The birth of the Paratethys during the Early Oligocene: From Tethys to an ancient Black Sea analogue?

Global and Planetary Change 49 (2005) 163 – 176 www.elsevier.com/locate/gloplacha

The birth of the Paratethys during the Early Oligocene: From Tethys to an ancient Black Sea analogue? H.-M. Schulz a,*, A. Bechtel b, R.F. Sachsenhofer b a

Institute for Geology and Paleontology, Technical University of Clausthal, Leibnizstrasse 10, D-38678 Clausthal-Zellerfeld, Germany b Department of Geosciences, Montanuniversita¨t Leoben, Peter-Tunner-Str. 5, A-8700 Leoben, Austria Received 14 May 2004; accepted 15 July 2005

Abstract Deeper water black shales, overlain by coccolith-bearing marlstones representing the incipient Paratethys (example: Early Oligocene; Austrian Molasse Basin), have sedimentary characteristics similar to those of the Holocene Black Sea since 7500 years bp. Framboid pyrite size, biomarker and C–N-isotope data additionally indicate that isolation of the Paratethys resulted in Black Sea-type characteristics during nannoplankton zone NP 23. In contrast to the estuarine circulation across the Bosphorus since 7500 years bp, marine conditions prevailed in the incipient Paratethys during NP 21/22. Nitrogen was fixed and low organic carbon accumulation rates prevailed. In both settings a vertical density water-column stratification was accompanied by photic zone anoxia, and by anaerobic methane oxidation in the Paratethys. In the Paratethys increased run off, starting in NP 22, led to estuarine circulation during NP 23. During this period cyclic blooms of calcareous nannoplankton resulted in high calcite accumulation rates which diluted the coeval clay sedimentation. Similar sedimentary features in the Black Sea and the Paratethys during the earliest Oligocene are result from opposite paleoceanographic developments, both leading to estuarine circulation patterns. In the Black Sea, permanent photic zone anoxic conditions were established 7500 years bp in response to the first invasion of saline Mediterranean waters into the former freshwater lake. In contrast, brackish surface water in the Paratethys resulted from nutrient-rich freshwater diluting the marine water body. D 2005 Elsevier B.V. All rights reserved. Keywords: Paratethys; Oligocene; Black Sea; organic carbon; estuarine; circulation

1. Introduction

* Corresponding author. Tel.: +49 5323 723112. E-mail addresses: [email protected] (H.-M. Schulz), [email protected] (A. Bechtel), [email protected] (R.F. Sachsenhofer). 0921-8181/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2005.07.001

The Eocene/Oligocene (E/O) boundary at about 33.9 My bp (Gradstein and Ogg, 2004; Gradstein et al., 2004) is regarded as a significant step in the global cooling during the Cenozoic (Shackleton and Kennett, 1975), and marks a prominent fossil greenhouse–icehouse transition. Contemporaneous to this climatic

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change, strong tectonic activity changed the Eurasian configuration (Fig. 1). The Paratethys became isolated and the configuration of the later Mediterranean Sea became apparent (Ba´ldi, 1980; Rusu, 1988).

c Paratethys

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Early Oligocene - Middle Kiscellian - Solenovian Study area

Incipient Black Sea basin

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Early Oligocene - Early Kiscellian - Pshekian

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Western Tethys

Sea Land Evaporites

Late Eocene - Priabonian - Beloglinian

Fig. 1. Paleogeography during the Late Eocene and Early Oligocene (redrawn after Ro¨gl, 1998). a = open circulation; b = initiation of Paratethys basins; c = Paratethys isolation. Note: See Popov et al. (2004) for a more detailed presentation of the paleogeography.

The concept of an isolated Parathetyan marginal sea is based on a specific faunal and floral development (dParatethys bioprovinceT). Formerly restricted to the Neogene (Laskarev, 1924), the dParatethysT concept was extended to the Paleogene (Ba´ldi, 1984, 1989; Rusu, 1988) and refers to the beginning dProtoparatethysT according Senes and Marinescu (1974), and Rusu (1988). The birth of the Paratethys as a marginal sea at the E/O boundary is reflected by the basin-wide occurrence of organic-rich sediments (Popov et al., 1993; Ro¨gl, 1998, 1999), which act as hydrocarbon source rocks in many parts of the Paratethys (Schmidt and Erdogan, 1996; Ziegler and Roure, 1999). This sedimentary break is also reflected by the change from highly diverse to monospecific faunal and floral biota (details in Ba´ldi, 1984 and Ro¨gl, 1998). Organic carbon accumulation during this period was a consequence of stagnant bottom water conditions (Popov and Stolyarov, 1996; Sissingh, 1997; Schulz et al., 2002). Moreover, black shale deposition in the Eastern Paratethys extended into the Early Miocene and, thus, represents the most extensive, longest anoxic event of the Cenozoic dNorth-PeritethysT history (Popov and Stolyarov, 1996). However, details of paleoceanographic processes along the E/O transition are still a matter of debate. A first description of sedimentary events in the Paratethys during its isolation from the Tethys was published by Ba´ldi (1984). The author speculated that oceanographic connections in the early Paratethys may be best described as analogues to those of the modern Black Sea. During the Early Oligocene, the only open connection of the Paratethys to the southern marine realm existed in the west (Fig. 1) and marine water inflow was continuously restricted due to narrowing of seaways. The early Oligocene dBlack Sea-typeT sequence described here (coccolith-bearing marlstone underlain by black shale) covered large areas of the Paratethys. Lithological similarities, especially of the coccolithbearing marlstones (Dynow Marlstone), are evident in isochronous sediments of Upper and Lower Austria (Ro¨gl et al., 2001) and in the Carpathians (Krhovsky´ et al., 1993). The study area is located in the Molasse Basin of Upper Austria, representing the western part of the Central Paratethys (Fig. 1). This region was chosen because it acted as a sensitive gateway for water

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data come from cores of the well Oberschauersberg 1 (Osch1, drilled in 1985; for location see Fig. 2). Although the thermal maturity of these sediments is low (R r b 0.35%), we combined information from several independent proxy signals in order to avoid misinterpretations due to diagenetic effects.

exchange with the open ocean, and provides signals indicative for basin isolation and oceanographic processes that were stored preferentially in these sediments. Aims of the present study are to reconstruct the paleoceanographic development from open marine to restricted conditions (Paratethys) and to test whether an ancient Black Sea-type marginal sea may have spread over the early Paratethys for about four million years (34–30 Ma bp). For this purpose, we extract information from new and published multi-proxy data sets, re-interpret these data and compare them with recent (paleo)oceanographic data and phenomena from the Black Sea (e.g. Calvert and Karlin, 1998; Michaelis et al., 2002; Peckmann et al., 2001). Our

13∞

100 km 11°

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Bohemian CZ Massif

2. Geologic overview Sediments from the western and central Paratethys occur today along the front of the Alpine–Carpathian nappe system. The Alpine Molasse Basin is an east– west trending foreland basin, which resulted from the

17°

Vienna

Molasse Basin

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S K

Crystalline rocks / Bohemian Massif 48° 48 48∞

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Periadr iatic Lin.

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47° 47 47∞

Linz

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Osch1

Di1 Hobg1 Imbricated Molasse

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Interval transit time (µsec/m) Eggerding Fm. Depth (m)

10 m

Dynow Marlstone

Schöneck Fm.

Hobg1

Tratt7

Fi1

Osch1

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Fig. 2. Location map of working area and sonic logs of the Scho¨neck Fm. and Dynow Marlstone in five wells analysed. Hobg1 = Hochburg1, Tratt7 = Trattnach7, Fi1 = Fischlham1, Osch1 = Oberschauersberg1, Di1 = Dietach1. For chronostratigraphy of Scho¨neck Fm., Dynow Marlstone and Eggerding Fm. see Fig. 3.

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subduction of the southern margin of the European plate beneath the Adriatic plate (Ziegler, 1987). The main deepening and widening of the Molasse Basin occurred during the Early Oligocene (Sissingh, 1997). Details on the geology and the Paleogene sediments of the Austrian Molasse Basin were summarized by Wagner (1996, 1998), Piller and Rasser (2001) and Steininger and Wessely (1999). The oldest sediments marking the initial evolution of the Molasse Basin are Late Eocene in age. During the Priabonian shallow marine sediments including corallinacean limestones onlapped northward onto fluvial and limnic deposits. The shallow marine sediments grade southwards into slope sediments with a rich fauna showing strong Tethyan–Mediterranean affinity (Steininger and Wessely, 1999). The Priabonian deposits are overlain by organic-rich rocks of the Scho¨neck Formation and by nannofossil chalks (Dynow Marlstone, middle Kiscellian, NP 23; Fig. 2). The deposition of the Scho¨neck Fm. across the lithotamnium

limestones reflects the rapid deepening of the basin in response to thrust loadings (Wagner, 1996). The sedimentation of the Dynow Marlstone was additionally controlled by reduced water salinities (Ba´ldi, 1984; Ro¨gl, 1998, 1999; Ro¨gl et al., 2001) and spread over the entire Paratethyan Basin. This sedimentary sequence—black shale of the Scho¨neck Fm. overlain by the coccolith-bearing Dynow Marlstone—resembles sediments in the Black Sea, which were deposited in this deep basin since 7500 years bp (Fig. 3). Finally, the Dynow Marlstone changes into predominantly marlstones with high organic carbon contents (Eggerding Fm.) during nannoplankton zone NP 24.

3. Results Rocks rich in organic carbon (TOC) were deposited in the early Paratethys. TOC values across the E/O boundary are about 2%, but may rise to more

Fig. 3. Organic carbon (TOC) and calcite content (both wt.%) in sediment cores from the Black Sea and the Central Paratethys. Information for the Black Sea deep-water core (BS4-9) are taken from Calvert and Karlin (1998). Here, freshwater muds (unit 1) are overlain by an organic-rich sapropel (unit 2) and a modern, finely laminated coccolith marl (unit 3). TOC and calcite data across the E/O-transition were determined on sediment cores from well Oberschauersberg 1 in Upper Austria (Schulz et al., 2002, 2004). White numbers in black arrows indicate the three sedimentary cycles. AS = Ampfing Sandstone.

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than 10% TOC in the black shale of the upper Scho¨neck Fm. (Fig. 3). TOC values abruptly decline at the top of this formation, and increase within the Dynow Marlstone to more than 3% TOC at the base of the Eggerding Fm. Strong scattering of the calcite contents in the lower part of the Scho¨neck Fm. is primarily caused by episodic planktonic and benthic foraminiferal blooms, but is also the result of authigenic carbonate formation (Fig. 3). The black shale unit of the upper Scho¨neck Fm. contains two calcareous marlstone intercalations. These units, composed of micritic laminae with a nodular structure (Schulz et al., 2002), resemble isochronous sediments from the Outer Carpathians. They are interpreted as zooplankton faecal pellets made up of coccolith debris from planktonic copepods (Haczewski, 1989) and point to coccolithophoride blooms prior to massive blooms in NP 23. The Dynow Marlstone is composed of three sedimentary cycles, each starting with carbonates made up by coccolithophorides and each ending with increasing shale and TOC contents. The increasing TOC contents of the last cyle in the Dynow Marlstone transfer to the organic-rich marlstones of the Eggerding Fm. The hydrogen index (HI) increases from 200 mgHC/gTOC to more than 600 mgHC/gTOC in the black shale unit of the Scho¨neck Fm. The HI values scatter from 400–600 mgHC/gTOC in the Dynow Marlstone Fm. and the lower Eggerding Fm. (Fig. 4). The predominant primary producer of sterols (sterane precursors) is algal phytoplankton, inhabiting the photic zone (Volkman, 1986). TOC-normalised aaasteranes and their ahh-isomers in the C27 to C29 range (summarized as steranes concentrations in Fig. 4) may be regarded as primary productivity proxies. According the results, the trophic level remained constantly high during deposition of the lower Scho¨neck Fm. The nutrient level was also not influenced by a volcanic activity near the E/O boundary indicated by discrete, thin tuff layers (Fig. 4). Within the black shale unit of the Scho¨neck Fm. the sterane concentrations show a significant decrease. This pattern suggests a sudden limitation in productivity of sterols producing organisms. Moderately high values in the Dynow Marlstone suggest an increase in the trophic level after black shale deposition. Additionally, low sterane values may also be a result of natural sulfurization of these biomarkers during early diagenesis

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(Kohnen et al., 1991). However, sulfurization of sterols was restricted due to low sulfur contents (3% S on average in the black shale of the Scho¨neck Fm. and 1% S on average throughout the Dynow Marlstone). Framboid pyrite growth acted as the main fixing process of available sulfur. Throughout the investigated sedimentary column a trimethylated C29-chroman (tri-MTTC) predominates by far over its dimethylated counterpart (di-MTTC), indicating euhaline to mesohaline (30–40x) conditions (Sinninghe Damste´ et al., 1987; Schwark and Pu¨ttmann, 1990). Nevertheless, the di- / tri-MTTC ratio increases within the lower part of the Scho¨neck Fm. and reaches 0.4 near the marlstone intercalations (zooplankton faecal pellets of planktonic copepods made up of coccolith debris) in the black shale interval (Fig. 4). Above the marly intercalations, the values abruptly decrease and remain on this low level until the top of the Dynow Marlstone. This pattern indicates a slight increase in the surface water paleosalinity throughout the Scho¨neck Fm. This scenario was followed by a massive fall in paleosalinity at the transition from the Scho¨neck Fm. to the Dynow Marlstone as a higher alkylation degree of chromanes is indicative for lower paleo-salinities. Apart from aromatization of the precursor h-carotene universally produced by phytoplankton (Koopmans et al., 1996), aryl isoprenoids are thought to be derived from isorenieratene. This carotenoid is a specific for the photosynthesis of the brown strain of green sulfur Chlorobiaceae bacteria (Summons and Powell, 1987). These organisms are phototrophic anaerobes and, thus, require both light and H2S for growth. In modern environments they appear in sulphate-containing water bodies which are sufficiently quiescent and organic-rich to enable sulfide production close to the photic zone (Summons, 1993). A trimethyl-substituted C14-aryl isoprenoid, a degradation product of isorenieratene, was not detected within the lower part of the Scho¨neck Fm., but its occurrence in the upper part and the Dynow Marlstone indicates photic zone anoxia during deposition of the younger sedimentary succession (Fig. 4). However, the applicability of aryl isoprenoids as proxies for photic zone anoxia, derived from green sulfur Chlorobiaceae, is restricted within the upper black shale interval of the Scho¨neck Fm. due to the controversial pyrite size distributions (Fig. 4). At the end of the deposition of

168 H.-M. Schulz et al. / Global and Planetary Change 49 (2005) 163–176 Fig. 4. Paleoceanographic proxy data across the E/O-transition in well Oberschauersberg 1 in Upper Austria. The euxinic to oxic–dysoxic classification is based on pyrite framboid size distributions according Wilkin et al. (1996): low mean values (Am) and low standard deviations r (Am) point to euxinic conditions, high values for both indicate oxic–dysoxic conditions. Besides clay mineralogy, all data are compiled from Schulz et al. (2002, 2004).

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the Scho¨neck Fm. the interpretation of framboid pyrite sizes indicates oxic to dysoxic bottom water, but maximum C14-aryl isoprenoid concentrations have been detected. Here, different precursors (probably h-carotene) of these biomarkers may have prevailed. Bulk rock nitrogen isotopes (d 15Ntot) are an additional proxy for the trophic level of the water column and its denitrification activity (e.g. Ganeshram et al., 2000, and references herein). In general, d 15Ntot values of 2x to + 4x predominantly point to nitrogen fixation (Fig. 4). However, the downcore variability in d 15Ntot indicates a direct interaction between the surface water trophic level and nitrogen isotope fractionation. This phenomenon is reflected by the parallel occurrence of low steranes concentrations and increasing d 15Ntot values in the black shale of the Scho¨neck Fm. Here, the observed trends reflect less N-isotope fractionation due to decreasing nutrient availability (according Altabet, 1996 and Calvert et al., 1992). An influence by increased denitrification activity can be ruled out due to the increased oxygen level which prevailed during the sedimentation of the upper black shale unit (see interpretation of framboid pyrite sizes in Fig. 4). A sudden sedimentary break marks the onset of Dynow Marlstone. Within each cycle in the Dynow Marlstone increasing d 13TOC values run parallel to increasing d 15Ntot values. This change points to initially enhanced primary productivity due to increased runoff (low di- / tri-MTTC in Fig. 4) which led to periods of increased nutrient utilization and massive blooms of calcareous nannoplankton. Carbon isotope ratios aid to discriminate between marine and terrestrial organic matter in sediments, and the identification of different land plants (e.g. Meyers, 1994; additional references in Rullko¨tter, 2000). As maceral analysis and HI values indicate that the organic matter composition remains fairly constant throughout the whole investigated sedimentary succession, up to the drop in the d 13TOC values to b 29x within the black shale unit of the Scho¨neck Fm. (Fig. 4) which coincides with the carbon isotopic signature of organic material produced at the E/O transition (Hayes et al., 1999; Hertelendi and Veto¨, 1991). A correlation between the low d 13TOC values b 29x and the minimum in sterane concentrations may additionally indicate that decreased primary productivities during sedimentation enhanced the nega-

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tive shift in the d 13TOC values. Two alternative scenarios have to be evaluated which may arise from the low d 13TOC values at the top of the Scho¨neck Fm.: (a) anaerobic methane oxidation (AOM) and (b) fluvial delivery of dissolved inorganic carbon and terrigenous organic matter both isotopically light. An interpretation of the negative shift of the d 13TOC values to b 29x in the lower black shale interval of the Scho¨neck Fm. due to AOM remains speculative as a clear carbon isotopic signal for this process is not efficiently transported by sinking particles into the underlying sediment (according results in the Black Sea according Wakeham et al., 2003). However, AOM can be excluded for the upper black shale interval in view of the results of framboid pyrite size analysis. Input of isotopically light dissolved inorganic carbon and terrigenous organic matter by fluvial activity would explain the change in size of framboid pyrite, but was not found in the relevant biomarker composition (see Schulz et al., 2002). The prominent climate change at the E/O boundary is reflected by a change in clay mineralogy (Fig. 4). Less kaolinite was formed and transported into the Paratethys as a result of decreasing tropical weathering intensities during the early Oligocene. A renewed increase during NP 23 may be attributed to changed circulation patterns and, thus, different source areas. Furthermore, the occurrence of smectite/illite mixed-layer minerals coincides with volcanic activity reported by Ba´ldi (1984), Rusu (1988), and von Blankenburg and Davies (1995).

4. Discussion Although significantly smaller, the modern Black Sea with its estuarine circulation pattern may be regarded as a recent analogue of the incipient Paratethys at the E/O boundary. Today, the Black Sea is the largest euxinic inland water basin on earth, being devoid of oxygen and containing abundant sulfide from about 100 m depth to the seafloor at about 2200 m (insert in Fig. 3). This elliptic basin has only a narrow opening to the shallow (b75 m deep) Bosphorus Strait; otherwise, it is a completely enclosed marginal sea. In comparison, the Paratethys had similar narrow openings to the south, west and north during the Early Oligocene (Fig. 1c, 5).

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4.1. Comparison of circulation pattern Dohmann (1991) presented an anti-estuarine circulation model for the early Oligocene western Paratethys (modified in Fig. 6a). This model is supported by the increasing di- / tri-MTTC ratios which suggest increasing surface salinities due to evaporation of marine water. According to Wagner (1996), predominant eastward directed winds during the Early Oligocene enhanced Tethyan water inflow from the west. Marine, highly diverse microfossil assemblages prevailed during NP 21 (e.g. Ro¨gl, 1998). The occurrence of a macrofauna dominated by endemic taxa (e.g. the pteropod Spiratella) during NP 22 in the Hungarian Paleogene basin, located east of the study area (Fig. 5), indicates first ingressions of boreal water (Ba´ldi, 1984). This event favoured a low diverse microfossil assemblage. During NP 22 freshwater influxes from the continent diluted the surficial Tethyan water, deteriorated the circulation pattern and resulted in brackish surface water salinities (decline of di- / tri-MTTC ratios; Fig. 4). As a consequence, a break-down of the water column stratification (see framboid pyrite interpretation in Fig. 4) led to a ventilation of the bottom water by oxygen (Fig. 6b). The geochemical findings (di- /

tri-MTTC ratios) are supported by micropaleontological data which indicate less marine influence from the Prealps in the west to the Carpathian flysch basin in the east (Ro¨gl, 1998). Ongoing narrowing of seaways during NP 23 (Fig. 1c) restricted the surficial inflow of salty water. Massive freshwater runoff spread as surface flows across the western Paratethys (low di- / tri-MTTC ratios; Fig. 4), which was inhabited by bivalves endemic in the Paratethys and a low diverse, monospecific microfossil assemblage. This estuarine circulation pattern resembles the modern situation in the Black Sea. Across the Bosphorus freshwater leaves the Black Sea as a surface flow which is undercut by inflowing of salty Mediterranean waters. In comparison with modern marginal seas (here: Black Sea, but also the Baltic Sea; see Feistel et al., 2003), a positive water balance can be assumed for the Central Paratethys during NP 23. 4.2. Comparison of stratification The sharp density stratification, accompanied by weak vertical circulation and mixing, inhibits ventilation of sub-pycnocline waters of the Black Sea from the surface as it did in the incipient Paratethys. Organic matter, continually sinking and decomposing, has thus

Fig. 5. Highly schematic paleogeograpy and seaways during the Early Oligocene (Kiscellian) in the Western and Central Paratethys (modified after Dohmann, 1991). Location of well Osch1 is represented by derrick symbol.

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Fig. 6. Paleoceanographic models for the Central Paratethys (upper NP 21 to NP 23). The influence by marine water and stratification pattern during (a) NP 21 was pertubated by influx of cold freshwater during NP 22 (b), which finally led to the establishment of an estuarine circulation pattern during NP 23 (c). This change was a consequence of tectonically initiated narrowing of seaways and coincides with global cooling after the E/O boundary. Circulation pattern during NP 23 (c) is compared with the modern Black Sea situation (since 7500 years bp). For location of cross sections see Fig. 5. Location of well Osch1 is represented by derrick symbol.

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led to development of a permanent anoxia and high concentrations of hydrogen sulfide below depths of 100–150 m. This permanent photic zone anoxia in the Black Sea, established 7500 years bp, marks the first invasion of saline Mediterranean waters (Jones and Gagnon, 1994). Photic zone anoxia in the Black Sea is highlighted by the occurrence of isorenieratene in sediments (Huang et al., 2000). A similar vertical density water-column stratification has to be assumed for the Paratethys, based on evidence of diagenetic products of isorenieratene (C14 aryl isoprenoids; Fig. 4).

the Black Sea, the modern laminated coccolith marl containing 2–6% TOC (unit 3) overlies a finely laminated sapropel (unit 2) containing 5–20% TOC. The boundary between the sapropel and the overlying modern laminated marls at 2700 years bp (Jones and Gagnon, 1994) marks the first invasion of the basin by coccolithophorids imported from the Mediterranean (Ross et al., 1970). In comparison, highly diverse nannoplankton floras existed prior to NP 23 in the Paratethys, but decreased salinities and an increased nutrient supply caused changes to monospecific massive coccolithophorid blooms during NP 23.

4.3. Comparison of sedimentation pattern The occurrence of the black shale unit in the Scho¨neck Fm. and the overlying coccolith-bearing, whitish Dynow Marlstone resembles the sedimentary history of the Black Sea since the last 7500 years (Fig. 3). In

4.4. Comparison of trophic level and organic carbon accumulation Calvert and Karlin (1998) demonstrated that the accumulation rate of organic carbon (TOC-AR) in the

Fig. 7. Comparison of oceanographic processes in the Black Sea (since 7500 years bp) and in the Central Paratethys (after E/O transition in well Oberschauersberg 1). Information regarding total organic carbon accumulation rates (TOC-AR) for the Black Sea deep-water core (BS4-9) are taken from Calvert and Karlin (1998). References about further Black Sea paleo-features are listed in the text. Paratethyan time scales according Ro¨gl (1996). AOM = Anaerobic oxidation of methane. A TOC-AR value of 0.19 gm 2 year 1 for the Paratethys-black shale unit in NP 22 (0.6 My duration, 1.35 m thickness) was calculated after Calvert and Karlin (1998) for direct comparison. The accumulation rates (AR) in (gm 2 year 1) derived from: AR = S  D  (1-P), where S = linear sedimentation rate = 2.3  10 4 cm/year, and porosity, P ~ 15%. Mean TOC content is about 5%, and organic matter density, D was calculated as (TOC, %  2) = 1.0 gcm 3 according Calvert and Karlin (1998). During sedimentation of the Dynow Marlstone (~ NP 23: ~ 2.3 My duration, ~ 6.50 m thickness), a TOC-AR of 0.09 gm 2 year 1 was calculated, where S = linear sedimentation rate = 2.83  10 4 cm year 1, and porosity, P ~ 20%. Mean TOC content is about 2%, and organic matter density, D was also calculated as (TOC, %  2) = 1.0 gcm 3.

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Black Sea sapropel (unit 2) is not significantly different from that in the coccolith marl of unit 3. TOC-AR for both units were less than 3 gm 2 year 1. Thus, the higher organic contents of the sapropel were caused by less dilution of the organic fraction rather than by better preservation of organic matter. The low TOCAR proposes that the Black Sea sapropel is not a necessarily characteristic of an anoxic basin. TOCAR of about 0.2 gm 2 year 1 prevailed during sedimentation of the upper black shale unit of the Scho¨neck Fm. (NP 22). Moreover, the sedimentation of the black shale interval is coupled to an abrupt decrease in sterane concentrations (Fig. 3), indicating low primary productivities. Primary productivities increased again with onset of Dynow Marlstone sedimentation, but mainly stimulated cyclic and massive blooms of coccolithophorides, which diluted the prevailing low TOC-AR (about 0.1 gm 2 year 1; for TOC-AR calculation see explanation in Fig. 7).

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5. Conclusions The Black Sea and the Mediterranean Sea were reconnected during the Holocene sea-level rise. Thus, the evolution of the modern Black Sea was controlled by the invasion of dense, saline Mediterranean water into the former freshwater lake since 7500 years bp. In contrast, the evolution of the initial Paratethys was mainly controlled by narrowing of seaways connecting it to the Tethys. Paleoceanographically, this led to ingressions of boreal water from the east initially as undercurrents (anti-estuarine circulation), then to ingressions of freshwater runoff overflowing and diluting the former Tethyan seawater (estuarine circulation). Both the scenarios for modern Black Sea and the early Paratethys during NP 23 led to comparable oceanographic and stratification characteristics (Fig. 6). Thus, the depositional processes in the Black Sea since 7500 years bp resemble those in the incipient western Paratethys during the early Oligocene.

4.5. Comparison of biochemical cycles The major present-day sink for methane generated in Black Sea sediments is anaerobic methane oxidation with lower amounts being oxidized through aerobic methanotrophy (Schouten et al., 2001). The anaerobic oxidation of methane is thought to involve methane oxidizing archaea in consortium with sulfate reducing bacteria (Boetius et al., 2000; Pancost et al., 2000). Considering the annual methane production in the sediments, more than 98% of this CH4 is oxidized anaerobically in the euxinic water column (Reeburgh et al., 1991). Evidence for this recent process are 13Cdepleted archaeal-derived compounds in particulate organic matter from the euxinic waters of the Black Sea (Schouten et al., 2001). We speculate that anaerobic methane oxidation may have also prevailed during deposition of the black shale unit of the Scho¨neck Fm. (d 13C b 28x; Fig. 4). Additionally, nitrogen was mainly fixed in Paratethyan sediments. A shift to higher d 15N values of + 3x in the black shale unit of the Scho¨neck Fm. reflects oligotrophic conditions as faster uptake kinetics cause preferential assimilation of 14 N (relative to 15N) when nutrients are abundant (Wada and Hattori, 1978). Furthermore, changes to heavier d 15N values in the single cycles of the Dynow Marlstone reflect decreasing nutrient levels.

5.1. Black Sea (7500–2700 years bp) vs. Central Paratethys (black shale unit in the Scho¨neck Fm.) In both depositional settings, photic zone anoxia caused by density stratification favoured the deposition of organic-rich rocks. This is despite low organic carbon accumulation rates. Increased runoff led to short-term oxygenation in the Paratethys at the end of this period. Anaerobic methane oxidation, characteristic for the modern Black Sea, remains speculative in the oligotrophic incipient Central Paratethys. In contrast to this period of the Black Sea, an anti-estuarine circulation pattern prevailed in the Central Paratethys and was changed by freshwater inflow. 5.2. Black Sea (since 2700 years bp) vs. Central Paratethys (Dynow Marlstone) Across the inlets of both marginal seas, estuarine circulation patterns established and massive blooms of coccolithophorides favoured the deposition of marlstones. Organic matter accumulation was slow and the organic matter became further diluted by precipitation of carbonates. Blooms of low salinity-adapted coccolithophorids in the Paratethys were favoured by fresh-

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water influx and an increased trophic level. Here, photic zone anoxia again established. In contrast, both processes continued in the Black Sea, furthermore accompanied by anaerobic methane oxidation. In the latter basin, blooms of coccolithophorids are due to an invasion of Emiliania huxleyii from the Mediterranean Sea. Due to low accumulation rates of organic carbon, sediments from both marginal sea settings are not necessarily characteristic of anoxic basins.

Acknowledgements The authors thank Rohoel-Aufsuchungs AG (Vienna) for providing core material and data. Technical assistance provided by colleagues at the Geological Departments in Clausthal and Leoben is gratefully acknowledged. We are grateful to W. Piller, P. A. Ziegler and G. Georgiev for their constructive reviews of the manuscript. Remarks to an earlier draft by P. M. Meyers, F. Steininger and important suggestions by F. Ro¨gl and an anonymous reviewer improved the manuscript.

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