The Permian-Triassic boundary in the Idrijca Valley (Western Slovenia): isotopic fractionation between carbonate and organic carbon at the P/Tr transition
Permsko-triasna meja v dolini Idrijce (zahodna Slovenija): izotopska frakcionacija med karbonatnim in organskim ogljikom na prehodu iz perma v trias
Tadej DOLENEC,12 Sonja LOJEN2 & Matej DOLENEC3 University of Ljubljana, Department of Geology, Aškerčeva 12, 1000 Ljubljana, Slovenia 2 Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia 3 Geoexp d.o. o., Slap 21, 4290 Tržič, Slovenia
Key words: P/Tr boundary, Idrijca Valley, stable isotopes, isotopic fractionation Ključne besede: meja perm-trias, dolina Idrijce, stabilni izotopi, izotopska frakcionacija
Abstract
The transition from Permian to Triassic is characterized by a well known prominent negative excursion of 613Ccarb. and 513Corg. reflecting global perturbations in the carbon cycle, a dramatic increase in isotopic fractionation between carbonate and organic carbon (A613Ccarb .org) indicating a sudden break-down in primary productivity, and the drastic disappearance of typically Upper Permian marine fauna.
The surprising shift of 4.15%o towards higher values in 613Corg. not reflected in the S13Ccarb. pattern and a reduction of A613Ccarb _OIJ! also by 4.15 %0 6 cm above the boundary may be most plausibly attributed to the unusual plankton blooms which could be the consequence of a subsequent recovery period in the biological system associated with an increase in oxidation conditions.
Kratka vsebina
Članek obravnava frakcionacijo ogljikovih izotopov na meji perm-trias v dolini Idrijce. Prehod iz perma v trias na raziskanem območju karakterizira negativna ogljikova anomalija, ki se odraža tako v karbonatih kot v organski snovi. Potek krivulje A513Ccarb _org, ki podaja frakcionacijo med karbonatnim in organskim ogljikom kaže, da je na permsko-triasni meji prišlo do nenadnega povečanje izotopske frakcionacije med obema vrstama ogljika. To je po našem mnenju posledica dramatičnega zmanjšanja primarne produkcije v takratnem morju, ki v raziskanem profilu tudi sovpada z drastičnim izginotjem tipične zgornjepermske faune. Hitra sprememba poteka frakcionacijske krivuje tik nad mejo je najverjeneje posledica ponovne vzpostavitve pogojev ugodnih za bohoten razvoj planktonskih organizmov, kar se odraža tudi v nenadnem zmanjšanju izotopske frakcionacije med karbonatnim in organskim ogljikom.
Introduction
Permo-Triassic (P/Tr) boundary events, which took place approximately 250 m.y.
ago, led to the most extensive mass extinction in the history of life. A number of possible explanations for this profound break in the evolution of life have been proposed,
such as volcanic activity, sea-level fluctuation, changes in sea water chemistry, an extra-terrestrial impact event and various related factors (Y o i c h i, 1994). The global events outlined above coincide with isoto-pic and elemental anomalies recorded in several P/Tr boundary sections all over the world. One of the most remarkable anomalies is the world-wide negative shift of 613C of inorganic and organic carbon across the P/Tr boundary (M a g a r i t z et al., 1992; Wang et al., 1994; W o 1 b a c h et al., 1994; F a u r e et al., 1995). A corresponding oxygen isotopic shift is more or less parallel, but less pronounced. Furthermore, significant shifts in sulphur (K a j i w a r a et al., 1994) and strontium isotopes (V e i z e r, 1989; Kram & Wedepohl, 1991) have also been recorded.
Fig. 1. Map showing the location of the studied region in Western Slovenia. Global paleogeo-graphy during the Permian-Triassic boundary interval is taken from S u n et al. (1989).
In this study we present the results of carbonate and organic carbon stable isotope analysis of the P/Tr boundary section in the Idrijca Valley, Western Slovenia (Fig. 1), and discuss their implications with respect to the nature and the causes of the boundary events at the end of the Permian in this part of the Western Tethys.
Geological setting and stratigraphy
In Western Slovenia, sedimentation proceeded concordantly across the P/Tr boun-
dary. The Middle Permian Val Gardena Formation of mostly fluvial origin is overlain by an approximately 250 m thick dark grey and black well bedded abundantly fossili-ferous shallow marine Upper Permian Žažar Formation. The uppermost unit of the Žažar Formation exposed in the Idrijca Valley is composed of 5 to 40 cm thick dark grey and black limestone layers interbed-ded with rare black shales. The total thickness of this unit is about 14 metres. Limestone microfacies are represented mostly by packstones and grainstones, partly wacke-stones very rich in microfossils; macrofos-sils, however, are very rare (Ramovš, 1986). The faunal composition displays gradual impoverishment of Upper Permian ta-xa moving upward towards the boundary and an abrupt disappearance at the boundary. The P/Tr boundary is sharp, not erosi-onal and is represented by an up to 0.8 cm thick clayey marl layer (PTB layer) overlying the black algal packstone. The PTB layer which marks the Permo-Triassic boundary contains spherules, which most probably represent spherical prashynophite algal skeletons diagenetically infilled by magnetite (Hansen et al., 1999). The deposition of the PTB layer most probably took place during a period of maximum eustatic fall and regression. It is followed by a light grey well bedded Lower Scythian sparitic limestone. The total thickness of the basal Scythian unit exposed in this area is about 13 metres. Upward, the sparitic limestone is followed by a laminated dolomitic limestone alternating with grey stylolithic dolomites. According to present knowledge the basal Scythian unit contains some conical tube-like fragments of fossils. They may be characteristic of the lowermost lower Tri-assic.
Materials and methods
The boundary profile in the Idrijca Valley was systematically sampled at 20 cm intervals, except in the vicinity of the paleon-tologically defined P/Tr boundary where sampling intervals were reduced to 10, 5 and 2 cm. The isotopic measurements were carried out on un-dolomitized limestone samples. Only unreciystallized or insignifi-
cantly recrystallized samples were used for isotopic measurements. Samples were obtained as a split of powder prepared from rock chips remaining after thin section preparation. Powdered rock samples for 6lsO and 513C analysis were prepared by overnight digestion in >100 % phosphoric acid at 50 °C. C02 gas released during acid treatment was cryogenically cleaned and analyzed for C isotopic composition on a Varian MAT-250 mass spectrometer. The S13C values were normalized by assuming 613C values of + 2.48 %o for the IAEA-CO-1 standard on the PDB scale.
For preparation of samples for the determination of total organic carbon, powdered whole rock samples were treated with heated 3M hydrochloric acid at 50 °C to react carbonates. Upon cessation of COs evolution excess acid was removed by-repeated washing (three to four times) with doubly distilled water until neutral. After the final decanting of water, the carbonate free residues were oven-dried at 50 °C. Organic carbon isotope ratios were measured in the carbonate-free residues in an Europa 20-20
Stable Isotope Analyser (Europa Scientific LTD.) with an ANCA-NT preparation module for on-line combustion of bulk solid samples and chromatographic separation of the gases. Organic carbon isotope values were calibrated using the IAEA-CH-7 standard with a S13C value of - 31.8 %o on the PDB scale.
The analytical precision based on multiple analysis of internal laboratory standards was ± 0.01 %o for 613Ccarb. and ± 0.008 %o for 513Corg, respectively. Overall analytical reproducibility of the isotopic data was ± 0.1 %o for carbonate carbon and ± 0.090 %o for organic carbon.
Results and discussion
The isotopic composition of organic and carbonate carbon exhibits synchronous shifts in an isotopic lighter direction only within a 50 cm thick interval, from the P/Tr boundary to - 50 cm downward (Fig. 2). These shifts imply a relatively rapid sequence of responses of the oceanic biota and
Fig. 2. Stable carbon isotope composition pf limestone (ô13Ccarb), total organic carbon (ô13C0„) and car-bonate-organic carbon isotope fractionation (ô13Ccarb _0rg ) across the Permian-Triassic boundary in the
Idrijca Valley section.
the global carbon cycle to the environmental stress imposed by the end Permian global events. Several questions are of interest: how does the isotopic composition of organic carbon in the P/Tr boundary sequence reflect the environmental stress on the marine biota, and what can the combined carbon isotopic composition of organic and carbonate carbon reveal about the sequence of global changes which affected the carbon cycle at the P/Tr transition?
To answer these questions, the A613Ccarb _org fractionation curve was constructed (Fig. 2). Hollander et al., (1993) have shown that the isotopic difference or carbon isotopic fractionation between carbonates and organic carbon AS13Ccarb._org.) can be used to evaluate major changes in the carbon cycle, removing the effect of changes in 613C of the dissolved inorganic carbon surface water reservoir. For the P/Tr transition in the Idrijca Valley section the decrease in isotopic composition of organic carbon by 3.51 %o (from - 26.21 to - 29.72 %0) and increasing A613Ccarb _org. by 3.01 %o (from 27.49 to 30.50 %>), as well as a decrease in 613C of carbonate carbon, can be explained by a decrease in primary production at the end of the Permian. This breakdown in the photosynthesis respiration cycle appears to coincide with a dramatic reduction of the accumulation ra-te (D o 1 e n e c et al., 1999) and an accelerated fall in sea level, also clearly indicated in the Karavanke Mountains (Dolenec et al., 1996; Hansen et al., 1999). A decrease in marine productivity appears to be consistent with the observed fossil record, which also suggests a drastic disappearance of Upper Permian marine fauna.
The surprising shift of 4.15 %o toward higher values in 613Corg. (from - 29.27 to -25.12 %o) and the reduction in A613Ccarb_org. also by 4.15 %o (from 30.50 to 26.35 %o) 6 cm above the P/Tr boundary in contrast indicate: a subsequent recovery period in the biological system, a decrease in oceanic pCO,, and an increase in surface water temperature or increase in influx of terrestrial isotopically heavier organic matter into the sedimentary basin. It appears that this positive organic carbon excursion occurred too suddenly to be explained as a result of any gradual processes. In fact, it is best interpreted as a result of unusual plankton
blooms during the subsequent recovery period in the biological system, leading to a drastic reduction in the concentration of dissolved C02 in surface waters. The variability of A613Ccarb _org during the remaining Scythian most probably suggests further fluctuations of primary productivity and insufficient recovery of higher consumers, as already mentioned by K a k u w a (1996).
A negative 613C excursion of inorganic and organic carbon isotopes and dramatic changes in the carbonate-organic carbon fractionation (A613Ccarb._org) across the P/Tr boundary indicate remarkable changes in the carbon cycle. Several possible explanations for these perturbations in the carbon cycle have been considered. They might have been caused by a combination of factors such as degradation and oxidation of organic matter (Magaritz & Holser, 1991; F a u r e et al., 1995), changes in biological productivity of the ocean surface (Magaritz et al., 1992; K a k u w a, 1996) or volcanic activity (Renne & Basu, 1991; Campbell et al., 1992). Veevers & Tewari (1995) suggested that volcanism along the Tethys margin together with that along the Panthalassan margin raised the background level of C02 in the atmosphere so that the spike of C02 from the end-Permian Siberian Traps finally triggered the Permian Triassic catastrophe. The results presented here most probably indicate that the combination of degradation and oxidation of large amounts of organic matter due to global marine regression and/or regional epeirogenetic uplift (F a u r e et al., 1995) together with volcanic activity seem to be the factors most responsible for the observed carbon isotopic changes and the terminal Permian productivity crash.
Conclusions
Isotopic data based on 513Ccarb, S13Corg and A613Ccarb _org values indicate well known perturbations in the carbon cycle and a dramatic decrease in primary productivity which is coincidental with an abrupt disappearance of Upper Permian marine fauna at the P/Tr boundary in the Idrijca Valley section.
A general and gradual decrease in 613C of carbonate and organic carbon across the
P/Tr boundary is consistent with an overall increase in atmospheric pC02 over a period of 10 m yr. (F a u r e et al., 1995) and seems to be related to the degradation and oxidation of a large amount of organic matter due to global marine regression.
A sharp negative 613Corg anomaly and the shape of 613Corg and A613Ccarb,.OIg. curves indicate a rapid interruption of these gradual process which could be attributed to the relatively short term events. These events accelerated the global changes in carbon cycling and caused a breakdown in the primary production. Disastrous environmental conditions, not yet explicitly understood, could be related to volcanic activity, most probably associated with the Siberian Traps volcanism which triggered these events with consequences not only for oceanic anoxia, but also for biological systems and for climatic changes. Decreasing S13Corg and increasing A613Ccarb._org values at the P/Tr boundary are consistent with a sudden reduction in primary productivity, while the increasing 613C018. and decreasing A513Ccarb..org. values 6 cm above the boundary indicate unusual plankton blooms which could be the consequence of the recovery period in the biological system associated with change to more oxidising conditions.
Acknowledgments
This study was financially supported by 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.
References
Campbell, I. H., C z a m a n s k e, G. K., Fedorenko, V. A.,Hill, R. L. & Stepa-n o v, V. 1992: Synchronism of the Siberian Traps and the Permian-Triassic boundary. - Science, 258, 1760-1763, New York.
D o 1 e n e c, T., B u s e r, S. & D o 1 e n e c, M. 1996: Stable isotope variations in the Permian-Triassic boundary sedimentary rocks from the Karavanke Mountains (Slovenia). V. M. Gold-schmidt Conference 1996, Heidelberg, Germany, Journal of Conference Abstract, pp. 137-138, Heidelberg.
D o 1 e n e c, T., L o j e n, S. & R a m o v s, A. 1999: The Permian-Triassic boundary in Western Slovenia (Idrijca Valley section): magnetostrati-graphy, stable isotopes and elemental variations (submitted).
F a u r e, K., d e W i t, M.J. & W i 1 1 i s, J. P. 1995: Late Permian global coal hiatus linked to 13C-depleted CO* flux into the atmosphere during the final consolidation of Pangea. - Geology 23, 507-510, Boulder.
Hansen, H. J., L o j e n, S., Toft, P., Dolenec, T., TongJ., Michaelsen, P. & Sarkar, A. 1999: Magnetic susceptibility and organic carbon isotopes of sediments across some marine and terrestrial Permo-Triassic boundaries. (submitted).
Hollander, D. J., M c K e n z i e, J. A. & K e n n e t h, J. H. 1993: Carbon isotope evidence for unusual plankton blooms and fluctuation of surface water CO, in "Strangelove Ocean" after terminal Cretaceous event. - Palaegeogr. Palaeo-clim. Palaeoecol., 104, 229-237, Amsterdam.
K a j i w a r a, Y., Y a k a m i t a, S., I s h i d a, K., I s h i g a, H. & I m a i, A. 1994: Development of a largely anoxic stratified ocean and its temporary massive mixing at the Permian/Triassic boundary supported by the sulfur isotopic record. -Palaeogeogr. Palaeoclim. Palaeoecol., Ill, 367-379, Amsterdam.
K a k u w a, Y. 1996: Permian-Triassic mass extinction event recorded in bedded chert sequence in southwest Japan. - Palaeogeogr. Palaeoe-clim. Palaeaecol., 121, 35-51, Amsterdam
K r a m m, U. & W e d e p o h 1, K. H. 1991: The isotopic composition of strontium and sulfur in seawater of Late Permian (Zechstein) age. -Chem. Geol., 90, 253-262, Amsterdam.
M a g a r i t z, M. & H o 1 s e r, W. T. 1991. The Permian-Triassic of the Gartnerkofel-1 Core (Carnic Alps, Austria): Carbon and Oxygen Isotope Variation. - Abh. Geol. B. A., 45, 149-163, Wien.
Magaritz, M., Krishnamurthy, R. V. & H o 1 s e r, W. T. 1992: Parallel trends in organic and inorganic carbon isotopes across the Permian/Triassic boundary.- Am. Jour. Sci., 292 727-739, New Haven.
R a m o v s, A. 1986: Marine development of the uppermost Zazar beds and the lowermost Scythian beds: In Permian and Permian-Triassic boundary in the South Alpine segment of the Western Tethys. IGCP Project 203, Excursion Guidebook, pp. 39-42.
R e n n e, PR. & B a s u, A. R. 1991: Rapid eruption of the Siberian traps flood basalts at the Permo-Triassic boundary. - Science, 253 176-177, New York.
Sun, S., L i, J., C h e n, H,Pen g, W., Hsu, K. J. & S h e 11 on, J. W. 1989: Mesozoic and Ce-nozoic sedimentary history of South China. -Am. Assoc. Pet. Geol. Bull., 73, 1247-1269, Tulsa.
V	e i z e r, J. 1989: Strontium isotopes in sea-water through time. - Ann. Rev. Earth. Planet Sci., 17, 141-167, Palo Alto.
V	e e v e r s, J. J. & T e w a r i, R. C. 1995: Per-mian-Carboniferous and Permian-Triassic ma-gmatism in the rift zone bordering the Tethyan margin of southern Pangea. - Geology, 23, 467-470, Boulder.
W a n g, E., G e 1 d s e t z e r, H. H. J. & K r u o s e, H. R. 1994: Permian-Triassic extinction: Organic 613C evidence from British Columbia, Canada. - Geology, 22, 580-584, Boulder.
W o 1 b a c h, W. S., R o e g g e, D. R. & G i 1 mour, I. 1994: The Permian-Triassic of the Gartnerkofel-1 core (Carnic Alps, Austria): Organic carbon isotope variation, In New Develo-
pments Regarding the KT Event and Other Catastrophes in Earth History. - LPI Contribution No. 825, Lunar and Planetary Institute, Huston, USA, pp. 138.
Y o i c h i, E. 1994: Patterns and paleoenvi-ronmental implications of end-Permian extinction of Rugosa in South China. - Palaeogeogr. Pa-laeoclimatol. Palaeoecol., 107, 165-177, Amsterdam.