The Permian–Triassic boundary in Western Slovenia (Idrijca Valley section): magnetostratigraphy, stable isotopes, and elemental variations

Chemical Geology 175 Ž2001. 175–190 www.elsevier.comrlocaterchemgeo

The Permian–Triassic boundary in Western Slovenia žIdrijca Valley section/ : magnetostratigraphy, stable isotopes, and elemental variations T. Dolenec a,b,) , S. Lojen b, A. Ramovsˇ a a

Faculty for Natural Sciences and Engineering, Department of Geology, UniÕersity of Ljubljana, AskercÕa ˇ ˇ 12, 61000 Ljubljana, SloÕenia b Joef Stefan Institute, JamoÕa 39, 61000 Ljubljana, SloÕenia

Abstract Stable isotope analyses of carbonate Ž d13C carb . and total organic carbon ŽTOC; d13 C org ., together with geochemical analyses of 54 major, minor, and trace elements and magnetic susceptibility measurements were carried out on whole rock samples of the undisturbed Permian–Triassic ŽPrTr. limestone boundary sequence in the Idrijca Valley ŽW. Slovenia.. At the PrTr boundary, there is a 0.8-cm thick clayey marl layer ŽPTB. showing a characteristic magnetic susceptibility pulse and considerable enrichment in most major, minor, and trace elements. The PrTr transition is characterized by Ž1. an abrupt decrease in sedimentation rate from 32 to 5 cmr100 ka, Ž2. a well known prominent negative excursion of d13 C carb and d13C org , reflecting global perturbations in the carbon cycle, and Ž3. the drastic disappearance of typically Upper Permian marine fauna. Although the shape of the d13C carb curve indicates gradual long-term change across the PrTr boundary, the sharp negative d13C org anomaly within an interval from 30 cm below to 6 cm above the boundary suggests an interruption of those gradual processes. Our observations for selected redox sensitive elements ŽMo, U, V and Zn., abundances for organic carbon and sulphur and the rare earth elements ŽREE. distribution in the boundary sequence, as well as the shape of the CerCe ) curve suggest that oceanic anoxia was typical of the Upper Permian, and that the transition to oxygenated conditions occurred at the PrTr boundary. Oxygenation at the PrTr transition is coincidental with the terminal phase of the Upper Permian marine regression. The shape of the CerCe ) curve indicates that the redox environmental conditions changed again in the earliest Triassic, resulting once more in oxygen deficient conditions. q 2001 Elsevier Science B.V. All rights reserved. Keywords: PrTr boundary; Idrijca Valley; Stable isotopes; Elemental geochemistry; Magnetostratigraphy

1. Introduction The Permian–Triassic ŽPrTr. boundary, approximately 250 Ma ago, is characterized by the most extensive mass extinction in the history of life. A ) Corresponding author. Faculty for Natural Sciences and Engineering, Department of Geology, University of Ljubljana, Askercva ˇ ˇ 12, 61000 Ljubljana, Slovenia.

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 and various related factors ŽYoichi, 1994.. Erwin Ž1994, 1996. proposed that the mass extinction at the end of the Permian is caused by a combination of these more or less co-occurring events noted above, operating in three phases. One is repre-

0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 0 0 . 0 0 3 6 8 - 5

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sented by a marine regression during the Late Permian, resulting in the destruction of many marine basins, reduction in habitat for many organisms, and increased climatic instability. The second phase involved the eruption of the Siberian Traps and further environmental degradation. The final phase may have started immediately prior to the boundary with the end of the Late Permian regression followed by a transgression in the lowermost Triassic. Different scenarios have been suggested as possible causes for the mass extinction. Wignall and Twitchett Ž1996. suggested that oceanic anoxia, both at low and high paleolatitudes, may have been responsible for the mass extinction in the Late Permian. Knoll et al. Ž1996. suggested hypercapnia, the influence of toxic concentrations of CO 2 , and perhaps, H 2 S, into surficial environments as cause for the Late Permian biological crisis. This influx resulted from a rapid overturn of the deep anoxic ocean. The global events outlined above coincide with isotopic and elemental anomalies recorded in numerous PrTr boundary sections worldwide. One of the most remarkable anomalies is a worldwide negative shift of d13 C recorded for inorganic and organic carbon across the PrTr boundary Že.g., Magaritz et al., 1992; Wang et al., 1994; Wolbach et al., 1994; Faure et al., 1995.. A corresponding, yet less pro-

Fig. 1. Map showing the location of the studied region in Western Slovenia. Global paleogeography during the PrTr boundary interval is taken from Sun et al. Ž1989..

nounced oxygen isotopic shift, is more or less parallel to the carbon isotope event. Furthermore, major shifts in sulphur ŽKajiwara et al., 1994. and strontium isotopic compositions ŽVeizer, 1989; Kramm and Wedepohl, 1991. have been recorded. In this study, we present results for the carbonate and organic carbon isotopic compositions and additional geochemical analyses for the PrTr boundary section in the Idrijca Valley, western Slovenia ŽFig. 1.. Together with the data for magnetic susceptibility, implications of these geochemical results will be discussed with respect to the nature and the causes of the PrTr boundary events in this part of the western Tethys.

2. Geological setting and stratigraphy In western Slovenia, a continuous sedimentary successions straddles across the PrTr boundary. The Middle Permian Val Gardena Formation of mostly fluvial origin is overlain by the 250-m thick, dark grey and black, well-bedded, and abundantly fossiliferous shallow marine Upper Permian aar Formation. The lower part of this limestone sequence is very rich in CaucasianrIndo-Armenian brachiopod fauna ŽTyloplecta yangtseensis, T. ricthofeni, T. callocrenea, T. sloÕenica, Tischernyschewia typica, Spinomarginifera, Linoproductus lineatus, Leptodus nobilis ., small Richthofenia lawrenciana bioherms and Waagenophyllum indicum biostromes. The faunal composition changes upward through the section with typical elements of the South Tyrolian Bellerophon fauna Ž Comelicania pl. sp.. appearing in the upper part of the Zazar Formation. The uppermost part of the Zazar Formation exposed in the Idrijca Valley is composed of 5–40 cm thick dark grey and black limestone layers interbedded with rare black shales. The total thickness of this unit is about 14 m. Limestone microfacies are represented mostly by packstones and grainstones, and partly by wackestones very rich in microfossils. Macrofossils, however, are very rare. According to Ramovsˇ Ž1986., the lower fossiliferous horizons are characterized by typical calcareous algae Ž Permocalculus fragilis, Gymnocodium bellerophontis ., foraminifers Ž Vermiporella nipponica, V. serbica, Hemigordius sp.,

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Glomospira sp., Ichtiolaria sp., Tuberitina sp., Pachyphloia sp., cf., Palaeonubecularia sp.. and echinoderms of Upper Permian age. Continuing upward in the section, the faunal assemblage consists of Gymnocodium bellerophontis, Archaeolithophyllum, Nodosinella, GlobiÕalÕulina graeca sp., Glomospira, Hemigordius, AmmoÕertella, Vermiporella nipponica, Permocalculus, small fine ribbed brachiopod shells, and echinoderm remains. The topmost part of this unit is characterized by a 20-cm thick black algal packstone containing fragments of Gymnocodiaceae, Glomospira sp., Nodosariids and echinoderm fragments. The faunal composition displays gradual impoverishment of Upper Permian taxa moving upward towards the boundary and an abrupt disappearance at the boundary. Lithostratigraphically, the PrTr boundary is represented by a sharp, yet not erosional contact, that consists of a clayey marl layer with a maximum thickness of 0.8 cm, overlying the black algal packstone. This clayey layer contains spherules, which most probably repre-

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sent spherical prashinophyte algal skeletons diagenetically infilled by magnetite ŽHansen et al., 1999.. The deposition of the PrTr boundary layer most probably occurred during a period of maximum eustatic sea level fall and regression, correlative with the sedimentation of red terrigenous sediments across the PrTr boundary in the Karavanke Mountains ŽDolenec et al., 1999a,b.. The boundary layer in the Idrijca Valley is overlain 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 m. The sparitic limestone is followed by a laminated dolomitic limestone alternating with grey stylolitic dolomites.

3. Materials and methods The boundary profile in the Idrijca Valley was systematically sampled at 20 cm intervals, except in

Table 1 d13 C carb and d13 C org values of limestone from the PrTr boundary section in the Idrijca Valley Sample no.

Depth Žm.

IT23 IT22 IT21 IT20 IT19 IT18 IT17 IT16 IT15 IT14 IT13 IT12 IT11 IT10 IT9 IT8 IT7 IT6 IT5 IT4 IT3 IT2 IT1

13.4 8.9 5.9 4.8 4.6 4.3 3.7 3.5 3.3 2.85 2.4 2.1 1.8 1.4 1.15 1.00 0.65 0.35 0.3 0.15 0.1 0.06 0.03

n.d. — Not determined.

d13 C carb y0.9 y0.32 0.29 0.14 y0.01 0.22 0.11 y0.10 0.10 0.26 0.22 0.66 0.44 0.59 0.42 0.73 0.75 0.79 0.95 0.98 1.18 1.23 1.29

d13 C org y27.85 n.d. y26.11 y26.08 n.d. n.d. y27.29 n.d. y28.12 y27.40 y26.42 y27.45 y27.84 y26.90 y27.27 y26.70 y26.25 y27.50 y26.19 y26.59 y26.35 y25.12 y26.85

Sample no.

Depth Žm.

d13 C carb

PTB IP1 IP2 IP3 IP4 IP5 IP6 IP7 IP8 IP9 IP10 IP11 IP12 IP13 IP14 IP15 IP16 IP17 IP18 IP19

0.01 y0.05 y0.20 y0.30 y0.45 y0.50 y0.55 y0.70 y0.90 y1.15 y1.65 y2.60 y4.10 y5.20 y7.50 y10.30 y12.50 y14.00 y16.00 y18.50

1.20 1.23 1.09 1.28 1.07 2.38 2.29 2.42 2.35 3.07 3.04 3.54 3.82 4.08 4.18 4.13 4.48 3.86 4.67 4.38

d13 C org y26.70 y29.27 y26.67 y26.21 n.d. y25.72 y25.82 y27.32 y26.44 y25.97 y27.5 y26.46 y25.91 y26.42 y25.70 y26.12 y26.46 y26.69 y25.96 y27.11

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the vicinity of the paleontologically defined PrTr boundary, where sampling intervals were reduced to 10, 5, and 2 cm. The isotopic measurements were carried out on clean whole rock limestone samples. Carbonate mineralogy phases were determined by X-ray diffractometry and by examination of thin sections by standard optical methods, including staining with Alizarin-red. The diagenetic history was evaluated by petrographic methods, as well as an assessment of paleontological characteristics. Only non- or insignificantly recrystallized samples were used for isotopic measurements. Powdered whole rock samples were prepared for isotope analyses of d18 O and d13 C by overnight digestion in 100%

phosphoric acid at 508C. CO 2 gas released during acid treatment was cryogenically cleaned, and the carbon isotopic composition was measured on a Varian MAT-250 mass spectrometer. d13 C values were normalized according to d13 C values of q2.48‰ vs. PDB for the IAEA-CO-1 standard. The carbon isotopic composition of total organic carbon ŽTOC., was measured on powdered whole rock samples pretreated with 3 M hydrochloric acid at 508C in order to remove carbonates. Isotope measurements on carbonate-free dry rock powders were performed on a Europa 20–20 Stable Isotope Analyser ŽEuropa Scientific. equipped with an ANCA-NT preparation module for on-line combustion of bulk

Table 2 Chemical data of limestone from the PrTr boundary section in the Idrijca Valley Depth Žm. 13.4 4.6 1.15 0.3 0.1 0.06 0.03 0.01 y0.05 y0.5 y1.15 y5.2 y12.5 y18.5

SiO 2 Ž%. 3.86 10.24 1.79 3.56 6.72 4.79 9.53 24.13 3.98 11.96 4.41 3.62 7.53 9.71

Al 2 O 3 Ž%.

Fe 2 O 3 Ž%.

MnO Ž%.

MgO Ž%.

CaO Ž%.

Na 2 O Ž%.

K 2O Ž%.

TiO 2 Ž%.

0.87 1.88 0.31 0.59 1.54 0.93 2.16 8.88 0.89 4.34 1.52 0.92 1.39 1.57

0.36 0.18 0.13 0.33 0.57 0.49 0.71 2.28 1.2 1.35 0.48 0.39 0.57 0.51

- 0.01 - 0.01 - 0.01 - 0.01 - 0.01 - 0.01 - 0.01 0.02 0.01 0.03 0.01 0.02 0.02 0.02

2.01 0.52 2.84 4.6 6.21 5.84 6.91 8.78 4 12.33 1.91 1.23 1.47 1.25

53.3 48.93 52.56 48.87 44.8 46.94 41 23.08 49.06 31.14 50.98 52.7 50.56 48.67

0.04 0.07 0.03 0.04 0.04 0.1 0.04 0.07 0.04 0.06 0.05 0.11 0.18 0.11

0.27 0.23 0.12 0.18 0.55 0.3 0.77 3.11 0.26 1.54 0.47 0.27 0.37 0.44

0.03 0.09 0.01 0.02 0.07 0.03 0.1 0.45 0.04 0.14 0.04 0.02 0.04 0.04

2.37 1.28 4.07 2.84 2.75 3.08 1.53 0.07 n.d. 0.67 1.38 2.91 1.86 3.43

0.17 0.12 0.11 0.14 0.29 0.24 0.37 0.36 n.d. 0.65 0.32 0.27 0.30 0.16

8.02 7.68 3.21 5.68 16.13 9.30 21.52 00.00 9.66 47.42 18.64 10.94 15.27 18.29

Co Žppm.

Cr Žppm.

Sc Žppm.

Cu Žppm.

Zn Žppm.

Ni Žppm.

La Žppm.

Ce Žppm.

Pr Žppm.

3 2 3 3 2 2 5 10 3 4 2 2 4 5

4 2 3 3 3 22 5 13 2 6 8 10 7 9

2 3 2 2 3 2 2 12 6 8 5 2 2 3

4.77 5.7 4.13 8.67 14.47 15.51 21.16 27.3 49.45 25.9 5.9 4.64 10.93 4.24

9.55 12.18 6.22 10.07 18.76 18.27 26.07 32.3 38.55 35.87 13.48 10.48 28.45 9.26

1.18 1.24 0.82 1.41 2.54 2.94 3.66 4.12 8.36 4.68 1.30 0.95 2.85 0.90

Depth Žm.

Zr Žppm.

V Žppm.

As Žppm.

13.4 4.6 1.15 0.3 0.1 0.06 0.03 0.01 y0.05 y0.5 y1.15 y5.2 y12.5 y18.5

12 38 5 11 20 12 27 115 21 23 9 7 11 7

6 7 2 3 8 7 13 48 11 37 21 13 17 19

1 1 1 13 32 30 7 38 12 11 5 2 3 3

2 2 1 1 2 2 4 6.1 3 4 2 2 2 2

5.8 8.9 6.2 6.2 9.9 6.5 13.2 4 6 24.3 12.8 8.9 10.1 15.4

0.9 1.4 0.6 0.6 1.8 1.2 2.7 7.6 1.7 4.8 1.7 1 1.6 1.9

C org. Ž%.

S Ž%.

Rb Žppm.

T. Dolenec et al.r Chemical Geology 175 (2001) 175–190

solid samples and chromatographic separation of the resulting gases. Organic carbon isotope values were calibrated using the IAEA-CH-7 standard with a d13 C value of y31.8‰ vs. PDB. All isotope analyses of whole rock carbonate and carbonate-free residue samples were performed at least as duplicate measurements. Analytical precision based on multiple analyses of internal laboratory standards was "0.01‰ for d13 C carb and "0.01‰ for d13 C org , respectively. Overall analytical reproducibility of the isotope data was "0.1‰ for carbonate carbon and "0.09‰ for organic carbon. Selected samples were analysed by inductively coupled plasma-mass spectrometry ŽICP-MS. for rare

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earth elements ŽREE; La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu., Rb, Nb, In, Mo, Sn, Sb, Cs, Ta, W, Pb, Bi, Th and U. Major ŽSi, Al, Fe, Mg, Ca, Na, K., minor ŽMn, P, Ti. and selected trace elements ŽBa, Sr, Y, Zr, Be, V. were measured after fusion with a mixture of lithium metaboraterlithium tetraborate by ICP with a thermo Jarrell-Ash Enviro II ICP, while abundances of Cu, Zn, Ag, Ni, Co, Cd, Sc, As, Au, Cr, Se and Hg were also determined by ICP spectrometry after total digestion by four acids Žnitric, hydrochloric, perchloric and hydrofluoric.. Concentrations of P, In, Ta, Pb, Bi, Be, Au, Sb, Hg and Se were generally lower than the detection limit of the selected ICPrMS andror ICP methods

Nb Žppm.

Mo Žppm.

Sn Žppm.

Sb Žppm.

Cs Žppm.

Hf Žppm.

W Žppm.

Th Žppm.

U Žppm.

Ba Žppm.

Sr Žppm.

Y Žppm.

0.60 1.72 0.08 0.20 1.35 0.51 2.19 11.00 1.26 2.44 0.76 0.36 0.82 0.91

0.27 0.13 0.24 - 0.01 - 0.01 0.22 0.61 0.60 2.46 5.93 5.13 2.91 3.25 1.90

0.20 - 0.2 1.50 - 0.2 3.10 0.30 - 0.2 1.90 1.60 1.40 - 0.2 0.30 - 0.2 0.40

0.14 0.06 0.14 0.10 0.36 0.20 0.60 0.53 0.18 0.17 0.17 0.30 0.37 0.03

0.49 0.59 0.37 0.49 1.22 0.79 1.66 6.90 0.80 4.75 1.56 0.89 1.35 1.30

0.33 1.16 0.35 0.31 0.35 0.27 0.63 3.20 0.90 0.47 0.16 0.19 0.26 0.17

2.10 0.27 0.63 0.90 - 0.01 0.19 0.61 1.50 0.61 0.50 0.17 1.59 0.39 0.87

0.78 1.79 0.41 0.68 1.63 0.99 2.31 8.50 1.79 4.03 1.40 0.94 1.44 1.42

0.75 1.08 0.44 0.62 0.98 0.76 1.31 4.09 1.83 4.30 4.18 3.41 4.24 3.20

17 22 16 52 41 19 55 250 37 88 32 22 22 28

1171 1354 486 495 792 414 454 403 584 387 658 1095 965 1294

5 6 7 9 13 16 18 17 37 28 5 4 10 5

Nd Žppm.

Sm Žppm.

Eu Žppm.

Gd Žppm.

Tb Žppm.

Dy Žppm.

Ho Žppm.

Er Žppm.

Tm Žppm.

Yb Žppm.

Lu Žppm.

CerCe ) Žppm.

0.99 1.08 0.94 1.45 2.26 2.72 3.14 2.27 6.46 3.81 1.08 0.93 0.37 0.82

0.16 0.17 0.12 0.22 0.34 0.42 0.45 0.46 1.01 0.63 0.16 0.13 2.03 0.13

0.95 1.1 0.88 1.31 1.95 2.41 2.54 2.67 5.18 4.08 1 0.81 0.37 0.8

0.53 0.56 0.51 0.77 1.12 1.24 1.38 1.74 2.76 2.29 0.55 0.4 0.14 0.36

0.07 0.08 0.06 0.08 0.12 0.16 0.18 0.26 0.29 0.28 0.08 0.05 0.98 0.05

4.93 5.38 4.24 6.5 11.11 13.83 15.61 15.7 38.08 20.59 5.89 4.33 12.07 4.14

1.05 1.21 0.75 1.38 2.15 2.62 2.9 2.95 6.97 4 1.15 0.81 0.46 1.05

0.28 0.26 0.18 0.28 0.51 0.63 0.64 0.54 1.49 0.82 0.21 0.19 2.05 0.16

0.17 0.21 0.18 0.26 0.41 0.47 0.54 0.57 1.02 0.79 0.19 0.14 1.05 0.15

0.47 0.67 0.45 0.58 0.77 0.8 1.06 1.63 1.8 1.9 0.46 0.27 0.14 0.38

0.06 0.08 0.06 0.07 0.10 0.13 0.17 0.24 0.26 0.25 0.06 0.04 1.17 0.06

0.92 1.05 0.78 0.66 0.71 0.62 0.68 0.63 0.44 0.75 1.12 1.15 1.09

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Ž0.01%, 0.1, 0.005, 5, 0.05, 1, 2, 1, 1 and 0.5 ppm, respectively.. TOC and total sulphur ŽWRS. were determined on an automated LECO CS-344 carbon–sulphur analyser. Prior to the analyses, powdered rock samples were repeatedly washed with distilled water to remove any soluble sulphate and then decarbonated with 5% HCl. Results for organic carbon were recalculated to whole rock by accounting for weight lost during decarbonation. Elemental analyses were carried out at Activation Laboratories, Ontario, Canada. Analytical precision and accuracy were better than "3% for major elements, "5% for REE, but between "5% and "10% for minor and remaining trace elements indicated by results for duplicate measurements of samples, as well as the MAG-1 ŽUS National Bureau of Standards. standard. Furthermore, CCH-1 limestone material ŽUniversite´ de Liege, ` Belgium. was analysed as laboratory standard. Sampling for magnetic susceptibility stratigraphy was conducted with 10 samples for each bed of the succession. The Upper Permian and Lower Scythian limestones were cut into blocks with a rectangular cross-section. In the laboratory, they were cut into 7-mm slices parallel to the bedding. During the cutting process, 3 mm of each slice was lost. The samples were crushed into pieces smaller than 1 cm and their magnetic susceptibility were measured using a KLY-2 Kappa bridge ŽGeofyzika Brno.. The values obtained were recalculated into SI units and plotted against stratigraphic height.

ary of Idrijca Valley, we used the magnetic susceptibility pattern as basis for estimating the sediment accumulation rate. Magnetic susceptibility stratigraphy was introduced by Hansen et al. Ž1993. for problems of high resolution stratigraphy across the KrT boundary and further applied to the PrTr boundary section in the Idrijca Valley ŽHansen et al., 1999.. Time-frequency analysis of the magnetic susceptibility curve suggests that at 20, 40, and 100 ka represent Milankovic cycles ŽHansen et al., 1993.. The pattern observed in the Idrijca Valley ŽFig. 2. consists of larger and smaller susceptibility peaks, with the former interpreted as representing 100 ka cycles. Each of the larger peaks consists of 4–5 smaller peaks, which were interpreted as 20 ka cycles. According to time-frequency analysis of the magnetic susceptibility pattern, the sediment accumulation rate is lowest in the upper Permian part of the Idrijca Valley section with 5 cmr100 ka between 15 cm below the boundary and the boundary level. From y15 to y32 cm, the rate is slightly higher at 14 cmr100 ka, while further down in the section to y180 cm, it is around 32 cmr100 ka. Following the boundary level, a sedimentation rate between 25 and 30 cmr100 ka characterizes the lowermost Scythian. The temporal evolution of the sedimentation rate displays a major reduction close to the boundary level. The possibility of a small hiatus will have to be investigated.

4. Results and discussion

The boundary section in the Idrijca Valley has been studied biostratigraphically by Ramovsˇ Ž1986.. However, due to the low abundance of conodonts, the PrTr boundary is represented by a lithological marker bed — the thin clayey marl layer. For the present discussion, it is important to note the gradual impoverishment of upper Permian fauna approaching the boundary and its abrupt disappearance at the boundary level. Upper Permian taxa show an abrupt extinction in the topmost, up to 20-cm thick, black upper Permian limestone bed underlying the clayey marl layer. This indicates that the extinction occurred at the level of regression. This is in contrast to the results of Wignall and Hallam Ž1992., which indicate that the abrupt disappearance of Per-

13

The isotopic composition of carbonate Ž d C carb . and TOC Ž d13 C org ., together with the elemental abundances are summarized in Tables 1 and 2. Magnetic susceptibility, isotope data, selected element concentrations and REE data across the PrTr boundary are presented in Figs. 2–5. Furthermore, geochemical and isotope data were subjected to correlation analysis using a CSS statistical package. The resulting correlation matrix is given in Table 3. 4.1. Magnetostratigraphy The study of global boundary events requires high resolution chronostratigraphy. For the PrTr bound-

4.2. Biostratigraphy

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181

Fig. 2. Stratigraphic plot of magnetic susceptibility of the PrTr boundary sequence from the youngest Permian to the oldest Triassic. The vertical line of the curve represents the actual measurements, while an asterisk represents a y3 point moving average. Magnetostratigraphy is from Hansen et al. Ž1999..

mian fauna in the Alps of northern Italy did not occur at the level of maximum regression, but shortly

above, associated with a major transgression within the Tesero Horizon. In the Idrijca Valley and in the

Fig. 3. Stable carbon isotope composition of carbonate Ž d13 C carb . and TOC Ž d13 C org . across the PrTr boundary in the Idrijca Valley section.

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T. Dolenec et al.r Chemical Geology 175 (2001) 175–190

Fig. 4. Depth profile of selected major, minor, and trace elements and CerCe ) values across the PrTr boundary in the Idrijca Valley section.

T. Dolenec et al.r Chemical Geology 175 (2001) 175–190

Fig. 5. PAAS-normalized REE patterns for the PrTr boundary section from the Idrijca Valley Žv Upper Permian limestone, ` Lower Triassic limestone, ' PTB layer..

Karavanke Mountains, the PrTr boundary sections record a rapid major marine flooding event above the boundary. In the Karavanke Mountains, the earliest Triassic marine transgression which spread anoxic water over the entire region followed the sedimentation of up to 25 m of red-coloured terrigenous material, representing the upper Permian marine regression in this region ŽDolenec et al., 1981, 1999a.. 4.3. Carbonate carbon stable isotopes The sedimentary sequence from the Idrijca Valley section displays a general decrease of about 4‰ in the carbonate carbon isotopic composition from the upper Permian to the lower Triassic. Superimposed on this longer-term temporal evolution is a larger change in d13 C carb toward lower values beginning about 5 m below the PrTr boundary ŽFig. 3.. Prior to this level, upper Permian limestone d13 C carb values range from q3.9‰ to q4.7‰. Similarly, high d13 C carb values of more than q3.5‰ have also been recorded for the upper Permian of the Karavanke Mountains ŽDolenec et al., 1999a,b. and numerous other upper Permian sections studied for carbon isotopes ŽBaud et al., 1989; Magaritz and Holser, 1991.. The accelerated decrease in d13 C carb , commencing about 5 m below the PrTr boundary level,

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continues across the boundary into the lower Scythian beds ŽFig. 3.. The shape of the d13 C carb curve undoubtedly reflects the well-known perturbations in the global carbon cycle at the PrTr boundary, coupled with a dramatic reduction of the sedimentation rate and the rate of organic carbon burial. It should be noted that no negative d13 C carb peak was recorded in the up to 13-m thick basal Scythian unit exposed in the Idrijca Valley. The absence of a negative d13 C anomaly either reflects a condensed section andror an as yet unrecognized unconformity. However, this interpretation is preliminary and has to await further detailed studies. However, in the Masore section Ž10 km NW of the Idrijca Valley section., a temporal trend in d13 C carb for the basal Scythian limestone resembles the one from the Idrijca Valley section, but displays two negative d13 C carb anomalies of y4.3‰ and y3.7‰, respectively. These agree well with the data from the Carnic Alps ŽMagaritz and Holser, 1991., where similar peaks were found at 17 and 22 m above the boundary ŽDolenec et al., 1999b.. We interpret these anomalies as unrelated to the PrTr boundary events. They are younger and may be related to the eustatic oscillation of the Tethys sea level associated with the pronounced oxidation of organic matter in the Upper Scythian. 4.4. Organic carbon stable isotopes The isotopic composition of TOC is much more variable compared to the isotopic composition of carbonate carbon. It displays Ž1. a less pronounced decrease in d13 C org of about 2‰ from the Upper Permian to the Lower Triassic, Ž2. a prominent negative anomaly at the PrTr boundary, and Ž3. several second order isotopic minima ŽFig. 3.. A major drop in d13 C org begins about 50 cm below the boundary, with a minimum value of y29.3‰ at the boundary level. This is followed by an abrupt shift to a more enriched value of y25.1‰, 6 cm above the boundary. Finally, the temporal trend records values, which are up to 2‰ lower than those recorded for the upper Permian. Although the overall temporal evolution Žas exemplified by the carbonate carbon isotope curve. suggests a long-term more gradual character, the presence of a sharp organic carbon d13 C negative anomaly indicates a short-term interruption of this pattern at the boundary.

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Table 3 Corelation matrix of the PrTr boundary limestone from the Idrijca Valley CerCe ) SiO 2

Na 2 O K 2 0

TiO 2

C org.

S

Rb

Nb

Mo

Sn

Sb

Cs

Hf

W

Th

U

Ba

Sr

Y

1.00 0.96 1.00 0.81 0.89 1.00 0.43 0.39 0.22 1.00 0.51 0.62 0.69 0.59 1.00 y0.89 y0.93 y0.88 y0.52 y0.84 1.00 0.13 0.01 y0.03 0.29 y0.30 0.10 1.00 0.92 0.99 0.91 0.39 0.68 y0.95 y0.03 1.00 0.95 0.98 0.87 0.26 0.56 y0.90 y0.04 0.97 1.00 y0.75 y0.77 y0.72 y0.17 y0.48 0.72 0.01 y0.73 y0.73 1.00 0.44 0.54 0.65 0.63 0.78 y0.68 0.04 0.58 0.41 y0.65 1.00 0.92 0.99 0.91 0.37 0.63 y0.93 0.02 1.00 0.97 y0.72 0.56 1.00 0.93 0.97 0.87 0.19 0.51 y0.86 y0.04 0.95 0.99 y0.69 0.35 0.95 1.00 y0.06 0.05 0.18 0.20 0.23 y0.11 0.17 0.08 y0.10 y0.31 0.69 0.11 y0.14 1.00 0.29 0.32 0.37 y0.10 0.48 y0.42 y0.59 0.34 0.36 y0.27 0.29 0.32 0.35 0.17 1.00 0.47 0.49 0.50 0.10 0.41 y0.52 0.08 0.54 0.54 y0.43 0.43 0.51 0.55 y0.11 0.53 1.00 0.89 0.97 0.91 0.50 0.72 y0.95 0.03 0.98 0.91 y0.75 0.69 0.98 0.89 0.25 0.32 0.47 1.00 0.85 0.87 0.79 0.04 0.37 y0.74 y0.11 0.83 0.94 y0.64 0.16 0.83 0.95 y0.30 0.35 0.44 0.75 1.00 0.13 0.21 0.18 0.23 y0.05 y0.07 y0.12 0.24 0.24 0.04 y0.13 0.24 0.27 y0.27 y0.25 0.15 0.16 0.24 1.00 0.95 0.99 0.93 0.32 0.63 y0.94 y0.02 0.99 0.98 y0.77 0.53 0.98 0.97 0.04 0.36 0.52 0.96 0.89 0.19 1.00 0.48 0.52 0.52 0.51 0.16 y0.40 0.52 0.51 0.40 y0.55 0.62 0.56 0.39 0.77 y0.03 0.21 0.62 0.21 y0.03 0.50 1.00 0.89 0.96 0.90 0.25 0.61 y0.90 y0.10 0.97 0.97 y0.67 0.43 0.97 0.97 y0.02 0.37 0.52 0.92 0.90 0.28 0.97 0.40 1.00 y0.14 y0.31 y0.47 y0.06 y0.74 0.51 0.38 y0.40 y0.31 0.20 y0.48 y0.37 y0.30 y0.14 y0.44 y0.38 y0.41 y0.23 0.26 y0.35 0.02 y0.43 1.00 0.21 0.28 0.62 0.02 0.65 y0.48 y0.22 0.31 0.26 y0.51 0.82 0.29 0.26 0.23 0.38 0.23 0.36 0.27 y0.23 0.37 0.09 0.30 y0.57 1.00 0.90 0.92 0.81 0.13 0.45 y0.82 y0.08 0.89 0.97 y0.70 0.25 0.89 0.98 y0.25 0.37 0.51 0.81 0.98 0.24 0.94 0.27 0.93 y0.25 0.24 0.84 0.91 0.87 0.51 0.58 y0.85 0.18 0.91 0.82 y0.75 0.71 0.93 0.80 0.43 0.19 0.39 0.96 0.64 0.11 0.89 0.80 0.83 y0.28 0.31 0.49 0.55 0.60 0.09 0.61 y0.63 y0.11 0.59 0.60 y0.25 0.26 0.57 0.60 y0.18 0.55 0.43 0.52 0.54 0.02 0.58 y0.02 0.63 y0.52 0.37 0.86 0.90 0.92 0.31 0.67 y0.90 y0.05 0.91 0.89 y0.79 0.66 0.90 0.88 0.06 0.29 0.64 0.89 0.79 0.18 0.93 0.47 0.86 y0.38 0.53 0.13 0.10 0.07 0.61 0.38 y0.26 0.10 0.10 y0.07 y0.27 0.70 0.10 y0.14 0.70 y0.01 y0.09 0.26 y0.33 y0.33 0.06 0.49 y0.08 y0.01 0.18 0.93 0.98 0.93 0.43 0.70 y0.96 y0.01 0.99 0.95 y0.77 0.65 0.98 0.93 0.15 0.35 0.52 0.99 0.81 0.15 0.98 0.56 0.93 y0.39 0.41 0.87 0.86 0.80 0.29 0.44 y0.78 0.09 0.87 0.87 y0.52 0.29 0.87 0.88 y0.15 0.18 0.53 0.81 0.80 0.32 0.86 0.41 0.88 y0.27 0.21 0.24 0.27 0.23 0.27 0.18 y0.25 0.45 0.29 0.24 y0.01 0.15 0.31 0.25 y0.09 y0.43 0.18 0.28 0.14 y0.06 0.24 0.24 0.23 y0.23 y0.04 0.78 0.89 0.93 0.16 0.61 y0.84 y0.14 0.89 0.86 y0.75 0.59 0.90 0.85 0.28 0.40 0.27 0.91 0.79 0.10 0.91 0.56 0.87 y0.40 0.50 0.30 0.36 0.71 y0.16 0.56 y0.49 y0.21 0.39 0.37 y0.66 0.78 0.38 0.39 0.14 0.40 0.32 0.40 0.43 y0.12 0.47 0.12 0.41 y0.52 0.96 0.49 0.53 0.78 0.15 0.64 y0.64 0.08 0.55 0.49 y0.75 0.85 0.54 0.49 0.34 0.43 0.49 0.60 0.44 y0.19 0.60 0.42 0.50 y0.48 0.88 0.25 0.30 0.66 y0.15 0.52 y0.43 y0.13 0.33 0.30 y0.67 0.82 0.31 0.32 0.20 0.37 0.31 0.35 0.35 y0.17 0.40 0.15 0.33 y0.48 0.97 0.17 0.22 0.60 y0.18 0.49 y0.37 y0.14 0.25 0.22 y0.60 0.83 0.24 0.24 0.21 0.34 0.25 0.28 0.28 y0.21 0.33 0.11 0.25 y0.47 0.97 0.19 0.23 0.60 y0.15 0.49 y0.38 y0.12 0.26 0.22 y0.61 0.83 0.24 0.24 0.22 0.33 0.23 0.29 0.28 y0.22 0.33 0.13 0.25 y0.45 0.97

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SiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2O TiO 2 C org. S Rb Nb Mo Sn Sb Cs Hf W Th U Ba Sr Y Zr V As Co Cr Sc Cu Zn Ni La Ce Pr Nd Sm

Al 2 O 3 Fe 2 O 3 MnO MgO CaO

Eu 0.10 0.15 0.53 y0.17 0.48 y0.32 y0.16 0.18 0.15 y0.51 0.78 0.16 0.17 Gd 0.09 0.14 0.52 y0.15 0.51 y0.33 y0.18 0.17 0.14 y0.48 0.80 0.15 0.15 Tb 0.20 0.26 0.62 y0.09 0.55 y0.42 y0.12 0.29 0.25 y0.59 0.82 0.27 0.26 Dy 0.25 0.32 0.65 0.04 0.64 y0.49 y0.13 0.35 0.29 y0.59 0.85 0.32 0.29 Ho 0.29 0.35 0.68 0.03 0.67 y0.53 y0.17 0.38 0.33 y0.60 0.83 0.36 0.33 Er 0.34 0.41 0.73 0.07 0.70 y0.58 y0.17 0.45 0.39 y0.64 0.84 0.42 0.39 Tm 0.51 0.58 0.83 0.16 0.75 y0.72 y0.10 0.61 0.56 y0.72 0.82 0.59 0.55 Yb 0.54 0.60 0.82 0.24 0.74 y0.73 y0.10 0.62 0.56 y0.75 0.81 0.60 0.55 Lu 0.55 0.60 0.84 0.20 0.73 y0.73 y0.06 0.62 0.58 y0.72 0.79 0.61 0.57 CerCe ) y0.13 y0.22 y0.46 0.08 y0.64 0.43 0.57 y0.28 y0.28 0.07 y0.20 y0.23 y0.29 Zr

V

As

Co

Cr

Sc

Cu

Zn

Ni

La

Ce

Pr

Nd

0.17 0.32 0.22 0.21 0.22 y0.21 0.25 0.21 0.33 0.23 0.21 0.19 y0.25 0.24 0.22 0.35 0.26 0.32 0.29 y0.21 0.35 0.27 0.36 0.26 0.40 0.29 y0.23 0.40 0.24 0.41 0.30 0.42 0.34 y0.22 0.44 0.26 0.42 0.31 0.49 0.38 y0.18 0.50 0.23 0.39 0.41 0.65 0.52 y0.15 0.66 0.27 0.43 0.35 0.66 0.53 y0.15 0.67 0.21 0.39 0.42 0.66 0.55 y0.15 0.67 0.27 y0.53 y0.23 y0.22 y0.35 0.03 y0.29 Sm

Eu

Gd

Tb

Dy

Ho

Er

0.03 y0.18 y0.45 0.97 0.04 0.17 y0.49 0.98 0.13 0.28 y0.50 0.98 0.17 0.32 y0.54 0.99 0.15 0.36 y0.57 0.99 0.19 0.43 y0.59 0.98 0.29 0.58 y0.60 0.93 0.32 0.58 y0.54 0.91 0.31 0.59 y0.56 0.91 0.43 y0.36 0.73 y0.74 Tm

Yb

Lu

T. Dolenec et al.r Chemical Geology 175 (2001) 175–190

Zr 1.00 V 0.70 1.00 As 0.60 0.37 1.00 Co 0.83 0.84 0.48 1.00 Cr y0.24 0.37 y0.25 0.16 1.00 Sc 0.86 0.94 0.55 0.94 0.22 1.00 Cu 0.82 0.76 0.39 0.81 y0.04 0.85 1.00 Zn 0.19 0.31 0.48 0.23 y0.08 0.25 0.21 1.00 Ni 0.80 0.89 0.51 0.83 0.13 0.91 0.70 0.15 1.00 La 0.39 0.36 0.42 0.61 0.01 0.47 0.33 y0.03 0.58 1.00 Ce 0.45 0.59 0.40 0.73 0.24 0.64 0.44 0.05 0.64 0.89 1.00 Pr 0.31 0.33 0.36 0.57 0.05 0.42 0.27 y0.04 0.53 0.99 0.91 1.00 Nd 0.24 0.27 0.33 0.50 0.06 0.34 0.19 y0.05 0.47 0.98 0.88 1.00 1.00 Sm 0.23 0.28 0.32 0.50 0.09 0.35 0.19 y0.05 0.47 0.98 0.89 0.99 1.00 1.00 Eu 0.17 0.18 0.33 0.44 0.05 0.28 0.11 y0.05 0.40 0.96 0.85 0.98 0.99 0.99 1.00 Gd 0.15 0.19 0.31 0.44 0.11 0.27 0.10 y0.07 0.38 0.96 0.85 0.98 0.99 0.99 1.00 1.00 Tb 0.25 0.30 0.36 0.52 0.10 0.38 0.21 y0.02 0.48 0.98 0.90 0.99 1.00 1.00 0.99 0.99 1.00 Dy 0.27 0.37 0.37 0.56 0.19 0.44 0.23 y0.01 0.53 0.96 0.92 0.98 0.98 0.98 0.97 0.97 0.99 1.00 Ho 0.32 0.38 0.41 0.59 0.17 0.48 0.27 y0.01 0.55 0.97 0.92 0.98 0.97 0.98 0.97 0.97 0.99 1.00 1.00 Er 0.37 0.44 0.44 0.63 0.17 0.53 0.31 y0.00 0.61 0.96 0.93 0.97 0.96 0.96 0.95 0.95 0.98 0.99 1.00 1.00 Tm 0.52 0.59 0.50 0.77 0.18 0.69 0.47 0.11 0.72 0.92 0.96 0.92 0.89 0.90 0.87 0.87 0.92 0.95 0.96 0.97 1.00 Yb 0.53 0.61 0.42 0.75 0.24 0.70 0.48 y0.01 0.74 0.89 0.95 0.90 0.87 0.88 0.83 0.84 0.89 0.93 0.94 0.96 0.98 1.00 Lu 0.54 0.61 0.45 0.78 0.19 0.71 0.52 0.08 0.72 0.91 0.96 0.91 0.88 0.89 0.85 0.85 0.91 0.94 0.95 0.96 0.99 0.99 1.00 CerCe ) y0.32 y0.03 y0.61 y0.36 0.25 y0.28 y0.23 y0.01 y0.33 y0.73 y0.48 y0.67 y0.67 y0.66 y0.70 y0.70 y0.68 y0.68 y0.72 y0.71 y0.66 y0.60 y0.63

CerCe )

1.00

185

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The variability of d13 C org in the upper Permian and lower Scythian could either reflect a variable source of organic matter or transgressive–regressive events during the Scythian. However, we interpret the second order d13 C org anomalies as phases of enhanced oxidation of organic matter, possibly resulting from a corresponding lowering of the sea level ŽMagaritz and Holser, 1991. and combined with changes in the proportions of continental and marine contributions of organic matter to the sedimentary environment. 4.5. Major, minor, and selected trace elements A significant feature of the PrTr boundary sequence is a remarkable enrichment of lithophiles ŽSi, Al, Mg, K, Mn, Rb, Cs, Nb, Sc, Zr, Hf, Cr, Th, U, V, Y, REE., siderophiles ŽFe, Co, Ni, Mo, Sn, C., and chalcophiles ŽCu, Zn, Pb, As, Sb, Bi, S. in an at least 60-cm thick column Žfrom y50 to q10 cm. across the boundary level ŽTable 2, Fig. 4.. However, due to limited sampling performed further away from the immediate boundary level, we cannot precisely define the extent of this enrichment. Highest concentrations for the elements were measured in the PrTr boundary layer. Enrichment factors in this layer, compared to the concentrations in the upper Permian and lower Triassic limestone range from 1.5 to 137. A sudden decrease in calcite content is indicated by a decrease of CaO content from 49.06% to 23.08% and an increase in detrital content at the boundary is coincidental with a dramatic change in sedimentation rate. The geochemical composition of the boundary sequence reflects the geochemical characteristics of the marine chemical fraction vs. the detrital fraction, most probably introduced into the sedimentary environment via eolian transport. Petrographic observations show that the noncarbonate, mostly terrigenous component composed of quartz, feldspars, muscovite, clay and heavy minerals, is responsible for loading of Si, Al, and K and their complement of trace elements. This is confirmed by a very strong correlation between Si, Al, and K with each other Ž0.92 F r F 0.99. and Ti, Rb, Nb, Cs, Hf, Zr, Sc, Th, Ba, V, Co, Ni and Cu Ž0.78 F r F 0.95.. The strong correlations of the mentioned elements with Al and

K Ž0.83 F r F 0.99. confirm the importance of clay minerals as one of the main carrier phases for trace elements. Iron is correlated positively with Al and K Ž0.89 F r F 0.91., as well as Ti, Rb, Nb, Cs, Hf, Zr, Sc, Th, Ba, V, Co, Ni and Cu Ž0.80 F r F 0.96.. This suggests that Fe is present in two forms, a terrigenous contribution Žwith Ti. and hydrogenous contribution Žwith Co, Ni, Cu, V.. This confirms a dual source of Fe and some of the trace metals ŽCo, Ni, Cu, V. which are present in metal sulphides in the anoxic environment, probably in association with authigenic pyrite, andror partly as pure metal sulphide phases. Dyrssen Ž1985. pointed out the possible role of precipitating Fe sulphides andror Fe oxidesrhydroxides as scavengers for trace metals. Co and Ni should be incorporated into Ni, Co pyrite Žbravoite. as a result of slower kinetics for water exchange than Fe. Thus, they should tend to adsorb onto or coprecipitate with iron sulphide phases ŽLuther and Morse, 1998.. The chalcophile elements, such as As, Sb, Sn and Cu, tend to concentrate in reducing environments in the form of sulphides andror are adsorbed on organic compounds ŽVine and Tourtelot, 1970; Berner, 1971; Calvert, 1976.. Their slight enrichment in the upper Permian limestone relative to the lower Scythian limestone most probably indicates more anoxic conditions during the upper Permian. This is also confirmed by higher concentrations of Mo, V, and U in the topmost Permian limestone as compared to the lower Scythian limestone. Emerson and Husted Ž1991. have shown that sediments accumulating in basins with sulphate reduction occurring in the bottom water are a major sink for elements such as Mo. Thus, an enrichment of Mo in sediments might be diagnostic for sulphate-reducing conditions in bottom waters at the time of deposition ŽJacobs et al., 1987; Emerson and Husted, 1991; Crusius et al., 1996.. Bulk chemical compositions do not yield information on the partitioning of trace elements among the various components of the sediment, because we believe that the presence of a terrigenous component does not mask the relationships between the redox sensitive elements, such as Mo, and the prevailing environmental redox conditions. Thus, the temporal pattern of the Mo concentration ŽFig. 4. suggests that

T. Dolenec et al.r Chemical Geology 175 (2001) 175–190

oceanic anoxia was more or less typical for upper Permian sedimentation of the Zazar Formation, and that the transition to more oxygenated conditions occurred at the PrTr boundary. This observation is also supported by the abrupt decrease in U and V contents in the boundary level and a depletion in chalcophile elements, such as Zn which starts at 50 cm below the boundary. Oxygenation at the PrTr boundary is coincidental with the maximum of the Upper Permian marine regression. Slightly higher concentrations of U and V approximately 0.3 and 4.6 m above the boundary suggest that redox conditions changed in the earlier Triassic, resulting again in oxygen deficiency. The depletion of Mo, V, U and Zn and their irregular distribution in lower Scythian beds as compared to the Zazar Formation indicates that environmental conditions in the lower Triassic were likely less reducing and less steady than during the deposition of the major part of the upper Permian. 4.6. Abundances of TOC and whole rock sulphur (WRS) TOC and WRS contents in the upper Permian range from 0.66 to 3.47 wt.%, and from 0.16 to 0.65 wt.%, respectively. For the boundary layer, concentrations at 0.07 and 0.36 wt.% have been measured. In the lower Scythian, TOC concentrations are generally higher Žfrom 1.28 to 4.07 wt.%., while the WRS content is slightly lower, with values between 0.11 and 0.37 wt.%, than in the upper Permian. The concentration profiles across the PrTr boundary display a substantial decrease in TOC at the boundary and an increase in WRS content at 50 cm below the boundary ŽTable 2, Fig. 4.. However, as a consequence of an abrupt decrease in WRS, both curves are parallel while approaching the boundary level. Above the PrTr boundary, a sudden increase in TOC and a further decrease in WRS are observed. The marked decrease in TOC content towards the PrTr boundary can be attributed to a decrease in the sedimentation rate coupled with decreased surface water productivity. Additionally, decreased preservation of organic matter results from increasing oxygenation of the sedimentary environment at the boundary. Many studies have shown that the TOC content in marine sediments is controlled by primary

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organic production, by preservation of organic matter due to anoxic conditions, and by the rate of sedimentation ŽDemaison and Moore, 1980; Canfield, 1989, 1992; Dersch et al., 1991.. Microscopic observations indicate that sulphur is present as pyrite, occurring as a replacement mineral in microfossils and as very minute framboids. Coarse-grained pyrite was also found in limestones below the PrTr boundary, but they are rare in limestones above the boundary. The relatively high WRS content of the upper Permian limestone could be interpreted as a sign of extensive euxinic conditions, while the pronounced enrichment in WRS 50 cm below the boundary may indicate an additional flux of sulphur. This could result from an increase in erosion, resembling a similar increase in S content at the PrTr boundary in the Carnic Alps ŽWolbach et al., 1994.. 4.7. REE The REE concentrations in the Upper Permian limestone range from 22.5 to 161.6 ppm Žaverage 68.4 ppm., while those of the Lower Scythian limestone are considerably lower and exhibit values mostly in the range from 19.5 to 92.7 ppm Žaverage 49.8 ppm.. The highest REE concentrations were measured for the immediate boundary ŽTable 2, Fig. 4.. The PAAS-normalized REE pattern is characterized by an enrichment of the middle REE and a variable Ce anomaly ŽFig. 5.. REE have been utilized as proxy signals for reconstructing the redox and pH conditions of ancient environments ŽDeBaar et al., 1988; Liu et al., 1988; Zhong and Mucci, 1995.. Parekh et al. Ž1977. observed a close similarity between REE distribution patterns in the calcite phase of marine limestone and normal seawater, with no subsequent diagenetic redistribution. According to Brookins Ž1989., some REE may also be incorporated into metal sulphides under very alkaline, S-rich, carbonate-poor conditions. The negative correlation of individual REE with Ca and Sr Žy0.32 F r F y0.73. may suggest that REE are not directly incorporated into the calcite lattice. Low to moderate correlations of REE with Si, Al, and K Ž0.10 F r F 0.62. indicate that these ele-

188

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ments are neither in close association with clay mineralogy nor with silicates. On the other hand, a moderate to strong correlation between REE and Fe and S Ž0.60 F r F 0.84. may indicate their preferential association with metal sulphides as already proposed by Wang et al. Ž1986., DeBaar et al. Ž1988. and Brookins Ž1989.. This is also indicated by the positive correlation between REE and Co Ž0.44 F r F 0.78., Ni Ž0.38 F r F 0.74., Sb Ž0.22 F r F 0.49., V Ž0.18 F r F 0.61., As Ž0.31 F r F 0.50., Mo Ž0.14 F r F 0.34. and Cu Ž0.1 F r F 0.52.. REE also positively correlate with Th Ž0.24 F r F 0.67.. The relatively close association with Mg Ž0.48 F r F 0.74. may be a result of the close association between Mg and S Ž r s 0.78. and Mn Ž r s 0.59. on one hand, and between REE and S, Fe and Mn on the other.

ˇ ˇ ciably anoxic during the deposition of the Zazar Formation. The transition from Permian to Triassic is characterized by an abrupt decrease of CerCe ) values, which suggests increased oxidizing conditions in the topmost Permian and in the basal Triassic sea. The CerCe ) curve reaches a minimum value of 0.44 at the boundary, followed by a sharp increase to higher values in the lowermost Triassic. The evolution of the CerCe ) curve indicates that the redox environment drastically changed again in the earliest Triassic, resulting in oxygen deficient conditions and an absence of significant deep water circulation. This anoxic event may be the result of a rapid short-term transgression in the earliest Triassic, which led to the spread of anoxic water on the epicontinental shelves ŽHallam, 1989; Wignall and Hallam, 1992..

4.8. Cerium anomaly 5. Conclusions Another notable feature of the PrTr boundary sequence is a pronounced and variable Ce anomaly with CerCe ) values between 0.44 and 1.15 ŽFig. 4.. The cerium anomaly was calculated using the following equation: CerCe ) s Ce N rŽLa N P PrN . 0.5, where subscript N indicates a ŽPAAS. normalized value ŽTaylor and McLennan, 1985.. The redox sensitivity of Ce has been used as an indicator of paleoceanographic conditions ŽHu et al., 1988; DeBaar et al., 1985, 1988; Elderfield, 1988; Sholkovitz and Schneider, 1991; German et al., 1995; Schijf and DeBaar, 1995., expressed either as a positive or negative Ce anomaly. Previous studies ŽWang et al., 1986; Wright et al., 1987; Liu et al., 1988. have suggested that Ce depletion in the carbonate phase reflects Ce depletion in bottom waters and vice versa. At times of anoxia, a larger fraction of the Ce entering the ocean will remain trivalent and will behave like other trivalent REE, resulting in a more normal REE pattern in the overall carbonate phase with no pronounced negative Ce anomaly. Conversely, in oxic oceans such as those of the present day, Ce is relatively depleted as it is rapidly precipitated from solution with metal oxides ŽWright et al., 1987; Elderfield, 1988.. This depletion is reflected in Ce depleted REE patterns in marine carbonates ŽHu et al., 1988.. The positive Ce anomaly in the upper Permian limestone ŽFig. 4. indicates that sea water was appre-

The transition from the Permian to the Triassic is characterized by an abrupt change in sedimentation rate from 32 to 5 cmrka, a decrease in calcite content, an increase in detrital fraction, and by a strong disturbance in the carbonate and organic carbon isotopic compositions. These anomalies are supposed to reflect environmental changes of global importance, which resulted in a terminal Permian productivity crash. We suggest a causal connection between the carbon isotopic anomalies and the endPermian marine regression, but the exact controls remain complex and enigmatic. Results for selected redox sensitive elements, C and S concentrations, REE distributions and the Ce anomaly suggest more or less anoxic conditions during the deposition of the upper Permian limestone, the culmination of anoxic conditions about 50 cm below the boundary, followed by oxygenated conditions persisting into the earliest Scythian. The temporal pattern for CerCe ) indicates that redox conditions change again approximately 4.6 m above the PrTr boundary, resulting once more in oxygen-deficient conditions. Although there is widespread disagreement about the causeŽs. of the end-Permian mass extinction, our data from the Idrijca Valley section indicate that the upper Permian marine fauna would have been heavily stressed and drastically reduced by the increase in

T. Dolenec et al.r Chemical Geology 175 (2001) 175–190

marine anoxia, which we have observed at 50 cm below the boundary, in addition to the culmination of the late-Permian marine regression.

Acknowledgements This study was financially supported by the Ministry of Science and Technology, Republic of Slovenia, and Geoexp d.o.o. Trzic ˇ ˇ Slovenia. To both these institutions, we express our sincere thanks. Reviews by Lee Kump and two anonymous reviewers greatly improved the manuscript. We thank Harald Strauss for correcting the English style.

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