1. Introduction
The timing and magnitude of graptolite bioevents have been the subject of numerous studies since the pioneering work by Koren’ (Reference Koren’1987), which for the first time considered diversity dynamics in relation to important global physical events. Loydell (Reference Loydell1994, Reference Loydell1998) discussed causal relationships between sea-level drawdowns and graptolite extinctions. Melchin, Koren’ & Štorch (Reference Štorch1998) revealed similar patterns in their summary of global Silurian graptolite faunal dynamics.
Although most studies have been focused on the Late Ordovician graptolite extinction and the subsequent early Silurian recovery and radiation (Koren’, Reference Koren’1991a ; Melchin & Mitchell, Reference Melchin, Mitchell, Barnes and Williams1991; Finney et al. Reference Finney, Berry, Cooper, Ripperdan, Sweet, Jacobsen, Soufiane, Achab and Noble1999; Chen et al. Reference Chen, Melchin, Sheets, Mitchell and Fan2005; Štorch et al. Reference Štorch, Mitchell, Finney and Melchin2011) or the mid-Homerian crisis (Jaeger, Reference Jaeger1991; Koren’, Reference Koren’1991b ; Lenz, Reference Lenz1993; Urbanek, Reference Urbanek1993), a number of other critical intervals were identified by Koren’ (Reference Koren’1993), Urbanek (Reference Urbanek1993), Loydell (Reference Loydell1994), Melchin (Reference Melchin1994), Štorch (Reference Štorch, Čejchan, Hladil and Štorch1995) and Melchin, Koren’ & Štorch (Reference Štorch1998).
It has been recognized for a long time (Bouček, Reference Bouček1953) that a distinct changeover and temporary impoverishment of graptolite fauna took place in late Aeronian time. Štorch (Reference Štorch, Čejchan, Hladil and Štorch1995) placed this graptolite mass extinction at the top of the Lituigraptus convolutus Biozone and named it the convolutus Event. Subsequently, Melchin, Koren’ & Štorch (Reference Štorch1998) examined data from the principal graptolite-bearing successions worldwide and concluded that the extinction reached its peak in the subsequent Stimulograptus sedgwickii Zone, with minimum diversity attained in the upper part of the zone.
Widespread evidence for the late Aeronian sea-level drawdown, recognized already by Leggett et al. (1991) and Johnson (Reference Johnson, Witzke, Ludvigson and Day1996) was summarized by Loydell (Reference Loydell1998). He correlated sedimentary successions from various palaeoplates and depositional settings by means of the then published graptolite records and showed that sediments indicating better oxygenation, higher input of siliciclastic material or carbonate deposition in otherwise siliciclastic successions were deposited during the sedgwickii graptolite Zone. The coincidence between lowered sea-level, a positive δ13C excursion and graptolite extinction in the upper Aeronian sedgwickii Zone has been further discussed by Loydell (Reference Loydell2007). Graptolite data, however, did not allow for higher resolution intrazonal correlation. The question remained as to whether the sea-level fell owing to a single or multiple eustatic event(s) and whether its major phase took place in the early, middle or even late part of the sedgwickii Zone. Since then, further sedimentological evidence of the sea-level fall has been found and more details of graptolite faunal dynamics combined with carbon isotope data have allowed for more precise dating and correlation of this turbulent period.
In the Prague Synform of the Barrandian area, the upper Aeronian strata are assigned to the upper part of the Želkovice Formation, which is dominated by black graptolitic shales (Kříž, Reference Kříž1975; Chlupáč et al. Reference Chlupáč, Havlíček, Kříž, Kukal and Štorch1998). The interval between the middle Aeronian L. convolutus Biozone and lowermost Telychian Rastrites linnaei Biozone has been little known and little exposed so far. No section spanned the whole interval and both new and historical localities (Bouček, Reference Bouček1930, Reference Bouček1953; Štorch, Reference Štorch2001; P. Štorch, unpub. Ph.D. thesis, Czech Geological Survey, Prague, 1991) comprised either limited strata adjacent to basalt sills or heavily weathered shales with poorly preserved graptolites. A complete succession was made temporarily accessible, however, in the highway tunnel north of Radotín (Štorch et al. Reference Štorch, Černý, Bohátka and Melichar2009) and provided a unique opportunity for detailed and multidisciplinary studies. Unweathered Llandovery strata, composed of black shales rich in stratigraphically important graptolites, have also yielded excellent material for total organic carbon (TOC) and carbon isotope analyses. Some parts of the late Aeronian–early Telychian succession are rich in pyrite and undergo rapid decomposition in outcrops; hence the rarity of surface exposures and the particular significance of the subsurface section used as a key reference in this paper.
We have exploited all data recovered from the Radotín tunnel and other present and past localities in different parts of the Prague Synform (Figs 1, 2) for quantitative assessment and detailed study of the sedgwickii Event, improving precision of its biostratigraphical correlation, and relative timing of the associated positive carbon isotope excursion and lithological changes.
Location of the upper Aeronian sections in the Silurian outcrop area of the Prague Synform.
Correlation chart of the upper Aeronian localities in the Prague Synform: biostratigraphy and lithology.
2. Methodology
A series of 33 bulk samples, each with a mass of 4–5 kg, was collected from a 7 m thick succession comprising the Pribylograptus leptotheca, Lituigraptus convolutus, Stimulograptus sedgwickii, Lituigraptus rastrum and lower Rastrites linnaei biozones (zones in the following text), exposed in a gallery of the Radotín tunnel. For technical reasons, the succession was sampled at slightly irregular intervals concentrated around the supposed level of graptolite extinction and carbon isotope excursion in the sedgwickii Zone (see Fig. 3 for sampling levels). Bulk samples were split in the laboratory and all determinable graptolite rhabdosomes (4,000 specimens, excluding small fragments) were employed in calculations.
Radotín tunnel section: lithological column, lithostratigraphical units, chronostratigraphy based upon graptolite biozones, δ13Corg and TOC profiles, and graptolite dominance and species diversity counted for respective samples (sampling levels). Dominance is calculated using Σ(ni/n)2 (ni is number of individuals of taxon i). Values higher than the median value are highlighted by grey shaded fields.
Basic biodiversity indices were calculated from graptolite quantitative data collected from the Radotín samples to evaluate the faunal and ecological changes within the studied stratigraphic interval. First of all, rarefaction curves were computed for all samples to investigate the effect of sample size upon species counts. Only samples having the rarefaction curves distinctly flattening out for the larger number of specimens were included in further analysis. This limited dataset was tested for correlation between species counts and sample size. Owing to the fact that it is not possible to presume generally the linearity of the anticipated relationship (i.e. species counts and sample size), the commonly used Pearson's correlation coefficient could not be used to assess the degree of correlation. For this reason two other correlation coefficients (Spearman's and Kendall's correlation coefficients) were used for the analysis. To characterize species richness across the studied interval we calculated Menhinick's and Margalef's richness indices and compared these data with rarefaction analysis. Dominance and Berger–Parker indices were also calculated for all samples to evaluate the evenness of the studied graptolite assemblages. Dominance was calculated using Σ(ni/n)2 where ni is number of individuals of taxon i and n is sample size. The Berger–Parker index is the number of individuals of the most common taxon divided by sample size. Correlations between species richness indices and evenness indices were evaluated using Spearman's and Kendall's correlation coefficients. Another of the goals was ascertaining the time trends of all of the above-mentioned biodiversity indices; these were calculated. The statistical significance of these trends was analysed by the Mann–Kendall test. This statistical method is a non-parametric test for the detection of a trend in a time series and has been widely used in environmental science because it is simple, robust and can cope with missing values and values below a detection limit. The relationship between the above-mentioned species biodiversity indices and the stratigraphical position of samples was analysed to evaluate faunal and ecological changes in the studied interval. Most statistical calculations were undertaken using the software package Past, ver. 2.08b (Hammer, Harper & Ryan, Reference Hammer, Harper and Ryan2001).
All 33 samples from the Radotín tunnel section were analysed for organic carbon isotopes and TOC. Entirely fresh hand specimens were cut and rock powder was prepared from a few grams of fresh sample. A few milligrams of rock powder were taken for TOC and total carbon isotope analyses. Before analyses, rock powders were decarbonatized with 10% HCl at 40 °C for several hours, then washed and dried. About 20 mg of rock powder were used for TOC and about 10 mg for isotope analyses. Samples were combusted in a Fisons 1108 elemental analyser coupled on-line to a Finnigan Mat 251 mass spectrometer via a ConFlo interface. As reference material, NBS 22 (Gulf oil, with δ13C value −29.75 ‰) and acetanilid (Analytical Microanalysis, UK) were measured. Accuracy and precision were controlled by replicate measurements of laboratory standards and were better than ± 0.1 ‰ (1 σ) for total carbon isotope analyses and better than ± 0.02 % (1 σ) for TOC content.
Biostratigraphical data from the Radotín tunnel were supplemented by extensive collections from large blocks dumped outside the tunnel. The latter served for further study on graptolite taxonomy and assemblages. Published and unpublished data from all other coeval sections in the Barrandian area available to the authors in the course of the last 30 years and the museum collections of B. Bouček have also been examined. Illustrated specimens from the first author's collection are housed in the Czech Geological Survey and bear the designation PŠ.
3. Localities
In the Barrandian area, natural exposures of upper Aeronian shales are very rare and of limited extent in comparison with the common exposures and subcrops of the solid, weathering resistent siliceous black shales and silicites of the middle Aeronian. The soft, clayey and micaceous upper Aeronian black shales are rich in pyrite and undergo rapid decomposition under surface or near-surface conditions. Thus, available sections (Figs 1, 2) are either within the weathering-resistant contact aureoles of neighbouring basalt sills or temporarily exposed by deep excavations or subsurface construction works.
3.a. Radotín tunnel
The sedimentary succession temporarily exposed in the highway tunnel (Fig. 4) north of Radotín, near Prague provided unique material for detailed and multidisciplinary studies. The unweathered Llandovery black shale succession, rich in well-preserved graptolites, also yielded excellent samples for TOC and carbon isotope analyses; hence this subsurface section is of particular significance. The tunnel also provided robust evidence of the middle and upper Aeronian and lower Telychian successions, including the sedgwickii Event. The lower part of the sedgwickii Zone and convolutus–sedgwickii zone boundary beds were studied for the first time in this section.
Middle Aeronian through to lowermost Telychian sedimentary rocks exposed in the face of the Radotín tunnel. No. 1 designates silty silicites and siliceous shales of the convolutus Zone, no. 2 points to pyrite-rich black shale of the lower sedgwickii Zone and no. 3 to a grey mottled silty bed in the middle part of the sedgwickii Zone. Micaceous black shale of the upper sedgwickii Zone is marked by no. 4. The black clayey shale (no. 5) immediately below the pale-coloured basal mudstone yielded graptolites of the rastrum Zone. The massive bed of pale-coloured, mottled carbonate mudstone, without graptolites, forms the base of the Litohlavy Fm. (no. 6). Lowermost Telychian graptolites of the linnaei Zone appear in the black claystone intercalations (no. 7) just above the basal mudstone. Photo by Jakub Bohátka.
3.b. Zdice – Barrande's Colony Lapworth
A shallow test-pit in a forested slope between Holý vrch Hill and Smutný Hill east of Zdice (c. N 49° 54′ 22.3″, E 13° 59′ 56.0″) exposed a 2 m thick succession comprising micaceous black shale of the upper sedgwickii Zone, fine black shale of the rastrum Zone and yellowish-green mudstone without graptolites. The moderately weathered upper Aeronian sedimentary rocks are underlain and overlain by basalt sills.
3.c. Zadní Třebaň
Black micaceous shales, c. 1 m thick, are sandwiched between two doleritic basalt sills on the rocky slope west of Zadní Třebaň railway station (N 49° 55′ 05.2″, E 14° 11′ 53.9″) and belong to the upper part of the sedgwickii Zone. A shallow test-pit excavated in the upper part of the same forested slope (N 49° 55′ 02.2″, E 14° 11′ 50.3″) exposed black shale with graptolites assigned to the newly erected rastrum Zone.
3.d. Želkovice
A test-pit excavated for B. Bouček in the 1940s (see Bouček, Reference Bouček1953) and subcrops examined by P. Štorch in the field south of Želkovice village (N 49° 52′ 33.1″, E 14° 02′ 20.1″) exposed black clayey shales of the uppermost sedgwickii and rastrum zones that are topped by pale-coloured mudstone, barren of graptolites and further separated by a basalt sill from the overlying succession with graptolites of the linnaei Zone.
3.e. Nové Butovice
Large building excavations in the Prague-Nové Butovice district (N 50° 02′ 50,5″; E 14° 20′ 29.0″) exposed a continuous sedimentary succession ranging from the upper Hirnantian through to the lower Telychian (see also Štorch, Reference Štorch2006, fig. 5). Most graptolites were poorly preserved in weathered upper Aeronian micaceous and clayey shales. The first silty bed was recorded high in the convolutus Zone, but a major siltstone bed, 0.3 m thick, occurs within the sedgwickii Zone.
Selected upper Aeronian graptolites from the Prague Synform. (a) Spirograptus andrewsi (Sherwin), PŠ 1010, rastrum Zone, Radotín tunnel; (b) Cephalograptus extrema Bouček & Přibyl, PŠ 1009, lower sedgwickii Zone, Radotín tunnel; (c) Torquigraptus pulcherrimus (Manck), PŠ 1013, upper sedgwickii Zone, Zdice – Col. Lapworth; (d) Lituigraptus rastrum (Richter), PŠ 1008, rastrum Zone, Želkovice; (e) Comograptus barbatus (Elles & Wood), PŠ 1007, upper sedgwickii Zone, Zdice – Col. Lapworth; (f) Parapetalolithus praecedens (Bouček & Přibyl), PŠ 1014, upper sedgwickii Zone, Zadní Třebaň; (g) Torquigraptus magnificus (Přibyl & Münch) (centre) and Stimulograptus sedgwickii (Portlock) (right, left and below), PŠ 1011, middle sedgwickii Zone, Radotín tunnel; (h) Petalolithus clandestinus Štorch, PŠ 1012, lower sedgwickii Zone, Radotín tunnel. All specimens, scale bar represents 3 mm.
3.f. Velká Ohrada
Pipeline excavations north of Řeporyje (N 50° 02′ 19.6″, E 14° 18′ 57.5″) provided a temporary exposure of uppermost Hirnantian through to upper Telychian strata (Štorch, Reference Štorch2006, fig. 6). Two stratigraphical unconformities have been recorded in the early Silurian black shale succession. The second break in sedimentation separated condensed micaceous laminites of the convolutus Zone from the overlying Aeronian/Telychian boundary mudstone. The sedgwickii and rastrum zones are both missing.
Composite range chart of the upper Aeronian graptolites recorded in the Prague Synform. In other areas the stratigraphic ranges of several taxa are either longer than shown (e.g. N. scalaris, Metacl. undulatus and St. sedgwickii records from Telychian succession of Wales; Loydell, Reference Loydell1993) or more restricted, such as ‘M.’ limatulus and Ceph. extrema in British sections (Zalasiewicz et al. 2006). Break in graptolite ranges between informal lower and upper parts of the sedgwickii Zone indicates silty mudstone level, barren of determinable graptolites.
3.g. Hýskov
A series of test-pits (N 50° 00′ 13.8″, E 14° 04′ 20.1″) runs through middle Llandovery basaltic volcaniclastic rocks, interbedded with tuffaceous shales and limestones, in a separate block preserved in a tectonized zone along the Prague Fault, c. 5 km NW of the nearest outcrop area of coeval black shale strata (Štorch, Reference Štorch2001). The local fauna, which is dominated by brachiopods (Havlíček, Reference Havlíček1977), trilobites (Šnajdr, Reference Šnajdr1978) and dendroid graptolites (Kraft, Reference Kraft1982), provides evidence for dysoxic and oxic bottom conditions, unique to the middle Llandovery succession of Central Europe. Graptoloid graptolites (Štorch, Reference Štorch2001), all referable to the lower sedgwickii Zone, are subordinate.
4. Results
4.a. Late Aeronian graptolite fauna and biostratigraphy
4.a.1. Stimulograptus sedgwickii Biozone
The sedgwickii Zone comprises an interval from the lowest occurrence of the zonal index graptolite to the lowest occurrence of Lituigraptus rastrum (Richter) (Fig. 5d). It is 1.7 m thick in the Radotín tunnel section and c. 1.1 m thick in the Velká Ohrada 1 section. The Zadní Třebaň and Zdice sections preserved only the upper part of the biozone, which corresponds roughly in thickness to that measured in the Radotín tunnel section (Fig. 2). Also the Llandovery volcanic–carbonate succession exposed by trenches near Hýskov (Štorch, Reference Štorch2001) yielded graptolites typical of the lower sedgwickii Zone assemblage encountered from the Radotín tunnel section, although the two sections differ in relative abundances of the taxa. The observed thickness of lime- and mudstones, excluding volcaniclastic rocks, is 6.5 m. See Figure 6 for the graptolite record combined from all available localities. At least two successive graptolite assemblages are recognized within the zone:
(a) In the lower part of the zone uncommon Stimulograptus sedgwickii (Portlock) (Fig. 5g) is usually associated with Petalolithus clandestinus Štorch (Fig. 5h), Torquigraptus magnificus (Přibyl & Münch) (Fig. 5g) and Pristiograptus variabilis (Perner). Cephalograptus extrema Bouček & Přibyl (Fig. 5b) is common in the Radotín tunnel section, whereas Neolagarograptus tenuis (Portlock) is common at Hýskov. Normalograptus scalaris (Hisinger), Pseudorthograptus insectiformis (Nicholson), Pristiograptus regularis (Törnquist), Campograptus lobiferus (McCoy), Torquigraptus? aff. cerastus (Hutt) and Torquigraptus decipiens (Törnquist) continued from the upper part of Lituigraptus convolutus Biozone. Pribylograptus cf. leptotheca (Lapworth), and ‘Monograptus’ limatulus Törnquist also persisted into the lowermost part of the sedgwickii Zone. The thin-walled, slightly proximally tapering Metaclimacograptus undulatus (Kurck) is common but further proliferates above this interval. Torquigraptus pulcherrimus (Manck) (Fig. 5c), Torquigraptus aff. linterni (Williams et al.) and Parapetalolithus praecedens (Bouček & Přibyl) (Fig. 5f) appear in high levels of the lower sedgwickii Zone.
(b) The upper part of the sedgwickii Zone (note that it was formerly referred to the middle and even lower parts of the zone by Bouček, Reference Bouček1953 and Štorch, Reference Štorch1994) is characterized in Barrandian sections by abundant graptolites with moderate to low taxonomic diversity. St. sedgwickii becomes more abundant and is accompanied by repeated blooms of Metacl. undulatus. Torquigraptus aff. linterni also increases in abundance; Comograptus barbatus (Elles & Wood) (Fig. 5e) and Pristiograptus pristinus Přibyl made their first occurrences at the base of this interval. T. magnificus makes its highest occurrence in the middle part; Nl. tenuis disappeared well below the top of the sedgwickii Zone. Pristiograptus variabilis (Perner) and P. pristinus continued through to succeeding zones. Parapet. praecedens is widespread, whereas records of Ceph. extrema are limited to the Zdice and Želkovice sections. Other typical taxa, although uncommon, are T. pulcherrimus and Com. barbatus. The uppermost part of the sedgwickii Zone in the present redefined usage is marked by the incoming of Torquigraptus involutus Lapworth and Spirograptus andrewsi (Sherwin) (Fig. 5a).
The graptolite assemblage of the sedgwickii Zone was originally identified by Lapworth (Reference Lapworth1878) from the upper Birkhill Shale Formation of Dob's Linn, Scotland, and named after a junior synonym of St. sedgwickii (Portlock, Reference Portlock1843): Monograptus spinigerus (Nicholson, Reference Nicholson1868). In the Barrandian area, a corresponding assemblage was identified by Bouček (Reference Bouček1930) at Barrande's former locality, Colony Lapworth near Zdice (present Zdice section), and designated as the Monograptus involutus Zone. Subsequently, Bouček (Reference Bouček1934) adopted St. sedgwickii as a zonal index fossil and provided a more detailed description of the zone and its graptolite assemblage (Bouček, Reference Bouček1953), based upon collections from Zdice, Želkovice, Libomyšl, Běleč and Zadní Třebaň. A distinctive graptolite assemblage, dominated by Lituigraptus rastrum (Richter), was reported from the upper part of Bouček's sedgwickii Zone by Bouček (Reference Bouček1953) and Štorch (Reference Štorch1994). This assemblage forms the basis of the Lituigraptus rastrum Biozone recognized in this paper.
4.a.2. Lituigraptus rastrum Biozone
The upper part of the Bohemian sedgwickii Zone in the sense of Bouček (Reference Bouček1934, Reference Bouček1953) and Štorch (Reference Štorch1994, Reference Štorch2006) is separated from the succeeding Rastrites linnaei Zone and lowest occurrences of Spirograptus guerichi Loydell, Štorch & Melchin by a barren mudstone in all Barrandian sections (see Kříž, Reference Kříž1975 and Štorch, Reference Štorch2006). The uppermost levels of their sedgwickii Zone have been already cited by Bouček (Reference Bouček1953) as having a specific assemblage dominated by ‘Rastrites’ rastrum. A new systematic revision of the graptolite fauna has revealed that Stimulograptus halli (Barrande) is present in this interval, which is thus correlatable, at least in part, with the Stimulograptus halli Zone used in the British Isles by Jones & Pugh (Reference Jones and Pugh1916), Loydell (Reference Loydell1991a , Reference Loydell1992) and Zalasiewicz et al. (Reference Zalasiewicz, Taylor, Rushton, Loydell, Rickards and Williams2009). Owing to difficulties in distinguishing between incomplete specimens – mostly distal fragments – of the two overlapping zonal index taxa (St. sedgwickii and St. halli), the question remained of how to separate formally the upper levels with a distinct graptolite assemblage from the underlying bulk of the sedgwickii Zone.
An alternative index graptolite was looked for. Instead of St. halli, the abundant and readily determinable Lituigraptus rastrum (Richter) (Fig. 5d) has been found to work well in practice in all relevant sections of the Barrandian area. The newly introduced L. rastrum Zone varies from 0.4 to 0.75 m thick in examined sections (Radotín tunnel, Nové Butovice, Želkovice, Karlík, Zdice, Zadní Třebaň).
The base of L. rastrum Zone, defined by the incoming of its abundant zonal index graptolite, roughly coincides with or pre-dates the lowest occurrence of Stimulograptus halli (Barrande) and slightly post-dates that of S. andrewsi. Rastrites schaueri Štorch & Loydell has its first occurrence in about the middle of the rastrum Zone. Several taxa, in particular Glyptograptus cf. incertus (Elles & Wood), Com. barbatus and T. involutus continued from the upper part of sedgwickii Zone to the lower part of L. rastrum Zone. St. sedgwickii itself and T. involutus also persisted into the rastrum Zone and Parapet. praecedens and S. andrewsi became more common. Other species not restricted to this zone are abundant Metacl. undulatus, P. variabilis, P. pristinus and St. halli.
4.b. Graptolite faunal dynamics
The Middle Aeronian Pribylograptus leptotheca and Lituigraptus convolutus zones exhibit ecologically well-balanced, high-diversity graptolite faunas with a particularly low degree of dominance and high values of evenness indices. Štorch (Reference Štorch1998) described 33 species from a single locality of the L. convolutus Zone at Tmaň near Beroun. The baseline extinction rate accelerated and diversity declined across the convolutus/sedgwickii zonal boundary (Rivagraptus, Neodiplograptus and most species of Campograptus and Rastrites became extinct). Some of the surviving taxa disappeared in the subsequent earliest part of the sedgwickii Zone (‘Monograptus’ limatulus group, Pribylograptus).
The early part of the sedgwickii Zone saw a rapid turnover of graptolite taxa (Fig. 6). Campograptus gave origin to Stimulograptus sedgwickii, the lowest occurrence of which defines the formal base of this zone. Torquigraptids proliferated as Torquigraptus decipiens and T.? aff. cerastus were replaced by T. magnificus, T. pulcherrimus and T. aff. linterni. Normalograptus (N. scalaris), the last representatives of ancorate Petalolithus (Pet. clandestinus) and those of the Rastrites peregrinus lineage (R. gracilis Přibyl) are still important elements of this assemblage. The unancorate Parapetalolithus (Parapet. praecedens) first appears in the uppermost part of this interval. Graptolite diversity decreased to 23 species if counted for the whole lower sedgwickii Zone. Diversity recorded for individual sampling levels dropped still more significantly. A low degree of dominance indicates persisting stable conditions whereas conspicuously low graptolite abundance, generally lower but fluctuating TOC and high pyrite content indicate changing environmental conditions. A siltstone bed encountered in the upper middle part of the sedgwickii Zone in the Radotín tunnel and Nové Butovice sections occurs at the same level as a further change in the graptolite fauna.
Above this bed, in the upper part of the sedgwickii Zone, graptolites become abundant, but the assemblages are commonly dominated by two species (St. sedgwickii and Metacl. undulatus) both persisting from the lower part of the sedgwickii Zone. Extinction and speciation rates are low in this interval (Fig. 6) and overall diversity declined further to 17 species.
Through much of the Lituigraptus rastrum Zone the assemblages are dominated by one or two species (Metacl. undulatus and L. rastrum). Dominance increased still further while overall diversity remained at the minimum of 17 species (Fig. 6). The graptolite crisis known as the sedgwickii Event (Melchin, Koren’ & Štorch, Reference Štorch1998) persisted through this level.
Barren mudstones, which separate the black shale of the rastrum Zone from that of the linnaei Zone of the lowermost Telychian (Figs 2, 3, 4), hide the subsequent recovery interval in the Barrandian area. Graptolite assemblages sub- and suprajacent to the barren mudstone, when correlated with coeval successions of Wales (after Loydell, Reference Loydell1991a ) and Spain (current research), suggest that the boundary interval of the halli and guerichi zones is barren of graptolites in the Barrandian area. Only four graptolite species were found both below and above the mudstone in the Radotín tunnel section. The first black shale intercalation above the mudstone hosts higher diversity and lower dominance assemblages, composed for the most part by likely descendants of the taxa recorded from the upper sedgwickii and rastrum zones. Surprisingly, a significant graptolite radiation probably commenced during the course of the more oxic sedimentation of this barren mudstone.
4.c. Biodiversity and time trend analyses
The effect of sample size upon species counts was investigated by rarefaction analysis for all samples from the Radotín section (Fig. 3). The analysis resulted in the need to reduce the dataset from 29 samples to 22 samples for subsequent biodiversity analyses (i.e. rarefaction curves flatten out well for these samples suggesting that further sampling would not increase the number of taxa substantially). The limited dataset contains two samples from the leptotheca Zone, six samples from the convolutus Zone, four samples from the lower sedgwickii Zone, seven samples from the upper sedgwickii Zone and three samples from the rastrum Zone. Tests of these 22 graptolite samples revealed no statistically significant correlation between species number and sample size in both used correlation coefficients (Spearman's and Kendall's).
To characterize species richness across the studied interval we compared the number of observed graptolite species in each sample and calculated Menhinick's and Margalef's richness indices. All of these data indicate that the graptolite species diversity is generally decreasing through the Radotín section (Fig. 7a, c). The analysis of time trends in all above-mentioned values (number of graptolite species found as well as in Menhinick's and Margalef's richness indices) across the whole studied stratigraphical interval (i.e. from leptotheca to rastrum zones) showed a statistically significant trend (p-values for all Mann–Kendall statistics are much lower than critical p-values for the 0.01 significance level). These results fit well with the general trend in number of graptolite species recorded in the Barrandian area in individual graptolite zones (Fig. 7d). The number of graptolite species in each of the five graptolite zones observed in the Radotín section as well as estimation of graptolite species diversity based on rarefaction analysis revealed the same decreasing trend from the convolutus to upper sedgwickii zones (Fig. 7d). However, statistical tests-based quantitative data from the Radotín section revealed statistically significant differences in the total number of graptolite species only between the leptotheca, convolutus and lower sedgwickii zones on the one hand and upper sedgwickii and rastrum zones on the other, even though the species richness of the convolutus Zone seems to be the highest (Fig. 7d). Estimations of graptolite species diversity based on values of Menhinick's and Margalef's richness indices for individual graptolite zones revealed a similar trend. Also in this case, a statistically significant change in the total biodiversity in the studied graptolite zones was revealed between the lower and upper sedgwickii Zone (Fig. 7e). On the other hand, evaluation of time trends of species richness indices for all samples from the leptotheca to lower sedgwickii zones (12 samples) by the Mann–Kendall test revealed a statistically significant decreasing trend for the Margalef's richness index (p < 0.05) and number of graptolite species found (p < 0.01). No statistically significant trend in species richness indices was found for data from the upper sedgwickii and rastrum zones. Taken together our analyses show a statistically significant decreasing trend in graptolite species diversity from the leptotheca to lower sedgwickii zones and subsequent distinct decrease in graptolite species diversity between the lower and upper parts of the sedgwickii Zone (Fig. 7a, b, d, e). Analyses of changes in graptolite species diversity inferred from summary data (i.e. from total number of graptolite species being present in each zone) are biased to some extent by the very likely different time duration of the graptolite zones.
Species richness and community evenness indices calculated for individual samples (a–c) from the Radotín tunnel section and for cumulative samples uniting all samples within a particular graptolite zone (d–f). Species richness is expressed as number of individual species in each sample (a, d), and is estimated by Menhinick's and Margalef's richness indices (b, e). Species diversity at 250 occurrences (based on rarefaction analysis) is also shown for cumulative samples (d, e). Evenness is expressed by simple dominance and Berger–Parker indices.
Analysis of evenness of graptolite communities also revealed a distinct trend. Both the dominance and Berger–Parker indices show an increasing distinct trend in dominance within graptolite assemblages across the studied interval (Fig. 7c). Analysis using the Mann–Kendall statistics showed a statistically significant increasing trend in the both these indices (p < 0.01) through the Radotín section (Fig. 7c). A more detailed view shows that values of both the dominance and Berger–Parker indices have quite different dispersion for data from the leptotheca to lower sedgwickii zones on the one hand and for data from the upper sedgwickii and rastrum zones on the other. In the first stratigraphical interval both the dominance and Berger–Parker indices vary only slightly. On the other hand, both indices increased dramatically (three times) in the upper sedgwickii and rastrum zones (Fig. 7c). The distinct increase of both the dominance and Berger–Parker indices in the latter stratigraphical interval is also observable in the summary data calculated for the studied graptolite zones (Fig. 7f). Correlation analysis using Spearman's and Kendall's coefficients between species richness indices and evenness indices revealed statistically significant negative correlations (p < 0.01) between dominance on the one hand and all species richness indices (number of observed graptolite species in each sample and calculated Menhinick's and Margalef's richness indices) on the other. Taken together, the analysis of evenness has shown that graptolite assemblages that are richer in species have significantly higher evenness. The graptolite assemblages of the leptotheca to lower sedgwickii zones had very low dominance in contrast with those of the upper sedgwickii and rastrum zones. Very high variability and generally high values of dominance are typical for the graptolite assemblages of the upper sedgwickii and rastrum zones (Fig. 7c, f).
4.d. Sedimentary signatures of the sedgwickii Event
The Radotín tunnel section exhibits its first significant change in sedimentation in the lowermost sedgwickii Zone (Fig. 3). It is associated with a considerable, but not abrupt impoverishment of the graptolite assemblage for several important graptolite clades. Siliceous black shales of the convolutus Zone are replaced by pyrite-rich, graptolite-poor shale with lower TOC (c. 3%). Uncommon graptolites refer this relatively thick level to the lower part of the sedgwickii Zone. It is overlain by a heavily bioturbated micaceous siltstone without any determinable graptolites, which, in turn, is succeeded by organic-rich (TOC c. 6%), finely laminated micaceous black shale with an abundant, but still lower diversity, graptolite fauna. Differences between the graptolite assemblage below and above the siltstone and the abrupt change in grain size and TOC suggest that an interval of nondeposition and/or erosion may be coupled with the deposition of the siltstone bed. Organic-rich micaceous black shales above the siltstone bed are assigned to the upper part of the sedgwickii Zone. The stratigraphical ranges of numerous graptolite species span the interval. Hence a short duration of the potential stratigraphical break, still within upper middle part of the sedgwickii Zone, can be inferred.
The lower part of the sedgwickii Zone is preserved in the Hýskov section and was deposited under considerably shallower and better-oxygenated bottom conditions than the otherwise black shale dominated Llandovery succession of the Prague Synform. Marly shales and limestones, including grainstones with coated grains, host rich and diverse benthic faunas (Štorch, Reference Štorch2001).
The sedimentary record indicates incipient sea-level drawdown in the early part of the sedgwickii Zone, which culminated abruptly in about the middle of the zone. A high content of early diagenetic pyrite preserved in concretions and planar bioturbation fillings as well as lower TOC in the lower part of the sedgwickii Zone in the Radotín tunnel are consistent with temporarily disrupted anoxia as inferred from the model discussed by Schieber (Reference Schieber2003).
In the Velká Ohrada section, the stratigraphical break is of much greater duration and late Aeronian sedimentary rocks are missing entirely. In a large east-central part of the Prague Synform, late Hirnantian siltstones are overlain directly by the pale-coloured mudstones of the basal Telychian (Štorch, Reference Štorch2006).
4.e. Carbon isotope excursion and its biostratigraphic dating
Carbon isotope sampling began at the base of the leptotheca Zone of the Želkovice Formation, about 6 m below the base of the Telychian Litohlavy Formation. The last sample was collected about 1 m above the base of the latter formation (Fig. 3). In total 33 samples were analysed for δ13Corg and TOC content in the Radotín tunnel section. Both of these vary through the interval studied but statistical analysis revealed no significant trend. The first positive shift in δ13Corg appears at 5.10 m, at the base of the sedgwickii Zone. Around the level of 6.2 m, in the middle of the sedgwickii Zone, δ13Corg values rapidly increase whereas TOC values decrease (Fig. 3). Correlation analysis does not support a significant statistical relationship between δ13C org and TOC values. Both limbs of the δ13C org excursion are within strata with a relatively high organic content and minor silt content. Samples from the middle of the siltstone (6.24 m) exhibit extremely high δ13C org values (−14.31 to −18.12 ‰), which must be considered, however, with reservation. The TOC values of these two samples are considerably lower (0.6%) than TOC values of other samples from the sedgwickii Zone (average value is about 4.8%). In addition the siltstone derived probably from a shallower environment could primarily contain small amounts of carbonate carbon. For these reasons, partial opening of the organic carbon isotope system of these samples during diagenesis cannot be dismissed. Some part of the δ13C excursion may not be preserved in the Radotín tunnel section owing to a possible stratigraphical break associated with the siltstone. δ13C returns to its baseline values in the rastrum Zone. Maximum TOC values (up to 7%) occur just above the siltstone and remain high in the rastrum Zone.
4.f. The Aeronian/Telychian boundary
A prominent, pale-coloured calcareous mudstone/claystone bed, the base of which marks the base of the Litohlavy Formation, separates the black shale with a L. rastrum Zone assemblage from the overlying black shale level with S. guerichi and Rastrites linnaei Barrande in all sections. The graptolite faunas of the two separate levels are readily recognizable since only a few species span the two levels (St. halli, R. schaueri, P. pristinus and P. variabilis). The black shale above the pale mudstone sees the incoming of R. linnaei, S. guerichi, Torquigraptus planus (Barrande), Torquigraptus obtusus (Schauer), Glyptograptus fastigatus Haberfelner, Glyptograptus auritus Bjerreskov, Parapetalolithus elongatus Bouček & Přibyl, Parapetalolithus palmeus (Barrande) and Pseudoplegmatograptus sp. Glyptograptus cf. incertus, Parapet. praecedens, Ceph. extrema, S. andrewsi and L. rastrum have all vanished from the fossil record. Considering the break in graptolite faunal composition and omission of some age-diagnostic taxa (e.g. Paradiversograptus runcinatus Lapworth), slow sedimentation must be considered likely for the graptolite-barren mudstone. It appears to correspond to both the upper part of the halli Zone sensu Loydell (Reference Loydell1991a ) and the Paradiversograptus runcinatus Subzone recognized by Loydell in the lowermost part of S. guerichi Zone in Wales and subsequently by Gutiérrez-Marco & Štorch (Reference Štorch1998) in the lowermost part of R. linnaei Zone in the Western Iberian Cordillera of Spain.
The degree of diachroneity of the base of the barren mudstone is negligible in the Prague Synform. It overlies black shale with a L. rastrum zonal assemblage in all localities except Velká Ohrada (Fig. 2) where the succession is interrupted by a regional break in sedimentation (Štorch, Reference Štorch2006). The thickness of the mudstone bed, however, varies from 0.5 m (e.g. Radotín tunnel) to 1.25 m at Nové Butovice. Kříž (Reference Kříž1975) reported a thickness of 0.3–4.10 m from unspecified boreholes in the SW part of the Silurian synform. Apart from subordinate black shale intercalations with a guerichi or linnaei Zone graptolite fauna, the same pale-coloured mudstone sedimentation continued well into the Spirograptus turriculatus Zone as documented for example in the classical Litohlavy section (Štorch, 1991). The maximum value reported for the mudstone thickness by Kříž (Reference Kříž1975) encompassed all of this succession including minor black shale intercalations. Prominent pale-coloured mudstone discussed by Kříž (Reference Kříž1975) and Štorch (Reference Štorch2006) gives evidence that oxic sedimentation predominated in the Barrandian area during post-sedgwickii Event recovery through to the early Telychian utilis extinction Event recognized by Loydell (Reference Loydell1994). Apparent links to graptolite mass extinctions and positive carbon isotope excursions are missing. Repeated oxic bands in the subsequent Litohlavy Formation account for cyclic, probably climatic changes, which influenced sedimentation in the Barrandian area up to the base of late Telychian Oktavites spiralis Zone.
The Parapetalolithus palmeus and Parapetalolithus hispanicus subzones were recognized within the classical Bohemian Rastrites linnaei Zone by Bouček (Reference Bouček1953). This graptolite fauna comes from black shale intercalations in the mudstone-dominated interval. The graptolite assemblages of the two subzones suggest that both are correlatable with only a limited, presumably upper, part of the standard lowermost Telychian S. guerichi Zone. Consequently, the base of Telychian Stage associated with the first appearance datum (FAD) of S. guerichi in graptolite-bearing successions worldwide, must be placed from a chronostratigraphical point of view in the barren mudstone bed, below the black shale intercalation with the lowest occurrence of Spirograptus guerichi (Parapet. palmeus Subzone) and, consequently, above the black shale with L. rastrum but without S. guerichi and R. linnaei.
4.g. Correlation with graptolite-rich upper Aeronian successions abroad
4.g.1. Thuringia
Schauer (Reference Schauer1967, Reference Schauer1971) listed most of the graptolite species recorded in the present study (such as Parapet. praecedens, Ceph. extrema, T. decipiens, St. sedgwickii, rare Com. barbatus and Nl. tenuis) from his sections but detailed correlation with the upper Aeronian succession of the Barrandian area may be partly biased by taxonomy. Apparent differences from Barrandian sections include R. hybridus ranging from the uppermost L. convolutus Zone through to the linnaei Zone. Upper Aeronian sections of the Barrandian area yielded Rastrites gracilis Přibyl and R. cf. erectus Hutt instead. Note that incomplete R. hybridus affected by tectonic strain may be easily misidentified as Rastrites peregrinus Barrande, which is a typical element of the convolutus Zone. Schauer (Reference Schauer1967) reported also Monograptus circularis Elles & Wood, albeit without description or illustration, which could be actually T. aff. linterni of Bohemian sections. T. magnificus, described and figured from the sedgwickii Zone of Thuringia by Přibyl & Münch (Reference Přibyl and Münch1943), was reported from the lower convolutus Zone by Schauer (Reference Schauer1967) and T. pulcherrimus was missing altogether. L. rastrum, identified by Törnquist (Reference Törnquist1907) in Raitzhain and Grobsdorf, was recorded by Schauer (Reference Schauer1967, Reference Schauer1971) in the upper sedgwickii Zone of his sections and thus tentatively correlates with the L. rastrum Zone of this paper and, at least in part, with the St. halli Zone of Loydell (Reference Loydell1991a ).
4.g.2. Denmark
In Bornholm, the black shale of the uppermost convolutus Zone with Ceph. cometa and Ceph. extrema is sharply overlain by ‘light grey, argillaceous and silty mudstone’ barren of graptolites but rich in pyritized planar burrows (Bjerreskov, Reference Bjerreskov1975). This interval grades upwards into laminated shale becoming darker and intercalated with bands of ‘nonfossiliferous green mudstone’. The graptolite assemblage of this dark, laminated shale with Glyptograptus auritus Bjerreskov, Parapet. palmeus, Paradiversograptus runcinatus and S. guerichi a.o. is correlatable with the basal Telychian lower linnaei and/or guerichi Zone. The upper Aeronian is represented by a light grey silty mudstone interval without graptolites and/or may even be partly missing owing to a possible break in sedimentation at the base of the silty mudstone.
4.g.3. Spain
In the Western Iberian Cordillera (Gutiérrez-Marco & Štorch, Reference Gutiérrez-Marco and Štorch1998) and Central Iberian Zone (Štorch et al. Reference Štorch, Gutiérrez-Marco, Sarmiento, Rábano, Gutiérrez-Marco and Rábano1998), the lower Silurian Los Puertos and Criadero quartzites are regularly overlain by black micaceous shales of the linnaei Zone. Also, the middle Aeronian convolutus Zone may be locally developed as a black shale interval within the quartzite succession (Gutiérrez-Marco & Štorch, Reference Gutiérrez-Marco and Štorch1998) but the upper Aeronian is always represented by quartzites. In the relatively deeper-water Silurian of the Ossa Morena Zone, however, black shale sedimentation commenced in the Akidograptus ascensus Zone and continued through much of the Silurian (Jaeger & Robardet, Reference Jaeger and Robardet1979). Black shales of the c. 6 m thick sedgwickii Zone are overlain by a complete succession of the halli Zone, which (according to David Loydell, pers. comm. 2010) comprises a significant interval with L. rastrum in its middle part. Graptolite assemblages as well as faunal dynamics are closely similar to those in Bohemian sections and allow for intrazonal correlation. The lower part of the sedgwickii Zone is marked by a low-diversity and low-dominance graptolite fauna with St. sedgwickii, Pet. clandestinus, Ceph. extrema and T. magnificus. Nl. tenuis proliferates in the middle part of the zone, above a 5 cm thick band of rusty, pyrite-rich silty mudstone, and it is associated with Com. barbatus and other typical species, including Parapet. praecedens. Further correlation with Bohemian successions would be premature since the El Pintado section is under detailed study by D. K. Loydell, P. Štorch and J. C. Gutiérrez-Marco. No remarkable changes in lithology have been encountered from the convolutus Zone through to the base of the halli Zone except for the pyrite-rich mudstone band in the middle part of the sedgwickii Zone, which may be interpreted as a temporary disruption of the anoxic regime.
4.g.4. Scotland
Toghill (Reference Toghill1968) assigned an 8.4 m thick interval of the Dob's Linn succession to the sedgwickii Zone; the lower part of this, however, is barren of graptolites. Graptolite-bearing black shales in the middle part of this interval have been further studied by Pannell, Clarkson & Zalasiewicz (Reference Pannell, Clarkson and Zalasiewicz2006) who identified St. sedgwickii, Nl. tenuis concentrated in several peaks of abundance, N. scalaris, pristiograptids, glyptograptids and petalolithids. Loydell (Reference Loydell1991b ) provided a range chart of the graptolites through the uppermost sedgwickii Zone and halli Zone at Dob's Linn, based on a re-examination of Toghill's collection. Williams et al. (Reference Williams, Zalasiewicz, Rushton, Loydell and Barnes2003) reported graptolite assemblages including St. sedgwickii, P. aff. regularis, Torquigraptus linterni Williams et al., T. magnificus, Streptograptus ansulosus (Törnquist), Rastrites cf. gracilis, Paradiversograptus capillaris (Carruthers), Ps. insectiformis?, Gl. cf. incertus and some undetermined taxa from various Scottish localities and referred them to the upper sedgwickii Zone. Carbon isotope data acquired from the Dob's Linn section by Melchin & Holmden (Reference Melchin and Holmden2006) revealed a prominent carbon isotope excursion in the upper Aeronian. The relatively condensed section, limited resolution of biostratigraphical data and limited sampling density, however, did not allow for more detailed determination of the structure and biostratigraphical dating of the isotope curve at this particular level.
4.g.5. England and Wales
Moderately diverse graptolite assemblages of the sedgwickii Zone occur in the Howgill Fells (Rickards, Reference Rickards1970) and Lake District (Hutt, Reference Hutt1974). A substantial part of the zonal succession, however, is lacking a graptolite record since graptolite-bearing black shale is confined to several intercalations within predominantly barren mudstones. In the Lake District area (Hutt, Reference Hutt1974) Nl. tenuis is associated with the lowest occurrences of St. sedgwickii. Rickards (Reference Rickards1976) noted that the first occurrences of Nl. tenuis are well above those of St. sedgwickii. Loydell (Reference Loydell1991a , Reference Loydell1992, Reference Loydell1993) studied the latest Aeronian strata of mid-Wales and recognized the Stimulograptus halli Zone erected by Jones & Pugh (Reference Jones and Pugh1916) between the sedgwickii Zone and early Telychian strata with Spirograptus guerichi. In the Barrandian area, the L. rastrum Zone is now recognized between the upper Aeronian sedgwickii Zone and lower Telychian linnaei Zone, at about the same level.
4.g.6. Libya and Algeria
Štorch & Massa (Reference Štorch2006) reported sedgwickii Zone assemblages with St. sedgwickii, Glyptograptus aff. incertus, Metaclimacograptus hughesi, P. regularis, P. cf. renaudi and P. cf. pristinus from the western Murzuq Basin and Al Qarqaf Arch of Libya. Pet. clandestinus, P. regularis and Nl. tenuis were taken as indices of the lower part of the biozone at Al Qarqaf Arch. A similar association was recognized by Štorch & Massa (Reference Štorch, Massa, Salem and Oun2003) from a drill core in the Ghadāmis Basin (north-central Libya). Pet. clandestinus has also been found in the Algerian Eastern Tassili-n Ajjer (referred to Petalolithus cf. wuxiensis by Štorch & Massa, Reference Štorch, Massa, Salem and Oun2003). No significant graptolite data are available for the remaining part of the upper Aeronian in North Africa.
4.g.7. Canada, Nova Scotia
Graptolite-bearing black shales developed in the Lower Ross Brook Formation of the Arisaig Group were assigned to the lower part of the sedgwickii Zone by Melchin (Reference Melchin2007). The graptolite assemblage, comprising metaclimacograptids, N. scalaris, Pet. clandestinus, P. regularis, Camp. lobiferus and Nl. tenuis, was referred to the Neolagarograptus tenuis Subzone. The black shale succession grades upwards into grey silty shale, which, according to Melchin (Reference Melchin2007), still belongs in the sedgwickii Zone. The c. 160 m thick, shallowing up succession is topped by oxic strata with lowermost Telychian brachiopods. Subsequent carbon isotope analyses revealed three positive shifts of δ13Corg (Melchin et al. Reference Melchin, MacRae, Frýda, Štorch, Bullock and Loydell2011) in the upper Aeronian sedimentary succession, which corresponds with the sedgwickii and presumably also halli (or rastrum) zones. The middle peak of this three-fold δ13Corg excursion reaches up to −25.5 ‰. Each of the peaks is associated with a phase of shallowing in the Nova Scotia succession (Melchin et al. Reference Melchin, MacRae, Frýda, Štorch, Bullock and Loydell2011). Despite somewhat different sea-level histories, the Barrandian area and Nova Scotia exhibit roughly corresponding stratigraphical levels and a remarkably similar three-fold pattern to the carbon isotope excursion. Also the graptolite assemblage assigned by Melchin to the lower sedgwickii Zone (Nl. tenuis Subzone) is particularly remarkable for its palaeobiogeographical affinities to Gondwanan graptolite faunas (Melchin, Reference Melchin2007).
4.g.8. Canada, Arctic Islands
The upper Aeronian of Cornwallis Island is marked by an interval of thinly laminated and even rippled dolostones in a generally black shale dominated lower Silurian succession. The same interval sees a prominent positive δ13Corg excursion, which Melchin & Holmden (Reference Melchin and Holmden2006) dated by graptolites to the lower sedgwickii Zone. The lower sedgwickii Zone of Melchin & Holmden (Reference Melchin and Holmden2006), however, represents the bulk of the sedgwickii Zone of this paper since the upper sedgwickii Zone of Melchin & Holmden is within strata between the lowest occurrence of St. halli and the lowest occurrence of the lowermost Telychian Spirograptus guerichi, and thus correlates with the halli Zone of Loydell (Reference Loydell1991a ) or rastrum Zone of this paper. Although the isotope and sedimentary record account for a single massive event comprising all of the sedgwickii Zone, the limited stratigraphical resolution of published graptolite data and limited sampling density may have obscured the more complex nature of this critical interval. Published graptolite records do not allow for precise biostratigraphical control in the Arctic Canadian sections at this particular level, but particularly high TOC values and the falling δ13Corg recorded in strata just above the peak of the late Aeronian positive excursion closely resemble the trends observed in the Radotín tunnel section (Fig. 3).
5. Discussion
Upper Aeronian successions in the Prague Synform and the eight other regions discussed above show a substantial decrease of graptolite diversity and rapid demise of many prolific middle Aeronian taxa. Loydell (Reference Loydell2007), considering his data from Wales (Loydell, Reference Loydell1993) and Dob's Linn of Scotland (Loydell, Reference Loydell1991b ), supposed that graptolite diversity dropped to minimum values in the sedgwickii Zone, whereas Melchin et al. (Reference Melchin, Koren’, Štorch, Landing and Johnson1998) draw the minimum diversity values in their upper sedgwickii Zone, which actually correlates with the halli Zone of Loydell (Reference Loydell1991a ) and rastrum Zone of this paper. Our data were sampled with higher stratigraphic resolution and suggest that faunal change started near to top of the convolutus Zone. The diversity decline culminated during the upper sedgwickii Zone (s.s.) and low diversity was maintained into the rastrum Zone. It must be considered that graptolite-bearing black shale referred to the rastrum Zone in the Prague Synform is the equivalent of a minor, presumably lower and/or middle part of the halli Zone of Wales. The post-extinction recovery interval and incipient adaptive radiation after the sedgwickii Event are hidden in the Bohemian succession within the graptolite-barren calcareous mudstone. This is probably why the increasing graptolite diversity reported by Loydell (Reference Loydell1991b , Reference Loydell2007) from the halli Zone has not been found in the Prague Synform.
Published data from the Welsh halli Zone (Loydell, Reference Loydell1991a , Reference Loydell1992, Reference Loydell1993) suggest that the species present, their relative abundances and evenness of graptolite communities show considerable similarities to the Bohemian succession. Up to 80% of graptolite rhabdosomes from bulk samples of the uppermost sedgwickii and lower rastrum zones in Radotín tunnel section belong to Metacl. undulatus. The same species predominated, although less dramatically, in Wales (32.8% in the halli Zone; Loydell, Reference Loydell1993, p. 151). Stimulograptus sedgwickii and/or St. halli are also abundant in both areas. On the other hand, abundance peaks of Nl. tenuis, recorded by Pannell, Clarkson & Zalasiewicz (Reference Pannell, Clarkson and Zalasiewicz2006) in the sedgwickii Zone at Dob's Linn, are absent in the Prague Synform.
Within the Prague Synform specific relative abundances of graptolite taxa have been observed in mixed graptolite-shelly fauna of a rather shallow and oxygenated lime-rich succession of the lower sedgwickii Zone at Hýskov (Štorch, Reference Štorch2001). There, Ceph. extrema is absent whereas Pet. clandestinus and Nl. tenuis are equally as common as T. magnificus and N. scalaris. The role of water depth, oxygenation and food resources in composition of the graptolite fauna remains obscure until depositional settings are well defined in further, stratigraphically and palaeogeographically relevant sections.
The graptolite crisis named the sedgwickii Event coincides with sedimentary signatures of changing environment. Anoxia weakened after the end of the convolutus Zone alongside an increasing rate of graptolite species extinction. TOC values decreased along with substantial increase of early diagenetic pyrite preserved in planar bioturbation fillings, concretions and lenticular beds. A high content of early diagenetic pyrite coupled with lower TOC is consistent with temporarily disrupted anoxia according to the model advocated by Schieber (Reference Schieber2003). Graptolite abundance dropped to a minimum but species richness decreased less dramatically owing to rapid replacement of the extinct species.
The middle part of the sedgwickii Zone is characterized by dominant silty material and intensive bioturbation. The difference between the graptolite fauna below and above the siltstone bed suggests the possibility of a distinct break in sedimentation associated with this level even though no unconformity has been detected and both the base and top of the siltstone are gradual rather than sharp. This level presumably represents the minimum sea-level and a sequence boundary recognized by Štorch (Reference Štorch2006). It is also correlatable with an apparent gap in sedimentation recorded in several sections in the Prague Synform (Fig. 3 and Štorch, Reference Štorch2006). The middle–late Aeronian gap in sedimentation encountered by Kaljo & Maartma (Reference Kaljo and Martma2000) and Kaljo et al. (Reference Kaljo, Martma, Männik and Viira2003) in platform sections of Estonia may have been associated with the same eustatic event, although high-resolution δ13Corg and graptolite data are missing. The graptolite fauna from the black shale subjacent to the siltstone matches the assemblage recorded in the shallow-water, carbonate-dominated succession of the sedgwickii Zone of the Hýskov section (Fig. 2). Štorch (Reference Štorch2001) explained those shallow-water limestones and volcaniclastic rocks in an otherwise black shale dominated succession of the Prague Synform as resulting from rapid growth of a local submarine volcano simultaneously with late Aeronian sea-level drawdown.
Black shales above the siltstone bed are still silty and coarsely micaceous. Graptolites became abundant, although species richness has decreased to a minimum and evenness values are also strongly decreased. TOC reached particularly high values (6–7%) above the siltstone (Fig. 3). Low species richness and low evenness of the assemblage combined with high TOC may be attributed to unfavourable conditions established by extensive algal/bacterial blooming. The organic fraction is dominated, apart from fragments of graptolite rhabdosomes, by amorphous organic matter of presumably algal origin. It is possible that algal blooms have been triggered by increased input of nutrients from windblown dust during a dry period of the glacial cycle envisaged by Melchin & Holmden (Reference Melchin and Holmden2006) and Loydell (Reference Loydell2007). A similar proliferation of amorphous organic matter was recorded above the lundgreni Event by Lenz et al. (Reference Lenz, Noble, Masiak, Poulson and Kozlowska2006) at Cornwallis Island, Arctic Canada. There is no apparent relationship between increased carbon burial and isotope composition in the Radotín tunnel section. There is no statistically significant relationship between δ13C org and TOC values. Considerably high TOC values in black silty shale of the upper sedgwickii Zone clearly post-dated the major mid-sedgwickii Zone δ13C org excursion (Fig. 3). Low TOC in mottled siltstone with the maximum positive shift of δ13C org is likely to reflect early oxidation of the organic matter.
High TOC values combined with low species richness and low evenness of the graptolite community in the upper sedgwickii Zone are consistent with substantial change in both absolute and probably also relative abundance of different primary producers. Different groups of palynomorphs and macroalgae may have different carbon isotope compositions and thus bearing on δ13C values (Lécuyer & Paris, Reference Lécuyer and Paris1997).
Carbon isotope curves based on data from Dob's Linn and Cape Manning (Cornwallis Island, Arctic Canada) exhibit a prominent positive excursion in δ13Corg and δ13Ccarb in the sedgwickii Zone (Melchin & Holmden, Reference Melchin and Holmden2006). The carbon isotope record seems to be more complex, however, than that outlined by Melchin and Holmden. In the Radotín tunnel section, the first positive shift in δ13Corg appeared in the convolutes–sedgwickii zone boundary beds, but for technical reasons only one sample could be taken from this interval (Fig. 3). A major positive shift in δ13Corg values (c. 7 ‰) followed in the middle part of the sedgwickii Zone and largely coincided with a silty incursion. Then δ13C org remained above baseline value through the upper sedgwickii Zone and after another minor positive shift dropped down to more negative values in the course of the rastrum Zone. Recent sampling of the far less condensed succession in NE Nova Scotia, Canada has shown a similar δ13Corg pattern. Three positive shifts of δ13Corg noted by Melchin et al. (Reference Melchin, MacRae, Frýda, Štorch, Bullock and Loydell2011) were dated by graptolites to the sedgwickii and presumably also halli (or rastrum) zones. The middle peak of this three-fold δ13Corg excursion is the most prominent one and the peaks are associated with phases of shallowing (Melchin et al. Reference Melchin, MacRae, Frýda, Štorch, Bullock and Loydell2011). Despite somewhat different palaeogeographical settings, the Prague Synform and Nova Scotia exhibit roughly corresponding stratigraphy and a similar three-fold pattern to the late Aeronian positive δ13Corg excursion.
The late Aeronian sedimentary record preserved in the Prague Synform matches the conception of glacio-eustatically driven changes in Silurian sea-level advocated by Loydell (Reference Loydell1998, Reference Loydell2007). The positive δ13C org excursion is correlative with changes in sedimentation, widespread unconformities and glacial deposits in South America recognized by Grahn & Caputo (Reference Grahn and Caputo1992), Díaz-Martínez & Grahn (Reference Díaz-Martínez and Grahn2007) and Díaz-Martínez et al. (Reference Díaz-Martínez, Vavrdová, Isaacson, Grahn, Gutiérrez-Marco, Rábano and García-Bellido2011).
A basin-wide interval of graptolite-barren calcareous mudstone with an organic carbon content of less than 0.1% separates the low-diversity uppermost Aeronian fauna of the rastrum Zone from the considerably richer assemblage of the Spirograptus guerichi Zone in the next black shale. Not only the post-extinction recovery interval, but also the Aeronian/Telychian boundary lie hidden with the graptolite-barren sedimentary rock. Although the GSSP is formally placed between the last appearance datum (LAD) of the brachiopod Eocoelia intermedia and FAD of E. curtisi, in practice the guerichi graptolite Zone is commonly used as the base of the Telychian (e.g. Zalasiewicz et al. Reference Zalasiewicz, Taylor, Rushton, Loydell, Rickards and Williams2009; Loydell, Reference Loydell2012).
6. Conclusions
(1) The upper Aeronian Stimulograptus sedgwickii Biozone of Bouček (Reference Bouček1953) and Štorch (Reference Štorch1994, Reference Štorch2006) is informally subdivided into lower and upper parts. A new Lituigraptus rastrum Biozone is recognized in the upper part of the former sedgwickii Zone. The new biozone correlates, in part, with the Stimulograptus halli Zone of Loydell (Reference Loydell1991a ) and Zalasiewicz et al. (Reference Zalasiewicz, Taylor, Rushton, Loydell, Rickards and Williams2009). Along with significant refinement of the late Aeronian biostratigraphy, the Radotín tunnel section gave access to a critical interval, which is either oxic and graptolite-barren or absent owing to nondeposition in most other Czech and foreign sections.
(2) Good graptolite control has enabled biostratigraphical correlation and dating of the late Aeronian sedgwickii Event of Melchin, Koren’ & Štorch (Reference Štorch1998), including coeval sedimentary changes and carbon isotope signatures, with a particularly high resolution, previously unavailable at this level. A detailed, quantitative study of graptolite faunal dynamics, coupled with the sedimentary record and δ13Corg data, has provided evidence for the complex and step-wise nature of the late Aeronian graptolite crisis, which extended through the whole sedgwickii Zone and well into the rastrum Zone. Our results suggest that the rapid increase in the extinction rate of graptolite species started near the end of the convolutus Zone and initially was partly balanced by an increased rate of graptolite speciation and/or immigration. This phase coincides with an initial positive shift in δ13Corg values and weakened anoxia in the early sedgwickii Zone. Incursion of silty material and a strong δ13Corg positive excursion suggest that major sea-level drawdown coincided with the middle part of the sedgwickii Zone. Environmental perturbations mirrored by the biotic crisis, however, progressed further into the late sedgwickii and rastrum zones as indicated by minimum species richness and decreasing evenness of the graptolite community. A major drop in evenness post-dated falling species richness but correlates with an abrupt increase in TOC values. A new, although minor, positive shift in δ13Corg values appeared as high as in the lowermost part of the rastrum Zone. The resulting carbon isotope curve resembles the multiple, three-fold pattern of the late Aeronian excursion recorded in Nova Scotia by Melchin et al. (Reference Melchin, MacRae, Frýda, Štorch, Bullock and Loydell2011).
(3) The combined biotic, sedimentary, carbon isotope and TOC record of the upper Aeronian succession of the Prague Synform indicates multiple sea-level drawdowns, culminating late in the middle sedgwickii Zone, which is consistent with the glacio-eustatic models proposed by Caputo (Reference Caputo, Landing and Johnson1998), Loydell (Reference Loydell1998) and Melchin & Holmden (Reference Melchin and Holmden2006).
(4) Mature, ecologically well-balanced, high-diversity graptolite assemblages reappear in the lowermost Telychian linnaei (or guerichi) Zone in the Prague Synform. The recovery interval is obscured, along with the Aeronian/Telychian boundary itself, in the graptolite-barren oxic mudstone that separates the black shales of the rastrum and linnaei zones.
Acknowledgements
We are very grateful to D. K. Loydell who improved the English and, together with J. Zalasiewicz, provided valuable comments on the manuscript. J. Rajlichová and Š. Manda drafted figures 1–3 and 6. This work is a contribution of research plan AV0Z30130516 carried out by Institute of Geology AS CR, v.v.i. Funding was provided by the Czech Science Foundation (GA CR) through Grant No. 205/09/0619.
