Multi-proxy record of the Austrian Upper Triassic Polzberg Konservat-Lagerstätte in light of the Carnian Pluvial Episode

We present a multi-proxy investigation of a lower Carnian basinal succession from Polzberg in the Northern Calcareous Alps (Lower Austria). A section comprising a unique Konservat-Lagerstätte was studied based on bio- and chemostratigraphy along with geophysical methods, yielding a detailed and robust stratigraphic calibration of the Polzberg succession. The Polzberg section revealed the paleoceanographic history and helped to identify a global climatic reversal, the Carnian Pluvial Episode. The age of the Upper Triassic Reingraben formation in the Northern Calcareous Alps is refined as the Austrotrachyceras austriacum Zone within the lower Carnian (Julian 2). Ammonoids and conodonts provide a detailed biostratigraphic subdivision that serves as a basis for analyses of the faunal distribution and the paleoenvironmental evolution of the Upper Triassic Reifling Basin. The succession includes lithological and facies changes similar to those of coeval units in the Tethys. The Carnian was characterized by a weak (~ 1‰) positive δ13C trend, punctuated by a negative shift during the lower Carnian corresponding to the initiation of the Carnian Pluvial Episode, a period representing the onset of early/late Carnian transitional global greenhouse conditions. Organic maturity parameters and the conodont alteration index (CAI) show that the thermal overprint of the Polzberg section is low. Biomarker proxies suggest that the organic matter of the uppermost Göstling formation is a mixture of marine and terrestrial material deposited in a dysoxic environment. Within the overlaying Reingraben formation, the amount of marine biomass decreased gradually upwards. Oxygen-depleted conditions, probably due to water-column stratification, continued during deposition of the Reingraben formation. Bacterial sulfate reduction played a major role in organic matter degradation.


Geological setting
The Upper Triassic outcrops at Polzberg are located on the western slope of Mount Schindelberg (1066 m), north of the Ois River, 4 km northeast of Lunz am See in Lower Austria.The locality Schindelberg is synonymous with the locality Polzberg (= Pölzberg 4,29,30 ; 1:50,000, geological map, sheet 71 Ybbsitz 32 , and sheet 72 Mariazell 33 ; Fig. 1).The northernmost tectonic elements of the Northern Calcareous Alps (NCA, Bajuvaric Units) in Lower Austria are the Frankenfels Nappe, followed to the south by the Lunz Nappe.Within the Lunz Nappe, the Reifling Basin 4,22 -an Upper Triassic intraplatform basin-is located between Polzberg and Großreifling.The exact position of the fossiliferous localities in the southern area of the Lunz Nappe within the lower, fossiliferous part of the Reingraben formation 34,35 was determined by GPS (global positioning system): N 47° 53′ 4.98″ and E 15° 4′ 28.15″, town Gaming, federal district Scheibbs.

Multi-proxy data on the Polzberg Konservat-Lagerstätte
The lithologies at Polzberg can be subdivided into three main lithostratigraphic units that, in stratigraphic order, are the Reifling Formation (formalized), the Göstling formation (= Göstlinger Kalke; not formalized; frequently noted as Göstling formation of the uppermost part of the Reifling Formation) and the Reingraben formation (= Reingraben Shales, Reingrabener Schiefer; not formalized; Fig. 2).The main lithological units are here understood as well-definable formations, but not yet formalized herein.

Lithology and microfacies
The Reifling Formation (− 4 to − 2 m, beds R1 and R2) was deposited on the paleoslope of the Reifling Basin 4 .It consists of light grey and grey-green nodular limestones composed of peloidal, filamentous wackestone and packstones (Figs. 3, 4).Bed-thickness (0.2-0.4 m) and carbonate content are stable throughout the formation.Rare fish and marine reptile remnants are visible.
At the base of Julian 2, the Reifling Formation (Trachyceras aonoides Zone, T. aonoides Subzone) was replaced by the Göstling formation (− 2 to 0 m, R3 and Göst top), which represents the Austrotrachyceras austriacum Zone (A. minor Subzone).The Göstling formation consists of black, mm-to cm-bedded calciturbiditic radiolarian limestones with organic-rich mudstones.The change from the Reifling Formation to dark, fine-laminated radiolaria wackestones to packstones of the Göstling formation is abrupt.At Polzberg, only about 2 m of radiolarian-rich Göstling limestones can be observed, whose thickness may have been reduced by tectonics.

Ammonoid biostratigraphy
No ammonoids are present in the Reifling Formation, and only rare trachyceratid specimens (Austrotrachyceras patroclus) are present in the Göstling formation.Austrotrachyceras patroclus is characteristic for the basal Austrotrachyceras austriacum Zone (A. minor Subzone).As described in previous papers 4,20,25 , the lowermost, fossiliferous part of the Reingraben formation can be dated based on the occurrence of Austrotrachyceras minor, Paratrachyceras haberfellneri and the accompanying Carnites floridus and Simonyceras simonyi, indicating a Julian 2 age (Julian 2/Ib; Fig. 2).The presence of these ammonoid members together with the mass occurrence of the benthic bivalve Halobia rugosa within the fossiliferous layers confirms the biostratigraphical age assignment as Julian 4,8,20,21,25 .

Palynology
The Reingraben formation contains terrestrial elements such as spores and pollen grains, and marine palynomorphs such as acritarchs, dinoflagellate cysts and chitinous remains including scolecodonts and foraminiferal linings (Fig. 6).Based on the palynological association the age is equal to the ammonoid A. austriacum Zone.The palynological composition corresponds to the densus-maljawkinae phase of Van der Eem 38 and to the Aulisporites astigmosus assemblage 39 , corresponding to the Duplicisporites continuus assemblage of Roghi 40 .
No obvious changes in the palynological association were found within the Reingraben formation of Polzberg, while the Lagenella martini assemblage is present in the basal part of the overlaying as in introduction Lunz Formation 8,39 .The palynology here is characterized by the presence of a rich terrestrial association mainly represented by the typical Carnian sporomorphs of southern Europe 8,39,[41][42][43] .Sporomorphs belonging to Azonotriletes (Concavisporites, Calamospora, Todisporites, Punctatisporites, Lunzisporites, Verrucosisporites and Lycopodiacidites), Zonotriletes and Cavatomonoletes (Kraeuselisporites and Aratrisporites, respectively) have been documented.The pollen association is composed of the monosaccites group characterized by the genera Enzonalasporites, Vallasporites, Patinasporites and Pseudoenzonalasporites, and of disaccites with the genera Alisporites, Ellipsovelatisporites and Ovalipollis.Among Circumpolles the main genera have been ascribed to Camerosporites and Duplicisporites.Monosulcates (Cycadopites) and Aletes are present but not abundant.Marine elements are composed by acritarchs (Micrhystridium sp.) and green algae belonging to the chlorophyta (Tasmanites spp.).The occurence of Azonotriletes and Aulisporites spp.reflects the presence of ferns, cycads and cycadalean swamp or marsh vegetation (floodplain vegetation), whereas the abundant presence of circumpolles and bisaccates indicate coastal pioneer vegetation and hinterland upland elments, respectively.

Stable carbon isotopes-δ 13 C carb
The δ 13 C carb values from the Polzberg section range from a minimum of − 7.89‰ (Po layer D in the Reingraben formation) to a maximum of + 3.05‰ (Po Reif R2; Reifling limestones) (Fig. 7, Supplementary Information), showing a stepwise significant negative trend.The δ 13 C carb values in the Reifling limestones range between + 2.64 and + 3.05‰ with a mean value of + 2.84‰, which decrease in the Göstling formation to + 0.80 to + 1.89‰ (mean of + 1.52‰).Successively, the δ 13 C carb values oscillate between − 1.72 and + 1.19‰ in the Reingraben formation, with a mean of − 0.04‰ (Fig. 7, Supplementary Table 1).The variation of δ 13 C carb values is lower in the lower versus upper Reingraben formation.The average δ 13 C carb value of intercalated dolostone layers is − 2.34‰, with one significatly more negative value of − 7.89‰ at Po layer D (shift of approx.2-7‰).
In the Reifling limestones at the base of the studied section, the values are very high (87-95%; Supplementary Table 1).In the Göstling limestones, they are lower (58-79%) and decrease towards the top.Values in the Reingraben formation (Fig. 7) decrease towards the more clayey top from ~ 80 to 6-7% in the uppermost layers.The intercalated grey dolostone layers contain 79.1% (layer F) to 86.9% (layer D) calcite equivalent .The decrease in the upper part of the section reflects the more clayey lithology.The correlation between the CaCO 3 content and grey-scale is weak (correlation coefficient R 2 = 0.39), with limestones being brighter than dark marls.

Rock-Eval data
These data are listed in the Supplementary Information.The HI value is plotted versus depth in Fig. 7. HI values of the Reifling limestones are very low (< 60 mgHC/gTOC), indicating the presence of kerogen Type III to IV.From left to right, ammonoid zonal scheme micro-and nannofossil data, logs and layer numbers, gamma-ray and magnetic susceptibility, CaCO 3 , total organic carbon (TOC), sulfur (S), carbon/ nitrogen ratio (C/N); isotope bulk rock δ 13 C and δ 18 O, organic material δ 13 C, isotope bulk rock δ 34

Molecular composition of hydrocarbons
Rock extracts from the organic matter-rich sample from the Göstling formation (Göst Top; 1.97% TOC) and seven samples from the Reingraben formation with relatively high organic matter contents (0.76-1.32% TOC) were investigated regarding their biomarker distribution.Concentrations and ratios of selected organic compounds are plotted versus stratigraphic height in Fig. 8 8A).
The upward increase in the relative proportion of long-chain n-alkanes is also evident in the ratio n-C 27+29+31 /n-C 15+17+19 , the so-called terrestrial/aquatic ratio (TAR 44 ).This ratio is significantly below 1 in the Göstling and lower part of the Reingraben formation; about 1 in the middle part of the Reingraben formation; and significantly above 1 in the upper part (Fig. 8B).
The carbon preference index (CPI; according to Bray and Evans 45 ) increases gradually upwards from 1.24 to 1.71 (Fig. 8B).Positive correlations exist between CPI and the amount of long-chain n-alkanes (R 2 = 0.79) and between CPI and TAR (R 2 = 0.65).
The pristane/phytane (Pr/Ph) ratio 46 is slightly above 1 in the sample from the Göstling formation (1.28) and in the Reingraben sample at 100 cm (1.06), and is significantly lower (0.27-0.67) in the remaining samples (Fig. 8C).
C 27 Diasteranes are present in all samples in lower amounts than their C 27 regular sterane counterparts.The C 27 diasterane/C 27 regular sterane ratio varies between 0.29 and 0.42 and is especially high in the Göst Top samples.

Magnetic susceptibility and gamma-ray profiles
The magnetic susceptibility (χ) and gamma-ray (GR) values differ markedly between the lower calcareous part of the Reifling and Göstling limestones (Trachyceras aonoides Zone) and the marly to shaly upper part (Austrotrachyceras austriacum Zone) of the Polzberg section, showing a significant trend throughout the entire section (Fig. 7, Supplementary Information).Low gamma-ray values (in cps) of the Reifling limestone log oscillate around a mean of 14.8, with a maximum of 23 at Po/R1 at − 3.7 m and minimum of 10 at Po/R2 at − 2.7 m (Fig. 7).Slightly increased gamma-ray values of the Göstling limestone log show a mean of 16.7, with a maximum of 25 at Po/R1 at − 3.7 m and minimum of 12 at Po/R2 at − 2.7 m (Fig. 7).Remarkably higher gammaray values of the Reingraben formation oscillate around a mean of 35.8 (maximum 56 at Po/layer D, minimum 25 at Po/170 cm and 210 cm), without a significant trend (Fig. 7).

Maceral analysis
Because of the generally low organic matter content, the maceral analysis can be at best considered semiquantitative.Nevertheless, percentages of maceral groups are presented in Fig. 8l.This figure shows that liptinite is the prevailing maceral group.Liptodetrinite and alginite are the main liptinite macerals, but sporinite occurs as well.Vitrinite and inertinite (including recycled vitrinite) are present in subordinate amounts.Overall, the organic matter is of autochthonous marine and allochthonous terrigenous origin.

Interpretation
Thermal maturity Rock-Eval parameters T max (average 431 °C) and PI (0.03) show that the organic matter is thermally immature.Maturity-related biomarker parameters (20S/(20S + 20R) ratio of C 29 steranes: avg.0.29; 22S/(22S + 22R) ratio of C 31 hopanes: avg.0.52) suggest a maturity at the transition from immature to early oil window.This agrees with vitrinite reflectance values of nearby coals from the overlaying as in introduction Lunz Formation (0.45-0.71%Rr 53 ) and of a nearby drift wood (Sau1 "Gagat"; 0.72 ± 0.03%Rr).In any case, a significant influence of diagenesis on organic and inorganic parameters can be excluded.

Organic matter richness
The light grey limestones of the Reifling Formation contain very low amounts of TOC (≤ 0.15%), suggesting poor conditions for organic matter preservation.Hence this interval is not further discussed here.
The laminated dark Göstling formation contains partly high TOC contents (max.1.97%).TOC/S ratios of these samples are typically high (> 2.8), suggesting sulfur limitation.TOC and carbonate content are not correlated.With relation to organic matter characteristics, three intervals can be distinguished within the Reingraben Shale.
• The lower interval (0-80 cm) is characterized by strongly varying but locally relatively high TOC (0.59- 1.32%) and HI values (78-293 mgHC/gTOC; Fig. 7).Clear differences exist between the abundance of OM in mm-thin shaly (< 10% carbonate) and marly layers (44-55% carbonate).TOC contents are low in shaly layers (< 0.8%), but increase with carbonate contents (Fig. 9A).In contrast, TOC contents are higher in marly layers (0.9-1.4%), but decrease with increasing carbonate content.S2 values show a strong correlation with TOC (R 2 = 0.97; Fig. 9B).The slope of the regression line suggests that the true HI is in the order of 470 mgHC/gTOC and significantly higher than those calculated from measured S2 values.This points to a strong mineral matrix effect 54 or the presence of two populations of organic matter: inert and reactive OM 55 .As data from different lithologies plot along the regression line, the data likely reflect the presence of largely inert, probably terrigenous OM (~ 0.5 %) and varying amounts of aquatic OM with a HI of ~ 470 mgHC/gTOC.
The general positive relation between TOC and carbonate contents for low carbonate samples (Fig. 9A) suggests that the amount of aquatic OM may be controlled by the productivity of calcareous organisms.In contrast, the high percentage of carbonate minerals in marly rocks results in dilution of OM (and a negative relation between calcite and TOC; Fig. 9A).
• The middle interval (100-200 cm) is characterized by a gradual upward decrease in TOC (1.29-0.85)and HI (206-116 mgHC/gTOC).The general TOC-calcite and TOC-S2 relations are similar to those in the lower interval (Fig. 9A).They show that the change of a positive effect of carbonate content on TOC to a negative diluton effect is at about 30% carbonate.The average HI of the aquatic organic matter in the middle interval is somewhat lower (~ 340 mgHC/gTOC) in the middle than in the lower interval (Fig. 9B).• The upper interval (202-340 cm) is characterized by the presence of thin carbonate layers with very low TOC contents (< 0.5%) intercalated into low carbonate background sediments with low to moderate TOC (0.76-0.94%).In this interval the dilution of OM by carbonate minerals is especially evident (Fig. 9A).The HI of the aquatic OM is ~ 225 mgHC/gTOC, while the HI of the "inert" OM may be on the order of 50 mgHC/ gTOC (Fig. 9B), suggesting the dominance of partly inert terrigenous OM in carbonate-rich layers.

Origin of organic matter
We used molecular parameters, Rock-Eval parameter HI, and maceral data to determine the organic matter type in samples from the uppermost Göstling and the Reingraben formations.Epicuticular waxes produce long-chain n-alkanes with a significant odd-to-even preference.Thus, both the ratio of long-to short-chain n-alkanes (TAR 44 ) and CPI 45 are often used to determine the relative amount of terrigenous organic matter 56 .This approach is also supported by the observed positive correlations between CPI and TAR (R 2 = 0.65) and between CPI and the amount of long-chain n-alkanes (R 2 = 0.79).Since CPI and TAR reflect the relative contribution of land-plants, it is unsurprising that HI is negatively correlated with TAR (R 2 = 0.52) and CPI (R 2 = 0.71).
Based on these observations, significant variations in organic matter input occurred during deposition of the studied interval: • The organic matter in the uppermost Göstling and lower Reingraben formation (< 80 cm) is characterized by a high amount of aquatic organisms mixed with land-plant derived organic matter.While lamalginite is present, petrographic evidence for extensive algal or bacterial mats is missing.Interestingly, the Göstling sample shows the highest concentration of C 29 steranes, often attributed to land-plants.However, C 29 steranes may also be produced by other organisms 56 .Furthermore, the content of liptinite macerals in the Göstling sample is surprisingly low.Considering that the total amount of macerals identified in this TOC-rich sample is lower than expected, much of the organic matter may not be visible under the microscope.While TAR and CPI are similar in the Göstling and the lower Reingraben samples (Fig. 8B), the steranes/hopanes ratio is much higher in Göstling (1.45 versus 0.21; Fig. 8G).Because hopanes are produced by prokaryotic organisms, this suggests a significantly higher contribution of bacterial biomass in the lower Reingraben formation.• In the middle part of the Reingraben formation (100-200 cm), HI is reduced (Figs.7, 9), suggesting a decrease in aquatic organic matter.This interpretation is supported by the observed moderately high TAR and CPI values in this interval.Slightly increased steranes/hopanes ratios (0.56-0.58;Fig. 8G) suggest a decrease in bacterial activity compared to the lower part of the Reingraben shale.• The upper interval at Reingraben (202-340 cm) displays the lowest HI (Figs. 7, 9) and the highest TAR and CPI values (Fig. 8B).This is clear evidence for a dominance of land-plant derived organic matter.This interpretation also agrees with the elevated C 19 /C 23 tricyclic terpanes ratio 57 (Fig. 8H) and a relatively high abundance of terrigenous vitrinite and inertinite macerals (Fig. 8L).
Overall, the relative proportion of terrigenous OM increases from the Göstling formation to the lower, middle and upper intervals of the Reingraben formation.Since the TOC contents decrease in the same direction, this reflects either a gradual decrease in marine OM productivity or a dilution of marine OM due to increased clastic www.nature.com/scientificreports/input rather than an absolute increase in terrigenious OM.High concentrations of 4-methylsteranes in Göstling and the lower Reingraben formation (Fig. 8e) suggest that dinoflagellates contributed to the aquatic biomass 58,59 (see also palynological results).

Redox and salinity conditions
The Pr/Ph ratio 46 and the homohopane index (HHI 48 ) are widely used redox parameters.Moreover, the concentration of aryl-isoprenoids has been used to determine photic zone anoxia 52 .The determined Pr/Ph ratios decrease from the Göstling to the upper Reingraben formation from 1.3 to 0.4 (Fig. 8c).This suggests dysoxic conditions in the lower part of the studied section and strictly anoxic conditions in the upper part.In contrast, the HHI suggests a gradual upward decrease in the degree of anoxia (Fig. 8F), which is also indicated by the stratigraphic trend of aryl-isoprenoids within the Reingraben formation (Fig. 8K).However, aryl-isoprenoids are rare in the sample from the top of the Göstling formation.Summarizing, different redox parameters show different depth trends.Although this may reflect temporally different redox conditions in different portions of the water column, these results should not be over-interpreted.Nonetheless, the laminated sediments were clearly deposited under oxygen-depleted conditions 20,21,24 .
The oxygen depletion was probably related to salinity stratification 20,21,24 .This interpretation is supported by high Gammacerane Index (GI) values, which peak (0.73) in Göstling formation but are also high in Reingraben samples (0.23-0.34;Fig. 8J).Interestingly, GI is slightly higher in the upper than in the lower and middle intervals of the Reingraben formation.
The moretane/hopane and the Ts/(Ts + Tm) ratios show a very strong negative correlation (R 2 = 0.96).Both ratios are maturity parameters, but are also strongly facies dependent 56 .For example, alkaline conditions favour formation of moretane over hopane and Tm over Ts 56 .Hence, high moretane/hopane and low Ts/(Ts + Tm) ratios in the upper Reingraben formation (Fig. 8H, I) may reflect an increase in pH.However, as both ratios are also highly correlated with C 19 /C 23 tricyclic terpanes ratios (Fig. 8H; R 2 = 0.89 and 0.88, respectively), an influence of organic matter type cannot be excluded.

Bacterial sulfate reduction
Very low δ 34 S values are characteristic for biogenic sulfate reduction (BSR).The vertical distribution of δ 34 S values suggests two upward trends with increasing degree of BSR for the Reingraben Shale (Fig. 7).A slightly lower degree of BSR is indicated for the dolomitic layers.The degree of BSR can be estimated independently using the Sulfate Reduction Index (SRI = (TOC + TS/1.33)/TOC) of Lallier-Verges et al. 60 .This value is high for Reingraben Shale samples (1.30-2.85).

Evolution of the Polzberg succession
The depositional environment was influenced by the evolution of climate and environmental conditions.The sequence shows three main lithologies and two main lithological transitions with the grey radiolaria limestones of the Reifling Formation, the transitional laminated black silicious limestones Göstling formation and the laminated argillaceous shaly Reingraben formation.Similar lithological sequences have been observed in Upper Triassic successions of several other localities in the NCA of Austria (e.g., Göstling, Grossau, Mendlingbach, Saugraben, Scheiblinggraben, Steinbachgraben, Rehberg 10,22 ).Although minor differences occur in the lithotypes and in their thicknesses, the lithological boundaries occur consistently at a regional scale.This is an evidence for a change in basinal sedimentation throughout the NCA and the western Tethys during the Late Triassic.
The grey-scale factor is indicative of a higher detrital content, marking an increased siliciclastic input in the clay versus the lower limestone deposits, especially in the Reingraben formation 4 .Grey scale values show a positive correlation with CaCO 3 contents (i.e.light grey with high carbonate, dark gray to black with low carbonate; R 2 = 0.39) and negatively correlate with TOC values as bright colours in the Reifling Formation and intercalated layers A-F correspond to low TOC (R 2 = 0.14) and S values (R 2 = 0.33).Dark lithologies go with increased TOC and S values respectively.The CaCO 3 content from rock samples (Po Reif base to Po 340 cm, Fig. 7) reflects a change from a radiolarian limestone-dominated (Reifling and Göstling formations) to a more argillaceous marly facies (Reingraben formation).These results corroborate those obtained from gamma-ray (GR) and magnetic susceptibility logs from the calcareous basal Reifling Formation over the dark laminated limestone of the Göstling formation to the arigillaceous mudstones of the Reingraben formation.Increasing values in GR and MS hint at a higher clay mineral content, supported by thin sections and microfacies analysis suggest an environment with fluctuating carbonate productivity or oscillating siliciclastic and terrestrial input.
Note that the correlation between TOC and N is very poor (e.g.R 2 = 0.28 for Reingraben formation), potentially indicating that some N is non-organic.Organic nitrogen derives mostly from peptides and amino acids of planktonic organisms 61 and can therefore be used as an indicator for planktonic productivity.These data are comparable to modern data by [62][63][64][65][66] who showed that the distribution and abundance of N are correlated to TOC.Nitrogen availability often limits biological production, both in terrestrial and in marine environments.Total nitrogen (TN) was determined in order to calculate C/N (TOC vs. TN) ratios.The C/N ratio reflects the origin of organic matter: organic matter (OM) of terrestrial origin typically results in C/N > 15, while marine OM features ratios of ≤ 5 65,67,68 .The low C/N ratio in the lowermost part of the Polzberg section within the light grey calcareous Reifling Formation suggests dominant marine organic matter and an increasing terrestrial influx in the upper part of the dark-laminated Göstling and argillaceous Reingraben formation, in agreement with the deposition on a continental margin.
Palynomorphs from the top Reifling Formation (Julian 1), over the Göstling (Julian 2/Ia) and Reingraben formations (Julian 2/Ib) to the siliciclastic Lunz Formation (Julian 2/II), show a trend from dryer climate to warmer, more humid conditions 8,39 .A temperature maximum was detected for the Reingraben formation after www.nature.com/scientificreports/ a positive shift of 2 °C at the transition from the Göstling to Reingraben deposits 8 .The Reingraben formation is characterized by terrestrial and marine palynomorphs (spores, pollen and acritarchs versus dinoflagellate cysts, respectively).Spores belong to ferns, cycads and cycadaleans and are indicative of swamp or marsh vegetation.The abundant presence of gymnosperm pollen grains (circumpolles and bisaccates) indicates coastal pioneer vegetation and hinterland upland elements.Acritarchs and chlorophyta are the marine elements.The palynomorphs show a transition from xerophytic to more hygrophytic assemblages, reflecting a climate change during the onset of the CPE.The δ 13 C bulk values (Fig. 7) are coherent with the evolutionary stable isotopic trends of normal marine deposits measured in other, comparable Carnian sections within the Tethyan Realm.In particular, the δ 13 C carb values in the Reifling limestones have a mean of + 2.84‰, decrease in the Göstling formation to a mean of + 1.52‰, and oscillate in the Reingraben formation (excl layers A-F) around a mean of − 0.04‰ (Fig. 7, Supplementary Information).This negative shift of the mean value of ca.1.5‰ δ 13 C bulk values in the lower Carnian Polzberg section occurs in the transitional zone between the Trachyceras aonoides and Austrotrachyceras zones (i.e.Julian1 to Julian 2 8 ).
The δ 13 C org values from the Lunz am See area including the Polzberg section were described in a study on carbon isotope records to examine carbon cycle perturbation from the Southern Alps (Dolomites, Italy) and the Transdanubian Range (Hungary) (Dal Corso et al.) 7 .This transitional phase, the Julian 1-Julian 2 boundary interval 7 mirrors the onset of the Carnian Pluvial Episode, manifested in the first negative carbon isotope excursion (CIE 1).This shift can reach 4‰ in terrestrial and biomarker material, and approx.2‰ in bulk organic matter.
δ 13 C org values in the Reifling limestones exhibit a mean of − 25.85‰, in the Göstling formation with a mean of − 27.52‰, and oscillate in the Reingraben formation around a mean of − 26.49‰ (Fig. 7, Supplementary Information).Hence, a negative shift of the mean of ca.1.67‰ δ 13 C org in the lower Carnian Polzberg section reflects the transitional zone of Julian1 to Julian 2, followed by a slight positive shift of ca.1.03‰.The shifts can be compared and are in the range of previously described fluctuations 2,7,8,69 .A negative shift of about 3.8‰ δ 13 C org was reported for the transition from the topmost Reifling Formation to the Göstling formation.δ 13 C org from the Polzberg section ranges from − 28.75 to − 23.45‰ (max range 5.3‰), similar to ranges given by Dal Corso et al. 7 with − 28.4‰ to − 21.8‰ (max range 6.6‰; compiled section Göstling and Polzberg) and by Mueller et al. 8 with − 28.5‰ to − 23.1‰ (max range 5.4‰; compiled section Göstling; Großreifling and Polzberg).These perturbations in carbon curves and dataset are linked to major changes in marine sedimentology, hence lithology, caused by an abrupt rise in rainfall and terrestrial weathering at the onset of the CPE.Intensified Pangaean monsoon activity led to enhanced terrestrial and freshwater influx into basinal environments, caused by the Wrangellia vulcanism, a Large Ingneous Province with mega flood basalts 1,2,7,8,14 .The negative shift in NCIE-1 (= CIE 1) mirrors an enhanced injection of δ 13 C-depleted CO 2 into the atmosphere-oceanic system [8][9][10]70,71 . At Pozberg, the onset of the Göstling formation with calciturbiditic, radiolaria limestones is contemporaneous with the demise of the Wetterstein carbonate platform, coinciding with a negative shift in δ 13 C carb of around 2‰ around this transition from the basal peloidal-filamentous Reifling Formation, over the laminated radiolaria-rich Göstling formation to the lowermost 5 m of the argillaceous Reingraben formation 8,10 .Note that a coeval rise of CCD was documented in the Lagonegro Basin (southern Italy) 6 .
The carbon isotope values are consistent with those observed in other Upper Triasic records of the western Tethyan Realm (Austria, Germany, Hungary, Italy, Spain, Tunisia, Turkey).The herein presented δ 13 C record exhibits the typical range of values for Carnian hemipelagic to basinal deposits, which in the present case are basal normal marine carbonates to brackish-influenced shales at the top.Single peaks and trends in the δ 13 C record are also important for correlation with other reference sections.
The variation of χ and GR within the sedimentary record of the Polzberg section is a direct function of the variation of the mineral contents (e.g., clay minerals, magnetic minerals).χ includes contributions from all minerals (diamagnetic, paramagnetic and ferrimagnetic) present in the sediment in proportion to their abundance.χ and GR values reflect the direct function of the terrigenous and siliciclastic input into the marine Reifling Basin.The considerable rise in χ and GR values from the basal Reifling limestones, over the Göstling limestone to the argillaceous and shaly lithologies of the Reingraben formation mirror the change of climatic conditions during the Julian 1/Julian 2 interval, with increased temperatures, humidification and terrestrial and freshwater influx.Subsequently, higher radioactivity values reflect a higher clay content toward the top of the section.Climatic conditions during the CPE with enhanced rainfall triggered erosive mechanisms on already existing terrestrial areas.This boosted the terrigenous, brackish and freshwater influx into intraplatform basins.These processes were associated with an enrichment of radiant minerals and trace elements.The peak if this redepositional phase was marked by the climax of siliciclastic influx at the Julian 2/II with the prograding deltaic deposition and total infill of the Reifling Basin with sandstones of the Lunz Formation (with famous plant fossils and coals).

Conclusions
Integrated high-resolution studies on the Polzberg section reveal the early Late Triassic paleoceanographic history of the intraplatform Reifling Basin area.Paleontological, lithological, geochemical and geophysical data show evidence for deposition during the onset of a humid and warmer episode in the Carnian, known as the Carnian Pluvial Episode (CPE).A comprehensive analysis of the Polzberg sections from the Northern Calcareous Alps established a reference section in the western Tethyan region, including the Polzberg Konservat-Lagerstätte.www.nature.com/scientificreports/ The documented lithological variations reflect the paleoenvironmental and climate change during the lower Carnian Austrotrachyceras austriacum Zone (Julian 2; Car 2).A lithological change from carbonate-dominated limestones, over black laminated limestones up to arillaceous to shaly deposits appears to be triggered by the Wrangellian Large Igneous Province (LIP).The enhanced LIP volcanism increased emissions of volcanic gases into the atmosphere, resulting in positive shifts of temperatures and rainfall along with negative shifts of δ 13 C. Carbonate systems (i.e.sedimentation) were interrupted or disappeared, terrestrial and freshwater influx increased, subsequently forming anoxia in basinal environments.Carbonate factories of the Wetterstein platform with microbially dominated ecosystems disappeared and shifted to less productive metazoan-dominated environments.Negative carbon isotope excursions (NCIEs) mirror the changes in the carbon precipitations.NCIE-1 marks the onset of the CPE in the Tethys Ocean and, at the Polzberg section, at the base of the Göstling formation at the transition from Julian 1 to Julian 2. Paleontological (ammonoids, bivalves, fish, conodonts, radiolaria, plants), lithological (thin sections, grey scale), geochemical (δ 13 C carb , δ 13 C org, δ 34 S, δ 15 N, CaCO 3 , TOC, N, S), organic matter (OM) and hydrocarbons (n-alkanes, steroids, hypanoids), and geophysical data (magnetic susceptibility, gamma-ray) clearly show a transition from a carbonate to an argillaceous world during the formation of the Polzberg Konservat-Lagerstätte deposited during the onset of the globally warmer and more humid CPE.This indicates that the Reifling Basin and the adjacent Wetterstein platforms were affected by peculiar oceanographic conditions during this episode.The results highlight the complexity of the western Tethyan (Mediterranean) oceanography and special depositional basinal environments during prominent climatic perturbations.This study adds an important piece to our understanding of Carnian paleoceanographic history and climate.High-resolution studies on biostratigraphy, geochemistry and geophysics of the Polzberg section and the formation of the Polzberg Konservat-Lagerstätte from the NCA provide a reliable reference for future analyses of paleontological and geological issues.
The organic matter here is a mixture of aquatic and terrigenous material.The relative amount of aquatic biomass peaks in the topmost sample from the Göstling formation and the lower part of the Reingraben formation and decreases upsection.Bacterial sulfate reduction was an important process in organic matter degradation.Biomarker proxies indicate relatively high pH values during deposition of the upper Reingraben formation.
The section suffered a low thermal overprint as suggested by CAI and organic maturity parameters.

Microfossils
A total of 52 thin-sections and 32 washed residue samples were made to study the microfacies and biostratigraphically important microfossils (conondontophorids, radiolaria, planktonic foraminifera).These data provide a high-resolution biostratigraphy, at least for the lowermost part of the section.Conodonts were investigated and photographed at the Department of Geoscience (University of Padova, Italy) and radiolaria at the Department of Physics and Geology (Università degli Studi di Perugia, Italy).Conodont and radiolaria specimens are stored in the Natural History Museum in Vienna (NHMW, Austria).

Lithology and facies
A total of 76 thin-sections were made to study the lithology, microfacies and biostratigraphically important microfossils (radiolaria, ostracods, planktonic foraminifera).

Geochemistry
Samples for CaCO 3 , TOC, S, and N content were collected with a spacing of 0.1-0.2m and combined with greyscale quantification.The samples were analyzed for total carbon (TC; %), total sulfur (TS; %) and total organic carbon (TOC; %) at the Chair Energy Geosciences (Montanuniversitaet Leoben; Austria) using an Eltra Helios CS elemental analyzer.TOC contents were measured on samples pre-treated with concentrated phosphoric acid.The accuracy of C and S measurements is better than 1%.Calcite equivalent percentages were derived using the equation calcite equiv.= 8.334 × (TC − TOC).The accuracy is better than 1%.In addition, calcium carbonate content was measured using a carbonate bomb technique 72 at the Institute for Earth Sciences (Karl-Franzens-University, Graz, Austria).The accuracy is in the order of 2-3% 73 .Main and trace elements were examined using the energy dispersive X-ray fluorescence (EDXRF) device Epsilon 5 (PANalytical).Each sample of 4 g, powdered < 60 µm, was mixed with 0.9 g wax (MERCK, Hoechst Wachs C Mikropulver) in a pebble mill Kugelmühle (Retsch MM200) and pelletized in a SPECAC press tool.

Organic geochemistry
Eight samples were selected based on their TOC content and their stratigraphic position for biomarker analyses (Chair Energy Geosciences; Montanuniversität Leoben; Austria).5 to 10 g of the sample material was extracted for about 1 h at 75 °C and 100 bar using dichloromethane as solvent and a Dionex ASE 350 Accelerated Solvent Extractor.The extract was concentrated to ⁓ 0.5 ml using a Zymark TurboVap 500 closed cell concentrator.Afterwards, asphalthenes were precipitated from a hexane: dichloromethane solution (80:1 according to volume) and separated by centrifugation.The hexane-soluble fractions were further separated into heterocompounds (NSO), saturated and aromatic hydrocarbons by medium-pressure liquid chromatography using a Köhnen-Willsch MPLC instrument 76 .Proportionate amounts of internal standards (n-tetracosane or squalene for aliphatics and 1,1ʹ-binaphthyl for aromatics) were added to each sample prior to measurement.
The n-alkanes and isoprenoids in the saturated hydrocarbon fractions were analyzed using a gas chromatograph (Trace GC-Ultra) with a flame ionization detector (GC-FID).The GC was equipped with a 50 m HP-PONA capillary column (inner diameter [i.d] 0.2 mm, 0.50 µm film thickness).After splitless sample injection at 270 °C, the oven temperature was increased from 70 to 310 °C followed by 35 min isothermal period.
Specific biomarker molecules in the saturated and aromatic fractions were analyzed by gas chromatographymass spectrometry (GC-MS) using a Thermo Scientific Trace GC-Ultra equipped with a 60 m TG/DB-5MS fused silica capillary column (i.d.0.25 mm; 0.25 µm film thickness) coupled to a ThermoFisher ISQ mass spectrometer.The GC oven temperature was initially programed from 40 °C/min, held for 2 min and ramped to 310 °C with 4 °C/min followed by an isothermal period of 40 min.The sample was injected in split mode with a split ratio of 20 at 260 °C using helium as carrier gas.The spectrometer was operated in the EI (electron ionization) mode with a mass-to-charge ratio (m/z) scan range from m/z 50 to m/z 650 (0.7 s total scan time).To determine methylsterane concentrations, we further investigated aliphatic fractions of selected samples using a Trace™ 1300 GC (ThermoFisher) equipped with a 60 m TG-5MS fused silica capillary column (i.e.0.25 mm; 0.25 µm film thickness) attached to a ThermoFisher TSQ9000 triple quadrupole GC-MS/MS.The oven was set to hold 40 °C for 2 min and then heated to 310 °C with 4 °C/min, followed by a constant temperature period of 40 min.The sample was injected splitless with an injector temperature of 310 °C.
The raw data were processed using Xcalibur and Chromeleon data systems.The respective retention time for each compound within the mass spectra or total ion current (TIC) were used for their identification in addition to comparing the mass spectra with published data.Correction factors were appropriately applied for mass chromatograms to correct for fragment ions, whereas for TIC chromatograms, absolute concentrations were determined from the peak areas of the saturated and aromatic fractions in relation to their internal standards per sample.
A semi-quantitative calculation of the target compounds was conducted based on results of total ion current (TIC) chromatograms.Absolute concentrations were determined for the methylsteranes using their TIC chromatograms (also detectable using m/z 231) set in relation to an internal standard (5β(H)-Cholane) per sample.

Maceral analysis
Maceral analysis was performed on samples selected for biomarker analysis using white and fluorescence light, a 50 × oil immersion objective, and a Leica DM 4P microscope.At least 1000 points were counted, but given the generally low organic matter content, the results can be at best considered semi-quatitative.
The grey-scale data (0 = black, max.255 = white, no dimension) indicate relative brightness intensity and were obtained by scanning 26 polished rock samples on a flatbed scanner (Epson Perfection 4990 Photo; method in Lukeneder et al. 66,77 ).
A total of 32 samples were measured for bulk stable isotopes (δ 18 O, δ 13 C); additionally, δ 13 C , δ 18 O and 87 Sr/ 86 Sr ratios of primary calcite were measured from a few ammonoid shells (original shell material aragonite) and one gastropod shell.
Isotopic analyses were performed in the Stable Isotope Laboratory at the Institute of Earth Sciences, Karl-Franzens-University, Graz, using an automatic Kiel II preparation line and a Finigan MAT Delta Plus mass spectromenter.Samples for the isotopic composition were drilled with a 0.3 mm-diameter dental drill.Samples were dried and reacted with 100% phosphoric acid at 70 °C: International standard NBS-18 and an internal laboratory standard were analysed continuously for accuracy control.The standard deviation was less than 0.08‰ δ 18 O and 0.04‰ for δ 13 C. Isotopic date are reported in conventional δ notation relative to the Vienna Pee Dee belemnite (V-PDB) standard in ‰ units and calculated with SMOW (standard mean ocean water).All isotopic data were measured at the Institute for Earth Sciences (Karl-Franzens-University, Graz, Austria) and are reported in conventional δ notation relative to the Vienna Pee Dee belemnite (VPDB) standard in ‰ units.
The ammonoid shell composition and internal microstructure were analysed at the laboratories of the Department of Material Sciences and Process Engineering (University of Natural Resources and Life Sciences, Vienna) by SEM imaging on a Quanta™ 250 FEG from FEI (environmental scanning electron microscope with a Shottky field emission source FEG-ESEM) with an EDS tool for microanalysis by energy-dispersive X-ray spectroscopy.
Low-field magnetic susceptibility (χ, MS) was measured directly in the field with a hand-held, highly sensitive (1 × 10 -7 , measured in SI units; results are indicated throughout as 'value × 10 -6 SI'), low-level SM-30 MS meter www.nature.com/scientificreports/(GF Instruments) bearing a 50-mm pick-up coil.Bed-by-bed measurements were conducted at distinct beds (e.g.limestone or dolomite layers).This approach was replaced by a measuring interval of between 5 and 50 cm where distinct beds were missing (e.g.clay deposits, Reingraben formation); 112 measurements of MS were performed in sediments of the Reifling Formation (n = 12), Göstling formation (n = 10) and Reingraben formation (n = 90).Each single bed was measured at least three times and the average calculated.To prevent measurement errors due to the sensitivity of the hand-held instruments (uneven terrain and therefore variable distance between coil and rock surface), a fresh smooth surface was prepared for each single point measured.

Figure 1 .
Figure 1.Locality map of the Upper Triassic Polzberg area.(A) Polzberg area (white star) within the Northern Calcareous Alps (NCA, Lower Austria).(B) Paleogeographic position of the Polzberg Reifling Basin with indicated outcrop position in (C).(C) Palinspastic position of the Reifling Basin and the Polzberg locality with indicated transverse section (D).(D) Transect through the Reifling Basin with reconstruction of basinal environments low oxygen conditions (-O 2 ) and surrounding land with plants.(E) Laminated character of the Reingraben Shales at the Polzberg Konservat-Lagerstätte.(F) Outcrop situation at Polzberg, Reifling Formation and basal Göstling formation.Unit boundary marked by a white dashed line.(G) Basal Reingraben formation at Polzberg.(F, G) images by AL April 2023.White asterisks: position of the Polzberg Konservat-Lagerstätte (POLZ).WCP, Wetterstein Carbonate Platform; Hds, Hallstatt deepwater swells.NCA area in gray.(B-D) not to scale.Adapted after Lukeneder & Lukeneder 4 .(B) adapted after Lukeneder et al. 27 .Artwork of map in (C) by Mathias Harzhauser, NHMW.Prepared by AL using CorelDraw X7; www.corel draw.com.

Figure 4 .
Figure 4. SEM images from different fresh lithological surfaces of the basal Reifling Formation, the Göstling formation and the Reingraben formation form the Polzberg deposits.(A) Limestone with angular broken surface of the middle part of the Reifling Formation, Reifling R2. (B) Limestones wirth cherty angular area in the center of the Göstling formation, Top Göst.(C) Intercalated calcareous layer with angular dolomite crystals within rare clay flakes in the basal Reingraben formation, Polz 0 cm carbonate.(D) Wavy laminated argillaceous layers with domoinant clay flakes in the basal Reingraben formation, Po 10 cm clay.(E) Calcareous dolostone layer with anular dolomite crystals in the middle part of the Reingraben formation, Po layer A. (F) Claystone layer with angular dolomite crystals swimming in the clayey matrix within the basel Reingraben formation, Po 10 cm.(G) Calcareous dolostone-layer with angular dolomite crystals covered by clay flakes located in the upper part of the Reingraben formation, Po layer F. (H) Fine wavy laminated claystones of the upper Reingraben formation, Po 340 cm.Polz, Polzberg; Top Göst, Top Göstling formation.Scale bars 10 µm.Prepared by AL using CorelDraw X7; www.corel draw.com.

1
). Relatively strong variations occur in the upper part of the section.The values of intercalated dolostone layers are significantly less negative (shift of 3-6‰), with a mean of − 4.19‰ (− 5.48‰ [Po layer D] to − 2.82‰ [Po layer A].

Figure 7 .
Figure 7. Compilation log (Po) and biostratigraphical, geochemical and geophysical data of the lower Carnian Polzberg locality.From left to right, ammonoid zonal scheme micro-and nannofossil data, logs and layer numbers, gamma-ray and magnetic susceptibility, CaCO 3 , total organic carbon (TOC), sulfur (S), carbon/ nitrogen ratio (C/N); isotope bulk rock δ13 C and δ18 O, organic material δ 13 C, isotope bulk rock δ 34 S, grey-scale, and hydrogen index (HI).CPE, Carnian Pluvial Episode, range marked by a vertical black bar.Prepared by AL using CorelDraw X7; www.corel draw.com.