Integrated stratigraphy (radiolarians, calcareous nannofossils, carbon and strontium isotopes) of the Sinemurian – Pliensbachian transition at Mt. Rettenstein,

A 95 m thick succession of grey siliceous limestone and marl on Mount Rettenstein in the Northern Calcareous Alps allowed us to study well-preserved and diverse radiolarian assemblages across the Sinemur-ian – Pliensbachian boundary. The distribution of 31 most characteristic radiolarian species is presented. Am-monites of the Jamesoni Subzone, the topmost subzone of the Jamesoni Zone, have been previously found in the upper part of the section. The radiolarian data are complemented with calcareous nannofossil biostratigraphy, and carbon and strontium isotope analyses of bulk carbonate samples. A negative stable carbon isotope excursion (CIE) occurs in the middle part of the section and is correlated to the supraregional Sinemurian – Pliensbachian Boundary Event. Radiolarian assemblages below the negative CIE belong to the C anutus rockfishensis – Wrangellium thurstonense and Jacus ? sandspitensis radiolarian zones. Katroma clara and several other species belonging to the subsequent Canutus tipperi – Katroma clara Radiolarian Zone first occur at the beginning of the negative CIE. The first occurrence of the nannofossil Similiscutum cruciulus , which defines the base of the NJ4 Calcareous Nannofossil Zone, was recognized near the top of the negative CIE, 10 m above the inferred stage boundary. The strontium isotope ratio is due to diagenetic alteration of bulk micrite systematically shifted to higher values, but clearly decreases across the stage boundary. This decrease is compatible with the trend of the standard 87 Sr/ 86 Sr curve established in skeletal calcite. This is the first time radiolarian zones across the Sinemurian – Pliensbachian boundary in Europe can be tied to calcareous nannofossil zones and chemostratigraphy and also calibrated to chronostratigraphy. These results contribute to the stability of Lower Jurassic radiolarian zones and will allow the recognition of the Sinemur-ian – Pliensbachian boundary in deep-marine successions where ammonites are absent.


Introduction
The ammonite zonation developed in NW Europe is currently the only biochronological scale that allows a precise identification of the Sinemurian -Pliensbachian boundary in a rock sequence (Gradstein et al., 2012).At the GSSP (Global Boundary Stratotype Section and Point) in Yorkshire, UK, ammonites are the primary and unique marker; other fossils (bivalves, foraminifera, palynomorphs and ostracods) are associated but provide little information for the definition of the GSSP since they are scarce or long-ranging taxa (Meister et al., 2006).Precise ammonite stratigraphy in different basins of Western Europe has been used as a base for high-resolution carbon isotope studies in the UK (Jenkyns et al., 2002;Meister et al., 2006;Korte and Hesselbo, 2011;Price et al., 2016;Schöllhorn et al., 2020a), France (Bougeault et al., 2017;Peti et al., 2017), Portugal (Duarte et al., 2014) and Spain (Gómez et al., 2016).A characteristic negative carbon isotope excursion (CIE) has been detected near the stage boundary and named the Sinemurian -Pliensbachian Boundary Event (S-P Event; Korte and Hesselbo, 2011).It has been proven that this negative CIE is at least of supraregional extent (Korte and Hesselbo, 2011;Price et al., 2016) and may serve as a valuable stratigraphic marker.In the Tethyan realm, the S-P Event has been recognized in shallow as well as in deep-marine carbonates in the Southern Alps (Franceschi et al., 2014(Franceschi et al., , 2019;;Schöllhorn et al., 2020b), in Morocco (Danisch et al., 2019;Mercuzot et al., 2020) and in the Himalayas (Han et al., 2021).
Two Lower Jurassic radiolarian zonations meet approximately at the Sinemurian -Pliensbachian boundary (Fig. 1).Both are composed of concurrent-range zones constructed with the Unitary Association (UA) Method.The Hettangian -Sinemurian zonation (Carter et al., 1998) was constructed exclusively with radiolarian collections from Haida Gwaii, British Columbia, Canada.The Pliensbachian to Aalenian zonation (Carter et al., 2010) is global but all radiolarian-bearing sections outside Haida Gwaii are devoid of ammonites.Both zonations are thus calibrated to North American ammonite zones.Consequently, the Canutus tipperi -Katroma clara Radiolarian Zone, which is correlated with the Tetraspidoceras recognitum Ammonite Assemblage of Pálfy et al. (1994), spans the Sinemurian -Pliensbachian transition but does not necessarily coincide with the stage boundary (Fig. 1).It is noteworthy that none of the sections included in the construction of these zones contain a continuous record of Late Sinemurian to Early Pliensbachian radiolarian assemblages.Among subsequently described radiolarian sections, only the Orbuklukeli section in the Mersin Mélange, Turkey, includes Sinemurian and earliest Pliensbachian assemblages (Tekin et al., 2020) but the preservation across the boundary is very variable and poor in most samples.
Calcareous nannofossils are also considered one of the most useful and diagnostic biostratigraphic tools for the Early Jurassic time interval (Bown and Cooper, 1998;Mattioli and Erba, 1999;Mattioli et al., 2013;Fraguas et al., 2015Fraguas et al., , 2018;;Peti et al., 2017).Nannofossil biozonations consist of interval zones delineated by bioevents (first or last occurrences of index species).According to the zonation of Bown and Cooper (1998) for NW Europe, Upper Sinemurian to Lower Pliensbachian rocks comprise NJ3 Crepidolithus crassus and NJ4 Similiscutum cruciulus Calcareous Nannofossil zones (Fig. 1).The base of the NJ4 Zone is marked by the first occurrence of Similiscutum cruciulus, as it has been recorded in many other European basins, for example in Italy (Mattioli and Erba, 1999), Northern Spain (Fraguas et al., 2015(Fraguas et al., , 2018) ) and France (Peti et al., 2017).The NJ3-NJ4 zones boundary has been calibrated to ammonite and carbon-isotope stratigraphy, and placed within the Jamesoni Ammonite Zone, above the base of the Pliensbachian stage   Pálfy et al., 1994;radiolarian zones from Carter et al., 1998radiolarian zones from Carter et al., , 2010; NW European calcareous nannofossil zones from Bown and Cooper, 1998).Radiolarian zones are calibrated to North American ammonites from Haida Gwaii, British Columbia, Canada; the grey field in the Lower Sinemurian marks the interval, in which ammonites exist (Coroniceras ammonite Zone) but radiolarians have not been found.The numerical age for the Bucklandi -Semicostatum boundary is uncertain.
T. Cifer et al. (Peti et al., 2017; Fig. 1).On Mount Rettenstein in the Northern Calcareous Alps, wellpreserved and diverse radiolarian assemblages have been discovered in a 95 m thick limestone-marl section that spans the Late Sinemurian to Early Pliensbachian time interval.This section offers a unique opportunity to study the actual superposition of radiolarian assemblages across the stage boundary.Here we document the distribution of selected radiolarian and all calcareous nannofossil taxa, and complement these data with bulk rock carbon and strontium isotope analyses.Thus, the primary aim of this paper is to reinforce the applicability of radiolarians as a tool to identify the Sinemurian -Pliensbachian boundary in deep-marine sedimentary successions.

Geological overview and previous biostratigraphic research in the study area
Mount Rettenstein is located in the southern part of the central Northern Calcareous Alps and, according to the regional tectonic "block model" by Frisch and Gawlick (2003), is part of the Upper Tirolic megaunit (Fig. 2A).It consists of three superposed tectonic units: Triassic rocks of the Werfen imbricates zone constitute the mountain's base and surrounding areas.Above this are Middle Jurassic Hallstatt Mélange rocks that are overlain by the topmost unit, the Rettenstein succession sensu stricto, which is the focus of this study (Auer et al., 2009; Fig. 2C).The Rettenstein succession begins with a Lower Jurassic succession of siliceous limestone and marl.These rocks, often informally called "Fleckenmergel" (e.g.Meister and Böhm, 1993), may belong to the Scheibelberg Formation or the Dürrnberg Formation (Cifer et al., 2020).The overlying ammonite rich, red condensed limestone and red marls T. Cifer et al. are both Early Jurassic in age and are often attributed to the Adnet Formation (Meister and Böhm, 1993).The succession continues with Middle Jurassic red calcareous marls (the Klaus Formation), which are overlain by the Rettenstein Debris Flow and radiolarites of the Ruhpolding Radiolarite Group.The top of this unit consists of several hundred meters of the Upper Jurassic Plassen Formation (for a more detailed description of the geological setting, see Auer et al., 2009 andCifer et al., 2020).
The Upper Sinemurian -Lower Pliensbachian Weitenhausgraben section presented here was studied at the base of the Rettenstein succession sensu stricto, in the grey siliceous limestone and marl succession (Fig. 2C).Palaeogeographically these Lower Jurassic sediments were deposited at the northwestern edge of the Neotethys (Fig. 2B).
Previous biostratigraphic research on Mt.Rettenstein was focused on rich ammonite faunas of the Adnet Group contained in a 10 m thick succession of red, slightly nodular marl and marly limestone.Therein, Meister and Böhm (1993) documented 10 successive ammonite horizons spanning from the upper part of the Ibex Zone (Luridum Subzone, approximately 2 m above the base) to the top of the Margaritatus Zone.A discontinuity characterized by a hardground separates the Margaritatus Zones from the overlying Toarcian beds with Hildoceras sp.In the upper part of the grey "Fleckenmergel", about 12 m below the contact with the "Adnet" limestone, these authors found Uptonia gr.jamesoni, the index species of the upper subzone of the Jamesoni Zone (Fig. 3).Higher in the section, about 8 m below the contact, they identified the lower horizon of the middle, i.e.Valdani Subzone of the Ibex Zone.The lithological boundary between the grey limestone below and the red "Adnet" limestone above most probably lies in the upper part of the Valdani Subzone.
The first radiolarian data from the Lower Jurassic limestone were based on six spot samples collected during detailed geological mapping of Mt.Rettenstein but their exact stratigraphic position within this thick succession of grey limestone and marl was not determined.An early Early Pliensbachian age was inferred for these samples, but a Late Sinemurian age could not be firmly excluded (Cifer et al., 2020).

Description of the studied section
The Weitenhausgraben section (47 • 27 ′ 09.21 ′′ N and 13 • 32 ′ 56.10 ′′ E) is a 95 m thick succession of alternating layers of marl, marly limestone and siliceous limestone (Figs. 3 and 4).The top of the section is placed at the sharp contact with the red nodular Adnet-type limestone (Fig. 2C).The succession is primarily grey in colour with three more reddish intervals in the lower part of the section (Fig. 3).Two 4 m thick reddish intervals occur in the first 20 m of the section; the third red interval extends from the 30 th to the 51 st meter of the section and is characteristically devoid of marl interlayers.In general, the limestone is well bedded with a bed thickness of up to 25 cm.The microfacies are similar through the entire section (Fig. 4).Two different microfacies types were determined; mudstone and wackestone.Both microfacies types show slight to moderate bioturbation.In the uppermost part of the section the sedimentary rocks are more recrystallized than in the lower part.The most common microfossils are sponge spicules and radiolarians but they are not abundant in every sample.Other grains are small echinoderm fragments and rare benthic foraminifera.Thin-shelled bivalves occur in some samples.The depositional environment of the studied succession is interpreted as hemipelagic.
-1.00 1.00 3.00  ).For calcareous nannofossil taxa only the first and last appearance is indicated in the stratigraphic range chart.Ammonite data from Meister and Böhm (1993).The colour of the sediment is indicated beside the stratigraphic chart.

Radiolarians
Our sampling was primarily focused on radiolarians, therefore only harder carbonate beds were selected.Forty limestone samples (Rö428 to Rö468) were collected and used for different analyses.To extract radiolarians, all samples were crushed to walnut size and dissolved in 8% acetic acid for 24 h.The residues were sieved through 63 μm mesh, dried and examined under a stereomicroscope at 50× magnification.

Calcareous nannofossils
Thirty smear samples were prepared for calcareous nannofossil analysis following the standard technique of Bown and Young (1998).Semi-quantitative analyses were performed with a Leica DMLP light microscope equipped with a Leica DFC 420 digital camera at 1250× magnification for biostratigraphic studies.In order to identify those species that are rare or very rare, 12 complete longitudinal transversals were analysed resulting in more than 2000 fields of view in each sample.Total abundance and degree of preservation of calcareous nannofossil assemblages, as well as the relative abundances of the species identified were analysed per samples (See Table 1 for further details).

Geochemical composition
Samples for the chemical analysis were prepared with an agate mortar and the analysis was carried out by Bureau Veritas, Vancouver, British Columbia, Canada using an ICP-ES (Inductively coupled plasma emission spectroscopy).The dissolution of the whole rock sample was done by Lithium borate fusion.Six duplicates and seven standards were used for quality control.

Stable carbon isotopes
The bulk carbonate samples were crushed, powdered and studied for stable carbon isotope composition using an automated carbonate preparation device (GASBENCH II) and a Thermo Finnigan delta plus XP continuous-flow mass spectrometer at the Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences.Three laboratory standards, calibrated using the NBS-18, NBS-19 and LSVEC reference materials (provided by the International Atomic Energy Agency), were used for sample standardization.These isotope compositions are expressed as δ 13 C values (in ‰) relative to V-PDB.As a test of external precision, the Harding Iceland Spar (Landis, 1983) sample was measured as unknown and yielded δ 13 C values of − 4.80 ± 0.05‰ (n = 20 in 5 measurement series).These values agree well with the published value of − 4.80 (Landis, 1983).

Strontium isotopes
For strontium isotope ratio analyses 30 bulk rock samples were selected in which the Sr content was greater than 300 ppm in 24 samples and between 230 ppm and 300 ppm in the remaining six samples.All bulk rock samples were taken from limestone beds.The measurements were performed with a Thermal Ionisation Mass Spectrometer (TIMS) TI-Box (formerly MAT 262; Spectromat) at the Institute of Geology, Mineralogy and Geophysics of the Ruhr-Universität Bochum (Germany).  8Sr/ 86 Sr analysis to detect the mass interference on mass 87 ( 87 Rb -87 Sr).For all samples measured here, rubidium was removed during the heating process of the filament; thus, no mass interference on mass 87 existed during the measurement ( 85 Rb = 0 mV signal intensity).For further details regarding the method, the reader is referred to Mueller et al. (2020).

Radiolarian biostratigraphy
Radiolarians were identified in six samples from the studied section (Rö437, Rö438, Rö439, Rö440, Rö448 and Rö453).All samples yielded rich, well-preserved assemblages with each containing up to 50 genera and about 100 identified species.Only 31 of the most characteristic taxa were selected for this stratigraphic study.They are illustrated in Fig. 5 and their distribution in the section is shown in Fig. 3.The complete inventory of all radiolarian assemblages along with systematic palaeontology will be published separately.
Throughout the entire section Tozerium filzmoosense Cifer, Pantanellium browni Pessagno and Blome, Anaticapitula anatiformis (De Wever), Anaticapitula omanensis Dumitrica, Foremania sandilandsensis gr.Whalen and Carter and Thurstonia robusta Cifer were identified.Anaticapitula anatiformis (De Wever) and Foremania sandilandsensis gr.Whalen and Carter are known to occur in the Sinemurian and Pliensbachian (Carter et al., 1998(Carter et al., , 2010)).Pantanellium browni Pessagno and Blome was zoned to the Hettangian to lowermost Sinemurian (Carter et al., 1998) but Pantanellium species, in general, are difficult to differentiate and are thus stratigraphically unreliable.The ranges of other species have not been well documented as yet.The lowest collected samples (Rö437, Rö438, Rö439 and Rö440, see Fig. 3) yielded several taxa that appear only in the Sinemurian.Bipedis douglasi Whalen and Carter restrains assignment to the Canutus rockfishensis -Wrangellium thurstonense and Jacus?sandspitensis zones (from UA 20 to UA 25 in Carter et al., 1998;UA 1 in Carter et al., 2010).The Jacus? sandspitensis Zone could not be distinguished because, as defined by Carter et al. (1998), only two species first appear in this zone (Jacus?sandspitensis Pessagno, Whalen and Yeh, and Paronaella cf.corpulenta De Wever) but neither were found in the studied section.Bipedis horiae Sugiyama, another stratigraphically significant species in these four samples is restricted to the Hettangian and Sinemurian in Japan (Matsuoka and Ito, 2019).Potentially important are some less well documented species that so far have not been found above the Sinemurian.These include Katroma hocakoeyensis Tekin, documented only from the Sinemurian of the Antalya Nappes in Turkey (Tekin, 2002), Paurinella liassica Tekin, known only in the Sinemurian (Tekin et al., 2020), Liassobetraccium spp., ranging from the Hettangian to the Sinemurian (O'Dogherty et al., 2009), Palaeosaturnalis parvus (Kozur and Mostler, 1990), recorded in the Hettangian type locality only (Kozur and Mostler, 1990) and Ares rettensteinensis Cifer, originally assigned to the Lower Pliensbachian (Cifer et al., 2020) but should probably be corrected to the Upper Sinemurian.Canoptum hettangicum (Kozur and Mostler, 1990) identified in sample Rö438 is the only welldocumented species for which the reported last occurrence does not reach the Upper Sinemurian.In Carter et al. (1998), this taxon was synonymized with Relanus reefensis Pessagno and Whalen, which was supposed to range only up to the Hettangianlowermost Sinemurian Crucella hettangica Zone.Here we demonstrate that Canoptum hettangicum (Kozur and Mostler, 1990) also appears in the Upper Sinemurian.In the studied section, some species are restricted to the lower four samples but are elsewhere known
Preservation classes: G = good (the majority of the specimens show their diagnostic characteristics and only some of them are slightly etched and/or overgrown), M = moderate (the majority of the specimens are recognizable, even if some of them are etched and/or overgrown and/or fragmented), B = poor (the majority of the specimens are heavily etched or/and overgrown and/or fragmented and the identification of the species is sometimes difficult), VB = very poor (only a few specimens are recognizable).
to cross the Sinemurian -Pliensbachian boundary.2010), but has been reported from Pliensbachian samples in other localities (Goričan et al., 2006;Tekin et al., 2020).Samples Rö448 and Rö453, taken from the upper part of the section, yielded several taxa that first appear in the Canutus tipperi -Katroma clara Zone.In addition to Katroma clara Yeh, these are Bagotum modestum Pessagno and Whalen, Lantus praeobesus Carter and Lantus obesus (Yeh).Parahsuum mostleri (Yeh), also identified in both samples, first appears in the Zartus mostleri -Pseudoristola megaglobosa Zone (Carter et al., 2010).This zone was defined with the appearance of 33 taxa but all except Parahsuum mostleri (Yeh) and some Praeconocaryomma species are missing in the studied section.It is thus likely that the range of Parahsuum mostleri (Yeh) should be extended downwards to the Canutus tipperi -Katroma clara Zone.Zhamoidellum sutnal (O'Dogherty), Naropa vi Hori, Whalen and Dumitrica and Podocapsa?abreojosensis Whalen and Carter were identified only in sample Rö453.Zhamoidellum sutnal (O'Dogherty) is known from the Lower Pliensbachian (O'Dogherty and Gawlick, 2008;Carter et al., 2010) and has previously been reported from Mount Rettenstein (Cifer et al., 2020).Naropa vi Hori, Whalen and Dumitrica is currently restricted to the Gigi fustis -Lantus sixi and Eucyrtidiellum nagaiae -Praeparvicungulla tlellensis zones but has been found previously only in Japan and Baja California Sur (Carter et al., 2010).Its range in the zonation may be incomplete.Podocapsa?abreojosensis Whalen and Carter first appears in the Canutus tipperi -Katroma clara Zone (Carter et al., 2010) as most other species in the upper part of the section.
The studied assemblages also revealed some evolutionary lineages that are useful for biostratigraphy (Fig. 3).Saitoum keki De Wever (Fig. 5-12) may have evolved from Saitoum aff.keki De Wever (Fig. 5-11), which has shorter feet and a longer apical horn as well as less numerous and more irregularly distributed pores.The genus Katroma yielded several species throughout the section, the most important are Katroma hocakoeyensis  and its possible descendent Katroma clara Yeh (Fig. 5

Calcareous nannofossil biostratigraphy
A total of 17 species belonging to seven genera were identified in the 30 smear samples (see Fig. 3, Table 1 and Appendix A for further details).Most samples are poorly preserved and yielded monospecific assemblages consisting of rare to extremely rare specimens of Schizosphaerella punctulata.A considerable increase in both, nannofossil assemblage abundance and diversity, as well as an improved preservation was noted between samples Rö454 and Rö459.They yielded up to eight species dominated by well-preserved specimens of Parhabdolithus liasicus liasicus, Tubirhabdus patulus and Crepidolithus crassus.The most abundant and stratigraphically significant taxa are illustrated in Fig. 6.
Given the observed variations in the calcareous nannofossil assemblages in terms of first occurrences and changes in the relative abundances of the index species, a nannobiohorizon identified as the first occurrence of Similiscutum cruciulus was observed in sample Rö451(Fig.3).This bioevent marks the boundary between the NJ3 Crepidolithus crassus and the NJ4 Similiscutum cruciulus Calcareous Nannofossil zones.Another important taxon is Calcivascularis jansae, which was found in sample Rö433.This species first occurs in the middle part of the NJ2b zone, which corresponds to the lower part of the Sinemurian.

Carbon and strontium isotopes
Stable carbon isotope values range from 0.64‰ to 2.22‰, and represent, based on the isotopic cross-plot, the primary signal (Figs. 3  and 7; Appendix C).Two major excursions are evident: one negative in the middle of the section and one positive near the top.From the base to about 40 m in the section δ 13 C values steadily increase from 0.64‰ to 2.00‰.Two positive shifts from this trend occur near the base of the section at about 5 m and 20 m, but are documented in only a few samples The values of 87 Sr/ 86 Sr (Fig. 3) range from 0.707562 ± 0.000005 to 0.708115 ± 0.000005, normalized to the standard NBS 987 value of 0.710248 (McArthur et al., 2012;Appendix C).They show a general downward trend from the base of the section towards the 50 m mark followed by a plateau in values in the upper part of the section.The 87 Sr/ 86 Sr values for comparison (Fig. 3) are taken from Jones et al. (1994) and Hesselbo et al. (2000) and are adjusted to the inferred position of the Sinemurian -Pliensbachian boundary in the studied section and the fact that the Jamesoni Zone extends at least up to the 85th meter in the section.The 87 Sr/ 86 Sr curve from Mount Rettenstein has a similar trend as the previously published curves but the values are elevated for a similar magnitude of 0.0002 to 0.0004 through the entire section (Fig. 3).

Regional correlation of the stable carbon isotope record
During the Sinemurian and Pliensbachian major palaeogeographic changes occurred in the Western Tethyan Realm that were accompanied by major perturbations in the global carbon cycle.The best documented perturbation is the Sinemurian -Pliensbachian Boundary Event (S-P Event) as recognized by Jenkyns et al. (2002) and more recently defined by Korte and Hesselbo (2011).This event is characterized by a negative carbon isotope excursion (CIE) that can be traced in different magnitudes at several localities, on a supraregional scale at least (Fig. 7).For example, Franceschi et al. (2019) documented decreasing values of δ 13 C carb from 2.2‰ to 0.8‰ in the sediments of the Tofino section, Lombardian Basin, Italy (Fig. 7).The Lombardian Basin was a deep depositional environment in the Western Tethyan Realm, with a similar depositional environment as the Mount Rettenstein succession.The Lombardian Basin represents a slightly different paleogeographic setting in proximity of a more productive carbonate platform, compared to the depositional environment of the succession studied herein.As a result, the sedimentation rate in the Lombardian Basin was higher due to a higher influx from the platform.The values of δ 13 C carb in the Lombardian Basin and at Mount Rettenstein are quite similar.Another example of a negative carbon isotope shift at the Sinemurian -Pliensbachian boundary can be found in the basin sediment sections in Morocco, which also recorded a 2‰ decrease in δ 13 C carb (Mercuzot et al., 2020).In general, the Sinemurian -Pliensbachian negative CIE can be identified in both platform and basin bulk carbonate samples, as well as in belemnite calcite and organic matter (e.g., Franceschi et al., 2014Franceschi et al., , 2019;;Gómez et al., 2016;Price et al., 2016;Mercuzot et al., 2020;Storm et al., 2020).
The stable carbon isotope curve from Mount Rettenstein is very similar to the curves from Viote and Tofino sections in Italy (Franceschi et al., 2014(Franceschi et al., , 2019) ) and to that from the Dorset section in England (Price et al., 2016).At all these localities stable carbon isotope values increase in the Upper Sinemurian to just below the negative CIE at the Sinemurian -Pliensbachian boundary (Fig. 7).Several authors (e.g.Franceschi et al., 2014;Schöllhorn et al., 2020aSchöllhorn et al., , 2020b;;Han et al., 2021) also documented that the negative shift of δ 13 C carb starts around the Sinemurian to Pliensbachian transition and continues into the Jamesoni Ammonite Zone.Conversely, Storm et al. ( 2020) documented a steady Fig. 7. Correlation of the δ 13 C carb curve from Mount Rettenstein with other δ 13 C carb curves.The Viote section at the Trento Platform does not exhibit an evident drop in δ 13 C carb values because the section contains an unconformity at the Sinemurian -Pliensbachian boundary.Note that the δ 13 C carb curve from Dorset (England) uses moving averages, whereas the curve from Mount Rettenstein does not.Note also different scale for thickness of the Tofino section in comparison to the other three localities.The sedimentation rate at the Tofino section was about three times higher than the sedimentation rate at the Weitenhausgraben section.
fall in stable carbon isotope values in wood from the late Oxynotum Ammonite Zone that persisted to the middle part of the Jamesoni Ammonite Zone.They documented the highest values of δ 13 C org before the S-P Event in the Oxynotum Zone.At the Weitenhausgraben section, the highest values of δ 13 C carb below the Sinemurian -Pliensbachian Boundary Event, at about the 35 th meter, could potentially be assigned to the Raricostatum Ammonite Zone (compare to Danisch et al., 2019) or to the Oxynotum Ammonite Zone (compare to Storm et al., 2020).Similar peaks were observed by Franceschi et al. (2019), but were not assigned to any specific time interval (Fig. 7).During the steady increase in δ 13 C carb values leading to the S-P Event we also recognize a relatively abrupt positive shift at about 20 m.This positive shift could potentially be related to the carbon isotope excursion in the Turneri Zone that is also documented in the Southern Alps (Masetti et al., 2017).
Another carbon isotope excursion that can be recognized at Mount Rettenstein occurs in the upper part of the section, where the values very gradually increase to about 1.20‰.Several authors have also recognized a positive excursion above the Sinemurian -Pliensbachian boundary.Mercuzot et al. (2020) documented the excursion in Morocco with a magnitude of 1 to 1.5‰.It was also recognized in Northern Spain (Rosales et al., 2001;Gómez et al., 2016), Portugal (Oliveira et al., 2006;Duarte et al., 2014), England (Price et al., 2016), and at the Tofino and Viote sections in Italy (Franceschi et al., 2014(Franceschi et al., , 2019)).This positive excursion is named after and is correlated with the Lower Pliensbachian Ibex Ammonite Zone.The δ 13 C carb curve at Mount Rettenstein has a similar trend in the upper part as the δ 13 C org data from Storm et al. (2020), where the values start to increase in the uppermost part of the Jamesoni Ammonite Zone, and reach a peak in the Ibex Ammonite Zone.This peak was recorded in the uppermost Valdani Ammonite Subzone (Price et al., 2016), which would also be in agreement with ammonite data from Mt. Rettenstein (Meister and Böhm, 1993).

Calibration of radiolarian zones to calcareous nannofossil zones and stable carbon isotopes
Radiolarian assemblages from samples Rö437, Rö438, Rö439 and Rö440 were placed in the Upper Sinemurian Canutus rockfishensis -Wrangellium thurstonense and Jacus?sandspitensis radiolarian zones.This assignment agrees with the δ 13 C carb values, which indicate that these samples are older than the S-P Event.The calcareous nannofossil taxon Calcivascularis jansae was found in sample Rö433.According to Bown and Cooper (1998), its first occurrence is located within the mid part of the NJ2b Mitrolithus elegans Calcareous Nannofossil Subzone, which represents the lower part of the Sinemurian stage (Fig. 1).According to our results, this bioevent is probably located below the oldest studied sample (Fig. 3).
Radiolarian assemblages from samples Rö448 and Rö453 were assigned to the Canutus tipperi -Katroma clara Radiolarian Zone and perhaps also the overlying Zartus mostleri -Pseudoristola megaglobosa Zone.Rö448 is in the negative CIE of the S-P Event, whose peak negative values are correlated to the base Pliensbachian, that is, to the Taylori Ammonite Subzone (Price et al., 2016).Katroma clara Yeh and several other characteristic radiolarian species (e.g.Bagotum modestum Pessagno and Whalen, Lantus praeobesus Carter, Lantus obesus (Yeh), Parahsuum mostleri (Yeh) and Zhamoidellum yehae Dumitrica (Fig. 3) first occur at this level.The first specimen of the calcareous nannofossil species Similiscutum cruciulus was identified in sample Rö451.The first occurrence of this taxon marks the base of the NJ4/NJT4 (NJT is used for calcareous nannofossil zones of the Tethyan realm; Mattioli and Erba, 1999;Peti et al., 2017) Similiscutum cruciulus Calcareous Nannofossil Zone.Fraguas et al. (2015Fraguas et al. ( , 2018) ) documented this event in several sections from Northern Spain and compared it to the biostratigraphic schemes proposed for NW Europe (Bown and Cooper, 1998), Italy and South France (Mattioli and Erba, 1999), and Portugal (Mattioli et al., 2013).In all these basins, they recognized the event lying nearly in the same stratigraphic position within the Jamesoni Zone.
In the Weitenhausgraben section, this calcareous nannofossil event was identified in the strata between the two recorded carbon isotope events.The negative CIE was identified approximately 10 to 15 m below the calcareous nannofossil event, whereas the positive CIE, referred to as the Ibex Event, is recognized approximately 25 m above the calcareous nannofossil event.The time interval between these carbon isotope events roughly correlates with the Lower Pliensbachian Jamesoni Ammonite Zone.Based on Uptonia gr.jamesoni reported from about 12 m below the boundary with the Adnet limestone (Meister and Böhm, 1993) and the position of the first occurrence of Similiscutum cruciulus at the top of the negative CIE, the nannofossil event can be more precisely placed in the Polymorphus Ammonite Subzone of the Jamesoni Ammonite Zone.

Bulk carbonate preservation and strontium isotopes
The main difference between this and other studies is the usage of bulk carbonate samples here, whereas Jones et al. (1994) and Hesselbo et al. (2000) used well-preserved shell material.In bulk carbonate samples, a positive offset of 87 Sr/ 86 Sr values is common and interpreted as diagenetic alteration in the presence of fluids enriched in radiogenic continent-derived strontium (Burke et al., 1982;Jones et al., 1994;Woodfine et al., 2008).Additional support for alteration of the bulk carbonate is provided by elevated Fe (between 3077 and 23,566 ppm) and Mn concentrations (between 232 and 1394 ppm; Appendix B), which are often used as indicators for diagenetic overprint (Brand and Veizer, 1980;Denison et al., 1994;Jones et al., 1994;Brand and Brenckle, 2001;Wang et al., 2018).Manganese values higher than 300 ppm and iron values higher than 1000 ppm were suggested as threshold to indicate alteration by Wang et al. (2018).Dolomitization is also expected to cause a positive shift of 87 Sr/ 86 Sr values (Onoue et al., 2016), but this factor can be excluded in the case of the Weitenhausgraben section.No evidence for dolomitization could be observed in the field, thin-sections and the CaO to MgO ratio (Appendix B).The enrichment in 87 Sr can be also caused by rubidium contamination, which is problematic in bulk rock samples that contain clay minerals (McArthur et al., 2012).Rubidium concentrations have not been measured directly in the present study; however, as described in the methods (3.2.5) all Rb was removed prior to starting the measurement of 87 Sr/ 86 Sr ratios.Consequently, Rb does not influence the Sr isotope ratios of the studied samples.Hence, the alteration of bulk carbonate is the most reasonable explanation for the higher 87 Sr/ 86 Sr ratios of the studied samples.
The 87 Sr/ 86 Sr values in the studied section are clearly too high for direct dating, but the general shape of the curve is similar to the standard 87 Sr/ 86 Sr curve (Fig. 3).A clear decrease in mean values across the inferred Sinemurian-Pliensbachian boundary correlates well with the shift to lower values in the strontium isotope ratio curve obtained from skeletal calcite (Jones et al., 1994).This parallelism supports the position of the stage boundary in the studied section as determined with radiolarians, calcareous nannofossils and stable carbon isotopes.

Conclusions
The Weitenhausgraben section on Mount Rettenstein is the first section known to have a continuous record of well-preserved radiolarians across the Sinemurian -Pliensbachian boundary.The boundary stratigraphic level is constrained with a negative CIE known as the S-P Event and corroborated with the base of the NJ4 Similiscutum cruciulus Calcareous Nannofossil Zone recognized 10 m above the stage boundary.Radiolarian zones across this boundary are for the first time calibrated to chronostratigraphy outside North America.The integrated bioand chemostratigraphic data confirm that Canutus rockfishensis -Wrangellium thurstonense to Jacus? sandspitensis radiolarian zones correspond to the Upper Sinemurian, and reveal that the base of the Canutus tipperi -Katroma clara Radiolarian Zone coincides with the base of the Pliensbachian Stage.These results contribute to the stability of Lower Jurassic radiolarian zones and will allow the recognition of the Sinemurian-Pliensbachian boundary in deep-marine successions where ammonites are missing.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig.1.Correlation of ammonite, radiolarian and calcareous nannofossil zones in the Sinemurian and Pliensbachian (numerical age of European ammonite zones fromStorm et al., 2020; North American ammonite zones fromPálfy et al., 1994; radiolarian zones fromCarter et al., 1998Carter et al., , 2010; NW European calcareous nannofossil zones fromBown and Cooper, 1998).Radiolarian zones are calibrated to North American ammonites from Haida Gwaii, British Columbia, Canada; the grey field in the Lower Sinemurian marks the interval, in which ammonites exist (Coroniceras ammonite Zone) but radiolarians have not been found.The numerical age for the Bucklandi -Semicostatum boundary is uncertain.

Fig. 2 .
Fig. 2. A. Location of the studied section in the structural overview of the Alpine orogen, showing the situation of the central Northern Calcareous Alps (after Frisch and Gawlick, 2003).B. Palaeogeographic map (modified after Gawlick and Missoni, 2019) with position of localities discussed in the text.C. Stratigraphy and lithology of the Mount Rettenstein succession sensu stricto (after Auer et al., 2009; Cifer et al., 2020).

Fig. 3 .
Fig. 3. Bulk carbon and strontium isotope values with occurrences of radiolarians and calcareous nannofossils at the Weitenhausgraben section, Mount Rettenstein.Strontium isotope values from this study are represented by three-point moving average values (measured values are given in Appendix C).For calcareous nannofossil taxa only the first and last appearance is indicated in the stratigraphic range chart.Ammonite data fromMeister and Böhm (1993).The colour of the sediment is indicated beside the stratigraphic chart.
For long-term reproducibility, two standards were used, the NIST NBS 987 and the USGS EN-1.Published values for these standards are: 0.710248 for NBS 987 (McArthur et al., 2012) and 0.709174 for USGS EN-1 (McArthur et al., 2012).The standards yielded mean values of 0.710246 ± 0.000002 (2σ SE; n = 159) for NBS 987 and 0.709165 ± 0.000004 (2σ SE; n = 124) for USGS EN-1 covering a time period from 2017 to 2021.To ensure that 87 Sr values are not enriched because of rubidium, the mass 85 ( 85 Rb) is constantly monitored during

Table 1
Distribution chart of calcareous nannofossil taxa in the Weitenhausgraben section, Mount Rettenstein showing the results obtained from the semiquantitative analyses performed on the studied samples.Calcareous nannofossil zones and biohorizons identified in this work are indicated.
(Carter et al., 2010)ria emmella(Cordey and Carter), which last appears in the Lower Pliensbachian Gigi fustis -Lantus sixi Zone(Carter et al., 2010), Praehexasaturnalis tetraradiatus Kozur and Mostler, last appearing in the lowermost Pliensbachian Canutus tipperi -Katroma clara Zone(Carter et al., 2010), and Palaeosaturnalis subovalis Kozur and Mostler, which was not included in the zonation ofCarter et al. ( -29).Katroma hocakoeyensis Tekin occurs only in the lower part of the section, whereas Katroma clara Yeh can be found only in the upper part of the section.Katroma clara Yeh has larger pores