenthal et al., 1997; Mashiotta et al.,1999; Lea et al., 1999; Lear et al., 2000; Elder-¢eld and Ganssen, 2000; Bailey et al., 2003), ormore if older work is considered (Chave, 1954), soour Mg trend with stratigraphic level (Fig. 5)might be interpreted as showing that the Hauteri-vian sea at Speeton was warmer in those timesthan it was in Valanginian or Barremian times.Comparison of the Mg (and Na) contents ofspecimens of Acroteuthis (Fig. 5) with equiva-lent-age specimens of other belemnites shows,however, that the di¡erences must be due simplyto di¡erential metabolic fractionation; Hibolitessimply incorporates more Mg and Na into its car-bonate than do the other genera (Figs. 4 and 5). Itis clear, therefore, that di¡erent belemnite generahave di¡erent temperature sensitivities to incorpo-ration of trace elements. Can we calibrate any orall of them for use in palaeo-temperature analysisand ice volume calculations? In Fig. 7 we showthe co-variance of Mg with N
18O in the three gen-era of belemnites of which we have many speci-mens (similar co-variance occurs for Sr and Nawith N
18O, which are not shown). In those of Au-lacoteuthis and Acroteuthis, concentrations of Mgincrease as N
18O becomes more negative, trendscompatible with both being related to tempera-ture, whilst in specimens of Hibolites no relationis seen. These trends are similar to those found inToarcian belemnites (McArthur et al., 2000; Bai-ley et al., 2003). Although our trace element dataare few and scatter somewhat, by assuming equi-librium precipitation of oxygen isotopes (or a con-stant o¡set from equilibrium) we deduce the trace
element/temperature dependencies for Aulacoteu-this and Acroteuthis given in Table 3. These valuesare similar to those found for modern biogeniccalcite. No temperature coe⁄cients are given forHibolites as N
18O in this genus does not correlatewith Na, Sr, or Mg.
The co-variance we show between Mg and N18O
might be interpreted as resulting from kineticfractionation, with faster calcite deposition lead-ing to isotopically lighter carbon and oxygen, andalso to more structural disorder in calcite so thatmore trace elements are incorporated at sites ofstructural defectiveness. Some evidence that thismight be so comes from the surprisingly goodrelation between N
13C and Mg in Aulacoteuthis(Fig. 7d), although the co-variance is eitherweak or absent in Valanginian specimens of Acro-teuthis (Fig. 7e) and specimens of Hibolites (Fig.7f). Were non-equilibrium fractionation a¡ectingour belemnites, however, a strong positive co-var-iance would be expected between N
18O and N13C.
In Aulacoteuthis, a (weak) relation occurs only iftwo outlying data (arrowed in Fig. 8a) are in-cluded in the consideration of co-variance, whichdoes not argue convincingly for the operation ofbiogenic fractionation in this genus. In specimensof Acroteuthis (Fig. 8b), there is no correlationbetween N
18O and N13C if all data are included,
none amongst the subpopulation that are ofHauterivian and Barremian age, and only aweak correlation amongst Valanginian belemnitesand one that is dependent for its presence onthree outliers (arrowed). Again, we feel that evi-dence for biogenic fractionation is too weak forits presence to be proven. The absence of anycorrelation between N
18O and N13C for Hibolites
is clear from Fig. 8c. We therefore attribute therelations seen in Fig. 7 between Mg concentra-tions and N
13C as being temperature e¡ects.
Table 3Coe⁄cients of temperature dependence of Sr, Na, and Mg concentrations in Acroteuthis and Aulacoteuthis, calculated from theslope of reduced major axis regressions to the N
18O and Mg data shown in Fig. 7
Genus Mg Sr Na Mg Sr Na(ppm/‡C) (ppm/‡C) (ppm/‡C) (%/‡C) (%/‡C) (%/‡C)
4.5.1. Palaeo-temperature implicationsThe abundance of ice-rafted debris decreases
from the Berriasian/Valanginian into the Hauteri-vian (Kemper, 1987; Frakes et al., 1992) and glen-donites also become rare from latest Valanginiantimes until the latest Aptian (Podlaha et al.,1998); these trends suggest a warming into theHauterivian. Such a warming is consistent withincreasing Mg concentrations in Acroteuthis, andthe lightening of its N
18O values through the no-ricum, amblygonium, and lower part of the regaleZone. It is also compatible with the lightening ofN18O in Hibolites through the same interval,although this increasing temperature is not re-£ected in its concentrations of Mg. In the upperpart of the regale Zone, the inversum Zone, andthe lower part of the speetonensis Zone, we haveonly the genus Hibolites to provide estimates ofconditions, but the small number of samples an-alysed suggests a decrease in temperature throughthese units to something approaching those at thebase of the Hauterivian (around 11‡C, assumingan ice-free world with N
18O of seawater of 31xSMOW). At higher levels in the Hauterivian, therecord is sparse and poorly constrained strati-graphically. For example, our samples from thegottschei Zone cannot be accurately positionedwithin it, so they are plotted at its mid-point ;the range of composition here, however, suggeststhat seawater temperature may have changed alittle through the zone, but had returned toaround 11‡C by earliest Barremian times. Fromthe base of the Barremian, temperature, as judgedfrom N
18O, increases upsection and reaches 20‡Cin the elegans Zone, before a sharp, and brief,excursion is seen to around 14‡C at 78^79 m:these isotopic trends are paralleled by trends inthe concentration of Mg in Barremian belemnites(Fig. 4), which are mostly of the genus Aulacoteu-this.
If our record of N18O (and Mg for some generain limited intervals) has any meaning, our dataare not in accordance with the arguments ofLini et al. (1992) that the Cretaceous ‘greenhouse’climate (having developed strongly in the Valan-ginian) continued through the Hauterivian andbeyond. Our trend does agree with the generalsense of the N
18O curve of Podlaha et al. (1998)
which, despite much scatter, shows temperatureswarming from the Hauterivian into the Barre-mian. At Speeton, an early Hauterivian increasein the temperature was soon reversed and notresumed until the beginning of the Barremian,through which a poorly de¢ned trend towardshigher temperatures culminates in the Barremian
Fig. 8. Relation of N18O to N13C in belemnites. The weak, or
absent, correlations between stable isotopic compositions ofcarbon and oxygen suggest that non-equilibrium (vital) e¡ectshave not in£uenced the isotopic compositions of these gen-era. Symbols as in Fig. 4.
elegans Zone. Whether the excursion to coolertemperatures, noted in the elegans Zone, is con-nected to OJP volcanism is something aboutwhich we have too few data to make speculationproductive. Finally, our data are too poorly con-strained temporally, and too few in number, tode¢ne trends in elemental and isotopic composi-tion that would allow us to infer, or calculate, icevolume from coupled elemental abundances andN18O records, but do suggest that more successmight be had using an appropriate genus of be-lemnite from more rapidly accumulating sedi-ments, where better temporal resolution mightbe found.
5. Conclusions
A pro¢le of 87Sr/86Sr in belemnite calcitethrough 85 m of Valanginian, Hauterivian, andBarremian strata at Speeton, Yorkshire, UK,identi¢es hiatuses in the sequences, allows quanti-¢cation of their duration, permits the duration ofammonite (and other) biozones to be determinedand shows that they di¡er by a factor of as muchas 18, and shows that the base of the Hauterivianat Speeton has an 87Sr/86Sr of 0.707380;0.000003. The timing of volcanism on the OJPcoincides with the point at which marine 87Sr/86Sr began a long-term decline from a maximumof 0.707493; 0.000004 in the Barremian elegansammonite Zone.
Elemental and stable isotopic compositions ofthe belemnite genera Hibolites, Aulacoteuthis andAcroteuthis suggest that their belemnite calciteprecipitated under conditions that yielded valuesof N18O within 0.4x of those expected for equi-librium conditions; specimens of Hibolites havevalues of N
18O o¡set from those of other generaby no more than 0.4x. Belemnites of the genusHibolites contain around 2000 ppm of Mg, abouttwice the concentrations in the genera Aulacoteu-this, Acroteuthis, Praeoxyteuthis, and Oxyteuthis.In specimens of Aulacoteuthis and Acroteuthis, thegood correlation between trace element contentand values of N
18O has allowed calculation ofthe (tentative) temperature dependencies of Mg,Na, and Sr concentrations of between 7 and
20%, depending on genus and element; the valuesare close to those found for modern biogenic car-bonate. With some re¢nement, such element/tem-perature relations may be used at palaeo-temper-ature proxies. The content of Na, Mg, and Sr, inspecimens of the genus Hibolites, shows no rela-tion to stable oxygen isotopic composition and sodoes not record palaeo-temperature. Future stud-ies of the trace element, and isotopic, compositionof belemnites should examine possible genera, orspecies, speci¢c e¡ects that might compromise pa-laeo-environmental interpretation.
Acknowledgements
The Radiogenic Isotope Laboratory at RHULis supported, in part, by the University of Londonas an intercollegiate facility. We thank ClintonRoberts and Sarah Houghton for assistance withthe Sr isotopic analysis, and Desmond Donovanfor useful discussions about belemnites and fordrawing to our attention many relevant referen-ces. The elemental analysis was done byJ.M.McA. using the NERC ICP-AES Facility atRHUL, with the permission of its Director, Dr.J.N. Walsh. We thank H. Erlenkeuser (Kiel, Ger-many) for providing stable isotopic data, PaulBown for the nannofossil zonations, and TimDenison and Jan Veizer for constructive reviews.This work was supported by NERC Grant NER/GS/2000/00598 to J.M.McA. and DFG grant Mu667/24 to J.M.
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