Biostratigraphy and paleoenvironments in a northwestern Tethyan Cenomanian-Turonian boundary section (Austria) based on palynology and calcareous nannofossils

Cretaceous Research 38 (2012) 103e112

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Biostratigraphy and paleoenvironments in a northwestern Tethyan Cenomanian-Turonian boundary section (Austria) based on palynology and calcareous nannofossils Polina Pavlishina a, Michael Wagreich b, * a b

Department of Geology, Palaeontology and Fossil Fuels, Sofia University ‘St. Kliment Ohridski’, Tzar Osvoboditel Str. 15, 1504 Sofia, Bulgaria Department of Geodynamics and Sedimentology, Center for Earth Sciences, University of Vienna, A e 1090 Vienna, Austria

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 July 2011 Accepted in revised form 3 February 2012 Available online 2 March 2012

The Ultrahelvetic Rehkogelgraben section of the Eastern Alps of Austria comprises a c. 5 m thick sequence of upper Cenomanian to lower Turonian pelagic sediments including black shales in a northwestern Tethyan paleogeographic setting. The following calcareous nannofossil events were recognized in stratigraphic order: The first occurrence (FO) of Quadrum intermedium, the LOs of Lithraphidites acutus and Corollithion kennedyi, the FO of Eprolithus octopetalus, and the FO of Quadrum gartneri. Thus, nannofossil zones UC 4, UC 5c, UC 6 and UC 7 could be recognized. Palynological investigations and palynofacies analysis give additional information especially from the black shale layers and the carbonate-free interval in the section. Five out of seven samples from this interval have yielded some low diversity but distinctive palynological associations. Sporomorphs dominate in the associations, being represented mainly by age diagnostic Normapolles species as Atlantopollis microreticulatus, Atlantopollis reticulatus together with Complexiopollis praeatumescens, Complexiopollis christae and Complexiopollis funiculus. The concurrent presence of these pollen species is regarded as characteristic for latest Cenomanian and early Turonian assemblages. A low diversity dinocyst association is identified, dominated by the Cyclonephelium compactum e Cyclonephelium membraniphorum complex, together with Isabelidinium and Circulodinium species. The dinocyst association is encountered in palynofacies rich in marine organic matter of granular amorphous composition. The palynofacies points to deposition in a distal, low-energy, stressed anoxic environment with high continental input. The palaeoenvironmental significance of the low diversity Cyclonephelium/ Eurydinium association is typical for Cenomanian-Turonian black shale sequences. Nannofossil indices and dinoflagellate associations indicate rather low-productivity, low-nutrient settings during at least the later part of OAE 2, and thus confirm interpretations based on foraminiferal assemblages. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Cenomanian-Turonian boundary Oceanic Anoxic Event 2 Nannofossils Dinoflagellates Palynomorphs Normapolles Paleoproductivity

1. Introduction The Cenomanian-Turonian boundary interval includes one of the most severe oxygen-deficiency crises in the Mesozoic, i.e. Oceanic Anoxic Event OAE 2 (Schlanger et al., 1987). It marks the end of the mid-Cretaceous super-greenhouse with its anoxic events that are characterized by extensive burial of organic carbon and a strong positive excursion of stable carbon isotope values

* Corresponding author. Tel.: þ43 1 4277 53465; fax: þ43 1 4277 9534. E-mail addresses: [email protected]fia.bg (P. Pavlishina), michael.wagreich@ univie.ac.at (M. Wagreich). 0195-6671/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.cretres.2012.02.005

(Leckie et al., 2002; Wagreich et al., 2011). Dinoflagellates and calcareous nannoplankton are among those plankton organisms groups that were intensively affected by the event leading to a significant faunal turnover with extinctions and radiations (e.g., Erba, 2004; Pearce et al., 2009). The Ultrahelvetic Rehkogelgraben section of the Eastern Alps comprises the first black-shale bearing Cenomanian-Turonian boundary interval reported so far from the Penninic Ocean, a northwestern Tethys branch (Wagreich et al., 2008; Gebhardt et al., 2010). Correlations to OAE 2 were possible via nannofossil biostratigraphy, planktonic foraminifera events, and d13C chemostratigraphy (Gebhardt et al., 2010). High organic matter contents (up to 5% total organic carbon) and predominance of marine

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organic matter (Wagreich et al., 2008) are similar to other (Tethyan) OAE 2 successions such as the Bonarelli-level in Italy. Biostratigraphy of the Rehkogelgraben section so far was based on calcareous nannofossil and planktic foraminifera. Palynological investigations and palynofacies analysis reported in this paper from selected samples that were also used for nannofossil biostratigraphy give additional information especially from the black shale layers and the carbonate-free interval in the section. In order to achieve the maximum and complete information from the palynological assemblages, dinocyst and sporomorph assemblages were simultaneously considered and palynofacies data are compared to the organic carbon content and Rock-Eval pyrolysis parameters. This paper then sheds light on the question of organic matter sedimentation and paleo-environmental conditions during OAE 2 archived in the nannofossils and palynomorph record in the special setting of this ocean basin (comp. Gebhardt et al., 2010). 2. Geological setting Pelagic sediments exposed in the Rehkogelgraben in Upper Austria (Fig. 1) form part of the Ultrahelvetic Unit of the fold-and-thrust belt of the Austrian Eastern Alps. This detached deep-water pelagic unit, together with the shallow-water deposits of the Helvetic shelf to the north, was originally part of the northern European margin of the Penninic Ocean, an oceanic branch that connected the northwestern Tethys to the Ligurian Ocean and the North Atlantic (Neuhuber et al., 2007). Sediments of the Ultrahelvetic Unit were deposited at the continental margin, in a pelagic slope environment, to the north of the abyssal Rhenodanubian (Penninic) Flysch basin (Faupl and Wagreich, 2000; Egger and Mohamed, 2010). The Cenomanian-Turonian part of the Rehkogelgraben section (coordinates WGS 84: 013 550 3000 E, 47 560 0800 N), initially described in Kollmann and Summesberger (1982), and its geological setting were reported in detail by Wagreich et al. (2008). The Rehkogelgraben section comprises tectonically disturbed pelagic and hemipelagic sediments of Aptian to Campanian age. Within the Cenomanian-Turonian boundary interval a 5 m thick succession of upper Cenomanian marl-limestone cycles is overlain by a black shale-claystone interval, followed by lower Turonian white to light

grey marly limestones with thin marl layers (Wagreich et al., 2008). Three black shale layers were logged and have total organic carbon (TOC) contents of up to 5% which display a predominance of marine organic matter based on Rock-Eval pyrolysis (Wagreich et al., 2008). Correlation of the black shale interval to Oceanic Anoxic Event 2 is possible via d13C chemostratigraphy and nannofossil and planktonic foraminifera biostratigraphy, and Wagreich et al. (2008) correlated the Rehkogelgraben record with several CenomanianTuronian boundary sections, notably Pueblo (the GSSP section for the base of the Turonian in the USA), the Bonarelli-level at Gubbio (Italy) and Eastbourne (U.K.) (see also Gebhardt et al., 2010). Foraminifers from the Rehkogelgraben section showed a specific evolution with the absence of clear disaster species and of high-productivity indicators and overall high foraminiferal diversities pointing to a special paleogeographic situation of the area with regional dysoxia, acting as a refuge during the Cenomanian-Turonian boundary environmental crisis (Gebhardt et al., 2010). Indications for oligotropy especially in the upper part of OAE 2 are also known elsewhere, e.g. at Eastbourne (UK; Paul et al., 1999; Linnert et al., 2011) and Wunstorf (northern Germany; Linnert et al., 2010) and also within some Tethyan sections (Erba, 2004). New nannofossil and palynological investigations together with palynofacies analysis give additional information especially from the black shale layers and the carbonate-free interval in the section. In order to achieve the maximum and complete information from the palynological assemblages, dinocyst and sporomorph assemblages are simultaneously considered and palynofacies data are compared to the organic carbon content. 3. Material and methods For detailed descriptions of procedures for calcareous nannofossil, i.e. standard smear slide preparation, and organic matter analysis such as TOC content and Rock-Eval pyrolysis see Wagreich et al. (2008). We refer to Burnett (1998) for nannofossil taxonomy. Five of seven samples (mainly from the black shale layers and the carbonate-free interval, see Fig. 2) were positively examined for their palynological content. Each sample of 100 g dry weight was processed using standard preparation techniques including HCl

Fig. 1. Position of the Rehkogelgraben section on a schematic tectonic map of the Eastern Alps (modified from Wagreich et al., 2008).

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Fig. 2. Stratigraphy of the Rehkogelgraben section based on calcareous nannofossil, planktonic foraminfera events (1) FO of H. helvetica based on disaggregated samples, (2) FO of H. helvetica based on thin sections, for details see Wagreich et al. (2008), and Gebhardt et al. (2010), and dinoflagellate marker species. Carbon isotope data from Wagreich et al. (2008) and Gebhardt et al. (2010). CTBI CIE ¼ Cenomanian-Turonian boundary interval carbon isotope excursion.

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and HF treatment, heavy liquid separation and KOH treatment. No oxidation was applied to the sample material. The residues were finally sieved through 6e8 mm nylon meshes. Strew mounts were made in glycerine jelly and are stored in the collections of the Sofia University ‘St. Kliment Ohridski’. The reader is referred to Williams et al. (1998) for dinoflagellate cyst taxonomy. 4. Results 4.1. Calcareous nannofossils We use a refined stratigraphy of the section which is mainly based on calcareous nannofossils and planktonic foraminifera (Wagreich et al., 2008) together with correlated carbon isotope excursions from bulk rock carbonate and organic carbon (Gebhardt et al., 2010). Wagreich et al. (2008) applied the nannofossil zonations of Perch-Nielsen (1985) and Burnett (1998). Here we give detailed nannofossil data (Figs. 2 and 3) on the section using 24 smear slide samples including the 17 samples reported already by Wagreich et al. (2008). The nannofossils are moderately to poorly preserved. Based on a visual evaluation of dissolution and etching (E) and overgrowth (O) based on Bown and Young (1998) E and O rankings of E2 (moderately etched) to O2 (moderately overgrown) and O3 (strongly overgrown) indicate all in all only a moderate to poor preservation of the calcareous nannofossil assemblages. Preservation thus has a significant impact on paleo-environmental interpretations based on nannofossils, i.e., a lower and an upper interval with overgrown nannofossils can be distinguished from a middle, carbonate-poor interval with etched nannofossils. A late Cenomanian age for the lower part of the section is indicated by the presence of the nannofossil marker species Lithraphidites acutus Verbeek and Manivit in Manivit et al., 1972, Eprolithus floralis (Stradner, 1962) Stover, 1966, and Corollithion kennedyi Crux, 1981 (nannofossil standard zones CC 10/UC4). The onset of the black shale e claystone interval lies above the last occurrence (LO) of L. acutus. Due to the lack of carbonate in this part of the profile (from 2.50 to 2.87 m), calcareous nannofossils data are missing in that interval. Above the barren lower part of the black shale interval, the FO of Eprolithus octopetalus, was recognized. The following FO of Q. gartneri indicates the base of nannofossil standard zone CC 11/UC 7 (lower Turonian). This age constraint suggests that the black shale can be attributed to the upper part of CC10 and UC5-6, respectively, spanning the Cenomanian-Turonian boundary interval (CTBI) as indicated by Kennedy et al. (2005). The succession of nannofossil events around the CenomanianTuronian boundary interval is still under discussion (Burnett, 1998), as different sections have given diverse results (e.g. Bralower, 1988; Paul et al., 1994; Lamolda and Mao, 1999; Luciani and Cobianchi, 1999; Hardas and Mutterlose, 2006; Tantawy, 2008; Linnert et al., 2010). Furthermore, the application of a detailed nannofossil biostratigraphy in the Rehkogelgraben section is hampered by the lack of carbonate. The main nannofossils events

recognized in the section are discussed below and correlated to other published sections. 4.1.1. LO of Corollithion kennedyi and Lithraphidites acutus The synchronous LO of several species, such as L. acutus and C. kennedyi, has not been described in other CTBI sections, and suggests that a stratigraphic gap exists above the last limestone bed below the black shales, spanning at least the interval from the LO of C. kennedyi to the LO of L. acutus. (i.e. 3 nannofossil subzones, UC3e to UC4b). This gap and some reworking was also recorded by planktonic foraminifera (Gebhardt et al., 2010) at the same level. Burnett (1998) used the LO of C. kennedyi as a subzonal marker (base UC3e) falling into the upper Cenomanian Calycoceras guerangeri ammonite zone, before the onset of the CIE, although Nederbragt and Fiorentino (1999) and Luciani and Cobianchi (1999) recorded its LO in the Oued Mellegue section in Tunisia above the onset of the isotopic event. The LO of Lithraphidites acutus defines the base of UC 5 of Burnett (1998). L. acutus is rare but consistently present in the section up to 2.44 m. Additional events around this time interval are the FO of Cylindralithus biarcus (defines the base of UC4a of Burnett, 1998), the LOs of Axopodorhabdus albianus (defines the base of subzone UC5b of Burnett, 1998) and Helenea chiastia (defines the base of zone UC6 of Burnett, 1998). All three species are extremely rare, i.e. H. chiastia has only single occurrences in two samples, or are due to overgrowth very difficult to identify (such as C. biarcus). Therefore, these events were not used in the applied zonation. However, it can be recognized that both A. albianus and H. chiastia are not present above the LOs of L. acutus and C. kennedyi and may have their LOs within the carbonate-free black shale interval. 4.1.2. FO of Quadrum intermedium Q. intermedium is herein reported to occur earlier, i.e. below the onset of the CIE than in most of the other studies dealing with nannofossils around the CT boundary (e.g. Hardas and Mutterlose, 2006). Its FO with forms showing 6 elements occur at 1.51 m, below the onset of the CIE (carbon isotope excursion). It is rare and discontinuous in its range at the onset. Here, mostly forms with six elements predominate, and forms with five elements occur at 2.95 m above the carbonate-free interval. Burnett (1998) used the FO of five-element forms of Q. intermedium (Fig. 3D) as a subzonal marker for the base of UC5c and placed the bioevent in the upper Cenomanian Neocardioceras juddii ammonite zone. Its FO below the FO of Eprolithus octopetalus is in accordance with Burnett (1998) and Hardas and Mutterlose (2006), but should occur later, between the 3rd and the 4th carbon isotope peak of OAE 2 CIE. 4.1.3. FO of Eprolithus octopetalus Burnett (1998) placed the FO of E. octopetalus in the lowermost Turonian, slightly above the FO of Watinoceras devonense, the ammonite suggested as the primary marker for the base Turonian

Fig. 3. Nannofossil marker species of the Rehkogelgraben section. Light microscope photographs (magnification 1000, oil immersion, all same scale). A. Eprolithus floralis (sample REH02/35, section meter 3.07); B. Lithraphidites acutus (REH02/29, section meter 2.15); C. Corrollithion kennedyi (sample REH02/25, section meter 0.88); D. Quadrum intermedium (REH03/6, section meter 4.20); E. Eprolithus octopetalus (REH03/6, section meter 4.20); F. Quadrum gartneri (REH03/9, section meter 5.10).

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(Kennedy et al., 2005), below the FO of Q. gartneri. Hardas and Mutterlose (2006) observed a reversed succession of these nannofossils events at the Demerara Rise. Forms with eight elements such as E. octopetalus showed to be present already at 1.51 m in the Rehkogelgraben section. Another nannofossils event, the FO of the seven element forms such as Eprolithus eptapetalus, may be present between the FO of E. octopetalus and Q. gartneri, defining the base of UC6b (Burnett, 1998). Seven element forms like Eprolithus eptapetalus are rarely present in the Rehkogelgraben section, again from 1.51 m onwards. However, due to strong overgrowth, these forms are hardly distinguishable from Lithastrinus moratus and/or the Lithastrinus septenarius group. Due to the problems in identification, these taxa were not used for further subzonation. 4.1.4. FO of Quadrum gartneri Q. gartneri (with four elements) defines the base of nannofossil zones CC 11/UC 7 (Perch-Nielsen, 1985; Burnett, 1998). Kennedy et al. (2005) proposed this event as a supplementary datum to date both the Cenomanian/Turonian boundary and the end of OAE2 (Tsikos et al., 2004). In Eastbourne and Pueblo it falls very close to the onset of decreasing d13C values in the upper part of the CIE, just above the 4th positive carbon isotope peak. This is in general in accordance with our findings and in contrast to Hardas and Mutterlose (2006) who found Q. gartneri below the 3rd carbon isotope peak, but reported also differing stratigraphic ranges between neighbouring sites at Demerara Rise, probably due to the rareness of this species. Concerning planktonic foraminifera datums, Q. gartneri occurs above the first occurrence (FO) of the planktonic foraminifera Helvetoglobotruncana helvetica (Bolli). This feature has generally not been observed in other sections (e.g. Luciani and Cobianchi, 1999) and may either indicate that the FO for Q. gartneri in this section is too high or that the FO of H. helvetica is (slightly) diachronous. As there are also taxonomic problems involved concerning the identification of H. helvetica (see also Gebhardt et al., 2010) we suggest that this may be an explanation for the inversion of the succession of this events. Q. gartneri is generally regarded as a good marker for the basal Turonian, although some authors, such as Bralower (1988) and Leckie et al. (2002), consider the FO of this marker species to already lie within the late Cenomanian. 4.1.5. Nannofossil zonation Based on the detailed data given above we are able to distinguish the following biozones according to the standard scheme of Burnett (1998) with slight modifications: UC4: base of section up to the LO of L. acutus. The following barren interval includes UC 5aeb. UC5ceUC6: From the FO of Q. intermedium (five elements) at 2.95 m to the FO of Q. gartneri UC7: defined by the FO of Q. gartneri at 4.20 m. 4.2. Palynological data Nearly all samples have yielded low diversity but distinctive palynological associations with stratigraphic and palaeoenvironmental significance. Terrestrial palynomorphs dominate the associations. They are represented mainly by pollen from the age diagnostic Normapolles group as well as single grains from the Tricolpites genus (Fig. 5). Gymnosperm pollen belonging to the genus Araucariacites are abundant, while fern spores are extremely rare. Marine elements are subordinate in quantity and are represented mainly by dinoflagellate cysts and foraminiferal linings.

Fig. 4. Preservation signals (etching E1 ¼ slightly, E2 ¼ moderately, E3 heavily etched; overgrowth O1 slightly, O2 ¼ moderately, O3 heavily overgrown) and relative quantitative distribution of nannofossil taxa, and nannofossil indices (NI ¼ nutrient index; NIP ¼ nannofossil index of productivity) in the Rehkogelgraben section based on selected samples. CTBI CIE ¼ Cenomanian-Turonian boundary interval carbon isotope excursion.

4.2.1. Terrestrially-derived palynomorphs The Normapolles group dominates and is comparatively well diversified throughout the entire section. Most profuse genera are Atlantopollis and Complexiopollis. The pollen assemblages are generally similar in all samples and comprise the following taxa: Atlantopollis microreticulatus Krutzsch, 1967, Atlantopollis reticulatus Krutzsch, 1967 together with Complexiopollis praeatumescens Krutzsch, 1959, Complexiopollis christae (Ameron, 1965) Krutzsch, 1967 and Complexiopollis funiculus Tschudy, 1973. They still associate with single grains from the Tricolpites genus in the older samples. These assemblages give valuable biostratigraphic information directly from the black shale-claystone carbonate-free interval. The Atlantopollis microreticulatus - Complexiopollis christae Association is recognized in the section. It is correlated to the Normapolles zonations for the Cenomanian-Turonian interval of Medus et al. (1980), Robaszynski et al. (1982) and Pavlishina and Minev (1996) from Southern France, Portugal and Bulgaria. Based on the established ranges of species, especially in ammonite calibrated sections, the concurrent presence of these pollen species is regarded as characteristic for the latest Cenomanian and especially early Turonian assemblages. Medus et al. (1980) pointed out the predominance of Atlantopollis representatives in the lower Turonian palynofloras from Southern France and Portugal and denominated them as assemblage with A. microreticulatus and A. reticulates. Pavlishina recognized the Atlantopollis microreticulatus - Complexiopollis christae Association in the scope of the lower Turonian Mammites nodosoides ammonite zone in Bulgaria (Pavlishina and Minev, 1996). Krutzsch (in Goczan et al., 1967) noticed that the representatives of Atlantopollis make their LO

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within the middle Turonian in Central Europe thus differentiating the lower Turonian palynofloras from the succeeding middle and upper Turonian ones. The species Complexiopollis christae and Complexiopollis funiculus are also biostratigraphycally important. Their FO is considered within the uppermost Cenomanian, in association with other Complexiopollis (C. praeatumescens, C. vulgaris) and continues in the Turonian, being characteristic enough for the Turonian palynofloras. The range of Complexiopollis christae is restricted to the Turonian stage (M. nodosoides, Collignoniceras woollgari and Subprionocyclus neptuni ammonite zones) and it’s LO is near the upper Turonian boundary (Pavlishina and Minev, 1996). So, the recognized Atlantopollis microreticulatus - Complexiopollis christae Association suggests early Turonian age for the black shale e claystone carbonate-free interval, thus confirming the already existing stratigraphic framework for the Rehkogelgraben section. The sporomorph assemblages are also dominated by gymnosperms from the Araucariacites genus. Pteridophyte spores are extremely rare. The dominance of Araucariacites in all samples has palaeoclimatic significance and gives signals for warm and humid climate. According to Doyle et al. (1982) this genus is related to a tropically-centered group, which is found in lowland deposits of Cretaceous age. 4.2.2. Dinocyst stratigraphic proxies Dinocysts have a poor record through the Cenomanian-Turonian boundary interval at the Rehkogelgraben section. However, a low diversity dinocyst association is identified in all studied samples. The association is dominated by the Cyclonephelium membraniphorum/Cyclonephelium compactum complex. The relative abundance of the first two taxa varies in the samples, but they are apparently more than the other species. Single specimens of Isabelidinium cooksoniae (Alberti) Lentin & Williams and Circulodinium distinctum (Deflandre and Cookson) Jansonius are encountered. Cyclonephelium membraniphorum as well as Isabelidinium cooksoniae are considered as typical Turonian species (Costa and Davey, 1992; Pearce et al., 2003). Their FO is registered within the lower Turonian and has correlation value in the type Turonian area (Robaszynski et al., 1982), in the Paris Basin and Central Europe (Foucher, 1979; Tocher and Jarvis, 1995) and in Bulgaria (Minev et al., 1996; Pavlishina and Minev, 1996). Their stratigraphic significance is reduced in the Rehkogelgraben section due to the relatively poor assemblages and especially due to the single occurrence of Isabelidinium cooksoniae. Circulodinium distinctum is a notable contributor to ‘blooms’ in the Turonian, but also with reduced stratigraphic significance. 5. Discussion 5.1. Dinocyst productivity indicators In our section the dinocyst Cyclonephelium membraniphorum/ Cyclonephelium compactum Association is encountered in all productive samples and in palynofacies rich in marine organic matter of granular amorphous composition (Fig. 5). The phytoclast group is composed mainly of small opaque lath, but also equidimensional particles. Equidimensional particles are not larger

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than the lath ones. The quantity of the amorphous organic matter increases in the black shale interval. According to Tyson (1993, 1995) large amounts of AOM results from a combination of high preservation rate and low-energy environments. The preservation of AOM is directly related to anoxic conditions and consequently, but not necessarily, correlated to high primary productivity. Following this, such palynofacies points to deposition in a distal, low-energy, stressed anoxic environment with high terrestrial input into the basin. Marshall and Batten (1988) first noted that the cyst association can be directly related to the litho- and palynofacies. They recognized the low diversity Cyclonephelium/Eurydinium association in the black shale sequences of Germany and eastern England, as well as in wells in central and southern North Sea. The association usually occurs in samples containing abundance of granular amorphous organic matter e a fact that they considered to reflect a more highly stressed environment with extension of anoxic conditions into the euphotic zone with reduced circulation. The Rehkogelgraben Cyclonephelium membraniphorum/Cyclonephelium compactum Association further correlates, especially concerning its diversity, to the already known OAE 2 dinocyst associations from English and French localities and the North Sea. Pearce et al. (2003, 2009) summarized the dinoflagellate cyst record and identified two distinct types of assemblages based on diversity and assemblage composition. The first one they termed the ‘SpiniferitesePalaeohystrichopora (SeP) Assemblage’ characterised by high diversities and abundances of gonyaulacoid (Spiniferites, Achomosphaera, Pterodinium) and peridinioid (especially Palaeohystrichophora infusorioides) cysts in the Anglo-Paris Basin, plus Chatangiella, Isabelidinium, Trithyrodinium in the North Sea. This assemblage contrasts to the low diversity CirculodiniumHeterosphaeridium (C/H) Assemblages that are typically dominated by gonyaulacoid cysts (Circulodinium, Heterosphaeridium). The (C/H) Assemblage shows a notable decrease and usually an absence of the dominant elements of the SeP Assemblage (Pearce et al., 2003). Pearce et al. (2003, 2009) analyzed these different types of dinocyst assemblages and used them as measure of surface water nutrient levels and temperature. In all these cases the CeH Assemblage is considered to be indicative for a stressed nutrientdepleted water environment. Following this, the encountered Rehkogelgraben Cyclonephelium membraniphorum/Cyclonephelium compactum Association could be attributed in general to the C-H types of assemblages. It is characterized by overall absence of high-productivity indicators and high dinocyst diversities. The association indicates low nutrient levels and the development of relatively less eutrophic water mass. Palynofacies data corroborate well suggesting low-energy, stressed anoxic environment. The composition of the dinocyst assemblages is possibly a response to the oxygen depletion and hyperstratification during the OAE 2 and the early Turonian thermal maximum. The low diversity dinocyst record at Rehkogelgraben shows some differences to the already published dinocyst successions from S Europe. Lamolda and Mao (1999) and Peyrot et al. (2011) reported relatively homogeneous dinocyst diversity pattern from the Ganuza section and three other Cenomanian-Turonian sections in the Castilian platform in Spain without any loss in diversity

Fig. 5. Selection of significant palynomorph species from the Rehkogelgraben section. All photomicrographs were taken using conventional light microscopy. A, B. Atlantopollis reticulatus (sample REH02/31, section meter 2.70); C, D. Atlantopollis microreticulatus (sample REH12, section meter 2.82); E, F. Complexiopollis christae (sample REH1, section meter 2.75); G, H. Complexiopollis funiculus (sample REH1, section meter 2.75); I, J, K. Complexiopollis praeatumescens (sample REH 04/2A, section meter 1.97); L. Tricolpites sp. (REH 04/2A, section meter 1.97); M. Cyclonephelium membraniphorum (sample REH02/31, section meter 2.70); N. Circulodinium distinctum (sample REH04/2A, section meter 1.97); O. Isabelidinium cooksoniae (sample REH02/31, section meter 2.70); P, Q. Palynofacies with granular amorphous organic matter including specimens of the C. membraniphorum e compactum Association (sample REH1, section meter 2.75).

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related to the OAE 2. Nevertheless, they all pointed that the Cenomanian-Turonian boundary event (CTBE) is manifested by higher proportion of Cyclonephelium spp. in the distal dinocyst assemblages and interpreted this to reflect possible anoxic conditions (Peyrot et al., 2011). For the Rehkogelgraben section nutrient levels have been estimated using foraminifera (Gebhardt et al., 2010) where very low abundance of planktic foraminifera together with absence of highproductivity indicators characterize the OAE 2 period and the black shales suggesting that nutrient supply was probably lower during the early Turonian. Nutrient availability was higher during times of black shale deposition, but without distinct eutrophication events. The low abundances of small benthic foraminifera were interpreted to indicate low oxic e dysoxic conditions at the sea floor (Gebhardt et al., 2010). 5.1.1. Comparison with TOC content and Rock-Eval pyrolysis parameters Palynofacies conclusions corroborate also well to the already published data from the organic matter analysis as TOC content and the Rock-Eval pyrolysis parameters (Wagreich et al., 2008). High amounts of amorphous organic matter and high values of TOC content occur during the black shale deposition and point to anoxic conditions within this stratigraphic interval. The Hydrogen Index (HI) shows comparatively high values, representing marine origin of the organic matter as it is evaluated during the microscopic investigations. It could be noted that in samples, where AOM is dominant, kerogen type II is recorded. 5.2. Productivity and nutrient measures based on nannofossils Based on quantitative nannofossil data, productivity and/or nutrient availability can be estimated by using various ratios of paleonevironment-sensitive nannofossils. First quantitative investigations, including calculation of fertility indices based on nannofossils, were applied to Cenomanian-Turonian sections from southern England (e.g. Jarvis et al., 1988; Lamolda et al., 1994; Paul et al., 1999; Gale et al., 2000). We used two indices as defined by several nannofossil workers. The nannofossil productivity index (NPI) was introduced by Eshet and Almogi-Labin (1996) as the log of the ratio of a highproductivity nannofossil taxa group (their group II: Glaukolithus (¼ Zeugrhabdotus in this study), Biscutum spp., Thoracosphaera saxea) divided by a low-productivity taxa group (their group I: Eiffellithus spp., Lithraphidites spp., Prediscosphaera spp., Thoracosphaera operculata, Vekshinella spp.). NPI values around and above zero characterize high-productivity settings, whereas values around 2 characterize low-productivity settings as defined by Eshet and Almogi-Labin (1996) in Campanian-Maastrichtian sections. Linnert et al. (2010) used the NPI value also on Cenomanian-Turonian boundary data. The second index used is the (nannofossil) nutrient index (NI) defined by Gale et al. (2000) by the quotient of Zeugrhabdotus ssp. and Biscutum ssp. divided by Watznaueria ssp. (see also Hardas and Mutterlose, 2006; Linnert et al., 2010). Low values of NI from 0 to 0.5 characterize low nutrient availability whereas values above 1 are typical for high nutrient availability and thus higher productivity (see also Linnert et al., 2010). We didn’t use modified definitions of fertility indices as used by Bornemann et al. (2005: NI) based on Aptian-Albian sections (NI) and Friedrich et al. (2005: calcareous nannofossil nutrient index, CNNI) from CampanianMaastrichtian sections as most of the taxa these authors used are not present during OAE 2 and thus missing from the Rehkogelgraben section.

NI values (as defined by Gale et al., 2000) in the Rehkogelgraben section stay below 0.40, and thus are indicative of relatively low nutrient availability throughout. Stable values before the onset of black shale deposition can be noted, from 0.16 to 0.32. After the nannofossil barren interval, values drop down within OAE 2 to 0.01e0.09, and do only slowly return to the higher pre-OAE 2 values as indicated in the last measured sample with a value of 0.16. NIP values show low values (0.51 to 0.09) below the carbon isotope excursion, then rise slightly to positive values around 0.10e0.30 in the lower part of OAE 2, and return again to lower values of 0.60 and 0.07 till the last sample which has again a slightly higher values of 0.15. NIP and NI values thus show a slightly different result, although the general picture is broadly similar. However, the NI index is more prone to preservation effects than the NIP index, as NI divides moderate dissolution resistant species by the most resistant taxa Watznaueria which may lead to lowered values due to diagenetic dissolution. NI values compare essentially taxa that have in general the same resistivity against dissolution. NI values show a drop during OAE 2, thus corroborate the findings of Gebhardt et al. (2010) who noted the absence of high-productivity foraminifers and inferred oligotrophic conditions throughout the interval. NIP values show a slight increase in productivity before the onset of OAE 2 and stay longer in the (relatively) higher productivity range than NI values, but also drop down already within the OAE 2 interval to negative values. Both indices indicate a recovery only above 5 m in the section, within the last sample. 5.3. Ocean surface temperature trend Several nannofossil taxa are interpreted as temperature sensitive, including Watznaueria ssp. which is regarded as a warm-water species (e.g. Perch-Nielsen, 1985). In addition, the genus Watznaueria is considered by some authors to be an oligotrophic nannofossil (e.g. Eleson and Bralower, 2005), or, by contrary, by other authors as a mesotrophic to eutrophic species (e.g. Lees et al., 2004). Due to the problem of selective preservation of Watznaueria ssp. (including Watznaueria barnesae and rare W. biporta) we didn’t use temperature indices based on that taxon. Due to taxonomic changes of Late Cretaceous nannofossil assemblages the calcareous nannofossil temperature index (CNTI) used by Bornemann et al. (2005) based on the ratio of the cool-water taxa including Tranolithus orionatus vs. warm-water taxon including Rhagodiscus asper, could not be used. Both Tranolithus and Rhagosdiscus are always very rare in our section and thus ratios calculated based only on these two taxa are not regarded as significant; slightly higher abundances of Tranolithus in general may indicate relatively cooler surface waters throughout the section following Bornemann et al. (2005). An increase and a peak of Watznaueria ssp. abundance can be seen in the lower to middle part of the OAE 2 interval at Rehkogelgraben, with peak abundances up to 80%. This may indicate warm water temperatures and includes the main two black shale intervals. Above that, abundance peaks of E. floralis (up to 46.5%) and a negative correlation to Watznaueria ssp. are recorded in the upper part of the carbon isotope excursion (CIE) interval (Fig. 4). Both taxa are regarded as relatively resistant against dissolution (e.g. Roth and Krumbach, 1986), so the inverse correlation in the OAE 2 interval may record a primary signal. Similar peaks of E. floralis are known at Eastbourne and other CT boundary sections (Bralower, 1988; Paul et al., 1994; Erba, 2004; Eleson and Bralower, 2005). E. floralis is regarded as a cooler-water index species (Erba, 2004; Melinte-Dobrinescu and Bojar, 2008) and thus may indicate a trend to cooling surface waters during the late stage of OAE 2 and shortly afterwards, in the early Turonian.

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6. Conclusions Based on calcareous nannofossil and dinoflagellates we redefine the stratigraphy of the Rehkogelgraben section in combination with the carbon isotope curve and planktonic foraminifera data given by Gebhardt et al. (2010). Significant nannofossil events are the last occurrences of Lithraphidites acutum and Corollition kennedyi, followed by the first occurrences of Quadrum intermedium, Eprolithus octopetalus and Quadrum gartneri. Based on nutrient and productivity indices for nannofossil and peculiar dinocyst associations we infer a low nutrient, oligotrophic environment for at least the middle and upper part of the OAE 2 interval as defined by the positive carbon isotope excursion. Palynology indicates an overall absence of high-productivity indicators and high dinocyst diversities. Palynofacies data corroborate well suggesting low-energy, stressed anoxic environment with high terrestrial input in the basin. Our results are in accordance with data for benthic foraminifera assemblages (Gebhardt et al., 2010) and indicate the presence of two oceanographic modes in relation to nutrient availability and productivity during OAE 2. Whereas Atlantic sites such as the Demerara Rise are characterized by high productivity especially in the lower part of the interval, some Tethyan sites like the Ultrahelvetic Rehkogelgraben section, and northern European “Boreal” sites like Eastbourne (UK) and Wunstorf (Germany) are characterized by oligotrophy for at least the greater part of OAE 2, despite the presence of (regional) black shales. This further sheds light on the possibility of regional differences in the development and response to OAEs and a concentration of high productivity and anoxia in the Atlantic such as suggested by regional oceanographic models as given by Trabucho Alexandre et al. (2010).

Acknowledgements Field work was supported by IGCP 555 grants of the Austrian Academy of Sciences. This is a contribution to IGCP project IGCP 555. We thank V. Vajda and an anonymous reviewer for their critical remarks that considerably improved the paper.

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