GeolCarp_Vol54_No4_217

CARBON ISOTOPIC SIGNATURE IN CARBONATE ROCKS 217

GEOLOGICA CARPATHICA, 54, 4, BRATISLAVA, AUGUST 2003

217228

UPPER CARBONIFEROUS TO LOWER TRIASSIC

CARBON ISOTOPIC SIGNATURE IN CARBONATE ROCKS

OF THE WESTERN TETHYS (SLOVENIA)

MATEJ DOLENEC

, BOJAN OGORELEC

and SONJA LOJEN

Department of Geology, Faculty of Natural Sciences and Engineering, University of Ljubljana, Akerèeva 12, 1000 Ljubljana,

Slovenia; matej.dolenec@s5.net

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,

C stratigraphy,

carbon stable isotopes, carbonates.

Introduction

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

carbonate signal.

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-

lease of

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 &

Cajhen 1978).

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).

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

O and

C of

the dolomite samples and a weak negative correlation (r = 0.22)

between

O and

C of the limestone samples of the Kara-

vanke Formation (Dolenec T. et al. 1999c), as well as a lack

of correlation between

O and

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-

preciably overprinted.

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

standard, was

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-

dard was

C ±0.01 (1

). Overall analytical reproducibili-

ty of the organic carbon isotopic data was ±0.1 .

Results

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

detrital components.

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

range of

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

C curve

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

C values

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

biostratigraphical studies.

Stable isotope data from the Idrijca Valley and Masore sec-

tions (Fig. 4) show an accelerating decrease of

C values

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-

pletion of

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

C values

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

events.

Discussion

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).

The carbon

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),

variable C

org.

carb.

export ratio changes into the marine sedi-

mentary carbon reservoir (Schidlowski 1987), as well as sea

level fluctuations (Magaritz & Stemmerik 1989). The sharp,

short-term

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

sediments.

The negative

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

dramatically

lowers the

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

from C

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

soil CO

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

carbonates with

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

soil CO

The positive

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

seawater

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

limestone. Their

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

therein).

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

2002).

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

C depleted

would have resulted in a decrease in atmospheric

values of up to a few per mil, ultimately resulting in

C en-

riched carbonates.

The sharp negative excursion of inorganic carbon

C val-

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

in the

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

changes in

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

release. Accord-

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

volcanism.

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

C paleoceanographic

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

C values

(Schidlowski 1987; Hollander & McKenzie 1991).

Conclusions

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-

crease of

C carbonate carbon values toward Upper Scyth-

ian and Anisian. We propose that the positive

C excursion

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.

The

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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