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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: j.mcarthur@ucl.ac.uk
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
J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285272
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
J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285 273
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
J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285274
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
J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285 275
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
J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285276
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
J.M. McArthur et al. / Earth and Planetary Science Letters 179 (2000) 269^285 277
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
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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.
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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.
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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.
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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.
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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]
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