Origin of rhythmic Albian black shales (Piobbico core, central Italy): Calcareous nannofossil quantitative and statistical analyses and paleoceanographic reconstructions



[1] The mid-Cretaceous (∼120–90 Ma) was a time of excess atmospheric CO2, greenhouse climate, and widespread O2 deficiency in the ocean. The Albian is punctuated by brief, intermittent episodes of anoxia/dysoxia, recorded as cyclic black shales in the western Tethys and Atlantic oceans. The Albian section of the Piobbico core (central Italy) contains 30 m (∼10 Ma) of rhythmic black shales that were sampled at a high resolution and examined for calcareous nannofossil assemblages and C and O stable isotopes. Unlike oceanic anoxic events, productivity was not the primary factor controlling the deposition of Albian rhythmic black shales. It is suggested that during warm humid climatic cycles, higher temperatures and/or increased precipitation and runoff produced density stratification at a regional scale. Recurrent lowered salinity in the early Albian and warming in the late Albian are credited with causing development of a pycnocline, resulting in slower rates of deep-water renewal and consequent anoxia.

1. Introduction

[2] During the mid-Cretaceous greenhouse period (∼120–90 Ma) [Deconto et al., 2000; Hay, 2008] the oceans were characterized by intermittent anoxia represented both by rhythmically bedded black shales [Herbert and Fischer, 1986; Erba, 1992; Beaudoin et al., 1996; Bellanca et al., 1996; Herrle, 2002; Galeotti et al., 2003; Mitchell et al., 2008] and a number of distinct oceanic anoxic events (OAEs) represented by discrete horizons of organic-rich facies [Schlanger and Jenkyns, 1976; Arthur et al., 1990; Leckie et al., 2002; Erba, 2004]. Early compilations of the stratigraphic distribution of Cretaceous black shales from deep-sea drilling cores and outcrops indicated their common occurrence and considerable stratigraphic range in the Aptian to Cenomanian interval and drew attention to the possible role of salinity stratification in relatively restricted basins [Ryan and Cita, 1977; Arthur and Natland, 1979; Jenkyns, 1980]. Despite the spatiotemporal differences between OAE-related and rhythmically bedded black shales, these two types of sediment are frequently, and erroneously, viewed as one and the same, albeit being profoundly different in character. OAEs record time intervals of prolonged (50–1250 ka) global anoxia to euxinia (i.e., with free H2S in the water column) with concomitant deposition of high amounts of marine organic matter, represented by meter-scale, carbonate-poor, Corg-rich black shales and biogenic silica [Pancost et al., 2004]. Conversely, rhythmic black shales comprise packets, typically many meters thick, of centimeter-scale, locally finely laminated and moderately Corg-rich claystone and marly claystone that apparently correspond with short (1–10 ka) episodes of local to regional anoxia/dysoxia.

[3] The Albian stage of the Cretaceous system has both OAE-related and rhythmically bedded black shales (Figure 1). Although initially viewed as regional subevents rather than global episodes [Arthur et al., 1990; Bréhéret, 1997], OAE1b (Paquier Event) may be represented in the Pacific domain by siliceous sediments and OAE 1d (Breistroffer Event) by an organic-rich level from a pelagic limestone in the Franciscan accretionary complex of California [Robinson et al., 2004, 2008]. OAE1c is probably more regional, although the general recovery of Cretaceous successions from the Pacific Ocean is extremely low, hampering estimates of the real extent of Albian anoxia. Albian rhythmic black shales are recognized in the Atlantic and western Tethys oceans (Figure 2) [e.g., Arthur et al., 1990; Bralower et al., 1993; Leckie et al., 2002], where intermittent anoxia/dysoxia was proven to be driven by orbital forcing [de Boer, 1991; Fischer et al., 1991; Herbert and Fischer, 1986; Erba and Premoli Silva, 1994; Fiet, 1998; Fiet et al., 2001; Grippo et al., 2004]. Two models have been proposed for Cretaceous OAEs and rhythmic black shales: (1) the enhanced productivity model [Weissert et al., 1985; Weissert and Lini, 1991; Erba, 1994, 2004; Erbacher et al., 1996; Jenkyns, 1999, 2003; Hochuli et al., 1999; Premoli Silva et al., 1999; Leckie et al., 2002; Snow et al., 2005] leading to an excess flux of organic matter to the seafloor with burial rate exceeding oxidation rate and (2) the stagnant basin model [Schlanger and Jenkyns, 1976; Bralower and Thierstein, 1984; Arthur et al., 1990; Herbert and Fischer, 1986; Fischer et al., 1991; Premoli Silva et al., 1989a, 1989b; Erba, 1992; Erbacher et al., 1996, 2001] resulting in enhanced preservation of organic matter because of oxygen depletion, similarly to formation of Plio-Pleistocene Mediterranean sapropels [Rossignol-Strick et al., 1982; Rohling, 1991; Rohling and Hilgen, 1991; Rohling and Thunnel, 1999; Meyers, 2006; Rohling et al., 2006]. Integrated geological data sets suggest that eutrophic conditions and excess primary productivity induced OAEs [Jenkyns, 1999], whereas water mass stratification was the primary cause of rhythmic black shales [Herbert and Fischer, 1986]. Three decades of dedicated research demonstrated that OAEs cannot be modeled on the basis of understanding of rhythmic black shales and vice versa, and that cyclic black shales are not identical, requiring further multiproxy studies aimed at identification of the specific paleoceanographic conditions triggering recurrent regional anoxia.

Figure 1.

Chronostratigraphic framework for OAEs and rhythmic black shales in the Aptian-Albian interval. Timescale is after Gradstein et al. [1995]; biostratigraphy and chemostratigraphy are after Erba [2004].

Figure 2.

(a) Mid-Cretaceous paleogeographic reconstruction showing the location of Piobbico section in western Tethys. The gray pattern corresponds to the area characterized by rhythmic deposition of black shales. (b) Detailed location of the Piobbico drill site in central Italy [after Erba, 1988].

[4] Calcareous nannoplankton have proven to be an excellent proxy of present and past surface water masses, since they are extremely sensitive to temperature, fertility, salinity and pCO2 (see Mutterlose et al. [2005] for a synthesis). Moreover, the type and degree of nannofossil preservation can provide crucial information on the early diagenetic history of pelagic carbonates, including bottom and interstitial water characteristics [Erba, 1992]. The combined study of Cretaceous calcareous nannofloras and stable carbon and oxygen isotopes has been adopted to reconstruct paleotemperatures and paleoproductivity in selected time slices [Herrle et al., 2003a], providing the best and most convincing evidence for coupled biotic and abiotic changes. However, such techniques have hitherto not been applied to long intervals of rhythmic black shales through the Albian.

[5] Here we investigate rhythmically bedded Albian black shales in the Umbria-Marche Basin (central Italy) (Figure 2), by means of calcareous nannofossil assemblages and bulk stable carbon and oxygen isotope ratios. Nannofossil quantitative and statistical analyses were used to identify the paleoecological indices of temperature and fertility of surface water masses at short- and long-term timescales. High-resolution characterization of nannofloras in couplets comprising black shales and pale limestones/marlstones was focused on the lower through lower upper Albian interval, improving and extending previous studies conducted on the upper Albian so-called “Amadeus segment” [Erba, 1992]. The objectives of our study are (1) to document Albian long-term trends in surface water fertility and temperature; (2) to understand the mechanisms of sedimentation and preservation of Albian rhythmically bedded black shales; and (3) to estimate the role of surface water temperature, fertility and salinity in the development of bottom water dysoxia-anoxia. In particular, we want to test the following hypotheses: (1) Are all rhythmic black shales caused by the same paleoceanographic change? (2) Are rhythmic black shales caused by one specific cause or by a combination of various causes? (3) Is productivity or stagnation the driving force? (4) Are the causes of rhythmic black shale deposition different from those producing OAEs?

2. Geological Setting and Lithology

[6] The Umbria-Marche sequence of the central Italian Apennines consists of a continuous succession of pelagic to hemipelagic sediments extending from the Jurassic through the Paleogene, whose sedimentary setting corresponds to a portion of the southern Tethyan passive margin lying in water depths estimated as a few kilometers [Bernoulli, 1972; Bernoulli and Jenkyns, 1974, 2009]. The Aptian-Albian Scisti a Fucoidi (or Marne a Fucoidi) Formation consists of a pelagic rhythmic varicolored sequence of marlstone, marly limestone, marly claystone and black shale, demonstrated to match precession, obliquity, short and long eccentricity cycles [Herbert and Fischer, 1986; Erba et al., 1989; Fischer et al., 1991; Erba and Premoli Silva, 1994; Herbert et al., 1995; Fiet et al., 2001; Grippo et al., 2004].

[7] In 1982, the Piobbico core was drilled at “Le Brecce,” located 3 km west of the town of Piobbico (Marche, Italy), at Km 33 of the Apecchiese State Road No. 257, on the left hydrographic side of the Biscubio stream (Figure 2). Coring penetrated the entire Scisti a Fucoidi Formation, including the upper transition to the Scaglia Bianca and the lower transition to the Maiolica. The total length of the core is 84 m with 98.8% recovery. The lithostratigraphy and biostratigraphy of the core were described by Erba [1988] and Tornaghi et al. [1989], its cyclostratigraphy by Herbert et al. [1995] and Grippo et al. [2004].

[8] Approximately 150 black shale layers were recovered in the Piobbico core: most of them are rhythmically intercalated with light gray to green marlstone, marly limestone and marly claystone in the Albian interval (Figure 3). They are usually finely laminated, with rare or no bioturbation, and with a total organic carbon (TOC) content varying between 0.1% and 1.5% wt (total weight) [Pratt and King, 1986].

Figure 3.

Carbon and oxygen isotope curves, paleotemperature fluctuations based on bulk δ18O data (scale ticks represent 1°C increments), nannofossil abundance, and factor 1 scores in the Albian interval of the Piobbico core. Lithostratigraphy, biostratigraphy, and cyclostratigraphy are after Erba [1992] and Grippo et al. [2004]. Smoothed curves are derived from Stineman function.

3. Materials and Methods

[9] The Albian interval of the Piobbico core, corresponding to nannofossil zones NC8 (Prediscosphaera columnata) and NC9 (Axopodorhabdus albianus) of Roth [1978] and Bralower et al. [1995], was sampled every 20 cm corresponding to 1 sample every 40 ka, following the cyclostratigraphic model of Herbert and Fischer [1986], Herbert et al. [1995] and Grippo et al. [2004]. High-resolution sampling was applied to 2 intervals characterized by rhythmically bedded black shales to derive a more detailed picture of the black shale–pale lithology cycles. A total of 32 black shales and adjacent lithologies were sampled from 37 to 27 m (segment A (zones NC8 and lower NC9)) and from 13 m to 11 m (segment B (zone NC9)) (Figure 3). The latter interval corresponds to the Amadeus segment of Erba [1992].

[10] A total of 243 samples of marlstone, marly limestone, marly claystone, limestone and black shale were prepared for nannofossil investigation using standard techniques. Calcareous nannofossils were investigated with a polarizing light microscope (cross-polarized and transmitted light), at 1250X magnification.

[11] About 300–350 individuals were counted in each smear slide and the relative abundance (% of the total nannofloras) of single taxa was used for Pearson's correlation coefficients and paleoecological reconstructions. By analogy with previous papers [Erba, 1992; Mattioli, 1997; Nederbragt and Fiorentino, 1999; Bucefalo Palliani et al., 2002] counts were also used to estimate the abundance of nannofossils as follows: Rare (R) = 1–5 specimens per field of view; Few (F) = 6–25 specimens per field of view; Common (C) = 26–45 specimens per field of view; Abundant (A) = >46 specimens per field of view.

[12] The software STATSOFT STATISTICA 6 was used for multivariate factor analysis (FA) (R-mode) varimax rotation with principal component extraction to determine the relationships between samples and variables, and to identify paleoceanographic and paleoecological indices. This approach yields a clearer signal with respects to paleoclimatic and paleoceanographic conditions than the evaluation of single-species abundances. Factor loadings represent relative relationships among individual taxa within the main factors, whereas factor scores denote the relationships within the sampled cases (lithological samples).

[13] For factor analysis, only taxa with mean abundance ≥1% (13 species) were involved in statistical analyses. However, because of their paleoecological significance, the rare species Eprolithus floralis and Staurolithites stradneri were also considered [e.g., Herrle, 2002]. Factor loadings between ±0.4 and ±0.5 were assigned to “associated taxa” and factor loadings > ±0.5 were assigned to “dominant taxa” [Malmgren and Haq, 1982]. The nutrient index (NI) of Herrle et al. [2003b] was adopted and partly modified. However, the temperature index (TI) of Herrle et al. [2003b] was not used, because of absence of most cold-water taxa in the studied material. Therefore we modified the original formula to include different cool-water species.

[14] To derive trends and short-term fluctuations, both nannofossil and stable isotope data were approximated using the Stineman function. The output of this function then has a geometric weight applied to the current point and ±10% of the data range, to arrive at the smoothed curve. For all data, the standard error at 95% of confidence was applied. The t test was used to assess whether the differences between black shales and other lithologies were significant.

[15] Bulk samples for isotopic analysis were first powdered, cleaned with 10% H2O2 followed by acetone, and then dried at 60°C. Powders were then reacted with purified orthophosphoric acid at 90°C and analyzed online using a VG Isocarb device and Prism Mass Spectrometer at Oxford University. Normal corrections were applied and the results are reported, using the usual δ notation, in per mil deviation from the PDB standard. Calibration to PDB was performed via our laboratory Carrara marble standard. Reproducibility of replicate analyses of standards was generally better than 0.1‰ for both carbon and oxygen isotope ratios. On the basis of δ18O data, paleotemperature trends were reconstructed following the formula of Erez and Luz [1983].

4. Results

4.1. Biostratigraphy

[16] The studied interval corresponds to the NC8 (P. columnata) and NC9 (A. albianus) nannofossil zones (Figure 3). The lowermost sample contains Prediscosphaera columnata, confirming that the base of the NC8 zone lies below the studied section [Erba, 1988, 1992]. Eiffellithus turriseiffelii is absent in the studied interval and, consequently, the top of the NC8 zone was not detected. A few biostratigraphic horizons were, however, recognized: the last occurrence (LO) of Rucinolithus terebrodentarius youngii at 38.40 m, the FO of Tranolithus orionatus (used to place the NC8b/NC8c subzonal boundary) at 37.53 m, the FO of Axopodorhabdus albianus at 33.49 m, the FO of Biscutum aff. B. constans (large specimens) at 30.72 m and the FO of Eiffellithus monechiae (used to place the NC9a/NC9b subzonal boundary) at 11.96 m (Figure 3).

4.2. Chemostratigraphy

[17] The carbon and oxygen isotope data show some degree of scatter, with all values falling within a 4‰ range (Figure 3). There is some degree of covariance of carbon and oxygen isotope values, conventionally interpreted as being due to mixing of a primary marine carbonate of approximately constant isotopic composition with variable quantities of a later diagenetic cement of a different isotopic composition [e.g., Marshall, 1992]. Introduction of isotopically light carbon into the ocean-atmosphere system accompanied by a rise in global temperatures can, however, produce a similar degree of correlation [e.g., Jenkyns, 2003]. It is notable that the range of isotopic values is considerably less extreme than those recorded from coeval deposits (Marne a Fucoidi) from the Gargano peninsula in southern Italy where diagenetic effects were more extreme [Luciani et al., 2004].

[18] The carbon isotope profile shows, with some departures, a drift from close to 3‰ at the base of the section to <2.5‰ close to the top, followed by an abrupt step up to higher values followed by a renewed downward trend. Such values are typical for those recorded in mid-Cretaceous pelagic carbonates and marlstones from elsewhere in Italy, Mexico, Switzerland and France [e.g., Bellanca et al., 1996; Bralower et al., 1999; Strasser et al., 2001; Herrle et al., 2003b, 2004] and shallow-water guyot carbonates from the mid-Pacific Mountains [Jenkyns, 1995; Jenkyns and Wilson, 1999], which suggests that they are broadly representative of seawater compositions. Carbon isotope ratios of carbonate sediments, which have undergone simple lithification without the introduction of externally derived fluids, are known to behave in a conservative manner [Scholle and Arthur, 1980; Marshall, 1992].

[19] The oxygen isotope ratios show scatter of a similar magnitude to that manifested by the carbon isotopes. The lower part of the section shows a gradual movement from ∼−1.75 to ∼−2.25, followed by a rise, fall and rise to ∼−2‰ at its top. The presence of these distinct trends, broadly independent of lithology, and the fact that diagenetically susceptible nannofossil taxa are relatively well preserved, is taken as evidence that the δ18O signature records a primary paleotemperature signal somewhat modified by the lithification process. Because the imprint of diagenesis typically introduces isotopically light cement [Marshall, 1992], the derived paleotemperatures, varying between 21 and 26°C, must be viewed as maxima. With respect to coeval sediments in the Vocontian Trough, southeast France [Herrle et al., 2003b] it is notable that, although the carbon isotope values are similar, the oxygen isotope values of the Piobbico sediments are greater by ∼1‰, suggesting a less modified geochemical signature in the Italian section.

4.3. Nannofossil Preservation and Paleoecology

[20] Calcareous nannofossils are abundant (Figure 3) and moderately well preserved throughout the studied interval. A total of 55 taxa were identified (see Appendix A) with a species richness (= number of species) of 23 taxa. The distribution and abundance of calcareous nannofossils are controlled by original factors and diagenetic overprint. Among paleoecological factors influencing nannofloral composition, surface water fertility and temperature play a major role. A few paleoenvironmental indices have been identified in mid-Cretaceous calcareous nannofossil assemblages and are routinely used to delineate paleoceanographic conditions as well as diagenetic modifications, as synthesized by Mutterlose et al. [2005].

[21] W. barnesiae is most resistant to diagenesis and its fluctuations in abundance have been proposed as a measure of preservation. Specifically, percentages of W. barnesiae higher than 40% [e.g., Roth and Bowdler, 1981; Thierstein and Roth, 1991] or 70% [Williams and Bralower, 1995] would indicate highly altered samples, where dissolution/overgrowth have selectively eliminated small, fragile and delicate taxa. However, W. barnesiae is also inversely correlated to mesoeutrophic taxa and, consequently, seems associated with low-fertility settings [Roth and Krumbach, 1986; Premoli Silva et al., 1989b, 1989c; Erba, 1992, 1994; Erba et al., 1992; Williams and Bralower, 1995; Tremolada et al., 2006].

[22] Despite high abundances of W. barnesiae throughout the studied interval, nannofossil assemblages are diversified and dissolution-susceptible taxa [e.g., Thierstein, 1980] are present and relatively abundant, independent of lithology. Moreover, in samples marked by strong diagenetic modification, the relative abundance of dissolution-susceptible coccoliths increases and the relative abundance of W. barnesiae decreases. We, conclude therefore, that the abundance of W. barnesiae preserves original paleoenvironmental signals.

[23] The small-sized and delicate taxa Z. erectus, D. rotatorius and B. constans are considered as nannoplankton mesoeutrophic taxa [e.g., Roth and Bowdler, 1981; Roth and Krumbach, 1986; Premoli Silva et al., 1989a, 1989b; Watkins, 1989; Erba et al., 1992; Coccioni et al., 1992; Williams and Bralower, 1995; Bellanca et al., 1996; Tremolada and Erba, 2002; Herrle, 2002, 2003; Herrle et al., 2003a, 2003b; Bornemann et al., 2005; Mutterlose et al., 2005; Tremolada et al., 2006]. R. asper was interpreted by several authors [Roth and Krumbach, 1986; Wise, 1988; Mutterlose, 1987, 1989; Erba et al., 1992; Herrle and Mutterlose, 2003; Herrle et al., 2003a, 2003b] as a warm-water species, and Z. diplogrammus and C. surirellus probably have similar paleoecological affinity [Erba, 1992; Herrle, 2002, 2003; Herrle et al., 2003a, 2003b]. High abundances of S. stradneri, E. floralis, R. parvidentatum, and Seribiscutum spp. were related to cold surface waters [Roth and Krumbach, 1986; Erba, 1992; Erba et al., 1992; Mutterlose, 1992; Herrle and Mutterlose, 2003; Herrle et al., 2003b].

4.4. Albian Long-Term Trends in Temperature and Fertility

[24] Nannofossil abundance (Figure 3) shows relatively high fluctuations, with average total abundance of 29 specimens/field of view. The highest abundances (up to 70 nannofossils/field of view) are recorded in the lower Albian and a general decrease is observed upward.

[25] The descriptive statistics of selected nannofossil taxa are synthesized in Table 1. Watznaueria barnesiae is the dominant species, comprising 28% and 82.4% (mean 54.8%) of the total assemblage. Biscutum constans shows abundances comprising between 0% and 26.3% (mean 6.6%) and Lithraphidites carniolensis exhibits mean abundances of 6.6% with peaks of 19.9%. Discorhabdus rotatorius on average represents 6.3% of the entire nannofloral assemblage, comprising between 0% and 22.7%. Rhagodiscus asper shows mean abundances of 5.1% with peaks of 12.3% and Rucinolithus irregularis represents 3.1% of the entire calcareous nannofloral assemblage with maximum abundances of 28%. Cretarhabdus surirellus exhibits abundance fluctuating between 0% and 10.3% (mean 2.4%) and Zeugrhabdotus embergeri represents average abundance of 1.8% of the total assemblage with peaks of 16%. Manivitella pemmatoidea, Zeugrhabdotus diplogrammus, Z. erectus, Z. elegans and Z. xenotus display average abundance between 1.3% and 1% of the entire calcareous nannofossil assemblage. The species Staurolithites stradneri and Eprolithus floralis show very low abundances (on average less than 1%) and they become extremely rare in the middle to upper Albian. However, these taxa were selected because of their paleoecological affinity for cold surface waters. Several other taxa, listed in Appendix A, are sparse and each represent less than 1% of total nannofloras.

Table 1. Descriptive Statistics of Selected Nannofossil Taxa Investigated in 243 Samples
SpeciesMean (%)Median (%)Minimum (%)Maximum (%)SDaStandard ErrorSkewnessKurtosis
  • a

    SD, standard deviation.

W. barnesiae54.855.628.082.411.20.7−0.05−0.7
B. constans6.
L. carniolensis6.
D. rotatorius6.
R. asper5.14.90.712.
R. irregularis3.
C. surirellus2.
Z. embergeri1.
M. pemmatoidea1.
Z. diplogrammus1.
Z. erectus1.−0.6
Z. elegans1.
Z. xenotus1.−0.4
E. floralis0.
S. stradneri0.

[26] Pearson's correlation coefficients derived for the selected taxa are reported in Table 2. W. barnesiae shows the highest negative correlation with D. rotatorius, B. constans and Z. erectus, which themselves are positively correlated. There is a positive correlation between Z. xenotus, Z. elegans and Z. erectus, as well as between R. irregularis and D. rotatorius. Z. embergeri is negatively correlated with Z. xenotus, Z. elegans, Z. erectus, L. carniolensis and positively correlated with R. irregularis. E. floralis exhibits a positive correlation with Z. embergeri and a negative correlation with B. constans. Z. diplogrammus shows a negative correlation with R. irregularis.

Table 2. Pearson's Correlation Coefficients Based on Data From 243 Samplesa
 W. barnesiaeR. asperB. constansD. rotatoriusZ. embergeriZ. elegansZ. erectusZ. xenotusZ. diplogrammusL. carniolensisM. pemmatoideaC. surirellusR. irregularisS. stradneriE. floralis
  • a

    Significant coefficients (p < 0.01) are in bold.

W. barnesiae1.00              
R. asper−0.231.00             
B. constans0.630.051.00            
D. rotatorius0.760.010.421.00           
Z. embergeri0.19−0.100.47−0.161.00          
Z. elegans−0.18−0.080.30−0.020.331.00         
Z. erectus0.50−0.050.500.360.340.351.00        
Z. xenotus−0.22−0.200.410.090.430.470.601.00       
Z. diplogrammus−−0.05−      
L. carniolensis−0.26−     
M. pemmatoidea−−0.12−    
C. surirellus−0.230.13−−−0.010.330.141.00   
R. irregularis0.34−0.01−0.100.340.14−0.190.01−−  
S. stradneri0.05−0.190.08−0.10−0.140.320.220.370.150.340.000.33−0.261.00 
E. floralis0.19−0.130.28−0.170.29−0.03−0.22−0.16−0.12−

[27] The FA (R-mode) varimax rotation was applied to percentages of the most abundant and paleoecological important nannofossil species. Three significant factors were extracted (Table 3), representing 53.3% of the total variance. Factor loadings are shown in Figure 4 according to factors 1 and 2, and factors 2 and 3.

Figure 4.

Factor 1 versus factor 2 and factor 2 versus factor 3 from factor analysis (R-mode) varimax normalized rotation with principal component extraction. Factor 1 is interpreted as the nannofossil abundance, factor 2 is interpreted as fertility of the surface water, and factor 3 is interpreted as temperature of the surface water.

Table 3. Factor Analysis Varimax Normalized Rotation With Principal Component Extraction Based on 15 Taxaa
FA VarimaxFactor 1Factor 2Factor 3
  • a

    Rotation is R-mode. Data are from 243 samples. Associates and dominant taxa are in bold.

W. barnesiae−0.090.93−0.11
R. asper0.350.140.57
L. carniolensis0.620.240.14
B. constans−0.310.700.22
D. rotatorius0.120.83−0.21
M. pemmatoidea0.02−0.020.63
Z. diplogrammus−
C. surirellus0.720.080.15
Z. elegans0.570.170.20
R. irregularis0.490.400.44
Z. erectus0.480.620.03
Z. xenotus0.790.320.01
Z. embergeri0.46−0.360.41
S. stradneri0.63−0.10−0.05
E. floralis0.15−0.35−0.19
Variance percent26.5416.4410.27

[28] The first factor (FA1, 26.54% of the total variance) shows the highest positive loadings for C. surirellus (dominant taxon), R. irregularis and Z. embergeri (associated taxa) and the highest negative loadings for Z. xenotus, S. stradneri, L. carniolensis, Z. elegans (dominant taxa) and Z. erectus (associated taxon) (Table 3 and Figure 4). Factor 1 scores are plotted in Figure 3 and exhibit a negative correlation (r = −0.62, p < 0.01) with the abundance of nannofossils.

[29] The second factor (FA2, 16.44% of the total variance) shows the highest positive loadings for D. rotatorius, B. constans, Z. erectus (dominant taxa) and R. irregularis (associated taxa) and the highest negative loadings for W. barnesiae (dominant taxa) (Table 3 and Figure 4). Factor 2 scores are plotted in Figure 5: they show high positive correlation (r = 0.93, p < 0.01) with the nutrient index (NI) of Herrle et al. [2003b]. The third factor (FA3, 10.27% of the total variance) shows the highest positive loadings for R. asper, M. pemmatoidea, Z. diplogrammus (dominant taxa) and the highest negative loadings only for two associated taxa (Z. embergeri and R. irregularis) (Table 3 and Figure 4).

Figure 5.

(a) Comparison between the nutrient index (NI) [Herrle et al., 2003b] (gray curve; symbols with bars of standard error) and factor 2 scores (black curve). (b) Comparison between the smoothed NI (gray curve) and the modified nutrient index (MNI) (black curve; symbols with bars of standard error). (c) Comparison between factor 3 scores (black curve) and the modified temperature index (MTI) (gray curve; symbols with bars of standard error). (d) Comparison between the MTI (gray curve) and paleotemperature variations as in Figure 3 (black curve). Smoothed curves are derived from Stineman function.

[30] To determine the NI, Herrle et al. [2003b] used Z. erectus and D. rotatorius as high-fertility taxa and W. barnesiae as a low-fertility taxon, whereas to determine the TI they used R. asper and Z. diplogrammus as warm-water taxa and Seribiscutum spp., Z. trivectis, R. parvidentatum and S. stradneri as cold-water forms. In the Albian interval of the Piobbico core, some cold-water taxa (Seribiscutum spp., Z. trivectis, R. parvidentatum) are absent. Only the cold-water species S. stradneri is present, albeit in very low abundance. E. floralis is considered a cold-water taxon [Roth and Krumbach, 1986; Erba et al., 1992] but was not included in the TI of Herrle [2002] and Herrle et al. [2003b].

[31] According to previous paleoecological reconstructions and the results of our factor analysis, we modified both fertility and temperature indices. In particular, the positive and negative loadings of the assemblage given by FA2 were used to determine surface water fertility and the modified nutrient index (MNI) was calculated as follows:

equation image

[32] The positive and negative loadings of the assemblages given by FA3 were used to reconstruct surface water temperatures and the modified temperature index (MTI) was calculated as follows:

equation image

[33] The MNI displays high positive correlation with factor 2 scores (r = 0.98; p < 0.01) and with Herrle's NI (r = 0.92, p < 0.01) (Figure 5). The MTI shows high correlation with factor 3 scores (r = −0.77, p < 0.01) and δ18O-derived paleotemperature trends (Figure 5). No correlation is present between MNI and MTI (r = 0.07, p < 0.001).

[34] Variations of bulk carbon and oxygen stable isotopes are compared with nannofossil indices of fertility and temperature in Figure 6. In the lower part of the Albian Piobbico core (from 40 m to 27 m) the smoothed carbon isotope curve presents higher values with a decrease followed by an increase in the uppermost interval. The δ18O smoothed curve displays a trend to more negative values, suggesting a general warming interrupted by an interval of relatively cooler conditions from 21 m to 12 m (Figure 6).

Figure 6.

Albian δ13C, δ18O, and nannofossil fertility (NI and MNI) and temperature (MTI) variations sampled at a 20-cm interval. Smoothed curves are derived from Stineman function.

[35] Fertility, as measured by the nannofossil MNI and NI, displays a general increase through the Albian. The MTI suggests relatively variable temperatures: the lower and lower middle Albian is moderately warm, reaching a maximum around 25 m, then a gradual decrease is recorded through the middle to upper Albian, and the uppermost interval displays another warming phase (Figure 6).

4.5. Short-Term Changes Associated With Rhythmic Black Shales

[36] Calcareous nannofloral assemblages show no systematic fluctuations in segment A and segment B (Figures 7 and 8) . Conversely, the fertility-related species D. rotatorius, B. constans and Z. erectus randomly display higher or lower abundances in black shales relative to adjacent lithologies. The same distribution patterns were observed for W. barnesiae, but with a trend opposite to that of the mesoeutrophic taxa. Only R. irregularis shows a decrease in abundance within most black shales, with higher abundances merely in 3 black shales. Moreover, R. irregularis reaches highest values in marly limestones (Figures 7 and 8). The warm-water taxa R. asper, Z. diplogrammus and M. pemmatoidea commonly increase in abundance within black shales, where the cold-water species S. stradneri and E. floralis are always extremely rare or absent.

Figure 7.

Nannofossil indicators of fertility and temperature fluctuations in segment A from 37 to 27 m (NC8 to NC9 nannofossil zones, early to middle Albian). Solid circles correspond to black shales.

Figure 8.

Nannofossil indicators of fertility and temperature fluctuations in segment B from 13 to 11 m (NC9 nannofossil zone, late Albian). Solid circles correspond to black shales.

[37] Fluctuations of δ13C and δ18O in alternating pale marlstones and black shales through segments A and B (Figure 9) show that C isotopic values vary randomly, whereas δ18O values are usually lower in black shales, especially in segment B.

Figure 9.

Fluctuations of δ18O, δ13C, nannofossil abundance, fertility (MNI and NI), and temperature (MTI) indices in alternating pale marlstones (open circles) and black shales (solid circles) in segments A and B. Because the amount of diagenetic cement cannot be quantified, only paleotemperature trends are illustrated.

4.5.1. Segment A (37–27 m)

[38] Nannofossils are common, with an average abundance of 40 specimens/field of view. Within black shales, high-abundance fluctuations are recorded relative to adjacent lithologies (Figure 9). Herrle's NI and the MNI show analogous trends, although the latter is shifted to slightly higher values: fertility is generally low, with a minor increase upward. The MNI values are commonly, but not invariably, higher in the black shales relative to adjacent lithologies (Figure 9). The MTI is characterized by relatively low values, implying warm conditions, with random fluctuations relative to black shales (Figure 9).

4.5.2. Segment B (13–11 m)

[39] Nannofossils in segment B are significantly less abundant (few to rare) than in segment A and a minor decrease is recorded upward (Figure 9). The NI [Herrle et al., 2003b] and MNI are very similar. As previously documented by Erba [1992], fertility indices are typically greatest in limestones and commonly show minima in the black shale intervals, characterized by values lower than the mean and the underlying lithology (Figure 9). Within black shale layers, the MTI usually suggests conditions warmer than those reconstructed for the other lithologies (Figure 9).

5. Discussion

5.1. Mid-Cretaceous Nannoplankton Paleoecology

[40] Factor 1 is interpreted as reflecting abundance of calcareous nannofloras (Figures 3 and 4): high positive loadings represent species occurring in intervals with low nannofossil abundance (C. surirellus, R. irregularis and Z. embergeri) and high negative loadings represent species occurring in intervals with high nannofossil abundance (Z. xenotus, L. carniolensis, S. stradneri, Z. erectus and Z. elegans). This implies that when nannofloral assemblages become larger, specific rather than all taxa increase in abundance. W. barnesiae, the most common taxon, is not relevant for the total abundance of calcareous nannofossils (Figure 4).

[41] Factor 2 is interpreted as related to surface water fertility: high positive loadings may represent mesoeutrophic conditions (more abundant D. rotatorius, B. constans, Z. erectus, R. irregularis) and high negative loadings (more abundant W. barnesiae) oligotrophic conditions (Figures 4 and 5). Factor 3 is interpreted as reflecting surface water temperature: high positive loadings (more abundant M. pemmatoidea, Z. diplogrammus, R. asper) may represent warmest conditions and high negative loadings (more abundant R. irregularis, Z. embergeri, E. floralis, S. stradneri) relatively cool conditions (Figures 4 and 5).

[42] Quantitative and statistical analyses indicate that R. irregularis is a potential proxy for paleoceanographic reconstructions. This taxon, in fact, seems to have a paleoecological affinity for mesotrophic and temperate waters (Figure 4) and is the only taxon with a systematic drop in abundance within rhythmic black shales (Figures 7 and 8), whereas others dissolution-susceptible and delicate taxa are present and occasionally common. We speculate that R. irregularis might be a stenohaline taxon that could not tolerate lowered salinity in surface waters.

5.2. Albian Long-Term Trends in Fertility and Temperature

[43] Nannofossil nutrient and temperature indices and C and O isotopic ratios indicate a general minor increase in fertility of surface waters during the Albian. As far as the paleotemperature are concerned, the data indicate a general warming during the early middle Albian, followed by temperate conditions (Figure 6). The absence of cold-water taxa such as R. parvidentatum and Seribiscutum spp. further indicates general warm-water conditions throughout the studied interval. However, the MTI records some fluctuations between warmest and less warm surface waters (Figure 6).

5.3. Short-Term Changes in Temperature and Fertility Relative to Black Shales

[44] The high-resolution study of the black shale–pale marlstone and limestone alternations of segment A (about 2.5 Ma long) and segment B (about 0.5 Ma long) revealed an absence of a systematic relationship between fertility and/or temperature and deposition of anoxic sediments because mesoeutrophic and warm-water taxa are variably abundant in black shales (Figures 7 and 8). Similarly, δ18O and δ13C changes are not systematic relative to black shales, although oxygen isotopes values tend to be lower in black shales of segment B (Figure 9). The MTI also suggests that most upper Albian black shales (segment B) were deposited under conditions warmer than adjacent intervals (Figure 9). Conversely, in the early to middle Albian (segment A) an increase in temperature was apparently not instrumental in generating anoxia (Figure 9).

[45] The t test was applied to all environmentally related parameters in order to assess the significance of differences between black shales and adjacent pale marlstones in both segments A and B (Table 4 and Figure 10). Only δ18O values are significantly more negative in black shales from both segments. Nannofossil fertility indices are significantly lower in black shales of segment B, but differences are not significant for segment A. Nannofossil paleotemperatures are significantly warmer in black shales of segment B, but temperature differences between black shales and pale marlstones are not significant in segment A (Figure 10 and Table 4).

Figure 10.

Box-and-whisker plots of MNI, MTI, and δ18O in pale marlstones (white boxes) and black shales (hatched boxes). Box represents the mean value plus or minus standard error; whiskers represent standard deviation. The p values of t test (see Table 4) are reported.

Table 4. One-Way t Test Including Values of Nannofossil Abundance, Single Taxon Relative Abundance, Temperature and Fertility Indices, Factor Scores, Carbon Isotopes, and Oxygen Isotopes in Pale Marlstones and Black Shalesa
Segment BMean MarlstoneMean Black Shalet ValueDegree of FreedompValid N MarlstoneValid N Black Shale
  • a

    Variables with significantly different values between the two lithological groups (p < 0.05) are in bold.

W. barnesiae55.4061.212.26680.034921
R. asper5.025.09−0.12680.914921
L. carniolensis3.713.620.15680.884921
B. constans2.253.02−1.47680.154921
D. rotatorius6.803.892.57680.014921
M. pemmatoidea0.941.412.10680.044921
Z. diplogrammus0.661.763.65680.004921
C. surirellus3.974.87−1.89680.064921
Z. elegans0.440.56−0.82680.414921
R. irregularis6.751.564.39680.004921
Z. erectus0.420.360.38680.714921
Z. xenotus0.010.000.66680.514921
Z. embergeri4.443.491.28680.214921
S. stradneri00-68-4921
E. floralis0.550.56−0.05680.964921
FS1 abundance1.170.951.85680.074921
FS2 fertility0.200.843.12680.004921
FS3 temperature0.850.414.52680.004921
NI [Herrle et al., 2003b]11.666.112.72680.014921
Nannofossil abundance7.368.61−1.23680.224921
Carbon stable isotopes2.372.47−0.72290.48265
Oxygen stable isotopes2.142.532.79290.01265
Segment AMean MarlstoneMean Black Shalet ValueDegree of FreedompValid N MarlstoneValid N Black Shale
W. barnesiae57.1752.092.16630.034421
R. asper4.394.43−0.17630.874421
L. carniolensis8.229.20−1.50630.144421
B. constans7.089.322.10630.044421
D. rotatorius5.765.540.23630.824421
M. pemmatoidea1.091.602.42630.024421
Z. diplogrammus1.641.560.39630.704421
C. surirellus1.211.73−1.49630.144421
Z. elegans1.451.75−1.22630.234421
R. irregularis2.141.500.63630.534421
Z. erectus1.161.923.52630.004421
Z. xenotus1.691.83−0.58630.564421
Z. embergeri0.971.38−1.95630.064421
S. stradneri0.380.54−1.35630.184421
E. floralis0.340.340.00631.004421
FS1 abundance−1.00−1.221.46630.154421
FS2 fertility−0.210.17−1.69630.104421
FS3 temperature−0.040.09−0.83630.414421
NI [Herrle et al., 2003b]11.3312.95−0.77630.444421
Nannofossil abundance43.6239.051.02630.314421
Carbon stable isotopes2.732.79−1.021170.319920
Oxygen stable isotopes2.012.172.081170.049920

[46] We conclude that periodic anoxic conditions were associated with warmer and less fertile surface waters during the late Albian (segment B), whereas lowered salinity characterized episodes of black shale deposition during the early to middle Albian (segment A) with no evidence of significant changes in surface water fertility and/or temperature.

5.4. Models for Rhythmic Anoxia During the Albian

[47] Segment B corresponds to the Amadeus segment [Erba, 1992], which may be considered representative, in whole or in part, of OAE1c [e.g., Leckie et al., 2002; Galeotti et al., 2003; Luciani et al., 2007], although this attribution may not be correct. The upper Albian interval of the Scisti a Fucoidi is controlled by Milankovitch cyclicity in the Tethyan region [Herbert and Fischer, 1986; Premoli Silva et al., 1989a; Fischer et al., 1991; Coccioni and Galeotti, 1993; Herbert et al., 1995; Erba and Premoli Silva, 1994; Cobianchi et al., 1997] and in the Saxony Basin [Fenner, 2001]. OAE1c was interpreted by Erbacher et al. [1996] as a “detrital oceanic anoxic event” characterized by high input of derived organic matter leading to deposition of black shales. Planktonic foraminiferal assemblages from the Amadeus Segment in the Fiume Bosso section and the equivalent segment I of the Piobbico Core [Erba, 1992] have been studied by Premoli Silva et al. [1989a, 1989b, 1989c], Tornaghi et al. [1989] and Galeotti [1998]. These authors suggested that temperature might have been the most important factor controlling the distribution of genera Ticinella and Biticinella, increasing in abundance during deposition of black shales. Benthic foraminiferal assemblages exhibit an increased abundance of small-sized species and elongate morphotypes within the orbitally modulated black shales [Coccioni and Galeotti, 1993]. This trend suggests that reduced ventilation of the seafloor was not accompanied by increased fluxes of organic matter during the deposition of black shales [Galeotti, 1998].

[48] The paleo-oxygenation curve through the Amadeus segment in the Piobbico Core [Erba and Premoli Silva, 1994] implies that sedimentation of black shales occurred under dysaerobic bottom water, as also documented by the relative abundance of vanadium and nickel [Tateo et al., 2000], but without the development of strictly anoxic conditions. The quantitative analysis of palynological residues from the Amadeus segment in the Fiume Bosso section suggests a strong enhancement of continental fluxes during the deposition of black shale [Kouwenberg et al., 1997].

[49] During the Albian, under warm and humid greenhouse conditions, precipitation and runoff cycles presumably played a major role in controlling the strength and dynamics of the halothermocline, a trend that was temporarily reinforced by increased temperatures. Changes in salinity and/or temperature of surface waters would have caused significant variations in their density and consequently generated stratification, inhibiting vertical mixing and deep-water ventilation.

[50] Our nannofloral and oxygen isotope data confirm that upper Albian rhythmic black shales (segment B) were deposited during short episodes of warmer conditions with no evidence of increased fertility. On the contrary, nannofossil indices suggest that surface water temperature did not systematically fluctuate during intermittent anoxia in the early Albian concomitant with rather stable low fertility (segment A). This supposition is supported by the results of the t test, indicating no significant differences of paleoecological indices between black shales and intervening pale marlstones. If paleotemperature of surface waters did not increase cyclically during times of bottom water oxygen depletion in the early Albian, then the lower δ18O values presumably imply a repetitive lowering of salinity.

[51] The results of our investigations suggest that Albian bottom water dysoxia-anoxia and deposition of black shales was not caused by one recurring specific change in surface water characteristics. On the contrary, Milankovitch climate cycles resulted in a cyclic reduction of surface water density deriving from variations in temperature and/or salinity.

[52] The Cretaceous was a time of significant change of ocean salinity [Hay et al., 2001, 2006] and a series of general circulation models indicate that surface seawater salinities were slightly higher (37.5‰) than those today [Barron et al., 1995; Bush and Philander, 1997; Norris and Wilson, 1998]. Hay [2008] speculates that average salinity of the Late Cretaceous was about the same as today (34.7‰), but in the Early Cretaceous (pre-Aptian interval) average salinities were possibly about 37.7‰. Wagner et al. [2008] calculate a range of salinities from 43‰ to 41‰ on the base of TEX86-SST and δ18O in the early Albian at Blake Nose Plateau (ODP Site 1049). Nannofloral assemblages should reflect such anomalously high salinities, presumably with depressed diversity and dominance of euryhaline taxa. However, no documentation of substantially different nannofloral assemblages is available for the interval of presumed major decrease in paleosalinity [Erba, 1992; Bralower et al., 1993; Herrle et al., 2003a, 2003b; Browning and Watkins, 2008].

[53] Paleosalinity reconstructions are particularly problematic, because of the number of basic assumptions necessary to separate paleotemperature and paleosalinity from δ18O data [Rohling and Bigg, 1998; Schmidt, 1999; Rohling, 2000]. The evaluation of Cretaceous paleosalinity changes is affected by even greater uncertainties, because of the large errors in estimates of temporal and spatial variations in seawater δ18O.

[54] Table 5 shows density values depending on temperature and salinity of water masses. We used these data to formulate possible models for inducing periodic black shale deposition under pycnocline-related stratification (Figure 11). For the late Albian, we adopted two different initial temperature-salinity conditions of 25°C and 35‰ (today's average at medium latitudes) and 30°C and 41‰ (possible late Albian average at medium latitudes) according to Hay [2008], respectively, and an increase in temperature was applied to three different simulations with constant or varied salinity. Significant changes in surface water density were obtained under increased temperatures, with the lowest density deriving from increased temperature and coeval runoff/precipitation-induced lowered salinity. If periodic warmer conditions caused excess evaporation and, therefore, increased salinity of surface waters, stratification would not develop, but downwelling of dense, warm and salty water would be predicted.

Figure 11.

Paleoceanographic models for the formation of pale limestones-marlstones (cases 1a and 1b) and black shales (cases 2a, 2b, and 3b) in the Albian. Cases 3a and 4b suggest downwelling of saline and possibly warmer surface waters preventing bottom anoxia.

Table 5. Density of Surface Water Masses Based on Variations of Temperature and Salinity
Temperature (C°)Salinity 43‰Salinity 41‰Salinity 38‰Salinity 35‰Salinity 33‰Salinity 31‰

[55] As far as lower to middle Albian black shales are concerned, no evidence of cyclic warming was obtained. Therefore, we simulated the possible oceanographic response under decreased (runoff and/or precipitation) and increased (evaporation) salinity of surface waters after initial temperature-salinity conditions of 24°C and 35‰ (today's average at medium latitudes, but slightly less warm than segment B) and 29°C and 38‰ (possible early Albian average at medium latitudes) according to Hay [2008], respectively. In this case, increasingly stronger stratification develops as progressive decrease in salinity is applied whereas, as for the late Albian, enhanced evaporation and higher salinity would cause sinking of dense salty surface waters.

[56] Increased precipitation and river runoff as well as higher aeolian input during the deposition of rhythmic Albian black shales were previously inferred, on the basis of organic matter composition and palynofacies of the Scisti a Fucoidi [Pratt and King, 1986; Fiet, 1998]. In fact, the organic matter is hydrogen-poor and partly of continental origin with high amounts of aeolian dust. Palynofacies investigations revealed the presence of abundant wood debris, spores and pollen, the latter often represented by Classopollis [Fiet, 1998]. The organic carbon-poor black shales are characterized by large amounts of dinocysts. Higher runoff coeval with black shale deposition is suggested by the common occurrence of freshwater algae such as Pediastrum, Scenedescus and various Tasmanitaceae, temporarily forming abundance peaks up to 90% of the total fraction [Fiet, 1998].

[57] The proposed models strongly support a significant role of lowered salinity leading to stratification as thought to be the case with Mediterranean sapropels regulated by Plio-Pleistocene monsoonal cycles [Rossignol-Strick, 1985; Rohling, 1991]. In the Albian, periodically, but not systematically, river discharge introduced higher volumes of nutrients and terrigenous input. In fact, the nannofossil distribution and statistical analyses suggest that enhanced productivity was not the primary factor controlling the formation and deposition of Albian rhythmic black shales. However, since calcareous nannoplankton are generally oligomesotrophic organisms, low abundance or absence of the mesoeutrophic nannofossil taxa in black shales might indicate a nutrient excess above the nannoplankton threshold.

6. Conclusions

[58] Quantitative and statistical analyses of nannofloral assemblages confirm the paleoecological affinity of several mid-Cretaceous taxa and indicate that modified paleofertility (MNI) and paleotemperature (MTI) indices might be more appropriate for low latitudes. Albian long-term trends suggest increasing surface water fertility and warm-water conditions, with some fluctuations between warmest and less warm surface waters.

[59] The integration of nannofossil and stable isotope proxies shows, for the first time, that Albian rhythmic anoxia/dysoxia was not caused by a recurrent change in one specific surface water characteristic. Deposition of black shales is associated with lowered salinity without significant changes in fertility and/or temperature in the early middle Albian, and to higher temperature with no evidence of increased fertility in the late Albian. We speculate that during intermittent warm humid climatic episodes, relatively higher temperatures produced a more intense thermocline and stronger water stratification. Alternatively, or concomitantly, increased precipitation, due to monsoonal cycles caused a salinity decrease and establishment of a halocline. R. irregularis is the only nannofossil species showing a drop in abundance in black shales and might be regarded as a stenohaline taxon, suffering under lowered salinity of surface waters.

[60] Our data suggest that stronger precipitation and freshwater runoff was more effective than warming during the early Albian, but warming might have been crucial in the late Albian. The resulting density stratification of water masses led to the development of a pycnocline, preventing vertical mixing: slower rates of deep-water renewal triggered dysaerobic-anaerobic conditions, further enhanced by lowered absorption of oxygen in warm surface water masses. The distribution of mesoeutrophic nannofossil taxa indicate that enhanced productivity was not the primary factor controlling the deposition of Albian rhythmic black shales. Conversely, similarly to Pleistocene sapropels, cyclic monsoonal climate induced higher runoff and stratification because of lowered salinity, possibly reinforced temporarily by warmer surface waters. While Cretaceous OAEs were primarily induced by global ocean fertilization, enhanced primary productivity and expansion of the oxygen minimum zone as well as lowered O2 under warm conditions [Jenkyns, 1999; Erba, 2004], rhythmic black shales are the result of cyclic reduction of surface water density deriving from variations in temperature and/or salinity, causing regional stratification.

Appendix A

[61] Taxonomic index of calcareous nannofossils observed in this study. Genera and related species are listed in alphabetic order. Authors and date of the original description and, when necessary, emendation are provided. See Perch-Nielsen [1985] and Bown [1998] for full information regarding taxonomy and authorships. Axopodorhabdus Wind and Wise in Wise and Wind [1976] A. albianus [Black, 1967] Wind and Wise in Wise and Wind [1976] Biscutum Black in Black and Barnes [1959] B. constans [Gòrka, 1957] Black in Black and Barnes [1959] B. aff. B. ellipticum [Gòrka, 1957] Grün in Grün and Allemann [1975] BraarudoshaeraDeflandre [1947] B. africanaStradner [1961] B. regularisBlack [1973] Calculites Prins and Sissingh in Sissingh [1977] Calculites sp. ChiastozygusGartner [1968] C. litterarius [Gòrka, 1957] Manivit [1971] CyclagelosphaeraNöel [1965] C. deflandrei [Manivit, 1966] Roth [1973] C. margereliiNöel [1965] CorollithionStradner [1962] C. achylosum [Stover, 1966] Thierstein [1971] CretarhabdusBramlette and Martini [1964] C. angustiforatus [Black, 1971] Bukry [1973] C. conicusBramlette and Martini [1964] C. coranadventisReinhardt [1966] C. crenulatusBramlette and Martini [1964] C. striatus [Stradner, 1963] Black [1973] C. surirellus [Deflandre, 1954] Reinhardt [1970] DiazomatolithusNöel [1965] D. lehmaniiNöel [1965] DiscorhabdusNöel [1965] D. rotatorius [Bukry, 1969] Thierstein [1973] EiffellithusReinhardt [1965] E. hancockiiBurnett [1998] E. monechiaeCrux [1991] EprolithusStover [1966] E. floralis [Stradner, 1962] Stover [1966] FlabellitesThierstein [1973] F. oblongus [Bukry, 1969] Crux in Crux et al. [1982] Haquius Roth [1978] H. circumradiatus [Stover, 1966] Roth [1978] HelicolithusNöel [1970] H. trabeculatus [Gòrka, 1957] Verbeek [1977] LithraphiditesDeflandre [1963] L. carniolensisDeflandre [1963] ManivitellaThierstein [1971] M. pemmatoidea (Deflandre in Manivit [1965]) Thierstein [1971] MicrostaurusBlack [1971] M. chiastus [Worsley, 1971] Grün in Grün and Allemann [1975] NannoconusKamptner [1931] Nannoconus truittiiBrönnimann [1955] PercivaliaBukry [1969] P. fenestrata [Worsley, 1971] Wise [1983] PrediscosphaeraRood, Hay and Barnard [1971] P. columnata [Stover, 1966] Perch-Nielsen [1984] P. cretacea [Arkangelskiy, 1912] Gartner [1968] P. spinosa [Bramlette and Martini, 1964] Gartner [1968] RhagodiscusRheinhardt [1967] R. achlyostaurion [Hill, 1976] Doeven [1983] R. angustus [Stradner, 1963] Reinhardt [1971] R. asper [Stradner, 1963] Reinhardt [1967] R. gallagheriRutledge and Bown [1996] R. pseudoangustusCrux [1987] R. splendens [Deflandre, 1953] Verbeek [1977] RucinolithusStover [1966] R. irregularis Thierstein in Roth and Thierstein [1972] R. terebrodentarius Applegate et al. in Covington and Wise [1987] R. terebrodentarius youngiiTremolada and Erba [2002] TranolithusStover [1966] T. orionatus [Reinhardt, 1966] Perch-Nielsen [1968] StaurolithitesCaratini [1963] S. mutterloseiCrux [1989] S. stradneri [Rood et al., 1971] Bown [1998] WatznaueriaReinhardt [1964] W. barnesiae [Black, 1959] Perch Nielsen [1968] W. biportaBukry [1969] W. britannica [Stradner, 1963] Reinhardt [1964] W. manivitaeBukry [1973] W. ovataBukry [1969] W. supracretacea [Reinhardt, 1965] Wind and Wise in Wise and Wind [1976] ZeugrhabdotusReinhardt [1965] Z. diplogrammus (Deflandre in Deflandre and Fert [1954]) Burnett in Gale et al., 1996 Z. elegans [Gartner, 1968] Burnett in Gale et al. [1996] Z. embergeri [Nöel, 1958] Perch-Nielsen [1984] Z. erectus (Deflandre in Deflandre and Fert [1954]) Reinhardt [1965] Z. xenotus [Stover, 1966] Burnett in Gale et al. [1996]


[62] We gratefully acknowledge constructive reviews by T. Bralower, an anonymous reviewer, and Jerry Dickens. D.T. and E.E. were funded through MIUR-PRIN 2007–2007W9B2WE_001. Norman Charnley undertook the isotopic analyses in Oxford.