Strontium isotope profile of the early Toarcian (Jurassic) oceanic anoxic event, the duration of ammonite biozones, and belemnite palaeotemperatures

Earth and Planetary Science Letters 179 (2000) 269^285 www.elsevier.com/locate/epsl

Strontium isotope pro¢le of the early Toarcian (Jurassic) oceanic anoxic event, the duration of ammonite biozones, and belemnite palaeotemperatures J.M. McArthur a; *, D.T Donovan a , M.F. Thirlwall b , B.W. Fouke c , D. Mattey b a

Department of Earth and Planetary Sciences, University College London, Gower Street, London WC1E 6BT, UK Department of Geology, Royal Holloway and Bedford New College, Egham Hill, Egham, Surrey TW20 0EX, UK Department of Geology, University of Illinois, 245 Natural History Building, 1301 W. Green Street, Urbana, IL 61801, USA b

c

Received 1 June 1999; received in revised form 30 March 2000; accepted 6 April 2000

Abstract We profile 87 Sr/86 Sr, N13 C, N18 O, Sr/Ca, Mg/Ca, and Na/Ca in belemnites through Pliensbachian and Toarcian strata on the Yorkshire coast, UK, which include the early Jurassic oceanic anoxic event. The 87 Sr/86 Sr profile shows that the relative duration of ammonite subzones differ by a factor of up to 30: the Lower Jurassic exaratum subzone is 30 times longer than the clevelandicum subzone because the exaratum subzone in Yorkshire, which contains the anoxic event, is condensed by a factor of between 6.5 and 12.2 times, relative to adjacent strata. Using our 87 Sr/86 Sr profile, the resolution in correlation and dating attainable in the interval is between þ 1.5 m and þ 15 m of section, and better than 0.25 Myr. In parts of the sequence, this stratigraphic resolution equals that attainable with ammonites. A new age model is provided for late Pliensbachian and early Toarcian time that is based on the 87 Sr/86 Sr profile. Through the sequence, the Sr/Ca, Mg/Ca, Na/Ca and N18 O of belemnite carbonate covary, suggesting that elemental ratios may be useful for palaeotemperature measurement. Our N13 Cbelemnite data splits into three the previously reported positive isotope excursion (to +6.5x) in the early Toarcian. We speculate that the excursion(s) resulted from addition to surface waters of isotopically heavy CO2 via ebullition of methanogenic CO2 from the sediment during early burial of organic rich ( s 10% TOC) sediments ß 2000 Elsevier Science B.V. All rights reserved. Keywords: strontium; isotope ratios; biozones; Ammonites; geochronology

1. Introduction We know in outline how marine 87 Sr/86 Sr has changed with time through the Phanerozoic ([1^ 3]; and refs. therein). For that part of the record

* Corresponding author. E-mail: [email protected]

when numerical age control is best (0^40 Ma), the change of 87 Sr/86 Sr with time is very close to being linear, when viewed at a resolution of 5 Myr (Fig. 1): the older record [2] shows a similar character, although temporally it is constrained less well. Marine 87 Sr/86 Sr is bu¡ered against short-term changes by the low concentration of Sr in river water and the large amount of Sr in seawater, facts re£ected in the long residence time of Sr in

0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 1 1 1 - 4

EPSL 5469 30-5-00

270

J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285

Fig. 1. Variation of 87 Sr/86 Sr against time for the period 0^ 40 Ma. Data from [8,11,65^70].

the oceans (V2 Myr); models of the response of marine 87 Sr/86 Sr to changing Sr £ux or 87 Sr/86 Sr values [4^6] also imply that short-term ( 6 2 Myr) changes in marine 87 Sr/86 Sr are strongly damped, except where catastrophic events strongly perturb the £uxes or 87 Sr/86 Sr of oceanic inputs or outputs [7]. Curves of 87 Sr/86 Sr against time (Fig. 1) are derived from the more fundamental relation between 87 Sr/86 Sr and stratigraphic level (place in a rock sequence). Pro¢les of 87 Sr/86 Sr with stratigraphic level are often non-linear because they re£ect the interplay of a variable sedimentation rate with a less variable rate-of-change-with-time of marine 87 Sr/86 Sr. In a sequence, discontinuities in 87 Sr/86 Sr with stratigraphic level reveal structural and sedimentological discontinuities that enable the recognition and quanti¢cation of stratigraphic gaps [8^11]. Linear relations between 87 Sr/ 86 Sr and stratigraphic level can occur only when both the rate-of-change-with-time of marine 87 Sr/ 86 Sr is constant and the sedimentation rate is constant. Such linear records can be used to estimate the relative durations of geological events recorded in the rock. Here we use this principle to estimate the relative duration of ammonite zones [9,12,13] and to show that they di¡er greatly. We pro¢le 87 Sr/86 Sr through upper Pliensbachian and lower Toarcian strata of the Yorkshire coast (UK), an interval that includes the early Toarcian oceanic anoxic event (OAE). We show that 87 Sr/86 Sr changes linearly with strati-

graphic level through much of the sequence and that this fact can be used to estimate the relative durations of ammonite biozones, and so the duration of the early Toarcian OAE. We ¢nd that the exaratum subzone (Jet Rock) of the classic Yorkshire sequence, in which the OAE is recognised, is condensed relative to neighbouring strata by a factor of between 6.5 and 12.2. We show that dense sampling for 87 Sr/86 Sr in the interval studied has provided a resolution in correlation and dating that is, for parts of the sequence, equal to that a¡orded by ammonites. Finally, we show that trends in Na/Ca, Mg/Ca and Sr/Ca in belemnite calcite closely track the N18 Obelemnite record and we speculate about the origins of these similarities. Finally, we note that the carbon isotopic record of belemnites parallels that of the sedimentary organic matter, but the positive isotopic excursion in both lags the peak of TOC in the sediments, and we propose a mechanism to explain this lagged relation. 2. Geological setting The geology of the Pliensbachian and Toarcian rocks of the Yorkshire coast is well known [14^ 23]. We collected belemnites from Hawsker Bottoms, Staithes, Port Mulgrave, Saltwick Bay, Runswick, Kettleness, and Blea Wyke (Peak), localities on the coast of Yorkshire within a few km of Whitby (ibid). Exposure in these wave-cut platforms and cli¡ sections is close to 100% and correlation between the separated sections is possible to better than decimeter level in the Toarcian, and to better than 50 cm in the Pliensbachian, via numerous marker beds of carbonate nodules, sideritic concretions, and distinctive lithologies. Our stratigraphical levels are based on [14^16] except for the variabilis Zone, which are based on [20]. Stratigraphic levels are referred to Blea Wyke for the variabilis Zone, and the crassum and ¢bulatum Subzones ; to Saltwick Bay from the base of the ¢bulatum Subzone to the base of the Toarcian (paltum Subzone); to Hawsker Bottoms for the hawskerense and apyrenum subzones; to Staithes for subzones stratigraphically lower than the apyrenum Subzone. Stratigraphic levels

EPSL 5469 30-5-00

J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285

are expressed as up (+values) or down (3values) from an arbitrary zero datum placed at the base of Bed 33 [16], a level that is 13.56 m above the Pliensbachian^Toarcian (P/T) boundary. We con¢rm the existence of a hiatus at the apyrenum/ gibbosus boundary [19] equivalent to 10.3 of strata (see later sections) and we adjust stratigraphic levels below this boundary by that amount. 3. Analytical methods and results Our samples were belemnites. They were cut for thin sections and from what remained, the apex, exteriors, apical line, and alveolus were removed using diamond cutting tools. The remains were fragmented (sub-mm), cleaned in 1.2 molar hydrochloric acid, washed in ultrapure water, and dried in a clean environment. Fragments were picked under the binocular microscope, to secure those judged to be best preserved, and were analysed for 87 Sr/86 Sr, N13 C, N18 O, Ca, Mg, Sr, Fe, Mn and Rb. For chemical analysis, samples were dissolved in 1.8 molar acetic acid. Concentrations of Rb were measured by furnace-AAS; other elements were analysed with ICP-AES. The precision of the analysis was better than þ 5%, but the reproducibility of results exceeded this for a few elemental analysis (notably Na) owing to natural variability of the sample's composition. For 87 Sr/86 Sr analysis, samples were dissolved in ultra-pure 6 M HNO3 , evaporated to dryness in order to oxidise organic matter, and converted to chloride salt by subsequent evaporation to dryness with ultra-pure 6 M HCl. Samples were then taken up in 2.5 M HCl and Sr was separated by standard methods of column chromatography. Values of 87 Sr/86 Sr were determined with a VG354 ¢ve-collector mass spectrometer using the multi-dynamic routines SRSQ and SRSLL that include corrections for isobaric interference from 87 Rb [24]. Data have been normalised to a value of 0.1194 for 86 Sr/88 Sr. The data were collected between July, 1996, and April, 1999. During data collection, the measured value for NIST 987 was within 0.000 035 of the value 0.710 248. Data in Table 1 are adjusted to a value of

271

0.710 2480 þ 0.000 0025 (2 S.E.M., n = 19) for NIST 987 which equals a value of 0.709 1746 þ 0.000 0032 (2 S.E.M., n = 19) for EN-1. Based upon replicated analysis of standards, the precision of our measurements (2 S.E.M.) was þ 15U1036 (n = 1), þ 11 (n = 2), þ 9 (n = 3) and þ 8 (n = 4). Total blanks were 6 2 ng of Sr. Sample contained s 5 Wg of Sr. Concentrations of Rb were too low to require correction for radiogenic 87 Sr. Analysis for N13 C and N18 O were carried out using an Isocarb system attached to a VG Prism stable isotope mass spectrometer. The data are presented in N notation with respect to the PDB standard. Analytical precision was 0.1x for both N13 C and N18 O with respect to repeat analysis of NBS-19. The results of the chemical and isotopic analyses are given in Table 1. We ¢t the data for 87 Sr/86 Sr and stratigraphic level using linear least-squares linear regression of 87 Sr/86 Sr on stratigraphic level; it is computational convenient, and is simpler than modelling with more rigorous ¢tting procedures such as LOWESS [2]. The method of ¢tting makes little di¡erence to our interpretation ; the use of polynomial regression improves data ¢ts, as judged by correlation coe¤cients (r2 ), by less than 2%, compared to linear least-squares regression. Nevertheless, we accept that there is no reason to suppose that nature conforms to algebraic rules. 4. Discussion 4.1. Sample preservation Thin sections examined in plane/polarised light, and using cathode-luminescence, showed that the belemnites contain pristine areas, but are altered on their exteriors, along the apical line, and along some growth rings, as has been reported before [25^28]. Altered areas were removed during sample preparation. The good repeatabillity of our isotopic measurements is strong evidence that our data represent accurately the marine 87 Sr/ 86 Sr of the interval studied. Further evidence of good preservation is the low concentration of Fe and Mn in samples (Table 1), and concentrations of Na, Sr, and Mg that are typical of well-pre-

EPSL 5469 30-5-00

Biozone

striatulum Sz. variabilis variabilis variabilis variabilis variabilis variabilis variabilis variabilis variabilis variabilis variabilis variabilis variabilis Zone crassum crassum crassum crassum crassum crassum Sz. ¢bulatum ¢bulatum ¢bulatum ¢bulatum ¢bulatum ¢bulatum ¢bulatum ¢bulatum ¢bulatum the ¢bulatum Sz. commune commune commune commune commune commune commune

Sample

Base of P3 P4 P5 P7 P8 P 10 P 11 P 12 P 14 P 15 P 17 P 20 Base of P 23 P 24 S 424 S 437A S 437B Base of S 436 P 25 P 28 P 29 S 435A S 432A S 428 S 423A S 422B Base of S 421A S 418C S 414B S 413A S 406 S 401 S 327

EPSL 5469 30-5-00

lii lii 72 72 72

54 54 54 54 53 48 46 44 44 44 38 36

59 54 53 53 51 51 50

71 xli xxxvi xxxiv 65 64 64 62 60

Bed No.

86.30 85.40 84.40 83.00 82.30 81.90 78.50 77.70 76.80 76.60 76.60 73.50 72.55 70.50 68.30 68.10 63.27 62.07 62.07 60.90 60.87 60.00 56.47 55.37 54.19 52.90 51.62 49.52 48.72 48.67 48.10 45.60 43.90 42.00 39.50 37.20 36.41

Stratigraphic level 156.55 155.79 154.94 153.75 153.15 152.81 149.92 149.24 148.48 148.31 148.31 145.67 144.86 143.12 141.25 141.08 136.97 135.95 135.95 134.96 134.93 134.20 131.19 130.26 129.26 128.16 127.07 125.29 124.61 124.56 124.08 121.96 120.51 118.90 116.77 114.82 114.14

Adjusted level

181.25 181.26 181.27 181.29 181.29 181.30 181.34 181.35 181.36 181.36 181.36 181.40 181.41 181.43 181.46 181.46 181.52 181.53 181.53 181.55 181.55 181.56 181.60 181.61 181.62 181.64 181.65 181.68 181.69 181.69 181.70 181.73 181.75 181.77 181.80 181.82 181.83

Numerical age

Sr/86 Sr

0.707 246 0.707 252 0.707 239 0.707 234 0.707 246 0.707 243 0.707 243 0.707 234 0.707 235 0.707 227 0.707 223 0.707 224 0.707 238 0.707 223 0.707 228 0.707 215 0.707 224 0.707 220 0.707 211 0.707 213 0.707 200 0.707 211 0.707 206 0.707 218 0.707 203 0.707 204 0.707 204 0.707 199 0.707 218 0.707 206 0.707 213 0.707 203 0.707 204 0.707 207 0.707 199 0.707 201 0.707 193

87

2 1 1 1 1 1 1

2 1 1 1 3 1 2 2 2

1 2 2 1 1

4.42 4.19 3.42 4.06 3.46 4.29 2.69

2.40 3.04 3.45 4.02 2.67 2.73 3.30 3.09 2.86

4.06 2.81 2.77 2.64

2.75 3.27 2.91 2.98 1.92 3.01 2.80 2.43 2.90 3.81 2.63 2.87

33.02 32.69 33.91 32.78 33.71 32.50 32.94

33.67 33.38 33.58 33.19 33.98 33.75 32.13 33.27 32.44

33.64 33.35 32.95 33.02

32.36 32.93 33.29 33.12 31.75 32.15 32.77 32.70 32.62 33.09 32.67 33.04

(x) (x)

(n) 1 2 1 3 1 2 2 1 2 2 2 2

N13 C N18 O

Sr

38.6 38.8 38.8 38.1 38.0 38.2 38.2

37.7 38.9 38.4 38.6 37.9 37.5 38.0 37.2 39.2

38.9 39.0 37.8 37.9 37.8

39.1 39.3 38.8 38.9 38.4 38.8 38.7 38.5 38.1 38.4 38.7 38.3

(%)

Ca

0.28 0.31 0.42 0.33 0.28 0.26 0.30

0.28 0.29 0.32 0.22 0.30 0.28 0.24 0.28 0.31

0.40 0.33 0.28 0.24 0.22

0.23 0.23 0.26 0.35 0.23 0.22 0.30 0.25 0.22 0.29 0.23 0.26

(%)

Mg

1372 1613 1374 1256 1347 1350 1255

1543 1471 1549 1287 1272 1303 1239 1319 1281

1383 1558 1185 1097 1333

1324 1444 1418 1642 1352 1419 1565 1355 1249 1533 1165 1373

(ppm)

Sr

Table 1 Isotopic and chemical data for belemnites from the late Pliensbachian and early Toarcian strata of the Yorkshire coast, UK

2508 2945 2720 2403 2668 2604 2444

2957 2609 3068 3613 2571 2765 1670 2721 2517

1552 2972 2331 2413 2936

2494 2558 2921 3055 2835 2688 3336 2541 2410 2927 2078 2557

Na

31 14 32 38 17 12 20

14 103 42 18 38 18 35 12 19

31 96 118 8

14 9 25 217 36 15 29 48 23 45 7 30

Fe

1 2 4 61 4 2 4

4 17 5 6 6 2 4 4 5

3 11 15 2

5 4 7 88 11 3 8 12 5 14 3 5

Mn

0.05 0.06 0.06 6 0.10 6 0.05 0.09 6 0.05

0.18 0.04 0.01 0.03 0.01 0.28 0.33 0.05 0.05

6 0.01 6 0.05 0.11 0.13

0.11 6 0.01 6 0.01

6 0.01 0.01 0.24 6 0.01 0.03 6 0.01 0.02 6 0.01

Rb

272 J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285

S 340 commune S 342 commune S 343 commune Base of the commune Sz. S 313A falciferum S 315 falciferum S 319B falciferum S 310 falciferum S 321 falciferum S 306 falciferum S 302 falciferum S 14A falciferum S 13 falciferum S 11A falciferum S 6B falciferum S 9B falciferum R8 falciferum R 6A falciferum R 7A falciferum R 10 falciferum R5 falciferum PM 103 falciferum PM 20 falciferum PM 17 falciferum PM 15 falciferum PM 16 falciferum PM 13 falciferum PM 18 falciferum Base of falciferum Sz. PM 7 exaratum PM 2C exaratum R 2C exaratum PM 8 exaratum S 1A exaratum R 4B exaratum S2 exaratum PM 104 exaratum PM 3 exaratum PM 105 exaratum

Table 1 (continued) I Sample Biozone

Bed No.

EPSL 5469 30-5-00

39 39 38 38 38 37 36 36 35 34

48 47 47 47 45 45 45 45 43 43 43 43 43 43 43 41 41 41 41 41 41 41 41 41

49 49 49

34.56 33.26 31.78 30.21 30.00 28.60 27.00 24.60 23.20 21.90 21.20 21.00 20.40 18.60 17.00 16.10 15.60 14.70 14.00 12.60 12.00 9.90 8.50 8.20 7.80 7.50 7.30 7.20 7.20 6.80 6.55 6.40 5.70 5.40 4.90 4.70 4.70 3.60 2.45

112.57 111.47 110.21 108.87 108.70 107.51 106.15 104.11 102.92 101.81 101.22 101.05 100.54 99.01 97.65 96.88 96.46 95.69 95.10 93.91 93.40 91.61 90.42 90.17 89.83 89.57 87.40 86.32 86.32 81.98 79.27 77.65 70.06 66.81 61.39 59.22 59.22 47.29 34.83

StratigAdjusted raphic level level 181.86 181.87 181.89 181.91 181.91 181.93 181.94 181.97 181.99 182.00 182.01 182.01 182.02 182.04 182.06 182.07 182.08 182.09 182.10 182.11 182.12 182.15 182.16 182.17 182.17 182.17 182.20 182.22 182.22 182.28 182.32 182.34 182.44 182.49 182.56 182.59 182.59 182.76 182.93

Numerical age

Sr/86 Sr

0.707 188 0.707 174 0.707 187 0.707 188 0.707 177 0.707 169 0.707 178 0.707 170 0.707 172 0.707 171 0.707 160 0.707 160 0.707 159 0.707 158 0.707 154 0.707 154 0.707 137 0.707 132 0.707 141 0.707 119 0.707 133 0.707 121 0.707 107

0.707 202 0.707 208 0.707 204 0.707 194 0.707 187 0.707 188 0.707 195 0.707 197 0.707 186 0.707 181 0.707 186 0.707 184 0.707 184 0.707 186

87

2 2 2 2 2 3 1 3 2 1

4 2 1 2 2 2 1 3 2 1 0 0 1 3 2 2 4 3 3 2 1 2 3 2

4.08 3.63 5.29 4.34 3.29 2.36 1.82

5.97 6.36

2.25 2.32 2.43 2.96 4.10 4.69 5.50 4.12 4.38 4.72 3.65 5.55 5.63

5.10 4.47 2.47

4.70 4.82

4.14 2.83 4.58

3.89 3.71

(%)

Ca

33.24 36.9 33.84 37.6 38.7 34.65 38.3 33.90 37.2 32.80 39.0 33.50 37.3 34.27 38.2 34.31 38.6 34.83 38.3

33.32 38.2 31.99 38.1 32.29 38.2 39.0 32.76 38.7 32.69 38.5 38.5 32.58 38.4 33.30 37.5 31.64 37.6 38.2 31.84 37.0 31.51 31.5 32.47 39.0 32.07 39.2 32.99 38.0 33.09 37.4 32.57 39.0 32.51 37.6 34.18 37.4 34.53 37.1 33.04 38.6 34.01 38.0 33.96 38.7

38.3 33.86 38.6 32.63 38.2

(x) (x)

(n) 1 1 1

N13 C N18 O

Sr

0.36 0.35 0.32 0.35 0.36 0.34 0.42 0.40 0.42 0.44

0.29 0.36 0.29 0.29 0.25 0.30 0.27 0.29 0.27 0.29 0.35 0.29 0.37 0.28 0.32 0.35 0.27 0.33 0.36 0.36 0.35 0.38 0.32 0.32

0.43 0.31 0.31

(%)

Mg

1417 1740 1392 1390 1652 1443 1679 1726 1802 1614

1454 1233 1453 1411 1400 1389 1204 1670 1443 1160 1503 1218 1227 1507 1380 1313 1564 1639 1511 1495 1104 1612 1653 1779

1339 1533 1398

(ppm)

Sr

2999 3535 2981 2960 3621 3208 3444 3366 3142 2893

3286 2997 2633 2851 2926 2902 2555 3342 3008 2611 2234 2698 2130 2554 2088 2240 3036 2963 2501 3031 2056 3044 3025 3566

3083 3151 2523

Na

159 77 114 78 75 21 166 13 65 52

15 29 8 8 11 9 13 16 69 38 48 20 149 111 74 62 36 28 103 21 197 43 138 100

39 8 13

Fe

23 13 37 18 13 6 23 5 18 6

61 5 3 2 6 61 3 10 6 13 16 8 15 21 18 16 11 6 20 8 45 9 32 11

4 61 3

Mn

6 0.10 0.10 0.15 0.20 6 0.06 6 0.10 6 0.10

6 0.10 6 0.06

0.02 0.06 0.04

6 0.05 6 0.05 6 0.05 0.10 0.09 6 0.10 6 0.05 6 0.07 6 0.07 0.31 0.1 0.15 6 0.07 0.20 0.09 0.10 6 0.07 0.06 6 0.10 0.07

6 0.01 6 0.10 6 0.05

Rb

J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285 273

Biozone

Bed No.

PM 111 exaratum PM 109 exaratum PM 21 exaratum PM 107 exaratum Base of exaratum Sz. PM 106 semicelatum PM 113 semicelatum PM 112A semicelatum PM 108 semicelatum K 117 semicelatum K 118A semicelatum PM 101 semicelatum K 121 semicelatum PM 102 semicelatum K 112B semicelatum Base of semicelatum Sz. K 111B tenuicostatum K 111C tenuicostatum Base of tenuicostatum Sz. K 108B clevelandicum Base of clevelandicum Sz. K 105A paltum K 107 paltum St 104 paltum HB 7 paltum HB 6 paltum HB 3 paltum HB 4 paltum HB 2 paltum HB 1 paltum Base of paltum Sz and P/T boundary at 313.56m St 109 hawskerense HB 11 hawskerense HB 8 hawskerense HB 14 hawskerense HB 12 hawskerense HB 13 hawskerense

Sample

Table 1 (continued)

EPSL 5469 30-5-00 313.98 314.10 315.92 317.47 317.57 317.63

58 41 40 38 38 38

17 16 14 45 45 44 43 43 43

20

27 27

32 32 32 32 31 31 31 31 30 29

2.25 1.80 0.90 0.15 0.00 30.45 30.65 30.80 31.00 31.68 32.08 32.90 33.18 34.06 34.54 35.36 36.07 36.07 38.10 38.08 39.70 39.77 39.91 311.69 312.53 313.01 313.07 313.09 313.25 313.32 313.56

34 34 34 34

Stratigraphic level

313.98 314.10 315.92 317.47 317.57 317.63

32.66 27.78 18.03 9.90 8.27 7.55 7.23 6.98 6.66 5.57 4.92 3.60 3.15 1.74 0.96 30.36 31.50 31.50 34.77 34.74 37.35 37.46 37.68 310.55 311.90 312.67 312.77 312.80 313.06 313.17 313.56

Adjusted level

183.63 183.64 183.77 183.87 183.88 183.89

182.96 183.03 183.16 183.28 183.30 183.31 183.31 183.32 183.32 183.34 183.34 183.36 183.37 183.39 183.40 183.42 183.43 183.43 183.48 183.48 183.51 183.52 183.52 183.56 183.58 183.59 183.59 183.59 183.59 183.59 183.60

Numerical age

Sr/86 Sr

0.707 077 0.707 075

0.707 068

0.707 071

0.707 070 0.707 076 0.707 085 0.707 079 0.707 072

0.707 104 0.707 108 0.707 085 0.707 103 0.707 094 0.707 085 0.707 101 0.707 089 0.707 093 0.707 093 0.707 081 0.707 091 0.707 088 0.707 097 0.707 082 0.707 085 0.707 086 0.707 088 0.707 080 0.707 079 0.707 078 0.707 081 0.707 078 0.707 072 0.707 068

87

0 1 1

0

1 1 1 2 0 1 2 1 1

1

2 2

1 1 2 1 1 1 1 1 1 2

1.29 1.64 2.21

1.59 1.63 1.46 1.09

3.17 1.96 2.82 2.10

2.33

2.49 2.53

1.68 2.91 3.52 1.92 2.55 3.18 2.29 2.20 2.41 3.88

2.00 3.09 3.31 2.00

(%)

Ca

38.6 37.8 39.1 38.5 39.3 38.6 39.0 38.5 39.1 39.1

38.6 39.5 39.0 38.9 39.0 38.5 39.2 39.1 38.9

32.89 39.1 39.2 38.9 38.7

31.04 38.7

30.53 31.37 30.90 31.09

0.03 31.27 30.57 31.02

30.45 39.2

31.05 38.6 30.65 38.6

33.64 32.76 31.72 31.49 31.09 0.08 31.31 31.55 0.69 0.87

33.51 37.7 38.5 38.4 33.80 38.7

(x) (x)

(n) 1 1 2 1

N13 C N18 O

Sr

0.25 0.37 0.34 0.26

0.19

0.18 0.17 0.19 0.19 0.18 0.23 0.20 0.20 0.21

0.18

0.20 0.17

0.41 0.21 0.25 0.26 0.24 0.22 0.30 0.21 0.17 0.19

0.40 0.48 0.33 0.31

(%)

Mg

987 1424 1577 1466

1158

1041 997 988 932 1054 1408 1161 1027 1012

1023

1047 1061

1269 1154 1321 1115 1146 1137 1342 1099 1025 999

1572 1618 1291 1542

(ppm)

Sr

2467 3503 3914 2738

2397

2226 1961 2142 2108 2449 2684 2123 2244 2149

2153

2161 2199

2665 2414 2779 2020 1991 2215 3223 2138 1821 2145

3266 3071 2825 3133

Na

9

13 22 22

15

16 9

34 47 47 27 19 31 19 17 34 16

9 95 238 39

Fe

6

3 8 13

8

7 3

12 12 13 10 3 7 3 4 18 5

61 8 69 4

Mn

6 0.10

6 0.1 0.11 0.08

0.06

6 0.06 0.09

6 0.01 6 0.06 6 0.06 6 0.06 6 0.06 6 0.06 6 0.01 0.09 6 0.01 0.11

6 0.10 6 0.10 6 0.06 6 0.06

Rb

274 J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285

Biozone

EPSL 5469 30-5-00

Bed No.

25 25 25

27 27

38 38 36 34 32

36 33 28 45 44

319.04 319.09 320.81 323.98 324.74 324.90 325.71 326.11 327.38 328.90 330.80 333.93 334.14 337.18 340.17 341.24 343.03 343.08 349.88 359.59

Stratigraphic level 319.04 319.09 320.81 323.98 329.74 329.90 330.71 336.41 337.68 339.20 341.10 344.23 344.44 347.48 350.47 351.54 353.33 353.38 360.18 369.89

Adjusted level

183.99 183.99 184.11 184.33 184.74 184.75 184.81 185.21 185.30 185.40 185.54 185.76 185.77 185.98 186.19 186.27 186.40 186.40 186.88 187.56

Numerical age

Sr/86 Sr

0.707 070 0.707 070 0.707 073 0.707 083 0.707 106 0.707 102 0.707 126 0.707 130 0.707 136 0.707 129 0.707 152 0.707 152 0.707 160 0.707 174 0.707 185 0.707 188 0.707 185 0.707 203 0.707 213 0.707 230

87

4 3 2

1 1

1 2 1 1 1

2.77 2.57 2.45

2.94 2.28

3.36 2.41 2.04 3.54 3.00

2.42 1.50

(%)

Ca

39.4 39.1 38.4 38.5 38.5

32.38 38.5 32.91 38.7 32.98 38.5

31.38 38.5 31.08 38.4

31.98 30.49 31.97 31.44 30.60

39.2 38.9 39.2 32.64 39.1 0.10 39.0

(x) (x)

(n) 1 1 1 1 1

N13 C N18 O

Sr

0.32 0.32 0.29

0.25 0.26

0.27 0.21 0.29 0.29 0.26

0.35 0.28 0.31 0.29 0.25

(%)

Mg

1451 1465 1329

1240 1187

1305 1119 1373 1388 1331

1496 1372 1470 1388 1100

(ppm)

Sr

3221 3089 2792

2819 2912

2945 2536 3193 3206 3354

3388 3314 3661 3224 2355

Na

14 16 39

32 22

251 29 45 54 72

159 22

Fe

6 3 22

23 5

19 13 12 21 24

27 9

Mn

6 0.10 0.07 0.07

0.07 0.08

0.10 6 0.06 0.08 0.07 6 0.1

6 0.1 0.11

Rb

Stratigraphic levels are on metres from the base of Bed 33 [14^16]. Samples numbers P are from Peak (Blea Wyke), R from Runswick, S from Saltwick Bay, St from Staithes, HB from Hawsker Bottom, R from Runswick, K from Kettleness.

Base of hawskerense Sz. HB 15 apyrenum HB 16 apyrenum HB 17 apyrenum St 116D apyrenum St 102A apyrenum Base of apyrenum Sz. St 119B gibbosus St 120 gibbosus St 121 gibbosus St 123 gibbosus St 126B gibbosus Base of gibbosus Sz. St 127A subnodosus St 128A subnodosus Base of subnodosus Sz. St 129 stokesi St 130 (1) stokesi St 131 stokesi Base of stokesi Sz.

Sample

Table 1 (continued)

J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285 275

276

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Fig. 2. Cross-plots of Fe, Mn, 87 Sr/86 Sr and N18 O in belemnite calcite from the Pliensbachian and Toarcian of the Yorkshire coast.

served biogenic carbonate [1,3,29]. Furthermore, there is no correlation between 87 Sr/86 Sr, N18 O, Fe, and Mn (Fig. 2). 4.2. Isotopic trends in

87

Sr/86 Sr

The 87 Sr/86 Sr of samples is plotted in Fig. 3 against biostratigraphy and stratigraphic level.

The data group into four segments (A^D, Fig. 4) that are modelled well by linear regression analysis. A further interval (from the P/T boundary at 313.56 to 320.8 m) contains a minimum in 87 Sr/86 Sr that is poorly de¢ned owing to a paucity of data, and another region (320.8 to 326.1 m) that contains a hiatus. Within each of the segments A^D, the rate of change of 87 Sr/86 Sr with stratigraphic level, R, is constant and reported here in units of change in 87 Sr/86 Sr (U106 ) per m of section. From the base of the sequence, 87 Sr/86 Sr declines linearly up-section (regression A; R = 33.85) to a level of 326.1 m, above which level a sharp decrease in 87 Sr/86 Sr con¢rms the presence of a hiatus at the apyrenum/gibbosus boundary [19]. The thickness of missing section is estimated to be 10.3 m, by extrapolating regression line A to the 87 Sr/86 Sr value of sample HB17 and measuring the o¡set on the x-axis (Fig. 4). Between 326.1 m and 313.2 m, data are too few to de¢ne the trend in 87 Sr/86 Sr where a minimum in 87 Sr/86 Sr occurs. From 313.2 m, 87 Sr/86 Sr increases linearly (regression B, R = 1.61) to the base of the exaratum subzone. There, R increases abruptly and remains unchanged (regression C, R = 10.4) to 7.5 m, which is 0.3 m into the base of the overlying falciferum subzone, a level where a lithological change may represent a sequence stratigraphic boundary [30]. At 7.5 m, R abruptly decreases and remains constant (regression D, R = 0.85) to the top of the section. The abrupt changes of R within the sequence result from abrupt changes in sedimentation rate that are superimposed on a rate of change of 87 Sr/ 86 Sr with time that is e¡ectively constant: the changes are too sharp to be caused by changing marine 87 Sr/86 Sr. Furthermore, the turning points are accompanied by sedimentological and biostratigraphic changes which con¢rm that they re£ect real events. The interval 0^+7.5 m (mainly exaratum subzone) must be condensed relative to adjacent strata as R in this unit is 12.2 times greater than it is in the overlying units and 6.5 times greater that it is in the immediately underlying units. It has been suggested [10] that the sequence might be condensed around this level, or contain a hiatus. That this is so is shown not only by our

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277

Fig. 3. Values of 87 Sr/86 Sr through the sequence. Open squares = data from [10] normalised to NIST 987 of 0.710 248 by addition of 0.000 022. Subzones shown in italics.

data, but also by the fact that the sequence from 0 to 7.2 m (the exaratum subzone; Jet Rock, Beds 33^40) contains numerous horizons of large carbonate concretions that exceed in size, by many orders of magnitude, those found elsewhere in our sampled sections. Spherical concretions 15 cm in diameter (Cannon Ball Doggers) mark the base of the Jet Rock and its top is marked by concretions 5 m in diameter and up to 1 m thick; the Jet Rock apart, concretions exceeding a diameter of 10 cm are uncommon. The large size of concretions in the Jet Rock con¢rms the interpretation drawn from the 87 Sr/86 Sr record that this unit is condensed relative to other parts of the sequence: condensation decreases burial rates and so increases the time during which nodules are supplied with Ca for growth by di¡usion from the sediment/water interface. 4.3. Relative duration of ammonite biozones Within the stratigraphic range of each of the

linear segment shown in Fig. 4 (A^D), the relative durations of biozones are represented by their relative stratigraphic thicknesses. Considering the Toarcian data, the di¡erent slopes of the regression lines (D, C, B, Fig. 4) represent di¡erent sedimentation rates, so the thicknesses (and so durations) can be made comparable by normalising R to a common value; we use R = 1. Adjusted thicknesses within D are 0.85 of their measured value, that of the Jet Rock increases by a factor of 10.4; adjusted thicknesses within B are 1.61 measured values. The normalised thickness for the Jet Rock is 10.4 times its actual thickness; had it accumulated at the same rate as the strata above or below it, it would have been 88 or 48.5 m thick respectively, rather than 7.2 m. After adjustment, the thicknesses of biozones in the Toarcian sequence re£ect the relative durations of biozones (Table 2) and they di¡er by a factor of 30. The relative durations of Pliensbachian biozones are less easily deduced; a linear model is inapplicable because of the hiatus at the apyrenum/gib-

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Fig. 4. Least-squares linear regression of 87 Sr/86 Sr onto stratigraphic level (H) measured in metres from datum, which is taken as 0 m at the base of Bed 33 of [15] i.e. base of the Cannon Ball Doggers. P/T boundary is at 313.56 m. When extrapolated, regression line A implies 10.3 m of strata are missing from the apyrenum/gibbosus boundary. Symbols distinguish data belonging to each regression line: 87 Sr/86 Sr = 0.707 1665+0.000 000 849 322H r2 = 0.88, n = 97 +85.4 to 7.5 m: 87 Sr/86 Sr = 0.707 0836+0.000 010 398 489H r2 = 0.88, n = 30 +7.5 to 0.0 m: 87 0.0 to 313.2 m: Sr/86 Sr = 0.707 0911+0.000 001 614 452H r2 = 0.68, n = 25 87 Sr/86 Sr = 0.707 031230.000 003 760 355H r2 = 0.95, n = 14 326.1 to 349.9 m:

bosus boundary and the minimum in 87 Sr/86 Sr that marks the latest Pliensbachian. We derive relative durations using an age model described below. 4.4. New age model and the numerical durations of biozones Currently, numerical ages are assigned to Jurassic stage boundaries in part by making stage durations proportional to the number of biozones they contain because biozones are assumed to be of equal duration. This assumptions thus underpin Mesozoic timescales [31] and derivatives such as the Jurassic 87 Sr/86 Sr curve ([2,10], Engkilde, personal communication, 1997) and estimates of the rates of sea level change during the early Toarcian [32,33]. Although these assumptions are accepted widely as probably being incorrect, and have been shown to be incorrect for restricted intervals where zonal duration has been quanti¢ed [9,12,13], they are widely used for lack of any other method of apportioning time to biozones. The ammonite Zones and Subzones within our sequence have durations that di¡er by as much as a factor of 30. This ¢nding requires that two new age models be developed for the interval, one for the Pliensbachian and another for the Toarcian.

We apportion time to Toarcian strata using the adjusted thicknesses and the tie-points of 183.6 Ma for the P/T boundary [34] and 181.4 Ma for the lower variabilis Zone [35]. For the Pliensbachian, we apportioned time on the basis of adjusted sediment thickness and tie-points at the P/T boundary and at the base of the stokesi Subzone, which has a numerical age of 187.56 Ma, calculated from its 87 Sr/86 Sr of 0.707 230 (Bailey, unpublished) and an average rate-of-change of 87 Sr/86 Sr with time of 30.000 040 per Myr for the interval [2], a rate close to that of 30.000 042 per Myr given elsewhere [13]. Our age models allow the numerical durations of Zones and Subzones to be determined (Table 2), but the estimates should be used with caution as they re£ect the timescale used to derive them. These age models show that the mean duration of the four youngest Pliensbachian Subzones is 0.67 Myr, whilst the mean duration for the four oldest Toarcian biozones is 0.075 Myr. That biozone duration changes by a factor of eight across the P/T boundary cannot entirely be an artifact of the age models as the numerical ages we use agree with independent estimates based on cyclostratigraphy [13]. Also, whilst the use of an alternative timescale [31] increases Toarcian durations by a factor of three and reduces Pliensbachian dura-

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tions by 30%, yielding mean durations of 0.08 (Toarcian) and 0.14 Pliensbachian), these are still di¡erent by a factor of about two. With the adopted timescale [34,35], the combined duration of the four oldest ammonite Subzones of the Toarcian totals 0.3 Myr, against previous estimates of between 0.9 and 1.1 Myr ([36], reported in [37]). The duration of the exaratum subzone (1.1 Myr) is longer than the 0.5 Myr previously thought [32,38,39], as is the Zone of H. falciferum, previously believed to be about 1 Myr in duration [10,32,37] and shown here to be about 1.4 Myr. In Germany, the H. exaratum Subzone of the H. falciferum Zone is subdivided into three [40^42]; upwards, these are; Harpoceras (Elegantuliceras Howarth) elegantulum; H. (Cleviceras Howarth) exaratum; H. (Cleviceras Howarth) elegans (the genera derived from [43]. These subzones are not used in the UK, but the ammonites elegantulum and elegans are present in the Yorkshire sequence [33,43]. In view of the considerable duration of the exaratum Subzone, it seems appropriate to use the German scheme.

279

4.5. Dating and correlation with strontium isotope stratigraphy Within the early Toarcian, 87 Sr/86 Sr changes with time at a rate of about 70U1036 per Myr (from data in Table 2). Given an isotopic resolution of þ 4U1036 (2 S.E.M. ; achievable with multiple analysis [7]), an uncertainty at 95% CI on the regression lines A^D (Fig. 4) of less than 16U1036 , and a compounded uncertainty of S.D.total = [(S.E.M.measurement )2 +(S.D.regression )2 ]1=2 , a temporal resolution of about þ 0.25 Myr should be achievable in correlation : given more analysis to reduce uncertainty of the regression, this ¢gure could be reduced by a factor of four. In practice, the ultimate numerical resolution will be dependent on the numerical age model used; other timescales [31,44] result in a ¢gure of around 0.5 Myr, rather than 0.25 Myr. The precision in ¢xing stratigraphic level, however, is not dependent on the age model and is about þ 1.5 m in the exaratum Subzone, about þ 15 m above it, and about þ 7 m below it (but above the P/T boundary).

Table 2 Durations of biozones Zone

Toarcian Thouarsense Variabilis Tie-Point Variabilis Bifrons Falciferum Tenuicostatum

Pliensbachian Spinatum Margaritatus

87 Sr/86 Sr base of biozone

Base of biozone (Ma)

fallaciosum striatulum

0.707 246

no Subzones crassum ¢bulatum commune falciferum exaratum semicelatum tenuicostatum clevelandicum paltum

0.707 223 0.707 213 0.707 206 0.707 194 0.707 159 0.707 094 0.707 085 0.707 080 0.707 078 0.707 073

181.25 181.40 181.43 181.55 181.69 181.91 182.22 183.30 183.42 183.48 183.51 183.60

0.186 0.113 0.144 0.217 0.312 1.080 0.119 0.061 0.036 0.086

5.2 3.2 4.0 6.1 8.8 30.3 3.3 1.7 1.0 2.4

hawskerense apyrenum gibbosus subnodosus stokesi

0.707 070 0.707 126 0.707 160 0.707 188 0.707 230

183.99 184.81 185.77 186.27 187.56

0.390 0.820 0.960 0.500 1.290

1.0 2.1 2.5 1.3 3.3

Subzone

Duration

Relative Duration

(Ma)

Relative durations of the Toarcian biozones are relative to that of the semicelatum Subzone. Relative durations of the Pliensbachian biozones are relative to that of the hawskerense Subzone. Derived from Table 1.

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water. For belemnites, environmental in£uences on N18 O values may dominate over species-speci¢c e¡ects or e¡ects of variable growth rate [22,27,28]. Nevertheless, since belemnites are extinct, we cannot calibrate the temperature response of their N18 O, so we do not calculate absolute palaeo-temperatures. An attempt to calibrate the isotopic composition of ambient water using an association of glendonites and belemnites [45] might be taken as indicating that a vital e¡ect of about 2x may exist for some Aptian belemnites. Nevertheless, we note that Sr/Ca, Na/Ca, and Mg/Ca values in our belemnites closely track changes in N18 O with stratigraphic level (Fig. 5) and correlate signi¢cantly with N18 O at the 1% level of signi¢cance (Fig. 6). The probable relation between temperature and the elemental composition of biogenic carbonate has drawn much interest [46^50] and the Mg/Ca values of belemnites have been used to estimate palaeotemperatures [51,52]. Our data show that Sr/Ca values may be more robust for this purpose than Mg/Ca, since the former correlates better with N18 O than does the latter (Fig. 6). Our element data were acquired solely for the purpose of assessing diagenetic al-

Fig. 5. Variation of N18 Obelemnite and Sr/Ca, Mg/Ca, Na/Ca with stratigraphic level. Filled circles, this paper; open triangles from [22] with their stratigraphic levels corrected by 1.78 m.

With replicate analysis, 87 Sr/86 Sr stratigraphy could subdivide the exaratum Subzone into about ¢ve subdivisions (7.2/1.5), a stratigraphic resolution better than that a¡orded by ammonites. 4.6. Belemnite palaeotemperatures from major elements? The N18 O of biogenic calcite re£ects the e¡ects of metabolic processes, the ambient water temperature, and the isotopic composition of ambient

Fig. 6. Correlation of N18 O with Sr/Ca, Mg/Ca and Na/Ca in belemnite calcite. All correlation coe¤cients are signi¢cant at the 1% level.

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teration. The correlations in Fig. 6 are therefore seen through compositional noise arising from the fact that we analysed a mixture of belemnite species and also sampled for elemental and isotopic analysis randomly within pristine areas of individual rostra, so the data are probably a¡ected by intra-rostral variations in both N18 O and elemental composition. Given that, the good correlations seen in Fig. 6 suggest that the use of belemnite composition for palaeotemperature work deserves further study. If metabolic e¡ects can be assessed [49], and if elemental/Ca values in belemnites re£ect only temperature, whilst belemnite-N18 O values re£ect variations in ice-volume as well as temperature, the combination of elemental and isotopic analysis may o¡er a tool to test for the existence or otherwise of signi¢cant ice volume during that period of time when belemnites £ourished (cf. [50]); for example, to test recent suggestions that polar ice may have existed during the Cretaceous period [53], especially Valanginian times [54]. 4.7. Carbon isotope trends An OAE is regarded as a short period of time during which occurred the widespread deposition of organic-rich sediments, although the terms widespread, short, and organic-rich are not de¢ned well [37^39]. Such events are supposedly accompanied by positive excursions in N13 C of marine carbon (Fig. 7; ibid) which may be of stratigraphic utility. For example, by locating the peak of the early Toarcian carbon isotope excursion within available biozonations [39] it was inferred that lower Toarcian ammonite zones might be diachronous between England and Italy. Conversely, it has been proposed that the carbon isotope excursion is diachronous [37]. The uncertainty could be resolved using Sr isotope stratigraphy, since our 87 Sr/86 Sr data ¢x the start of the OAE (base of Bed 34) at an 87 Sr/86 Sr of 0.707 085 þ 0.000 016 and the end (base of Bed 36) at an 87 Sr/86 Sr value of 0.707 122 þ 0.000 016. The uncertainties are at 95% CI of regression B (Fig. 4) and could be reduced to around þ 0.000 004 [7,55] by replicate analysis of samples

281

Fig. 7. Variation of N13 Cbelemnite and TOC with stratigraphic level. TOC data from [39]. Arrows A, B, mark minima of N13 Cbelemnite and arrows C, D mark excursions to near-normal values within a positive excursion. Filled circles, this paper; open triangles from [22], with stratigraphic levels corrected by 1.78 m. Ammonite subzones denoted by lower case letter, see Fig. 3 for key.

from the stratigraphic limits of the OAE. From our age model, we estimate that the OAE, if de¢ned as occupying the entire time recorded by beds 34 and 35, existed for 0.52 Myr (Table 1). Our carbon isotopic trend (Fig. 7) con¢rms that already published for the Yorkshire interval [23,39] but adds the detail that the major positive peak in the upper exaratum Subzone and higher is interrupted by two short returns to near-normal N13 Cbelemnite at C and D in Fig. 7, suggesting instability in the mechanism of spike generation. Another apparent maximum in N13 Cbelemnite in the semicelatum Subzone is probably an artifact of two bracketing isotopic minima (A and B, Fig. 7). Of the four minima in N13 Cbelemnite (A^D, Fig. 7), one is coincident with the middle exaratum Subzone, where sediment TOC reaches a maximum. Such a coincidence of maximum TOC and minimum N13 C occurs also in a Toarcian sequence in SW Germany [42], where the TOC maximum occurs in the semicelatum Subzone, rather than the mid exaratum Subzone as in Yorkshire. In both Yorkshire and Germany the N13 C minimum is explained as arising either through upwelling of deeper water [39] or from episodic mixing into surface waters of deeper sub-pycnal water [23,42], the deeper water in each case being made isotopically light by mineralisation of organic matter.

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The major positive excursion in N13 Cbelemnite peaks in the uppermost exaratum Subzone, some metres above the peak in sedimentary TOC% (Fig. 7). Positive isotopic excursions are ascribed to the removal from the oceans of large amounts of isotopically light carbon as organic matter (N13 CV325x) into black shales [39,56] or methane hydrates [57], which leaves oceanic carbon isotopically heavy. The large amounts of organic matter responsible for the early Toarcian excursion have not been located and, within the positive excursion, negative excursions occur to about +3.5x at 5.55 m (72 m normalised) and to about +2.5x at about 15.5 m (95 m normalised). These returns to near-normal isotopic values suggest that the positive isotopic excursion was easily reversed and so may have been local, rather than global, in origin. Furthermore, whilst the isotopic maximum reaches +6.5x in Yorkshire belemnites, it is not discernible in the isotopic composition of carbonate from an equivalent stratigraphic interval in SW Germany, and is there only just discernible in organic matter, peaking at 1x above background values. We speculate, therefore, that the positive excursions result from local responses to the burial of organic matter. Methanogenesis yields isotopically light methane (360x) and isotopically heavy CO2 (+15x). Processes within the sediment may mix isotopic signals from di¡erent redox zones and so yield a present-day range that is mostly between 310 and +10x [58,59], but values up to +19x have been measured for CO2 from pore water of sediments from the Baltic Sea [60,61]. Ebullition of this mixed gas from the sediments would have added isotopically heavy CO2 to the overlying water column, because it is a soluble reactive gas, whilst less soluble methane would have escaped from the system [60^64]. Beds 34 and 35 (3.5 m combined thickness, s 12% TOC) are the beds in the sequence both richest in TOC and of a thickness su¤cient to make them quantitatively important as long-term methane sources. Onset of methanogenesis would not have occurred until the organic matter in Beds 34 and 35 had been buried beneath the zone of sulfate reduction. The N13 DIC in the overlying water column could not re£ect isotopically heavy values until methanogene-

sis started. As N13 Cbelemnite ¢rst exceeds +4x in Bed 37, about 1 m (compacted) above Bed 35 (Table 1), we estimate from our age model (Tables 1 and 2) that burial to this depth took about 160 kyr to accomplish, and that the peak excursion in N13 Cbelemnite occurred after about 500 kyr. As our mechanisms are unlikely to in£uence a substantial thickness of water column (for mass balance reasons), the fact that positive isotopic excursions are recorded in belemnite calcite implies that the water in which they lived was shallow. 5. Conclusions 1. Using 87 Sr/86 Sr values for age assignment, we show that the early Toarcian OAE persisted for about 0.52 Myr (Table 1; timescale in [34]). 2. The durations of early Toarcian ammonite Subzones di¡ered by factors of up to 30; i.e. from 0.036 Myr for the clevelandicum Subzone to 1.08 Myr for the exaratum Subzone (timescale of [34]). This ¢nding has implications for the way Mesozoic numerical timescales are made; until now, they have assumed that ammonite biozones are of equal duration. 3. The Sr isotope curve for the interval provides a theoretical resolution of þ 0.25 Myr for dating strata in much of the early Toarcian. 4. Values of Sr/Ca, Mg/Ca, Na/Ca and N18 O in belemnite covary and may record palaeotemperatures. 5. Positive excursions in N13 Cbelemnite stratigraphically just above the early Toarcian OAE may result from ebullition of isotopically heavy CO2 that was generated by methanogenesis of organic rich sediments during shallow burial. 6. A consequence of (5) is that positive isotopic excursions in N13 C, in biogenic calcite or organic matter, will not be precisely synchronous worldwide, since the rate of burial will govern the time taken to reach the zone of methanogenesis for organic-rich sediments. 7. Using our approach of detailed pro¢ling of 87 Sr/86 Sr with stratigraphic level, the durations of other OAEs should be determinable.

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Acknowledgements The Radiogenic Isotope Laboratory at RHUL is supported, in part, by the University of London as an intercollegiate facility. We thank Michael Engkilde (Copenhagen) for providing initial isotopic data and Paul Wignall for advice in the ¢eld. Gerry Ingram, Mark Brownless, and Sarah Houghton helped with the isotopic measurements. Tony Osborn did the elemental analysis, mostly using the NERC ICP-AES Facility at RHUL, with the permission of its Director, Dr. J.N. Walsh. We thank Jan Veizer, Mike Talbot and an anonymous reviewer for constructive critiques of the script.[FA] References [1] J.M. McArthur, Recent trends in strontium isotope stratigraphy, Terra Nova 6 (1994) 331^358. [2] R.J. Howarth, J.M. McArthur, Statistics for strontium isotope stratigraphy: a robust LOWESS ¢t to the marine strontium isotope curve for the period 0 to 206 Ma, with look-up table for the derivation of numerical age, J. Geol. 105 (1997) 441^456. [3] J. Veizer, D. Buhl, A. Deiner, S. Ebneth, O.G. Podlaha, P. Bruckschen, T. Jasper, C. Korte, M. Schaaf, D. Ala, K. Azmy, Strontium isotope stratigraphy: potential resolution and event correlation, Palaeogeogr. Palaeoclimatol. Palaeoecol. 132 (1997) 65^77. [4] M.R. Palmer, H. Elder¢eld, Sr isotope composition of sea water over the past 75 Myr, Nature 314 (1985) 526^528. [5] D.A. Hodell, P.A. Mueller, J.A. McKenzie, G.A. Mead, Strontium isotope stratigraphy and geochemistry of the late Neogene ocean, Earth Planet. Sci. Lett. 92 (1989) 165^178. [6] F.M. Richter, K.K. Turekian, Simple models for the geochemical response of the ocean to climatic and tectonic forcing, Earth Planet. Sci. Lett. 119 (1993) 121^131. [7] J.M. McArthur, M.F. Thirlwall, M. Engkilde, W.J. Zinsmeister, R.J. Howarth, Strontium isotope pro¢les across K/T boundary sequences in Denmark and Antarctica, Earth Planet. Sci. Lett. 160 (1998) 179^192. [8] K.G. Miller, M.D. Feigenson, D.V. Kent, R.K. Olson, Upper Eocene to Oligocene isotope (87Sr/86Sr, d18O, d13C) standard section, Deep Sea Drilling Project Site 522, Paleoceanography 3 (1988) 223^233. [9] J.M. McArthur, M.F. Thirlwall, A.S. Gale, M. Chen, W.J. Kennedy, Strontium isotope stratigraphy in the Late Cretaceous: numerical calibration of the Sr isotope curve and intercontinental correlation for the Campanian, Paleoceanography 8 (1993) 859^873.

283

[10] C.E. Jones, H.C. Jenkyns, S.P. Hesselbo, Strontium isotopes in Early Jurassic seawater, Geochim. Cosmochim. Acta 58 (1994) 1285^1301. [11] E.E. Martin, N.J. Shackleton, J.C. Zachos, B.P. Flower, Orbitally-tuned Sr isotope chemostratigraphy for the late middle to late Miocene, Paleoceanography 14 (1999) 74^ 83. [12] J.M. McArthur, W.J. Kennedy, M. Chen, M.F. Thirlwall, A.S. Gale, Strontium isotope stratigraphy for the Late Cretaceous: direct numeric calibration of the Sr isotope curve for the US Western Interior Seaway, Palaeogeogr. Palaeoclimatol. Palaeoecol. 108 (1994) 95^119. [13] G.P. Weedon, H.C. Jenkyns, Cyclostratigraphy and the Early Jurassic timescale: data from the Belemnite Marls, southern England, Geol. Soc. Am. Bull. 111 (1999) 1823^ 1840. [14] M.K. Howarth, Domerian of the Yorkshire coast, Proc. Yorks. Geol. Soc. 30 (1955) 147^175. [15] M.K. Howarth, The Jet Rock Series and the Alum Shale Series of the Yorkshire coast, Proc. Yorks. Geol. Soc. 33 (1962) 381^422. [16] M.K. Howarth, The stratigraphy and ammonite fauna of the Upper Liassic Grey Shales of the Yorkshire coast, Bull. Br. Mus. (Nat. Hist.) Geol. 24 (1973) 235^277. [17] J.T. Greensmith, P.F. Rawson, S.E. Shalaby, An association of minor ¢ning upwards cycles and aligned gutter marks in the Middle Liass (L. Jurassic) of Yorkshire, Proc. Yorks. Geol. Soc. 42 (1980) 525^538. [18] J.H. Powell, Lithostratigraphical nomenclature of the Liass Group in the Yorkshire Basin, Proc. Yorks. Geol. Soc. 45 (1984) 51^57. [19] A.S. Howard, Lithostratigraphy of the Staithes Sandstone and Cleveland Ironstone formations (Lower Jurassic) of north-east Yorkshire, Proc. Yorks. Geol. Soc. 45 (1985) 261^275. [20] S.P. Hesselbo, H.C. Jenkyns, A comparison of the Hettangian to Bajocian successions of Dorset and Yorkshire, in: P.D. Taylor (Ed.), Field geology of the British Jurassic. Geological Society of London (1995) 105^150. [21] J.H.S. Macquaker, K.G. Taylor, A sequence stratigraphic interpretation of a mudstone-dominated succession: the Lower Jurassic Cleveland Ironstone Formation, UK, J. Geol. Soc. Lond. 153 (1996) 759^770. [22] G. Saelen, P. Doyle, M.R. Talbot, Stable isotope analysis of belemnite rostra from the Whitby Mudstone Formation, England: surface water conditions during deposition of a marine black shale, Palaios 11 (1996) 97^117. [23] G. Saelen, R.V. Tyson, M.R. Talbot, N. Telnaes, Evidence of recycling of isotopically light CO2 (aq) in strati¢ed black shale basins: contrasts between the Whitby Mudstone and Kimmeridge Clay formations, United Kingdom, Geology 26 (1998) 747^750. [24] M.F. Thirlwall, Long-term reproducibility of multicollector Sr and Nd isotope ratio analysis, Chem. Geol. (Isot. Geosci. Sect.) 94 (1991) 85^104. [25] G. Saelen, T.V. Karstang, Chemical signatures in belemnites, N. Jb. Geol. Pala«ont. Abh. Bd 177 (1988) 333^346.

EPSL 5469 30-5-00

284

J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285

[26] G. Saelen, Diagenesis and construction of the belemnite rostrum, Palaeontology 32 (1989) 765^798. [27] O.G. Podlaha, J. Mutterlose, J. Veizer, Preservation of N18 O and N13 C in belemnite rostra from Jurassic/Early Cretaceous successions, Am. J. Sci. 298 (1998) 324^347. [28] O.G. Podlaha, J. Mutterlose, J. Veizer, Preservation of N18 O and N13 C in belemnite rostra from Jurassic/Early Cretaceous successions, Erratum Am. J. Sci. 298 (1998) 534. [29] J. Veizer, Trace elements and isotopes in sedimentary carbonates, Rev. Miner. 11 (1983) 265^300. [30] P.B. Wignall, Model for transgressive black shales?, Geology 19 (1991) 167^170. [31] F.M. Gradstein, F.P. Agterberg, J.G. Ogg, J. Hardenbol, P. Van Veen, J. Thierry, Z. Huang, A Mesozoic Time Scale, J. Geophys. Res. 99 (1994) 24051^24074. [32] A. Hallam, Estimates of the amount and rate of sea-level change across the Rhaetian^Hettangian and Pliensbachian^Toarcian boundaries (latest Triassic to early Jurassic), J. Geol. Soc. Lond. 154 (1997) 773^779. [33] J.C.W. Cope, Discussion on estimates of the amount and rate of sea-level change across the Rhaetian^Hettangian and Pliensbachian^Toarcian boundaries (latest Triassic to early Jurassic), J. Geol. Soc. Lond. 155 (1998) 421^422. [34] J. Pa¨lfy, P.L. Smith, J.K. Mortensen, A revised numeric time scale for the Jurassic, Proceedings of the International Symposium on Jurassic Stratigraphy, Vancouver, September 1998. [35] J. Pa¨lfy, R.R. Parrish, P.L. Smith, A U^Pb age from the Toarcian (Lower Jurassic) and its use for time scale calibration through error analysis of biochronologic dating, Earth Planet. Sci. Lett. 146 (1997) 659^675. [36] P.C. Graciansky, G. Dardeau, T. Dunont, T. Jacquin, D. Marchand, R. Mouterde, P.R. Vail, Depositional sequence cycles, transgressive-regressive facies cycles, and extensional tectonics: examples from the southern Subalpine Jurassic basin, France, Bull. Ge¨ol. Fr. 164 (1993) 709^718. [37] A.P. Jime¨nez, C. Jime¨nez de Cisneros, P. Rivas, J.A. Vera, The early Toarcian anoxic event in the westernmost Tethys (Subbetic): paleogeographic and paleobiogeographic signi¢cance, J. Geol. 104 (1996) 399^416. [38] H.C. Jenkyns, The early Toarcian (Jurassic) anoxic event: stratigraphic, sedimentary and geochemical evidence, Am. J. Sci. 288 (1988) 101^151. [39] H.C. Jenkyns, C.J. Clayton, Lower Jurassic epicontinental carbonates and mudstones from England and Wales: chemostratigraphic signals and the early Toarcian anoxic event, Sedimentology 44 (1997) 687^706. [40] W. Ku«spert, Environmental changes during oil shale deposition as deduced from stable isotope ratios, in: G. Einsele, A. Seilacher (Eds.), Cyclic and Event Strati¢cation, Springier, 1982. [41] W. Riegraf, G. Werner, F. Lo«rcher, Die Posidonienschiefer. Biostratigraphie, Fauna und Fazies des su«dwestdeutschen Untertoarciums (Lias e), Stuttgart: Ferdinand Enke, 1984, 195 pp., 12 pls.

[42] S. Schouten, H. Van Kaam-Peters, W.I.C. Rijpstra, M. Schoell, J.S. Sinninghe Damste, E¡ects of an oceanic anoxic event on the stable carbon isotopic composition of early Toarcian carbon, Am. J. Sci. 300 (2000) 1^22. [43] M.K. Howarth, The ammonite family Hildoceratidae in the Lower Jurassic of Great Britain, Monograph of the Palaeontographical Society, London (1992) 1^200, pls. 1^ 38. [44] W.B. Harland, R.L. Armstrong, A.V. Cox, L.E. Craig, A.G. Smith, D.G. Smith, A Geologic Time Scale 1989, Cambridge University Press, 1990, 263 pp. [45] J.L. DeLurio, L.A. Frakes, Glendonites as a palaeoenvironmental tool: Implications for early Cretaceous high latitude climates in Australia, Geochim. Cosmochim. Acta 63 (1999) 1039^1048. [46] S. de Villiers, G.T. Shen, B.K. Nelson, The Sr/Ca-temperature relationship in coralline aragonite ^ in£uence of variability in (Sr/Ca)seawater and skeletal growth-parameters, Geochim. Cosmochim. Acta 58 (1994) 197^208. [47] Y. Rosenthal, E.A. Boyle, N. Slowey, Temperature control on the incorporation of magnesium, strontium, £uorine, and cadmium into benthic foraminiferal shells from Little Bahama Bank: prospects for thermocline palaeoceanography, Geochim. Cosmochim. Acta 61 (1997) 3633^3643. [48] D.J. Sinclair, L.P.J. Kinsley, M.T. McCulloch, High resolution analysis of trace elements in corals by laser ablation ICP-MS, Geochim. Cosmochim. Acta 62 (1998) 1889^1901. [49] L.M.A. Purton, G.A. Shields, M.D. Brazier, G.W. Grime, Metabolism controls Sr/Ca ratio in fossil aragonitic mollusks, Geology 27 (1999) 1083^1086. [50] C.H. Lear, H. Elder¢eld, P.A. Wilson, Cenozoic deep-sea temperatures and global ice volume from Mg/Ca in benthic foraminiferal calcite, Science 287 (2000) 269^272. [51] T.S. Berlin, D.P. Naydin, V.N. Saks, R.V. Teis, A.V. Khabakov, Jurassic and Cretaceous climate in northern USSR, from palaeotemperature determinations, Int. Geol. Rev. 9 (1967) 1080^1092. [52] N.A. Yasamanov, Paleothermometry of Jurassic, Cretaceous, and Palaeogene periods of some regions of the USSR, Int. Geol. Rev. 23 (1981) 700^706. [53] P.W. Ditch¢eld, High northern palaeolatitude Jurassic^ Cretaceous palaeotemperature variation: New data from Kong Karls Land, Svalbard, Palaeogeogr. Palaeoclimatol. Palaeoecol. 130 (1997) 163^175. [54] G.D. Price, A.H. Ru¡ell, C.E. Jones, R.M. Kalin, J. Mutterlose, Isotopic evidence for temperature variation during the early Cretaceous (late Ryazanian^mid Hauterivian), J. Geol. Soc. Lond. 157 (2000) 335^344. [55] A.J. Crame, J.M. McArthur, D. Pirrie, J.B. Riding, Strontium isotope correlation of the basal Maastrichtian stage in Antarctica to the European and US standard biostratigraphic schemes, J. Geol. Soc. Lond. 156 (1999) 957^964. [56] S.O. Schlanger, H.C. Jenkyns, Cretaceous oceanic anoxic events: causes and consequences, Geol. Mijnb. 55 (1976) 179^184.

EPSL 5469 30-5-00

J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285 [57] G.R. Dickens, J.R. O'Neil, D. Rae, R.M. Owen, Dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the Palaeocene, Paleoceanography 10 (1995) 965^971. [58] M.J. Whiticar, E. Faber, M. Schoell, Biogenic methane formation in marine and freshwater environments: CO2 reduction versus acetate fermentation ^ Isotope evidence, Geochim. Cosmochim. Acta 50 (1986) 693^709. [59] M.J. Whiticar, Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane, Chem. Geol. 161 (1999) 291^314. [60] C.S. Martens, D.A. Albert, M.J. Alperin, Biogeochemical processes controlling methane in gassy coastal sediments ^ Part 1. A model coupling organic matter £ux to gas production, oxidation and transport, Cont. Shelf Res. 18 (1998) 1741^1770. [61] C.S. Martens, D.A. Albert, M.J. Alperin, Stable isotope tracing of anaerobic methane oxidation of gasses in sediments of Eckernfo«rde Bay, German Baltic Sea, Am. J. Sci. 299 (1999) 589^610. [62] N.J. Fendinger, D.D. Adams, D.E. Glotfelty, The role of gas ebullition in the transport of organic contaminants from sediments, Sci. Total Environ. 112 (1992) 189^201. [63] B.B. Curry, T.F. Anderson, K.C. Lohmann, Unusual carbon and oxygen isotopic ratios of ostracodal calcite from last interglacial (Sangamon episode) lacustrine sediment in Raymond basin, IL, USA, J. Paleolimnol. 17 (1997) 421^ 435.

285

[64] M.V. Martinova, The release of gases from sediments, International Review of Hydrobiology, No. SISI (1998) 249^256. [65] K.G. Miller, M.D. Feigenson, J.D. Wright, B.M. Clement, Miocene isotope reference section, Deep Sea Drilling Project Site 608: an evaluation of isotope and biostratigraphic resolution, Paleoceanography 6 (1991) 33^52. [66] D.A. Hodell, P.A. Mueller, J.R. Garrido, Variations in the strontium isotopic composition of seawater during the Neogene, Geology 19 (1991) 24^27. [67] D.A. Hodell, F. Woodru¡, Variations in the strontium isotopic ratio of seawater during the Miocene: stratigraphic and geochemical implications, Paleoceanography 9 (1994) 405^426. [68] J.W. Farrell, S.C. Clemens, L.P. Gromet, Improved chronostratigraphic reference curve of late Neogene seawater 87 Sr/86 Sr, Geology 23 (1995) 403^406. [69] G.A. Mead, D.A. Hodell, Controls on the 87 Sr/86 Sr composition of seawater from the middle Eocene to Oligocene: Hole 689B, Maud Rise, Antarctica, Paleogeography 10 (1995) 327^346. [70] G.M. Henderson, D.J. Martel, R.K. O'Nions, N.J. Shackleton, Evolution of seawater 87 Sr/86 Sr over the last 400 ka: the absence of glacial/interglacial cycles, Earth Planet. Sci. Lett. 128 (1994) 643^651.

EPSL 5469 30-5-00