Sea-level changes and geochemical anomalies across the Cenomanian–Turonian boundary: Pecı́nov quarry, Bohemia

Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 265–285

Sea-level changes and geochemical anomalies across the Cenomanian–Turonian boundary: Pecı´nov quarry, Bohemia ˇ ech b, David Ulicˇny´ a,*, Jana Hladı´kova´ b, Moses J. Attrep Jr. c, Stanislav C Lenka Hradecka´ b, Marcela Svobodova´ d a Department of Geology, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic b Czech Geological Survey, Kla´rov 131/3, 118 21 Prague 1, Czech Republic c Los Alamos National Laboratory, Chemical Science and Technology, MS J 514, Los Alamos, NM 87545, USA d Geological Institute, Czech Academy of Sciences, Rozvojova´ 135, 165 00 Praha 6, Czech Republic Received 9 February 1995; accepted 19 March 1997

Abstract Relationships between geochemical anomalies, sea-level change and other events were studied in the Cenomanian–Turonian boundary interval in the Pecı´nov quarry in southwestern part of the Bohemian Cretaceous Basin (Czech Republic). A major 3rd-order sea-level rise at the base of the late Cenomanian M. geslinianum Zone was followed by deposition of organic-enriched mudstones in a succession of parasequences, deposited in response to high-frequency (4th-order) sea-level fluctuations and recording a stepwise decrease in bottom oxygenation towards intensely dysaerobic conditions. A complex d13C excursion occurs in total organic matter of the late Cenomanian deposits. A sequence boundary of latest Cenomanian age in the Pecı´nov section is correlated to a global sea-level fall during the N. juddii Zone. A renewed sea-level rise occurred during the early Turonian W. coloradoense Zone and reached maximum flooding during the M. nodosoides Zone. During the early Turonian, bottom waters were generally aerobic, and the positive d13C excursion waned early in the W. coloradoense Zone. The magnitude of the d13C excursion, more than 4‰, is approximately the same as in North America and Northern Africa, confirming that it was controlled by a global paleoceanographic mechanism. The absence of anoxia and a d13C anomaly during the peak flooding of early Turonian age suggests that widespread deposition of organic-enriched deposits, as well as the positive shift in d13C generally did not depend on the absolute elevation of sea level but, rather, on the area of newly flooded land during transgression. Abrupt, small-scale shifts in d13C towards higher values coincide with the flooding surfaces of parasequences and may reflect either regional changes in the proportion of marine and terrestrial organic matter or rapid global changes in isotopic composition of marine organic matter related to high-frequency sea-level changes. Abundances of Ir, Sc, Cr, V and other elements previously reported as forming anomalous concentrations in the boundary interval showed no enrichment in the Bohemian section. This is most probably due to the large distance of the depositional site from the presumed volcanic source of element-enriched deep waters in the proto-Caribbean region. Anomalous concentration of Mn at the base of the Turonian deposits was caused by diagenetic incorporation of Mn into siderite. © 1997 Elsevier Science B.V. Keywords: eustasy; Oceanic Anoxic Event; sequence stratigraphy; Cretaceous; stable isotopes Corresponding author. Tel.: +4202 21952157, fax: +4202 291425; e-mail: [email protected] 0031-0182/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 00 3 1 -0 1 8 2 ( 9 7 ) 0 0 0 55 - 2

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1. Introduction The Cenomanian–Turonian (C–T ) boundary interval is marked by the highest Phanerozoic sea level, a peak greenhouse climate, a famous oceanic anoxic event, and a number of geochemical anomalies, including a pronounced positive d13C excursion and anomalously high concentrations of many elements. The C–T boundary is associated with an extensive extinction of marine biota ( Kauffman, 1984; Elder, 1989). Most of the geochemical anomalies in the C–T deposits are thought to result from the interplay between sea-level change, marine productivity and anoxia (e.g. Arthur et al., 1987; Loutit et al., 1988; Pratt et al., 1991). The C–T boundary event became a model of an oceanic anoxic event caused by a sea-level rise (Arthur et al., 1987), and has also been applied to deposits of different age (e.g. the Frasnian–Famennian boundary, Joachimski and Buggisch, 1993). On the other hand, disagreements and contrasting interpretations concern virtually every aspect of the C–T boundary interval, including the sea-level history and the causes of anoxia and geochemical anomalies (cf. Jeans et al., 1991; Ulicˇny´, 1992a; Paul et al., 1994). In recent research the most important problems concerning the geochemical anomalies at the C–T boundary can be divided into the following categories: 1.1. Relationships between anoxia and sea-level change According to Arthur et al. (1987), a global sealevel rise close to the C–T boundary created new epicontinental seas on flooded cratons, leading to an abrupt increase in marine organic productivity. This process, coupled with increased oceanic stratification (due to rather equable climate and warm, saline bottom waters, sinking from tropical shelves), caused the expansion of the oxygen minimum zone (OMZ). The anoxic event and the related d13C anomaly were formerly assigned to the early Turonian ‘‘peak transgression’’ (Scholle and Arthur, 1980). More detailed biostratigraphic research showed that the anoxic event began after a sea-level rise during the late Cenomanian

Metoicoceras geslinianum ammonite Zone. The question remains why maximum anoxia as well as the peak d13C excursion were reached (and waned) before the early Turonian maximum flooding (cf. Pratt, 1985; Jarvis et al., 1988). Jeans et al. (1991) proposed that the anoxic conditions were caused by a major sea-level fall during the late Cenomanian. This idea was shared by Paul and Mitchell (1994) and Paul et al. (1994). Therefore, the validity of the ‘transgression model’ for the origin of anoxia and for the d13C anomaly still remains controversial. 1.2. Cause and timing of the d13C anomaly The global positive d13C excursion, recorded in both carbonate and organic matter (e.g. Scholle and Arthur, 1980; Hayes et al., 1989), is explained as a result of two coupled processes: preferential extraction of the lighter 12C isotope, due to increased marine plankton productivity in surface waters, and burial of unoxidized marine organic matter (rich in 12C ), due to widespread anoxic bottom conditions. Contrary to the most common interpretations, which employ increased productivity as the primary control of anoxia and d13C excursion, Paul and Mitchell (1994) proposed a marked reduction of primary productivity during the late Cenomanian. Jenkyns et al. (1994) and Mitchell and Paul (1994) suggested that the carbon isotope curves are generally a measure of the area of shelf seas available for organic carbon production and burial, and the rate of transgression. 1.3. The magnitude and structure of the d13C anomaly Marked variations found in the overall magnitude of the d13C excursion in different European sections led Hilbrecht et al. (1992) to propose that regional differences in upwelling intensity, causing differences in nutrient supply and primary productivity, were responsible for different levels of enrichment in 13C in the sedimentary record of different localities. In contrast, Gale et al. (1993) demonstrated that the carbon-isotopic profiles in the Western Interior and British Chalk are very similar to each other and the d13C peaks occur in

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consistent positions relative to biostratigraphic marker horizons in both regions. This supports a global rather than a local control on the detailed structure of the d13C excursion. The causes of the small-scale and apparently globally synchronous variations in d13C, superimposed on the overall late Cenomanian–early Turonian excursion (Gale et al., 1993), are not yet well understood. 1.4. Anomalous elemental concentrations Orth et al. (1988) reported two iridium peaks, associated with anomalous concentrations of Cr, Sc, Mn and other elements in the Upper Cenomanian. These anomalies coincided with the lower part of the d13C excursion and with steps in extinction of marine biota ( Elder, 1989). Orth et al. (1993) argued for an earthbound rather than extraterrestrial impact source for the elemental anomalies. Most probably the source was intensified submarine volcanism and/or increased circulation of deep, metal-rich waters associated with a sea-level rise and opening of new circulation pathways for deep oceanic waters. Another cause may have been a decrease in the rate of sedimentation of continental detritus due to the sea-level rise. Pratt et al. (1991) focused on the behaviour of manganese in the late Cenomanian and proposed that Mn, as a redox-sensitive element, was remobilized from oxidic marine sediments during the expansion of the oxygen minimum zone, and redistributed into shallow epicontinental deposits. The ultimate cause of the elemental anomalies in the C–T boundary interval still remains unknown. Finding solutions to the problems outlined above requires extending multi-disciplinary studies to other regions than those studied so far, and setting up a high-resolution inter-regional correlation network. The results of our study of a C–T boundary section in Bohemia are a contribution to this effort.

2. Methods In the Pecı´nov quarry near Nove´ Strasˇecı´, 40 km northwest of Prague ( Fig. 1), a type locality of the Pecı´nov Member ( Ulicˇny´, 1992b), a detailed sedi-

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mentological, geochemical, and paleontological study was focused on the C–T interval, spanning the ammonite zones Metoicoceras geslinianum, Neocardioceras juddii, Watinoceras coloradoense and the lower part of the Mammites nodosoides Zone. A sequence-stratigraphic interpretation of the C–T interval provided a basic framework for considerations of relationships between sea-level change, oxygen depletion, and geochemical events. Throughout this paper, the European scheme of ammonite zones is used (cf. Kennedy and Cobban, 1991). Sequence-stratigraphic terminology follows that of Van Wagoner et al. (1990). The terminology of Rhoads and Morse (1971) is used to characterized paleo-oxygen levels as aerobic, dysaerobic, and anaerobic. The sedimentological description of the section was supplemented by a petrological study of thin sections. Percentages of clastic quartz >0.05 mm, glauconite and siderite were determined by point counting of 500 points per thin section. Carbonate and organic carbon contents were determined by Geoindustria Central Laboratories, Prague, by coulometric titration (C , C ) and total carb the LECO combustion-infrared instrumentation (S). The TOC content was calculated as a difference between total and carbonate carbon. The percentage of total organic carbon ( TOC ) and the C/S ratio were used as a measure of bottom oxygenation during sedimentation. Carbon isotope analyses were performed only on TOC, because of the paucity of carbonate over most of the section. Carbonates were removed from the samples by hot diluted phosphoric acid. After washing and drying, the residue was oxidized in flow of oxygen at 950°C, water formed during oxidation was frozen. Nitrogen oxides which formed were reduced by hot Cu to N . Isotope 2 measurements of pure carbon dioxide were performed on a Finnigan MAT-251 mass spectrometer in the Czech Geological Survey, Prague. The reproducibility of results was 0.1‰. A separate set of samples for common and trace element analyses by instrumental neutron activation analysis (INAA) was taken from the same location as the samples used in petrological and carbon isotope study but a closer spacing was used, especially across lithologic boundaries.

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Fig. 1. (a) The Bohemian Cretaceous Basin in the tectonic framework of the Bohemian Massif. Cenozoic sedimentary cover omitted. (b) Paleogeographic setting of the Bohemian Cretaceous Basin in Central Europe during the early Turonian peak flooding, simplified after Ziegler (1990).

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Abundances for 40 common and trace elements were determined by INAA using the procedure described by Minor et al. (1981) at the Los Alamos Omega West Reactor. Radiochemical methods used to determine iridium required 1–2 g samples be irradiated for 7 h in a flux of approximately E+13 n/sec/cm2. Following 1 week of radioactive decay, the samples were dissolved within strong mineral acid and iridium carrier, purified by ion exchange chromatography, precipitated and counted (see Attrep et al., 1991 for details). Supplementary microprobe analyses were performed on Camebax microprobe at the Faculty of Science, Charles University, on samples which showed anomalous Mn abundances. The study of bivalve and foraminiferal assemblages was focused on the record of sea-level changes and bottom oxygenation in benthic communities. Foraminifera were isolated from samples of 1 kg weight using usual methods of sieving through 0.02–0.01 mm sieves and silk. Foraminiferal abundance is given as number of specimens per sample, diversity was determined using the Simpson equation. Palynological data were used as a proxy for the ratio of terrestrial and marine components of TOC.

and the overlying Bı´la´ Hora Formation (early Turonian) by prominent bounding surfaces (Prazˇa´k, 1989; Ulicˇny´ et al., 1993). The stratigraphic nomenclature of this interval is discussed ˇ ech et al. (1980) and Ulicˇny´ (1992b); Ulicˇny´ by C et al. (1993) used the former informal name ‘‘siltstone facies of the Peruc–Korycany Formation’’ for the Pecı´nov Member. The omission surface between the Pecı´nov Member and the Bı´la´ Hora Formation is an expression of a hiatus, encompassing the N. juddii Zone, and perhaps part of the M. geslinianum Zone in the central part of the basin. Previous studies of the C–T boundary interval in Bohemia (Prazˇa´k, 1989; Ulicˇny´ et al., 1993) focused on numerous borehole cores recovered from the central parts of the basin. Studies of surface sections of the Pecı´nov Member are complicated by a lack of well exposed outcrops, because of easy weathering and slumping of the mudstones. The faces of the active Pecı´nov quarry, ˇ ech previously described by Svoboda (1985) and C and Knobloch (1989), provide fresh exposures of the C–T boundary succession and make the Pecı´nov section the best exposed C–T succession in Bohemia.

3. The C–T boundary in the Bohemian Cretaceous Basin

4. Sedimentology and sequence stratigraphy 4.1. The Pecı´nov Member

The Bohemian Cretaceous Basin, formed during the early Late Cretaceous, was a relatively shallow, narrow seaway connecting the Boreal and Tethyan realms across the topographic high of the Bohemian Massif (Fig. 1). Unlike most European epicontinental basins, characterized by pelagic carbonate-dominated deposition, the Bohemian seaway was characterized by mudstone, calcareous siltstone and marlstone facies during the Cenomanian–Turonian interval. The onset of the C–T boundary events over most of the basin is marked by a profound facies change at the base of the late Cenomanian M. geslinianum Zone. It coincides with the base of dark grey to black offshore mudstones of the Pecı´nov Member, separated from both the underlying Korycany Member (early late Cenomanian)

The Cenomanian–Turonian boundary succession at Pecı´nov begins at the base of the Pecı´nov Member, which rests on the underlying subtidal to intertidal deposits of an estuarine succession of the Korycany Member (Fig. 2). The mudstonedominated Pecı´nov Member is divided by a number of omission surfaces into units characterized by an upward increase in siliciclastic component (fine sand and coarse silt, Fig. 3). These units were interpreted as parasequences sensu Van Wagoner et al. (1990), and numbered in a consecutive order, which serves only descriptive purposes and has no formal stratigraphic meaning. The basal surface of the Pecı´nov Member, and of Parasequence 1, is a well pronounced flooding surface sensu Van Wagoner et al. (1990), separat-

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Fig. 2. Lithology, regional lithostratigraphy, sequence stratigraphy and macrofossil biostratigraphy of the Cenomanian–Turonian boundary interval at Pecı´nov. The interpretation of sequence stratigraphy is linked to fluctuations in paleo-oxygen levels in the Pecı´nov Cenomanian–Turonian boundary succession. The Pecı´nov Member is underlain by a highstand systems tract of the ZC 2.4 sequence (Calycoceras naviculare Zone). ?N. juddii=possible occurrence of Neocardioceras juddii Zone; W. colo=Watinoceras coloradoense Zone. Base of the Mammites Zone is uncertain: it may extend down to the middle part of unit 2 of the Bı´la´ Hora Formation. TST=transgressive systems tract; HST=highstand systems tract.

ing shallow-marine to supratidal regressive facies below from the offshore mudstones above it. Parasequence 1 is based by a structureless, gravelly sandstone, 0.3–0.5 m thick, which locally contains shell moulds and mudstone intraclasts. We interpret it as a transgressive lag. The sandstone, locally displaying wave ripples, is overlain by grey silty

mudstones, rich in macrofossils, which form the bulk of parasequence 1. Parasequence 2 has at its base a locally scoured and burrowed omission surface, overlain by 0.3–0.4 m thick, sandy, bioturbated glauconitic layer, rich in pyrite concretions. The bulk of parasequence 2 is a black mudstone which gets

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Fig. 3. Percentages of clastic quartz (coarse silt to sand ), glauconite, siderite and pyrite throughout the Cenomanian–Turonian boundary interval at Pecı´nov. SB/TS=sequence boundary coinciding with a transgressive surface; MFS=maximum flooding surface; PS=parasequence.

gradually sandier upward (Fig. 3). The high content of pyrite, present as concretions about 1–3 cm in diameter, at the top of the parasequence 2 ( Fig. 3) is probably a late diagenetic phenomenon. Parasequence 3 rests on a sharp, intensely bioturbated omission surface. The basal 0.2 m of parasequence 3 is relatively light-coloured compared to the black mudstones below. The fine sand content in the mudstones of parasequence 2 decreases abruptly to almost zero at the base of parasequence 3, indicating starvation in coarser clastics; also, dense bioturbation suggests a considerably slower depositional rate. Parasequence 3 gets sandier upward and becomes a sandy siltstone, moderately to densely bioturbated (Chondrites), with an increased amount of glauconite. The base of parasequence 4 is defined by an abruptly decreased siliciclastic component in the silt to fine sand fraction, and a decrease in the density of bioturbation. The mudstones are dark grey to black, and contain a very low amount of carbonate (around 1% CaCO ). The preserved 3 thickness of this parasequence varies between different places in the quarry because of a slight erosional relief at the base of the overlying Bı´la´ Hora Formation.

4.2. The Bı´la´ Hora Formation Because of the hemipelagic nature of the facies that make up this formation, parasequences could not be identified and the Bı´la´ Hora Formation is divided into three units, separated by omission surfaces ( Fig. 2). Unit 1, up to 0.6 m thick, is a bed of sandy, glauconitic siltstone, which contains abundant phosphorite clasts (mostly remains of phosphatized Thalassinoides burrows) at its base. Very small (0.03–0.1 mm) rhombs of siderite are abundant in the pore spaces and burrows. Unphosphatized bivalve fossils, mostly oysters, occur in the glauconitic bed, which is densely penetrated by Chondrites. Coalified and partly pyritized wood debris is abundant, as well as pyrite concretions. The top of unit 1 is marked by an abrupt lithologic change and locally colonized by inoceramid bivalves. Unit 2 is a light grey, calcareous siltstone showing an upward increase in carbonate. An indistinct omission surface, probably a minor flooding surface, overlain by a weak glauconite concentration, occurs approximately 1 m above the base of the unit 2. The contact between units 2 and 3 is a sharp

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surface, associated with intense bioturbation (Chondrites) at the base of unit 3. Fish teeth and scales are concentrated at this surface. Ammonites and mytiloids are relatively more common on this surface relative to the rest of the Bı´la´ Hora Formation. Unit 3 is the main body of the Bı´la´ Hora Formation. In the Pecı´nov quarry it is incompletely exposed and has a thickness of approximately 12 m. It is formed by yellowish marlstones, containing between 25% and 30% CaCO ( Fig. 4). 3 The exposures at Pecı´nov are characterized by alternating more and less weathered beds, due to more or less rhythmic variations in CaCO . 3 4.3. Interpretation of bottom oxygenation and sea level changes The low values of the C/S ratio ( TOC vs. pyrite sulphur) suggest that the deposition of the Pecı´nov Member took place generally in dysaerobic or anaerobic conditions (Figs. 4 and 5; cf. Raiswell and Berner, 1985). In the cross-plot of TOC against S ( Fig. 5), the Cenomanian samples (Pecı´nov Member) plot in the field of oxygendepleted sediments, and the regression line passes above the origin, showing relative enrichment in

Fig. 5. Total organic carbon ( TOC ) plotted vs. sulphur (S). Upper regression line is for Cenomanian samples (Pecı´nov Member), the lower line for Turonian samples (Bı´la´ Hora Formation). Basal Turonian denotes samples from Unit 1 of the Bı´la´ Hora Formation. See text for further explanation.

pyrite sulphur. The most common occurrence of pyrite in framboidal aggregates, as well as extensive burrowing by Chondrites, indicate oxygen-depleted

Fig. 4. Carbonate, total organic carbon and the C/S ratio across the Cenomanian–Turonian boundary, Pecı´nov. The CaCO equivalent 3 was calculated from weight percentage of carbonate C, corrected for percentage of siderite in Unit 3. Note logarithmic scale for the C/S ratio curve.

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depositional conditions during sedimentation of the Pecı´nov Member (Raiswell and Berner, 1985; Ekdale, 1985). The parasequences 1–3 are arranged in a retrogradational pattern. Because of the minimum content of sand and silt at the base of parasequence 3 we tentatively place the maximum flooding surface at the base of parasequence 3 ( Fig. 2) and interpret parasequences 1 and 2 as a transgressive systems tract. Also, the rapid fluctuation in d13C (see Fig. 7) in a very short stratigraphic interval above its base suggests stratigraphic condensation; however, the assumption that parasequences 3 and 4 are parts of a highstand systems tract remains hypothetical because it cannot be confirmed by down-dip correlation. Units 1 and 2 of the Bı´la´ Hora Formation are interpreted as a transgressive systems tract, overlying a sequence boundary which coincides with a transgressive surface ( Fig. 2). This interpretation of the basal surface of the Bı´la´ Hora Formation is supported by the influx of coarser clastics recorded in unit 1, the presence of shallow-water fauna (oysters), and an erosional relief, documented from a nearby locality by Valecˇka and Skocˇek (1991). From the eastern part of the basin, Ulicˇny´ (1992b) reported intertidal lowstand deposits immediately beneath the Bı´la´ Hora Formation, documenting a forced regression of latest Cenomanian age. No lowstand deposits were preserved at Pecı´nov, most probably due to reworking by subsequent transgression (cf. Van Wagoner et al., 1990). We correlate the sequence boundary at the top of the Pecı´nov Member with the late Cenomanian sealevel fall during the N. juddii Zone. This sea-level fall is now documented from a number of locations, including the Western Interior (Leithold, 1994); the Anglo–Paris Basin (Juignet and Breton, 1992; Hancock, 1993); southeast France (Malartre and Ferry, 1993), and Israel (Bogoch et al., 1994). Juignet and Breton (1992) included this sea-level fall into the Haq et al. (1988) coastal onlap curve as the division between the ZC 2.5a and 2.5b 3rdorder cycles (Fig. 2). The base of the hemipelagic rhythmic succession of unit 3 is a clear maximum flooding surface formed during a period of marked starvation in

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clastic input. Unit 3 contains around 0.1% TOC; the C/S ratio increases from the base of the Bı´la´ Hora Formation upward ( Fig. 4); in addition, paleontologic evidence (see below) indicates aerobic bottom conditions.

5. Paleoenvironmental changes recorded in macrofauna and microfauna 5.1. Molluscan assemblages A rich and diverse molluscan assemblage occurs in situ in the parasequence 1 of the Pecı´nov Member. Epifaunal suspension feeders (Pseudoptera) and shallow-burrowing suspension feeders (Cucullea, Modiolus, Liopistha and Protocardia) dominate the bivalve community, whereas infaunal deposit-feeders are less abundant (Nuculana). Members of this association are known from brachyhaline as well as normal marine shoreface deposits (Scott, 1974; Fu¨rsich, 1994). The occurrence of this association together with deep-burrowing bivalves (Panopea and Tellina) and nectobenthic (Metoicoceras and Calycoceras) and pelagic ammonites (Placenticeras), suggest a relatively shallow offshore environment, characterized by aerobic or only slightly dysaerobic conditions. The overlying parasequence 2 is characterized by a drop in diversity of the molluscan assemblage and a decrease in abundance of both deep- and shallow-burrowing bivalves. This suggests deeper bathymetric conditions of parasequence 2 and oxygen-restricted bottom conditions. However, the presence of shallow-burrowing bivalves and nectobenthic ammonites (Metoicoceras) throughout parasequence 2 shows that the bottom conditions were not fully anaerobic. Macrofossils are almost missing in parasequences 3 and 4, which we interpret to be the consequence of anoxic bottom conditions. The absence of macrofossils in these parasequences is probably not due to diagenetic destruction of macrofossils in the mudstones, because diagenetic dissolution of carbonate occurred to some extent throughout the whole Pecı´nov Member. Moreover, a slight increase in carbonate content occurs in parasequence 4. Also, the softness of the substrate,

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which may influence the character of the benthic community ( Wignall, 1993), shows no significant changes throughout the succession. In general, the changes in molluscan communities of the Pecı´nov Member document a stepwise deepening of the depositional environment, coupled with a stepwise decrease in bottom oxygenation ( Fig. 2). Intensely dysaerobic to anaerobic bottom conditions probably existed during the deposition of parasequences 3 and 4. Unit 1 of the Bı´la´ Hora Formation is characterized by colonization by benthic fauna, especially abundant oysters, indicating a return of oxygenated bottom conditions, which persisted throughout the Turonian part of the section, in spite of the rising sea level. Mammitid ammonites, indicating deep-water conditions (Batt, 1993), and eurytopic cosmopolitic inoceramid bivalves (Mytiloides mytiloides) document maximum deepening at the base of unit 3. This surface is also characterized by the relatively most abundant occurrence of ammonites.

The lithology of the Bı´la´ Hora Formation was much more favorable for the preservation of intact foraminiferal assemblages. The minimum in abundance, number of species and planktonic foram diversity, occurring in the basal glauconitic bed (unit 1), is interpreted to be the consequence of prolonged sediment reworking and probably also diagenesis ( Fig. 6). The overlying deposits show an increase in foraminifera abundance, number of species, and planktonic diversity (Fig. 6), as well as in the percentage of calcareous benthic taxa (not shown). The base of the unit 3, yellow marls and limestones, is marked by a pronounced peak in foram abundance, number of species and planktonic diversity. This is evidence of a maximum flooding, accompanied by starvation in clastic input (Fig. 2). In general, the changes in the assemblages of foraminifera in the Bı´la´ Hora Formation provide evidence for oxygenated bottom conditions, accompanying the sea-level rise and maximum flooding during the early Turonian.

5.2. Foraminifera The interpretation of foraminiferal assemblages of the Pecı´nov Member is complicated by diagenetic dissolution of calcareous tests in the mudstones. In particular, the scarce occurrence of planktonic foraminifera is ascribed to selective destruction of the microfossils during diagenesis, and has no paleoenvironmental implications. The Pecı´nov Member assemblage is dominated by agglutinated foraminifera. Calcareous benthic species make up 10–20% of the assemblage. Throughout the section, the agglutinated foraminifera include low-oxygen tolerant taxa as lituolids (Ammobaculites), trochamminids, and textulariids (cf. Koutsoukos et al., 1990). However, for the assessment of changes in bottom oxygenation between parasequences 1 and 3, the foraminiferal assemblages do not allow as good a degree of resolution as the molluscs (see above). The marked drop in foraminiferal abundance, number of species, and benthic species diversity at the base of parasequence 4 was probably caused by a decrease in bottom oxygenation that accompanied the pulse in sea-level rise ( Fig. 6 and 7).

6. Carbon isotope record The record of d13C in TOC shows a pronounced positive excursion, spanning the whole thickness of the Pecı´nov Member. The beginning of the isotope anomaly is marked by a gradual increase in d13C above the base of the Pecı´nov Member, followed by a stepwise rise and maximum values of −21.8‰ at the base of parasequence 3. The anomaly wanes upward, with a minor peak corresponding to the base of parasequence 4. The base of the Bı´la´ Hora Formation is characterized by an abrupt drop in d13C values, followed by a gradual decline to normal Turonian values between −27 and −26‰. The overall magnitude of the anomaly is approximately 4.5‰, similar to the magnitude of carbon isotope excursions measured in organic matter elsewhere (4.2‰, Tarfaya Basin, Morocco, Thurow et al., 1988; 4.1–4.2‰, Western Interior, U.S., Pratt, 1985; Hayes et al., 1989). The d13C curve from Pecı´nov is characterized by its detailed structure, showing a number of

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Fig. 6. Changes in foraminiferal assemblages in the Cenomanian–Turonian boundary interval at Pecı´nov.

Fig. 7. Curve of d13C of total organic matter ( TOC ) plotted against the C–T boundary section at Pecı´nov. On the right, the same curve is divided into increments corresponding to particular sequence-stratigraphic units. fs=flooding surface; arrows pointing to the right indicate a sea-level rise on a particular bounding surface, those pointing to the left indicate a sea-level fall. The flooding surface below the MFS in the Bı´la´ Hora Formation is a minor flooding surface occurring within Unit 2.

lower-order fluctuations superimposed on the overall excursion. Abrupt shifts of more than 1‰ towards higher d13C values occur immediately above the parasequence boundaries within the Pecı´nov Member ( Fig. 7). The marked omission surface (a sequence boundary coinciding with a

transgressive surface) at the base of the Bı´la´ Hora Formation corresponds to an abrupt decline in d13C and is followed by final waning of the isotope excursion. The omission surfaces within the Bı´la´ Hora Formation (within unit 2 and at the base of unit 3) are associated with a minor d13C peak.

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7. Abundances of selected elements In the Pecı´nov C–T section we focused on concentrations of elements which have been reported by earlier studies as showing anomalous abundances in the C–T interval (Orth et al., 1988; Orth et al., 1993): Ir, Sc, Cr, V, Ti, Mn, Co, Ni, and Au. Departures from background values of other elements found within the section were also studied. Iridium values, ranging from 0.01 to 0.07 ppb, show a slight tendency to decrease upward (Fig. 8). The lower values above the base of the Bı´la´ Hora Formation are interpreted as being due to change in lithology, from mudstones below to marls above. In general, the values of Ir abundance are lower than the background values from sections where Ir anomalies occur (cf. Orth et al., 1993). No significant departures from the background values occur throughout the section. In addition, the ratio of Ir to Al (representing clay content), which reduces the effect of carbonate content variations in the section, shows no marked variation throughout the section, with the exception of one sample just above the base of the Bı´la´ Hora Formation. This sample is a phosphorite pebble, composed almost exclusively of francolite, and therefore has a low Al content. Other elements that were found anomalously concentrated together with Ir by Orth et al. (Orth et al., 1988; Orth et al., 1993), also show no significant excursions in the Pecı´nov section. Abundances of Cr, Sc and V in the section are shown in Fig. 8. Concentrations of some rare earth elements in the sample of phosphorite are elevated compared the values in the marlstones or mudstones. Dobesˇ et al. (1987) reported similar enrichments in REEs in Turonian phosphorites of the Bohemian Cretaceous. This enrichment of phosphorite in REEs is explained by the incorporation the REEs into the structure of apatite. The vertical profile of U abundance ( Fig. 8) shows high concentration of U (35 ppm) in the phosphorite pebble. This corresponds to usually high concentrations of U in Bohemian Cretaceous phosphorites, interpreted by Dobesˇ et al. (1987) to be a consequence of diagenetic remobilization of U from the sediment

and its incorporation into phosphorite as a carbonate complex. Manganese shows a pronounced anomaly (up to 1700 ppm) just above the C–T boundary, in the basal glauconitic bed of the Bı´la´ Hora Formation ( Fig. 9). This peak is also clear in the plot of Mn/Ca ratio, which eliminates the influence of the overall increase in Mn caused by the onset of carbonate sedimentation. However, this anomaly occurs in early Turonian deposits dated to the late Watinoceras Zone, coinciding with the end of or slightly post-dating the carbon isotope excursion. Therefore, it does not correlate stratigraphically to the Mn anomalies reported by Pomerol (1983), Pratt et al. (1991) and Orth et al. (1993), which coincide with elevated concentrations of other trace elements and occurred approximately 0.5 Ma earlier, during the late Cenomanian M. geslinianum/S. gracile and N. juddii Zones. In the Pecı´nov C–T section, the Mn peak is clearly associated with specific mineralogy of unit 1 of the Bı´la´ Hora Formation. The pore spaces of this glauconitic bed are filled with small (0.05–0.1 mm) siderite rhombs, especially abundant in Chondrites burrows. The maximum content of siderite in some parts of the greensand was 30%. The Mn contents in these siderite rhombs range between 0.43% and 0.63%, with an average of 0.5%. Siderite most probably formed during early diagenesis of the glauconitic bed, after its homogenization by intense bioturbation and subsequent drowning. Factors that favoured siderite formation were: an anoxic environment below the sediment–water interface; low pH, resulting from organic matter decay; and abundant glauconite as the source of Fe. The Fe contents (Fig. 9) clearly correlate with the percentage of glauconite and siderite, showing a higher value (14% Fe) at the base of the Bı´la´ Hora Formation than in the glauconitic bed at the base of parasequence 2 in the Pecı´nov Member (approximately 6% Fe). Manganese, as a redox-sensitive element, was remobilized from surrounding sediment under anoxic conditions, which make it highly mobile, and incorporated in siderite, which formed by replacement of the micritic matrix. Because of the mineralogic control and stratigraphic position of the Mn anomaly in the Pecı´nov

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Fig. 8. Abundances of selected elements plotted against the C–T boundary section at Pecı´nov. See text for further explanation.

section we interpret it as a local diagenetic phenomenon.

8. Discussion 8.1. Sea-level change and bottom oxygenation Along with the stepwise increase in depth, represented by the parasequences of the Pecı´nov Member, the living conditions of benthic communities deteriorated as the paleo-oxygen level decreased. The correlation of a stepwise decrease in bottom oxygenation with a stepwise sea-level

rise, documented in the Pecı´nov Member, confirms that the late Cenomanian anoxic event was driven by a sea-level rise, not a fall in sea level, as proposed by Jeans et al. (1991) and Paul et al. (1994). The apparent steps in oxygen depletion may also be an effect of hiatuses at parasequence boundaries, which may seem to ‘punctuate’ a more gradual process of oxygen minimum zone expansion (cf. discussion in Hart and Leary, 1991). The peak flooding in the early Turonian was not associated with anaerobic conditions at Pecı´nov, in contrast to the preceding late Cenomanian transgression, which did not reach such a high sea level and led to marked oxygen

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Fig. 9. Abundances of Mn, Fe, and Mn/Ca and Mn/Fe ratios plotted against the C–T boundary section at Pecı´nov. See text for further explanation.

depletion. The relative changes in water depth, indicated by facies changes across the late Cenomanian and early Turonian transgressive sur-

faces at Pecı´nov, suggest that the global increase in area of epicontinental seas during the late Cenomanian M. geslinianum transgression was

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definitely much larger than during the early Turonian (cf. similar data of Leithold, 1994; Elder et al., 1994). We agree with Jenkyns et al. (1994) that the solution to this problem could be that the development of anoxia is not related to the absolute elevation of sea level but, rather, to the increase in area of newly flooded land relative to the area flooded during the preceding sea-level highstand. 8.2. Cause and timing of the d13C anomaly The stepwise rise in d13C values in the Pecı´nov Member correlates with the sea-level rise that began during the M. geslinianum Zone. The highest recorded values correlate with a presumed maximum flooding interval at the base of parasequence 3. Tentative correlation to the d13C profile from Dover suggests that the base of parasequence 3 might correlate to the d13C peak of the Plenus Marls bed 8 and the base of parasequence 4 to the high values of the N. juddii Zone (A. Gale, pers. commun.). This correlation, however, is speculative because the stratigraphic extent of the hiatus at the base of the Bı´la´ Hora Formation is not exactly known. We interpret the abrupt drop in d13C at the base of the Bı´la´ Hora Formation as a consequence of the eustatic sea-level fall in the N. juddii Zone, which caused recycling of previously deposited organic carbon back into the ocean reservoir, and therefore led to the waning of the d13C excursion (cf. Arthur et al., 1987; Ulicˇny´, 1992a). Because of the good correlation of the d13C drop with the sequence boundary, we favour this explanation rather than that of Weissert and Lini (1994), who suggested a significant role for carbonate production, reinvigorated during a sea-level highstand, in the d13C decrease. The absence of a pronounced d13C anomaly during the peak early Turonian flooding (cf. Ulicˇny´, 1992a; Jenkyns et al., 1994) probably has the same cause as the absence of anoxia, discussed above; that is, a small relative increase in the flooded area. The data from the Pecı´nov C–T section do not allow speculation about the relative importance of productivity or preservation of organic matter in

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triggering the d13C excursion. However, our data allow us to reaffirm that both the oxygen depletion and the d13C excursion were caused by the late Cenomanian sea-level rise, as also suggested by data from many other regions. We see no evidence for a sea-level fall being the cause of oxygen depletion and the positive d13C excursion, as proposed by Jeans et al. (1991) and Paul et al. (1994). In agreement with Jenkyns et al. (1994) and Mitchell and Paul (1994), we believe that the positive and negative d13C excursions reflect positive and negative fluctuations in flooded area and, perhaps, in the rate of transgression. Furthermore, the fine-scale structure of the d13C anomaly at Pecı´nov suggests that such a relationship also exists on a scale of high-frequency (4th-order) sea-level fluctuations (see below). 8.3. Magnitude and structure of the d13C anomaly The similarity in magnitudes of the d13C org excursions in the Bohemian Cretaceous and with those in Africa and North America is firm evidence that the magnitude of the d13C anomaly was generally controlled by a global paleoceanographic mechanism, not by regional differences in paleoproductivity, as proposed by Hilbrecht et al. (1992). The different magnitudes of the d13C shift in the bulk carbonate data used by Hilbrecht et al. (1992) result most probably from diagenetic overprint, inadequate sampling intervals, hiatuses or condensed sections, or a combination of some of these factors. The critical role of diagenesis and incompleteness of the sedimentary record in preservation of the d13C signal was illustrated by Ulicˇny´ et al. (1993). An important phenomenon is the coincidence of abrupt positive shifts in d13C and parasequence boundaries. This may be explained either by: (1) local or regional fluctuations in input of terrigenous organic matter; or (2) by high-frequency (tens to hundreds of thousand years) changes in isotopic composition of marine organic matter. (1) The d13C of TOC in fluvial and marsh mudrocks of the Bohemian Cenomanian varies between −25‰ and −24‰ (J. Hladı´kova´, unpubl. data). In a simple shallowing-upward succession, such as a para-

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sequence, an increase in terrestrial input of organic matter with time might result in a gradual shift towards the more ‘terrestrial’ values up-section. Fig. 10 shows that, in a parasequence beginning before the onset of the d13C anomaly, the upward gradient of d13C would be from normal marine values of −26‰ towards higher values of terrestrial TOC. In contrast, a parasequence which formed during the main phase of the d13C excursion should show an upward decrease from anomalously high marine values around −23 or −22‰ towards relatively lower ter-

restrial d13C values. This might correspond to the d13C profiles of parasequences 1–4 of the Pecı´nov Member. Unfortunately, because of generally low contents of TOC in the Pecı´nov section, the Rock-Eval pyrolysis gave no results usable for interpretation of changes in composition of TOC. The results of palynological analysis are inconclusive as well: a marked upward increase in the percentage of terrestrial palynomorphs shows a clear pattern only in parasequence 2 ( Fig. 11). This, however, correlates to no vertical gradient in d13C. The results of palynologic study of other para-

Fig. 10. Illustration of one possible interpretation of the small-scale sea-level fluctuations in d13C at Pecı´nov by fluctuating regional input of marine and terrestrial organic matter. Parasequence A began by a flooding at a time of normal d13C values in marine organic matter, whereas parasequence B started accumulating when the marine TOC had an anomalously high d13C composition. The increase in the terrestrial component of TOC would result in a shift towards ‘terrestrial’ values (between −25‰ and −24‰) in each case. Compare with parasequences 1 and 3 in Fig. 8. However, in the Bı´la´ Hora Formation, such shifts also occur on flooding surfaces but outside the range of terrestrial values.

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sequences show that fluctuations in the ratio of terrestrial to marine elements are commonly greater within a parasequence than at parasequence boundaries (Fig. 10). So far, any significant change in the proportion of marine compared to terrestrial organic matter has been neither proven nor excluded. A detailed geochemical study of the composition of organic matter at Pecı´nov will be undertaken to solve this question. (2) The alternative interpretation is that the highfrequency shifts in d13C reflect changes in the isotopic composition of marine organic matter. This would imply a global paleoceanographic mechanism causing changes in fractionation (e ) between CO and primary p 2 organic matter, most likely fluctuations of pCO in the atmosphere (cf. Arthur et al., 2

Fig. 11. The percentage of terrestrial palynomorphs (pollen and spores) in the total palynospectrum across the Cenomanian–Turonian boundary at Pecı´nov. Samples from parasequence 1 were devoid of recognizable palynomorphs, probably due to poor preservation. The curve is interrupted at bounding surfaces of sequence-stratigraphic significance. Note that parasequence 4 in the sampled section (right) is approximately 30 cm thicker than in the original section: this set of samples was taken later than the other samples, approximately 200 m south of the original section.

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1988; Hayes et al., 1989). This idea is interesting, especially because high-frequency shifts in d13C in the Anglo–Paris Basin and northern Spain have been correlated to pelagic limestone–marl and chalk–marl rhythms by Paul et al. (1994). Recent studies show that the limestone–marl and similar rhythms in the pelagic record may reflect a high-frequency sea-level change of the Milankovitch-band periodicity (Juignet and Breton, 1992), and may be time equivalents of parasequences in shelf environments ( Elder et al., 1994). In case of the smallscale, high-frequency d13C fluctuations, it is important to estimate the periodicity of the cycles to which they are linked because that might imply the rate of reaction of the e to a p change in pCO , resulting from burial of 2 organic carbon during a short-term sea-level fluctuation. The comparison of the d13C profiles of Pecı´nov and Dover (cf. Gale et al., 1993) suggests that the Pecı´nov parasequences reflect a longer periodicity than the pelagic rhythmites, believed to be controlled by the precession cycle. Data from Gale (1995) suggest 357 ka, and the absolute dates of Obradovich (1991) give a figure of 700 ka for the time span of the M. geslinianum and N. juddii zones. The four parasequences preserved in the Pecı´nov Member probably span the whole M. geslinianum Zone, and perhaps a part of the N. juddii Zone. Based on the time scale of Gale (1995), the periodicity of the sea-level changes reflected in the Pecı´nov parasequences, may be estimated as close to 90 ka. A very similar periodicity, close to the 100 ka eccentricity signal, is found in the parasequences described by Leithold (1994) in the C–T interval in the Western Interior. It should be noted, however, that it is uncertain how much time is missing at the hiatal surfaces which separate the Pecı´nov parasequences, and also the lack of reliable biostratigraphic data makes any periodicity estimates very speculative. Regardless of whether the actual duration of the rhythms was around 100 ka or shorter, it is important to note that, if the short-term d13C fluctuations at Pecı´nov are indeed a response to a global forcing mechanism, then the carbon isotope composition

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of marine organic matter and carbonate may have reacted to sea-level changes much more quickly than expected so far. 8.4. Elemental abundances (2) The elements known to display anomalous concentrations in some C–T sections either do not show any anomalies in the Pecı´nov section (Ir, Cr, Sc and V ), or show variations linked to the specific mineralogy of the sediment. This absence of anomalous concentrations in the elements studied, except Mn, in the Pecı´nov section corresponds to the data from the Pulawy section, Poland (Peryt et al., 1994). Weak peaks for Ir, Sc and Cr (about 10% of North American concentrations) have only been reported so far from Western Europe. Orth et al. (1993) hypothesized that the weakening up to disappearance of elemental anomalies in an eastward direction may be a consequence of an increasing distance from a hot spot or a spreading centre, which was the source of element-enriched waters in the protoCaribbean region. The results from Pecı´nov seem to be in line with this hypothesis, as well as the absence of any anomalous concentration of Mn in the late Cenomanian part of the section. The interpretation of elemental anomalies in the C–T interval favoured by Pomerol and Mortimore (1993) and Orth et al. (1993) focuses on the activity of a submarine volcanic source of elemental enrichment. However, recent studies showed that other factors should also be taken into account, including condensed sedimentation ( Wallace et al., 1991; Wang et al., 1992), and the redox sensitivity of many elements (especially Mn, and also platinum group elements), which may generally lead to element redistribution at redox and facies boundaries during sedimentation and/or diagenesis (cf. Pratt et al., 1991; Wang et al., 1993). The enrichment in Mn in the basal Turonian deposits at Pecı´nov illustrates the importance of diagenetic processes in forming some elemental anomalies.

9. Conclusions (1) The stepwise deterioration of benthic commu-

(3)

(4)

(5)

(6)

nities in the transgressive and (probably) highstand systems tracts of the M. geslinianum Zone at Pecı´nov reaffirms that the oxygen depletion during the late Cenomanian was driven by a sea-level rise. The magnitude of the d13C excursion, more than 4‰, is approximately the same in Bohemia and in North America or Northern Africa. This confirms that the magnitude of the excursion was controlled by a global paleoceanographic mechanism, not by local differences in marine productivity. The waning of the d13C excursion is interpreted as being due to exposure and erosion of organic-enriched rocks and the recycling of organic carbon back into the oceanic reservoir after a sea-level fall in the latest Cenomanian N. juddii Zone. The absence of anoxia and a d13C anomaly during the peak flooding of early Turonian age suggests that widespread deposition of organic-enriched deposits, as well as the positive shift in d13C generally did not depend on the absolute elevation of sea level but, rather, on the increase in flooded area during transgression. The broad d13C curve at Pecı´nov shows a number of superimposed abrupt shifts towards higher values, which coincide with the flooding surfaces of parasequences. It is not yet clear whether these fluctuations reflect regional changes in the proportion of marine and terrestrial organic matter or a change in the isotopic composition of marine organic matter in response to high-frequency sea-level changes. Ir, Sc, Cr, V and other elements enriched in some sections of the C–T boundary interval showed no anomalous concentration in the Pecı´nov section. This was most probably because of the large distance of the depositional site from the presumed volcanic source of element-enriched deep waters in the protoCaribbean region. The anomalously high abundance of Mn at the base of the Turonian deposits at Pecı´nov was caused by incorporation of Mn into siderite during early diagenesis.

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Acknowledgements The authors wish to express their gratitude to ˇ eske´ lupkove´ za´vody, a.s., for the kind permisthe C sion to work in the Pecı´nov quarry; and to their staff members, especially Mr. Sochor and Mr. Kubec, for the assistance they provided. D. Ulicˇny´, ˇ ech were partly supported M. Svobodova´ and S. C by a Czech Republic Grant Agency grant No. 205/1744/94 to the senior author. M. Attrep received support from the Geosciences Research Program in the U.S. Department of Energy’s Office of Basic Energy Sciences. We also wish to thank Andy Gale and the reviewers, Helmut Weissert and Malcolm Hart, for their suggestions and constructive criticisms which significantly helped to improve the paper.

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