Carbon-Cycle Perturbation in the Middle Jurassic and Accompanying
Changes in the Terrestrial Paleoenvironment
Stephen P. Hesselbo, Helen S. Morgans-Bell, Jennifer C. McElwain,1 P. McAllister Rees,2
Stuart A. Robinson, and C. Elizabeth Ross3
Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, United Kingdom
(e-mail: stephen.hesselbo@earth.ox.ac.uk)
ABSTRACT
Carbon-isotope analyses of fossil wood from the Middle Jurassic Ravenscar Group, Yorkshire, NE England, reveal a
significant excursion toward light isotopic values (d13C change of ⫺3 to ⫺4‰) at about the Aalenian-Bajocian boundary
(∼174 Ma). A positive carbon isotopic excursion is also shown for the middle Bajocian (∼170 Ma) but is less clearly
defined. These isotopic patterns are very similar to the few published marine carbonate records available for this
time, in particular one based on belemnites from the Hebrides basin, NW Scotland, and others from pelagic limestones
in Italy. The similarity of the terrestrial and marine isotope curves is an indication that the observed isotopic signal
is a global phenomenon. Through parts of the Ravenscar Group (the Scarborough Formation), supplementary data
from bulk organic carbon and palynofacies analysis confirm that isotopic curves based on bulk analyses may be
strongly influenced by the balance of terrestrial versus marine organic matter present in the samples. The negative
isotope excursion at the Aalenian-Bajocian boundary marks a change from charcoal to coal as the dominant preservational mode of the macroscopic wood fossils, which is interpreted here as a shift to a more continuously humid
climate in the Early Bajocian. Upsection, charcoal once again becomes common, reflecting a return to more fireprone (presumably seasonally arid) environments in the middle Bajocian. Paradoxically, floral assemblages associated
with the lithological unit in which the negative excursion occurs display characteristics that would normally be
interpreted as adaptations to water stress brought about by relative aridity or salinity. Preliminary analyses of leaf
stomatal densities show some evidence of raised pCO2 relative to background values at about the level of the negative
excursion.
Introduction
Large-scale and rapid paleoenvironmental changes
are commonly associated with perturbations in the
global carbon cycle and thus also in the isotopic
ratios in both the oceanic and atmospheric carbon
reservoirs. Prime examples are the Early Toarcian,
Early Aptian, and Cenomanian-Turonian oceanic
anoxic events (Hasegawa 1997; Gröcke et al. 1999;
Hesselbo et al. 2000; Jahren et al. 2001; Ando et al.
2002); the Paleocene-Eocene Thermal Maximum
(Koch et al. 1992; Dickens et al. 1995; Beerling and
Manuscript received February 19, 2002; accepted August 5,
2002.
1
Department of Geology, Field Museum of Natural History,
1400 South Lake Shore Drive, Chicago, Illinois 60605-2496,
U.S.A.
2
Department of Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, U.S.A.
3
Shell International Exploration and Production B.V. Volmerlaan 8, Postbus 60, 2280 AB Rijswijk, The Netherlands.
Jolley 1998); and some mass extinctions (Magaritz
et al. 1992; Arens and Jahren 2000; Hesselbo et al.
2002). Terrestrial plant fossils commonly provide a
useful material to analyze because the contained
carbon is drawn directly from the atmosphere.
However, it is also true that modern terrestrial
plants show a great variety of carbon isotopic values depending on biophysical and biochemical factors such as water stress, photosynthetic pathway,
or degree of recycling of respired CO2 (e.g., as reviewed in Bocherens et al. 1993; Schleser 1999;
Beerling and Royer 2002). In addition, interpretation of the fossil record may be further complicated
by taphonomic and diagenetic factors (e.g., Van Bergen and Poole 2002). Thus, an important aspect of
any stratigraphic analysis of terrestrial carbonisotope profiles will be comparison of datasets from
more than one location or paleoenvironment so
[The Journal of Geology, 2003, volume 111, p. 259–276] 䉷 2003 by The University of Chicago. All rights reserved. 0022-1376/2003/11103-0002$15.00
259
260
S . P. H E S S E L B O E T A L .
Figure 1. Stratigraphic and paleogeographic orientation. A, Detail of the region of the Laurasian Seaway. Gray
tone p sea. (Simplified from Ziegler 1990.) Locations discussed in text and referred to in figure 6: 1, NE Yorkshire,
England; 2, Hebrides, Scotland; 3, Umbria-Marche, Italy. During the Jurassic, the paleolatitude of Yorkshire was ∼40⬚N
(Smith et al. 1994). B, Global paleogeogaphy for the Middle Jurassic (simplified from Smith et al. 1994). Box indicates
area shown in A.
that the diverse potential influences on isotopic
compositions can be properly assessed. If closely
similar patterns are observed in many coeval sections, a global change in the carbon-isotopic composition of the exchangeable carbon reservoir may
be safely inferred.
Although there exist detailed wood-based isotopic records for limited periods of Mesozoic time,
much of this interval is virtually devoid of relevant
data, and this lack hinders a more general understanding of the inherent variability in the operation
of the Mesozoic carbon cycle and associated paleoclimatic impacts. In this article, we present evidence from the classic plant-bearing Middle Jurassic Ravenscar Group, Yorkshire, NE England
(54⬚30⬘N, 0⬚30⬘W), which suggests that major paleoenvironmental changes took place in the Middle
Jurassic (at about the Aalenian-Bajocian boundary).
Specifically, we report evidence for a terrestrial
(wood-based) carbon-isotope excursion that occurs
coincidentally with an environmentally significant
change in the preservational style of wood fossils
and that accompanies changes in plant fossil assemblages. The isotopic profiles from the Ravenscar Group are then compared with other data from
the marine realm, and, last, we examine whether
estimates of relative changes in pCO2 derived from
stomatal densities are compatible with possible
mechanisms for the origins of the carbon-isotope
excursions and inferred paleoenvironmental
changes that took place at the time.
Geological Setting and Material and Methods
Fluvio-deltaic deposits are a significant component
of the Ravenscar Group, a succession that was produced in association with a rifted domal structure
centered on the North Sea (fig. 1; Hallam and Sellwood 1976; Hancock and Fisher 1981; Ziegler 1981,
1990; Underhill and Partington 1993). Marine
strata intercalated within the Ravenscar Group allow some parts to be assigned variably precise biostratigraphic ages (fig. 2). Diverse assemblages of
fossil plants occur in all the nonmarine units and
also, more sporadically, in some of the marine units
(for recent summary, see Van Konijnenburg-Van
Cittert and Morgans 1999). The principal systematic descriptions of the plant macrofossils are by
Black (1929), Harris (1961, 1964, 1969, 1979), and
Harris et al. (1974). In all, some 260 species (mostly
leaves, fructifications, and seeds) have been recovered from the Ravenscar Group. Fossil wood is also
abundant through most of the Ravenscar Group,
particularly in the nonmarine units (e.g., Cope
1993), although only a few of these specimens are
taxonomically identifiable or have provided paleoclimatically significant structural information
(Morgans 1999; Morgans et al. 1999).
In this study, we collected macroscopic wood fossils through all units of the Ravenscar Group and
aimed to achieve a minimum sample spacing of 0.5
m (an aim achieved for much of the succession as
shown in fig. 2). Through intervals of the succession where this sample resolution proved impos-
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C A R B O N - C Y C L E P E R T U R B AT I O N
sible (e.g., through much of the marine Scarborough
Formation), bulk sediment samples were instead
collected.
A total of 282 macroscopic wood fragments (preserved as coal or charcoal) were collected from the
Ravenscar Group (and the contiguous Dogger and
Cornbrash Formations). Samples were washed in
distilled water before being air dried and then powdered with a pestle and mortar. In each case, about
1 cm3 of powdered sample was decarbonated by
leaving it overnight with ∼10 mL3 of 3 M HCl. If
necessary, this process was repeated until dissolution of all carbonate had taken place. The samples were then repeatedly washed with deionized
water until neutrality was reached. After being air
dried, or oven dried at 60⬚C, ∼3 mg of each sample
was measured into an 8 # 6 mm tin capsule and
placed in a Europa Scientific Limited CN Biological
sample converter connected to a 20–20 stableisotope gas-ratio mass spectrometer at the Archaeology Research Laboratory, University of Oxford.
Using this process, the carbon-isotope ratio of each
sample was determined against an internal nylon
standard (d13 Cnylon p ⫺26.2 Ⳳ 0.2‰). The data are
expressed as parts per mil (‰) deviation from the
Peedee Belemnite standard. Data “outliers” were
subjected to repeat analysis to verify their values.
In addition to the wood samples, 65 bulk-rock
samples collected from the Scarborough Formation
were analyzed. Total organic carbon (TOC) and
CaCO3 contents were determined before any isotopic analysis. Samples were cleaned with deionized water, dried, and crushed to a fine powder.
Duplicate subsamples (four from each sample) were
weighed into ceramic boats. Two of these subsamples were roasted in an oven in air at 420⬚C for ∼12
h to remove organic carbon. All four subsamples
were then placed in turn into a Strohlein Coulomat
702 and heated to 1220⬚C in a stream of pure oxygen to release carbon (carbonate and organic) for
analysis. The CO2 produced was absorbed by a solution of barium perchlorate, which caused change
in pH. To restore the solution to the original pH,
electrolysis was used. The quantity of electricity
(coulomb) required to do this determines the percentage of carbon. The difference between the
amount of carbon determined in unroasted and preroasted samples provided an estimate of TOC. This
was verified using the replicate pair of samples. Reproducibility of samples was generally better than
0.1%. The percentage of inorganic carbonate (expressed as % CaCO3) was calculated by multiplying
the average carbon in the preroasted sample by
8.333⬘. For isotopic analysis, each sample was prepared in the same way as for wood fragments in-
261
cluding carbonate dissolution, but depending on
TOC content, 20–35 mg of each sample were
weighed out.
For stomatal density counts, cuticle and whole
coalified leaves were extracted from the rock matrix by soaking samples in water for 12–24 h, followed by sieving and washing of the disintegrated
sediments using a 0.5-mm sieve. Those fragments
of cuticle in the same size range as the remaining
rock matrix were separated by water flotation into
petri dishes where they were then sorted. The samples that did not break down in water were soaked
in 30% HCl for 12–24 h and then repeatedly washed
with deionized water until neutrality was reached;
the material was then sieved in water. Stomatal
densities were calculated according to standard procedures (McElwain et al. 1995; Poole and Kürschner
1999) using a Leica epifluorescence microscope
(#400 magnification) within a 0.189 mm2
graticule.
Carbon-Isotope Stratigraphy of the
Ravenscar Group
The carbon-isotope results are presented in figure
2. Two features of the wood-based curve are of particular note. First, there is an abrupt negative excursion that occurs near the base of the Sycarham
Member (between 55 and 60 m height in fig. 2; see
fig. 3 for overview of the relevant exposure). This
excursion remains well defined even if the most
extreme data point (which was subjected to replicate analysis) is removed from the dataset. Second,
carbon-isotope values become relatively heavy (i.e.,
less negative) at the top of the Scarborough Formation and into the base of the Scalby Formation
(at about 145 m height in fig. 2). Full data tables
are available from The Journal of Geology’s Data
Depository.
Wood fossils are rare through the transgressiveregressive facies sequence represented by the Scarborough Formation, and over this interval, isotope
data from bulk sedimentary organic carbon supplement the wood-based data points. Although the
bulk data clearly define an upward trend toward
lighter and then back to heavier isotope values,
these changes coincide exactly with marked sedimentary facies changes, and we interpret the pattern to reflect changes in the abundance of organic
components from terrestrial to marine and back to
terrestrial dominance. This interpretation is confirmed by consideration of palynofacies data that
indicate increasing marine palynomorph content
up to the middle of the Scarborough Formation followed by a trend of decreasing abundance to the
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C A R B O N - C Y C L E P E R T U R B AT I O N
top of the formation (Gowland and Riding 1991;
Ross 1999; fig. 4). Recognizable marine palynomorphs (leiospheres, tasmanitids, acritarchs, dinoflagellate cysts, and zoomorphs) track changes in
d13Corg values through the formation, but it is the
content of transparent yellow fluorescent structureless organic matter (SOM) that changes in sufficient proportions to effect the observed isotopic
shift of some 3‰. Transparent yellow fluorescent
SOM is typically associated with poorly oxygenated marine environments and is primarily sourced
from phytoplankton (Tyson 1993). Additionally, the
few isotope data from wood samples in the middle
of the Scarborough Formation do not deviate from
background wood values (fig. 2), which further suggests that no major change in the isotopic values
of the ocean-atmosphere system characterized this
interval.
Wood-based isotopic data from the top of the
Scarborough Formation and in the base of the
Scalby Formation show a good deal of scatter, but
analyses from adjacent localities consistently show
the presence of specimens with relatively heavy
isotopic values (fig. 5). In more detail, it is noteworthy that the lowest indication of less negative
carbon-isotope values is in the Blow Gill Member
of the Scarborough Formation and that similar values also occur in the lower part of the Moor Grit
at the base of the Scalby Formation. The carbonisotope values therefore provide no support for the
idea of a major stratigraphic break between the
Scarborough Formation and the Scalby Formation
(cf. Leeder and Nami 1979).
The age of the negative isotopic excursion must
be close to the Aalenian-Bajocian boundary, which
is dated ca. 174 Ma by Pálfy et al. (2000). An earliest
Bajocian (discites Zone) age was assigned to the
underlying Eller Beck Formation by Bate (1967) on
the basis of ostracod faunas in laterally contiguous
263
strata, although other authors have assigned the
Eller Beck Formation to the latest Aalenian (concavum Zone) on the same basis (e.g., Parsons 1980).
The overlying Lebberston Member has been assigned to the discites Zone by all recent authors
on the basis of similar lithostratigraphical correlations with marine strata to the south of the basin
(Parsons 1980; Powell and Rathbone 1983; Gaunt
et al. 1992). The poorly defined positive excursion
can be assigned to the middle Bajocian with some
confidence because ammonites indicative of the
humphresianum Zone are well known from the
Scarborough Formation (Parsons 1973, 1980), and
this equates to an age of ca. 170 Ma, using the timescale of Pálfy et al. (2000).
Comparison of our dataset with the carbonisotope stratigraphy of marine successions is of
some importance if a case is to be made that the
observed isotopic patterns are caused by global
rather than local paleoenvironmental, taphonomic,
or diagenetic factors. Detailed Middle Jurassic
carbon-isotope curves based on marine materials
exist for two settings (figs. 1, 6): marine siliciclastic
strata in the Hebrides basin, Scotland (Jenkyns et
al. 2002), and pelagic-carbonate sequences from
Umbria-Marche, Italy (Bartolini and Cecca 1999;
Bartolini et al. 1999; Zempolich and Erba 1999).
The overall patterns are best exemplified by the
Calcari a Posidonia in the Terminilletto section in
Italy, which represents the Aalenian and Early Bajocian interval (fig. 6). In this section, there is a
1.5‰ shift in carbon-isotope values of bulk carbonates, from the lightest values at about the AalenianBajocian boundary to heaviest values in the middle
Bajocian. An additional dataset from the nearby
Presale section shows similarly light carbonisotope values in beds assigned to the Middle–
Late Aalenian, although it should be noted that calibration of these sections to standard ammonite zo-
Figure 2. Summary graphic log of the Ravenscar Group, NE Yorkshire coast, made up from sections at Blea Wyke,
from Hayburn Wyke to Burniston Bay, and at Cayton Bay and Gristhorpe Bay. (Full details of the succession are given
in fig. 7. Sample locations can be obtained from The Journal of Geology’s Data Depository.) Heights are based on our
own field measurements, supplemented by data from the Ravenscar borehole, which were used to give total thickness
of the Saltwick, Cloughton, and Scalby Formations (Eschard et al. 1991). M p marine units, within an otherwise
nonmarine sequence. L.M. p Lebberston Member; M.G.M. p Moor Grit Member. Refer to tables 1 and 2 for stomatal
density data. The stomatal density data of McElwain (1997) are based on examination of Harris’s collection in the
Natural History Museum, London, and because this collection is not precisely located stratigraphically, large vertical
error bars are shown (relevant data enclosed by balloons). Stomatal density work carried out for this study relies on
material collected from specific plant beds: H p Hayburn Wyke Plant Bed; E p Negative Excursion Horizon; S p
Solenites Bed. Other well-known plant beds are shown in figure 7. Approximate numeric ages from the Pálfy et al.
(2000) timescale. The Cornbrash, at ∼196 m in the section, is a condensed limestone, and close juxtaposition of fossils
of different ages probably explains the spread of carbon-isotope values within the unit.
264
S . P. H E S S E L B O E T A L .
Figure 3. Field photographs of the Sycarham Member at Iron Scar (TA 015968) showing the exposure at the level
of the negative excursion. a, Overview of the section between 57 and 61 m height in figure 7. The surface on which
the researchers are standing is the top of the sandstone channel fill. b, Detail of the exposure at the negative excursion
level; a distinctive band of centimeter-sized siderite nodules occurs through the middle of the exposure seen in this
view.
nations has yet to be achieved with any confidence.
Belemnites from Bearreraig Burn, Skye, Hebrides
basin, which is an auxiliary stratotype for the
Aalenian-Bajocian boundary (Morton and Hudson
1995; Pavia and Enay 1997; McArthur et al. 2000),
show a marked negative excursion in the Early Bajocian (discites Zone) followed by a return to more
positive values higher in the section (Jenkyns et al.
2002). A more wide-ranging compilation of data
(Jenkyns et al. 2002, their fig. 7), which includes
analyses of belemnites from the Global Stratotype
Section at Cabo Mondego, Portugal, confirms ex-
ceptionally light carbon-isotope values at about the
Aalenian-Bajocian boundary.
Long-range correlation thus supports interpretation of the wood data as a global phenomenon, but
are alternative mechanisms for producing stratigraphic trends such as those observed in the Ravenscar Group also plausible? No clear relationships exist between carbon-isotope values and the
wood taxa identified by Morgans (1999; H. S. Morgans, unpublished data), nor is there any systematic
difference in isotopic values between wood preserved as coal and that preserved as charcoal (fig.
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C A R B O N - C Y C L E P E R T U R B AT I O N
265
Figure 4. Comparison of isotopic data from bulk sedimentary organic carbon with quantitative palynofacies data
from the Scarborough Formation. The palynofacies data shown here indicate that the d13Corg signal tracks changes in
the abundance of organic components.
2). Relatively heavy carbon-isotope values can be
produced in plants growing in water-stressed or saline environments (e.g., Nguyen Tu et al. 1999), and
because some facies of the Ravenscar Group are
indicative of coastal environments, such effects
might be expected. However, water or salt stress
cannot explain the Early Bajocian negative excursion, and the middle Bajocian positive excursion is
recognizable in wood from both the top of the marine Scarborough Formation and the coarse fluvial
Scalby Formation and so cannot, for example, simply be related to derivation from mangrove-type
plants.
On the basis of these comparisons and arguments, together with evidence from other woodbearing Mesozoic successions in which similar correlations to marine isotopic curves can be
unambiguously demonstrated (Gröcke et al. 1999;
Hesselbo et al. 2000; Jahren et al. 2001; Ando et al.
2002; Hesselbo et al. 2002), we have some confidence that the signal recorded from the Ravenscar
Group represents global changes in the isotopic
composition of atmospheric CO2, biospheric carbon, and also the (much larger) carbon reservoir of
the shallow ocean. It has been claimed (Arens et
al. 2000; Gröcke 2002) that a quantitative relationship of predictive value exists between d13Cplant and
d13Catmosphere, but Beerling and Royer (2002) have
clearly demonstrated that the putative relationship
consistently failed to predict d13Catmosphere where it
was known by independent means. Furthermore, it
is unclear from the original dataset (Arens et al.
2000) how the potentially complementary effects
of d13Catmosphere and pCO2 could be satisfactorily
separated.
In relation to the stratigraphic sequences, we
note that although the isotopic shifts appear abrupt
in the Ravenscar Group, they are much more gradual in the marine sections, including the Italian
pelagic carbonates. Therefore, it is likely that depositional rates for the Ravenscar Group were highly
episodic and the excursion interval is a relatively
condensed section.
Accompanying Environmental Changes
A detailed graphic log for the major part of the Ravenscar Group is shown in figure 7. The four nonmarine units of the Ravenscar Group were deposited under somewhat similar depositional and
paleoclimatic regimes. Broadly, all were laid down
in swampy fluvio-deltaic environments and contain palynofacies indicative of saline and freshwater influence (Hancock and Fisher 1981; Livera
and Leeder 1981; Fisher and Hancock 1985; Ross
1999). The Sycarham Member can be distinguished
from the other three nonmarine units in that all
contained lithofacies—above and below the negative isotopic excursion—show some palynological
266
S . P. H E S S E L B O E T A L .
Figure 5. Details of carbon-isotope data at the transition between the Scarborough Formation and the Scalby
Formation at two localities on the Yorkshire coast: White Nab (TA059865) and Long Nab–Hundale Point (treated as
one location, TA027943 to TA027947, standard Ordnance Survey grid reference). Figure 2 includes only data from
Long Nab–Hundale Point. WNIM p White Nab Ironstone Member; BGM p Blow Gill Member. “Prism” nomenclature
for Moor Grit is from Eschard et al. (1991).
evidence of saltwater influence, even in the channel sandstones (Hancock and Fisher 1981).
Paleoenvironmental information is also contained in the pattern of coal versus charcoal abundance through the Ravenscar Group. The data plotted in figure 2 clearly show that the majority of the
macroscopic wood fossils in the Cloughton Formation are preserved as coal, whereas overlying and
underlying units contain a mixture of coal and
charcoal. Coal is easily identified in hand specimen
as a dense, shiny, black structureless material; in
contrast, charcoal is much less dense, preserves
original wood structure, and has a lustrous appearance (see e.g., illustrations in Morgans 1999 and
Gröcke 2002). Were the charcoal-bearing strata deposited in environments that were particularly fire
prone or, alternatively, are there overriding taphonomic factors at play?
Many previous studies have attempted to assign leaf floras to upland or lowland areas of
growth on the basis of their taxonomic composition (e.g., Hill 1974; Van Konijnenberg-Van Cittert and Van der Burgh 1996). Similarly, attempts
have been made to assign fossil wood remains to
particular environments of growth. In the case of
the Ravenscar Group, Cope (1993) interpreted the
(charcoalified) conifer-ginkgophyte-cycadophyte
assemblages within the Scalby Formation as representing a fire-controlled climax community of
possible upland origin. Typically, charcoal suggests a seasonally dry climate where thunderstorms and lightning at the end of the dry season
triggered wildfires (Harris 1958), whereas coal is
indicative of continuous precipitation through
the annual cycle and the presence of suitable environments for accumulation and preservation of
plant remains (e.g., Lottes and Ziegler 1994). Noting that the Gristhorpe Plant Bed contained relatively little charcoal, Scott (2000) has suggested
that differences in charcoal abundance between
the Cloughton Formation (which contains the
Gristhorpe Plant Bed) and the overlying Scalby
Formation could have been due either to overall
increasing seasonal aridity, as supported by treering work of Morgans et al. (1999), or to a change
in dominance of plant fossils from lowland to
upland varieties. Unfortunately, coalification
prevents recognition of any tree rings in the specimens from the critical post–negative-excursion
section in the Sycarham Member of prime interest in this study.
Although we cannot conclusively rule out the
Figure 6. Carbon-isotope data from Middle Jurassic successions arranged in S-N profile (see also figs. 1, 2). The main negative carbon-isotope excursion at
about the Aalenian-Bajocian boundary is shown by a thick gray line. Also evident from this figure is a middle Bajocian positive isotopic excursion and a
possible negative excursion at the Toarcian-Aalenian boundary.
Figure 7. Detailed graphic log of the Ravenscar Group along a 4-km stretch of coast between Hayburn Wyke and
Burniston Wyke, NE Yorkshire, England. From 21 to 33.5 m at TA012969; from 37 to 61 m at TA016968 to TA018963;
from 67 to 86 m at TA020959 to TA021955 (this part of the succession shows great lateral variability); from 86 to
106.3 m at TA021955 to TA020951; from 106.3 to 133.9 m at TA020951 to TA026949; from 133.9 to 144.5 m at
TA026949 to TA027947; from 144.5 to 160.4 m at TA028943 to TA028932. (Locations use the standard Ordnance
Survey grid reference.) The major plant beds of Black (1929), Harris (1961, 1964, 1969, 1979), and Harris et al. (1974)
are indicated. Vertical boxes between 54.5 and 55.5 m, 56.5 and 58.5 m, and at about 71 m indicate where samples
were taken in cases of significant lateral facies variability. WNIM p White Nab Ironstone Member; BGM p Blow
Gill Member.
269
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S . P. H E S S E L B O E T A L .
possibility that the patterns we observe through the
whole Ravenscar Group are caused by changes in
the source regions of the preserved plants, this appears very unlikely because there is no discernible
broad correlation between preservational style and
lithofacies. For example, marine-influenced coastal
mudstones are common at the top of the Saltwick
Formation, within the Sycarham Member, within
the Cloughton Formation, and even within the
Scalby Formation, and yet these lithological units
are characterized by very different patterns of charcoal and coal occurrence. Thus, we prefer to interpret these patterns in terms of fire susceptibility of
the flora and floral debris, linked to strongly seasonally arid versus weakly seasonally arid climates.
Floral Changes in the Cloughton Formation
Ranges of plant taxa through the Ravenscar Group
have been discussed by Harris (1952) on the basis
of two decades of careful collecting. He noted that
the Cloughton Formation (comprising the Sycarham through Gristhorpe Members)—the substantial portion of which postdates the negative isotope
excursion—is characterized by fossil-plant assemblages that contrast with those occurring above and
below. Specifically, Harris (1952) observed that
some ginkgophytes and cycadophytes were absent
from the Cloughton Formation, whose beds instead
contain an abundance of cuticle fragments from a
then undescribed conifer, possibly Pagiophyllum
kurrii (Harris 1979, p. 28–34) or Pagiophyllum maculosum (Harris 1979, p. 41–45). Our own observations from samples at the level of the negative
excursion confirm that conifers and ferns dominate
the assemblages. However, as recognized by Harris,
the paleoenvironmental significance of these differences is not obvious. In all parts of the Ravenscar
Group, there are plants with “tough, xeromorphic
leaves, intermediate plants and delicate-looking
ones along with large numbers of swamp equisetales” (Harris 1952, p. 211), and on this basis, Harris
suggested that any accompanying climatic fluctuations were only slight or nonexistent. Indeed, differences between the units may be due at least in
part to taphonomic processes.
In this study, we have reconsidered the floral distributions throughout the units comprising the
Ravenscar Group. Figure 8 shows a breakdown of
species by genera, families, orders, or morphological categories (cf. Rees et al. 2000, 2002), expressed
as percentages of the total species (N) in each stratigraphic unit (data derived from Harris 1961, 1964,
1969, 1979; Harris et al. 1974). Different higher tax-
onomic levels used were chosen to reveal any inherent climate signals. So, sphenophytes and pteridosperms were broken down to the genus level,
ferns are shown by family, and ginkgophytes by
order. For cycadophytes and conifers, only the microphyllous morphological category is shown.
Taphonomic processes play a major role in the
different patterns observed between the marine and
nonmarine units. For example, the marine beds are
characterized by an absence of ferns combined with
relatively large numbers of microphyllous cycadophytes and conifers, a pattern that can easily be
explained in terms of comparative robustness.
However, there are also significant differences between the nonmarine units that are difficult to explain in terms of taphonomy alone. On the basis
of the floral diversity and compositional patterns
shown in figure 8, the lower and upper Saltwick
and Gristhorpe units comprise one group, and the
Sycarham and lower and upper Scalby units a second group. Compared with units in the second
group, those in the first have higher total diversity
as reflected in the numbers of plant species (fig. 8).
They also have relatively high fern family diversity
and percentage abundances, as well as higher
sphenophyte genus diversity. The genus Sagenopteris, which unusually for Ravenscar Group pteridosperms shows nonxeromorphic characteristics,
is present in the first group but not the second. By
contrast, units in the second group have relatively
high percentage abundances of microphyllous
thick-cuticled conifers. Overall, at first sight, the
first group (characterizing the Saltwick and Gristhorpe units) is suggestive of higher-diversity, “wetter” environments, whereas the second group (characterizing the Sycarham and Scalby units) suggests
lower-diversity, “drier” environments.
In view of the clear dominance of coal over charcoal in the section that immediately postdates the
negative excursion—principally the Sycarham
Member—there is an apparent contradiction with
considerations of the subtle floral evidence. One
possibility is that floras lumped into the Sycarham
Member actually show distinct stratigraphic distributions above and below the excursion that are
not resolved in Harris’s floral lists. Another possibility is that apparently xeromorphic characteristics actually relate to other environmental factors
such as raised atmospheric CO2 (cf. Retallack 2002).
This issue can be further investigated only on the
basis of bed-by-bed collection of new leaf fossils
through the excursion interval.
Journal of Geology
C A R B O N - C Y C L E P E R T U R B AT I O N
271
Figure 8. Floral distribution through the lithological units that comprise the Ravenscar Group. Data derived from
Harris (1961, 1964, 1969, 1979) and Harris et al. (1974); see these works for a complete floral list. This figure shows
only part of the floral dataset (i.e., those groups most likely to reveal a climatic signal). The selected floral groups
(genera, families, orders, or morphological categories; cf. Rees et al. 2000, 2002) are expressed as percentages of the
total number of species (N) present in each unit. Floras from locations at the base of the Saltwick Formation (e.g.,
Hasty Bank, Roseberry Topping) are assigned to the “lower” Saltwick Formation; “lower” and “upper” Scalby Formation equate to Moor Grit and Long Nab Members, respectively. The floral diversity allows the succession to be
divided into two subtle groups: one comprising the Saltwick Formation and Gristhorpe Member and another comprising the Sycarham Member and Scalby Formation, possibly representative of environmental fluctuations.
Stomatal Densities and CO2
Global shifts in the carbon-isotopic composition of
the atmosphere and ocean should have been accompanied by substantial fluctuations in the CO2
content of the atmosphere. In all suggested mechanisms for generating global carbon-isotope excursions, lighter carbon-isotope values coincide with
increased atmospheric pCO2, and positive excursions coincide with reduced pCO2 (e.g., Hesselbo et
al. 2002). In fossil-plant-bearing successions, the
CO2 content of the atmosphere may be gauged by
analysis of leaf stomatal density and, especially, the
density values normalized to epidermal cell density, the stomatal index (Beerling 1999; McElwain
2001). The effectiveness of this technique has been
extensively demonstrated for fossil, subfossil, and
modern plant material (McElwain 1998; McElwain
et al. 1999 and references therein), which show that
there is a strong inverse relationship between leaf
stomatal density (or index) and atmospheric CO2
272
Table 1.
S . P. H E S S E L B O E T A L .
Stomatal Density Data from Selected Beds within the Ravenscar Group
Stratigraphic level
Solenites Bed (99.08 m)
Solenites Bed (98.98 m)
Solenites Bed (98.88 m)
Excursion Horizon (58.15 m)
Hayburn Wyke Plant Bed (27.5 m)
Average
stomatal
density
SD
SE
N
104.8
103.8
111.7
36.9
28.1
32.6
30.5
20.8
3.8
6.5
4.0
3.5
54
25
59
35
61.9
16.4
2.3
51
Source of
stomatal data
Solenites
Solenites
Solenites
Brachyphyllum, Geinitzia,
unknown cuticle fragments
Solenites
Note. See figures 2 and 7 for stratigraphic locations. Average stomatal density is measured in stomata/mm2. N refers to the total
number of stomatal count observations made.
concentration. This relationship has been used to
reconstruct high-resolution CO2 curves for the Holocene (e.g., Indermühle et al. 1999; Rundgren and
Beerling 1999) and low-resolution curves for longer
intervals of time (Retallack 2001, 2002).
To test whether substantial fluctuations of CO2
content occurred alongside the negative carbonisotope excursions, samples containing leaf cuticles of the conifers Brachyphyllum and Geinitzia
and the czekanowskialean Solenites were collected
from key horizons within the Ravenscar Group
(figs. 2, 7). Accurate stomatal density measurements were made on the basis of counts of stomatal
pores in each sample (number of stomata per mm2
area of leaf surface). These new stomatal density
data can be compared with data previously generated from ginkgophytes (Ginkgo and Baiera) and
conifers (Brachyphyllum and Pagiophyllum) from
Harris’s collections (McElwain and Chaloner 1996;
McElwain 1997; tables 1, 2). Although McElwain
and Chaloner (1996) were also able to determine
stomatal indices from the museum specimens,
samples collected for this study were too poorly
preserved to allow clear identification of epidermal
cell outlines, required in the calculation of stomatal index. Thus, herein we discuss only stomatal
density.
Data from material in Harris’s collections, which
are not precisely located stratigraphically, show relatively little change in stomatal density through
the constituent formations, although there is a suggestion of low stomatal densities in the Cloughton
Formation, relative to both the Saltwick and Scalby
Formations (fig. 2; table 2). In contrast, there is considerable variation in stomatal density exhibited by
the new samples collected below, coincident with,
and above the negative carbon-isotope excursion.
Reduced stomatal densities in the Cloughton Formation suggest relatively high atmospheric pCO2.
However, very high stomatal densities from the Solenites Bed (ca. 100 m height in fig. 2) strongly con-
trast with the data from the whole Gristhorpe
Member. Not only are these stomatal density values high in comparison to normal background levels for the Middle Jurassic (fig. 2), but they are also
substantially higher than values from the same species of Solenites recorded from the Hayburn Wyke
Plant Bed (fig. 2). What these new data clearly question is any inference of constant pCO2 throughout
the Middle Jurassic, which might otherwise be concluded from previous work on cuticles from the
Ravenscar Group (McElwain and Chaloner 1996).
Discussion
The coincidence of major perturbations in the carbon cycle and pulses of basaltic magmatism have
become increasingly clear as better age constraints
are achieved for large igneous provinces (e.g., Hesselbo et al. 2000, 2002; Pálfy and Smith 2000). However, the association of the excursion reported in
this article and any large igneous province cannot
be demonstrated. Even though radiometric data
from some of the Karoo-Ferrar basalts appeared previously to show a Late Aalenian or Early Bajocian
age, more recent recalibration and new Ar-Ar data
have demonstrated that the youngest Karoo-Ferrar
flows are around 180–179 m.yr. old and belong to
the Toarcian (Jones et al. 2001).
Intriguingly, the Aalenian–Early Bajocian interval does appear to coincide with the birth of the
Pacific Plate and also a major pulse of subductionrelated magmatism (Bartolini and Larson 2001) and
continental deformation around the margins of
Pangea (e.g., as expressed in the Jurassic stratigraphy of the western United States; Bjerrum and
Dorsey 1995). Moreover, consideration of biochronologic and radiometric ages from AlpineMediterranean sections indicate ocean spreading of
the Alpine Tethys in Early Bajocian times accom-
Journal of Geology
Table 2.
C A R B O N - C Y C L E P E R T U R B AT I O N
Stomatal Density Data (stomata/mm2) from Selected Formations and Members within the Ravenscar Group
Brachyphyllum crucis:
Stomata/mm2
SE
Brachyphyllum mamillare:
Stomata/mm2
SE
Pagiophyllum kurrii:
Stomata/mm2
SE
Pagiophyllum maculosum:
Stomata/mm2
SE
Pagiophyllum ordinatum:
Stomata/mm2
SE
Baiera furcata:
Stomata/mm2
SE
Ginkgo huttoni:
Stomata/mm2
SE
Source.
273
Scalby
Formation
Gristhorpe
Member
Sycarham
Member
Saltwick
Formation
57.2
2.9
33.5
4.3
41.6
5.9
44.7
1.1
45.9
8.2
27.5
2.9
30.8
2.8
37.2
2.3
…
…
43.8
2.4
41.7
1.9
42.6
1.1
…
…
47.8
6.4
44.9
2.6
53.7
2.6
50.7
1.5
48.5
1.5
55.4
2.3
54.3
4.2
41.0
3.0
…
…
…
…
55.4
5.2
57.0
2.3
66.3
4.0
…
…
60.3
2.6
McElwain 1997, from Harris Collections.
panying deposition of radiolarian-rich strata in oceanic and ocean-margin settings (Bartolini and
Cecca 1999; Bill et al. 2001). Thus, the origins of
the carbon-isotope anomalies may lie in the profound changes to carbon fluxes stemming from
these magmato-tectonic events and their sedimentary consequences. In view of the gradual isotopic
changes inferred from tethyan carbonates, an explanation in terms of the catastrophic dissociation
of gas hydrates is untenable (cf. Dickens et al. 1995;
Hesselbo et al. 2000).
The negative excursion in the earliest Bajocian
section of the Ravenscar Group corresponds to a
significant and sustained change in local paleoenvironmental conditions from fire prone to fire resistant; this situation lasted on the order of a few
million years. This change is in apparent opposition
to subtle variations in the floral assemblages if typically xeromorphic characteristics are related solely
to water stress.
ACKNOWLEDGMENTS
Conclusions
A major carbon-cycle perturbation in the Early Bajocian (Middle Jurassic) is recognized as a global
phenomenon on the basis of its carbon-isotope expression in both terrestrial organic matter and marine carbonate. Within the Yorkshire Middle Jurassic, carbon-isotope data generated from fossil wood
samples track global atmospheric carbon-isotope
shifts, whereas variations in a bulk organic matter
isotope curve through part of the section are
strongly affected by the nature of the constituent
organic matter (marine dominated palynofacies coincide with lighter carbon-isotope values).
We acknowledge support from a combined Natural
Environment Research Council (NERC research
grants GST/02/1346 and GST/06/1346) industrial
consortium (Rapid Global Geological Events project) and from NERC studentships to S. A. Robinson (GT4/98/224/ES) and C. E. Ross (GT4/94/230/
G). Organic carbon-isotope analyses were carried
out at the Radiocarbon Accelerator Unit at the University of Oxford; we give thanks to M. Humm and
T. O’Connell. We also thank O. Green and S. Wyatt
for expert assistance in the laboratory; R. Spicer,
H. Jenkyns, and R. Woodfine for helpful scientific
discussion; and referees A. H. Jahren and A. N.
Other for their insightful comments.
274
S . P. H E S S E L B O E T A L .
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