GEOLOGIJA 41, 17-27 (1998), Ljubljana 1999
The Permian-Triassic boundary in the Karavanke Mountains
(Slovenia): Stable isotope variations in the boundary carbonate
rocks of the Košutnik Creek and Brsnina section
Permsko-triasna meja v Karavankah: variabilnost izotopske sestave v kar-
bonatnih kamninah Košutnikovega potoka in
Brsnine
Tadej Dolenec' \ Stanko Buser', Matej Doleneć
' University of Ljubljana, Department of Geology, Aškerčeva 12, 1000 Ljubljana,
Slovenia
^ Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
' Geoexp d.o.o., Slap 21, 4290 Tržič, Slovenia
Key words: P/Tr boundary, Karavanke Mountains, oxygen and carbon stable iso-
topes
Ključne besede: meja perm-trias, Karavanke, stabilni izotopi kisika in ogljika
Abstract
The stable isotope composition of the Upper Permian and Lower Triassic beds at
two locations (Košutnik Creek and Brsnina) in the southern Karavanke Mountains
has been used to investigate б''С and б^^О variations of the well exposed undis-
turbed marine carbonate sequence across the Permian-Triassic boundary. The
lithostratigraphic boundary between the Lower Triassic-Scythyan and underlying
Upper Permian beds is transitional and no exact line can be drawn between them.
The transition from Permian to Triassic is characterized by a major shift in carbon-
ate carbon б'^С and ö'^O from heavier to lighter values. The results suggest that the
carbon isotope variability at the P/Tr boundary reflects global changes in the car-
bon cycle and/or climatic changes, probably controlled by the Upper Permian
regression and further eustatic oscillations of the Tethys sea level and by tectonic,
The corresponding ò'^O variability should be regarded as indication of seawater
oxygen isotopie composition, salinity and temperature changes, changes in carbon-
ate mineralogy of the rocks, postdepositional alterations or some combinations of
all the mentioned possibilities.
Kratka vsebina
Članek obravnava variabilnost izotopske sestave kisika in ogljika v karbonatnih
kamninah na meji perm-trias v Košutnikovem potoku in pri Brsnini v Karavankah.
Za raziskano območje je značilna neprekinjena sedimentacija na prehodu iz perma
v trias. Meja med zgornjepermskimi in spodnjesktiskimi plastmi ni točno določena.
Predpostavljamo, da poteka med tankoplastnatim sivim mikritnim dolomitom, ki
ne vsebuje značilnih fosilov in rdečo, pretežno klastično sedimentno sekvenco,
debelo od 5 m v Koštunikovem potoku in do 25 m na Brsnini. Za prehod iz perma v
trias je značilna negativna kisikova in ogljikova anomalija. Variabilnost izotopske
18_Tadej Dolenec, Stanko Buser & Matej Dolenec
Introduction
The Permian-Triassic (P/Tr) boundary events which took place approximately 250
Ma ago, led to one of the most extensive mass extinctions in the history of the life.
Their causes are not yet v^ell known. The most plausible current explanation for this
extinction appears to evolve multiple elements such as volcanism-induced cooling,
extraterrestrial impact and global anoxia (Erwin, 1994). It was already demon-
strated that at the P/Tr boundary a reflection of worldwide collapse of terrestrial
ecosystems with accompanying loss of standing biomass is indicated by unparalleled
abundances of fungal remains (Brinkhuis & Visscher, 1994). Studies of
several P/Tr boundary sections all over the world show that the transition from Per-
mian to Triassic is characterized by a negative б''С excursion of inorganic and organ-
ic carbon isotopes (M a g a r i t z et al., 1988, 1992; Baud et al., 1989; M a g a r i t z
& Holser, 1991; Magaritz & Stemmerik, 1989; Erwin, 1993; Wang
et al., 1994; F a u r e et al., 1995; Dolenec et al., 1981). A considerable enrich-
ment of light carbon isotopes in marine carbonates and in organic matter is associat-
ed with many extinction related boundaries not only the P/Tr. The corresponding
oxygen isotope anomaly is sometimes more or less parallel but usually less pro-
nounced. Parallel behaviour of oxygen and carbon isotopes in marine carbonates
may suggest some common driving mechanisms (Verhagen et al., 1990). It could
be related to the dominance of meteoric water during the marine regression (Ver-
hagen et al., 1990), oxidation of marine organic matter and accompanying kinetic
oxygen isotope fractionation (Grusczczynski et al., 1989). According to
Schräg et al. (1995) the oxygen isotopie composition of diagenetically unaltered
bulk carbonates primarily reflects the temperature and/or isotopie composition of
the seawater Based on this assumption the major б^''0 excursions thus could be relat-
ed to global climatic changes, as well as changes in the isotopie composition of the
ocean water.
The particular aims of this study have been to complete the previous investiga-
tions of the P/Tr boundary in the Karavanke Mountains (Dolenec et al., 1981) and
to confirm the systematic changes during the Permian-Triassic transition.
Geological setting and stratigraphy
In the southern Karavanke Mountains, at Košutnik Creek and Brsnina (Fig. 1.)
sedimentation continued concordantly across the P/Tr boundary. The biostratigraph-
ic and lithostratigraphic boundary between the Lower Triassic (Scythian) beds and
the underlying Upper Permian beds is transitional and no exact line can be drawn
between them (Fig. 2. ). The Middle Permian Val Gardena Formation of mostly fluvial
origin is overlain by a 270 m thick Upper Permian carbonate sequence that was
sestave ogljika na meji P/Tr v raziskanih profilih odraža globalne spremembe v
ogljikovem ciklusu, in klimatske spremembe, ki so najverjetneje posledica zgornje-
permske regresije ter kasnejših spodnjetriasnih eustatičnih nihanj morske gladine.
Variabilnost izotopske sestave kisika v mejni sekvenci pa je po našem mnenju
najverjetneje posledica različnih faktorjev, tako variabilnosti izotopske sestave kisi-
ka v takratni morski vodi, slanosti, temperature kot mineralne sestave kamnin in
post sedimentacijskih sprememb.
The Permian-Triassic boundary in the Karavanke Mountains
19
Fig. 1. Map showing location of the Karavanke Mountains (1 - Košutnik Creek section, 2 -
Brsnina section)
named the Karavanke Formation (B u s e r et al., 1986). The basal unit of this
sequence is represented by an up to 70 m thick evaporitic facies composed of cellular
dolomite (rauhw^acke) which alternates w^ith rare black bituminous shales and grey
vuggy dolomites. In the lov^er part of the basal unit, only in the Košutnik Creek a 1.5
m thick sequence of well bedded black bituminous biomicritic limestone was found.
According to B u s e r (1974; 1980) it contains tiny sulfur geodes, Bellerophon gas-
tropods and numerous microfossils {Gymnocodium bellerophontis, Permocalculus
fragilis, Velebitela triplicata, Mizzia velebitana and Glomospira sp.) that permitted to
prove for the first time the Upper Permian age of the Karavanke Formation. The
evaporitic sequence is overlain by a thick succession (up to 200 m) of fossiliferous
biomicritic dolomites probably deposited in an open lagoon and shallow shelf envi-
ronment. The Upper Permian age of these beds is indicated by calcareous algal
assemblages {Mizzia cornuta, Permocalculus sp., Connexia sp.), as well as by very
common small foraminifers which belong to Glomospira sp., Agathamina sp. and
Hemigordius sp. (R a m o v š, 1986). About 70 to 80 m below the P/Tr boundary a
porphyrite dyke of Middle Triassic age cuts the Upper Permian beds.
The P/Tr boundary is placed arbitrarily at the end of the sedimentation of the well
bedded grey dolomicrite. It is followed by a red coloured more or less terrigenous
sequence predominantly composed of well bedded siltstones, mudstones and sand-
stones, alternating with micritic dolomites that contain no characteristic fossils. The
sequence was deposited in a very shallow evaporitic part of the basin, into which
abundant terrigenous material was transported. Its thickness is about 5 m in
Košutnik Creek and 25 m at Brsnina. In the investigated area these beds are overlain
mostly by dark grey and brown micritic and sparitic limestones intercalated with
oolitic limestone, marls and shales.
Methods
The boundary profiles in the Košutnik Creek and at Brsnina were systematically
sampled at 1 and 5 m intervals, except in the vicinity of the biostratigraphically and
lithostratigraphically defined P/Tr boundary where sampling intervals were reduced
to 20, 10 and 5 cm. The relative stratigraphie position of the samples and the analyti-
20
Tadej Dolenec, Stanko Buser & Matej Dolenec
Fig. 2. Stratigraphie section of the Upper Permian and Schythian beds in
the Karavanke Mountains, Tržič section (after Dolenec et al., 1981)
The Permian-Triassic boundary in the Karavanke Mountains_21
Results and discussion
After the Middle Permian period during which the predominantly clastic Val Gar-
dena Formation was deposited in continental environment gradual subsidence affect-
ed the extensive area of the Karavanke Mountains. The subsidence was followed by a
vast marine transgression of the Tethys Sea from the SE to the NW. A transgression is
consistent with conditions observed all over Europe during the late Permian. Docu-
mentations of a transgression exist not only in the Zechstein basin, but also to the
south in the Tethys (Assereto et al., 1973). It was a time of climatic and geo-
graphic changes from continental to marine environment which are reflected in an
intertongued lithofacies. In the Lower part of the Upper Permian thin sandy
dolomite layers interfinger with the topmost Val Gardena shales and sandstones
(B u s e r, 1980). The thickness of this basal unit which grades upward into the evap-
oritic sequence of the Upper Permian is about 5 m. The carbonate rocks of the evap-
oritic sequence (Fig. 3.) show the variation of б'-*С mostly in the range between + 0.69
and + 3.83 %o (PDB) and б^«0 between + 24.31 and + 26.93 %o (SMOW). Similar values
have been also found in the basal Upper Permian evaporitic unit of the southern
Karavanke Mountains at Tržič (Dolenec et al., 1981). Outside this range is the
sandy dolomite of the basal unit which is distinctly depleted in "C (б^^С = - 2.50 %o)
and '^O (б'''0 = + 21.44 %o). This depletion is probably related to the precipitation from
low salinity solutions with a predominant component of meteoric water and/or post-
depositional isotopie alteration. The oxygen isotopie composition of the evaporitic
sequence is not as high as expected from the recent evaporitic environments. Modern
dolomites from the Arabian Gulf have a б^*0 range mostly from + 30.4 to + 34.3 %o,
while those from the Baffin Bay are even slightly heavier with б^^О between + 34.5 to
+ 35.5 %o (T u c k e r, 1990). Such values are consistent with carbonate minerals for-
mation from hypersaline marine derived fluids (Perkins et al., 1994). The
observed depletion of the evaporitic sequence in '"O suggests the influx of fresh water
into the evaporitic basin and extensive meteoric diagenesis which also lead to the
various distribution phenomena. Thus б^^^О values are to be regarded as indicators of
seawater and pore fluids isotopie composition, as well as temperature and changes in
mineralogy. Studies of carbonate rocks have shown that the oxygen-isotope system is
more subject to exchange during diagenesis and burial metamorphism than the car-
bon isotope system (Magaritz, 1975; 1983). By analysing the least visibly alterat-
ed samples from the evaporitic sequence we attempt to minimise this effect. Thus we
can suppose that although the post depositional changes more or less altered the
cal results can be seen in Fig. 2. The isotopie measurements were carried out on
whole rock samples, carefully selected by using thin sections. The samples were ana-
lyzed in the Jožef Stefan Institute, Ljubljana, Slovenia, following the modified pro-
cedure of (M c Crea, 1950); carbonate samples were reacted overnight with 100 %
phosphoric acid at 50 °C. The CO2 gas generated was isotopically analyzed using a
Varian MAT 250 isotopie ratio-mass spectrometer Therefore the data reflect a
weighted average of the isotopie composition of the entire carbonate components in
the dolomite or limestone. All б^'C and б^^О values were reported in standard per mill
(%o) notation relative to the PDB and SMOW standards. The analytical reproducibili-
ty of duplicate samples was always better than ±0.1 %o for both carbon and oxygen
isotope composition.
22
Tadej Dolenec, Stanko Buser & Matej Dolenec
Fig. 3. б''С and б'^0 data of the Košutnik Creek section
original oxygen and carbon isotopie composition, the primary paleoceanographic sig-
nal was not completely overprinted. Parallel behaviour of the б'^С and б^^О curves
and a relatively high correlation (r = 0.89) between б^'C and б^^О suggest a transition
from a terrestrial to shallow marine evaporitic conditions.
The transition from Middle Permian to Upper Permian is characterized by a con-
siderable enrichment of carbonates with ''C (from - 2.50 to + 3.83 %o) and '"O (from
+ 21.44 to + 26.93 %o). Positive б''С and б"'0 excursions started when a transgressive
sea flooded the vast alluvial Middle Permian landscape. The general hypothesis pro-
posed to explain positive б"С shifts is that during the marine transgression, the
expansion of shallow shelf areas increased the organic carbon burial rate and
enriched the ocean in '-'C (Magaritz & Stemmerik, 1989; Compton et
al., 1990; F a u r e et al., 1995). In terms of the corresponding isotopie changes, peri-
ods of high-sea level are reflected in enrichment of ^'C in carbonates (H a 1 1 a m,
1992). According to the previous interpretation we suggest that a positive б^'C shift at
Middle Permian-Upper Permian transition resulted from changes in the burial rate
of organic carbon which began with the transgression of the Tethys Sea. The corre-
sponding oxygen isotope excursion is similar to those in б'*С and also indicates
changes from terrestrial to marine-evaporitic conditions.
Moving upward in the sections the sedimentary facies of the Upper Permian beds
evolved from more or less restricted lagoon dolomites alternated with sabkha gypsum
The Permian-Triassic boundary in the Karavanke Mountains
23
toward open lagoon-shallow shelf biomicrite dolomite, suggesting a transgressive
trend of the Tethys Sea. The isotopie data show a slight enrichment in ''C and ^^0 of
biomicrite dolomite relative to the rocks of the evaporitic unit with б"С and б^^О val-
ues mostly in the range from + 2.0 to + 3.1 %o, and from + 27.3 to + 28.3 %o. Out of this
range is the host rock of the porphyrite dyke enriched in light carbon ^^C and oxygen
isotopes (up to - 1.3 %o for б^^С and up to + 17.8 %o for б^^О). However, the ^^C and
^"0 alteration zones are only about 4 m thick.
Isotopie composition similar to those of the biomicrite dolomite has been also
reported from the Upper Permian sections of the Garnie Alps (Magaritz &
Holser, 1991), as well as from the Upper Permian beds at Tržič (Dolenec et
al., 1981). The relatively high б^•^C values of these carbonate rocks can be related to
the worldwide high storage of organic matter during the Late Paleozoic (Maga-
ritz & Holser, 1991), while б^'*0 values may reflect to some extent the tempera-
ture of the Tethys Sea water and /or dolomitizing solution.
The transition from Permian to Triassic is characterized by an abrupt shift of б^^С
and б^^^О toward lower values. In terms of amplitude, the depletion of б^-^С and б^'*0
across the P/Tr boundary is of about 4 %o for б^'C and 7 %o for б^^О. A detailed sam-
pling of the boundary interval in the Košutnik Creek and at Brsnina shows that the
б^^С and б^^О anomalies are not confined strictly to the lithostratigraphically and
biostratigraphically proposed boundary. A major global drop of б"С and б^^О begins
Fig. 4. б"С and б'^0 data of the Brsnina section
about 30 m below the boundary. In Carnic Alps a decrease of б^^С begins about 60 m
(Magaritz & Holser, 1991), while in the Idrijca Valley the same shift of б"С
starts only 5 m below the P/Tr boundary (Dolenec & Ramovš, 1996). The б"С
curve reaches the peak value of - 1.86 %o about 8 m below the boundary and after that
24_Tadej Dolenec, Stanko Buser & Matej Dolenec
suffers a succession of additional drop at the end of the Permian and two in the low-
ermost Scythian, before settling to more normal values which are 1 to 2 %o lower rel-
ative to those in the Upper Permian (Fig. 4.). If б'^С values are to be regarded as indi-
cators of changes in the oxidation and reduction system of carbon the shape of the
carbon curve thus reflects at least two separate phases of subaeral oxidation of
organic matter at the end of the Permian and two similar phases in the Lower
Scythian. These phases may be related to the eustatic oscillations of the Tethys Sea
level, as well as to the local fluxes of isotopically light organic derived carbon in a
depositional environment which slightly disturbed the global carbon isotope signal.
During marine regressions shelves were exposed to increased erosion and oxidation
of organic carbon, producing a negative б'^С shift (T a p p a n, 1968; Mackenzie
& Piggot, 1981; Comp t on et al., 1990; Magaritz & Holser, 1991;
F a u r e et al., 1995). The end Permian carbon isotopie perturbances in the investi-
gated area span an interval of about 50 m, whereas in the Idrijca Valley where lime-
stone sedimentation proceeded concordantly across the P/Tr boundary the same shift
of б'^С happens over about 10 m of section (Dolenec & Ramovš, 1996). We
suggest that the observed general decrease of б'^С values at the P/Tr transition is
probably associated with the global Late Permian marine regression which led to the
destruction of terrestrial and marine ecosystems which related in a ^^C depleted CO2
flux into the atmosphere (Magaritz & Holser, 1991; F a u r e et al., 1995).
This CO2 then equilibrated with the ocean waters, ultimately resulting in a
enriched reservoir in ocean waters and therefore '^C depleted carbonates.
The variations in б'^0 across the P/Tr boundary show slightly different trends with
respect to those in б'^С. After a decrease from + 27.55 to a minimum value of + 25.22
%o about 10 m below the boundary, the variability of б^''0 in the interval straddling
the P/Tr boundary is fairly uniform (between + 25.30 and + 26.05 %o). A decrease of
б^^О in the topmost Permian is best explained by a change from marine to desultory
evaporitic conditions affected by an excessive input of terrigenous material and by
local freshening of waters due to the influxes of more or less isotopicallly modified
continental waters into the sedimentary basin.
It is important to note that significant facies changes at the P/Tr boundary con-
firm the extensive regression and several second order transgressive-regressive cycles
probably controlled by eustatic oscillations of the sea level and tectonic. The climatic
conditions were presumably hot and arid to semi-arid and then changed to a some-
what more humid in the Lower Scythian (Assereto et al., 1973).
The end Permian regression was preceeded by a rapid early Triassic transgression
which brought a shallow epicontinental sea over the entire region of the Karavanke
Mountains. A shallow continental shelf extended during the time interval between
the Upper Permian and the Anisian stage from Slovenia to the adjacent Alpine
regions (Assereto et al., 1973; Broglio Loriga et al., 1979) and Dinarides
(Mudrenovid, 1980; B u s e r, 1987). The sedimentation became more or less
unified throughout the entire region. During this time micritic, sparitic and oolitic
limestones, as well as marls and shales were formed. Carbon isotopie composition of
this stratigraphie unit indicates a gradual enrichment in '^C. According to previous
interpretations we speculate that this enrichment in the regional sense probably
coincided with deposition of organic matter in shelf sediment during high sea level
stand, and/or with slightly cooling events in the Lower Scythian.
It is interesting to note that the Scythian limestones are considerably depleted in
'^O (from 3 to 8 %o) relative to the Upper Permian as well as Lower Scythian dolomite
The Permian-Triassic boundary in the Karavanke Mountains_25
Conclusions
The results we have presented in this study indicate that the transition from Per-
mian to Triassic is characterized by a strong disturbance in the global carbon cycle
accompanied by changes in б'^0 values in the boundary carbonate rocks. The nega-
tive б'-'С excursion is interpreted as reflecting an increased terminal Permian marine
regression which resulted in a '-^C depleted CO2 flux into the atmosphere. This CO2
then equilibrated with the ocean waters, ultimately resulting in '^C enriched carbon-
ates. On the other hand the positive б''С shift at the Middle Permian-Upper Permian
transition is attributed to the Upper Permian marine transgression with correspond-
ing enrichment of carbonates in "C. The shape of the carbon isotope curve reflects at
least two separate phases of subaerial oxidation of organic matter at the end of the
Permian and two similar phases in the Lower Scythian. These phases may be related
to the eustatic oscillations of the Tethys sea level and/or local changes in the propor-
tions of continental and marine contributions of organic matter into the sedimentary
basin which slightly overprinted and masked the changes in global carbon isotope
composition.
of the boundary zone. Extrapolation of high temperature data yields values which
indicate that dolomite which formed in isotopie equilibrium under sedimentary tem-
peratures should be enriched in '"O relative to calcite by3-6%o(Sheppard &
Schwartz, 1970). M c K e n z i e (1981) showed that the enrichment of the natu-
rally occurring dolomite in '"O over sedimentary calcite is + 3.2 %o at 35 "C. However,
the oxygen isotopie composition of coexisting sedimentary dolomite-calcite occur-
rences was often found to be similar (Botz & von der Bore h, 1984). Our data
show that the Scythian limestone are depleted in "'O up to 4 %o relative to the Scythi-
an dolomite, and up to 8 %o as compared to the Upper Permian dolomite. The isotopie
composition of these limestones shows the variation of б^*0 mostly in the range
between + 20.57 and + 23.95%o and б'^С between + 0.34 and + 1.96 %o. Similar depleted
values have been also observed in Scythian and Upper Permian limestone of the Idri-
jca Valley (Dolenec & Ramovš, 1996). These values are also considerably
depleted (up to 7%o) relative to the marine limestones of Recent age (F a u r e, 1977).
Such depletion cannot be interpreted only in terms of seawater temperature. It may
have also been caused by a change in б'"0 of the seawater, decrease of salinity as well
as by postdepositional alteration. Although there are several problems with regard to
the interpretation of the differences in the isotopie composition between dolomites
and limestones we suggest that the cause of the observed ^^O depletion in limestones
could be related to changes in carbonate mineralogy, salinity and oxygen isotopie
water composition, as well as to the diagenetic modifications.
The changes in carbonate mineralogy thus coincided with the drop in б^^О from
values of around + 26.05 to + 22.45 %o. Note that the corresponding б"С signal is not
changed and is preserved in both dolomite and limestone. After reaching the mini-
mum value of + 20.57 %o the б'*'0 curve returns to slightly more positive values of
+ 23.95 %o, and then gradually decreases again. The shape of the б^^О curve thus proba-
bly indicates a slightly cooling trend - more humid conditions during the deposition of
the grey limestone followed by a warmer period. A weak negative correlation between
б'''0 and б"С (r = - 0.24 for 39 samples) in the Lower Scythian limestone suggests some
different driving mechanisms which affected the isotopie composition of this unit.
26_Tadej Dolenec, Stanko Buser & Matej Dolenec
Acknowledgements
This research was performed with financial support from the Ministry of Science
and Technology - Republic of Slovenia and Geoexp. d.o.o. Tržič, Slovenia. To both
these institutions we express our sincere thanks.
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