Integrated stratigraphy of Early Aptian black shales ... - Ingenta Connect

Newsletters on Stratigraphy, Vol. 45/2, 115–137 Published online May 2012

Article

Integrated stratigraphy of Early Aptian black shales in the Boreal Realm: calcareous nannofossil and stable isotope evidence for global and regional processes C. Bottini1* and J. Mutterlose1 With 8 figures, 1 plate and 2 appendices Abstract. The Early Aptian is marked by an event of widespread anoxia in the oceans, known as Oceanic Anoxic Event (OAE)1a. During this time the Lower Saxony Basin (LSB), constituting the southern extension of the Boreal-Arctic Sea, was affected by the deposition of finely laminated black shales of the Fischschiefer (FS) considered to be the product of OAE 1a. This study focuses on Upper Barremian–Lower Aptian sediments from three different localities in northern Germany encompassing the FS. The proposed integrated litho-, bio-, and chemo-stratigraphy provides an accurate time control for correlation and for detecting the timing of the processes that affected the LSB during the deposition of the FS. The paleoecological and paleoclimatic reconstructions based on calcareous nannofossils indicate that sedimentation during the Late Barremian was mostly depending on regional conditions related to the paleogeography of the LSB. The deposition of the FS was instead mainly driven by mechanisms operating on a global scale and associated to OAE 1a: a warming event, also detected at low latitudes, was accompanied by high primary productivity and influx of cosmopolitan taxa through new seaways opened to the Tethys. In the late Early Aptian local factors prevailed again on sedimentation although paralleled by a decrease in temperature documented at different latitudes which probably favoured the migration of Boreal species southwards. Key words. Fischschiefer, Early Aptian, Calcareous nannofossils, Biostratigraphy, Paleoceanography, Carbon isotope

1.

Introduction

The Barremian–Early Aptian interval is characterized by large variations in environmental, paleoceanographic and paleobiological conditions. A greenhouse climate was accompanied by high primary productivity and by accelerated evolutionary rates among calcareous nannoplankton (e. g. Erba 1994, Sinton and Duncan 1997,

Larson and Erba 1999, Leckie et al. 2002, Jenkyns 2003, Erba 2004, Ando et al. 2008). The combination of these factors favoured the deposition of organic-rich black shales at global scale resulting in the Early Aptian Oceanic Anoxic Event (OAE) 1a (~ 120 Ma). During the Early Aptian, the Lower Saxony Basin (LSB), northwest Germany, was characterized by the deposition of finely laminated black shales known as

Authorʼs address: 1 Institute for Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Universitätsstraße 150, 44801 Bochum, Germany * Corresponding author’s E-Mail address: [email protected] © 2012 Gebrüder Borntraeger, Stuttgart, Germany DOI: 10.1127/0078-0421/2012/0017

www.borntraeger-cramer.de 0078-0421/2012/0017 $ 5.75

116 C. Bottini and J. Mutterlose Fischschiefer (FS). Based on integrated stratigraphy, the FS has resulted to be coeval with the lithological expression of OAE 1a in the Tethys (Selli Level) and in other localities (e. g. Bischoff and Mutterlose 1998, Mutterlose et al. 2009, Malkoč et al. 2010). Although the FS is the expression of OAE 1a in the Boreal Realm, it is not fully understood if the paleoclimatic and paleoecological changes which influenced the deposition of the Selli Level (and its equivalents) at low latitudes, also affected the deposition of the FS in the LSB. Paleoecological studies suggested the deposition of the FS to be mainly related to regional conditions concerning the paleogeography of the LSB (shallow and epicontinental sea). Warm conditions during the Late Barremian–Early Aptian (Mutterlose et al. 2010), accompanied by slightly reduced salinity (Bischoff and Mutterlose 1998) and high nutrients content in surface waters (Habermann and Mutterlose 1998) have been considered to be responsible for the development of anoxic conditions in bottom waters. It is, however, not clear to what extent local factors influenced and/or promoted the deposition of the laminated black shales in the LSB. A prerequisite for understanding the paleoecological and paleoclimatic evolution of the LSB within the supra-regional contest, is to rely on a constrained stratigraphic framework.

The aim of this study is to: a) develop an integrated stratigraphy for two sections in northern Germany based on calcareous nannofossils and carbon isotope across the Upper Barremian–Lower Aptian interval, b) to reconstruct the paleoclimatic, paleoecological and paleoceanographic factors that controlled the deposition of the FS in the LSB by using calcareous nannofossils as paleoecological and paleoclimatic tracers, c) to understand the interaction between global changes, associated with OAE 1a, and regional factors during the deposition of the FS by comparing the litho-, bio- and chemo-stratigraphic records from the LSB with those from the North Sea, SE France, Italy and Pacific Ocean.

2.

Geological Setting

In the Early Cretaceous, northwest Europe was composed of a number of marine epicontinental basins, which nowadays cover the area of the North Sea, the Netherlands, northern Germany and Poland (Fig. 1A). The southernmost of these basins, the LSB, formed the southern extension of the Boreal-Arctic Sea, paleogeographically located between the Boreal Realm in the north and the Tethys in the south. The LSB was about 280 km long and 80 km wide. During the Bar-

Fig. 1. A) Paleogeographic map of the Aptian of northwest Europe (from Malkoč et al. 2010). Below, enlargement showing the Lower Saxony Basin (modified from Kemper 1995) and paleo-position of studied sections: Alstätte 1, Hoheneggelsen KB9 core and Rethmar. B) Bio-, and litho-stratigraphy of the Barremian and Aptian in northwest Europe (modified from Bischoff and Mutterlose 1998, Malkoč et al. 2010). Stratigraphic ranges of the sections are indicated.

Integrated stratigraphy of Early Aptian black shales

remian and Aptian the central part of the basin was characterized by the deposition of more than 2,000 m of sediments mostly consisting of shales and mudstones, while shallow water sediments were deposited along the margins (e. g. Kemper 1979, Mutterlose 1992a, Mutterlose and Böckel 1998, Mutterlose and Bornemann 2000, Mutterlose et al. 2009).

3.

Material and methods

For this study we have analyzed samples from three different localities situated in northern Germany encompassing the Upper Barremian to the Lower Aptian: Rethmar, Alstätte 1, and Hoheneggelsen KB9 (Fig. 1B). Rethmar and Alstätte 1 have been investigated for calcareous nannofossil biostratigraphy, for CaCO3 and Total Organic Carbon (TOC) content (146 bulk-rock samples) and for carbon isotope (123 bulk-rock samples). All three sections have been studied for calcareous nannofossil paleoecology (210 samples; the material is stored at the Ruhr-University Bochum). The biostratigraphic and geochemical characterization of Hoheneggelsen KB9 is presented in the work by Heldt et al. (2012).

3.1

Sections

Rethmar: The section is situated 20 km southeast of Hannover (52° 18.37⬘ N; 10° 01.21⬘ E) (Fig. 1). Paleogeographically it occupied a rather distal basin position, the coastline being situated about 50 km further south. The succession consists of 144 m thick Barremian and 50 m thick Early Aptian mudstones, including a gap of 3 m covering the Barremian/Aptian boundary interval (Mutterlose and Wiedenroth 1995). The Barremian and lowermost Aptian are characterized by an alternation of finely laminated organic-rich black shales and dark claystones followed by pale marlstones in the middle Aptian. Finely laminated dark mudstones between the interval from 147 to 149.5 m correspond to the FS. The interval from 120 to 144 m can biostratigraphically be assigned to the late Barremian Oxyteuthis germanica and Oxyteuthis depressa belemnite zones, and from 147 m to the end of the section to the Early Aptian Neohibolites ewaldi belemnite Zone (Keupp and Mutterlose 1994). Carbon and oxygen isotope on belemnite revealed low values across the Upper Barremian and high values above the FS (Malkoč et al. 2010). A total of 100 samples were collected with a resolution of ca. 1 sample/40 cm.

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Alstätte 1: The outcrop is located in northwest Germany (52° 09⬘ 04.4⬙ N; 6° 54⬘ 37.7⬙ E) (Fig. 1). It exposes 15 beds (89–104) with a total thickness of 16 m through the Lower Aptian. The section mostly consists of mudstones and marlstones assigned to the N. ewaldi belemnite Zone and Deshayesites tenuicostatus ammonite Zone (Hoffmann and Mutterlose 2011). Finely laminated dark mudstones in the lower part of the section (bed 96) having a thickness of 1.8 m, correspond to the FS. The sediments of Alstätte 1 reflect neritic conditions, probably with a coastline being 10–15 km further to the northwest. A total of 93 samples were collected with a resolution of ca. 1 sample/10 cm across the FS. Hoheneggelsen KB9: The borehole was drilled about 30 km southeast of Hannover (re 35 81 683, h 57 85 392) (Fig. 1). It cored 23 m spanning the Early Aptian consisting of claystones and laminated claystones including ca. 2 m (from 17–15 m) of finely laminated marlstones identified as the FS (Kemper 1995). The top consists of 2.4 m deposits of Quaternary age. A total of 41 samples have been collected, with a resolution of ca. 1 sample/20 cm across the FS.

3.2

Bulk rock and carbon-isotope analyses

The Rethmar and Alstätte 1 sections were investigated for TOC and CaCO3 content processing 70 and 76 samples, respectively. TOC analyses were performed with a Deltatronik coulometer at the Ruhr-University Bochum. The TOC content was calculated by subtracting the wt% of total inorganic carbon (TIC) from wt% total carbon (TC) for a given sample: TOC = TC-TIC. The TC content was determined in dried sediments and TIC in dried and acidified (phosphoric acid) sediments. The CaCO3 content was calculated assuming the inorganic carbon measured in the samples to be in the form of CaCO3. Carbon-isotope analyses of bulk organic matter were performed on 60 samples from Rethmar and 63 samples from Alstätte 1. The analyses were performed at the GeoZentrum Nordbayern in Erlangen (Germany) with an elemental analyser (Carlo-Erba 1110) connected online to ThermoFinnigan Delta Plus mass spectrometer. Reproducibility of the analyses was checked by replicate analyses of international standards (USGS 40) and was better than 앐 0.04‰ (1s). Carbon-isotope analyses of bulk carbonate rock were determined only for Rethmar, since the carbonate content for most of Alstätte 1 samples was not suit-

118 C. Bottini and J. Mutterlose able for analyses (⬍ 6 wt%). The results are given in the conventional delta notation with respect to the Vienna PeeDee Belemnite (V-PDB) standard. Measurement reproducibility was better than 앐 0.05 ‰ (1s).

3.3

Calcareous nannofossils

Calcareous nannofossils were investigated on settling slides for 89 samples from Rethmar, 80 from Alstätte 1, and 41 from Hoheneggelsen KB9. Slides were prepared following the random technique of Geisen et al. (1999) using between 15–25 mg of dried samples. Common smear slides were prepared in order to estimate the required weight of sample and to detect preservation changes caused by preparation. The cover slide was fixed with Norland Optical Adhesive to an object slide. Calcareous nannofossils were investigated using an Olympus-BH2 light microscope with crosspolarized-light at a magnification of 1250x. At least 300 nannofossil specimens were counted in each sample. In addition, two random traverses were studied for each sample to detect rare and/or biostratigraphically important taxa. Both absolute and relative abundances of calcareous nannofossils were reconstructed: 1) absolute abundances (number of specimens/g sediment) were calculated applying the formula proposed in Bollmann et al. (1999); 2) relative abundances are represented by the percentage of the single taxon. Identification of taxa follows the taxonomic concepts of Perch-Nielsen (1985) and Bown (1998). Calcareous nannofossil preservation has been evaluated on the basis of the degree of etching and overgrowth. The diversity of the nannoflora has been determined using the parameter of species richness and heterogeneity (Shannon Index; Shannon and Weaver 1949), calculated using the software MVSP 3.1. The Shannon Index (heterogeneity) considers the quantitative composition of the nannofossil assemblages. High values of this index stand for highly diverse assemblages whereas low values represent less diverse assemblages dominated by a single species.

4.

Results

4.1

Bulk-rock and carbon-isotope analyses

Rethmar (Fig. 2, Appendix 1): The carbonate content is low below the FS (~ 2 %), around 13 % across the FS and much higher in the upper part of the section being around 25%. The TOC values are between 0.05 and

2 % across the Upper Barremian, and up to 9.5 % during the FS. Above the FS there is an interval of lower values (3 %) followed, around 150 m, by another peak up to 9 %. The d13Corg record is stable around – 25‰ for the Upper Barremian. At 146.85 m, coinciding with the base of the FS, d13Corg is characterized by low values around – 28 ‰. From 154 m onwards values are mostly constant around – 23 ‰ followed by a relative decrease around – 24 ‰. The d13Ccarb record is not available for the lower part of the section due to the low CaCO3 content. It is, however, possible to recognize in the d13Ccarb similar trends to those traced for the d13Corg referring to the interval between the FS and above it. Low d13Ccarb values, around – 2 ‰, characterize the FS, followed by values around 2 ‰ above it. Alstätte 1 (Fig. 3, Appendix 2): The carbonate content varies from 0.1 % to 25 %. The FS shows an average of 2 %, the highest values are reached in the upper part of the section (20 %). The TOC content varies between 0 and 3 %. The black shale of the FS have a TOC content between 2 and 3 %. Above the FS the values decrease but they rise up again (~ 2 %) in correspondence of bed 99 (6.47 m). Below the FS d13Corg values are around – 24.5 ‰. At 4.18 m, coinciding with the base of the FS, d13Corg displays a marked negative shift to – 27.5 ‰. Values stay extremely negative up to 5.7 m where they start to increase. From 7.7 m onwards values are constant around – 24 ‰.

4.2

Calcareous nannofossil biostratigraphy

Nannofossil schemes for the Boreal Realm have been proposed by Jakubowski (1987), Mutterlose (1992b), Bischoff and Mutterlose (1998), Bown et al. (1998), Ainsworth et al. (2000), Jeremiah (2001), and Malkoč et al. (2010). We applied the biozonation scheme of Bischoff and Mutterlose (1998) since it is based on a high-resolution study of sections in northern Germany, Isle of Wight and North Sea. For completeness, we also report (Figs. 2–3) the zonation proposed by Bown et al. (1998) which relies on the compilation of several studies on Boreal sections although the calibration across the Lower Aptian interval is probably not ideal for a high resolution study across the LSB. According to both schemes the studied sediments of Alstätte 1 cover the mid-Lower Aptian interval, while the Rethmar section encompasses the Upper Barremian to mid-Lower Aptian. The most important and common nannofossil taxa discussed in the paper are figured in Plate 1.


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Fig. 2. Integrated litho-, bio-, and chemo-stratigraphy of Rethmar. Calcareous nannofossil zonation is after Bown et al. (1998), BC zones, and Bischoff and Mutterlose (1998). On the right vertical distribution of calcareous nannofossil taxa important for biostratigraphy.

Rethmar (Fig. 2): Chiastozygus litterarius is found in sample 277-1 (129.7 m) defining the base of the Chiastozygus litterarius Zone which overlies the Vagalapilla matalosa (Broinsonia matalosa) Zone (BC17 of Bown et al. 1998). The base of the Flabellites oblongus Zone is uncertain since samples from 137.9 to 143.8 m are barren. There is also a gap of ca. 3 m between sample 299 and 351. The LO of Nannoconus abundans in sample 284-1 (134.5 m) approximates the Barremian/Aptian boundary (as proposed by Jakubowski 1987, Mutterlose 1992b, Bown et al. 1998; Malkoč et al. 2010) and the base of the BC18 of Bown et al. (1998). The FO of Rhagodiscus angustus has been observed in

the FS at 18.2 m. Eprolithus floralis/Eprolithus apertior have their FO in sample 371-1 (154.8 m), defining the base of the Eprolithus floralis Zone. In the same sample also Braarudosphaera hockwoldensis has its FO. The FO of Lithraphidites houghtonii occurs in sample 371-2 (155.1 m) and the LO in sample 394 (170.9 m). The LO of Micrantholithus hoschulzii is found at 162 m, and defines the base of the Nannoconus truittii Zone although no N. truittii acme is detected. Within the N. truittii Zone, and precisely in sample 377 (163.4 m), Farhania varolii has its FO and defines the base of the BC19 of Bown et al. (1998). The Repagulum parvidentatum acme starts at 167 m.

120 C. Bottini and J. Mutterlose

Fig. 3. Integrated litho-, bio-, and chemo-stratigraphy of Alstätte 1. Calcareous nannofossil zonation is after Bown et al. (1998), BC zones, and Bischoff and Mutterlose (1998). On the right vertical distribution of calcareous nannofossil taxa important for biostratigraphy.

Plate 1. Calcareous nannofossil micrographs (XPL, cross-polarised light): 1 Watznaueria barnesiae (Black 1959) PerchNielsen 1968, Sample H. KB9 3 m. 2 Watznaueria britannica (Stradner 1963) Reinhardt 1964, Sample H. KB9 3 m. 3 Cyclagelosphaera margerelii Noël 1965, Sample H. KB9 15.85 m. 4 Rhagodiscus asper (Stradner 1963) Reinhardt 1967, Sample H. KB9 15.53 m. 5 Rhagodiscus angustus (Stradner 1963) Reinhardt 1971, Sample H. KB9 15.42 m. 6 Chiastozygus litterarius (Górka 1957) Manivit 1971, Sample H. KB9 15.53 m. 7 Cretarhabdus surirellus (Deflandre 1954) Reinhardt, 1970, Sample R. 371/1. 8 Repagulum parvidentatum (Deflandre and Fert 1954) Forchheimer 1972, Sample R. 416/1. 9 Staurolithites stradneri (Rood et al. 1971) Bown, 1998, Sample R. 373/6. 10 Zeugrhabdotus erectus (Deflandre in Deflandre and Fert 1954) Reinhardt 1965, Sample R. 373/6. 11 Zeugrhabdotus xenotus (Stover, 1966) Burnett in Gale et al. 1996, Sample R. 379/1. 12 Crucibiscutum hayi (Black 1973) Jakubowski 1986, Sample H. KB9 15.53 m. 13 Biscutum constans (Górka 1957) Black in Black and Barnes 1959, Sample R. 371/1. 14 Discorhabdus ignotus (Górka 1957) Perch-Nielsen 1968, Sample H. KB9 3 m. 15 Rotelapillus laffittei (Nöel 1957) Nöel 1973, Sample H. KB9 15.53 m. 16 Flabellites oblongus (Bukry 1969) Crux in Crux et al. 1982, Sample H. KB9 15.53 m. 17 Eprolithus apertior Black 1973, Sample H. KB9 3 m. 18 Eprolithus floralis (Stradner 1962) Stover 1966, Sample H. KB9 3 m. 19 Farhania varolii (Jakubowski 1986) Varol 1992, Sample R. 373/6. 20 Assipetra infracretacea (Thierstein 1973) Roth 1973, Sample H. KB9 5.8 m. 21 Braarudosphaera africana Stradner 1961, Sample H. KB9 5.8 m. 22 Braarudosphaera hockwoldensis Black 1992, Sample R. 371/1. 23 Micrantholithus obtusus Stradner 1963, Sample H. KB9 5.8 m. 24 Nannoconus abundans Stradner and Grün 1963, Sample R. 271/1. 25 Nannoconus spp. (narrow canal) Sample H. KB9 5.8 m. 26 Nannoconus spp. (wide canal), Sample H. KB9 5.8 m. 27 Nannoconus spp. (wide canal) Sample H. KB9 5.8 m. 28 Nannoconus truittii Brönnimann 1955, Sample H. KB9 3.57 m. 29 Lithraphidites houghtonii Jeremiah 2001, Sample H. KB9 15.53 m. 30 Lithraphidites carniolensis Deflandre 1963, Sample H. KB9 15.53 m.


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122 C. Bottini and J. Mutterlose Alstätte 1 (Fig. 3): C. litterarius is found at the base of the section which extends up to 3.15 m (C. litterarius Zone) where the FO of F. oblongus defines the base of the F. oblongus Zone (BC18 of Bown et al. 1998). The FO of R. angustus has been observed just below the FS at 3.53 m. Eprolithus floralis/Eprolithus apertior have their FO at 7.39 m defining the base of the E. floralis Zone. The FO of L. houghtonii is also found in bed 100 (7.39 m). The position of the base of the N. truittii Zone is uncertain because M. hoschulzii is rare. The upper part of the section is marked by the FO of F. varolii in sample at 14.5 m (BC19 of Bown et al. 1998) and by the R. parvidentatum acme starting at 12 m.

4.3

Calcareous nannofossil preservation, abundance and diversity

In all three studied sections the genus Watznaueria is represented mainly by W. barnesiae and W. britannica. Rhagodiscus spp. includes R. asper and R. angustus. Zeugrhabdotus spp. is represented by Z. erectus, Z. diplogrammus, Z. xenotus, Z. scutula and Z. elegans. The genus Nannoconus is composed by narrow-canal species as N. steinmannii and N. colomi, and wide-canal forms as N. truittii, N. bucheri, N. kamptneri, N. grandis, N. abundans, N. elongatus. The Assipetra group consists of A. infracretacea and its large-sized morphotype (⬎ 7.5 mm) A. infr.larsonii; the Rucinolithus group con-

Fig. 4. Vertical distribution of absolute (shaded curve) and relative abundances (black curve) of selected taxa of calcareous nannofossils in Rethmar, plotted against litho-, bio and chemostratigraphy. Shaded areas in gray represent barren/almost barren samples. The legend is as reported in Fig. 2.


sists of R. terebrodentarius and its large-sized morphotype (⬎ 7.5 mm) R. terebr.youngii. Braarudosphaera spp. (B. africana, B. hockwoldensis) and Micrantholithus spp. (M. hoschulzii, M. obtusus) were grouped together in the “pentaliths” group. The genus Cretarhabdus is mostly represented by C. surirellus, followed by C. striatus and C. angustiforatus. Rethmar (Fig. 4): Calcareous nannofossil assemblages show moderately – poorly preserved specimens below sample 285, moderate-well preserved ones across the FS and well preserved ones in the overlying sediments. Intervals of barren/almost barren samples (shaded areas in Fig. 4) are identified from 137.9 to 147.32 m and from 150.8 to 154.1 m. The average absolute abundance of calcareous nannofossil is of 0.33 ҂ 109 specimens/g below the FS, and of 2.76 ҂ 109 specimens/g in the upper part of the section. The simple diversity (total number of species in each sample) is mostly constant except for intervals of almost barren samples. The Shannon Index values vary from 0.32 to 1 in very poorly preserved samples, from 1 to 2 in samples with poor preservation whereas in well preserved samples they range from 2 to 2.2. Watznaueria barnesiae is the most common taxon. The average abundance below bed 364 is 65% with samples having up to 90%. In the upper part of the section the average is around 40%. Rhagodiscus asper, B. constans, Z. erectus and D. ignotus are the second most abundant taxa. Rhagodiscus asper is more abundant in

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the FS and especially above it, with an average of 25%. Biscutum constans and D. ignotus show an average of 10%, Z. erectus of 15% without major fluctuations. Nannoconids are found in the interval corresponding to the FS and between bed 371 and 380, where they display a relative abundance below 3%. Assipetra infracretacea and R. terebrodentarius are found in the FS and around bed 374 with peaks of 2%. Cretarhabdus spp. shows peaks of 30% across the FS and above it. Pentaliths are represented by rare specimens within the FS and between the overlying beds 371 and 380. Repagulum parvidentatum is relatively abundant in the upper part of the section (2.5%) as well as Crucibiscutum hayi, showing percentages up to 3%. Staurolithites stradneri is present throughout the section except for barren/almost barren intervals, and shows an average of 2%. Alstätte 1 (Fig. 5): Calcareous nannofossil preservation is generally moderate-good, except for the interval below 2.57 m (for this reason the counting of calcareous nannofossils has not been conducted on this interval) and for intervals of barren/almost barren samples (shaded areas in Fig. 5). The absolute abundance has an average of 0.342 ҂ 109 specimens/g and the simple diversity is mostly constant with 20 species per sample. The Shannon Index varies from 2.12 to 2.37 in well preserved samples, whereas the index of samples with poor preservation ranges from 1.12 to 2. The most common taxon W. barnesiae shows constant relative

Fig. 5. Vertical distribution of absolute (shaded curve) and relative abundances (black curve) of selected taxa of calcareous nannofossils in Alstätte 1 plotted against litho-, bio and chemostratigraphy. Shaded areas in gray represent barren/almost barren samples. The legend is as reported in Fig. 3.

124 C. Bottini and J. Mutterlose

Fig. 6. Vertical distribution of absolute (shaded curve) and relative abundances (black curve) of selected taxa of calcareous nannofossils in Hoheneggelsen KB9 plotted against litho- and biostratigraphy (from Heldt et al. 2012). Shaded areas in gray represent barren/almost barren samples.

abundances throughout the studied interval. The average abundance is 34 %, the highest percentages (70– 80%) are reached in samples within the FS. Zeugrhabdotus erectus, R. asper, B. constans and D. ignotus are the second most abundant taxa, with an average of 13%, 12%, 6% and 5 %, respectively. Nannoconids are found in the interval corresponding to the FS, where they display relative abundances below 3 %. Assipetra infracretacea and R. terebrodentarius are found only above 7.3 m with percentages around 1– 2%. Across the FS Cretarhabdus spp. shows a marked increase with percentages around 5 %. Pentaliths are represented by rare specimens within the FS. Repagulum parvidentatum is relatively abundant in the upper part of the section (1.2 %), whereas C. hayi presents peaks in abundance up to 3–6 % from the FS up to the end of the section. Staurolithites stradneri is common throughout the section with an average of 2 %. Hoheneggelsen KB9 (Fig. 6): Calcareous nannofossil preservation is good, barren/almost barren samples

(shaded areas in Fig. 6) are identified only below the FS. The species richness is around 25 and the Shannon Index values vary from 2.16 to 2.6 in well preserved samples and from 1.2 to 2 in samples with poor preservation. The average abundance of W. barnesiae is 27 %, the highest percentages (50–60 %) are reached just below and above the FS in correspondence of poorly preserved samples. Rhagodiscus asper and B. constans are the second most abundant taxa, with an average of 13.6 % and 7.7 %, respectively. Nannoconids are absent below the FS. Within the FS and above 11.37 m they display percentage of 4 %. Assipetra infracretacea, R. terebrodentarius are more abundant within the FS with 4 %. Pentaliths are found mostly within the FS, with percentages of 2–3%. Cretarhabdus spp. increases across the FS up to 10% as well as C. hayi showing percentages around 20%. Repagulum parvidentatum is relative abundant in the upper part of the section (1–3 %). Staurolithites stradneri is common throughout the section with an average of 2 %.


5.

Discussion

5.1

Integrated stratigraphy

In the last decades many studies have been conducted on the Barremian – Aptian of Boreal sections aimed at providing an integrated stratigraphy for correlation with records from the Tethys, the North Sea, Pacific and Atlantic Oceans. The dataset generated for the LSB (e. g. Bischoff and Mutterlose 1998, Habermann and Mutterlose 1998, Mutterlose et al. 2009, Malkoč et al. 2010) allowed to determine that the laminated black shales of the FS are the sedimentary expression of OAE 1a and are coeval with the Selli Level and its equivalents. The characteristic isotopic signature of OAE 1a consists of a negative carbon-isotope excursion which, in the Tethys, coincides with the lowest stratigraphic levels of the organic-rich black shales of the Selli Level, followed by a large positive excursion extending after the black shales (e. g. Menegatti et al. 1998, van Breugel et al. 2007, Ando et al. 2008, Millán et al. 2009, Kuhnt et al. 2011).

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In this study, we propose a stratigraphic framework for Rethmar and Alstätte 1 based on the integration of biostratigraphic data (calcareous nannofossils), lithological information and carbon-isotope record. Calcareous nannofossil biostratigraphy (Fig. 7): Calcareous nannofossils allow an accurate zonation across the Barremian/Aptian boundary, especially for the FS. The FO of C. litterarius predates the Barremian/Aptian boundary, while the LO of N. abundans approximates the Barremian/Aptian boundary. The results confirm the FS to be enclosed by the FO of F. oblongus and the FOs of E. floralis/E. apertior as proposed by Mutterlose (1992b), Bischoff and Mutterlose (1998), Habermann and Mutterlose (1998), and Malkoč et al. (2010). The sections investigated show other similarities concerning FOs and LOs of calcareous nannofossils here compared with the records from other sites across the LSB: 1) the FO of R. angustus found prior to the FS (Mutterlose 1992b, Habermann and Mutterlose 1998, Malkoč et al. 2010, Heldt et al. 2012) or within the FS (Bischoff and Mutterlose 1998);

Fig. 7. Correlation of the studied sites: Alstätte 1, Rethmar, Hoheneggelsen KB9. Biostratigraphy, d13C and TOC data for Hoheneggelsen KB9 are from Heldt et al. (2012).

126 C. Bottini and J. Mutterlose 2) the FO of B. hockwoldensis found within the FS (Mutterlose 1992b; Malkoč et al. 2010; Heldt et al. 2012; this work) or just below the first black shale (Bischoff and Mutterlose 1998); 3) the FO of F. varolii postdating those of E. floralis/E. apertior (this work); and 4) the acme of R. parvidentatum found within the N. truittii Zone (Malkoč et al. 2010; this work). The comparison suggests that these events are mainly consistent across the LSB, and differences are probably related to preservation. Chemostratigraphy (Fig. 7): In the carbon-isotope record of Rethmar and Alstätte 1 a negative excursion is traced in the d13Corg curve and covers the entire FS. This anomaly can derive from alteration (sulfatereducing and methanogenic bacteria) of the organic matter of the sediments, however, we consider this scenario to be unlikely on the basis of the following observations: 1) the d13Corg record is given by a large number of data points and, in both sections, the trend is remarkably uniform; 2) the absolute values, comprised between – 24 and – 28 ‰, are comparable to records from contemporaneous basins elsewhere (e. g. Menegatti et al. 1998, van Breugel et al. 2007, Stein et al. 2011, Heldt et al. 2012); 3) no high correlation is observed between the presence of organic-rich layers, their TOC contents, and the carbon-isotope evolution (for Alstätte 1: R2 = 0.35 (n = 80); for Rethmar: R2 = 0.6 (n = 54)). The d13Ccarb record of Rethmar shows fluctuation patterns which are consistent with those of other coeval sections (e. g. Menegatti et al. 1998, Rückheim et al. 2006a, b, Kuhnt et al. 2011, Held et al. 2012). Diagenesis, generated through decomposition of organic-matter may have led to addition of 12C and therefore be responsible for the negative values across the FS (around –2.5 ‰). Nannofossils at this horizon are compositionally consistent with those in the levels above, showing no evidence of dissolution. Moreover these samples have between 12–60 % of CaCO3 contents and 0.2–6% of TOC, and the composition is not different from that of other samples analyzed. For these reasons, the selective diagenesis does not seem to be the sole cause for the observed negative shift. The typical isotopic trends of the Upper Barremian– Lower Aptian are recognizable in the sites investigated, especially because of the isotopic negative shift across the FS, followed by the positive anomaly. The d13Corg of Alstätte 1 and Rethmar and the d13Ccarb of Rethmar shows fluctuations which can be correlated with those defined by Menegatti et al. (1998). The

d13Corg of Rethmar is stable around –25‰ for the Upper Barremian (C2). At the base of the FS (146.85 m) d13Corg is characterized by marked negative values around –28‰ (C3). The onset of the C3 is missing due to a gap. Close to the top of the FS values start to rise being around –26‰ followed by a second negative shift to –28‰. This interval is interpreted as including segments C4 and C5. At 152.5 m, d13Corg shows an increase towards less negative values (C6). From 154 m onwards values are mostly constant around –23‰ (C7) followed by a relative decrease around –24‰ (C8). In the d13Ccarb record of Rethmar it is not possible to recognize the single segments between C3–C5, instead segments C6 is clearly represented and is followed by segment C7. In Alstätte 1 d13Corg values are stable around – 24.5 ‰ below the FS (C2). At the base of the FS (4.18 m) d13Corg displays a marked negative shift to – 27.5 ‰ (C3). The negative excursion is quite expanded and it is not possible to distinguish between single segments C4 and C5. At 5.7 m values start to increase (C6), and from 7.7 m onwards they are constant around – 24 ‰ (C7). Integrated litho-, bio- and chemo-stratigraphy (Fig. 7): In Alstätte 1 and Rethmar the onset of the negative carbon-isotope excursion (C3) is well approximated by the FO of F. oblongus and coincides with the first laminated black shale of the FS. The following interval, comprised between segment C4 and C6, corresponds instead with different lithologies overlying the FS which are characterized by lack of lamination. In Alstätte 1 a bed of pale mudstone with lower TOC content is followed by darker mudstones rich in organic matter. A similar alternation above the FS is detected in Rethmar indicated by variations in the TOC content. In Hoheneggelsen KB9, laminated sediments cover the C3–C6 interval (Heldt et al. 2012). In another section of the LSB, A39, the FS consists of two intervals of laminated black shales extending up to the middle of segment C6 and intercalated by pale marlstones (Mutterlose et al. 2009; Malkoč et al. 2010).These observations indicate that the sediments rich in organic matter cover the same interval – from C3 to the end of C6 – but the upper part is not everywhere represented by lamination (characteristic for the FS facies). The end in the sedimentation of organic rich sediments in Alstätte 1, Rethmar as well as in Hoheneggelsen KB9 (Heldt et al. 2012) and A39 (Malkoč et al. 2010) coincides with the onset of the chemostratigraphic segments C7 and is well approximated by the FOs of E. floralis/E. apertior.


5.2

Correlation with records from outside the LSB

The stratigraphic dataset collected for Alstätte 1 and Rethmar is correlated with records from the North Sea, SE France, Italy and Pacific Ocean (Fig. 8). The sequence of bio-events recognized in the sections investigated finds correspondence with that identified in the North Sea area (Ainsworth et al. 2000, Jeremiah 2001, Rückheim et al. 2006a, b). In the North Sea, however, the number of bio-events covering the Barremian– Aptian interval is much higher, probably influenced by different paleoceanographic settings (i. e. open-ocean conditions with good connections with other basins). The sedimentary expression of OAE 1a in SE France, in Italy and in the Pacific Ocean is sandwiched by the FO of Hayesites irregularis and the FO of E. floralis (e. g. Erba 1994; Bergen 1998; Erba et al. 1999; Herrle and Mutterlose 2003). In the sections investigated H. irregularis has not been detected, but in other Boreal sections rare specimens have been found below the FS close to the FO of F. oblongus (Bischoff and Mutterlose 1998, Malkoč et al. 2010). As for as the LSB (this

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work), the FO of E. floralis approximates the onset of the C7 (falling between the middle of the C6 and the onset of the C7) in the North Sea (Rückheim et al. 2006a), in SE France (e. g. Herrle and Mutterlose 2003, Herrle et al. 2004, Heimofer et al. 2006, Kuhnt et al. 2011) and in Italy (e. g. Erba et al. 1999, Bellanca et al. 2002, Luciani et al. 2006). An exception is represented by the record from NE Iran where E. floralis is found in the upper part of the Selli Level Equivalent (Mahanipour et al. 2011). The major differences are detected in the FO of R. angustus, found below the black shales of OAE 1a (e. g. Habermann and Mutterlose 1998, Kuhnt et al. 2011, Mahanipour et al. 2011, this work), above them (e. g. Bellanca et al. 2002) or within the FS (e. g. Bischoff and Mutterlose 1998). Calcareous nannofossil assemblages of the studied sections show a distinctive change across the Barremian/Aptian boundary detected also in other Boreal sections (Mutterlose 1992b, Bischoff and Mutterlose 1998, Mutterlose and Böckel 1998, Habermann and Mutterlose 1998, Ainsworth et al. 2000, Jeremiah 2001, Rückheim et al. 2006a, b, Malkoč et al. 2010). Compared to the record of the low latitudes, the Bar-

Fig. 8. Correlation of Rethmar with record from Italy (Cismon core), SE France (composite section of Gare de Cassis, La Marcouline, Camping section at Roquefort-La Bédoule) and Pacific Ocean (DSDP Site 463). Biostratigraphy, magnetostratigraphy, d13Ccarb, d13Corg and TOC data for the Cismon core are from van Breugel et al. (2007) and Erba et al. (2010). Biostratigraphy and d13Ccarb for SE France are from Kuhnt et al. (2011). Biostratigraphy, magnetostratigraphy, d13Ccarb, d13Corg and TOC data for DSDP Site 463 are from Erba et al. (1994), Larson and Erba (1999), and Ando et al. (2008).

128 C. Bottini and J. Mutterlose remian is populated by endemic taxa which became extinct (e. g. N. abundans) close to the Barremian/Aptian boundary. The assemblages of the Aptian are instead dominated by cosmopolitan taxa like C. litterarius, F. oblongus, B. hockwoldensis, R. angustus, E. floralis, and E. apertior which in that time had their first occurrence. Also nannoconids, although rare, are represented in the Lower Aptian by typical Tethyan forms. A speciation event, over the same time interval, has been detected in the Tethys, SE France and Pacific Ocean accompanied by an increase in abundance of planktonic foraminifers and radiolarians (e. g. Erba 1994, Bergen 1998, Erba 2004). The change in calcareous nannofossil assemblages from endemic to cosmopolitan forms is, however, peculiar for the Boreal Realm. This major change has been explained as constituting the breakdown of provincialism and opening towards the Tethys (Mutterlose 1992a, Bischoff and Mutterlose 1998, Mutterlose and Böckel 1998, Habermann and Mutterlose 1998). The dataset presented in our work provide further evidence at support of this interpretation. The sequence of nannofossil events (LOs, FOs) observed in northern Germany across the Aptian, is in good accordance with that of the North Sea, SE France, Italy and Pacific Ocean, especially concerning the events which approximate the onset and the end of OAE 1a. The correlation between the litho-, bio-, and chemo-stratigraphic data from the Boreal Realm and records from the low latitudes indicates that the onset of the deposition of the FS is coeval across the LSB and it is also coeval with that of the Selli Level and its equivalents, on the contrary, the top of the FS is diachronous. The correspondence between the laminated sediments of the FS and the Selli Level (and its equivalents), all marked by the negative carbon-isotope excursion, is suggestive of a common driving mechanism for anoxia. The negative excursion, marking the early phase of OAE 1a, has been interpreted resulting from a large input of isotopically light carbon in the ocean/atmosphere system on a global scale through intensified volcanogenic CO2 emissions during the Ontong Java and Manihiki Plateau emplacements and methane liberation from gas-hydrate dissociation (e. g. Menegatti et al. 1998, Larson and Erba 1999, Heimhofer et al. 2003, van Breugel et al. 2007, Méhay et al. 2009, Tejada et al. 2009). It is possible that, after the early phase of OAE 1a, corresponding to major environmental changes, regional factors were prevailing in the LSB and locally affecting the sedimentation leading, in some localities, to deposition of organic rich sediments without lamination.

5.3

Paleoecology

Calcareous nannofossil absolute abundances mostly mirror the relative abundances indicating that distribution of taxa in the sediment is homogenous (Figs. 4– 6). An exception is made for the lower part of Rethmar where low absolute abundances are paralleled by low CaCO3 content. In this interval the trends of the dominating taxa are better highlighted by the relative abundances. In both Rethmar and Alstätte 1 the absolute abundance shows a positive correlation (R2 = 0.7, n = 67) with the CaCO3 content but no preferential increase in samples having high TOC content. In all three sections investigated (Figs. 4–6), samples are characterized by relatively high percentages of Watznaueria spp. This oceanic genus is the most common and it has been assigned to warm and oligotrophic surface waters (e. g. Williams and Bralower 1995, Watkins et al. 1996, Kessels et al. 2003, Bornemann et al. 2005). It is also the most dissolution resistant species, and assemblages containing more than 40% are thought to be heavily altered (Roth and Bowdler 1981, Roth and Krumbach 1986, Premoli Silva et al. 1989, Williams and Bralower 1995). In the studied sections, however, samples having relatively high abundance of Watznaueria spp. show common dissolution prone species, such as Zeugrhabdotus spp., Biscutum spp. and D. ignotus. Thus, except for intervals of barren/almost barren samples and for the lowermost part of the Rethmar section, the relatively high percentages of Watznaueria spp. are thought not to be related to poor preservation. A Shannon Index of 2–2.50 in Hoheneggelsen KB9 and Alstätte 1 indicates good preservation and well diversified assemblages (excluding barren samples). Low Watznaueria spp. abundances have been detected in Hoheneggelsen KB9 across the FS in samples characterized by relatively high total abundances. Low abundances of Watznaueria spp. can be suggestive of increased nutrients in surface waters. Higher abundances in Rethmar and Alstätte 1 may partially reflect a preservation signal, since the record in the equivalent interval is characterized by moderate preservation compared to the good preservation in Hoheneggelsen KB9. Rhagodiscus spp. is considered as a warm water species instead E. floralis, R. parvidentatum and S. stradneri are considered to prefer cold surface water (e. g. Roth and Krumbach 1986, Premoli Silva et al. 1989, Erba 1992, Mutterlose 1992a, Street and Bown 2000, Herrle et al. 2003, Erba 2004). The shelf genus Biscutum spp. along with Z. erectus and D. ignotus are


interpreted as proxies for high primary productivity reflecting meso-eutrophic surface water conditions (e. g. Roth and Krumbach 1986, Watkins 1989, Erba 1992, Mutterlose et al. 2005, Tiraboschi et al. 2009). Crucibiscutum hayi has no clear affinity, but it has been proposed to indicate cold surface water (Mutterlose and Kessels 2000, Street and Bown 2000, Rückheim et al. 2006b). In the studied sections similar fluctuations between C. hayi and R. parvidentatum seems to support this hypothesis. During the Upper Barremian Rhagodiscus spp. is less abundant compared to the Lower Aptian and is paralleled by relatively common S. stradneri. In the lowermost part of the Aptian, including the FS, Rhagodiscus spp. is much more abundant suggesting that this interval was probably characterized by higher temperature compared to the Upper Barremian (Figs. 4–6). After the FS we assist to the progressive increase in abundance of C. hayi, R. parvidentatum and E. floralis (and of related species E. apertior and F. varolii). As mentioned above these taxa are cold water taxa, thus their increase in abundance may imply a decrease in temperature around the end of the C6 followed by a progressive cooling, up to the end of segment C7. Across this interval (C6–C7), however, Rhagodiscus spp. is still relatively abundant. It is possible that the interval marked by the FOs of E. floralis, E. apertior and F. varolii paralleled by the increase in abundance of R. parvidentatum and C. hayi, represents the “turning point” from warm conditions (during the FS) to cooler temperature which characterizes the Upper Aptian– Lower Albian (e. g. Menegatti et al. 1998, Luciani et al. 2006, Rückheim et al. 2006b, Ando et al. 2008, Millán et al. 2009, Kuhnt et al. 2011). The absence of significant variations in abundance of S. stradneri across the FS may indicate that this taxon tolerate a broader range of surface water temperature. The presence of E. floralis, E. apertior, F. varolii and R. parvidentatum in the equivalent interval at lower latitudes (e. g. Erba et al. 1999, Herrle and Mutterlose 2003, Kuhnt et al. 2011) can be suggestive of a migration of Boreal species southwards under cooler temperatures. Warmer conditions reconstructed for the FS on the basis of calcareous nannofossil data are in agreement with a rapid and pronounced temperature rise recorded by independent proxies in other Boreal sections (Mutterlose et al. 2010, Heldt et al. 2012) and in openocean settings (e. g. Jenkyns 2003, Pucéat et al. 2003, Schouten et al. 2003, Ando et al. 2008, Erba et al. 2010). This observation is a further indication that the mechanisms operating during OAE 1a were capable to

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influence the climate on a global scale probably imposing “super” greenhouse conditions. Paleotemperature variations reconstructed for the Tethys, Pacific and South Atlantic Oceans also provided evidence for cooler interludes encompassing the C4–C6 interval (e. g. Hochuli et al. 1999, Dumitrescu et al. 2006, Mìllan et al. 2009, Kuhnt et al. 2011, Erba et al. 2010). However, similar fluctuations are not traced by calcareous nannofossils in the Boreal Realm. The abundances of B. constans, Z. erectus and D. ignotus indicate that fertility of surface water was high since the Upper Barremian, and remained relatively high during the deposition of the FS and thereinafter (Figs. 4–6). In the Tethys and Pacific Oceans, the OAE 1a interval shows an increase in abundance of fertility marker species mainly in correspondence of the early phase of OAE 1a (Erba et al. 2010). This discrepancy can derive from peculiar conditions of the LSB. The major nutrient supply was probably via continental runoff and, due to restricted conditions, surface water fertility was easily maintained. Accelerated weathering rates were probably promoted by global warming, as reconstructed for the OAE 1a in the Tethys and Pacific Oceans on the basis of independent proxies (e. g. Tejada et al. 2009, Blättler et al. 2011). Influxes of Tethyan water masses may have played a role in sustaining high productivity also after the FS but without deposition of organic-rich layers. Low TOC content in the uppermost part of the Early Aptian indicates that the basin was characterized by oxygenation of bottom waters probably maintained by enhanced water circulation. The Lower Aptian is known to be marked by a major decrease in the abundance of nannoconids recognized world-wide. It started in the latest Barremian with the “nannoconid decline”, followed shortly before the deposition of OAE1a black shales by the “nannoconid crisis” (Erba 1994). Nannoconid “decline” and “crisis” have also been detected in the Boreal Realm (Erba 1994, Habermann and Mutterlose 1998, Rückheim et al. 2006a, b) but in the studied sections it is not possible to confirm or exclude the presence of the “nannoconid crisis” due to low preservation across the Barremian/Aptian boundary. The sporadic presence of nannoconids in the FS and especially after it, may correspond to the “recovery” of these taxa and correlate to the return of nannoconids detected in the Tethys and Pacific Oceans (e. g. Erba 1994, Erba and Tremolada 2004, Erba et al. 2010). Their presence is also indicative of possible influxes from the Tethys into the LSB (e. g. Mutterlose 1992a, Habermann and Mutterlose 1998, this work).

130 C. Bottini and J. Mutterlose The genera Assipetra and Rucinolithus are heavily calcified nannoliths compared to the average size of coccoliths and have no clear paleoecological affinity. They have been considered to indicate nutrient-rich surface waters, or to be peculiar CaCO3 forms precipitated during extreme climatic conditions and/or physical-chemical changes in ocean chemistry (Tremolada and Erba 2002, Erba et al. 2010). The presence of Assipetra and Rucinolithus in the FS and above it does not seem to follow a specific pattern but abundances (0–5%) are much lower compared to the Tethys (5– 20%) and the highest percentages are reached above the FS. Pentaliths are interpreted to prefer coastal areas and low salinity (e. g. Roth 1994). In the studied sections they are rare but most of the time they are found in samples containing nannoconids and show peaks in abundance within and above the FS. As already proposed in other works (Mutterlose 1992b, Bischoff and Mutterlose 1998, Habermann and Mutterlose 1998), the presence of pentaliths can be suggestive of temporary influx of water masses from the Tethys. In addition, accelerated runoff related probably to more humid conditions (especially during the FS) may have favoured these taxa by reducing surface water salinity. Evidence for reduced salinity also come from palynomorphs (Below and Kirsch 1997, Mutterlose and Böckel 1998) and dinoflagellate cysts (Keupp and Mutterlose 1994), which are consistent with input of terrigenous matter (Littke et al. 1998, Habermann and Mutterlose 1998). The paleo-affinity of Cretarhabdus spp. is still uncertain, but seems to be linked to paleoceanographic conditions during black shales deposition. In environments with stressful conditions in terms of variations in surface water parameters (nutrients, salinity, temperature) and/or reduced oxygen availability in the lower photic zone, the genus Cretarhabdus dominates the assemblages (Linnert et al. 2011). The large increase of Cretarhabdus spp. across the FS of the three studied sections seems to support this interpretation.

5.4

Reconstruction of the paleoceanographic evolution of the LSB across the OAE 1a: global versus local factors

The comparison between paleoecological and paleoclimatic information deduced in this work and the dataset available for other sites in the LSB, SE France, the North Sea, Italy and Pacific Ocean, allow to distinguish between global processes affecting the sedi-

mentation in the LSB and local factors depending on the paleogeographic evolution of the basin. Three main phases can be distinguished during the Late Barremian – Early Aptian: Phase 1 – Late Barremian: During the Late Barremian the LSB was characterized by prevailing restricted conditions (Mutterlose 1992a, Mutterlose and Böckel 1998, Habermann and Mutterlose 1998, Mutterlose et al. 2009). Calcareous nannofossil assemblages were dominated by endemic taxa (Mutterlose 1992a, Mutterlose and Böckel 1998, Habermann and Mutterlose 1998, this work) and the climate was less warm than in the Early Aptian (Mutterlose et al. 2010, this work) characterized by arid conditions (Keupp and Mutterlose 1994, Mutterlose et al. 2009). Reduced water circulation resulted in time intervals of water stratification, consequent expansion of the O2-minimum layer and local sedimentation of laminated organic-rich horizons (Blätterton). In general the sediment deposition during the Late Barremian was mostly affected by regional conditions related to the paleogeography of the LSB rather than by processes acting on a global scale. Phase 2 – Early Aptian FS: During the Early Aptian the LSB was a “stress” environment characterized by the deposition of the FS corresponding to the OAE 1a. Global warming, also affecting the Boreal Realm, was probably responsible for enhanced freshwater runoff from land areas and consequent increased input of nutrients in the LSB. Acceleration of weathering rates has been proposed also for the Selli event in the Tethys and Pacific Oceans on the basis of independent proxies (e. g. Tejada et al. 2009, Blättler et al. 2011). Although the inflow of open-ocean water masses from the Tethys was maintaining surface water circulation, the basin configuration still did not allow much bottom water exchange. The decrease of surface water salinity combined with high productivity favoured the development of anoxic conditions. Thus the deposition of the FS were mainly triggered by factors operating on a global scale in connection to OAE 1a, only partially promoted by local factors related to the paleogeography of the basin. The depositional environment is similar to that of the Barremian black shales (Blättertone), but the paleogeographic-climatic conditions and the mechanisms involved are different. The Early Aptian is also marked by extinction of endemic forms and the flourishing of cosmopolitan taxa which started around the Barremian/Aptian boundary (Mutterlose 1992a, b, Keupp and Mutterlose 1994, Bischoff and Mutterlose 1998, Mutterlose and Böckel


1998, Habermann and Mutterlose 1998, this work). The occurrence of new species in the LSB is related to accelerated speciation rates and to the opening of new seaways to the Tethys (Mutterlose 1992a, b, Keupp and Mutterlose 1994, Bischoff and Mutterlose 1998, Mutterlose and Böckel 1998, Habermann and Mutterlose 1998, this work). Rapid and intense global warming, associated with increased precipitation and consequent runoff, probably induced sea level rise. At the same time, equable climatic conditions between the Boreal Realm and the low latitudes favoured the migration of new taxa from South to North. Under these stress conditions, detected in both realms, calcareous nannoplankton responded with accelerated evolutionary rates. Thus, the change in calcareous nannofossil assemblages reflects both local variations related to the paleogeographic evolution of the LSB towards more pelagic and “more open marine” settings and variations happening on the global scale. Phase 3 – late Early Aptian: The end in the deposition of the FS was controlled by local conditions. Sedimentation was characterized by high content of organic carbon but laminated sediments were not extended to all basin. A cooling, also traced at low latitudes, affected the Boreal Realm after the deposition of the FS (e. g. Menegatti et al. 1998, Luciani et al. 2006, Rückheim et al. 2006b, Ando et al. 2008, Millán et al. 2009, Kuhnt et al. 2011, this work). This climatic change probably represents the recovery phase after the major perturbation of OAE 1a and constitutes a “turning point” towards cooler temperature registered in the Upper Aptian and Lower Albian. The cooling was most likely accompanied by migration of Boreal taxa from North to South through the seaways to the Tethys. Conditions of surface water were still mesoeutrophic, but enhanced water circulation were maintaining oxygenation of the bottom water thus preventing the deposition of black shales.

shallow coastal waters. The d13C excursions (segments C3–C7), encompassing the FS, are well constrained by the FOs of calcareous nannofossil F. oblongus and E. floralis/E. apertior. In the sections investigated, the onset of FS black shale deposition occurs coeval, and corresponds to the base of the Selli Level (and its equivalents). In contrast the top of the FS is diachronous. The correspondence between laminated sediments of the FS and the Selli Level confirms the FS to be the product of OAE 1a. Paleoecological and paleoclimatic information based on calcareous nannofossils deduced in this work provide a more complete picture of the evolution of the LSB during the Late Barremian – Early Aptian. The integration with information available for the low latitudes allow to distinguish between global processes, and local factors related to the paleogeographic evolution of the basin. Sedimentation during the Late Barremian was mostly depending on regional conditions rather than processes acting on a global scale. The deposition of the FS was instead triggered by mechanisms operating on a global scale, associated to OAE 1a, partially promoted by local evolution of the LSB. Regional factors controlled the end in the sedimentation of the FS and of the overlying beds. The evidence of accelerated evolutionary rates among calcareous nannoplankton and the development of cosmopolitan species in the LSB after the Barremian/ Aptian boundary, provide further support to the opening of new seaways to the Tethys and to climatic changes happening on a global scale. Acknowledgements. We thank M. Joachimski for undertaking carbon-isotope analyses (GeoZentrum Nordbayern, Erlangen), and T. Goral for carbon analyses (Ruhr-University Bochum). We are grateful to A. Bornemann and to an anonymous reviewer for their useful comments.

7. 6.

Conclusions

The stratigraphic framework (litho-, bio-, and chemostratigraphy) proposed provide a more accurate stratigraphic framework suitable for correlation and for a supra-regional characterization of the OAE 1a expressed in northern Germany. The dataset indicates that the entire global perturbation of the C-cycle associated with OAE 1a is recorded in the centre of the LSB as well as in the margin, in

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Taxonomic Index

A full list of all calcareous nannofossil taxa cited in the text, figures and plate is given below. Assipetra Roth, 1973 Assipetra infracretacea (Thierstein, 1973) Roth, 1973 Assipetra infracretacea larsonii Tremolada and Erba, 2002 Biscutum Black in Black and Barnes, 1959 Biscutum constans (Górka 1957) Black in Black and Barnes, 1959 Braarudosphaera Deflandre, 1947

132 C. Bottini and J. Mutterlose Braarudosphaera africana Stradner, 1961 Braarudosphaera hockwoldensis Black, 1992 Broinsonia Bukry, 1969 Broinsonia matalosa (Stover, 1966) Burnett in Gale et al., 1996 Chiastozygus Gartner, 1968 Chiastozygus litterarius (Górka, 1957) Manivit, 1971 Cretarhabdus Bramlette and Martini, 1964 Cretarhabdus angustiforatus (Black, 1971) Burky, 1973 Cretarhabdus striatus (Stradner, 1963) Black, 1973 Cretarhabdus surirellus (Deflandre, 1954) Reinhardt, 1970 Crucibiscutum Jakubowski, 1986 Crucibiscutum hayi (Black, 1973) Jakubowski, 1986 Cyclagelosphaera Noël, 1965 Cyclagelosphaera margerelii Noël, 1965 Discorhabdus Noël, 1965 Discorhabdus ignotus (Górka, 1957) Perch-Nielsen, 1968 Eprolithus Stover, 1966 Eprolithus apertior Black, 1973 Eprolithus floralis (Stradner, 1962) Stover, 1966 Farhania Varol, 1992 Farhania varolii (Jakubowski, 1986) Varol, 1992 Flabellites Thierstein, 1973 Flabellites oblongus (Bukry, 1969) Crux in Crux et al., 1982 Hayesites Manivit, 1971 Hayesites irregularis (Thierstein, 1972) Applegate et al., 1987 Lithraphidites Deflandre, 1963 Lithraphidites carniolensis Deflandre, 1963 Lithraphidites houghtonii Jeremiah, 2001 Manivitella Thierstein, 1971 Manivitella pemmatoidea (Deflandre in Manivit, 1965) Thierstein, 1971 Micrantholithus Deflandre in Deflandre and Fert, 1954 Micrantholithus hoschulzii (Reinhardt, 1966) Thierstein, 1971 Micrantholithus obtusus Stradner, 1963 Nannoconus Kamptner, 1931 Nannoconus abundans Stradner and Grün, 1963 Nannoconus steinmannii Kamptner, 1931 Nannoconus bucheri Brönnimann, 1955 Nannoconus colomii (De Lapparent, 1931) Kamptner, 1938 Nannoconus kamptneri Brönnimann, 1955 Nannoconus truittii Brönnimann, 1955 Nannoconus grandis Deres and Acheriteguy, 1981 Nannoconus elongatus Brönnimann, 1955

Repagulum Forchheimer, 1972 Repagulum parvidentatum (Deflandre and Fert, 1954) Forchheimer, 1972 Rhagodiscus Reinhardt, 1967 Rhagodiscus angustus (Stradner, 1963) Reinhardt, 1971 Rhagodiscus asper (Stradner, 1963) Reinhardt, 1967 Rotelapillus Nöel, 1973 Rotelapillus laffittei (Nöel, 1957) Nöel, 1973 Rucinolithus Stover, 1966 Rucinolithus terebrodentarius Applegate Bralower Covington and Wise, 1987 Rucinolithus terebrodentarius youngii Tremolada and Erba, 2002 Staurolithites Caratini, 1963 Staurolithites stradneri (Rood et al., 1971) Bown, 1998 Watznaueria Reinhardt, 1964 Watznaueria barnesiae (Black, 1959) Perch-Nielsen, 1968 Watznaueria britannica (Stradner, 1963) Reinhardt, 1964 Zeugrhabdotus Reinhardt, 1965 Zeugrhabdotus diplogrammus (Deflandre in Deflandre and Fert, 1954) Burnett in Gale et al., 1996 Zeugrhabdotus elegans (Gartner, 1968) Burnett in Gale et al., 1996 Zeugrhabdotus erectus (Deflandre in Deflandre and Fert, 1954) Reinhardt, 1965 Zeugrhabdotus scutula (Bergen, 1994) Rutledge and Bown, 1996 Zeugrhabdotus xenotus (Stover, 1966) Burnett in Gale et al., 1996

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Manuscript received: November 26, 2011; rev. version accepted: March 12, 2012.

136 C. Bottini and J. Mutterlose Appendix 1–2. Carbon-isotope, TOC and CaCO3 data.

A

Rethmar

Rethmar

m

bed

196.6 195 192.8 190.9 189.6 189 188.5 188.2 186.5 185.8 184.7 184.2 183.8 181 179.5 179 178.1 177.5 175.8 175.5 175 174.8 174.5 173.5 172.2 171.9 171.5 170.9 170.6 170.5 169.8 169.2 167.9 167.1 166.5 166.1 166.05 166 165.3 165.2 165.1 164.95 164.9 164.1 163.9 163.8 163.4 163.1 162.6 162.5 162.1 162 161.8

451 447 441 433 431 429 428 427 423 422 420 418 416 411 408 408 407 406 401 400 399 398 397 397 397 396 395 394 393 392 390 388 385 385 385 384 384 384 383 382 381 380 379 379 379 378 377 377 377 377 376 375 374

1 x x 1 1 1 x 1 1 x 1 1 1 1 3 2 1 x x 1 x 1 3 2 1 1 x 1 x 1 1 1 3 2 1 3 2 1 1 1 1 1 3 2 1 1 4 3 2 1 1 1 1

d13Ccarb d13Corg TOC CaCO3 (VPDB ‰) (VPDB ‰) (wt%) (wt%)

m

bed

3.19 3.08 3.09 2.57 2.27 2.28 2.31 1.97 x 2.59 2.71 x x 2.78 2.96 x 2.36 x x 2.46 2.04 2.48 2.77 x 2.93 2.43 2.05 0.65 3.17 2.95 2.38 2.34 3.82 3.88 3.85 2.05 2.02 2.13 3.66 3.32 2.72 x 0.74 2.47 2.18 3.91 2.97 3.45 3.65 3.78 4.22 4.03 2.03

161 160 159 157.8 157.7 157.6 157.5 157 156.5 154.1 154 153.9 153.1 152.95 152.5 152.2 151.9 151.2 150.8 149.4 149.2 149 148.8 148.6 148.5 148.1 148 147.75 147.6 147.48 147.32 147.16 147 146.95 146.9 146.85 142.9 141.8 141.5 140 137 135.9 134.5 132.5 131.5 129.7 128.9 127.8 125.8 122.1 121.4 120.5

373 373 373 373 373 373 373 372 372 370 369 369 368 367 366 365 365 364 363 360 359 359 359 358 357 356 356 356 354 354 353 353 352 351 351 351 297 297 297 295 291 289 287 283 281 277 275 273 269 267 265 263

–24.42 – x – x –24.47 – x – x –24.12 – x – x –24.84 – x – x –23.73 –23.95 –24.21 – x – x – x – x –24.49 – x – x –24.29 – x – x –23.84 – x – x –24.44 – x – x –24.24 – x – x –23.28 – x – x –23.13 – x – x –22.60 – x –23.45 – x – x – x – x –23.54 – x – x –23.79 –23.37 –22.67

0.49 x x 0.07 x x 0.52 x x 0.89 x x 0.53 8.35 x 0.21 x x x 0.45 x x 0.14 x x 0.27 x x 0.47 x x 0.19 x x 0.25 x x 0.03 x x x 0.17 x x 0.05 x x 0.15 x x 0.69 0.02 0.17

15.74 19.80 15.30 21.24 22.20 17.50 26.82 26.70 12.90 16.74 14.40 6.30 5.91 19.74 x 21.49 14.80 8.30 31.80 23.07 31.10 21.30 15.99 8.50 26.00 23.82 x x 28.99 x 34.80 32.57 16.00 17.70 22.57 25.90 30.30 31.99 24.30 22.60 27.30 25.49 46.30 28.70 46.23 23.50 28.00 30.99 25.40 32.90 19.49 35.99 46.06

d13Ccarb d13Corg TOC CaCO3 (VPDB ‰) (VPDB ‰) (wt%) (wt%) 7 6 5 4 3 2 1 2 1 1 2 1 1 1 1 2 1 1 1 1 3 2 1 1 1 3 2 1 2 1 2 1 1 3 2 1 3 2 1 1 1 1 1 1 1 1 1 2 3 1 1 1

– 3.95 – 3.76 – 4.08 – 3.83 – 4.48 – 4.48 – 3.47 – 3.91 – 2.4 – 0.39 – x – x –17.45 –2.16 – x – x – x – x – x – x – x –2.03 – x –23.59 – 1.1 –1.81 – x – x – x –2.66 – x – x – x – x – x – x – x – x – x – x – x – x – x – x – x – x – x – x – x – x – x – x

–23.14 –22.82 –23.16 – x – x – x –22.67 –22.24 –23.29 – x –24.86 – x –24.66 –24.69 –27.23 –25.91 –27.76 –26.53 –25.85 –26.48 – x –26.55 – x –27.17 –26.11 –26.59 –28.01 – x –27.12 –27.65 –27.01 – x –26.61 –27.45 – x –27.24 –24.94 –25.21 – x –25.01 – x – x –25.02 – x – x –24.97 – x – x –24.93 – x – x –24.78

0.24 0.25 0.24 x x x 0.78 0.51 0.17 0.14 x 2.16 0.21 4.51 7.20 2.71 8.55 5.74 3.41 3.26 x 6.85 x 3.01 1.43 5.96 9.19 x 7.35 6.95 4.39 8.95 2.30 6.76 x 4.14 1.52 0.77 x 1.13 x x 2.41 x x 0.89 x x 1.11 x x 0.96

41.48 32.15 26.32 24.10 26.00 28.80 27.66 31.90 56.64 67.22 1.10 0.33 77.05 12.99 0.01 0.24 0.42 0.08 0.06 1.50 1.30 37.98 2.50 68.72 12.74 19.83 14.74 4.40 14.16 16.58 0.25 0.88 0.22 0.07 1.30 0.10 0.05 2.67 2.60 1.08 4.20 5.30 3.25 3.20 4.10 2.33 1.30 5.90 3.25 1.20 2.00 3.08

Integrated stratigraphy of Early Aptian black shales A

A

Alstätte 1

Alstätte 1

137

m

TOC (wt%)

CaCO3 (wt%)

d13Corg (VPDB ‰)

m

TOC (wt%)

CaCO3 (wt%)

d13Corg (VPDB ‰)

14.4 13.7 13.6 13.59 13.56 13.45 13.44 13.42 13.39 13.33 12.99 12.79 12.59 12.25 12.19 12 11.79 11.71 11.39 11.17 10.99 10.63 10.59 10.19 10.09 9.79 9.55 9.39 9.23 9.01 8.99 8.59 8.47 8.19 7.93 7.79 7.5 7.39 7.12 6.99 6.86 6.75 6.73 6.6 6.47 6.17 5.994 5.946

1.23 0.81 1.06 x 0.75 x 0.17 0.34 x 0.21 x 0.71 x 0.57 x 0.67 0.46 0.70 x 1.27 x 0.87 x x 0.66 x 0.76 0.61 0.40 0.67 0.16 0.17 0.51 0.04 0.59 0.23 0.41 x 1.65 1.12 1.83 0.54 2.43 1.56 1.11 1.28 0.40 0.74

1.33 6.83 5.58 x 5.50 x 7.08 6.66 x 20.16 x 15.99 x 13.91 x 10.41 9.08 13.49 x 12.16 x 18.49 x x 13.58 x 11.41 5.91 8.16 8.66 6.33 5.58 8.66 5.83 7.91 6.00 7.83 x 2.08 0.67 0.33 0.33 0.50 1.50 1.42 0.25 0.25 1.17

–24.36 – x – x – x – x –24.85 – x – x –24.68 –24.19 –24.81 –24.80 –24.23 –24.09 –23.92 – x –24.15 –23.75 –24.55 –23.91 –24.38 –24.35 –24.77 –24.26 –24.27 –24.39 –24.37 –24.19 –24.22 –24.46 –24.02 –24.43 – x –24.14 –24.42 –23.68 –24.96 – x –25.17 –25.07 –25.35 –25.22 –25.22 –25.53 –25.32 –26.79 –25.27 –26.42

5.898 5.85 5.8 5.802 5.754 5.706 5.658 5.48 5.38 5.28 5.18 5.18 5.08 4.98 4.88 4.78 4.68 4.58 4.48 4.38 4.28 4.28 4.18 4.08 3.98 3.88 3.88 3.78 3.7 3.53 3.33 3.15 2.57 2.27 1.97 1.67 1.36 1.29 0.91 0.53 0.15

0.68 0.72 0.77 0.95 0.75 1.37 0.81 1.26 1.31 1.36 1.49 1.78 1.61 1.55 1.29 1.66 2.74 2.10 1.93 1.63 x 1.51 1.51 1.61 1.51 1.35 1.78 1.81 0.36 1.29 0.87 1.07 0.01 0.69 1.12 1.18 0.92 1.31 1.15 1.20 1.30

0.08 0.42 0.25 0.25 0.42 1.75 0.08 1.33 1.50 2.75 2.08 2.58 2.75 0.83 1.58 1.50 3.92 6.66 1.50 2.92 x 2.25 1.83 2.42 1.83 1.67 1.17 1.42 0.58 3.50 2.92 3.17 0.67 1.25 1.00 1.58 1.17 0.33 0.00 0.42 1.42

–26.24 –26.50 –26.36 –26.61 –26.75 –27.34 –26.10 –27.48 –27.38 –27.55 –27.27 –27.76 –27.77 –27.52 –26.90 –27.33 –26.49 –27.65 –27.26 –27.22 –27.38 –27.27 –27.28 –26.97 –26.98 –26.64 –26.67 –27.14 –26.27 –25.05 –24.67 –24.99 –25.15 –24.64 –24.85 –24.49 –24.75 –24.51 –24.82 –24.77 –25.39