Strontium isotope pro¢le of the early Toarcian (Jurassic ... · Strontium isotope pro¢le of the...

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, UKb Department of Geology, Royal Holloway and Bedford New College, Egham Hill, Egham, Surrey TW20 0EX, UK

c Department of Geology, University of Illinois, 245 Natural History Building, 1301 W. Green Street, Urbana, IL 61801, USA

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

Abstract

We profile 87Sr/86Sr, N13C, N18O, Sr/Ca, Mg/Ca, and Na/Ca in belemnites through Pliensbachian and Toarcian strataon the Yorkshire coast, UK, which include the early Jurassic oceanic anoxic event. The 87Sr/86Sr profile shows that therelative duration of ammonite subzones differ by a factor of up to 30: the Lower Jurassic exaratum subzone is 30 timeslonger than the clevelandicum subzone because the exaratum subzone in Yorkshire, which contains the anoxic event, iscondensed by a factor of between 6.5 and 12.2 times, relative to adjacent strata. Using our 87Sr/86Sr profile, theresolution in correlation and dating attainable in the interval is between þ 1.5 m and þ 15 m of section, and better than0.25 Myr. In parts of the sequence, this stratigraphic resolution equals that attainable with ammonites. A new agemodel is provided for late Pliensbachian and early Toarcian time that is based on the 87Sr/86Sr profile. Through thesequence, the Sr/Ca, Mg/Ca, Na/Ca and N18O of belemnite carbonate covary, suggesting that elemental ratios may beuseful for palaeotemperature measurement. Our N13Cbelemnite data splits into three the previously reported positiveisotope excursion (to +6.5x) in the early Toarcian. We speculate that the excursion(s) resulted from addition tosurface waters of isotopically heavy CO2 via ebullition of methanogenic CO2 from the sediment during early burial oforganic 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 87Sr/86Sr haschanged with time through the Phanerozoic ([1^3] ; and refs. therein). For that part of the record

when numerical age control is best (0^40 Ma), thechange of 87Sr/86Sr with time is very close to beinglinear, 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 87Sr/86Sr is bu¡ered against short-termchanges by the low concentration of Sr in riverwater 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

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

EPSL 5469 30-5-00

Earth and Planetary Science Letters 179 (2000) 269^285

www.elsevier.com/locate/epsl

the oceans (V2 Myr); models of the response ofmarine 87Sr/86Sr to changing Sr £ux or 87Sr/86Srvalues [4^6] also imply that short-term (6 2 Myr)changes in marine 87Sr/86Sr are strongly damped,except where catastrophic events strongly perturbthe £uxes or 87Sr/86Sr of oceanic inputs or outputs[7].

Curves of 87Sr/86Sr against time (Fig. 1) arederived from the more fundamental relation be-tween 87Sr/86Sr and stratigraphic level (place in arock sequence). Pro¢les of 87Sr/86Sr with strati-graphic level are often non-linear because theyre£ect the interplay of a variable sedimentationrate with a less variable rate-of-change-with-timeof marine 87Sr/86Sr. In a sequence, discontinuitiesin 87Sr/86Sr with stratigraphic level reveal struc-tural and sedimentological discontinuities that en-able the recognition and quanti¢cation of strati-graphic gaps [8^11]. Linear relations between 87Sr/86Sr and stratigraphic level can occur only whenboth the rate-of-change-with-time of marine 87Sr/86Sr is constant and the sedimentation rate is con-stant. Such linear records can be used to estimatethe relative durations of geological events re-corded in the rock. Here we use this principle toestimate the relative duration of ammonite zones[9,12,13] and to show that they di¡er greatly.

We pro¢le 87Sr/86Sr through upper Pliens-bachian and lower Toarcian strata of the York-shire coast (UK), an interval that includes theearly Toarcian oceanic anoxic event (OAE). Weshow that 87Sr/86Sr changes linearly with strati-

graphic level through much of the sequence andthat this fact can be used to estimate the relativedurations of ammonite biozones, and so the du-ration of the early Toarcian OAE. We ¢nd thatthe exaratum subzone (Jet Rock) of the classicYorkshire sequence, in which the OAE is recog-nised, is condensed relative to neighbouring strataby a factor of between 6.5 and 12.2. We show thatdense sampling for 87Sr/86Sr in the intervalstudied has provided a resolution in correlationand dating that is, for parts of the sequence, equalto that a¡orded by ammonites. Finally, we showthat trends in Na/Ca, Mg/Ca and Sr/Ca in be-lemnite calcite closely track the N18Obelemnite recordand we speculate about the origins of these sim-ilarities. Finally, we note that the carbon isotopicrecord of belemnites parallels that of the sedimen-tary organic matter, but the positive isotopic ex-cursion in both lags the peak of TOC in the sedi-ments, and we propose a mechanism to explainthis lagged relation.

2. Geological setting

The geology of the Pliensbachian and Toarcianrocks of the Yorkshire coast is well known [14^23]. We collected belemnites from Hawsker Bot-toms, Staithes, Port Mulgrave, Saltwick Bay,Runswick, Kettleness, and Blea Wyke (Peak), lo-calities on the coast of Yorkshire within a few kmof Whitby (ibid). Exposure in these wave-cut plat-forms and cli¡ sections is close to 100% and cor-relation between the separated sections is possibleto better than decimeter level in the Toarcian, andto better than 50 cm in the Pliensbachian, vianumerous 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 basedon [20]. Stratigraphic levels are referred to BleaWyke for the variabilis Zone, and the crassum and¢bulatum Subzones; to Saltwick Bay from thebase of the ¢bulatum Subzone to the base of theToarcian (paltum Subzone); to Hawsker Bottomsfor the hawskerense and apyrenum subzones; toStaithes for subzones stratigraphically lowerthan the apyrenum Subzone. Stratigraphic levels

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

EPSL 5469 30-5-00

J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285270

are expressed as up (+values) or down (3values)from an arbitrary zero datum placed at the baseof Bed 33 [16], a level that is 13.56 m above thePliensbachian^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 stratigraphiclevels below this boundary by that amount.

3. Analytical methods and results

Our samples were belemnites. They were cut forthin sections and from what remained, the apex,exteriors, apical line, and alveolus were removedusing diamond cutting tools. The remains werefragmented (sub-mm), cleaned in 1.2 molar hy-drochloric acid, washed in ultrapure water, anddried in a clean environment. Fragments werepicked under the binocular microscope, to securethose judged to be best preserved, and were ana-lysed for 87Sr/86Sr, N13C, N18O, Ca, Mg, Sr, Fe,Mn and Rb.

For chemical analysis, samples were dissolvedin 1.8 molar acetic acid. Concentrations of Rbwere measured by furnace-AAS; other elementswere analysed with ICP-AES. The precision ofthe analysis was better than þ 5%, but the repro-ducibility of results exceeded this for a few ele-mental analysis (notably Na) owing to naturalvariability of the sample's composition. For87Sr/86Sr analysis, samples were dissolved in ul-tra-pure 6 M HNO3, evaporated to dryness inorder to oxidise organic matter, and convertedto chloride salt by subsequent evaporation to dry-ness with ultra-pure 6 M HCl. Samples were thentaken up in 2.5 M HCl and Sr was separated bystandard methods of column chromatography.Values of 87Sr/86Sr were determined with a VG-354 ¢ve-collector mass spectrometer using themulti-dynamic routines SRSQ and SRSLL thatinclude corrections for isobaric interference from87Rb [24]. Data have been normalised to a valueof 0.1194 for 86Sr/88Sr. The data were collectedbetween July, 1996, and April, 1999. Duringdata collection, the measured value for NIST987 was within 0.000 035 of the value 0.710 248.Data in Table 1 are adjusted to a value of

0.710 2480 þ 0.000 0025 (2 S.E.M., n = 19) forNIST 987 which equals a value of 0.709 1746þ 0.000 0032 (2 S.E.M., n = 19) for EN-1. Basedupon replicated analysis of standards, the preci-sion of our measurements (2 S.E.M.) wasþ 15U1036 (n = 1), þ 11 (n = 2), þ 9 (n = 3) andþ 8 (n = 4). Total blanks were 6 2 ng of Sr. Sam-ple contained s 5 Wg of Sr. Concentrations of Rbwere too low to require correction for radiogenic87Sr. Analysis for N13C and N18O were carried outusing an Isocarb system attached to a VG Prismstable isotope mass spectrometer. The data arepresented in N notation with respect to the PDBstandard. Analytical precision was 0.1x for bothN13C and N18O with respect to repeat analysis ofNBS-19. The results of the chemical and isotopicanalyses are given in Table 1.

We ¢t the data for 87Sr/86Sr and stratigraphiclevel using linear least-squares linear regression of87Sr/86Sr on stratigraphic level ; it is computation-al convenient, and is simpler than modelling withmore rigorous ¢tting procedures such as LOW-ESS [2]. The method of ¢tting makes little di¡er-ence to our interpretation; the use of polynomialregression improves data ¢ts, as judged by corre-lation coe¤cients (r2), by less than 2%, comparedto linear least-squares regression. Nevertheless, weaccept that there is no reason to suppose thatnature conforms to algebraic rules.

4. Discussion

4.1. Sample preservation

Thin sections examined in plane/polarised light,and using cathode-luminescence, showed that thebelemnites contain pristine areas, but are alteredon their exteriors, along the apical line, and alongsome growth rings, as has been reported before[25^28]. Altered areas were removed during sam-ple preparation. The good repeatabillity of ourisotopic measurements is strong evidence thatour data represent accurately the marine 87Sr/86Sr of the interval studied. Further evidence ofgood preservation is the low concentration of Feand Mn in samples (Table 1), and concentrationsof Na, Sr, and Mg that are typical of well-pre-

EPSL 5469 30-5-00

J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285 271

Tab

le1

Isot

opic

and

chem

ical

data

for

bele

mni

tes

from

the

late

Plie

nsba

chia

nan

dea

rly

Toa

rcia

nst

rata

ofth

eY

orks

hire

coas

t,U

K

Sam

ple

Bio

zone

Bed

No.

Stra

tig-

raph

icle

vel

Adj

uste

dle

vel

Num

er-

ical

age

87Sr

/86Sr

SrN13

CN18

OC

aM

gSr

Na

Fe

Mn

Rb

(n)

(x)

(x)

(%)

(%)

(ppm

)

Bas

eof

stri

atul

umS

z.86

.30

156.

5518

1.25

0.70

724

6P

3va

riab

ilis

5485

.40

155.

7918

1.26

0.70

725

21

2.75

32.

3639

.10.

2313

2424

9414

56

0.01

P4

vari

abili

s54

84.4

015

4.94

181.

270.

707

239

23.

273

2.93

39.3

0.23

1444

2558

94

0.01

P5

vari

abili

s54

83.0

015

3.75

181.

290.

707

234

12.

913

3.29

38.8

0.26

1418

2921

257

0.24

P7

vari

abili

s54

82.3

015

3.15

181.

290.

707

246

32.

983

3.12

38.9

0.35

1642

3055

217

886

0.01

P8

vari

abili

s53

81.9

015

2.81

181.

300.

707

243

11.

923

1.75

38.4

0.23

1352

2835

3611

0.03

P10

vari

abili

s48

78.5

014

9.92

181.

340.

707

243

23.

013

2.15

38.8

0.22

1419

2688

153

60.

01P

11va

riab

ilis

4677

.70

149.

2418

1.35

0.70

723

42

2.80

32.

7738

.70.

3015

6533

3629

80.

02P

12va

riab

ilis

4476

.80

148.

4818

1.36

0.70

723

51

2.43

32.

7038

.50.

2513

5525

4148

126

0.01

P14

vari

abili

s44

76.6

014

8.31

181.

360.

707

227

22.

903

2.62

38.1

0.22

1249

2410

235

P15

vari

abili

s44

76.6

014

8.31

181.

360.

707

223

23.

813

3.09

38.4

0.29

1533

2927

4514

0.11

P17

vari

abili

s38

73.5

014

5.67

181.

400.

707

224

22.

633

2.67

38.7

0.23

1165

2078

73

60.

01P

20va

riab

ilis

3672

.55

144.

8618

1.41

0.70

723

82

2.87

33.

0438

.30.

2613

7325

5730

56

0.01

Bas

eof

vari

abili

sZ

one

70.5

014

3.12

181.

430.

707

223

P23

cras

sum

lii68

.30

141.

2518

1.46

0.70

722

81

38.9

0.40

1383

1552

P24

cras

sum

lii68

.10

141.

0818

1.46

0.70

721

52

4.06

33.

6439

.00.

3315

5829

7231

36

0.01

S42

4cr

assu

m72

63.2

713

6.97

181.

520.

707

224

22.

813

3.35

37.8

0.28

1185

2331

9611

60.

05S

437A

cras

sum

7262

.07

135.

9518

1.53

0.70

722

01

2.77

32.

9537

.90.

2410

9724

1311

815

0.11

S43

7Bcr

assu

m72

62.0

713

5.95

181.

530.

707

211

12.

643

3.02

37.8

0.22

1333

2936

82

0.13

Bas

eof

cras

sum

Sz.

60.9

013

4.96

181.

550.

707

213

S43

6¢b

ulat

um71

60.8

713

4.93

181.

550.

707

200

22.

403

3.67

37.7

0.28

1543

2957

144

0.18

P25

¢bul

atum

xli

60.0

013

4.20

181.

560.

707

211

13.

043

3.38

38.9

0.29

1471

2609

103

170.

04P

28¢b

ulat

umxx

xvi

56.4

713

1.19

181.

600.

707

206

13.

453

3.58

38.4

0.32

1549

3068

425

0.01

P29

¢bul

atum

xxxi

v55

.37

130.

2618

1.61

0.70

721

81

4.02

33.

1938

.60.

2212

8736

1318

60.

03S

435A

¢bul

atum

6554

.19

129.

2618

1.62

0.70

720

33

2.67

33.

9837

.90.

3012

7225

7138

60.

01S

432A

¢bul

atum

6452

.90

128.

1618

1.64

0.70

720

41

2.73

33.

7537

.50.

2813

0327

6518

20.

28S

428

¢bul

atum

6451

.62

127.

0718

1.65

0.70

720

42

3.30

32.

1338

.00.

2412

3916

7035

40.

33S

423A

¢bul

atum

6249

.52

125.

2918

1.68

0.70

719

92

3.09

33.

2737

.20.

2813

1927

2112

40.

05S

422B

¢bul

atum

6048

.72

124.

6118

1.69

0.70

721

82

2.86

32.

4439

.20.

3112

8125

1719

50.

05B

ase

ofth

e¢b

ulat

umS

z.48

.67

124.

5618

1.69

0.70

720

6S

421A

com

mun

e59

48.1

012

4.08

181.

700.

707

213

24.

423

3.02

38.6

0.28

1372

2508

311

0.05

S41

8Cco

mm

une

5445

.60

121.

9618

1.73

0.70

720

31

4.19

32.

6938

.80.

3116

1329

4514

20.

06S

414B

com

mun

e53

43.9

012

0.51

181.

750.

707

204

13.

423

3.91

38.8

0.42

1374

2720

324

0.06

S41

3Aco

mm

une

5342

.00

118.

9018

1.77

0.70

720

71

4.06

32.

7838

.10.

3312

5624

0338

61

60.

10S

406

com

mun

e51

39.5

011

6.77

181.

800.

707

199

13.

463

3.71

38.0

0.28

1347

2668

174

60.

05S

401

com

mun

e51

37.2

011

4.82

181.

820.

707

201

14.

293

2.50

38.2

0.26

1350

2604

122

0.09

S32

7co

mm

une

5036

.41

114.

1418

1.83

0.70

719

31

2.69

32.

9438

.20.

3012

5524

4420

46

0.05

EPSL 5469 30-5-00


Tab

le1

(con

tinu

ed)

I Sam

ple

Bio

zone

Bed

No.

Stra

tig-

raph

icle

vel

Adj

uste

dle

vel

Num

er-

ical

age

87Sr

/86Sr

SrN13

CN18

OC

aM

gSr

Na

Fe

Mn

Rb

(n)

(x)

(x)

(%)

(%)

(ppm

)

S34

0co

mm

une

4934

.56

112.

5718

1.86

0.70

720

21

38.3

0.43

1339

3083

394

60.

01S

342

com

mun

e49

33.2

611

1.47

181.

870.

707

208

13.

893

3.86

38.6

0.31

1533

3151

86

16

0.10

S34

3co

mm

une

4931

.78

110.

2118

1.89

0.70

720

41

3.71

32.

6338

.20.

3113

9825

2313

36

0.05

Bas

eof

the

com

mun

eS

z.30

.21

108.

8718

1.91

0.70

719

4S

313A

falc

ifer

um48

30.0

010

8.70

181.

910.

707

187

44.

143

3.32

38.2

0.29

1454

3286

156

16

0.05

S31

5fa

lcif

erum

4728

.60

107.

5118

1.93

0.70

718

82

2.83

31.

9938

.10.

3612

3329

9729

56

0.05

S31

9Bfa

lcif

erum

4727

.00

106.

1518

1.94

0.70

719

51

4.58

32.

2938

.20.

2914

5326

338

36

0.05

S31

0fa

lcif

erum

4724

.60

104.

1118

1.97

0.70

719

72

39.0

0.29

1411

2851

82

0.10

S32

1fa

lcif

erum

4523

.20

102.

9218

1.99

0.70

718

62

4.70

32.

7638

.70.

2514

0029

2611

60.

09S

306

falc

ifer

um45

21.9

010

1.81

182.

000.

707

181

24.

823

2.69

38.5

0.30

1389

2902

96

16

0.10

S30

2fa

lcif

erum

4521

.20

101.

2218

2.01

0.70

718

61

38.5

0.27

1204

2555

133

60.

05S

14A

falc

ifer

um45

21.0

010

1.05

182.

010.

707

184

35.

103

2.58

38.4

0.29

1670

3342

1610

60.

07S

13fa

lcif

erum

4320

.40

100.

5418

2.02

0.70

718

42

4.47

33.

3037

.50.

2714

4330

0869

66

0.07

S11

Afa

lcif

erum

4318

.60

99.0

118

2.04

0.70

718

61

2.47

31.

6437

.60.

2911

6026

1138

130.

31S

6Bfa

lcif

erum

4317

.00

97.6

518

2.06

038

.20.

3515

0322

3448

160.

1S

9Bfa

lcif

erum

4316

.10

96.8

818

2.07

02.

253

1.84

37.0

0.29

1218

2698

208

0.15

R8

falc

ifer

um43

15.6

096

.46

182.

080.

707

188

12.

323

1.51

31.5

0.37

1227

2130

149

156

0.07

R6A

falc

ifer

um43

14.7

095

.69

182.

090.

707

174

32.

433

2.47

39.0

0.28

1507

2554

111

210.

20R

7Afa

lcif

erum

4314

.00

95.1

018

2.10

0.70

718

72

2.96

32.

0739

.20.

3213

8020

8874

180.

09R

10fa

lcif

erum

4112

.60

93.9

118

2.11

0.70

718

82

4.10

32.

9938

.00.

3513

1322

4062

160.

10R

5fa

lcif

erum

4112

.00

93.4

018

2.12

0.70

717

74

4.69

33.

0937

.40.

2715

6430

3636

116

0.07

PM

103

falc

ifer

um41

9.90

91.6

118

2.15

0.70

716

93

5.50

32.

5739

.00.

3316

3929

6328

60.

06P

M20

falc

ifer

um41

8.50

90.4

218

2.16

0.70

717

83

4.12

32.

5137

.60.

3615

1125

0110

320

60.

10P

M17

falc

ifer

um41

8.20

90.1

718

2.17

0.70

717

02

4.38

34.

1837

.40.

3614

9530

3121

80.

07P

M15

falc

ifer

um41

7.80

89.8

318

2.17

0.70

717

21

4.72

34.

5337

.10.

3511

0420

5619

745

PM

16fa

lcif

erum

417.

5089

.57

182.

170.

707

171

23.

653

3.04

38.6

0.38

1612

3044

439

0.02

PM

13fa

lcif

erum

417.

3087

.40

182.

200.

707

160

35.

553

4.01

38.0

0.32

1653

3025

138

320.

06P

M18

falc

ifer

um41

7.20

86.3

218

2.22

0.70

716

02

5.63

33.

9638

.70.

3217

7935

6610

011

0.04

Bas

eof

falc

ifer

umS

z.7.

2086

.32

182.

220.

707

159

PM

7ex

arat

um39

6.80

81.9

818

2.28

0.70

715

82

5.97

33.

2436

.90.

3614

1729

9915

923

60.

10P

M2C

exar

atum

396.

5579

.27

182.

320.

707

154

26.

363

3.84

37.6

0.35

1740

3535

7713

60.

06R

2Cex

arat

um38

6.40

77.6

518

2.34

0.70

715

42

38.7

0.32

1392

2981

114

37P

M8

exar

atum

385.

7070

.06

182.

440.

707

137

24.

083

4.65

38.3

0.35

1390

2960

7818

60.

10S

1Aex

arat

um38

5.40

66.8

118

2.49

0.70

713

22

3.63

33.

9037

.20.

3616

5236

2175

130.

10R

4Bex

arat

um37

4.90

61.3

918

2.56

0.70

714

13

5.29

32.

8039

.00.

3414

4332

0821

60.

15S

2ex

arat

um36

4.70

59.2

218

2.59

0.70

711

91

4.34

33.

5037

.30.

4216

7934

4416

623

0.20

PM

104

exar

atum

364.

7059

.22

182.

590.

707

133

33.

293

4.27

38.2

0.40

1726

3366

135

60.

06P

M3

exar

atum

353.

6047

.29

182.

760.

707

121

22.

363

4.31

38.6

0.42

1802

3142

6518

60.

10P

M10

5ex

arat

um34

2.45

34.8

318

2.93

0.70

710

71

1.82

34.

8338

.30.

4416

1428

9352

66

0.10

EPSL 5469 30-5-00


Tab

le1

(con

tinu

ed)

Sam

ple

Bio

zone

Bed

No.

Stra

tig-

raph

icle

vel

Adj

uste

dle

vel

Num

er-

ical

age

87Sr

/86Sr

SrN13

CN18

OC

aM

gSr

Na

Fe

Mn

Rb

(n)

(x)

(x)

(%)

(%)

(ppm

)

PM

111

exar

atum

342.

2532

.66

182.

960.

707

104

12.

003

3.51

37.7

0.40

1572

3266

96

16

0.10

PM

109

exar

atum

341.

8027

.78

183.

030.

707

108

13.

0938

.50.

4816

1830

7195

86

0.10

PM

21ex

arat

um34

0.90

18.0

318

3.16

0.70

708

52

3.31

38.4

0.33

1291

2825

238

696

0.06

PM

107

exar

atum

340.

159.

9018

3.28

0.70

710

31

2.00

33.

8038

.70.

3115

4231

3339

46

0.06

Bas

eof

exar

atum

Sz.

0.00

8.27

183.

300.

707

094

PM

106

sem

icel

atum

323

0.45

7.55

183.

310.

707

085

11.

683

3.64

38.6

0.41

1269

2665

3412

60.

01P

M11

3se

mic

elat

um32

30.

657.

2318

3.31

0.70

710

11

2.91

32.

7637

.80.

2111

5424

1447

126

0.06

PM

112A

sem

icel

atum

323

0.80

6.98

183.

320.

707

089

23.

523

1.72

39.1

0.25

1321

2779

4713

60.

06P

M10

8se

mic

elat

um32

31.

006.

6618

3.32

0.70

709

31

1.92

31.

4938

.50.

2611

1520

2027

106

0.06

K11

7se

mic

elat

um31

31.

685.

5718

3.34

0.70

709

31

2.55

31.

0939

.30.

2411

4619

9119

36

0.06

K11

8Ase

mic

elat

um31

32.

084.

9218

3.34

0.70

708

11

3.18

0.08

38.6

0.22

1137

2215

317

60.

06P

M10

1se

mic

elat

um31

32.

903.

6018

3.36

0.70

709

11

2.29

31.

3139

.00.

3013

4232

2319

36

0.01

K12

1se

mic

elat

um31

33.

183.

1518

3.37

0.70

708

81

2.20

31.

5538

.50.

2110

9921

3817

40.

09P

M10

2se

mic

elat

um30

34.

061.

7418

3.39

0.70

709

71

2.41

0.69

39.1

0.17

1025

1821

3418

60.

01K

112B

sem

icel

atum

293

4.54

0.96

183.

400.

707

082

23.

880.

8739

.10.

1999

921

4516

50.

11B

ase

ofse

mic

elat

umS

z.33

5.36

330.

3618

3.42

0.70

708

5K

111B

tenu

icos

tatu

m27

36.

073

1.50

183.

430.

707

086

22.

493

1.05

38.6

0.20

1047

2161

167

60.

06K

111C

tenu

icos

tatu

m27

36.

073

1.50

183.

430.

707

088

22.

533

0.65

38.6

0.17

1061

2199

93

0.09

Bas

eof

tenu

icos

tatu

mS

z.33

8.10

334.

7718

3.48

0.70

708

0K

108B

clev

elan

dicu

m20

38.

083

4.74

183.

480.

707

079

12.

333

0.45

39.2

0.18

1023

2153

158

0.06

Bas

eof

clev

elan

dicu

mS

z.33

9.70

337.

3518

3.51

0.70

707

8K

105A

palt

um17

39.

773

7.46

183.

520.

707

081

13.

170.

0338

.60.

1810

4122

2613

36

0.1

K10

7pa

ltum

163

9.91

37.

6818

3.52

0.70

707

81

1.96

31.

2739

.50.

1799

719

6122

80.

11St

104

palt

um14

311

.69

310

.55

183.

560.

707

072

12.

823

0.57

39.0

0.19

988

2142

2213

0.08

HB

7pa

ltum

453

12.5

33

11.9

018

3.58

0.70

706

82

2.10

31.

0238

.90.

1993

221

08H

B6

palt

um45

313

.01

312

.67

183.

590

39.0

0.18

1054

2449

HB

3pa

ltum

443

13.0

73

12.7

718

3.59

0.70

707

01

1.59

30.

5338

.50.

2314

0826

84H

B4

palt

um43

313

.09

312

.80

183.

590.

707

076

21.

633

1.37

39.2

0.20

1161

2123

HB

2pa

ltum

433

13.2

53

13.0

618

3.59

0.70

708

51

1.46

30.

9039

.10.

2010

2722

44H

B1

palt

um43

313

.32

313

.17

183.

590.

707

079

11.

093

1.09

38.9

0.21

1012

2149

Bas

eof

palt

umS

zan

dP

/Tbo

unda

ryat

3313

.56m

3313

.56

3313

.56

183.

600.

707

072

St10

9ha

wsk

eren

se58

313

.98

313

.98

183.

630.

707

071

1.29

31.

0438

.70.

1911

5823

979

66

0.10

HB

11ha

wsk

eren

se41

314

.10

314

.10

183.

640

1.64

HB

8ha

wsk

eren

se40

315

.92

315

.92

183.

770.

707

068

2.21

32.

8939

.10.

2598

724

67H

B14

haw

sker

ense

383

17.4

73

17.4

718

3.87

039

.20.

3714

2435

03H

B12

haw

sker

ense

383

17.5

73

17.5

718

3.88

0.70

707

71

38.9

0.34

1577

3914

HB

13ha

wsk

eren

se38

317

.63

317

.63

183.

890.

707

075

138

.70.

2614

6627

38

EPSL 5469 30-5-00


Tab

le1

(con

tinu

ed)

Sam

ple

Bio

zone

Bed

No.

Stra

tig-

raph

icle

vel

Adj

uste

dle

vel

Num

er-

ical

age

87Sr

/86Sr

SrN13

CN18

OC

aM

gSr

Na

Fe

Mn

Rb

(n)

(x)

(x)

(%)

(%)

(ppm

)

Bas

eof

haw

sker

ense

Sz.

3319

.04

3319

.04

183.

990.

707

070

HB

15ap

yren

um36

319

.09

319

.09

183.

990.

707

070

139

.20.

3514

9633

88H

B16

apyr

enum

333

20.8

13

20.8

118

4.11

0.70

707

31

38.9

0.28

1372

3314

HB

17ap

yren

um28

323

.98

323

.98

184.

330.

707

083

139

.20.

3114

7036

61St

116D

apyr

enum

453

24.7

43

29.7

418

4.74

0.70

710

61

2.42

32.

6439

.10.

2913

8832

2415

927

60.

1St

102A

apyr

enum

443

24.9

03

29.9

018

4.75

0.70

710

21

1.50

0.10

39.0

0.25

1100

2355

229

0.11

Bas

eof

apyr

enum

Sz.

3325

.71

3330

.71

184.

810.

707

126

St11

9Bgi

bbos

us38

326

.11

336

.41

185.

210.

707

130

13.

363

1.98

39.4

0.27

1305

2945

251

190.

10St

120

gibb

osus

383

27.3

83

37.6

818

5.30

0.70

713

62

2.41

30.

4939

.10.

2111

1925

3629

136

0.06

St12

1gi

bbos

us36

328

.90

339

.20

185.

400.

707

129

12.

043

1.97

38.4

0.29

1373

3193

4512

0.08

St12

3gi

bbos

us34

330

.80

341

.10

185.

540.

707

152

13.

543

1.44

38.5

0.29

1388

3206

5421

0.07

St12

6Bgi

bbos

us32

333

.93

344

.23

185.

760.

707

152

13.

003

0.60

38.5

0.26

1331

3354

7224

60.

1B

ase

ofgi

bbos

usS

z.33

34.1

433

44.4

418

5.77

0.70

716

0St

127A

subn

odos

us27

337

.18

347

.48

185.

980.

707

174

12.

943

1.38

38.5

0.25

1240

2819

3223

0.07

St12

8Asu

bnod

osus

273

40.1

73

50.4

718

6.19

0.70

718

51

2.28

31.

0838

.40.

2611

8729

1222

50.

08B

ase

ofsu

bnod

osus

Sz.

3341

.24

3351

.54

186.

270.

707

188

St12

9st

okes

i25

343

.03

353

.33

186.

400.

707

185

42.

773

2.38

38.5

0.32

1451

3221

146

60.

10St

130

(1)

stok

esi

253

43.0

83

53.3

818

6.40

0.70

720

33

2.57

32.

9138

.70.

3214

6530

8916

30.

07St

131

stok

esi

253

49.8

83

60.1

818

6.88

0.70

721

32

2.45

32.

9838

.50.

2913

2927

9239

220.

07B

ase

ofst

okes

iS

z.33

59.5

933

69.8

918

7.56

0.70

723

0

Stra

tigr

aphi

cle

vels

are

onm

etre

sfr

omth

eba

seof

Bed

33[1

4^16

].Sa

mpl

esnu

mbe

rsP

are

from

Pea

k(B

lea

Wyk

e),

Rfr

omR

unsw

ick,

Sfr

omSa

ltw

ick

Bay

,St

from

Stai

thes

,H

Bfr

omH

awsk

erB

otto

m,

Rfr

omR

unsw

ick,

Kfr

omK

ettl

enes

s.

EPSL 5469 30-5-00


served biogenic carbonate [1,3,29]. Furthermore,there is no correlation between 87Sr/86Sr, N18O,Fe, and Mn (Fig. 2).

4.2. Isotopic trends in 87Sr/86Sr

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

The data group into four segments (A^D, Fig.4) that are modelled well by linear regressionanalysis. A further interval (from the P/T bound-ary at 313.56 to 320.8 m) contains a minimum in87Sr/86Sr that is poorly de¢ned owing to a paucityof data, and another region (320.8 to 326.1 m)that contains a hiatus. Within each of the seg-ments A^D, the rate of change of 87Sr/86Sr withstratigraphic level, R, is constant and reportedhere in units of change in 87Sr/86Sr (U106) perm of section.

From the base of the sequence, 87Sr/86Sr de-clines linearly up-section (regression A;R =33.85) to a level of 326.1 m, above whichlevel a sharp decrease in 87Sr/86Sr con¢rms thepresence of a hiatus at the apyrenum/gibbosusboundary [19]. The thickness of missing sectionis estimated to be 10.3 m, by extrapolating regres-sion line A to the 87Sr/86Sr value of sample HB17and measuring the o¡set on the x-axis (Fig. 4).Between 326.1 m and 313.2 m, data are too fewto de¢ne the trend in 87Sr/86Sr where a minimumin 87Sr/86Sr occurs. From 313.2 m, 87Sr/86Sr in-creases linearly (regression B, R = 1.61) to thebase of the exaratum subzone. There, R increasesabruptly and remains unchanged (regression C,R = 10.4) to 7.5 m, which is 0.3 m into the baseof the overlying falciferum subzone, a level wherea lithological change may represent a sequencestratigraphic boundary [30]. At 7.5 m, R abruptlydecreases and remains constant (regression D,R = 0.85) to the top of the section.

The abrupt changes of R within the sequenceresult from abrupt changes in sedimentation ratethat are superimposed on a rate of change of 87Sr/86Sr with time that is e¡ectively constant: thechanges are too sharp to be caused by changingmarine 87Sr/86Sr. Furthermore, the turning pointsare accompanied by sedimentological and biostra-tigraphic changes which con¢rm that they re£ectreal events. The interval 0^+7.5 m (mainly exara-tum subzone) must be condensed relative to ad-jacent strata as R in this unit is 12.2 times greaterthan it is in the overlying units and 6.5 timesgreater that it is in the immediately underlyingunits. It has been suggested [10] that the sequencemight be condensed around this level, or containa hiatus. That this is so is shown not only by our

Fig. 2. Cross-plots of Fe, Mn, 87Sr/86Sr and N18O in belem-nite calcite from the Pliensbachian and Toarcian of theYorkshire coast.

EPSL 5469 30-5-00


data, but also by the fact that the sequence from0 to 7.2 m (the exaratum subzone; Jet Rock, Beds33^40) contains numerous horizons of large car-bonate concretions that exceed in size, by manyorders of magnitude, those found elsewhere in oursampled sections. Spherical concretions 15 cm indiameter (Cannon Ball Doggers) mark the base ofthe Jet Rock and its top is marked by concretions5 m in diameter and up to 1 m thick; the Jet Rockapart, concretions exceeding a diameter of 10 cmare uncommon. The large size of concretions inthe Jet Rock con¢rms the interpretation drawnfrom the 87Sr/86Sr record that this unit is con-densed relative to other parts of the sequence:condensation decreases burial rates and so in-creases the time during which nodules are sup-plied with Ca for growth by di¡usion from thesediment/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 relativedurations of biozones are represented by their rel-ative stratigraphic thicknesses. Considering theToarcian data, the di¡erent slopes of the regres-sion lines (D, C, B, Fig. 4) represent di¡erentsedimentation rates, so the thicknesses (and sodurations) can be made comparable by normal-ising R to a common value; we use R = 1. Ad-justed thicknesses within D are 0.85 of their meas-ured value, that of the Jet Rock increases by afactor of 10.4; adjusted thicknesses within B are1.61 measured values. The normalised thicknessfor the Jet Rock is 10.4 times its actual thickness;had it accumulated at the same rate as the strataabove or below it, it would have been 88 or 48.5m thick respectively, rather than 7.2 m. After ad-justment, the thicknesses of biozones in the Toar-cian sequence re£ect the relative durations of bio-zones (Table 2) and they di¡er by a factor of 30.The relative durations of Pliensbachian biozonesare less easily deduced; a linear model is inappli-cable because of the hiatus at the apyrenum/gib-

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

EPSL 5469 30-5-00


bosus boundary and the minimum in 87Sr/86Srthat marks the latest Pliensbachian. We deriverelative durations using an age model describedbelow.

4.4. New age model and the numerical durations ofbiozones

Currently, numerical ages are assigned to Juras-sic stage boundaries in part by making stage du-rations proportional to the number of biozonesthey contain because biozones are assumed to beof equal duration. This assumptions thus under-pin Mesozoic timescales [31] and derivatives suchas the Jurassic 87Sr/86Sr curve ([2,10], Engkilde,personal communication, 1997) and estimates ofthe rates of sea level change during the earlyToarcian [32,33]. Although these assumptionsare accepted widely as probably being incorrect,and have been shown to be incorrect for restrictedintervals where zonal duration has been quanti¢ed[9,12,13], they are widely used for lack of anyother method of apportioning time to biozones.

The ammonite Zones and Subzones within oursequence have durations that di¡er by as much asa factor of 30. This ¢nding requires that two newage models be developed for the interval, one forthe Pliensbachian and another for the Toarcian.

We apportion time to Toarcian strata using theadjusted thicknesses and the tie-points of 183.6Ma for the P/T boundary [34] and 181.4 Ma forthe lower variabilis Zone [35]. For the Pliens-bachian, we apportioned time on the basis of ad-justed sediment thickness and tie-points at theP/T boundary and at the base of the stokesi Sub-zone, which has a numerical age of 187.56 Ma,calculated from its 87Sr/86Sr of 0.707 230 (Bailey,unpublished) and an average rate-of-change of87Sr/86Sr with time of 30.000 040 per Myr forthe interval [2], a rate close to that of30.000 042 per Myr given elsewhere [13].

Our age models allow the numerical durationsof Zones and Subzones to be determined (Table2), but the estimates should be used with cautionas they re£ect the timescale used to derive them.These age models show that the mean duration ofthe four youngest Pliensbachian Subzones is 0.67Myr, whilst the mean duration for the four oldestToarcian biozones is 0.075 Myr. That biozoneduration changes by a factor of eight across theP/T boundary cannot entirely be an artifact of theage models as the numerical ages we use agreewith independent estimates based on cyclostratig-raphy [13]. Also, whilst the use of an alternativetimescale [31] increases Toarcian durations by afactor of three and reduces Pliensbachian dura-

Fig. 4. Least-squares linear regression of 87Sr/86Sr onto stratigraphic level (H) measured in metres from datum, which is taken as0 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, re-gression line A implies 10.3 m of strata are missing from the apyrenum/gibbosus boundary. Symbols distinguish data belonging toeach regression line:

+85.4 to 7.5 m : 87Sr/86Sr = 0.707 1665+0.000 000 849 322H r2 = 0.88, n = 97+7.5 to 0.0 m : 87Sr/86Sr = 0.707 0836+0.000 010 398 489H r2 = 0.88, n = 300.0 to 3313.2 m : 87Sr/86Sr = 0.707 0911+0.000 001 614 452H r2 = 0.68, n = 253326.1 to 3349.9 m : 87Sr/86Sr = 0.707 031230.000 003 760 355H r2 = 0.95, n = 14

EPSL 5469 30-5-00


tions by 30%, yielding mean durations of 0.08(Toarcian) and 0.14 Pliensbachian), these are stilldi¡erent by a factor of about two.

With the adopted timescale [34,35], the com-bined duration of the four oldest ammonite Sub-zones of the Toarcian totals 0.3 Myr, against pre-vious estimates of between 0.9 and 1.1 Myr ([36],reported in [37]). The duration of the exaratumsubzone (1.1 Myr) is longer than the 0.5 Myrpreviously thought [32,38,39], as is the Zone ofH. falciferum, previously believed to be about1 Myr in duration [10,32,37] and shown here tobe about 1.4 Myr. In Germany, the H. exaratumSubzone of the H. falciferum Zone is subdividedinto three [40^42]; upwards, these are; Harpoce-ras (Elegantuliceras Howarth) elegantulum ; H.(Cleviceras Howarth) exaratum ; H. (ClevicerasHowarth) elegans (the genera derived from [43].These subzones are not used in the UK, but theammonites elegantulum and elegans are present inthe Yorkshire sequence [33,43]. In view of theconsiderable duration of the exaratum Subzone,it seems appropriate to use the German scheme.

4.5. Dating and correlation with strontium isotopestratigraphy

Within the early Toarcian, 87Sr/86Sr changeswith time at a rate of about 70U1036 per Myr(from data in Table 2). Given an isotopic resolu-tion of þ 4U1036 (2 S.E.M.; achievable withmultiple analysis [7]), an uncertainty at 95% CIon the regression lines A^D (Fig. 4) of less than16U1036, and a compounded uncertainty ofS.D.total = [(S.E.M.measurement)2+(S.D.regression)2]1=2,a temporal resolution of about þ 0.25 Myr shouldbe achievable in correlation: given more analysisto reduce uncertainty of the regression, this ¢gurecould be reduced by a factor of four. In practice,the ultimate numerical resolution will be depen-dent on the numerical age model used; othertimescales [31,44] result in a ¢gure of around 0.5Myr, rather than 0.25 Myr. The precision in ¢xingstratigraphic level, however, is not dependent onthe age model and is about þ 1.5 m in the exara-tum Subzone, about þ 15 m above it, and aboutþ 7 m below it (but above the P/T boundary).

Table 2Durations of biozones

Zone Subzone 87Sr/86Sr base ofbiozone

Base ofbiozone

Duration Relative Duration

(Ma) (Ma)

ToarcianThouarsense fallaciosum

striatulum 0.707 246 181.25Variabilis Tie-Point 181.40Variabilis no Subzones 0.707 223 181.43 0.186 5.2Bifrons crassum 0.707 213 181.55 0.113 3.2

¢bulatum 0.707 206 181.69 0.144 4.0commune 0.707 194 181.91 0.217 6.1

Falciferum falciferum 0.707 159 182.22 0.312 8.8exaratum 0.707 094 183.30 1.080 30.3

Tenuicostatum semicelatum 0.707 085 183.42 0.119 3.3tenuicostatum 0.707 080 183.48 0.061 1.7clevelandicum 0.707 078 183.51 0.036 1.0paltum 0.707 073 183.60 0.086 2.4

PliensbachianSpinatum hawskerense 0.707 070 183.99 0.390 1.0

apyrenum 0.707 126 184.81 0.820 2.1Margaritatus gibbosus 0.707 160 185.77 0.960 2.5

subnodosus 0.707 188 186.27 0.500 1.3stokesi 0.707 230 187.56 1.290 3.3

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

EPSL 5469 30-5-00


With replicate analysis, 87Sr/86Sr stratigraphycould subdivide the exaratum Subzone into about¢ve subdivisions (7.2/1.5), a stratigraphic resolu-tion better than that a¡orded by ammonites.

4.6. Belemnite palaeotemperatures from majorelements?

The N18O of biogenic calcite re£ects the e¡ectsof metabolic processes, the ambient water temper-ature, and the isotopic composition of ambient

water. For belemnites, environmental in£uenceson N18O values may dominate over species-speci¢ce¡ects or e¡ects of variable growth rate [22,27,28].Nevertheless, since belemnites are extinct, we can-not calibrate the temperature response of theirN18O, so we do not calculate absolute palaeo-tem-peratures. An attempt to calibrate the isotopiccomposition of ambient water using an associa-tion of glendonites and belemnites [45] might betaken as indicating that a vital e¡ect of about2x may exist for some Aptian belemnites.Nevertheless, we note that Sr/Ca, Na/Ca, andMg/Ca values in our belemnites closely trackchanges in N18O with stratigraphic level (Fig. 5)and correlate signi¢cantly with N18O at the 1%level of signi¢cance (Fig. 6). The probable relationbetween temperature and the elemental composi-tion of biogenic carbonate has drawn much inter-est [46^50] and the Mg/Ca values of belemniteshave been used to estimate palaeotemperatures[51,52]. Our data show that Sr/Ca values may bemore robust for this purpose than Mg/Ca, sincethe former correlates better with N18O than doesthe latter (Fig. 6). Our element data were acquiredsolely for the purpose of assessing diagenetic al-

Fig. 6. Correlation of N18O with Sr/Ca, Mg/Ca and Na/Ca inbelemnite calcite. All correlation coe¤cients are signi¢cant atthe 1% level.

Fig. 5. Variation of N18Obelemnite and Sr/Ca, Mg/Ca, Na/Cawith stratigraphic level. Filled circles, this paper; open trian-gles from [22] with their stratigraphic levels corrected by 1.78m.

EPSL 5469 30-5-00


teration. The correlations in Fig. 6 are thereforeseen through compositional noise arising from thefact that we analysed a mixture of belemnite spe-cies and also sampled for elemental and isotopicanalysis randomly within pristine areas of individ-ual rostra, so the data are probably a¡ected byintra-rostral variations in both N18O and elemen-tal composition. Given that, the good correlationsseen in Fig. 6 suggest that the use of belemnitecomposition for palaeotemperature work deservesfurther study.

If metabolic e¡ects can be assessed [49], and ifelemental/Ca values in belemnites re£ect onlytemperature, whilst belemnite-N18O values re£ectvariations in ice-volume as well as temperature,the combination of elemental and isotopic analy-sis may o¡er a tool to test for the existence orotherwise of signi¢cant ice volume during thatperiod of time when belemnites £ourished (cf.[50]) ; for example, to test recent suggestionsthat polar ice may have existed during the Creta-ceous period [53], especially Valanginian times[54].

4.7. Carbon isotope trends

An OAE is regarded as a short period of timeduring which occurred the widespread depositionof organic-rich sediments, although the termswidespread, short, and organic-rich are not de-¢ned well [37^39]. Such events are supposedly ac-companied by positive excursions in N13C of ma-rine carbon (Fig. 7; ibid) which may be ofstratigraphic utility. For example, by locatingthe peak of the early Toarcian carbon isotopeexcursion within available biozonations [39] itwas inferred that lower Toarcian ammonite zonesmight be diachronous between England and Italy.Conversely, it has been proposed that the carbonisotope excursion is diachronous [37]. The uncer-tainty could be resolved using Sr isotope stratig-raphy, since our 87Sr/86Sr data ¢x the start of theOAE (base of Bed 34) at an 87Sr/86Sr of0.707 085 þ 0.000 016 and the end (base of Bed36) at an 87Sr/86Sr 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 aroundþ 0.000 004 [7,55] by replicate analysis of samples

from the stratigraphic limits of the OAE. Fromour age model, we estimate that the OAE, if de-¢ned as occupying the entire time recorded bybeds 34 and 35, existed for 0.52 Myr (Table 1).

Our carbon isotopic trend (Fig. 7) con¢rms thatalready published for the Yorkshire interval[23,39] but adds the detail that the major positivepeak in the upper exaratum Subzone and higher isinterrupted by two short returns to near-normalN13Cbelemnite at C and D in Fig. 7, suggesting in-stability in the mechanism of spike generation.Another apparent maximum in N13Cbelemnite inthe semicelatum Subzone is probably an artifactof two bracketing isotopic minima (A and B, Fig.7). Of the four minima in N13Cbelemnite (A^D, Fig.7), one is coincident with the middle exaratumSubzone, where sediment TOC reaches a maxi-mum. Such a coincidence of maximum TOCand minimum N13C occurs also in a Toarcian se-quence in SW Germany [42], where the TOC max-imum occurs in the semicelatum Subzone, ratherthan the mid exaratum Subzone as in Yorkshire.In both Yorkshire and Germany the N13C mini-mum is explained as arising either through up-welling of deeper water [39] or from episodic mix-ing into surface waters of deeper sub-pycnal water[23,42], the deeper water in each case being madeisotopically light by mineralisation of organicmatter.

Fig. 7. Variation of N13Cbelemnite and TOC with stratigraphiclevel. TOC data from [39]. Arrows A, B, mark minima ofN13Cbelemnite and arrows C, D mark excursions to near-nor-mal values within a positive excursion. Filled circles, this pa-per; open triangles from [22], with stratigraphic levels cor-rected by 1.78 m. Ammonite subzones denoted by lower caseletter, see Fig. 3 for key.

EPSL 5469 30-5-00


The major positive excursion in N13Cbelemnite

peaks in the uppermost exaratum Subzone, somemetres above the peak in sedimentary TOC%(Fig. 7). Positive isotopic excursions are ascribedto the removal from the oceans of large amountsof isotopically light carbon as organic matter(N13CV325x) into black shales [39,56] or meth-ane hydrates [57], which leaves oceanic carbonisotopically heavy. The large amounts of organicmatter responsible for the early Toarcian excur-sion have not been located and, within the posi-tive 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). Thesereturns to near-normal isotopic values suggestthat the positive isotopic excursion was easily re-versed and so may have been local, rather thanglobal, in origin. Furthermore, whilst the isotopicmaximum reaches +6.5x in Yorkshire belem-nites, it is not discernible in the isotopic compo-sition of carbonate from an equivalent strati-graphic interval in SW Germany, and is thereonly just discernible in organic matter, peakingat 1x above background values. We speculate,therefore, that the positive excursions result fromlocal responses to the burial of organic matter.

Methanogenesis yields isotopically light meth-ane (360x) and isotopically heavy CO2

(+15x). Processes within the sediment may mixisotopic signals from di¡erent redox zones and soyield a present-day range that is mostly between310 and +10x [58,59], but values up to +19xhave been measured for CO2 from pore water ofsediments from the Baltic Sea [60,61]. Ebullitionof this mixed gas from the sediments would haveadded isotopically heavy CO2 to the overlyingwater column, because it is a soluble reactivegas, whilst less soluble methane would have es-caped from the system [60^64]. Beds 34 and 35(3.5 m combined thickness, s 12% TOC) are thebeds in the sequence both richest in TOC and of athickness su¤cient to make them quantitativelyimportant as long-term methane sources. Onsetof methanogenesis would not have occurred untilthe organic matter in Beds 34 and 35 had beenburied beneath the zone of sulfate reduction. TheN13

DIC in the overlying water column could not re-£ect isotopically heavy values until methanogene-

sis started. As N13Cbelemnite ¢rst exceeds +4x inBed 37, about 1 m (compacted) above Bed 35(Table 1), we estimate from our age model (Ta-bles 1 and 2) that burial to this depth took about160 kyr to accomplish, and that the peak excur-sion in N13Cbelemnite occurred after about 500 kyr.As our mechanisms are unlikely to in£uence asubstantial thickness of water column (for massbalance reasons), the fact that positive isotopicexcursions are recorded in belemnite calcite im-plies that the water in which they lived was shal-low.

5. Conclusions

1. Using 87Sr/86Sr values for age assignment, weshow that the early Toarcian OAE persistedfor about 0.52 Myr (Table 1; timescale in[34]).

2. The durations of early Toarcian ammoniteSubzones di¡ered by factors of up to 30; i.e.from 0.036 Myr for the clevelandicum Subzoneto 1.08 Myr for the exaratum Subzone (time-scale of [34]). This ¢nding has implications forthe way Mesozoic numerical timescales aremade; until now, they have assumed that am-monite biozones are of equal duration.

3. The Sr isotope curve for the interval provides atheoretical resolution of þ 0.25 Myr for datingstrata in much of the early Toarcian.

4. Values of Sr/Ca, Mg/Ca, Na/Ca and N18O inbelemnite covary and may record palaeotem-peratures.

5. Positive excursions in N13Cbelemnite stratigraphi-cally just above the early Toarcian OAE mayresult from ebullition of isotopically heavyCO2 that was generated by methanogenesis oforganic rich sediments during shallow burial.

6. A consequence of (5) is that positive isotopicexcursions in N13C, in biogenic calcite or or-ganic matter, will not be precisely synchronousworldwide, since the rate of burial will governthe time taken to reach the zone of methano-genesis for organic-rich sediments.

7. Using our approach of detailed pro¢ling of87Sr/86Sr with stratigraphic level, the durationsof other OAEs should be determinable.

EPSL 5469 30-5-00


Acknowledgements

The Radiogenic Isotope Laboratory at RHULis supported, in part, by the University of Londonas an intercollegiate facility. We thank MichaelEngkilde (Copenhagen) for providing initial iso-topic data and Paul Wignall for advice in the¢eld. Gerry Ingram, Mark Brownless, and SarahHoughton helped with the isotopic measurements.Tony Osborn did the elemental analysis, mostlyusing the NERC ICP-AES Facility at RHUL,with the permission of its Director, Dr. J.N.Walsh. We thank Jan Veizer, Mike Talbot andan anonymous reviewer for constructive critiquesof the script.[FA]

References

[1] J.M. McArthur, Recent trends in strontium isotope strat-igraphy, Terra Nova 6 (1994) 331^358.

[2] R.J. Howarth, J.M. McArthur, Statistics for strontiumisotope stratigraphy: a robust LOWESS ¢t to the marinestrontium isotope curve for the period 0 to 206 Ma, withlook-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 resolu-tion and event correlation, Palaeogeogr. Palaeoclimatol.Palaeoecol. 132 (1997) 65^77.

[4] M.R. Palmer, H. Elder¢eld, Sr isotope composition of seawater 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 thelate Neogene ocean, Earth Planet. Sci. Lett. 92 (1989)165^178.

[6] F.M. Richter, K.K. Turekian, Simple models for the geo-chemical response of the ocean to climatic and tectonicforcing, Earth Planet. Sci. Lett. 119 (1993) 121^131.

[7] J.M. McArthur, M.F. Thirlwall, M. Engkilde, W.J. Zin-smeister, R.J. Howarth, Strontium isotope pro¢les acrossK/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 Site522, Paleoceanography 3 (1988) 223^233.

[9] J.M. McArthur, M.F. Thirlwall, A.S. Gale, M. Chen,W.J. Kennedy, Strontium isotope stratigraphy in theLate Cretaceous: numerical calibration of the Sr isotopecurve and intercontinental correlation for the Campanian,Paleoceanography 8 (1993) 859^873.

[10] C.E. Jones, H.C. Jenkyns, S.P. Hesselbo, Strontium iso-topes 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 latemiddle 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 LateCretaceous: direct numeric calibration of the Sr isotopecurve for the US Western Interior Seaway, Palaeogeogr.Palaeoclimatol. Palaeoecol. 108 (1994) 95^119.

[13] G.P. Weedon, H.C. Jenkyns, Cyclostratigraphy and theEarly 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 ShaleSeries of the Yorkshire coast, Proc. Yorks. Geol. Soc. 33(1962) 381^422.

[16] M.K. Howarth, The stratigraphy and ammonite fauna ofthe 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 associ-ation of minor ¢ning upwards cycles and aligned guttermarks in the Middle Liass (L. Jurassic) of Yorkshire,Proc. Yorks. Geol. Soc. 42 (1980) 525^538.

[18] J.H. Powell, Lithostratigraphical nomenclature of theLiass Group in the Yorkshire Basin, Proc. Yorks. Geol.Soc. 45 (1984) 51^57.

[19] A.S. Howard, Lithostratigraphy of the Staithes Sandstoneand Cleveland Ironstone formations (Lower Jurassic) ofnorth-east Yorkshire, Proc. Yorks. Geol. Soc. 45 (1985)261^275.

[20] S.P. Hesselbo, H.C. Jenkyns, A comparison of the Het-tangian to Bajocian successions of Dorset and Yorkshire,in: P.D. Taylor (Ed.), Field geology of the British Juras-sic. Geological Society of London (1995) 105^150.

[21] J.H.S. Macquaker, K.G. Taylor, A sequence stratigraphicinterpretation of a mudstone-dominated succession: theLower Jurassic Cleveland Ironstone Formation, UK,J. Geol. Soc. Lond. 153 (1996) 759^770.

[22] G. Saelen, P. Doyle, M.R. Talbot, Stable isotope analysisof belemnite rostra from the Whitby Mudstone Forma-tion, England: surface water conditions during depositionof a marine black shale, Palaios 11 (1996) 97^117.

[23] G. Saelen, R.V. Tyson, M.R. Talbot, N. Telnaes, Evi-dence of recycling of isotopically light CO2 (aq) in strati-¢ed black shale basins: contrasts between the WhitbyMudstone and Kimmeridge Clay formations, UnitedKingdom, Geology 26 (1998) 747^750.

[24] M.F. Thirlwall, Long-term reproducibility of multicollec-tor Sr and Nd isotope ratio analysis, Chem. Geol. (Isot.Geosci. Sect.) 94 (1991) 85^104.

[25] G. Saelen, T.V. Karstang, Chemical signatures in belem-nites, N. Jb. Geol. Pala«ont. Abh. Bd 177 (1988) 333^346.

EPSL 5469 30-5-00


[26] G. Saelen, Diagenesis and construction of the belemniterostrum, Palaeontology 32 (1989) 765^798.

[27] O.G. Podlaha, J. Mutterlose, J. Veizer, Preservation ofN18O and N13C in belemnite rostra from Jurassic/EarlyCretaceous successions, Am. J. Sci. 298 (1998) 324^347.

[28] O.G. Podlaha, J. Mutterlose, J. Veizer, Preservation ofN18O and N13C in belemnite rostra from Jurassic/EarlyCretaceous successions, Erratum Am. J. Sci. 298 (1998)534.

[29] J. Veizer, Trace elements and isotopes in sedimentary car-bonates, Rev. Miner. 11 (1983) 265^300.

[30] P.B. Wignall, Model for transgressive black shales?, Geol-ogy 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 TimeScale, J. Geophys. Res. 99 (1994) 24051^24074.

[32] A. Hallam, Estimates of the amount and rate of sea-levelchange across the Rhaetian^Hettangian and Pliens-bachian^Toarcian boundaries (latest Triassic to earlyJurassic), J. Geol. Soc. Lond. 154 (1997) 773^779.

[33] J.C.W. Cope, Discussion on estimates of the amount andrate of sea-level change across the Rhaetian^Hettangianand Pliensbachian^Toarcian boundaries (latest Triassic toearly Jurassic), J. Geol. Soc. Lond. 155 (1998) 421^422.

[34] J. Palfy, P.L. Smith, J.K. Mortensen, A revised numerictime scale for the Jurassic, Proceedings of the Internation-al Symposium on Jurassic Stratigraphy, Vancouver, Sep-tember 1998.

[35] J. Palfy, R.R. Parrish, P.L. Smith, A U^Pb age from theToarcian (Lower Jurassic) and its use for time scale cali-bration 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 se-quence cycles, transgressive-regressive facies cycles, andextensional tectonics: examples from the southern Subal-pine Jurassic basin, France, Bull. Geol. Fr. 164 (1993)709^718.

[37] A.P. Jimenez, C. Jimenez de Cisneros, P. Rivas, J.A.Vera, The early Toarcian anoxic event in the westernmostTethys (Subbetic): paleogeographic and paleobiogeo-graphic 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 epicontinentalcarbonates and mudstones from England and Wales: che-mostratigraphic signals and the early Toarcian anoxicevent, Sedimentology 44 (1997) 687^706.

[40] W. Ku«spert, Environmental changes during oil shale dep-osition as deduced from stable isotope ratios, in: G. Ein-sele, A. Seilacher (Eds.), Cyclic and Event Strati¢cation,Springier, 1982.

[41] W. Riegraf, G. Werner, F. Lo«rcher, Die Posidonienschie-fer. Biostratigraphie, Fauna und Fazies des su«dwestdeut-schen Untertoarciums (Lias e), Stuttgart: FerdinandEnke, 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 an-oxic event on the stable carbon isotopic composition ofearly Toarcian carbon, Am. J. Sci. 300 (2000) 1^22.

[43] M.K. Howarth, The ammonite family Hildoceratidae inthe Lower Jurassic of Great Britain, Monograph of thePalaeontographical 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 palaeoenvi-ronmental tool: Implications for early Cretaceous highlatitude climates in Australia, Geochim. Cosmochim.Acta 63 (1999) 1039^1048.

[46] S. de Villiers, G.T. Shen, B.K. Nelson, The Sr/Ca-temper-ature relationship in coralline aragonite ^ in£uence ofvariability in (Sr/Ca)seawater and skeletal growth-parame-ters, Geochim. Cosmochim. Acta 58 (1994) 197^208.

[47] Y. Rosenthal, E.A. Boyle, N. Slowey, Temperature con-trol on the incorporation of magnesium, strontium, £uo-rine, and cadmium into benthic foraminiferal shells fromLittle Bahama Bank: prospects for thermocline palaeo-ceanography, Geochim. Cosmochim. Acta 61 (1997)3633^3643.

[48] D.J. Sinclair, L.P.J. Kinsley, M.T. McCulloch, High res-olution analysis of trace elements in corals by laser abla-tion 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 mol-lusks, Geology 27 (1999) 1083^1086.

[50] C.H. Lear, H. Elder¢eld, P.A. Wilson, Cenozoic deep-seatemperatures and global ice volume from Mg/Ca inbenthic 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 northernUSSR, from palaeotemperature determinations, Int.Geol. Rev. 9 (1967) 1080^1092.

[52] N.A. Yasamanov, Paleothermometry of Jurassic, Creta-ceous, and Palaeogene periods of some regions of theUSSR, Int. Geol. Rev. 23 (1981) 700^706.

[53] P.W. Ditch¢eld, High northern palaeolatitude Jurassic^Cretaceous palaeotemperature variation: New data fromKong 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. Mut-terlose, Isotopic evidence for temperature variation duringthe 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, Stron-tium isotope correlation of the basal Maastrichtian stagein Antarctica to the European and US standard biostrati-graphic schemes, J. Geol. Soc. Lond. 156 (1999) 957^964.

[56] S.O. Schlanger, H.C. Jenkyns, Cretaceous oceanic anoxicevents: causes and consequences, Geol. Mijnb. 55 (1976)179^184.

EPSL 5469 30-5-00


[57] G.R. Dickens, J.R. O'Neil, D. Rae, R.M. Owen, Dissoci-ation of oceanic methane hydrate as a cause of the carbonisotope excursion at the end of the Palaeocene, Paleocea-nography 10 (1995) 965^971.

[58] M.J. Whiticar, E. Faber, M. Schoell, Biogenic methaneformation 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 systematicsof bacterial formation and oxidation of methane, Chem.Geol. 161 (1999) 291^314.

[60] C.S. Martens, D.A. Albert, M.J. Alperin, Biogeochemicalprocesses controlling methane in gassy coastal sediments ^Part 1. A model coupling organic matter £ux to gas pro-duction, oxidation and transport, Cont. Shelf Res. 18(1998) 1741^1770.

[61] C.S. Martens, D.A. Albert, M.J. Alperin, Stable isotopetracing of anaerobic methane oxidation of gasses in sedi-ments 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 ofgas ebullition in the transport of organic contaminantsfrom sediments, Sci. Total Environ. 112 (1992) 189^201.

[63] B.B. Curry, T.F. Anderson, K.C. Lohmann, Unusual car-bon and oxygen isotopic ratios of ostracodal calcite fromlast interglacial (Sangamon episode) lacustrine sediment inRaymond basin, IL, USA, J. Paleolimnol. 17 (1997) 421^435.

[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. Clem-ent, Miocene isotope reference section, Deep Sea DrillingProject Site 608: an evaluation of isotope and biostrati-graphic resolution, Paleoceanography 6 (1991) 33^52.

[66] D.A. Hodell, P.A. Mueller, J.R. Garrido, Variations inthe strontium isotopic composition of seawater duringthe Neogene, Geology 19 (1991) 24^27.

[67] D.A. Hodell, F. Woodru¡, Variations in the strontiumisotopic ratio of seawater during the Miocene: strati-graphic and geochemical implications, Paleoceanography9 (1994) 405^426.

[68] J.W. Farrell, S.C. Clemens, L.P. Gromet, Improved chro-nostratigraphic reference curve of late Neogene seawater87Sr/86Sr, Geology 23 (1995) 403^406.

[69] G.A. Mead, D.A. Hodell, Controls on the 87Sr/86Sr com-position of seawater from the middle Eocene to Oligo-cene: Hole 689B, Maud Rise, Antarctica, Paleogeography10 (1995) 327^346.

[70] G.M. Henderson, D.J. Martel, R.K. O'Nions, N.J.Shackleton, Evolution of seawater 87Sr/86Sr over the last400 ka: the absence of glacial/interglacial cycles, EarthPlanet. Sci. Lett. 128 (1994) 643^651.

EPSL 5469 30-5-00


Strontium isotope pro¢le of the early Toarcian (Jurassic ... · Strontium isotope pro¢le of the...

Documents

Transcript of Strontium isotope pro¢le of the early Toarcian (Jurassic ... · Strontium isotope pro¢le of the...