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Journal of the Geological Society, London, Vol. 166, 2009, pp. 431–445. doi: 10.1144/0016-76492007-177.
431
Palaeoenvironmental significance of carbon- and oxygen-isotope stratigraphy of
marine Triassic–Jurassic boundary sections in SW Britain
CHRISTOPH KORTE
1
*, STEPHEN P. HESSELBO
1
, HUGH C. JENKYNS
1
,
ROSALIND E. M. RICKABY
1
& CHRISTOPH SPO
¨
TL
2
1
Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
2
Institut fu
¨
r Geologie und Pala
¨
ontologie, Universita
¨
t Innsbruck, Innrain 52, A-6020 Innsbruck, Austria
*Corresponding author (e-mail: korte@geo.ku.dk)
Abstract: Carbon-isotope stratigraphy is a useful tool for stratigraphic correlation, especially for strata
deposited during major perturbations of the carbon cycle that affected the marine, terrestrial and atmospheric
reservoirs. For the Triassic–Jurassic boundary, effectively defined by a first-order mass extinction, major
fluctuations in carbon-isotope values have been well documented, but these datasets have generally been
derived from bulk-rock samples. Hence, the extent to which features of the isotopic curve reflect diagenetic
alteration or changing proportions of constituent materials is unconstrained. Here, carbon- and oxygen-isotope
data are presented from well-preserved oyster shells (Liostrea) comprising low-magnesium calcite, a mineral
species relatively resistant to diagenetic alteration. Samples were obtained from Lavernock Point, Glamorgan,
Wales, a coastal section close to a candidate stratotype for the base of the Jurassic at St Audrie’s Bay,
Somerset, England. The carbon-isotope signature from St Audrie’s Bay, previously defined on the basis of
analysis of bulk organic matter, is confirmed by our new data. Major features are (1) the upper part of an
‘initial’ negative isotope excursion in the lowest part of the section, followed by (2) a pronounced positive
excursion, and (3) an extended ‘main’ negative isotope excursion in the highest part of the section. The data
confirm that the carbon-isotope stratigraphy previously documented from bulk organic matter in SW England
records the chemical composition of the contemporaneous seawater. Bulk carbonates sampled over the same
interval near Lyme Regis, England, show similar trends to those from oyster calcite in the lower part of the
study section, but there are more
13
C-depleted values up-section. These lower values probably result from an
admixture of primary and diagenetic carbonate. Palaeotemperatures calculated from oxygen-isotope values
from Lavernock Point oyster shells are relatively cool at the beginning of the positive carbon-isotope
excursion, and increased by up to 10 8C during the main negative carbon-isotope excursion. The new results
are compatible with the view that positive carbon-isotope excursions correspond to times of low atmospheric
carbon dioxide content, whereas negative carbon-isotope excursions correspond to times of high atmospheric
carbon dioxide content, as is also found to be the case during the Early Jurassic (Toarcian) Oceanic Anoxic
Event. The Mg/Ca and Sr/Ca ratios and ä
18
O of investigated Liostrea hisingeri show no correlation,
supporting data from modern bivalves that indicate that incorporation of Mg and Sr is controlled mainly by
factors other than temperature.
The Triassic–Jurassic (T–J) transition, c. 200 Ma ago, was a
time of mass extinction that affected both the marine and
continental biota (Hallam & Wignall 1997, and references there-
in). The patterns of environmental change that accompanied the
extinction, and the cause or causes, are vigorously debated (see,
e.g. Hesselbo et al. 2007a, for summary). As is the case for
many other episodes of major environmental change, the T–J
boundary is characterized by major perturbations to the carbon
cycle, demonstrated by carbon-isotope profiles generated from
several parts of the world (McRoberts et al. 1997; Pa´lfy et al.
2001, 2007; Ward et al. 2001, 2007; Hesselbo et al. 2002, 2004;
Guex et al. 2004; Galli et al. 2005, 2007; Kuerschner et al.
2007; Williford et al. 2007). Although there is generally good
agreement between the shapes of curves from different regions, a
shortcoming of the carbon-isotope profiles published hitherto is
that they are almost entirely based on analyses of bulk organic
matter or bulk carbonate, where the exact nature, origin and
proportion of the components is indeterminate. The few existing
data from diagenetically unaltered marine skeletal low-Mg
calcite are not sufficient to construct stratigraphically extended
isotope curves (e.g. van de Schootbrugge et al. 2007). Here,
analyses of shell material from the oyster Liostrea hisingeri
(Nilsson) are presented, carefully screened for diagenetic altera-
tion, and collected from numerous horizons across and above the
T–J boundary in SW Britain.
Similarities between the T–J boundary events and those
occurring during Phanerozoic Oceanic Anoxic Events have been
highlighted by a number of workers (e.g. Cohen & Coe 2002;
Jenkyns et al. 2002). Apart from the development of large-
amplitude carbon-isotope excursions, similarities include anoma-
lies and trend reversals in Sr and Os isotope records (Korte et al.
2003; Cohen & Coe 2007), and rapid turnover or crises amongst
shallow-marine carbonate-producing organisms (Hautmann 2006;
Kiessling et al. 2007; Tomasˇovy
´
ch & Siblı´k 2007). It has been
shown for other intervals of major environmental change that
carbon-isotope fluctuations are correlated with marine and atmo-
spheric palaeotemperature fluctuations, and such co-variations
have to be taken into account during consideration of the forcing
functions involved (e.g. Jenkyns 2003). Although data for
palaeotemperature changes accompanying T–J boundary events
are debated (e.g. McElwain et al. 1999; Hubbard & Boulter
2000), van de Schootbrugge et al. (2007) have demonstrated the
potential for oxygen-isotope and Mg/Ca analyses of the oysters
from St Audrie’s Bay to indicate palaeo-seawater temperature, in
this case documenting a temperature increase coincident with a
shift to lighter carbon-isotope values over some c. 3.5 m of strata.
Herein, oxygen-isotope and elemental concentration data are
presented for well-preserved low-Mg-calcite oysters for three
sections in SW Britain spanning the T–J boundary. These new
records greatly extend the dataset from the St Audrie’s Bay
section (van de Schootbrugge et al. 2007) both in terms of
quantity and temporal range.
Geological setting
Oysters have been collected at the highest level of stratigraphic
resolution possible from the upper Lilstock Formation and lower
Blue Lias Formation at Lavernock Point, Glamorgan, South
Wales (Fig. 1; Table 1), as well as from St Audrie’s Bay and
Watchet, Somerset, SW England. The lithostratigraphy of these
sections has been described by Richardson (1905, 1911), Whit-
taker (1978), Whittaker & Green (1983), Waters & Lawrence
(1987), Hesselbo et al. (2004) and Hounslow et al. (2004).
Lavernock Point represents the best location to obtain oysters at
a high resolution across the T–J interval and through lower
Hettangian strata. This section is biostratigraphically well cali-
brated (Trueman 1920; Hodges 1994) and is easily compared
with the succession at St Audrie’s Bay where the ä
13
C
org
curve
of Hesselbo et al. (2002) was generated. Additionally, whole-
rock carbonates were sampled from the Pinhay Bay section, near
Lyme Regis (Devon, SW England; Fig. 1). The stratigraphy of
this section has been described by Richardson (1906), Lang
(1924), Hallam (1960a), Hesselbo & Jenkyns (1995), Wignall
(2001a) and Hesselbo et al. (2004).
Materials and methods
Although it was not possible in every case to identify the genus
and/or species of the sampled shell fragments, most probably all
isotope data originate from Liostrea hisingeri Nilsson because
this form is the only oyster described from the sampled levels
(Richardson 1905; Trueman 1920; Hodges, pers. comm.). Oyster
shells consist generally of an outer simple prismatic layer and a
prominent inner foliated layer (Carter 1990; Hautmann 2006).
Both layers consist, at least for Jurassic oysters, primarily of
calcite and, particularly in the case of foliated layers, of low-Mg
calcite (LMC). Aragonitic elements may also have been present
in some oyster shells, but their expression appears to have been
suppressed after the earliest Jurassic (Hautmann 2006). In the
present study, only material from the foliated layers was
analysed.
It is thought that bivalves secrete their shells extracellularly
(Lowenstam & Weiner 1989), although studies of artificially
damaged specimens of the modern oyster Crassostrea virginica
(Gmelin) have shown that living hemocytes are present during
the rapid growth of prismatic and foliated LMC layers during
Fig. 1. (a) Triassic–Jurassic
palaeogeography (modified from Ziegler
1990; McHone 2000; Hesselbo et al. 2002).
CAMP, Central Atlantic Magmatic
Province. White square indicates UK study
area. (b) Location map for Lavernock Point,
Watchet, St Audrie’s Bay, and Lyme Regis
(modified after Hesselbo et al. 2004). (c)
Summary of pre-existing bulk organic
matter carbon-isotope data from St Audrie’s
Bay (Hesselbo et al. 2002) showing the
principal features discussed in the text. T–J
boundary is positioned on the basis of
correlation using carbon isotopes with the
proposed GSSP at Kuhjoch, Austria (details
provided by Hillebrandt, pers. comm.).
C. KORTE ET AL.432
regeneration (Mount et al. 2004), a phenomenon that could be
intracellular. However, whatever the nature of the process
involved, it has been observed that C. virginica precipitates the
prismatic and foliated shell layers close to oxygen- and carbon-
isotope equilibrium with ambient water (Surge et al. 2001).
Low-Mg calcite is relatively resistant to diagenetic alteration,
thus minimizing the potential resetting of the primary geochem-
ical signal from seawater (e.g. Veizer et al. 1997a,b, 1999;
Jenkyns et al. 2002). Observed ä
18
O and ä
13
C variations for
modern (Andrus & Crowe 2000; Surge et al. 2001) and ancient
(Kirby et al. 1998; Kirby 2000) oysters reflect climatic and/or
environmental changes. Despite the inherent resistance of low-
Mg calcite to post-depositional alteration, in the present study
each oyster was screened by chemical and optical techniques.
The methods used include optical microscopy, scanning electron
microscopy (SEM) and elemental analysis by inductively coupled
plasma-atomic emission spectroscopy (ICP-AES). The textural
observations and elemental data have been utilized to evaluate
the isotopic data.
Oysters were prepared by flaking shell fragments using a
needle. Splinters of the foliated layers were inspected and
handpicked under a binocular microscope. Weathered fragments,
attached sedimentary grains, and crack fillings were rejected.
Optically well-preserved shell material was checked by SEM to
obtain information on microstructural characteristics. Only fo-
liated shell layers with smooth surfaces were regarded as being
well preserved. The preservation of foliated structure indicates
that post-depositional dissolution and reprecipitation were negli-
gible (Fig. 2).
The Sr and Mn elemental concentrations (Table 1) were
determined at the Ruhr-University Bochum by ICP-AES, either
on aliquots of phosphoric acid remaining after reaction of shell
splinters for evolution of CO
2
(GasBench method), or on
additional separated shell splinters generated during sample
preparation for the isotope work (Prism II method). International
limestone standard reference materials (CCH-1, GBW 03105)
were analysed for Sr and Mn along with the samples, and the
accuracy and precision of these elemental analyses was better
than 3% and 1%, respectively. Obtained Sr and Mn concentra-
tions were utilized to evaluate the pristine preservation of LMC
shell material (Brand & Veizer 1980). Although variations of
these elements exist for palaeo-seawater throughout the Phaner-
ozoic (Steuber & Veizer 2002), and are concentrated to different
degrees in modern bivalves (Vander Putten et al. 2000), the
content of Sr and Mn provide further information regarding the
degree of preservation of the shells. With increasing diagenetic
influence, Sr is removed and Mn is added (Brand & Veizer
1980). To be consistent with a number of previous studies (Korte
et al. 2003, 2005a,b, 2006, 2008), samples with less than 400
ìgg
1
Sr and/or more than 250 ìgg
1
Mn were classified as
being altered, and their ä
18
O values are considered suspect. It
should be emphasized that none of the selection criteria are
perfectly reliable and the use of only one criterion (e.g. trace-
element concentrations) might be misleading when evaluating
the extent of alteration. The final evaluation of samples as either
pristine or altered was therefore based on optical and micro-
structural criteria, as well as trace-element chemistry (Table 1).
Only carbon- and oxygen-isotope data from well-preserved
oysters will be considered in the following presentation, discus-
sion and interpretation.
Samples of about 2 mg of shell fragments (oysters) or fine-
grained carbonate (whole rocks) were analysed isotopically for
ä
18
O and ä
13
C at the Department of Earth Sciences, University
of Oxford, using a VG Isogas Prism II mass spectrometer with an
on-line VG Isocarb common acid-bath preparation system. In the
instrument, the powdered sample is reacted with purified phos-
phoric acid (H
3
PO
4
)at908C. Calibration to the V-PDB standard
via NBS-19 is made daily using the Oxford in-house (NOCZ)
Carrara Marble standard. Reproducibility of replicated standards
is typically better than 0.1‰ for ä
13
C and ä
18
O.
Single shell splinters (,0.5 mm) with a mass of about 0.1 mg
were prepared in c. 1 mm steps in the growth direction for six
well-preserved oysters from the Lavernock Point section to
examine intra-shell variation in ä
13
C and ä
18
O values. Surface
layers were removed and sampling started about 1 mm below the
surface. These considerably smaller samples were analysed for
ä
18
O and ä
13
C on a GasBench II linked to a ThermoFinnigan
Delta
plus
XL mass spectrometer at the Institut fu
¨
r Geologie und
Pala¨ontologie of the Universita¨t Innsbruck (Spo¨tl & Vennemann
2003). The long-term precision (1 s.d.) is better than 0.06‰ for
ä
13
C and better than 0.08‰ for ä
18
O. Carbon- and oxygen-
isotope values were calibrated against V-PDB and are reported in
the standard ‰ notation (Tables 1 and 2).
Some carbonate samples (Table 1; c. 0.2 mg) were analysed at
the Institut fu
¨
r Geologie, Mineralogie und Geophysik, Ruhr-
Universita¨t Bochum also using a GasBench II linked to a
ThermoFinnigan Delta S mass spectrometer. The results from the
carbonate samples were corrected to the nominal values for the
carbon- and oxygen-isotope standards CO-1 and CO-8. Reprodu-
cibility was better than 0.1‰ for both ä
13
C and ä
18
O. Carbon-
and oxygen-isotope values were calibrated against V-PDB and
are reported in the standard ‰ notation (Table 1). Tests in all
three laboratories demonstrated that NBS-19 and NOCZ showed
similar results and are within 0.1‰ for both ä
13
C and ä
18
O.
Shell splinters of about 0.25 mg were powdered for the Mg/Ca
ratio measurements. To be consistent with previous work, the
samples were cleaned by a procedure developed and revised by
Boyle & Keigwin (1985) whereby ferromanganese oxides and
organic matter were removed (see Rickaby & Halloran 2005).
After a weak acid leach (0.001M HNO
3
), the samples were
dissolved in 0.5 ml 0.22M HNO
3
and centrifuged for 5 min at
5000 r.p.m. Analysis was carried out by inductively coupled
plasma-mass spectrometry (ICP-MS) at Oxford University, using
a PerkinElmer PESCIEX ELAN 6100 DRC Quadrupole ICP-MS
system fed by a low-uptake (100 ìl min
1
) spray-chamber
nebulizer (see Rosenthal et al. 1999). Data were collected for
four masses:
46
Ca,
26
Mg,
88
Sr, and
55
Mn with the ICP-MS
system set to peak hopping mode using a method adapted from
Harding et al. (2006).
Results
Carbon and oxygen isotopes
Most of the oysters originate from the Lavernock Point section
(Fig. 1) and, for this locality, the lithology, stratigraphy, and
oxygen- and carbon-isotope data are shown in Figure 3. The
lowest well-preserved oysters retrieved originated from the lower
Langport Member and collection was continued up through the
Langport Member and the Blue Lias Formation, the latter
including the P. planorbis, C. johnstoni and the W. portlocki
ammonite subzones of the P. planorbis and A. liasicus ammonite
zones of the Hettangian (Fig. 3). The lowest well-preserved
oysters originated from the base of Bed A, from the lower
Langport Member. A correlation from Lavernock to the proposed
Global Stratotype and Stratigraphic Point (GSSP) at Kujoch,
Austria (Hillebrandt, pers. comm.), cannot be accomplished on
the basis of biostratigraphy because the UK sections lack
TRIASSIC–JURASSIC BOUNDARY 433
Table 1. Positions, material analysed, carbon- and oxygen-isotope values, Mg/Ca and Sr/Ca ratios, and Mn and Sr concentrations of the analysed
samples
Sample Height above
base Langport
Mbr (cm)
Material ä
13
C
(‰ V-PDB)
ä
18
O
(‰ V-PDB)
Isotope
laboratory
Mg/Ca
(mM M
1
)
Sr/Ca
(mM M
1
)
Mn
(ìgg
1
)
Sr
(ìgg
1
)
Pristine oysters
LAV 109 36 Oyster 2.87 0.42 Ox 9.03 0.58 237 431
LAV 109 36 Oyster 3.30 0.46 Bo 8.42 0.55 237 431
LAV 109 b 36 Oyster 3.00 0.39 Ox nd nd 237 431
LAV 109 36 Oyster 3.76 0.18 Inn nd nd 237 431
LAV 110 91 Oyster 2.24 0.09 Bo 5.08 0.55 242 570
LAV 110 91 Oyster 2.89 0.12 Ox 5.44 0.63 242 570
LAV 261 91 Oyster 3.18 0.88 Bo 4.08 0.55 126 597
LAV 110-1 91 Oyster 2.83 0.10 Inn 4.98 0.55 216 472
LAV 110-1 91 Oyster 2.86 0.34 Inn 7.10 0.55 216 472
LAV 261 91 Oyster 3.36 0.96 Inn 4.03 0.52 126 597
LAV 261 91 Oyster 3.29 0.55 Inn 4.13 0.57 126 597
LAV 257 171 Oyster 3.51 0.19 Bo nd nd 174 530
LAV 257 171 Oyster 3.55 0.12 Bo nd nd 174 530
LAV 258 171 Oyster 4.63 1.62 Bo 2.28 0.48 126 570
LAV 257 171 Oyster 3.94 0.04 Inn nd nd 174 530
LAV 262 275 Oyster 3.62 0.49 Bo nd nd 80 560
LAV 262 275 Oyster 4.11 0.35 Bo 5.50 0.56 44 538
LAV 264 285 Oyster 3.62 0.08 Bo nd nd 81 482
LAV 204 290 Oyster 4.04 0.05 Bo 4.69 0.55 185 585
LAV 204 290 Oyster 4.04 0.05 Bo 6.29 0.50 185 585
LAV 204 290 Oyster 4.04 0.05 Bo 5.39 0.57 185 585
LAV 204 290 Oyster 4.04 0.05 Bo 6.29 0.50 185 585
LAV 203-1 303 Oyster 3.02 0.17 Bo nd nd 32 518
LAV 203-1 303 Oyster 3.31 0.02 Ox nd nd 32 518
LAV 200 332 Oyster 4.34 0.81 Ox 11.57 0.69 57 646
LAV 200 332 Oyster 4.53 0.65 Bo 9.92 0.61 57 646
LAV 201 332 Oyster 4.77 1.02 Ox 4.48 0.52 53 608
LAV 200 332 Oyster 4.45 0.36 Inn 5.23 0.56 57 646
LAV 205 362 Oyster 3.68 0.87 Ox nd nd 83 515
LAV 206 403 Oyster 3.88 0.80 Ox 6.87 0.52 64 465
LAV 206 403 Oyster 3.88 0.80 Ox 13.33 0.62 64 465
LAV 207-1 403 Oyster 3.69 0.66 Ox 9.56 0.53 48 576
LAV 207-2 403 Oyster 4.00 0.34 Bo 8.55 0.54 64 566
LAV 207-2 403 Oyster 4.16 0.48 Ox nd nd 64 566
LAV 207-1 403 Oyster 2.93 1.06 Inn 5.59 0.55 48 576
LAV 208 425 Oyster 4.25 0.22 Bo 6.55 0.50 31 594
LAV 209 448 Oyster 2.98 0.93 Ox nd nd 189 547
LAV 210-1 451 Oyster 3.33 1.17 Ox 6.34 0.46 128 531
LAV 210-1 451 Oyster 3.33 1.17 Ox 7.89 0.51 128 531
LAV 210-2 451 Oyster 3.43 1.09 Inn 5.43 0.50 67 496
LAV 210-2 451 Oyster 3.55 1.18 Ox nd nd 67 496
LAV 218 477 Oyster 3.17 1.25 Bo 5.63 0.50 93 574
LAV 218 477 Oyster 3.38 1.18 Ox nd nd 93 574
LAV 212 497 Oyster 3.26 0.38 Ox 7.62 0.52 50 580
LAV 212 497 Oyster 3.26 0.38 Ox 9.59 0.58 50 580
LAV 220 500 Oyster 3.95 0.82 Ox nd nd 41 537
LAV 213-1 515 Oyster 2.54 0.43 Bo nd nd 75 527
LAV 213-3 515 Oyster 3.24 0.24 Bo 5.39 0.46 44 965
LAV 214-2 526 Oyster 3.51 0.27 Ox 5.73 0.51 17 498
LAV 214-2 526 Oyster 3.52 0.16 Inn 4.65 0.51 17 498
LAV 215-1 537 Oyster 2.76 0.46 Ox nd nd 37 480
LAV 215-2 537 Oyster 3.02 0.54 Ox 7.44 0.51 53 501
LAV 215-2 537 Oyster 3.02 0.54 Ox 9.23 0.54 53 501
LAV 216 605 Oyster 2.93 1.19 Bo nd nd 15 638
LAV 216 605 Oyster 3.07 1.05 Inn 6.07 0.55 15 638
LAV 217-1 624 Oyster 2.15 1.79 Ox 4.30 0.56 29 583
LAV 217-1 624 Oyster 2.51 1.25 Bo 5.76 0.63 29 583
LAV 219 648 Oyster 2.45 0.69 Inn 5.77 0.54 110 434
LAV 232 877 Oyster 2.16 1.09 Bo nd nd 5 506
LAV 232 877 Oyster 1.86 1.06 Inn 3.96 0.51 5 506
LAV 230 882 Oyster 2.36 1.78 Bo nd nd 52 517
LAV 233 898 Oyster 1.99 1.74 Bo nd nd 10 590
LAV 235-A 968 Oyster 1.71 0.97 Bo nd nd 15 500
LAV 235-B 968 Oyster 2.01 0.06 Bo nd nd 46 430
LAV 237 1102 Oyster 2.48 0.99 Bo nd nd 21 461
( continued)
C. KORTE ET AL.434
relevant ammonites. However, on the basis of the carbon-isotope
data available from St Audrie’s Bay, together with the new data
presented here, the T–J boundary should lie within the Langport
Member at Lavernock.
The general trend in ä
13
C values through the Lavernock Point
section is marked by a prominent positive excursion, which starts
from c. 3‰ in the lower Langport Member, increasing by about
2‰ to highest values of c. 5‰ in the lower Blue Lias, followed
by a 3‰ decrease to c. 2‰ just below the base of the planorbis
Zone (Fig. 3). The carbon-isotope values remain relatively low
up to the portlocki Subzone of the liasicus Zone (with slight
variations in the johnstoni Zone).
Highly resolved ä
13
C profiles in the growth directions of the
shells for six well-preserved oysters from Lavernock Point are
shown in Figure 4 and Table 2: two of these samples were also
profiled perpendicular to growth direction. Carbon-isotope values
show a range of about 1‰ in LAV 110 (mean: 2.8 0.2‰,
n ¼ 44), less than 1‰ in LAV 258 (mean: 4.5 0.2‰, n ¼ 8),
about 0.6‰ in LAV 201 (mean: 4.9 0.2‰, n ¼ 8), more than
1.1‰ in LAV 208 (mean: 3.8 0.4‰, n ¼ 8), nearly 1.3‰ in
LAV 210 (mean: 3.5 0.3‰, n ¼ 26) and about 1.4‰ in LAV
219 (1.8 0.4‰, n ¼ 28). The ä
13
C values perpendicular to the
growth direction vary much less: about 0.6‰ in both LAV 201
(mean: 4.9 0.2‰, n ¼ 7) and LAV 210 (mean: 3.6 0.1‰,
n ¼ 16).
Carbon-isotope values from the Lyme Regis section (Fig. 5;
Table 3), based on analysis of whole-rock carbonate, are about 3
0.5‰ within the lower Langport Member. A decrease in
values of these samples starts in the upper Langport Member,
falling to about 1‰ just below the base of the planorbis Zone.
This declining trend continues, reaching –0.2‰ at the top of the
planorbis Subzone. The carbon-isotope values remain low (c.
0‰) in the johnstoni and portlocki Subzones, but it should be
noted that only one ä
13
C value was obtained for each of these
two units.
The ä
18
O values from Lavernock Point oysters are about 0‰
(1‰) in the lower Langport Member, increase somewhat to
highest values of more than 1.6‰ in the upper Langport Member
and decline sharply (with some reversals) to values of about
2‰ in the portlocki Subzone of the Blue Lias Formation. The
ä
18
O variations in shell-growth direction and perpendicular-to-
growth direction of each of the six well-preserved oysters show
the features described below (Fig. 4, Table 2). The oxygen-
isotope values vary by about 2.1‰ in LAV 110 (mean: 0.2
0.5‰, n ¼ 44), by more than 2‰ in LAV 258 (mean: +0.6
0.9‰, n ¼ 8), by only 0.2‰ in LAV 201 (mean: 1.0 0.1‰,
n ¼ 8), by about 1.1‰ in LAV 208 (mean: 0.3 0.4‰,
n ¼ 8), by 0.7‰ in LAV 210 (mean: 1.0 0.2‰, n ¼ 26) and
by more than 1.6‰ in LAV 219 (1.5 0.5‰, n ¼ 28). The
ä
18
O variations perpendicular to the growth direction are 0.2‰
Table 1. ( continued )
Sample Height above
base Langport
Mbr (cm)
Material ä
13
C
(‰ V-PDB)
ä
18
O
(‰ V-PDB)
Isotope
laboratory
Mg/Ca
(mM M
1
)
Sr/Ca
(mM M
1
)
Mn
(ìgg
1
)
Sr
(ìgg
1
)
LAV 238 1102 Oyster 1.69 1.46 Bo nd nd 84 449
LAV 237 1102 Oyster 1.73 1.12 Inn 9.14 0.53 21 461
LAV 243 1239 Oyster 2.04 0.89 Bo nd nd 80 456
LAV 244 1249 Oyster 2.73 1.52 Bo nd nd 48 607
LAV 246 1473 Oyster 2.59 1.41 Bo nd nd 16 480
LAV 246 1473 Oyster 2.78 1.26 Inn 3.43 0.55 16 480
LAV 247 1552 Oyster 2.32 2.05 Bo nd nd 173 560
LAV 249 1708 Oyster 1.61 2.03 Bo nd nd 84 433
LAV 249 1708 Oyster 2.01 1.24 Bo nd nd 89 425
LAV 256 1748 Oyster 1.26 1.79 Bo nd nd 127 466
LAV 255-C 1781 Oyster 1.89 1.88 Bo nd nd 190 520
LAV 255-C 1781 Oyster 1.98 1.39 Inn 2.91 0.55 190 520
LAV 252 2075 Oyster 1.37 1.98 Bo nd nd 103 525
LAV 251-A 2080 Oyster 1.82 1.81 Bo nd nd 180 480
LAV 251-B 2080 Oyster 1.70 2.23 Bo nd nd 61 500
SAB 165-1 415 Oyster 2.23 1.29 Ox 5.14 0.45 7 471
Wa 1 285 Oyster 3.64 0.71 Ox nd nd 49 550
Wa 2 325 Oyster 3.22 0.35 Ox 8.69 0.52 57 574
Altered oysters
LAV 107-1 4 Oyster 0.59 1.45 Ox nd nd 1373 687
LAV 210-3 451 Oyster 3.19 1.31 Ox nd nd nd nd
LAV 211 470 Oyster 4.06 0.10 Ox nd nd 440 570
LAV 213-2 515 Oyster 2.03 2.03 Bo nd nd 338 411
LAV 214-1 526 Oyster 2.20 1.00 Bo nd nd 227 386
LAV 231 885 Oyster 2.25 1.49 Bo nd nd nd nd
LAV 245-B 1473 Oyster 2.37 1.17 Bo nd nd 53 388
LAV 255-B 1781 Oyster 1.80 1.62 Bo nd nd 267 519
SAB 100-1 (below) 265 Oyster 2.72 2.76 Ox nd nd 1636 1100
SAB 100-2 (below) 265 Oyster 4.11 3.68 Ox nd nd 1392 681
SAB 179 (below) 1115 Oyster 3.49 10.35 Bo nd nd 817 463
Wa BBL 0 Oyster 5.59 6.22 Ox nd nd 743 347
Bulk rocks
LAV 110-1 91 Bulk rock 0.43 5.19 Bo nd nd nd nd
LAV 210 451 Bulk rock 0.87 4.67 Bo nd nd nd nd
LAV 201 332 Bulk rock 0.10 5.19 Bo nd nd nd nd
LAV 219 648 Bulk rock 0.77 3.50 Bo nd nd nd nd
LAV, Lavernock Point; SAB, St Audrie’s Bay; Wa, Watchet; Ox, Oxford; Bo, Bochum; Inn, Innsbruck; nd, not determined.
TRIASSIC–JURASSIC BOUNDARY 435
in LAV 201 (mean: 0.9 0.1‰, n ¼ 7) and more than 0.4‰
in LAV 210 (mean: 1.2 0.1‰, n ¼ 16).
To aid discussion of the oxygen- and carbon-isotope trends, all
ä
13
C and ä
18
O values from well-preserved oysters at Lavernock
Point and from the St Audrie’s Bay area (including the previously
published data of van de Schootbrugge et al. (2007)) are
compiled in Figure 6, together with the whole-rock carbonate
ä
13
C data from the Lyme Regis section.
The isotopic values of well-preserved oyster shells collected
from the same stratigraphic level, and therefore geologically
coeval, show spreads of more than 1‰ for both ä
13
C and ä
18
O
(Figs 3, 4 and 6). Such variation is also seen in single shells and
is the norm for modern oysters (Surge et al. 2001), reflecting
seasonal environmental and temperature change. Therefore, when
discussing long-term changes in environmental parameters, the
general trends enclosed in the envelopes shown in Figure 6
should be considered. It should be noted that the scatter of
carbon- and oxygen-isotope values for the six oysters fit perfectly
within the general trends, constituting evidence that both ä
13
C
and ä
18
O values represent a primary seawater signal.
Mg/Ca and Sr/Ca ratios
Mg/Ca and Sr/Ca ratios have been generated for 48 well-
preserved oyster shells (Table 1) and these vary in the range 2.3–
13.3 mmol mol
1
and 0.4–0.7 mmol mol
1
, respectively. It has
been reported that Mg and Sr concentrations in bivalve shells
depend on ambient seawater temperature and increase with
warming (Dodd 1965; see also Stecher et al. 1996). Notably,
Klein et al. (1996a) documented that for the modern bivalve
Mytilus trossulus (Gould) the Mg/Ca ratios tend to reach higher
values with rising seawater temperature. However, in a more
extensive study Vander Putten et al. (2000) found that Mg/Ca
ratios in Mytilus edulis (Linnaeus) shell calcite showed consider-
able deviations from those expected from water temperature
alone, implying that metabolic processes are involved in Mg
incorporation into the calcite framework. For strontium also it is
most likely that water temperature is not the unique control on
the Sr/Ca ratios in bivalve shells because, in modern bivalves,
metabolic or kinetic effects and/or seawater chemistry have also
been shown to be important (Klein et al. 1996b; Vander Putten et
al. 2000; Lorrain et al. 2005). Work on other modern bivalve
groups has added further weight to the view that their Mg/Ca
and Sr/Ca ratios are unreliable palaeotemperature proxies (Freitas
et al. 2006).
The temperature dependence of Mg/Ca and Sr/Ca ratios in
Early Jurassic oysters was assessed by plotting these ratios
against ä
18
O (Fig. 7), a procedure that assumes that ä
18
Ois
overwhelmingly controlled by seawater temperature (discussed
further below). There are no significant correlations, and there-
Fig. 2. SEM images of well-preserved foliated layers of oyster shells. (a) LAV 110; (b) LAV 204; (c) LAV 219; (d) LAV 251. Scale bars represent 2 ìm.
C. KORTE ET AL.436
Table 2. Highly resolved ä
13
C and ä
18
O profiles of six well-preserved
Liostrea samples
Sample Section Direction Distance
(mm)
ä
13
C
(‰ V-PDB)
ä
18
O
(‰ V-PDB)
LAV 110-1 1 Grow. 0.50 3.13 0.34
LAV 110-1 1 Grow. 1.25 3.13 0.83
LAV 110-1 1 Grow. 1.90 2.96 0.46
LAV 110-1 1 Grow. 8.25 2.89 0.58
LAV 110-1 1 Grow. 8.71 2.56 1.26
LAV 110-1 1 Grow. 9.14 3.07 0.24
LAV 110-1 1 Grow. 9.14 3.12 0.14
LAV 110-1 1 Grow. 9.48 3.08 0.07
LAV 110-1 1 Grow. 9.56 3.11 0.14
LAV 110-1 1 Grow. 9.86 3.05 0.40
LAV 110-1 1 Grow. 10.20 2.91 0.14
LAV 110-1 1 Grow. 10.97 3.04 0.26
LAV 110-1 1 Grow. 11.48 3.00 0.48
LAV 110-1 1 Grow. 11.90 2.56 0.39
LAV 110-1 1 Grow. 12.45 2.39 0.59
LAV 110-1 1 Grow. 12.96 2.43 0.40
LAV 110-1 1 Grow. 13.73 2.74 0.26
LAV 110-1 1 Grow. 14.24 2.82 0.37
LAV 110-1 1 (b) Grow. 1.80 2.93 0.54
LAV 110-1 1 (b) Grow. 5.00 2.72 0.20
LAV 110-1 1 (b) Grow. 6.15 2.99 0.45
LAV 110-1 1 (b) Grow. 6.25 2.81 0.16
LAV 110-1 1 (b) Grow. 7.25 2.88 0.76
LAV 110-1 1 (b) Grow. 9.27 2.85 0.46
LAV 110-1 1 (b) Grow. 10.29 2.61 0.69
LAV 110-1 1 (b) Grow. 10.75 2.96 0.55
LAV 110-1 1 (b) Grow. 11.01 2.49 0.29
LAV 110-1 1 (b) Grow. 11.65 2.35 0.12
LAV 110-1 1 (b) Grow. 12.67 2.44 0.53
LAV 110-1 1 (b) Grow. 13.43 2.79 0.49
LAV 110-1 1 (b) Grow. 13.81 2.76 0.86
LAV 110-1 2 Grow. 0.85 2.82 0.24
LAV 110-1 2 Grow. 1.75 2.98 0.54
LAV 110-1 2 Grow. 2.65 2.26 0.77
LAV 110-1 2 Grow. 3.65 2.57 0.01
LAV 110-1 2 Grow. 5.15 2.74 0.65
LAV 110-1 2 Grow. 5.30 2.83 0.69
LAV 110-1 2 Grow. 6.10 2.83 0.58
LAV 110-1 2 Grow. 6.50 2.96 0.02
LAV 110-1 2 Grow. 7.40 2.87 0.31
LAV 110-1 2 Grow. 7.65 2.63 0.50
LAV 110-1 2 Grow. 8.50 2.97 0.07
LAV 110-1 2 Grow. 10.50 2.71 0.04
LAV 110-1 2 Grow. 11.50 2.25 0.39
LAV 201 1 Grow. 0.75 4.74 0.94
LAV 201 1 Grow. 1.40 4.65 1.05
LAV 201 1 Grow. 1.80 4.55 1.01
LAV 201 1 Grow. 2.75 4.92 0.92
LAV 201 1 Grow. 4.25 4.89 0.98
LAV 201 1 Grow. 4.50 5.14 0.89
LAV 201 1 Grow. 5.10 4.95 1.08
LAV 201 1 Grow. 6.05 5.05 0.87
LAV 201 2 Perp. 0.80 5.07 0.79
LAV 201 2 Perp. 2.20 4.69 1.02
LAV 201 2 Perp. 3.75 4.50 0.99
LAV 201 2 Perp. 3.90 4.98 0.89
LAV 201 2 Perp. 5.05 4.99 0.82
LAV 201 2 Perp. 6.00 4.83 0.84
LAV 201 2 Perp. 7.50 5.05 0.81
LAV 208-p 1 Grow. 3.50 3.56 0.57
LAV 208-p 1 Grow. 5.50 3.89 0.46
LAV 208-p 1 Grow. 7.30 3.61 0.73
LAV 208-p 1 Grow. 9.00 3.62 0.60
LAV 208-p 1 Grow. 10.00 3.14 0.56
LAV 208-p 1 Grow. 12.50 3.91 0.02
LAV 208-p 1 Grow. 14.50 4.18 0.17
LAV 208-p 1 Grow. 17.00 4.26 0.33
LAV 210-2 1 Grow. 0.90 3.78 0.93
LAV 210-2 1 Grow. 0.90 3.71 1.31
LAV 210-2 1 Grow. 1.00 3.07 0.75
LAV 210-2 1 Grow. 2.00 3.46 1.02
LAV 210-2 1 Grow. 2.50 3.35 1.21
LAV 210-2 1 Grow. 2.60 3.12 1.19
LAV 210-2 1 Grow. 3.80 3.46 0.87
LAV 210-2 1 Grow. 4.40 2.97 1.15
LAV 210-2 1 Grow. 4.85 3.47 0.99
LAV 210-2 1 Grow. 4.90 3.04 1.23
LAV 210-2 1 Grow. 5.80 3.57 0.92
LAV 210-2 1 Grow. 6.00 4.13 0.88
LAV 210-2 1 Grow. 7.50 3.85 0.92
LAV 210-2 1 Grow. 7.50 3.31 1.00
LAV 210-2 1 Grow. 7.60 3.67 0.70
LAV 210-2 1 Grow. 8.50 3.82 0.84
LAV 210-2 1 Grow. 9.50 3.55 0.98
LAV 210-2 1 Grow. 10.05 4.01 0.63
LAV 210-2 1 Grow. 11.00 3.63 0.79
LAV 210-2 1 Grow. 11.30 3.79 0.96
LAV 210-2 1 Grow. 12.50 3.52 0.93
LAV 210-2 1 Grow. 12.50 3.70 1.09
LAV 210-2 1 Grow. 13.60 3.65 0.92
LAV 210-2 1 Grow. 14.35 2.88 0.92
LAV 210-2 1 Grow. 14.40 3.54 0.96
LAV 210-2 1 Grow. 15.50 3.51 0.96
LAV 210-2 2 Perp. 3.00 3.78 0.96
LAV 210-2 2 Perp. 4.50 3.69 1.13
LAV 210-2 2 Perp. 6.00 3.64 1.13
LAV 210-2 2 Perp. 6.90 3.53 1.34
LAV 210-2 2 Perp. 8.25 3.52 1.11
LAV 210-2 2 Perp. 4.50 3.65 1.14
LAV 210-2 2 Perp. 3.20 3.80 0.94
LAV 210-2 2 Perp. 3.85 3.74 1.05
LAV 210-2 2 Perp. 9.20 3.52 1.08
LAV 210-2 2 Perp. 10.50 3.59 1.16
LAV 210-2 2 Perp. 11.50 3.48 1.17
LAV 210-2 2 Perp. 11.10 3.50 1.25
LAV 210-2 2 Perp. 12.50 3.54 1.17
LAV 210-2 2 Perp. 13.50 3.55 1.27
LAV 210-2 2 Perp. 15.15 3.40 1.03
LAV 210-2 2 Perp. 15.00 3.24 1.41
LAV 219 1 Grow. 0.80 2.30 0.93
LAV 219 1 Grow. 1.70 1.64 2.08
LAV 219 1 Grow. 3.00 1.69 1.65
LAV 219 1 Grow. 4.25 1.43 2.30
LAV 219 1 Grow. 5.50 2.63 0.67
LAV 219 1 Grow. 6.00 1.89 1.10
LAV 219 1 Grow. 7.30 1.94 1.56
LAV 219 1 Grow. 6.50 1.56 1.80
LAV 219 1 Grow. 7.50 2.09 1.37
LAV 219 1 Grow. 8.50 1.75 1.82
LAV 219 1 Grow. 9.85 2.43 1.04
LAV 219 1 Grow. 11.00 1.79 1.84
LAV 219 1 Grow. 11.50 2.40 0.84
LAV 219 1 Grow. 13.50 2.40 0.71
LAV 219 1 Grow. 12.50 2.01 1.24
LAV 219 2 Grow. 0.85 2.20 0.71
LAV 219 2 Grow. 1.50 1.51 1.32
LAV 219 2 Grow. 2.60 1.43 1.84
LAV 219 2 Grow. 3.50 1.38 2.07
LAV 219 2 Grow. 5.50 1.57 1.62
LAV 219 2 Grow. 6.50 1.28 1.82
LAV 219 2 Grow. 4.50 1.31 2.21
LAV 219 2 Grow. 7.50 1.36 1.94
LAV 219 2 Grow. 8.50 1.24 1.95
LAV 219 2 Grow. 9.50 1.70 1.44
LAV 219 2 Grow. 10.50 1.82 1.17
LAV 219 2 Grow. 11.50 2.34 0.73
LAV 219 2 Grow. 12.50 2.30 0.86
LAV 258 1 Grow. 4.50 4.89 1.64
LAV 258 1 Grow. 6.50 4.69 1.45
LAV 258 1 Grow. 7.50 4.46 1.36
LAV 258 1 Grow. 8.50 4.58 0.89
LAV 258 1 Grow. 14.50 4.29 0.23
LAV 258 1 Grow. 15.00 4.18 0.11
LAV 258 1 Grow. 22.50 4.47 0.43
LAV 258 1 Grow. 24.50 4.21 0.52
Grow., growth direction; Perp., perpendicular to growth direction.
TRIASSIC–JURASSIC BOUNDARY 437
fore it can be inferred that Mg/Ca and Sr/Ca ratios of the
investigated Mesozoic Liostrea hisingeri are determined mainly
by factors other than temperature. It is notable that these results
are in contrast to those of van de Schootbrugge et al. (2007),
who reported a distinct increase of oyster-calcite Mg/Ca and Sr/
Ca ratios with decreasing ä
18
O values, but this was based on a
much smaller dataset.
Discussion
Significance for correlation of T–J boundary strata
Carbon-isotope shifts reflect perturbations in the Earth’s carbon
cycle at a variety of spatial scales (Kump & Arthur 1999). It is
understood that seawater ä
13
C is controlled by the burial and re-
oxidation of
12
C-enriched organic matter within the ocean–
Fig. 3. Stratigraphic section for Lavernock Point, showing sample locations and carbon- and oxygen-isotope values for well-preserved oyster (Liostrea)
shell calcite. T–J boundary position is less precisely definable in comparison with St Audrie’s Bay, but on the basis of carbon-isotope stratigraphy must lie
somewhere within the marly upper beds of the Langport Member.
C. KORTE ET AL.438
atmosphere system, linked to several factors, such as atmospheric
CO
2
levels, nutrient supply, sedimentation rates, net primary
productivity, biological isotope fractionation or sea-level changes
(Scholle & Arthur 1980; Jenkyns 1996; Hayes et al. 1999; Kump
& Arthur 1999; Jarvis et al. 2006). Additional mechanisms
suggested to add significant masses of isotopically light carbon
include input of volcanic mantle-derived CO
2
into the ocean–
atmosphere system (Hansen 2006), sudden release of methane
from gas hydrates (Dickens et al. 1997; Hesselbo et al. 2000),
thermal metamorphism of organic-rich sediments (Svensen et al.
2004; McElwain et al. 2005) or overturn of
12
C-enriched
(anoxic) oceanic bottom waters (e.g. Ku
¨
spert 1982; Knoll et al.
1996).
Many carbon-isotope excursions are global in scale, and are
registered in a wide range of marine and continental deposits
such as platform carbonates, calcareous pelagic sediments,
organic-rich shales, terrestrial palaeosols and lacustrine deposits.
The coeval nature of the ä
13
C fluctuations makes it possible to
use the peaks and troughs for trans-continental stratigraphic
correlation for many stage boundaries (e.g. Permian–Triassic
boundary; Korte & Kozur 2005), and this has also been proposed
for the T–J boundary through the correlation of ä
13
C
org
fluctua-
tions of several sections in North America and Europe (Hesselbo
et al. 2002, 2004; McRoberts et al. 2007). The carbon-isotope
stratigraphy from the well-preserved oysters from T–J boundary
sections presented here (Fig. 6), show the same features as
illustrated by Hesselbo et al. (2002, 2004) for St Audrie’s Bay
(SW England), Kuerschner et al. (2007) for the Tiefengraben
section (Austria), Ward et al. (2007) for New York Canyon
Ferguson Hill (Nevada, USA), and Williford et al. (2007) for
Kennecott Point (Queen Charlotte Islands, British Columbia,
Canada). The same major carbon-isotope fluctuations in the well-
preserved marine oyster carbonate described here are also seen
in bulk organic material (see Fig. 1), namely: (1) a sharp ‘initial’
negative excursion (here seen only in relatively low carbon
isotope values at the base of the Langport Member), followed by
(2) a ‘boundary’ positive excursion, and (3) an extended ‘main’
negative excursion.
It is noteworthy that the amplitudes of the carbon-isotope
excursions are much larger (commonly twice the size) for the
marine organic matter than for the carbonates, a common feature
of carbon-isotope anomalies characterizing several Mesozoic–
Cenozoic events (Arthur et al. 1988; Pagani et al. 2006; Hesselbo
et al. 2007a). The general shape of the curve has been identified
in other Triassic–Jurassic boundary sections for marine carbo-
nates (e.g. McRoberts et al. 1997; Pa´lfy et al. 2001; Galli et al.
2005; van de Schootbrugge et al. 2007), but insufficient sampling
density, lack of organic-matter-based records for the same strata,
the occurrence of depositional hiatuses, or the effects of signifi-
cant sediment redeposition, have made the identification of the
negative–positive–negative geometry of the excursion in these
sections uncertain.
The new data presented here demonstrate that the isotopic
signature described is a seawater signal, and the similarity of the
ä
13
C records from carbonate and organic matter from different
sections in the Tethyan and in North American regions strongly
suggests that the carbon-isotope excursions are of global extent.
Consequently, further definition of a high-resolution carbon-
isotope stratigraphy has significant potential for improved corre-
lation of a time interval beset with problems using only
biostratigraphy.
In addition to insufficient sampling density and/or depositional
hiatuses, diagenetic alteration must be considered when using the
carbon-isotope curve for stratigraphic correlation. For the Lang-
port Member and the lower Blue Lias Formation, carbon-isotope
values from whole-rock carbonate at Lyme Regis show super-
ficially the same ä
13
C trend as oysters (Fig. 6), but the bulk-rock
data are much more
13
C-depleted in the planorbis, johnstoni and
portlocki subzones higher in the Blue Lias (about 2‰ lower than
stratigraphically equivalent oysters). These isotopically lighter
Fig. 4. ä
18
O and ä
13
C variations of foliated
layers in growth direction for three well-
preserved Liostrea shells. Two cut sections
have been analysed from sample LAV 110
(lower Langport Member), and one of these
has been sampled for different depths below
the surface (section 1: open circles, c.1mm
below the surface; grey circles, c. 1.3 mm
below the surface). All other sections have
been sampled c. 1 mm below the surfaces
to avoid contamination.
TRIASSIC–JURASSIC BOUNDARY 439
whole-rock ä
13
C data are most probably diagenetically compro-
mised because these carbonate beds are partly concretionary in
origin, with
12
C-enriched carbon originating, at least in part (Fig.
8: dark grey field), from oxidized organic matter (Hallam 1960b;
Weedon 1986; Sheppard et al. 2006). On the other hand, the data
from the organic-lean Langport Member can be interpreted as
more accurately representing the positive ä
13
C excursion, and
these relatively heavy values show that bulk-rock data, in this
case deriving from massive micritic limestones, can preserve the
primary seawater signature. Such carbonates were deposited as
lime mud and peloids and lithified during diagenesis in an
essentially closed system, such that the final ä
13
C values of these
limestones were probably derived from the dissolving solid phase
of the original lime mud (see Veizer 1983). A correlation
between Lyme Regis, St Audrie’s Bay and Lavernock Point,
based on carbon-isotopes, implies that the Langport Member at
Lyme Regis is coeval with argillaceous facies in the Langport
Member and Blue Lias at the other two localities (see Hesselbo
et al. 2004).
Palaeoenvironmental change at the T–J boundary
The relationship between the T–J environmental crisis and flood-
basalt volcanism of the Central Atlantic Magmatic Province (Fig.
1) has been much discussed, including the possibility that
volcanogenic CO
2
was responsible for the negative carbon-
isotope excursions (McHone 1996; Marzoli et al. 1999; Pa´lfy et
al. 2000; Wignall 2001b; Courtillot & Renne 2003; Pa´lfy 2003;
Nomade et al. 2007). Bulk-rock organic-carbon data from St
Audrie’s Bay show an abrupt ‘initial’ negative ä
13
C excursion
(Fig. 6). Such apparently sudden negative shifts could be
accounted for by a local hiatus obscuring a more gradual change
(see Hesselbo et al. 2002; Lucas et al. 2007), but the preponder-
ance of records showing abrupt change (e.g. Hesselbo et al.
2002; Guex et al. 2004; Ward et al. 2007; Williford et al. 2007)
implies either a relatively sudden event or an extraordinary
synchroneity of sedimentary condensation.
Rather than being produced directly by volcanogenic CO
2
(see
Self et al. 2006), such sudden negative carbon-isotope shifts are
probably accounted for by the release of isotopically light carbon
through other, less direct, mechanisms, such as those proposed
for the Palaeocene–Eocene boundary and Toarcian Oceanic
Anoxic Event: namely, massive methane release from gas
hydrates (Dickens et al. 1995, 1997; Hesselbo et al. 2000; Kemp
et al. 2005), and/or the intrusion of large sills into organic
carbon-rich sedimentary strata (Svensen et al. 2004, 2007;
McElwain et al. 2005).
In the case of the T–J boundary, intrusion of Central Atlantic
Fig. 5. Lithology and bulk ä
13
C
carb
data for the section at Pinhay Bay, Lyme Regis, SW England.
C. KORTE ET AL.440
Magmatic Province magma into organic-rich lacustrine sediments
of the deep and extensive Triassic rift systems could have caused
similar effects. A large proportion of the Central Atlantic
Magmatic Province intrusive bodies are either eroded or deeply
buried beneath later Mesozoic deposits on the continental
margins (McHone 1996) and the nature of the material into
which the magma was intruded is generally unknown. In the
Newark Basin, one of these Triassic rift basins, dolerite sills are
locally intruded into Carnian lacustrine black shales of the
Lockatong Formation, which have undergone intense thermal
metamorphism (Van Houten 1969, 1971). Thus, although the
extent to which this may have been an effective process in
generation of isotopically light thermogenic methane is uncer-
tain, it is known that such processes occurred on at least a local
scale during T–J boundary time.
Positive carbon-isotope excursions are usually explained by
accelerated burial of organic carbon (Scholle & Arthur 1980;
Kump & Arthur 1999). For the pronounced earliest Jurassic
positive excursion (Fig. 6), an origin through globally accelerated
carbon burial cannot at present be demonstrated, largely because
the stratigraphic record for this time interval is so fragmentary.
At St Audrie’s Bay, although the equivalent strata are not
Table 3. Whole-rock carbon- and oxygen-isotope data for the Pinhay
Bay, Lyme Regis section
Sample Height above base of
Langport Mbr (cm)
ä
13
C
(‰ V-PDB)
ä
18
O
(‰ V-PDB)
LM 1 0 3.28 3.35
LM 2 39 3.53 2.93
LM 3 60 3.61 2.79
LM 4 93 3.36 3.41
LM 5 110 3.47 3.36
LM 6 138 3.55 3.16
LM 7 161 3.31 3.44
LM 8 185 4.03 1.76
LM 9 207 3.39 2.96
LM 10 243 3.88 2.23
LM 11 280 3.56 2.86
LM 12 319 3.58 2.94
LM 13 332 2.92 3.46
LM 14 354 2.92 3.93
LM 15 369 3.88 2.34
LM 16 420 3.70 2.46
LM 17 431 3.77 1.90
LM 18 459 3.73 2.19
LM 19 507 3.32 3.19
LM 20 520 2.86 4.56
LM 21 601 3.07 2.92
LM 22 628 3.16 2.86
LM 23 640 2.39 3.14
LM 24 649 2.43 2.81
LM 25 674 2.21 3.04
LM 26 709 1.60 4.58
LM 27 736 1.89 2.90
LM 28 759 1.39 2.08
LM 29 783 1.59 2.17
LM 30 796 1.52 1.98
LM 31 811 1.52 2.86
LM 32 848 1.58 3.16
LM 33 858 1.55 3.09
LM 34 869 1.60 2.79
LM 35 884 1.02 3.93
LM 36 909 1.11 3.09
LM 37 921 1.36 2.30
LM 38 948 1.19 2.23
LM 39 975 0.77 2.36
LM 40 994 0.80 2.32
LM 41 1083 0.45 1.53
LM 42 1114 0.01 2.62
LM 43 1182 -0.18 1.90
LM 44 1315 0.01 2.12
LM 45 1447 -0.14 1.89
Isotope laboratory: Oxford.
Fig. 6. ä
13
C and ä
18
O values for all well-preserved oysters for the
Triassic–Jurassic transition from Lavernock Point, Watchet and St
Audrie’s Bay (including data from van de Schootbrugge et al. 2007), and
carbon-isotope data from whole-rock carbonates from Lyme Regis. Bulk
organic ä
13
C data (Hesselbo et al. 2002, 2004) are also plotted for
comparison. Correlations between St Audrie’s Bay, Watchet and
Lavernock are based on lithostratigraphy and biostratigraphy. Correlation
between Lyme Regis and the other localities is based upon
biostratigraphy for the Blue Lias and carbon-isotope data for the
Langport Member. Ages are assigned to the data points based on the
position of samples at each locality within the recognized ammonite
subzones (see Jenkyns et al. 2002), with the addition of a notional
subzone to span the gap between the base of the planorbis Subzone and
the proposed (lower) base of the Hettangian. Data points in the Triassic
are plotted assuming a constant sedimentation rate through the earliest
Jurassic at Lavernock Point and St Audrie’s Bay.
TRIASSIC–JURASSIC BOUNDARY 441
markedly enriched in organic carbon, they are strongly laminated
(Hesselbo et al. 2004), indicating bottom-water anoxia and the
potential for enhanced organic matter burial at other localities.
Oxygen-isotope values in the calcitic shells of oysters are
controlled by the seawater ä
18
O, pH and temperature (Zeebe &
Wolf-Gladrow 2001). Assuming that seawater pH was similar to
the present-day value and seawater ä
18
O was about –1.2‰ in an
ice-free world (Zachos et al. 2001), bottom-water temperatures,
using the equation of O’Neil et al. (1969), were between ,7 and
14 8C for deposition of the upper Langport Member and between
c. 12 and 22 8C for the planorbis, johnstoni and portlocki
ammonite Subzones. Assuming no change in the local seawater
ä
18
O over the interval in question, these data indicate a tempera-
ture increase of more than 8 8C for the bottom waters in the
vicinity of the Lavernock Point locality. Starting from relatively
cool initial temperatures, a warming occurred in concert with the
shift towards negative carbon-isotope values.
It might be argued that the decreasing oxygen-isotope trend
that occurs leading into the planorbis Zone was caused, at least
in part, by a lowering of the local or global seawater ä
18
O rather
than a rise in temperature. Changes in global seawater ä
18
O
related to interaction of seawater with the lithosphere are on
multi-million year time scales (Gregory 1991; Veizer et al. 1999)
and can be excluded as an explanation. Short-term lowering of
the seawater ä
18
O could be caused by melting of continental ice,
in the manner observed for the post-Eocene ‘icehouse’ (Shackle-
ton & Opdyke 1973). The extent of continental ice during
supposed ‘greenhouse’ times is a matter of much speculation and
few data (Price 1999; Miller et al. 2005). Climate modelling
studies that have been carried out for the T–J boundary interval
have been aimed at testing the environmental response of the T–
J world to increased rather than decreased atmospheric CO
2
partial pressures; nevertheless, polar regions of the Earth experi-
ence polar ice-forming conditions in simulations with 2 3 pre-
industrial atmospheric CO
2
content (Huynh & Poulsen 2005). It
is notable, however, that the lightening in oxygen-isotope values
observed in the present study would have corresponded to a
eustatic sea-level rise of some 200 m if generated by waning ice
sheets (see Miller et al. 2005) and, although a sea-level rise is
compatible with sequence stratigraphic interpretations for this
time interval, the large magnitude is not (Hesselbo 2008, and
references therein).
Lastly, a localized change in salinity may be postulated, either
from normal marine to freshwater conditions, or from hypersa-
line to normal marine conditions. An upward trend towards
greater freshwater influence is implausible because ammonites
appear at the culmination of the trend to lighter isotope values,
the exact opposite of what would be expected. An upward trend
from hypersaline to normal marine waters can also be ruled out
because Jurassic oysters and their modern counterparts are
intolerant of hypersaline conditions, occupying brackish to
normal marine habitats (Hendry & Kalin 1997; Fu
¨
rsich 1993). In
an isotopic study of Middle Jurassic oysters from England,
Hendry & Kalin (1997) explained heavy oxygen isotopes in
shells from nearshore settings as a result of evaporative concen-
tration within low-salinity waters of hydrodynamically closed
lagoons, a palaeogeographical setting that is very different from
the one considered here.
Faunal evidence, particularly the occurrence of corals, echino-
derms and conodonts, led Swift (1995) and Swift and Martill
(1999) to argue in favour of a normal marine environment for
the Langport Member in southern Britain. The absence of other
Fig. 7. Cross-plots for ä
18
O against Mg/Ca
and Sr/Ca.
Fig. 8. Cross-plot for ä
13
C against ä
18
O values of all pristine and altered
oysters (Lavernock Point, Watchet and St Audrie’s Bay) as well as bulk-
rock samples (Lyme Regis and Lavernock Point). It should be noted that
altered oysters as well as bulk-rock samples tend to show lower ä
18
O and
partly also lower ä
13
C values than unaltered oysters.
C. KORTE ET AL.442
typically normal marine taxa such as brachiopods and ammonites
may be due to the severe ecological disturbance that occurred
during the Triassic–Jurassic boundary mass extinction rather
than simple salinity control. Celestine in the Langport Member
of Devon was interpreted by Hesselbo & Jenkyns (1995) as a
possible replacement of evaporitic gypsum. However, this ob-
servation is of only marginal relevance to interpretation of the
Lavernock oyster oxygen-isotope data, which come from a
different facies and a distant location.
With regard to a detailed comparison between the carbon-
isotope and oxygen-isotope curves presented in this study, a
small difference in the position of the positive peaks is detected,
with the peak in oxygen-isotope values occurring a few deci-
metres lower in the section than the peak in carbon-isotope
values. However, these differences may reasonably be regarded
as a consequence of the variability exhibited within single shells
and between shells taken from the same horizon. The data do not
clearly define a phase lag between the oxygen- and carbon-
isotope curves.
The relationship between palaeotemperature change (as in-
ferred from oxygen-isotope values) and carbon-isotope fluctua-
tions is similar to that observed for the Toarcian Oceanic Anoxic
Event (Hesselbo et al. 2007b), during which high temperatures
coincided with strongly negative carbon-isotope values, and
lower temperatures with more positive carbon-isotope values
(and enhanced organic-carbon burial causing drawdown of atmo-
spheric CO
2
). The T–J boundary results are compatible with the
interpretation that positive carbon-isotope excursions correspond
to times of low atmospheric carbon dioxide content, and negative
carbon-isotope excursions correspond to times of high atmo-
spheric carbon dioxide content, implying significant swings in
the balance between carbon drawdown through organic-matter
burial and recycling of isotopically light carbon from endogenic
or exogenic sources.
Conclusions
Major carbon-isotope fluctuations are preserved within low-Mg
calcite shells of oysters collected at T–J boundary sections in
SW Britain. These fluctuations follow trends previously estab-
lished from bulk organic-matter samples from the same sedimen-
tary basin, and thus confirm that the observed isotopic signals
capture characteristics of the local water masses. Furthermore,
the similarity to carbon-isotope curves generated from other
basins suggests that the signal is global. The amplitude of the
carbonate carbon-isotope fluctuations is about half that observed
from organic matter, a feature common to other Mesozoic and
Early Cenozoic events affecting the global carbon cycle. The T–
J boundary isotopic excursions occurred coincident with Central
Atlantic Magmatic Province continental flood-basalt volcanism,
and may have been triggered by the intrusion of mantle-derived
melts into carbon-rich sedimentary deposits and/or dissociation
of gas hydrates triggered by a global rise in temperature. The
pronounced positive carbon-isotope anomaly is associated with
relatively heavy seawater oxygen-isotope values, indicating cool
conditions, possibly associated with enhanced organic-carbon
burial and drawdown of atmospheric carbon dioxide. The subse-
quent trend towards lighter seawater oxygen-isotope values
indicates returning warmer temperatures that parallel a return to
lighter carbon-isotope values and inferred build-up of atmo-
spheric carbon dioxide. Mg/Ca and Sr/Ca ratios show no signifi-
cant correlation to ä
18
O, indicating that Mg and Sr incorporation
in the investigated Liostrea hisingeri is controlled mainly by
factors other than temperature.
We acknowledge P. Hodges (Cardiff) for scientific discussion in the field,
and N. Charnley, J. Arden, C.-J. de Hoog (all Oxford), U. Schulte, W.
Gosda (both Bochum) and M. Wimmer (Innsbruck) for stable-isotope and
elemental analyses. We thank the Deutsche Akademie der Naturforscher
Leopoldina (BMBF-LPD 9901/8-116) for contributions to the financing
of this project. P. Olsen is thanked for scientific discussion. Finally, we
thank J. Pa´lfy, I. Jarvis and an anonymous referee for their insightful
critical comments.
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