Belemnites of Valanginian, Hauterivian and Barremian Age: Sr-Isotope Stratigraphy, Composition (87Sr/86Sr, N13C, N18O, Na, Sr, Mg), and Palaeo-Oceanography

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Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2004) 253^272 www.elsevier.com/locate/palaeo

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;Ã, 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 21August 2002; received in revised form 31July 2003; accepted 5 September 2003

Abstract

We present new data on 87Sr/86Sr, N13C and N18O, and elemental compositions of belemnites from 85 m of Valanginian, Hauterivian and Barremian strata at Speeton, Yorkshire, eastern England. The 87Sr/86Sr data provide a global standard for 87Sr/86Sr isotopic dating, and correlation to the biostratigraphic schemes of NW Europe. Values of 87Sr/86Sr increase from 0.707380 þ 0.000003, at the base of the Hauterivian, to 0.707493 þ 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 87Sr/86Sr 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 87Sr/86Sr 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 N18O 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 of marine palaeo-temperatures. Specimens of the genus Hibolites have N18O 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 N18O 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 N18O and the content of Na, Sr, and Mg suggests that incorporation of these

* Corresponding author. E-mail address: [email protected] (J.M. McArthur).

PALAEO 3220 12-12-03 254 J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2004) 253^272 trace elements in these genera is largely controlled by temperature. The dependency of concentration on temperature ranges from 7 to 20% per degree Celsius, if equilibrium fractionation of oxygen isotopic composition is assumed, so the Mg, Na and Sr content of these genera may be used as palaeo-temperature proxies. The trace element content of Hibolites shows no relation to stable oxygen isotopic composition and so does not record palaeo-temperature. ß 2003 Elsevier B.V. All rights reserved.

Keywords: belemnites; 87Sr/86Sr; Sr isotope stratigraphy; N18O; N13C; Boreal; Tethyan; Cretaceous; Hauterivian; Barremian

1. Introduction 1997; Price and Sellwood, 1997; Podlaha et al., 1998; Price et al., 2000; van de Schootbrugge et Interpreting Earth history for Early Cretaceous al., 2000; Rosalesd et al., 2001;(Niebuhr and times has proven a challenging task, but one that Joachimski, 2002). In view of this, we follow Pod- is of interest because of the change to ‘green- laha et al. (1998) in asking whether belemnite cal- house’ conditions in the early-to-mid-Cretaceous cite forms in equilibrium, or in disequilibrium, (Lini et al., 1992), when the northern hemisphere with its ambient water, and so whether such in- was divided into Boreal and Tethyan biotic terpretations are valid. We do so by examining Realms. Of particular interest is the degree to the chemical and isotopic composition of Early which the emplacement in Barremian/Aptian Cretaceous belemnites of both Boreal and times of the World’s largest igneous province, Tethyan a⁄nities that are proven to be well-pre- the Ontong Java Plateau (OJP), contributed to served, not least because they retain their original environmental change. There is some evidence values of 87Sr/86Sr. We show that our belemnites that it did have an impact on climate, biotic di- precipitated calcite under conditions that closely versity and ocean-wide patterns of sedimentation approximated isotopic equilibrium for oxygen, (Larson and Erba, 1999; references therein). and that the Na, Sr, and Mg contents of belem- These authors acknowledge the di⁄culty of link- nites hold promise as palaeo-temperature proxies. ing OJP volcanism to world events, but amongst the evidence that does so is the onset of the decline of marine 87Sr/86Sr in the Late Barremian, at 2. Geological setting about the time of onset of OJP volcanism (ibid). We have determined a precise record of 87Sr/86Sr 2.1. Stratigraphy and faunas through much of Barremian time to test the link between emplacement of the OJP and the 87Sr/ The Speeton Clay Formation is exposed at Fil- 86Sr record. We extend the record back through ey Bay, Speeton, in Yorkshire, northeastern Eng- Hauterivian time, and a fragment of Valanginian land (Fig. 1). The formation (Fig. 2) comprises time, in order to provide a standard curve for about 100 m of interbedded marine claystones dating and correlation with 87Sr/86Sr in this inter- and calcareous mudrocks of which we sampled val. Such a record will assist the integration of 85 m. The sediments rest unconformably on the Boreal and Tethyan histories which have been Kimmeridge Clay Formation which is of Volgian hampered by marked £oral and faunal di¡erences age (Rawson et al., 1978). The stratigraphical suc- between the realms, particularly during latest Ju- cession and biozonations shown in Fig. 2 are ac- rassic and earliest Cretaceous times. companied by a plot of 87Sr/86Sr in belemnites Also of interest is the fact that palaeo-environ- against their stratigraphic level in order to show mental interpretations, for this and other periods, in outline the distribution of our samples through often invoke palaeo-temperatures determined the section. The sequence has a number of strati- from the oxygen isotopic composition of belem- graphic breaks and condensed intervals, the most nite calcite (Hudson and Anderson, 1989; Ander- notable of which is at the base of the Hauterivian son et al., 1994; Saelen et al., 1996; Ditch¢eld, part of the section where the Upper Valanginian

PALAEO 3220 12-12-03 J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2004) 253^272 255

Fig. 1. Upper ¢gure shows the present location and geology of the Speeton area. Lower ¢gure shows the palaeo-geography of northwest Europe during Early Cretaceous times (adapted from Mutterlose, 1998). The locality at Speeton occupied a position proximal to both the Tethyan and Boreal Realms, with in£ux of Hibolites (Tethyan) belemnites from the south as sea level rose in early Hauterivian times.

PALAEO 3220 12-12-03 256 J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2004) 253^272

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. is absent (Fig. 2). Breaks and condensed intervals of their belemnite fauna. The A beds (Aptian^ are represented by phosphatic nodule beds, con- Albian) and the C beds (Hauterivian) are domi- centrations of belemnites, and intensely glauco- nated by characteristically Tethyan belemnites of nitic levels. The base of the Ryazanian (base of the genera Neohibolites (A beds) and Hibolites Bed E) is marked by a major transgression (Raw- (C beds). The B beds (Barremian; Rawson and son and Riley, 1982) which probably corresponds Mutterlose, 1983) and the D beds (upper Ryaza- to a maximum £ooding surface (Haq et al., 1987; nian ^ lowermost Hauterivian) are dominated by Ru¡ell, 1991). The early Hauterivian period was Boreal belemnites (e.g. Rawson, 1973; Mutter- marked by a series of minor regressive episodes lose, 1992) of the genera Acroteuthis (D beds), (Ru¡ell, 1991). Oxyteuthis, Praeoxyteuthis, and Aulacoteuthis The strata at Speeton were divided into four (B beds), with small Hibolites co-occurring in major units, beds A^D, labelled from the top the B beds. Overlap of forms is very limited: down by Lamplugh (1889) on the characteristics e.g. a few specimens of the Boreal genus Acroteu-

PALAEO 3220 12-12-03 Table 1 Isotopic 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 N18OCaMgSrNaFeMn

(m) Means + 3 n (x)(x) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

Barremian Top B 115.0 P. bidentatum Hibolites sp. 0.707428 10.16 30.1 39.9 1865 1105 1385 13 3

Barremian 1681 84.9 P. denckmanni Bed 45 Oxyteuthis brunsvicensis 0.707478 5 5 2 0.45 30.17 38.8 1835 985 1260 141 253^272 9 (2004) 202 Palaeoecology Palaeoclimatology, Palaeogeography, / al. et McArthur J.M. Barremian 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 denkmanni Barremian 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 4 Barremian 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 4 Barremian 1116 78.5 P. elegans LB1A 0.4 m up from base Aulacoteuthis descendens 0.707482 10.64 31.76 39.1 1590 1135 1325 130 5 Barremian 1157 78.5 P. elegans LB1A 0.4 m up from base Aulacoteuthis descendens 0.707487 11.31 30.36 40.0 1030 948 555 141 5 Barremian 1130 78.5 P. elegans LB1A 0.4 m up from base Aulacoteuthis descendens 0.707488 10.60 31.63 38.9 1585 1230 996 10 3 Barremian 1091 78.5 P. elegans LB1A 0.4 m up from base Aulacoteuthis descendens 0.707483 112 1.45 30.59 39.0 1195 1060 763 133 4 Barremian 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 4 Barremian LB1A small 78.2 P. elegans LB1A base Aulocateuthis sp. 0.707486 11.60 30.21 40.8 1390 1000 522 10 5 Barremian 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 elegans Barremian 2001/5 77.6 P. ¢ssicostatum LB1B, 1.7 m above base Bed LB1D Aulocoteuthis compressa 1.23 31.26 40.3 1150 1090 645 7 4 (Mutterlose) 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 5 AAO32 12-12-03 3220 PALAEO Barremian 2001/3 77.5 P. ¢ssicostatum LB1B, 1.7 m above base Bed LB1D Aulocoteuthis descendens 0.707483 10.36 31.61 41.1 1585 1175 1250 7 4 (Stolley) Barremian 2001/2 77.2 P. ¢ssicostatum LB1B, 1.7 m above base Bed LB1D Aulacoteuthis sp. ??? 30.16 31.36 39.3 1905 1295 1315 63 7 Barremian 2001/1 76.8 P. ¢ssicostatum LB1D, 0.9 m above base Aulacoteuthis sp. (juvenile) 0.707483 10.90 31.52 39.6 1235 1165 593 108 5 Barremian LB1D base 76.3 P. ¢ssicostatum LB1D base Aulacoteuthis sp. 0.75 31.51 40.5 1465 1230 867 12 4 Barremian LB1F 64 cm 75.2 P. ¢ssicostatum LB1F 1 cm from top Aulacoteuthis sp. 0.707482 4 4 2 1.15 31.64 39.6 1645 1280 1250 122 5 Barremian LB1F 1 cm B 74.6 P. ¢ssicostatum LB1F 1 cm up B Aulacoteuthis sp. 1.72 31.05 40.0 1120 985 593 54 7 Barremian LB1F 1 cm A 74.6 P. ¢ssicostatum LB1F 1 cm up A Aulacoteuthis sp. 0.707480 1 Barremian LB2A174.7 P. ¢ssicostatum LB2A110cm from top Aulacoteuthis sp. 0.707477 10.85 31.55 41.2 1485 1170 768 12 5 Barremian CB 190 73.2 P. ¢ssicostatum Bed LB2AI Aucacoteuthis absolutiformis 0.707479 6 5 3 1.04 30.85 39.8 1315 1080 936 6 3 Barremian LB2B 69.4 P. ¢ssicostatum LB2B Praeoxyteuthis sp. 0.707476 6 6 3 0.22 30.67 40.0 1350 1330 1270 15 7 Barremian 1605 68.1 P. ¢ssicostatum 30 cm above base LB2C.II Aulocoteuthis speetonensis 0.707479 9 7 3 0.64 30.51 40.4 1393 1055 816 16 4 Barremian 1603 68.0 P. ¢ssicostatum 20 cm above base LB2C.II Aulocoteuthis speetonensis 0.707479 5 4 3 0.63 30.08 38.9 1375 1130 842 8 4 Barremian XVIII Top/33 67.7 P. ¢ssicostatum Top of Bed LB3A Aulocoteuthis descendens 0.707474 112 2.65 0.00 40.2 835 937 552 4 4 Barremian 1213 65.1 P. ¢ssicostatum LB3A Praeoxyteuthis pugio 0.707475 10.94 30.25 39.5 990 1240 778 85 4 Barremian 1420 65.1 P. ¢ssicostatum LB3A Praeoxyteuthis pugio 0.70747110.26 32.12 41.4 1400 1190 805 92 5 Barremian 777 65.1 P. ¢ssicostatum LB3A Praeoxyteuthis pugio 0.70747110.93 30.70 40.0 1400 1310 913 83 4 Barremian 1261 65.1 P. ¢ssicostatum LB3A Praeoxyteuthis pugio 0.707480 2 2 3 0.63 30.11 39.5 1520 998 987 82 4 62.45 Base ¢ssicostatum Barremian CB76 61.8 P. rarocinctum LB3C Praeoxyteuthis pugio 0.707476 4 4 2 1.25 30.50 38.8 1370 1155 873 130 4 Barremian 1414 60.0 P. rarocinctum Bed LB4C Praeoxyteuthis pugio 0.7074818 6 6 30.29 0.10 40.4 1140 1210 642 193 18 Barremian 1655 55.5 P. rarocinctum LB5D Hibolites obtusirostris 0.707478 1 1 2 0.75 0.24 39.3 1750 995 1090 35 3 Barremian 1326 54.9 P. rarocinctum Bed LB5D Praeoxyteuthis jasiko¢ana 0.707475 1 1 2 1.84 0.49 39.6 922 1110 675 6 6 Barremian 1433 54.9 P. rarocinctum Bed LB5D Hibolites minmus 0.707478 7 6 3 0.70 0.13 38.7 3325 1230 2010 162 4 54.40 Base raraocinctum Barremian 1537 54.0 C. variabilis 10 cm above base Bed LB5E Praeoxyteuthis jasiko¢ana 0.707467 1 1 2 1.58 0.45 40.4 1030 1220 728 26 5 Barremian 1538 54.1 C. variabilis 5 cm above base Bed LB5E Praeoxyteuthis jasiko¢ana 0.707472 2 1 3 1.15 0.20 41.0 959 1120 635 18 6 Barremian 1219 53.6 C. variabilis Top of Bed C1 Hibolites jaculoides 0.707463 4 3 3 0.97 0.18 39.3 1730 1115 1375 8 4 Barremian 2003/1736 53.4 C. variabilis Bed C1A Acroteuthis rawsoni 0.97 0.03 38.9 1015 1195 647 61 8 Barremian 2003/1737 53.4 C. variabilis Bed C1A Acroteuthis rawsoni 2.32 0.17 39.4 935 1180 587 20 4 Barremian 1011 53.9 C. variabilis Base of Bed C1 Hibolites jaculoides 0.707472 1 1 2 0.75 0.26 38.6 1905 1310 1705 9 3 Base Barremian 51.85 Base variabilis Hauterivian C2D 51.8 S. marginatus Bed C2D Hibolites jaculoides 0.707452 1 0.81 0.68 40.0 1555 1165 1450 9 4 257 258 Table 1( Continued).

Stage Sample No. Level Zone Bed No. Specimen type 87Sr/86Sr N13C N18OCaMgSrNaFeMn

(m) Means + 3 n (x)(x) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

Hauterivian C2E 51.5 S. marginatus Bed C2E Hibolites jaculoides 0.707462 2 2 2 0.96 30.26 39.8 1285 1300 1595 11 4 Hauterivian 1294 50.9 S. marginatus 20 cm above base C2F Hibolites jaculoides 0.707456 10.45 30.02 39.7 1900 1185 1585 8 4

50.70 Base marginatus 253^272 (2004) 202 Palaeoecology Palaeoclimatology, Palaeogeography, / al. et McArthur J.M. Hauterivian CB2 (C4) 44.0 gottschei Bed C4 Hibolites jaculoides 0.707455 0 0 2 30.16 0.13 40.0 3030 1225 1805 0 0 Hauterivian CB3 (C4) 44.0 gottschei Bed C4 Hibolites jaculoides 0.707459 1.42 0.26 39.7 2205 1285 1790 180 6 Hauterivian CB4(C4) 44.0 gottschei Bed C4 Hibolites jaculoides 0.707452 4 4 2 1.05 0.74 39.6 1675 1145 1485 172 5 Hauterivian CB6(C4) 44.0 gottschei Bed C4 Hibolites jaculoides 0.43 0.42 39.7 1875 1065 1650 138 5 Hauterivian CB13(C4) 44.0 gottschei Bed C4 Hibolites jaculoides 0.70744110.02 30.33 39.0 2870 1070 1650 170 5 Hauterivian CB14(C4) 44.0 gottschei Bed C5 Not known 1.38 0.01 39.3 1860 1225 1450 138 4 Hauterivian CB15(C4) 44.0 gottschei Bed C4 Hibolites jaculoides 0.707451 8 8 1.55 0.58 38.2 1320 1225 1455 152 4 38.80 Base gottschei Hauterivian C6 28.3 S. speetonensis Bed C6 Hibolites jaculoides 0.707428 1 0.98 0.53 39.6 2135 1070 1525 0 9 26.00 Base speetonensis Hauterivian 2550 25.9 S. inversum 20 cm above base Bed C7A Hibolites sp. 0.707431 1 0.57 0.67 39.7 2490 1540 1915 48 5 Hauterivian C7E 24.7 S. inversum Top of Bed C7E Hibolites jaculoides 0.707426 3 3 2 1.54 0.53 40.7 2185 1145 1505 6 0 Hauterivian C7 24.3 S. inversum Base Bed C7E Hibolites jaculoides 0.707430 4 4 2 1.24 0.39 39.1 1260 1165 1600 19 3 Hauterivian 2367 23.7 S. inversum 25 cm above base Bed C7G Hibolites jaculoides 0.707430 8 6 3 0.79 0.63 38.6 1565 1155 1510 14 3 23.10 Base inversum AAO32 12-12-03 3220 PALAEO Hauterivian SB9 21.5 E. regale C8B. 80% up 0.707413 2 2 2 1.12 0.51 40.4 2135 1560 2060 46 8 Hauterivian 2200 22.5 E. regale 140 cm above base Bed C8 Hibolites sp. 0.707414 1.18 0.46 39.5 2080 1295 1555 18 4 Hauterivian 2188 22.3 E. regale 126 cm above base Bed C8 Hibolites jaculoides 0.707425 3 6 3 0.45 0.19 38.9 2170 1245 1930 6 3 Hauterivian 2169 22.1 E. regale 105 cm above base Bed C8 Hibolites jaculoides 0.707418 5 5 3 0.05 30.09 39.5 2310 1300 1670 15 15 Hauterivian 2144 21.9 E. regale 80 cm above base Bed C8 Hibolites jaculoides 0.707409 5 8 4 1.18 30.34 39.8 1990 1350 1620 21 4 Hauterivian 2042 21.5 E. regale 40 cm above base Bed C9A Hibolites sp. 0.707411 3 5 3 0.60 0.62 39.7 2510 1255 1580 19 6 Hauterivian C9/68 18.6 E. regale C9 Hibolites jaculoides 0.707421 3 3 2 1.23 0.59 38.3 2375 1115 1630 10 6 Hauterivian C 9/69 18.6 E. regale C9 Hibolites jaculoides 0.707425 1 1.49 0.09 39.6 2275 1200 1625 14 4 Hauterivian 1914 18.2 E. regale 175 cm above base Bed C9D Hibolites sp. 0.707405 0 0 2 1.60 30.27 39.0 2135 1215 1580 9 5 Hauterivian SB8 18.0 E. regale C9D. Middle 0.707406 2 2 3 3.21 30.66 39.1 1685 1275 1535 11 4 Hauterivian 2003/1733 18.0 E. regale C9D. Middle Acroteuthis sp. 1.26 31.02 38.5 1200 1315 882 9 6 Hauterivian 1871 17.8 E. regale 130 cm above base Bed C9D Hibolites sp. 0.707411 11 11 3 1.12 0.30 40.1 1690 1235 1475 10 5 Hauterivian 1796 17.0 E. regale 55 cm above base Bed C9D Hibolites sp. 0.707409 1 1.33 0.37 39.2 2010 1150 1495 151 20 Hauterivian 17.61 16.7 E. regale 20 cm above base Bed C9D Hibolites jaculoides 0.707405 1 1.24 0.33 39.8 1800 1300 1560 32 5 Hauterivian 2003/1732 16.4 E. regale C10 5 cm from top Acroteuthis sp. 1.19 30.25 38.7 930 1400 988 51 5 Hauterivian 16.71 15.5 E. regale 80 cm above base Bed C10 Hibolites sp. 0.707398 1 1 3 0.25 0.24 39.1 2120 1045 1420 20 7 Hauterivian 2003/1825 15.6 E. regale Mid C 10 Acroteuthis sp. 1.29 30.09 39.4 1062 1390 1125 14 3 Hauterivian 2003/1831 15.6 E. regale Mid C 11 Acroteuthis sp. 1.02 30.12 40.2 1005 1560 1035 14 5 Hauterivian 16.26 15.1 E. regale 40 cm above base Bed C10 Hibolites sp. 0.707405 5 5 2 0.81 0.50 39.4 2325 1090 1675 22 4 Hauterivian 15.29 13.4 E. regale 100 cm above base Bed C11B Hibolites jaculoides 0.707405 1 1 2 0.90 0.32 38.8 2095 1035 1480 48 6 Hauterivian 14.99 13.1 E. regale 70 cm above base Bed C11B Hibolites jaculoides 0.707392 2 2 2 1.42 0.62 38.7 2530 1240 1890 12 4 Hauterivian 14.70 12.8 E. regale 40 cm above base Bed C11B Hibolites jaculoides 0.707393 1 1.19 0.29 39.7 1880 1145 1370 42 4 Hauterivian 14.52 12.6 E. regale 20 cm above base Bed C11B Hibolites sp. 0.707388 9 9 2 1.23 0.56 38.5 1975 1075 1670 23 3 Hauterivian 1438 12.4 E. regale Base Bed C11B Hibolites jaculoides 0.707388 1 2 3 1.52 0.68 39.0 1790 1160 1525 121 4 12.40 Base regale Hauterivian CB30(D1) 12.3 noricum^amblygonium D1 Hibolites jaculoides 0.707394 1 0.86 0.79 40.4 2070 1115 1440 51 4 Hauterivian 1376 11.9 E. amblygonium Base Bed D2B Acroteuthis (A.) cf. acmonoides 0.707383 1 0.61 0.11 40.3 616 1085 648 0 4 Hauterivian 1362 11.7 E. amblygonium 20 cm above base Bed D2D Acroteuthis (A.) paracmonoides 0.707379 1 0.88 0.17 40.1 653 1110 758 15 4 p. Hauterivian 1341 11.5 E. amblygonium Base Bed D2D Acroteuthis (A.) paracmonoides 0.707380 1 0.88 0.46 39.3 597 1080 781 11 3 p. Table 1( Continued).

Stage Sample No. Level Zone Bed No. Specimen type 87Sr/86Sr N13C N18OCaMgSrNaFeMn

(m) Means + 3 n (x)(x) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)

Base Hauterivian 11.50 Base amblygonium Valanginian SB7 11.3 polyptychites D2E. 80% up. Acroteuthis sp. 0.707344 4 4 3 30.92 30.09 40.0 921 1205 918 0 4 3 3 Valanginian 1297 11.3 polyptychites 170 cm above base Bed D2E Acroteuthis sp. 0.707333 1 0.24 0.46 39.9 593 1165 747 3 253^272 6 (2004) 202 Palaeoecology Palaeoclimatology, Palaeogeography, / al. et McArthur J.M. Valanginian 1236 11.4 polyptychites 110 cm above base Bed D2E Acroteuthis (A.) acmonoides 0.7073317 7 2 30.72 0.10 40.2 772 1170 739 7 7 Valanginian D2E 10.9 polyptychites Mid D2E Acroteuthis sp. 0.7073411 30.27 30.25 39.8 929 1080 866 3 10 Valanginian Sp 1195 11.0 polyptychites 70 cm above base Bed D2E Acroteuthis sp. 0.707327 1 30.31 0.04 40.6 718 1150 825 0 14 Valanginian Sp 1181 10.8 polyptychites 55 cm above base Bed D2E Acroteuthis sp. 0.707337 4 8 3 30.32 30.34 39.8 740 1155 801 0 4 Valanginian Sp11.21 10.2 polyptychites Top of Bed D3A Acroteuthis sp. 0.707330 1 30.85 30.16 39.3 1325 1105 1010 16 12 Valanginian D3A bel 10.2 polyptychites D3A top Not identi¢ed 0.707343 2 2 2 0.08 30.16 39.6 719 1080 671 19 8 Valanginian SB6 10.1 polyptychites D3A. Middle Acroteuthis sp. 0.707334 6 6 4 30.22 30.40 40.3 645 1310 1005 4 4 Valanginian D3A 10.1 polyptychites D3A Acroteuthis sp. 0.707339 9 6 3 0.76 30.35 39.8 712 1145 743 12 12 Valanginian 1056 9.6 polyptychites 50 cm above base Bed D3B Acroteuthis sp. 0.707342 2 2 2 31.38 30.86 39.2 1240 1115 956 8 11 Valanginian D3B 9.5 polyptychites D3B. Oyster 0.707336 8 5 3 Valanginian SB5 9.2 polyptychites D3B. 15% up Acroteuthis sp. 0.707339 8 8 3 30.59 0.07 40.4 484 1265 780 0 13 Valanginian D3C 9.0 polyptychites D3C Oyster 0.707335 6 6 3 Valanginian D3D 8.9 polyptychites D3D Acroteuthis sp. 0.31 30.08 39.4 956 1015 896 3 3 8.55 Base polyptychites Valanginian D4A 8.2 paratollia D4A Acroteuthis sp. 0.707336 3 3 2 0.63 0.58 40.5 375 1035 568 3 5 AAO32 12-12-03 3220 PALAEO Valanginian SB4 8.2 paratollia D4A. Middle Acroteuthis sp. 0.707338 7 7 3 30.42 30.01 39.9 497 1210 703 4 9 Valanginian SB3 7.6 paratollia D4C. Middle Not known 0.707303 8 7 4 0.43 30.43 39.2 4811290 941 21 3 Valanginian 2001/12 8.0 paratollia D4C, 1.60 m above base Acroteuthis (A) subquadratoides 30.35 30.09 40.5 586 1270 701 161 15 (Swin) Valanginian 2001/11 7.8 paratollia D4C, 1.40 m above base Acroteuthis (A) explanoides 0.04 30.49 39.0 530 1270 848 16 5 (Pav) Valanginian 2001/10 7.8 paratollia D4C, 1.35 m above base Acroteuthis (A) explanoides 30.74 30.35 40.0 749 1060 913 77 6 (Pav) Valanginian 2001/9 7.6 paratollia D4C, 1.20 m above base Acroteuthis (A) cf. explanoides 30.04 0.06 40.2 585 991645 0 3 Valanginian 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 4 Valanginian 2001/7 6.9 paratollia D4C, 0.5 m above base Acroteuthis (A) explanoides 30.71 30.63 38.0 1010 1200 745 99 11 (Pav) Valanginian 2001/6 6.7 paratollia D4C, 0.3 m above base Acroteuthis (A) explanoides 0.12 30.20 39.5 432 1220 777 58 10 (Pav) Valanginian 2001/15 6.4 paratollia Base D4C A. (A) kemperi (Pickney) 30.53 30.59 40.0 594 1255 637 5 5 Valanginian 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 15 Base Valanginian 6.10 Base paratolia Base D4D Berriasian SB2 3.1 P. albidum D6A. Middle Not known 0.707265 5 3 5 30.63 0.73 39.2 566 1050 690 7 7 Berriasian SB11.5 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 given for 87Sr/86Sr are the maximum and minimum deviations from the mean of n replicates, or þ 0.000015 for singlet analysis. 259 260 J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2004) 253^272 this occur in the low and top C beds (Table 1; composition can be interpreted in terms of ocean Hibolites beds; Rawson, 1973; Mutterlose et al., temperature. 1987; Mitchell, 1992). While Neohibolites is the ¢rst belemnite with a truly cosmopolitan distribution, the opportunistic taxon Hibolites periodi- 3. Samples, analytical methods and results cally migrated northwards during the late Valan- ginian and Hauterivian. This eurythermal genus Our samples comprise 26 of the belemnites re- (Mutterlose, 1988) underwent an endemic evolu- ported in Price et al. (2000) with 84 additional tion in the Boreal sea. Out of the Tethyan stock, a belemnites collected at Speeton by the authors. group evolved with small guards that forms a The stratigraphic levels of samples are given in continuous lineage from the Endemoceras regale Table 1, along with compositional data and iden- ammonite Zone (late early Hauterivian) to the ti¢cations mostly to the genus level. No samples Late Barremian (Mutterlose, 1988). were obtained from the upper 80% of the speetonensis Zone as this unit is very poorly exposed. 2.2. Palaeogeography We sampled 85 m of section, spanning mostly the Hauterivian to lower Barremian interval, but in- During the Early Cretaceous, Speeton was lo- cluding small slices of Valanginian sediment at the cated on the southwestern margin of the Southern base of the section. This thickness of sediment North Sea basin at a palaeo-latitude of about 40^ represents a depositional period of about 7 myr 45‡N (Fig. 1; also, see Ziegler, 1982; Knox, 1991; (timescale of Gradstein et al., 1995) at an overall Rawson, 1992). The area known now as north- sedimentation rate of 11 m per myr in the Hau- west Europe formed the southernmost extension terivian/Barremian. Given that our sample posi- of the Boreal^Arctic sea, with seaways extending tioning is certain to no better than about 10 cm, towards the Tethys in the south (Ziegler, 1982; and is sometimes much worse, our temporal res- Mutterlose, 1992). As the area lay at the margin olution in sampling is no better than þ several of the Boreal Realm (Fig. 1), it was in£uenced by thousand years. in£uxes of nanno£ora and fauna from both Belemnites were prepared for analysis by using realms (Rawson, 1973; Mutterlose, 1992). For diamond cutting tools to remove the apex, exteri- Early Cretaceous times, the strong separation of ors, apical line, and alveolus. The remains were the Boreal and Tethyan Realms has been ascribed fragmented (sub-mm), cleaned in 1.2 M hydro- to a physical division resulting from the disposi- chloric acid, washed in ultra-pure water, and tion of continents coupled to changes in sea level dried in a clean environment. Fragments were and the deepening of connecting seaways (Fig. 1; picked under the binocular microscope to secure Doyle, 1987; Haq et al., 1987; Ru¡ell, l991; Mut- those judged to be best preserved, ground to a terlose, 1992) and, alternatively, to pronounced powder, and analysed for 87Sr/86Sr, N18O, N13C, di¡erences in temperature between these provin- Ca, Mg, Na, Sr, Ba, Fe, Mn, and Rb. The results ces (e.g. Rawson, 1973; Stevens, 1973). Attempts are in Table 1. to resolve these con£icting views have, in part, For chemical analysis, subsamples were dis- resorted to classical interpretations of stable solved in 1.2 M hydrochloric acid. Concentrations isotopic palaeo-temperatures (e.g. Price et al., of Rb were measured by furnace-AAS; other ele- 2000) and carbon isotopic considerations (Weis- ments were analysed with ICP-AES. The precision sert and Lini, 1991), which suggest that seasonally of the analysis was better than þ 5%. For 87Sr/ cold ocean temperatures, and limited polar ice, 86Sr analysis, subsamples were dissolved in ultra- existed during the Early Cretaceous (but see also pure 6 M nitric acid, evaporated to dryness in Bennet and Doyle, 1996). Such interpretations order to oxidise organic matter, and converted rely on the assumption that biogenic calcite (in- to chloride salt by subsequent evaporation to dry- cluding that of belemnites) was precipitated in ness with ultra-pure 6 M hydrochloric acid. Sam- equilibrium with seawater and that its isotopic ples were then taken up in 2.5 M hydrochloric

PALAEO 3220 12-12-03 J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2004) 253^272 261 acid and Sr was separated by standard methods of is indicative of good sample preservation (Fig. 1). ion-exchange chromatography. Values of 87Sr/86Sr Our samples meet these tests of preservation: they were determined with a VG-354 ¢ve-collector are visually well-preserved, have isotopic and ele- mass spectrometer using the multi-dynamic rou- ment compositions that replicate well, have low tine SRSQ that includes corrections for isobaric concentrations of Fe and Mn (Table 1), and interference from 87Rb (Thirlwall, 1991). Data have concentrations of Na, Sr, and Mg, typical have been normalised to a value of 0.1194 for of well-preserved belemnites (Saelen and Kar- 86Sr/88Sr and a value of 0.710248 for NIST 987, stang, 1989; McArthur et al., 2000; van de which equals a value of 0.709175 for EN-1 in our Schootbrugge et al., 2000), so we take it that laboratory. Based upon replicated analysis of our samples retain their original elemental and NIST 987, the precision of our measurements isotopic compositions. (2 S.E.M.) was þ 15U1036 (n =1), þ11n (= 2), þ9 (n =3)and þ8 (n = 4). In practice, the uncer- 4.2. Isotopic trends in 87Sr/86Sr tainty on replicate measurements from the sample-picking stage was generally better than these The 87Sr/86Sr of samples is plotted in Figs. 2 ¢gures. Mean 87Sr/86Sr are reported in Table 1, and 3 against biostratigraphy and stratigraphic together with the maximum and minimum devia- level. Samples from the gottschei Zone could not tions from the mean of replicate analyses and the be positioned accurately within the zone, so they number, n, of replicates for each sample. Total are plotted at its mid-point. The data group into blanks were 6 0.2 ng of Sr and subsample con- three segments. The 87Sr/86Sr value of the lower- tained around 5 Wg of Sr. Concentrations of Rb most two samples (SB1, SB2 mean 0.707265) in- were too low to require correction for radiogenic dicates a late Berriasian (Ryazanian) age of 138.3 87Sr. Data for N13C and N18O were provided by H. Ma +1.3/30.7 (numerical ages from McArthur et Erlenkeuser of Kiel University, with additional al., 2001). An interval without preserved marine data being taken from Price et al. (2000) with faunas separates the lowermost two samples from corrections. For isotopic data, analytical precision SB3 (Table 1) which has an 87Sr/86Sr age that is was 0.1x for both N13C and N18O with respect to earliest Valanginian (136.5 þ 0.4 Ma). Above this repeat analysis of NBS-19. The results of the level, between 8.2 m (in D4A) and 11.5 m (top of chemical and isotopic analyses are given in Ta- D2E), which includes the uppermost Paratollia ble 1. beds and the Polyptichites beds, 18 samples have an invariant 87Sr/86Sr of 0.707338 þ 0.000002 (2 s.e., n = 38), which gives an 87Sr/86Sr age of 4. Discussion 135.3 þ 0.3 Ma, suggesting a hiatus of 1.3 þ 0.5 Ma, or an extremely condensed section, occurs 4.1. Sample preservation within beds D4A-C. This 87Sr/86Sr value corre- lates to the Valanginian Karakaschiceras biassa- Cathodoluminescence, trace element composi- lensis SZ (McArthur and Janssen, in preparation), tion, examination in thin section and by SEM and the invariance of 87Sr/86Sr between 8.2 and (Veizer, 1974, 1983; Saelen and Karstang, 1989; 11.5 m constrains the duration of this interval Jones et al., 1994; McArthur, 1994; Podlaha et to 9 0.2 myr, since marine 87Sr/86Sr increased al., 1998) and discordancy of 87Sr/86Sr are useful by about +0.000020 per myr during Valanginian for identifying alteration in samples. In particular, time (Jones et al., 1994; McArthur et al., 2001; di¡erent (sub)samples from a level that give the Price and Grocke, 2002). same 87Sr/86Sr are likely to be recording an orig- A hiatus (Rawson, 1971; Fig. 2) separates the inal 87Sr/86Sr value (Jones et al., 1994; McArthur, Valanginian Bed D2E from the overlying Hauteri- 1994) and so the original compositions for other vian Bed D2D (base of the amblygonium Zone). parameters both elemental and isotopic. Likewise, Across this boundary, an increase of 87Sr/86Sr in a well-de¢ned trend in 87Sr/86Sr through a section 0.000042 (Fig. 3) represents a gap of some 2.0 myr

PALAEO 3220 12-12-03 262 J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2004) 253^272

(McArthur et al., 2001). The base of the Hauteri- vian, at 11.5 m, has an 87Sr/86Sr of 0.707380 þ 0.000003 and a rate of change of +0.000008 per metre. Upsection, the rate declines to e¡ectively zero within the gottschei Zone, steepens markedly through the uppermost Hauterivian marginatus Zone and lowermost Barremian variabilis Zone, and £attens through the rarocinctum Zone (close to 0.70748) and most of the ¢ssicostatum Zone. Thereafter, it increases through the upper third of the latter and the lower elegans Zone, and peaks in the middle elegans Zone (0.707493 þ 0.000004), after which it decreases. We have one sample from the topmost Barremian (P. bidentatum Zone) at Speeton (Table 1) which has an 87Sr/ 86Sr of 0.707430; thus, 87Sr/86Sr continues to decline above the denckmanni Zone.

4.3. A link between 87Sr/86Sr and Ontong Java Volcanism?

Larson and Erba (1999) attribute the decline in 87Sr/86Sr from the Late Barremian to the late Ap- tian to the e¡ects of volcanism between about 125 and 120 Ma that was associated with the emplacement of the basalts of the OJP. Our data show that marine 87Sr/86Sr peaked in the mid-elegans ammonite Zone which, according to the scheme given in Bown (1998), is about one-third of Chron CM3 from its top. This is early in Boreal nannofossil Zone BC15, and is in the lower part of Tethyan nannofossil Zone CC6. This stratigraphic placement has a numerical age of around 124^125 Ma (on the revised timescale of Larson and Erba, 1999), an age range that is coincident with that given for the onset of OJP volcanism by Larson and Erba (1999, especially their ¢gure 7) after 87 86 Fig. 3. Detailed Sr/ Sr record of the sequence at Speeton, their reinterpretation of numerical ages and bio- Yorkshire, UK. (a) Lower Barremian to Hauterivian, (b) Hauterivian, (c) lowest Hauterivian, Valanginian and Ber- stratigraphy of materials associated with the for- riasian. Uncertainty on 87Sr/86Sr is better than þ 0.000015 mation of the OJP. A 3.5 km thick sequence of for a single analysis. Data for 87Sr/86Sr are reported to NIST volcanic lavas on Malaita gives a similar mean 987 of 0.710248 and EN-1 of 0.709175. Data of Jones et al. age of around 125 þ 2 (Tejada et al., 2002) after (1994) are normalised to NIST 987 by addition of 0.000022. recalculation to FCT = 28.15 Ma and Mnhb-1 of Note the hiatus at 11.5 m between Valanginian strata and the overlying Hauterivian strata. 523.0 used in Larsen and Erba (1999), although it is not clear what part of the volcanic stratigraphy of the OJP they represent. Citing Bralower et al. (1997), Larson and Erba (1999) state that the decline in 87Sr/86Sr started

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0.5^1myr after the level of magnetic reversal M0. global MOR production), that were themselves Our data show that the decline started in the the main driver for changes in marine 87Sr/86Sr upper part of M3, some 3 myr before that time, during the Cretaceous. and that it coincides with the time of eruption of the ¢rst sea£oor basalts (so far known) on the 4.4. Relative duration of ammonite biozones OJP (Larson and Erba, 1999; Tejada et al., 2002). As hydrothermal circulation would have The 87Sr/86Sr pro¢le through the Hauterivian commenced before the onset of sea£oor eruption and Barremian at Speeton does not change line- of basalt, i.e. as the basaltic source approached arly with stratigraphic height (Figs. 2 and 3): 87Sr/ the sea£oor, our demonstration that the decline 86Sr changes more rapidly through ammonite in 87Sr/86Sr started earlier than proposed by Lar- zones that are lithologically thin, than it does son and Erba (1999) ¢ts better with their proposal through zones that are thick. For example, in than did their age estimated for the onset of de- the three thickest zones (speetonensis, gottschei, cline in 87Sr/86Sr. Nevertheless, the magnitude and ¢ssicostatum) 87Sr/86Sr increases little, whilst the timing of any hydrothermal £ux associated with increase is more rapid in the variabilis Zone, pos- the rising OJP magmas are matters of speculation. sibly through the underlying marginatus Zone, Furthermore, the onset of an increased hydrother- and through the noricum^amblygonium zones, all mal in£ux to the oceans would be bu¡ered by the of which are thin in comparison. These variable existing mass of oceanic Sr, so 87Sr/86Sr would be rates of change in 87Sr/86Sr with stratigraphic level unlikely to change before at least 1myr had result from changing sedimentation rates through passed (Richter and Turekian, 1993). More inter- the sequence (cf. McArthur et al., 2000). Stronti- estingly, marine 87Sr/86Sr continued to decline for um isotope pro¢les through Hauterivian strata of some 3^5 myr after the cessation of the ¢rst phase the Vocontian basin, southeastern France (van de of OJP volcanism (Bralower et al., 1997)toa Schootbrugge, in review; McArthur et al., unpub- minimum around 115 Ma (the shortness of which lished data) show that marine 87Sr/86Sr increased is possibly an artefact of age calibration). If the linearly with stratigraphic position, and so with onset of OJP volcanism made marine 87Sr/86Sr time. We use this assumption of linearity to com- decline, its cessation (or substantial diminution) pute the zone thicknesses assuming a constant would have arrested that decline. The fact that rate of sedimentation (and so rate of change it did not might suggest that OJP volcanism was with time of 87Sr/86Sr). The results are shown in just one global event of many (e.g. changes in Table 2 and reveal that the recalculated relative

Table 2 Relative durations of ammonite biozones for Hauterivian and early Barremian time, calculated assuming a linear rate of change of 87Sr/86Sr in seawater through the interval (see text for justi¢cation) Zone base 87Sr/86Sr of zone base and Thickness (m) 2 s.e. uncertainty Measured to 87Sr/86Sr elegans 0.707485 (3) ¢ssicostatum 0.707476 (3) 15.8 5.6 rarocinctum 0.707474 (3) 8.11.2 variabilis and the base of the Barremian 0.707461(3) 1.5 8.1 marginatus 0.707454 (3) 2.2 4.4 gottschei 0.707450 (3) 11.9 2.5 speetonensis 0.707434 (3) 12.8 10 inversum 0.707424 (2) 2.9 6.2 regale 0.707388 (4) 10.7 22.5 noricum^amblygonium and the base of the Hauterivian 0.707380 (4) 0.9 5 Measured thickness 66.7 m, change in 87Sr/86Sr 0.000105.

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thicknesses, and so relative durations, of Hauteri- vian and Barremian ammonite zones di¡er by fac- tors up to 18. The measured thicknesses do not re£ect the relative durations of the zones. Missing strata within the Valanginian sequence at Speeton preclude the use of 87Sr/86Sr pro¢les in this way in that 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 de Schootbrugge 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-climate proxy that re£ects both ice volume and temperature, and if the Sr/Ca and Mg/Ca values of belemnites are temperature dependent (Berlin et al., 1967; Yasamanov, 1981), as is the case for many modern 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), the combination of both should allow us to test for the existence of signi¢cant ice volume during the Early Cretaceous (cf. the approach of Lear et al., 2000 using foraminifera). In view of this, it is worthwhile trying to establish which groups of belemnites, if any, do faithfully record palaeo- oceanographic conditions, and which do not. To help this quest, we show elemental and isotopic compositions of belemnites from Speeton as cross plots in Fig. 4 and against stratigraphic level in Figs. 5 and 6.

Fig. 4. Cross plots of element concentrations for Acroteuthis, Aulacoteuthis, and Hibolites. The co-variation of Na with Mg is good in Aulacoteuthis (open circles), less good and of a steeper slope in Hibolites (triangles) and poor in Acroteu- this (¢lled circles are Valanginian specimens, diamonds are Hauterivian specimens). In Aulacoteuthis and Acroteuthis,Sr co-varies closely with Na and Mg, but in Hibolites, it co- varies with neither.

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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 contents of Mg and Na, and heavier values, by some 0.4x in N18O, than do others. Of the trace element concentrations, those of Sr change least through the section. Symbols as in Fig. 4.

N18O in belemnites: the use of N18O values of non-equilibrium fractionation during calci¢cation belemnites to calculate palaeo-temperatures is be- (e.g. McConnaughey, 1989a,b) has proven intract- set by problems viz. salinity e¡ects, biological able. The elemental and isotopic compositions of fractionation, diagenetic alteration, and uncer- many calcite-secreting groups, e.g. molluscs tainty regarding the isotopic composition of the (Mitchell et al., 1994), the rudist bivalve Torreites ocean, which are readily acknowledged by most (but not other rudists; Steuber, 1999) and bra- authors. The problem introduced by diagenetic chiopods (to some degree; Carpenter and Loh- alteration was highlighted by Longinelli (1969), mann, 1995; Curry and Fallick, 2002), suggest Spaeth et al. (1971) and Veizer (1974); methods that biological fractionation occurs during calci¢- have been developed to identify, and so avoid, it cation so, by analogy, it probably a¡ects belem- (Veizer, 1983; McArthur, 1994; Podlaha et al., nite compositions and confounds some of their 1998). The problem of ice volume is commonly records of ambient conditions. Some authors resolved in the Mesozoic by assuming an ice-free have attempted to identify non-equilibrium frac- world and so an ocean composition of around tionation by comparing di¡erent belemnite genera 31x (SMOW, which is 31.2% PDB): it would (e.g. Ditch¢eld, 1997; Price and Sellwood, 1997), be solved were we able to develop a robust Mg/Ca or minimise it by using one species alone (Nie- thermometer as such a thermometer would be in- buhr and Joachimski, 2002), whilst others note dependent of ice volume. Attempts to address the that well-preserved biogenic carbonate from dif- salinity issue have sometimes appealed to palaeo- ferent groups (ammonites, bivalves, belemnites) climate modelling (e.g. Price and Sellwood, 1997). gives di¡erent palaeo-temperatures (Anderson et The problem posed by the possible operation of al., 1994). Given the above, the observation that

PALAEO 3220 12-12-03 266 J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2004) 253^272 belemnite palaeo-temperature trends through time rences of Hibolites and Acroteuthis at Speeton are are noisy (Podlaha et al., 1998) is no surprise, separated by between 20 and 50 cm in this slowly especially when the possible e¡ect on N18Oof accumulating section (stratigraphic placement is short-term climate change is also considered not available for Svalbard), so they are separated (ibid). in time by between 18 000 and 45 000 years. This In our section, specimens of Acroteuthis of early time span is ample for climate, and so local ocean Hauterivian and earliest Barremian age are iso- temperature, to change. Nevertheless, it requires topically lighter in N18O by about 0.4x (on aver- that the Boreally derived (northern) genus Acro- age) than are specimens of Hibolites of equivalent teuthis preferred warmer water than did the age (Figs. 5 and 6). The di¡erence in N18O con- Tethyan-derived (southern) genus Hibolites. trasts with that between Hibolites (mean N18O Whatever the real reason for the di¡erences +0.6 þ 0.4; 2 s.d., n = 14) and Acroteuthis (mean noted above, we note that they are small. We +1.2 þ 0.8; 2 s.d., n = 5), from the Early Creta- also note that between 53.4 and 55.5 m in the ceous Tordenskjoldberget Member of the Kong- section (Table 1), the mean N18O of three speci- sÖya Formation of Svalbard, that was recorded mens of P. jasiko¢ana (0.38x), four of Hibolites by Ditch¢eld (1997). Furthermore, specimens of (0.2x) and two of Acroteuthis (0.10x) are very Hibolites and Belemnopsis, of Late Jurassic age close (Table 1, Fig. 4), a fact that suggests the (Price and Sellwood, 1997), have values of N18O o¡sets seen lower in the section are caused, at that di¡er by only 0.17x. The di¡erences (if least in part, by real temperature di¡erences, real) might be due to di¡erences in metabolic frac- rather than metabolic e¡ects. In summary, the tionation, to slight di¡erences in the depth of hab- fact that several genera of belemnites at Speeton itat, given that 0.4^0.6x represents only about give N18O values that are within 0.4x of each 1.6^2‡C in temperature, to di¡ering salinity in other suggests that the N18O values of our belem- each area, or to short-term climate change, but nites are recording at least the major trend in pa- it is di⁄cult to reconcile the opposite sense of laeo-temperature through our section, and may be the di¡erences between Acroteuthis and Hibolites re£ecting more subtle changes as well. in Speeton and Svalbard with any of these alter- What are those trends? The N18O values of be- natives. Considering climate change, the co-occur- lemnite calcite are around 30.2 þ 0.4x in the Valanginian, where specimens are exclusively at- tributed to the genus Acroteuthis. At the Hauteri- vian/Valanginian boundary (at 11.5 m), which is marked by a time gap of 2.0 myr (see earlier sections), values jump to around +0.4x (for Acro- teuthis) and to around +0.8x (for Hibolites). They become more negative upsection to around a level of about 19^22 m (middle of the regale Zone), where these genera record values of 31.02x (Acroteuthis) and 30.27x (Hibolites) respectively. At 24 m (lower inversum Zone), specimens of Hibolites again record values around x 87 86 Fig. 6. Variation of N18O with stratigraphic height in the lower +0.3 . Between 11.5 and 24 m, Sr/ Sr in- Hauterivian. From the base of the Hauterivian at 11.5 m, creases by 0.000044, a change that represents a N18O in both Acroteuthis and Hibolites becomes more nega- period of close to 2 myr. In belemnites of Barre- tive upsection, whilst Hibolites de¢nes a return to original mian age, values become lighter upsection and 18 values of N O by 24 m after peaking around 19^22 m. The reach 31.76x in the Barremian elegans Zone interval 11.5^24 m represents about 2 myr (based on the change in 87Sr/86Sr) and the N18O record shows a pronounced (ignoring one outlier at 65 m in the basal ¢ssico- warming to 17‡C and return to cool temperature (10‡C) dur- statum Zone), immediately after which values ing that time period. Symbols as in Fig. 4. brie£y and abruptly lighten to around 30.2 to

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Fig. 7. Relation of Mg to N18O and N13C in the belemnite genera Aulacoteuthis, Acroteuthis, and Hibolites.InAulacoteuthis,Mg concentrations increase as N18O becomes more negative, trends that are compatible with increasing temperature of ambient seawater. 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 as in Fig. 4.

30.6x. The implications of these data for pa- in the genus Hibolites, no co-variance is seen be- laeo-temperatures through the section are dis- tween Sr and Mg, a weak co-variance may be cussed later. present between Na and Sr, whilst the co-variance Trace elements: the contents of Mg, Na, and Sr between Mg and Na is su⁄ciently convincing to in our belemnites show a large range but, in speci- be thought real. Concentrations of both Mg and mens of Acroteuthis and Aulacoteuthis, all three Na are higher in specimens of Hibolites, which are elements correlate positively, albeit with varying mostly of Hauterivian age, than in other genera degrees of closeness, a co-variance shown many (Figs. 4 and 5) which are (mostly) of Valanginian times before for biogenic carbonate. In contrast, or Barremian age. On the three trace elements,

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Table 3 Coe⁄cients of temperature dependence of Sr, Na, and Mg concentrations in Acroteuthis and Aulacoteuthis, calculated from the slope of reduced major axis regressions to the N18O and Mg data shown in Fig. 7 Genus Mg Sr Na Mg Sr Na (ppm/‡C) (ppm/‡C) (ppm/‡C) (%/‡C) (%/‡C) (%/‡C) Acroteuthis 160 85 100 20 7 12 Aulacoteuthis 100 40 105 8 13 13 No coe⁄cients are calculated for Hibolites as there is no co-variance of N18O with Mg (Fig. 7). concentrations of Sr show the least change with element/temperature dependencies for Aulacoteu- stratigraphic level. this and Acroteuthis given in Table 3. These values Concentrations of Mg in modern biogenic cal- are similar to those found for modern biogenic cite increase with increasing temperature by be- calcite. No temperature coe⁄cients are given for tween 5% and 10% per degree (Nu«rnberg et al., Hibolites as N18O in this genus does not correlate 1996; Rosenthal et al., 1997; Mashiotta et al., with Na, Sr, or Mg. 1999; Lea et al., 1999; Lear et al., 2000; Elder- The co-variance we show between Mg and N18O ¢eld and Ganssen, 2000; Bailey et al., 2003), or might be interpreted as resulting from kinetic more if older work is considered (Chave, 1954), so fractionation, with faster calcite deposition lead- our Mg trend with stratigraphic level (Fig. 5) ing to isotopically lighter carbon and oxygen, and might be interpreted as showing that the Hauteri- also to more structural disorder in calcite so that vian sea at Speeton was warmer in those times more trace elements are incorporated at sites of than it was in Valanginian or Barremian times. structural defectiveness. Some evidence that this Comparison of the Mg (and Na) contents of might be so comes from the surprisingly good specimens of Acroteuthis (Fig. 5) with equiva- relation between N13C and Mg in Aulacoteuthis lent-age specimens of other belemnites shows, (Fig. 7d), although the co-variance is either however, that the di¡erences must be due simply weak or absent in Valanginian specimens of Acro- to di¡erential metabolic fractionation; Hibolites teuthis (Fig. 7e) and specimens of Hibolites (Fig. simply incorporates more Mg and Na into its car- 7f). Were non-equilibrium fractionation a¡ecting bonate than do the other genera (Figs. 4 and 5). It our belemnites, however, a strong positive co-var- is clear, therefore, that di¡erent belemnite genera iance would be expected between N18O and N13C. have di¡erent temperature sensitivities to incorpo- In Aulacoteuthis, a (weak) relation occurs only if ration of trace elements. Can we calibrate any or two outlying data (arrowed in Fig. 8a) are in- all of them for use in palaeo-temperature analysis cluded in the consideration of co-variance, which and ice volume calculations? In Fig. 7 we show does not argue convincingly for the operation of the co-variance of Mg with N18O in the three gen- biogenic fractionation in this genus. In specimens era of belemnites of which we have many speci- of Acroteuthis (Fig. 8b), there is no correlation mens (similar co-variance occurs for Sr and Na between N18O and N13C if all data are included, with N18O, which are not shown). In those of Au- none amongst the subpopulation that are of lacoteuthis and Acroteuthis, concentrations of Mg Hauterivian and Barremian age, and only a increase as N18O becomes more negative, trends weak correlation amongst Valanginian belemnites compatible with both being related to tempera- and one that is dependent for its presence on ture, whilst in specimens of Hibolites no relation three outliers (arrowed). Again, we feel that evi- is seen. These trends are similar to those found in dence for biogenic fractionation is too weak for Toarcian belemnites (McArthur et al., 2000; Bai- its presence to be proven. The absence of any ley et al., 2003). Although our trace element data correlation between N18O and N13C for Hibolites are few and scatter somewhat, by assuming equi- is clear from Fig. 8c. We therefore attribute the librium precipitation of oxygen isotopes (or a con- relations seen in Fig. 7 between Mg concentra- stant o¡set from equilibrium) we deduce the trace tions and N13C as being temperature e¡ects.

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4.5.1. Palaeo-temperature implications The 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 Valanginian times until the latest Aptian (Podlaha et al., 1998); these trends suggest a warming into the Hauterivian. Such a warming is consistent with increasing Mg concentrations in Acroteuthis, and the lightening of its N18O values through the noricum, amblygonium, and lower part of the regale Zone. It is also compatible with the lightening of N18OinHibolites through the same interval, although this increasing temperature is not re- £ected in its concentrations of Mg. In the upper part of the regale Zone, the inversum Zone, and the lower part of the speetonensis Zone, we have only the genus Hibolites to provide estimates of conditions, but the small number of samples analysed suggests a decrease in temperature through these units to something approaching those at the base of the Hauterivian (around 11‡C, assuming an ice-free world with N18O of seawater of 31x SMOW). At higher levels in the Hauterivian, the record is sparse and poorly constrained strati- graphically. For example, our samples from the gottschei Zone cannot be accurately positioned within it, so they are plotted at its mid-point; the range of composition here, however, suggests that seawater temperature may have changed a little through the zone, but had returned to around 11‡C by earliest Barremian times. From the base of the Barremian, temperature, as judged from N18O, increases upsection and reaches 20‡C in the elegans Zone, before a sharp, and brief, excursion is seen to around 14‡C at 78^79 m: Fig. 8. Relation of N18OtoN13C in belemnites. The weak, or these isotopic trends are paralleled by trends in absent, correlations between stable isotopic compositions of the concentration of Mg in Barremian belemnites carbon and oxygen suggest that non-equilibrium (vital) e¡ects (Fig. 4), which are mostly of the genus Aulacoteu- have not in£uenced the isotopic compositions of these gen- this. era. Symbols as in Fig. 4. If our record of N18O (and Mg for some genera in limited intervals) has any meaning, our data which, despite much scatter, shows temperatures are not in accordance with the arguments of warming from the Hauterivian into the Barre- Lini et al. (1992) that the Cretaceous ‘greenhouse’ mian. At Speeton, an early Hauterivian increase climate (having developed strongly in the Valan- in the temperature was soon reversed and not ginian) continued through the Hauterivian and resumed until the beginning of the Barremian, beyond. Our trend does agree with the general through which a poorly de¢ned trend towards sense of the N18O curve of Podlaha et al. (1998) higher temperatures culminates in the Barremian

PALAEO 3220 12-12-03 270 J.M. McArthur et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 202 (2004) 253^272 elegans Zone. Whether the excursion to cooler 20%, depending on genus and element; the values temperatures, noted in the elegans Zone, is con- are close to those found for modern biogenic car- nected to OJP volcanism is something about bonate. With some re¢nement, such element/tem- which we have too few data to make speculation perature relations may be used at palaeo-temper- productive. Finally, our data are too poorly con- ature proxies. The content of Na, Mg, and Sr, in strained temporally, and too few in number, to specimens of the genus Hibolites, shows no rela- de¢ne trends in elemental and isotopic composition to stable oxygen isotopic composition and so tion that would allow us to infer, or calculate, ice does not record palaeo-temperature. Future stud- volume from coupled elemental abundances and ies of the trace element, and isotopic, composition N18O records, but do suggest that more success of belemnites should examine possible genera, or might be had using an appropriate genus of be- species, speci¢c e¡ects that might compromise pa- lemnite from more rapidly accumulating sedi- laeo-environmental interpretation. ments, where better temporal resolution might be found. Acknowledgements

5. Conclusions The Radiogenic Isotope Laboratory at RHUL is supported, in part, by the University of London A pro¢le of 87Sr/86Sr in belemnite calcite as an intercollegiate facility. We thank Clinton through 85 m of Valanginian, Hauterivian, and Roberts and Sarah Houghton for assistance with Barremian strata at Speeton, Yorkshire, UK, the Sr isotopic analysis, and Desmond Donovan identi¢es hiatuses in the sequences, allows quanti- for useful discussions about belemnites and for ¢cation of their duration, permits the duration of drawing to our attention many relevant referen- ammonite (and other) biozones to be determined ces. The elemental analysis was done by and shows that they di¡er by a factor of as much J.M.McA. using the NERC ICP-AES Facility at as 18, and shows that the base of the Hauterivian RHUL, with the permission of its Director, Dr. at Speeton has an 87Sr/86Sr of 0.707380 þ J.N. Walsh. We thank H. Erlenkeuser (Kiel, Ger- 0.000003. The timing of volcanism on the OJP many) for providing stable isotopic data, Paul coincides with the point at which marine 87Sr/ Bown for the nannofossil zonations, and Tim 86Sr began a long-term decline from a maximum Denison and Jan Veizer for constructive reviews. of 0.707493 þ 0.000004 in the Barremian elegans This work was supported by NERC Grant NER/ ammonite Zone. GS/2000/00598 to J.M.McA. and DFG grant Mu Elemental and stable isotopic compositions of 667/24 to J.M. the belemnite genera Hibolites, Aulacoteuthis and Acroteuthis suggest that their belemnite calcite precipitated under conditions that yielded values References of N18O within 0.4x of those expected for equilibrium conditions; specimens of Hibolites have Anderson, T.F., Popp, B.N., Williams, A.C., Ho, L.-Z., Hud- values of N18O o¡set from those of other genera son, J.D., 1994. The stable isotopic records of fossils from by no more than 0.4x. Belemnites of the genus the Peterborough Member, Oxford Clay Formation (Juras- sic), UK: palaeoenvironmental implications. J. Geol. Soc. Hibolites contain around 2000 ppm of Mg, about London 151, 125^138. twice the concentrations in the genera Aulacoteu- Bailey, T.R., Rosenthal, Y., McArthur, J.M., van de Schoot- this, Acroteuthis, Praeoxyteuthis, and Oxyteuthis. brugge, B., Thirlwall, M.F., 2003. Paleoceanographic In specimens of Aulacoteuthis and Acroteuthis, the changes of the late Pliensbachian^Early Toarcian interval: good correlation between trace element content a possible link to the genesis of an Oceanic Anoxic Event. N18 Earth Planet. Sci. Lett., 212, 307^320. and values of O has allowed calculation of Bennet, M.R., Doyle, P., 1996. Global cooling inferred from the (tentative) temperature dependencies of Mg, dropstones in the Cretaceous ^ fact or wishful thinking? Na, and Sr concentrations of between 7 and Terra Nova 8, 182^185.

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