CARBON ISOTOPIC SIGNATURE IN CARBONATE ROCKS 217
GEOLOGICA CARPATHICA, 54, 4, BRATISLAVA, AUGUST 2003
UPPER CARBONIFEROUS TO LOWER TRIASSIC
CARBON ISOTOPIC SIGNATURE IN CARBONATE ROCKS
OF THE WESTERN TETHYS (SLOVENIA)
, BOJAN OGORELEC
and SONJA LOJEN
Department of Geology, Faculty of Natural Sciences and Engineering, University of Ljubljana, Akerèeva 12, 1000 Ljubljana,
Geological Survey of Slovenia, Dimièeva 14, 1000 Ljubljana, Slovenia
Department of Environmental Sciences, Joef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
(Manuscript received April 11, 2002; accepted in revised form March 11, 2003)
Abstract: The carbon isotope composition of carbonate rocks spanning an approximately 1650 m thick interval ranging
from Upper Carboniferous to Lower Triassic, together with pedogenic carbonate carbon and organic carbon isotope data
of Middle Permian plant remains, were used to evaluate the carbon isotope evolution of the Late Paleozoic in the West-
ern Tethys. The results indicate a positive carbon isotope event across the Carboniferous-Permian (C/P) boundary, a
negative shift of
C values at the end of the Lower Permian, a positive
C excursion at the Middle Permian-Upper
Permian transition and the well known worldwide perturbations of the carbon cycle at the end of the Permian marked by
a prominent decrease of about 4 at the Permian-Triassic (P/Tr) boundary, followed by a gradual increase of
values from the Lower Scythian to the Anisian. It is suggested that the shape of the
C curve records changes in the
carbon cycle that reflecting variations in factors such as burial rate and oxidation of organic carbon due to sea-level
oscillations, which could be slightly overprinted by local processes influencing carbon isotopic composition in particular
sedimentary environments. However, the marked disturbances in the carbon cycle across the P/Tr boundary most prob-
ably resulted from a combination of factors such as an accelerated oxidation of organic carbon due to the terminal phase
of the Upper Permian marine regression, in combination with volcanic activity and a possible expulsion of methane from
stored hydrates, as well as from a crash in primary bioproductivity.
Key words: Western Tethys, Slovenia, Upper CarboniferousLower Triassic, Permian-Triassic boundary,
carbon stable isotopes, carbonates.
In the short-term, a high-resolution carbon isotope record at a
frequency lower than the mixing time of the oceans (ca. 1000
yr.) is crucial when evaluating changes of marine productivity,
while the interpretation at frequencies greater than the mixing
time will reflect the transient partition between different water
masses. The latter is the so-called internal fractionation
within the marine reservoir of dissolved inorganic carbon
(Corfield 1994). On timescales longer than this, any change in
C of carbonate carbon reflects the fractionation of carbon
isotopes between different carbon reservoirs (Berger & Vin-
cent 1986; Shackleton 1987). This may be thought of as ex-
ternal fractionation. Both internal and external fractionation
of carbon isotopes may contribute to excursions in the
C enrichment in marine carbonates correlating with the in-
crease of pCO
in the atmosphere has been interpreted as a
consequence of an increase in organic carbon storage either on
land (Shackleton 1987) or on newly flooded continental
shelves (Magaritz & Stemmerik 1989; Faure et al. 1995). In
contrast, slower burial or rapid oxidation of organic carbon
will result in lower
C values of marine carbonates due to re-
C depleted CO
to the atmosphere (Magaritz &
Stemmerik 1989; Faure et al. 1995).
In this study the carbon isotope stratigraphy of selected
stratigraphic sections from Western Slovenia the Karavanke
Mountains (Dolanova soteska, Koutnik Creek and Brsnina),
Julian Alps (Straa Hill), Sava Folds (irovski vrh, Sv. Valen-
tin) and Idrija region was used to examine their depositional
environment and to provide further insights into the carbon iso-
topic evolution of the western part of the Tethys Ocean from
the latest Carboniferous to the earliest Triassic (Fig. 1). The
stratigraphic sections spanning an approximately 1650 m thick
stratigraphic interval, ranged from Upper Carboniferous to
Lower Triassic (Fig. 2). In addition, we also discuss the nature
and the causes of the well known perturbations in the carbon
cycle at the P/Tr boundary, which are more or less coincidental
with the most severe extinction of marine and terrestrial organ-
isms in the history of life (Erwin 1993; Wignall & Hallam
1996; Knoll et al. 1996; Musashi et al. 2001; Wignall &
Twitchett 2002; Berner 2002 and references therein).
Some previously published works by Dolenec T. (1973),
Dolenec T. & Ramov (1996); Dolenec T. et al. (1981, 1998,
1999a,b,c; 2001) and Dolenec T. & Lojen (2000) on stable
organic and inorganic carbon of the Permian and Triassic, to-
gether with preliminary studies on the carbon isotopic com-
position of plant remains from the Val Gardena Formation
(Dolenec T. & Pezdiè 1986) serve as the background against
which the present geochemical study was carried out.
218 DOLENEC, OGORELEC and LOJEN
Geological setting and stratigraphy
On the basis of a major tectonic event within the Permian
(Saalian orogenetic phase), the late to post-Variscan sedimen-
tary sequence of the Southern and Eastern Alps is divided into
two evolutionary cycles (see Krainer 1993, and references
therein). The sediments of the lower cycle (Late Carbonifer-
ous/Early Permian) were deposited in discrete, isolated inter-
montane basins, which were filled with different sediments
and volcanic rocks. In the Southern Karavanke Mountains and
in the Julian Alps the sequence of the first cycle is represented
by deltaic, shallow marine to deep-marine clastic and carbon-
ate sediments of the Late Carboniferous Auering Group (Ra-
mov 1976) and the Dolanova Soteska Limestone Member
(Buser & Forke 1995) ranging from Late Moscovian to Late
Artinskian. The thickness of these sediments is up to 800 m
(Fig. 2, section 3). The Dolanova Soteska Limestone Mem-
ber is introduced as a white, pale red to red limestone unit,
named the Trogkofel Limestone in previous literature (Ramov
1976). Conodont fauna together with fusulinids indicate an
older age (Asselian) for these limestones than was previously
thought (Buser & Forke 1995).
With a hiatus caused by block faulting (Saalian orogenetic
phase), the Tarvis Breccia and Middle to Upper Permian sedi-
ments of the second cycle, which are more widely distributed
and not restricted to discrete basins, conformably overlie Low-
er Permian sediments. The thickness of the Tarvis Breccia,
which has been regarded as scarp-foot fan deposits and proxi-
mal debris-flows (Krainer 1993), varies in the Southern Kara-
vanke Mountains from a few metres up to 150 metres (Buser &
Freshwater calcite cements of the Tarvis Breccia indicate a
subaerial exposure corresponding to a regressive event at the
end of the first cycle, after the deposition of the Trogkofel For-
Fig. 1. A Map showing the location of the studied sections in Slovenia. B Global paleogeography during the P/Tr is taken from Sun et
al. (1989) . C The location of the investigated area is shown by a black point (1 Brsnina, 2 Koutnik Creek, 3 Dolanova soteska, 4 Straa
Hill, 5 Sv. Valentin, 6 irovski vrh, 7 Idrijca Valley, 8 Masore). The present extension of the ancient Julian and Dinaric Carbonate Plat-
forms, together with the intermediate Slovenian Basin, is taken from Buser & Debeljak (1996).
CARBON ISOTOPIC SIGNATURE IN CARBONATE ROCKS 219
Fig. 2. Upper Carboniferous to Lower Triassic carbonate carbon isotopic record from the Southern Karavanke Mountains, Julian Alps,
Sava Folds and Western Slovenia: 1, 2 Upper Permian and Lower Triassic succession of the Southern Karavanke Mountains ¯ (Upper
Permian Karavanke Formation, Lower Scythian and Anisian beds Brsnina and Koutnik Creek section); 3 Upper Carboniferous,
Lower and Middle Permian sedimentary sequence (Dolanova soteska section: Auering Group, Dolanova Soteska Limestones, Tarvis
Breccia Unit: H Dolanova Soteska Limestone fragments, F limestone cement); 4 Middle Permian Neoschwagerina Limestone
(Straa Hill Julian Alps); 5 Middle Permian pedogenic carbonates Val Gardena Formation (Sv. Valentin Sava Folds); 6
Middle Permian pedogenic carbonates and playa lake dolomites ¢ (irovski vrh Sava Folds); 7 Upper Permian aar Formation
and Lower Scythian beds (Idrijca Valley section Western Slovenia); 8 Upper Permian aar Formation and Lower Scythian beds
(Masore section Western Slovenia). Description of rock types see in the text.
220 DOLENEC, OGORELEC and LOJEN
mation (Buggisch & Flügel 1980). In the Karavanke Moun-
tains the Tarvis Breccia is overlain by the up to 10 m thick
basal conglomerates of the Val Gardena Formation, which is
followed by prevalent sandstones, accompanied by conglom-
erates, siltstones and claystones. The Val Gardena Formation
attains the greatest thickness (up to 600 m) in the irovski vrh
area (Fig. 2, sections 5 and 6) which belongs to the western
part of the Sava Folds (Omaljev 1967). The sediments are of
fluvial, playa lake and shallow-marine origin (Buggisch 1978;
Ori 1986) and were deposited in an arid and semiarid climate.
Interbedded siltstones and claystones often contain pedogenic
carbonate nodules and rare thin dolomite, as well as gypsum
layers (Skaberne 1995). The gypsum layers are interpreted as
being formed in playa lakes (Drovenik 1970). In the Julian
Alps, near Lake Bled (Straa Hill), a shallow-water
Neoschwagerina reef limestone (Fig. 2, section 4) time-
equivalent to the Val Gardena Formation was deposited
(Flügel et al. 1984).
In the Southern Karavanke Mountains the Middle Permian
Val Gardena Formation is overlain by an up to 270 m thick
Upper Permian evaporitic-dolomitic sequence (Fig. 2, sec-
tions 1 and 2) referred to as the Karavanke Formation (Buser
1974). The boundary between the two formations is transi-
tional and characterized by thin sandy red dolomite layers al-
ternating with the topmost Val Gardena shales and sandstones
(Fig. 3, section 1), indicating a widespread and slowly pro-
gressive transgression of the Bellerophon Sea on the territory
of the present Dinarides (Krainer 1993 and references there-
in). At this time the formation of the extensive Slovenian Car-
bonate Platform began in the area of the Southern Alps and
the Dinarides (Buser 1996). The thickness of the transitional
unit, which grades upward into the Karavanke Formation, is
about 5 m (Dolenec T. et al. 1981). The basal unit of the Kara-
vanke Formation is represented by an up to 70 m thick
evaporitic sequence composed of cellular dolomite (rau-
hwacke), which alternates with rare black and slightly bitumi-
nous marls, and grey vuggy dolomites. Sulphate minerals are
no longer present; they were entirely replaced by calcite. The
evaporitic sequence is overlain by a 200 m thick succession of
fossiliferous biomicritic dolomites (Buser 1974; Dolenec T. et
al. 1999c). The Late Permian age of these beds is indicated by
calcareous algal assemblages, as well as by foraminifers (Ra-
mov 1986). The lithostratigraphic boundary between the Up-
per Permian Karavanke Formation and the Lower Triassic
(Scythian) beds is placed at the end of the sedimentation of
the well-bedded grey dolomicrite (Dolenec T. et al. 1999c). It
is followed by a red-coloured partly terrigeneous evaporitic
sequence, predominantly composed of thin-bedded siltstones,
mudstones and sandstones alternating with micritic dolomites,
showing the impressions of gypsum crystals (Fig. 3, section
2). These dolomites contain no characteristic fossils and so
could be any age within the P/Tr interval. The earliest Triassic
beds represent a predominantly terrigenous sequence deposit-
ed in an extremely shallow sea, which gradually became a
wide, extensive mud flat (Assereto et al. 1973), most probably
indicating a short term regression or sea level fluctuation at
the P/Tr boundary. Its thickness varies between 5 m in the Ko-
utnik Creek section and 25 m in the Brsnina section. In the
investigated area of the Southern Karavanke Mountains these
beds are mostly overlain by an around 200 m thickness of
Lower Triassic dark-grey and brown micritic and sparitic
limestones and dolomites intercalated with oolitic limestone,
marls and shales. Ooids were formed in intertidal channels
and deltas (Dolenec T. et al. 1981). Episodically intercalated
supratidal sediments and a clastic influx in the Lower Triassic
sedimentary succession most probably indicate eustatic sea-
level changes and tectonics (Assereto et al. 1973; Broglio-
Loriga et al. 1983; Brandner et al. 1984). Anisian dolomite
conformably overlies the Scythian beds (Fig. 2, sections 1 and
2; Fig. 3, section 1). Similar transgressiveregressive events
have also been recognized in the Scythian sequence of the
Upper Austro-Alpine units (Krainer 1993 and references
therein). The thickness of the dolomite, which contains algal
remains and foraminifers and which was only partly included
in the present study, is over 200 m (Dolenec T. et al. 1981).
In Western and Central Slovenia the Val Gardena Forma-
tion is overlain by an approximately 250 m thick dark-grey to
black bedded and fossiliferous shallow marine limestone
(Fig. 2, sections 7 and 8; Fig. 4), the aar Formation (Ram-
ov 1958). The lower part of this limestone contains a rich
brachiopod fauna, small bioherms and coral-patch reefs (Ra-
mov 1986). The faunal composition displays gradual impov-
erishment of the Upper Permian taxa moving upward towards
the P/Tr boundary and an abrupt disappearance at the bound-
ary level (Dolenec T. et al. 2001). Although indications of
shallowing are present over a broad region, sequence strati-
graphic analysis of the Idrijca Valley section revealed no evi-
dence of emergence or pronounced sea-level changes across
the boundary (Dolenec T. & Lojen 2000). The boundary is
represented by a thin <0.8 cm clayey marl layer (Permian-Tri-
assic boundary PTB layer) overlying black Upper Permian
algal packstones. The PTB layer shows a characteristic mag-
netic susceptibility pulse (Hansen et al. 1999, 2000) and con-
siderable enrichment in most minor and trace elements (Dole-
nec T. et al. 2001). It also contains spherules, which most
probably represent prasinophyte algal skeletons, diagenetical-
ly infilled by magnetite (Hansen et al. 1999, 2000). A detailed
study of the P/Tr boundary revealed that the PTB layer lies
within an approximately 15 cm thick unit of oolitic grain-
stone, bioclastic grainstone and dolomicrite, which represents
an equivalent to the well known Tesero Oolite Horizon (Dole-
nec M. 2000; Dolenec M. & Ogorelec 2001). Faunal assem-
blages (foraminifers, calcareous algae) at the base of this unit
are consistent with an Upper Permian age for the lower part of
the Tesero Oolite Horizon. In contrast, the dolomicrite with
rare ooids, which immediately overlies the PTB layer, con-
tains conical tube-like fossils, most probably of Earlandia sp.,
and is supposed to be of Early Scythian age (Dolenec M.
2000). The deposition of the P/Tr boundary layer most proba-
bly occurred during a period of maximum eustatic sea-level
fall and regression, correlated with the sedimentation of red
terrigeneous sediments across the P/Tr boundary in the Kara-
vanke Mountains (Dolenec T. et al. 1998). A laminated dolo-
micritic limestone alternating with grey stylolitic dolomite
overlies the Tesero Oolite Horizon. In the Masore section the
stylolitic dolomite is covered by greyish-green and reddish
CARBON ISOTOPIC SIGNATURE IN CARBONATE ROCKS 221
calcareous micaceous shale and sandstone, including lenses of
oolitic limestone (Fig. 4, section 8). The thickness of the Low-
er Scythian dolomite is about 100 m.
Material and methods
A total of 380 carbonate samples and 10 samples of plant
remains were collected in eight sections numbered 18 for
isotopic analysis (Fig. 1 and 2). The carbonate carbon isotopic
measurements were carried out on undolomitized limestone
and uncalcitized dolomite samples, cellular dolomite, pe-
dogenic carbonates, as well as on separated components of
Tarvis Breccia calcite cement. The mineralogy of the carbon-
ate phases was determined by X-ray diffractometry and by ex-
amination of thin sections by standard optical methods, in-
cluding staining with Alizarin-reds. All samples were also
evaluated by petrographic methods to assess their diagenetic
history. Thin-section examination showed that the limestone
and dolomite samples analysed are without crack-filling
sparites and un-weathered, without evidence of meteoric-wa-
ter diagenesis. By selecting the least visibly weathered and re-
crystallized samples from the investigated sections, we at-
tempted to minimize possible post-depositional effects. A
weak positive correlation (r = 0.40) between
the dolomite samples and a weak negative correlation (r = 0.22)
C of the limestone samples of the Kara-
vanke Formation (Dolenec T. et al. 1999c), as well as a lack
of correlation between
C values of the carbonate
samples of the aar Formation (unpublished), most probably
suggests that the isotopic composition of the investigated car-
bonate rocks has not been seriously altered after their forma-
tion and that the primary paleoceanographic signal is not ap-
Therefore, we suppose that the carbon isotope data can re-
flect the carbon isotopic composition of the original marine
composition. Samples were obtained as a split of powder pre-
pared from rock chips remaining after thin section prepara-
tion. In order to speed up the reaction time and to ensure com-
plete reaction of carbonates, powdered rock samples for
analysis were prepared by overnight digestion in excess
100 phosphoric acid at 50 °C. CO
gas released during
acid treatment was cryogenically cleaned and analysed for
carbon isotopic composition on a Varian MAT 250 mass
spectrometer. All whole rock and calcite cement samples were
analysed in duplicate or triplicate. The results are reported in
the conventional delta notation as deviation from the
VPDB (Vienna PeeDee Bellemnitela americana) standard. The
C values were normalized assuming
C values of +2.48
for IAEA-CO-1 standard on the VPDB scale. The analytical
precision, based on multiple analysis of an internal laboratory
C ±0.01 (1
). Overall analytical reproduc-
ibility of the carbonate carbon isotopic data was ±0.1 .
Middle Permian plant remains from dark-grey and grey Val
Gardena sandstones were selected from previously crushed
samples by hand under a binocular microscope. The organic
carbon plant remains were then powdered in an agate mortar
and treated with 3 M hydrochloric acid at 50 °C to react the
carbonates. Upon cessation of CO
evolution, excess acid was
removed by repeated washing (three to four times) with dou-
bly distilled water to neutral pH. After the final decantation of
water, the carbonate-free residue, mostly composed of plant
remains, was oven-dried at 50 °C. Organic carbon isotope ra-
tios were measured in a Europa 20-20 Stable Isotope Analyser
(Europa Scientific Ltd.) with an ANCA-NT preparation mod-
ule for on-line combustion of bulk solid samples and chro-
matographic separation of gases. Organic carbon isotope val-
ues were calibrated using the IAEA-CH-7 standard with a
C value of 31.8 on the VPDB scale. The results are re-
ported in the conventional delta notation as deviation from
the VPDB standard. The analytical precision for organic car-
bon based on multiple analysis of an internal laboratory stan-
C ±0.01 (1
). Overall analytical reproducibili-
ty of the organic carbon isotopic data was ±0.1 .
A schematic presentation of the carbon isotope stratigraphy
together with the lithostratigraphic development of sedimen-
tary rocks from the Upper Carboniferous to the Lower Trias-
sic, is shown in Fig. 2. In Figs. 3 and 4 detailed sections and
carbon isotope stratigraphy across the Middle Permian/Upper
Permian boundary in the Karavanke Mountains and across the
P/Tr boundary in the Karavanke Mountains and in Western
Slovenia are also presented.
The shape of the
C curve for the sedimentary sequences
of the lower (Late Carboniferous/Early Permian) and upper
cycle (MiddleLate Permian to Early Triassic) is character-
ized by a series of distinct carbon isotope changes (Fig. 2, sec-
tion 3). There is a gradual positive
C excursion from 2.9
to +3.9 at the transgressive C/P boundary in the Karavanke
Mountains, followed by a moderately variable
C signal in
the Lower Permian sedimentary sequence containing some
well developed clastic-carbonate cycles. The
C values of
the Lower Permian Dolanova Soteska Limestone Member
range from +1.2 to +5.8 , with most of the values falling be-
tween +3.2 and +5.2 . The most depleted
C values were
found in sandy limestone samples containing high amounts of
At the Lower/Middle Permian transition in the Karavanke
Mountains (Fig. 2, section 3) the negative excursion of
values is stratigraphically associated with the Tarvis Breccia
Horizon. The carbon isotopic composition of the limestone
cement of Tarvis Breccia from the Karavanke Mountains
ranges between 5.8 and 4.7 , while the
C values of
isotopically altered Dolanova Soteska Limestone fragments
cluster between 0.9 and +0.6 (Fig. 2, section 3).
The carbon isotopic composition of playa lake dolomite
from the Val Gardena Formation falls between 3.3 and
5.7 , while that of pedogenic carbonates clusters between
9.5 and 5.8 with an average value of 7.9 (Fig. 2,
sections 5 and 6). These values are up to 14.7 lighter than
time-equivalent Neoschwagerina limestone with
C in the
range from +4.8 to +5.2 (average +4.9 ) (Fig. 2, sec-
tion 4). The
C values of plant remains in the Val Gardena
Formation from irovski vrh were found to be between 22.1
and 21.7 (Dolenec T. & Pezdiè 1986). A slightly wider
222 DOLENEC, OGORELEC and LOJEN
C values (from 23.4 to 21.3 ; average
22.8 ) was measured in additional samples during this
study. Similar carbon isotopic composition values with
in the range between 23.8 and 21.2 were also reported
for Permian coals and particulate organic matter from selected
coalfields in South Africa (Faure et al. 1995).
The transition from Middle Permian to Upper Permian in
the Karavanke Mountains is characterized by a positive
excursion from 2.5 to +3.8 observed at the base of the
Karavanke Formation (Fig. 2, section 1 and Fig. 3, section 1).
The dolomites of the basal evaporitic sequence show variation
C values mostly in the range between +0.7 and +3.8 .
In the overlying shallow-shelf biomicritic dolomite the
values range between +2.0 and 3.0 . Slightly higher
values (+2.5 to +3.5 ) have been reported from the dolo-
mitized Upper Permian Bellerophon Formation of the Carnic
Alps (Magaritz & Holser 1991). In Western Slovenia the Up-
per Permian limestone of the aar Formation (Idrijca Valley
and Masore section) exhibits variations of
C values mostly
in the range between +3.9 and +5.5 (Fig. 2, section 7, 8
and Fig. 4). These values are up to 2.5 higher than those of
the Karavanke Formation.
The Permian to Triassic transition in the Karavanke Moun-
tains is characterized by a prominent negative
C shift of
Fig. 3. Variations in lithology and carbonate carbon isotopic composition from Middle Permian to Anisian in Koutnik Creek section (1) and
Brsnina Section (2). Detail from Fig. 2. a transitional unit composed of sandy red dolomite alternating with shales and sandstones; b an
evaporitic sequence composed of cellular dolomite intercalates with black bituminous shales and grey vuggy dolomites; c light-grey fos-
siliferous biomicritic dolomite; d a red partly terrigeneous sequence composed of siltstones, mudstones and sandstones alternating with
micritic dolomites showing impressions of gypsum crystals; e dark-grey and brown micritic and sparitic limestone intercalated with
oolitic limestone, marls and shales; f grey dolomicrite and dolosparite. Legend to lithology see Fig. 4.
CARBON ISOTOPIC SIGNATURE IN CARBONATE ROCKS 223
about 4 (Fig. 2 and 3). A major drop of
C values begins
approximately 15 m below the lithologically proposed bound-
ary. In the Carnic Alps the decrease of
C begins about
60 m below (Magaritz & Holser 1991), while in Western Slo-
venia (Idrijca Valley and Masore section) the same shift of
C values starts only 5 m below the P/Tr boundary (Dolenec
T. & Ramov 1996). In the Brsnina and Koutnik Creek sec-
tion (Fig. 3) of the Karavanke Mountains the
reaches a first minimum peak value of 1.9 about 8 m be-
low the boundary, a second minimum of 1.6 at the bound-
ary itself, followed by a positive excursion of +0.6 in the
basal Scythian, after which values are 1 to 2 lower than in
the Upper Permian. The remaining Scythian and Lower Ani-
sian carbonates are characterized by a general long-term grad-
ual increase in
C values by 2 (Fig. 2 and 3, section 1).
C values of nearly 300 samples of Mesozoic limestones
and dolomites from the same areas range mostly from 1.8 to
+3.7 with an average of about +2 (Ogorelec et al.
1999). These values are distinctly lower than the
of Permian carbonates.
The position of the negative
C peak anomaly may indi-
cate that the P/Tr boundary in the Karavanke Mountains does
not correspond to the lithologically defined boundary and
should be placed a little further down the section. This is be-
cause in the Carnic Alps a dramatic
C drop occurs right af-
ter the stratigraphic P/Tr boundary which is placed within the
lowermost 0.5 m of the 4 m thick Tesero Oolite Horizon
(Holser et al. 1991). This horizon was not found in the investi-
gated sections of the Karavanke Mountains. However, this in-
terpretation is preliminary and has to await further detailed
Stable isotope data from the Idrijca Valley and Masore sec-
tions (Fig. 4) show an accelerating decrease of
which begins about 5 m below the P/Tr boundary. This accel-
Fig. 4. Variation in lithology and carbonate carbon isotopic composition in the Upper Permian and Lower Scythian in the Masore section
(8) and Idrija section (7). Detail from Fig. 2.
224 DOLENEC, OGORELEC and LOJEN
erating decrease of
C values continues across the boundary
into the Lower Scythian beds. In terms of amplitude the de-
C values across the P/Tr boundary in both sec-
tions is about 4 (from +4.1 to 0.1 in the Idrijca Valley
section and from +4.0 to 0.5 in the Masore section) and is
similar to that found in the Karavanke Mountains. It is impor-
tant to note that there is no evidence of a shift of
back to their pre-excursion level. However, in the Masore sec-
tion, where Lower Scythian beds are better exposed than in
the Idrijca Valley section, two negative
C peak anomalies
of 4.3 and 3.7 (Fig. 4, section 8), similar to those report-
ed from the Carnic Alps (Magaritz & Holser 1991) were
found at 17 and 22 m above the boundary (Dolenec T. et al.
1999b). They are younger and unrelated to the P/Tr boundary
The gradual positive
C shift at the C/P boundary may re-
flect the transgressive trend of the Tethys Sea. The general hy-
pothesis suggested to explain positive
C shifts of carbonate
carbon is that the expansion of shallow shelf areas increased
the organic carbon burial rate and enriched the ocean water in
C (Magaritz & Stemmerik 1989).
C values of the Lower Permian Dolanova
Soteska Limestone Member are up to 3.8 higher relative to
the Permian limestone average values (+2 ) proposed by
Veizer et al. (1980). However, similarly high
C values be-
tween +2 and +5 were also observed in Lower Permian
limestones of the northern Yukon Teritory, while those of the
basal Lower Permian limestones in the Sverdrup Basin in
Canada range from +4 to +7 (Beauchamp et al. 1987) and
are markedly heavier than known time-equivalent limestones
elsewhere in the world (Keith & Weber 1964; Galimov et al.
1975; Veizer et al. 1980). Pronounced
C enrichments have
also been observed in the Lower Permian of Tasmania (Rao &
Green 1982). We suggest that the carbon isotopic composi-
tion of the Dolanova Soteska Limestone Member could re-
flect a Lower Permian primary
C marine water enrichment.
It can be explained as a result of increased organic carbon
storage during this time. Extensive deposition of coal in the
Upper Carboniferous and LowerMiddle Permian sediments
has been regarded as an important site of organic carbon accu-
mulation (Bluth & Kump 1991; Faure et al. 1995). The most
reasonable explanation for the marked variations observed in
the Lower Permian
C record is a change in the burial frac-
tion of organic carbon in marine sediments (Kump 1991),
export ratio changes into the marine sedi-
mentary carbon reservoir (Schidlowski 1987), as well as sea
level fluctuations (Magaritz & Stemmerik 1989). The sharp,
C depleted spikes in this succession were most
probably caused by pronounced local fluxes of isotopically
light organic-derived carbon in the depositional environment
during low sea level episodes due to oxidation of terrestrial
C excursion across the Lower/Middle Per-
mian transition observed in the Karavanke Mountains could
be explained by a regression of the Tethys Sea due to Saalian
movements. The observed
C values of Tarvis Breccia un-
doubtedly indicate a subaerial exposure on the top of the low-
er cycle sedimentary sequence, which was also documented
by Buggisch & Flügel (1980) for the Carnic Alps. During sub-
aerial exposure decay of organic matter in soil zones at expo-
sure surfaces generates larges volumes of
C enriched CO
with very low
C values of about 2.5 (Heydari et al.
2001). Leaching of such isotopically light CO
C composition of percolating ground water, fi-
nally resulting in
C depletion of precipitating carbonate ce-
ment, as well as in decreasing the
C of carbonate rock frag-
ments, due to the effects of pronounced meteoric diagenesis
(Pelechaty et al. 1996).
The carbon isotopic composition of pedogenic carbonates
C average value ~8 ) shows that they are enriched to
about 15 relative to the carbon isotopic composition of the
overlying flora (
C average value ~23 ). Terrestrial plants
in the Permian used only the C
photosynthetic pathway (Tho-
masson et al. 1986) and the
C values of resultant organic
matter fall around 27 (Quade et al. 1995). The mean
value of CO
plants is also about 27 (Quade et al.
1995). These data indicate that the carbon isotopic composi-
tion of Permian C
plants is up to 4 more positive than the
average value for C
plants. It has been demonstrated that in-
creasing temperature, aridity, irradiation, a decreasing canopy
effect and osmotic stress all increase the
C value of C
plants (Faure et al. 1995).
It is generally accepted that the carbon isotopic composition
of pedogenic carbonates is primarily determined by that of
which is a mixture of two components: atmospheric
and plant derived CO
(Cerling et al. 1991). In modern soils,
the effect of atmospheric CO
penetration and equilibrium
isotope fractionation during mineral precipitation leads to soil
C values 15 heavier than those in the
overlying flora (Cerling et al. 1989; Quade et al. 1989). Ac-
cording to the considerations outlined above, we suppose that
pedogenic carbonates from the Val Gardena Formation pre-
cipitated mostly in isotopic equilibrium with Middle Permian
C excursion at the transition from Middle
Permian to Upper Permian in the Karavanke Mountains most
probably resulted from increases in the burial rate of organic
carbon due to the marine transgression. This heavy carbon en-
richment started together with transgression of the Tethys Sea
on to the vast alluvial Middle Permian landscape and indi-
cates changes from a terrestrial to a marine evaporitic environ-
ment (Dolenec T. et al. 1998). Documentation of this trans-
gression exists not only to the south of the Tethys, but also to
the north in the Zechstein Basin (Assereto et al. 1973).
The carbonate carbon isotopic composition of dolomites of
the Karavanke Formation, which show up to 2.5 lower
C values relative to the limestone of the aar Formation,
could be attributed to biogenic carbon input from the sur-
rounding land and/or deceleration in the rate of organic car-
bon burial in the sedimentary environment of the Karavanke
Formation. Furthermore, according to some studies (Patterson
& Walter 1994), evaporation can also result in lowering of
C composition and the
C values of precipitat-
ing carbonates. The impressions of gypsum crystals in the
CARBON ISOTOPIC SIGNATURE IN CARBONATE ROCKS 225
basal evaporitic succession and in the partly terrigenous,
evaporitic P/Tr boundary sedimentary sequence suggest that
evaporation in combination with a biogenic carbon input may
be more a probable mechanism to explain the light carbon iso-
topic composition of the Karavanke Formation. On the con-
trary the limestone of the aar Formation acquired its carbon
isotopic composition predominantly through isotopic equilib-
rium with Upper Permian atmospheric CO
. It is interesting to
note that the carbon isotopic composition of the Upper Permi-
an limestone of the aar Formation below the P/Tr boundary
is similar to that of the Lower Permian Dolanova Soteska
Limestone Member and Middle Permian Neoschwagerina
C values are relatively high (mostly in the
range between +2.5 and +5.5 ) and extremely stable
(Fig. 2, sections 14, 7 and 8). We suppose that these values
could be related to the worldwide high storage of organic mat-
ter during the Late Paleozoic. This interpretation is supported
by deposition of vast amounts of coal and organic matter in
sedimentary rocks from the Upper Carboniferous to the end of
the Permian, when a global and abrupt break in coal formation
and/or preservation occurred (Faure et al. 1995 and references
In discussing the Permian-Triassic extinction events, sever-
al mechanisms for variations and dramatic perturbations in the
C values of marine carbonates have been suggested, such
as burial and erosion of organic carbon (Magaritz et al. 1992;
Faure et al. 1995), variations of sea-level changes and salinity
(Magaritz & Stemmerik 1989; Hallam & Wignall 1999), vari-
ability in primary production and a productivity crash (Wang
et al. 1994; Kakuwa 1996; Wignall & Twitchett 1996), volca-
nic activity (Renné & Basu 1991; Wignall & Hallam 1993;
Renné et al. 1995; Veevers & Tewari 1995), widespread an-
oxia (Wignall & Hallam 1992; Isozaki 1994; Wignall &
Twitchett 2002), addition of
C-rich deep-water to the sur-
face ocean (Heyerdary et al. 2000), large input of gas hydrate
into the ocean-atmosphere system (Erwin 1993; Faure et al.
1995; Musashi et al. 2001), as well as an extraterrestrial im-
pact event (Xu & Zheng 1993; Kaiho et al. 2001; Berner
The long term general decrease in the
C values of marine
carbonates, which starts several metres below the stratigraphic
P/Tr boundary and is most probably associated with the in-
creased terminal Permian marine regression, is considered to
trace the release of a
C depleted CO
flux to the atmosphere
due to oxidation of buried organic carbon and peat deposits,
as well as the possible expulsion of oil and gas from the fore-
land basins along the peripheral margins of the entire super-
continent (Faure et al. 1995). Expulsion of such
would have resulted in a decrease in atmospheric
values of up to a few per mil, ultimately resulting in
The sharp negative excursion of inorganic carbon
ues at the boundary indicates a rapid interruption of these
gradual processes and was most probably triggered by a major
rapid volcanogenic input of isotopically light carbon released
during the eruption of the Siberian flood basalts dated as con-
temporaneous with the P/Tr boundary (Renné & Basu 1991).
Veevers & Tewari (1995) suggested that volcanism along the
Tethys and Panthalasian margin raised the level of CO
atmosphere so that the spike of CO
from the end Permian Si-
berian Traps finally triggered the Permian-Triassic catastro-
phe. More recently Berner (2002) proposed that short-term
C at the P/Tr boundary are best explained by a
combination of mass mortality from an impact or radiation
blast together with methane release and CO
ing to Berner (2002), the pressure wave most probably arising
from a bolide impact could have triggered both the release of
methane from stored hydrates and the initiation of Siberian
The changes in the carbon cycle across the P/Tr boundary
presented here are most likely related to end Permian volcanic
activity, together with degradation and oxidation of organic
matter due to the terminal phase of the Upper Permian marine
regression. However, methane release from stored hydrates
should also be taken into account. In the Karavanke Moun-
tains the global
C record across the boundary is supposed
to be slightly perturbed due to local factors influencing the
carbon isotopic composition of precipitating carbonates.
However, in Western Slovenia the
signal seems to be undisturbed and thus records a global de-
crease in the
C values of surficial ocean water and carbon-
ates, which coincides with the greatest extinction of marine
and terrestrial organisms in the Phanerozoic.
Two negative excursions of the
C values in the Lower
Scythian of the Masore section are interpreted as reflecting
two separate phases of subareal oxidation of organic matter,
which could be related to stages in the lowering of the Tethys
sea level and/or local fluxes of isotopically light organic-de-
rived carbon in a depositional environment. The shape of the
C curve for the Upper Scythian and Lower Anisian is most
probably related to increased primary productivity of the
ocean water and the sequestration of organic matter in sedi-
ments. Deposition of
C-rich organic matter in sediments re-
sults in an overall increase in
C values of ambient seawater
and therefore those of precipitating carbonates. Only ex-
changes between reduced and oxidized carbon reservoirs can
explain such long term changes in carbonate
(Schidlowski 1987; Hollander & McKenzie 1991).
Stable isotope analyses of carbonates from stratigraphic se-
quences in Western Slovenia ranging from Upper Carbonifer-
ous to Anisian revealed a marked enrichment in
C of Permi-
an limestones relative to Mesosoic carbonates, thus revealing
a further example of globally increased organic carbon stor-
age during the Permian.
The pattern of carbonate carbon
C values indicates a
positive carbon isotope event across the C/P boundary, a
negative carbon isotope event at the topmost Lower Permi-
an, a positive shift of
C values at the Middle Permian to
Upper Permian transition and the well known global negative
C anomaly at the P/Tr boundary, followed by a gradual in-
C carbonate carbon values toward Upper Scyth-
ian and Anisian. We propose that the positive
at the C/P boundary and at the Middle to Upper Permian tran-
sition, as well as the gradual increase of
C values from
226 DOLENEC, OGORELEC and LOJEN
Scythian to Anisian, could result from an enhanced organic
carbon burial rate on continental shelves due to marine trans-
gressions. This hypothesis is supported by the positive corre-
lations observed between
C changes and sea-level fluctua-
tions in stratigraphic sections of the Karavanke Mountains. In
contrast, the negative
C shift at the topmost Lower Permian
is well correlated with a subaerial exposure of these beds due
to marine regression caused by the Saalian orogenic phase.
C pattern for the Upper Permian seems to require a
more detail explanation. We propose that the gradual decrease
towards the P/Tr boundary was a result of increased erosion
and oxidation of organic carbon during an enhanced Upper
Permian marine regression and amalgation, as well as periph-
eral deformation of Pangea (Faure et al. 1995). The abrupt and
relatively short-term negative
C shift of about 4 at the
P/Tr boundary most probably resulted from a combination of
events which accelerated the changes in the global carbon cy-
cle, such as erosion and oxidation of organic carbon, a release
of methane from stored hydrates together with volcanic activi-
ty in the Siberian region, as well as along the Tethys and Pan-
thalasia margins and a sudden reduction in primary productiv-
ity. These events accelerated the input of isotopically light
into the seawateratmosphere system during the Permi-
an-Triassic transition and caused further changes in the global
carbon cycle. These changes are coincidental with the global
environmental changes at the P/Tr boundary and the greatest
mass extinction in the Phanerozoic.
Acknowledgments: The Ministry of Education, Science and
Sport, Republic of Slovenia, UNESCO IGCP Project No.
386 and Geoexp d.o.o., Triè, Slovenia, financially supported
this study. To all these institutions we express our sincere
thanks. The manuscript was greatly improved by the construc-
tive comments of Dr. Paul B. Wignall, Dr. Otília Lintnerová
and one anonymous reviewer.
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