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The mid-Valanginian Weissert Event as recorded by calcareousnannoplankton in the Vocontian Basin
Emanuela Mattioli a,⁎, Bernard Pittet a, Laurent Riquier b, Vincent Grossi a
a Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement (UMR 5276, CNRS) Université Lyon 1, Ecole Normale Supérieure Lyon, Campus scientifique de la Doua, 69622 Villeurbanne, Franceb Institut des Sciences de la Terre Paris (ISTEP), UMR CNRS 7193 Université Pierre et Marie Curie— Paris 6, 4 Place Jussieu, 75252 Paris Cedex 05, France
Themid-ValanginianWeissert Event represents one of themost significant paleoceanographic events of the EarlyCretaceous and is characterized by a major perturbation of the carbon cycle testified by a positive carbon isotopeshift, a crisis of both neritic and pelagic carbonate producers, and climatic fluctuations. Here we propose apaleoreconstruction of paleoenvironmental changes that occurred in the reference Vergol and La Charce sections(Vocontian Basin, SE France) during theWeissert Event based on the analysis of calcareous nannofossil absoluteabundance and assemblages. These latter are compared to newly acquired and already published sedimentolog-ical and organic geochemical analyses. Our approach is novel for the time interval considered. Indeed, PrincipalComponent Analysis was applied for the first time to the entire nannofossil assemblage to reconstruct environ-mental conditions, instead of using paleoecological preferences of single species. The comparison of calcareousnannofossils and biomarker analyses indicates that a phase of severe sea–water stratification occurred prior tothe carbon positive excursion of theWeissert Event in the Vocontian Basin. Thiswas followed by a raise in fertilityof surface waters, as attested by increased nannofossil abundances, in particular, of meso–eutrophic taxa, andbiomarkers likely produced by dinoflagellates. This increase in fertility was likely triggered by a humid climateand enhanced continental input of clays and nutrients to surface oceanic waters. Calcareous nannofossils alsoproved to react to sea-level changes that occurred in the Valanginian, as inferred bypreviousworks. Species likelyinhabiting proximal areas were recorded in higher proportions during a major sea-level drop in the PeregrinusAmmonite Zone, Late Valanginian. The results of this study also permit to revise previously proposed paleoeco-logical affinities of somenannofossil species.We suggest thatWatznaueria barnesiae, one of themost-widely usedspecies in the literature, should be used cautiously because of its high plasticity with respect to environmentalconditions. Also, nannoconids that are usually regrouped in papers with paleoceanographic purposes shouldbe analyzed separately because they show distinct species-specific ecological preferences.
The Valanginian (139.4–133.9 Ma; Gradstein et al., 2012) recordsthe earliest major perturbation of the global carbon cycle of the Creta-ceous System: the mid-Valanginian Weissert Event (Erba et al., 2004)reflected by δ13Ccarb and δ13Corg positive excursions. The major positivecarbon isotope shift of the Valanginian occurred after a long-lasting pe-riod of relative quiescence during the Late Jurassic-earliest Cretaceous(Erba et al., 2004; Föllmi et al., 2006). The Cretaceous shifts in δ13C arecommonly associated with an enhanced deposition of organic matterin marine sediments, possibly linked to the development of oceanicdysoxic or anoxic conditions. However, the mid-Valanginian WeissertEvent appears to be different from the other Cretaceous δ13C perturba-tions because significant organic matter-rich deposits did not occur in
attioli).
the oceanic realm, and no clear evidence of anoxia is recorded(Westermann et al., 2010; Kujau et al., 2012). The positive shift in δ13Cduring the Valanginian is possibly linked to the deposition of organicmatter as coals in various continental areas (Budyko et al., 1987;Ziegler et al., 1987; McCabe and Totman Parrish, 1992; Rees McAllisteret al., 2004; Westermann et al., 2010), although the age of these de-posits is controversial. These deposits were favored by humid climateconditions (Frakes and Francis, 1988; Frakes et al., 1992; Price et al.,1998; Fesneau et al., 2009) starting a few thousands years before thecarbon isotope shift (Gréselle et al., 2011; Kujau et al., 2012). Such con-ditions occurred in a period of low sea level probably triggering subaer-ial exposure of large epicontinental areas (Gréselle and Pittet, 2010)where vegetation could develop. Humid conditions could have thusfavored 12C sequestration by plants, and organic matter storage in con-tinental environments (Westermann et al., 2010; Kujau et al., 2012). Inaddition, higher trophic levels in shallow seas favored biotic assem-blages producing less carbonate than tropical communities (Funk
et al., 1993; Föllmi et al., 1994; Gréselle and Pittet, 2010; Bonin et al.,2012), thus implying a decrease in the storage capacity of 13C-enriched carbonate carbon on platforms (Föllmi et al., 2006;Westermann et al., 2010). The positive shift in δ13C of the Valanginianoccurred simultaneously to a cooling of marine waters (Podlaha et al.,1998; Pucéat et al., 2003), with the highest δ13C values reached whenthe temperature was the lowest (McArthur et al., 2007; Gréselle et al.,2011; Barbarin et al., 2012). These conditions likely also played a rolein the observed changes of carbonate producing biota in shallowwater environments (Gréselle et al., 2011; Bonin et al., 2012).
Climate changes throughout the Valanginian and the developmentofmore humid conditionshave probably impactedweathering intensityon lands and enhanced the delivery of nutrients to surface oceanic wa-ters (Erba et al., 2004; Duchamp-Alphonse et al., 2007, 2011; Gréselleet al., 2011; Barbarin et al., 2012). Since diatoms did not diversify signif-icantly until the Late Cretaceous (Round et al., 1990), calcareous nanno-plankton and dinoflagellates were among the main primary producersin the oceans during the studied time interval. Fluctuations of trophicconditions in surfacewaters deeply affected the calcareous nannoplank-ton community. Changes in nannoplankton assemblages can thus beused as a proxy to reconstruct paleoenvironmental conditions inmarinesurface waters (Williams and Bralower, 1995; Melinte and Mutterlose,2001; Bersezio et al., 2002; Reboulet et al., 2003; Erba and Tremolada,2004; Erba et al., 2004; Kessels et al., 2006; Duchamp-Alphonse et al.,2007; Bornemann and Mutterlose, 2008).
The paleoecology of a few selected species of calcareous nannofossilsof the Early Cretaceous is relatively well assessed, and fluctuationsin relative abundance of these species served as indicators ofpaleoceanographic conditions (Williams and Bralower, 1995; Melinteand Mutterlose, 2001; Bersezio et al., 2002; Reboulet et al., 2003; Erbaand Tremolada, 2004; Erba et al., 2004; Duchamp-Alphonse et al.,2007; Bornemann and Mutterlose, 2008). However, entire calcareousnannofossil assemblages have been rarely considered in studies ofEarly Cretaceous (Mutterlose, 1996; Herrle et al., 2003; Watkins et al.,2005; Browning and Watkins, 2008). In the present study, changes inthe entire nannofossil assemblage preserved in sediments from theVocontian Basin were investigated. Stratigraphical variations innannofossil assemblages combined with absolute nannofossil abun-dances and fluxes, clay accumulation rates and δ13Ccarb data availablefor the studied sections were used to infer changes in productivityin marine surface waters during the mid-Valanginian WeissertEvent and to characterize the impact of climate changes on primarypaleoproductivity in the Vocontian Basin. The analysis of lipid bio-markers (namely, sterane/hopane and pristane/phytane ratios) inselected samples together with already available data of organic bio-markers helped to support changes in primary productivity and redoxconditions at this site.
2. Geological setting
The Vocontian Basin was a relatively small (about 150 km in width;Cotillon et al., 1980), eastward gulf located along the NW margin ofTethys in the Jurassic and Cretaceous. The basin was bounded to theWest by the Paleozoic “Massif Central” and it was surrounded by car-bonate platforms where carbonate mud was produced and exportedoffshore (Reboulet et al., 2003; Gréselle and Pittet, 2010) (Fig. 1A).The Vocontian Basin has been considered as analogous of the presentEuropean margin of the Atlantic Ocean, composed of a series of tiltedblocks deepening eastwards to the deeper Tethyan Ocean (Lemoine,1984, 1985). Ferry (1990) suggested that the evolution of the areawas linked to an aborted rift basin. In this view, the Vocontian Basinmay be considered as a sort of pull-apart basin. The basin probablyattained its maximum bathymetry (~500–800 m) in the Early Creta-ceous (Hauterivian–Barremian; Wilpshaar et al., 1997; Mattioli et al.,2008).
The studied successions of Vergol and La Charce were located in thecentral part of the Vocontian Basin (Fig. 1B), and are well-dated byammonite and calcareous nannofossil biostratigraphy (Reboulet et al.,1992, 2003; Reboulet, 1996; Reboulet and Atrops, 1999; Gardin, 2008;Barbarin et al., 2012). These two successions can be correlated to eachother, but also to the well-studied parastratotypic section of Angleslocated ca. 100 km south of the analyzed localities (Gréselle et al.,2011). In order to obtain a continuous record of the Valanginian intervaland to avoid local sedimentary perturbations such as slumps, the twosections were combined in a composite section.
The lower Valanginian is characterized by carbonate-rich marl-limestone alternations. A decrease of the carbonate content is observedin the Fuhri horizon of the Biassalense ammonite Subzone (AS)(Fig. 1C). This interval is also characterized by the occurrence of fourcentimeter-thick levels enriched in organic matter, the Barrande layers(Reboulet et al., 2003). These levels, although relatively thin, are record-ed in several sections of the Vocontian Basin and can be consideredas stratigraphical marker beds (Reboulet et al., 2003). The lower–upper Valanginian boundary is identified as the limit between theCampylotoxus and Verrucosum ammonite Zones (AZ), and laysca. 4 m above the NK3A/NK3B nannofossil subzones. The clay-richupper Valanginian interval is interrupted by a 10 m thick carbonate-rich bundle, the “Faisceau Médian”, which is present throughout theVocontian Basin (Cotillon et al., 1980). Sampling of the lower part ofthe succession is in Vergol, starting from the Nicklesi AS, the loggingand sampling were effectuated in the La Charce succession (Fig. 1C).
3. Material and methods
Samples for calcareous nannofossil analysis were taken in marl-limestone alternations starting from the base of the lower ValanginianCampylotoxus AZ up to the lowermost levels of the lower HauterivianRadiatus AZ. Sample spacing was irregular. Contiguous limestone bedsand marly interbeds were sampled at a low frequency (each couple ofsamples taken every meter to 10 m spaced) all along the compositesection (Fig. 1C), but two intervals were sampled with a higher resolu-tion (1 sample every 5 cm), namely: 1) the transition between thecarbonate-rich and the carbonate-poor marl-limestone alternations(Campylotoxus and Biassalense ASs; ~15 m), and 2) the “FaisceauMédian” in the Peregrinus AS (~10 m).
A total of 174 microscope slides were prepared according to therandom-settling technique (Beaufort, 1991; Williams and Bralower,1995; Geisen et al., 1999), slightly modified as described in Olivier et al.(2004). This technique allows absolute quantification of nannofossilsper gram of rock. Briefly, a homogeneous suspension is made with~20 mg of dried rock-powder and water. The suspension is let in a set-tling device during 24 h. After decantation of the powder on a glassslide, water is very slowly evacuated. Once dried, slides are mountedon microscope slides using Rhodopass. Slides were observed under aZEISS AXIOSKOP 40 polarizing light microscope at a magnification of1000×. On average, 320 nannofossils per sample (from 200 up to 516specimens depending on the richness of the sample), both coccolithsand nannoliths, were counted over a variable surface area (0.001 to0.023 cm2) according to the richness of nannofossils in the slide. In onesingle sample, only 123 specimens could be counted, due to the paucityof nannofossils. After counting, a larger surface of the slide was analyzedin order to retrieve very rare species and have a correct estimation ofspecies richness. A total of 110 different taxa were taxonomically deter-mined. Three preservation classes (relatively poor, moderate and good)were recognized on the basis of etching and overgrowth of the speci-mens, as established by Roth (1981).
Principal Component Analysis (PCA) was used to treat and interpretnannofossil assemblages. PCA allows interpreting complex data setsand reducing a large data matrix composed of several variables to asmall number of factors representing the main modes of variations(Beaufort and Heussner, 2001). In order to avoid the closed-sum
Fig. 1. (A) Large-scale and (B) zoom of paleogeographic reconstruction of the western Tethys in the Valanginian and position of the studied transect from the proximal Jura–DauphinoisPlatform to the Vocontian Basin and to the distal margin of the Provence Platform (after Gréselle and Pittet, 2010). (C) Stratigraphic log, sample position, and carbon isotope data from theVergol and La Charce composite section. The interval containing the Barrande Layers and the Weissert event are also shown.
474 E. Mattioli et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 414 (2014) 472–485
problem using percentages of species, these were transformed into logbefore applyingPCA. PCAwas computedwith the StatView5.0 software.The extraction method for eigenvalues was Orthotran/Varimax. Someclusters were created by grouping the species belonging to the samegenus and having significant correlations. Thus, Cyclagelosphaera
margerelii and C. rotaclypeata, Biscutum constans and B. ellipticum,all the species of the genus Rhagodiscus except R. asper, all theZeugrhabdotus, all the Staurolithites, the Cretarhabdus plus Retecapsa,all the Tubidiscus, all the Rucinolithus, and all the Conusphaera(C. mexicana and C. rothii) were considered for PCA. The three species
of Nannoconus (N. steinmannii, N. kampteneri, N. globulus), the most fre-quently and regularly retrieved in the assemblage were also integratedto the analysis. The matrix in our study consists of the log-transformedrelative abundances of 27 taxa representing more than 90% of the totalcounted nannofossils. The rare and discontinuously recorded taxa werenot included in the analysis.
The Shannon Index (H; Shannon and Weaver, 1949) was alsoapplied to the assemblage in order to have a mathematical measureof species diversity in the analyzed nannofossil community. This iscalculated as follows:
SH ¼ −Σ Pi � ln Pi½ �ð Þ
i ¼ 1
where i is a given species, S is the species richness, Pi is the relativeabundance of each species, and N is the number of specimens countedin each sample. The PCA factorial scores and their stratigraphic distribu-tionwere then compared to the diversity index, to absolute nannofossilabundance and flux, to clay accumulation rate and to δ13Ccarb curvesavailable for the studied sections (Hennig et al., 1999; Gréselle et al.,2011; Kujau et al., 2012).
Two samples were selected for Total Organic Carbon content(wt.%TOC) and lipid biomarker analyses. The first one is from theFuhri horizon of the Biassalense AS, in the marls just above the 4thBarrande level in the uppermost part of the lower Valanginian. The sec-ond sample belongs to the interval comprised between the NK3A/NK3Bboundary and the transition between the lower and upper Valanginian,and belongs to the interval of positive excursion of δ13C values (Fig. 1C).The newly acquired data were thus compared to those produced byKujau et al. (2012) (Fig. 1C). The two analyzed samples were dried inan oven at 90 °C for 24 h, then they were powdered in a rotating diskmill. Total carbon (TC, wt.%) was determined using a LECO IR 212 ana-lyzer by combustion of 100 mg of sample at 1050 °C; the released CO2
was quantified by infrared detector. Calcium carbonate content(wt.%CaCO3) was measured by reacting 100 mg of sample with HCl;the released CO2 was determined by pressure measurement. Totalorganic carbon content (wt.%TOC) was calculated by the differencebetween total carbon and carbonate carbon. The precision for theseanalyses is ±0.1% for total organic carbon.
Biomarkers were extracted from powdered sediment (5–7 g) withdichloromethane/methanol (7:1, v/v) using a soxhlet. Asphalteneswere separated from the bulk extract by precipitation overnight withheptane. The resulting maltene fraction was chromatographed over awet packed column of inactivated (4% H20) silica (Kieselgel 60) andthe aliphatic hydrocarbons were eluted with n-hexane. The aliphaticfraction was analyzed by gas chromatography (GC) and GC–mass spec-trometry (GC–MS). In both cases the separation of the compounds wasachieved with a HP-5MS capillary column (30 m × 0.25 mm × 0.25 μm)and the oven GC temperature was programmed from 60 °C (1 min) to130 °C at 20 °C min−1, and then to 300 °C (held 30 min) at 4 °C min−1.Helium was used as a carrier gas at constant flow (1.1 ml min−1). Massspectra of alkanes were obtained using a MD800 Voyager spectrometerinterfaced to an HP6890 gas chromatograph.
4. Results
4.1. Preservation
Nannofossils are generally well-preserved in most of the samples ofthe Vergol section, as recognized under both light (Fig. 2) and scanningelectron microscopes. At La Charce, the preservation is moderate togood. Etching was not observed and, in few samples, some large taxasuch as Haquius showed secondary overgrowth. In some cases, frag-mentation of nannofossils was observed. Tiny and delicate coccoliths,such as representatives of Zeugrhabdotus or Staurolithiteswere observed
with pristine central area structures (Fig. 2). A slightly poorer preserva-tionwas observed in levels B1, B2 and B3 (Fig. 3A and B), probably relat-ed to the higher organic-matter content of these intervals. Speciesrichness is indeed weak in these three levels (Fig. 3B). In the other sam-ples, preservation seems to be quite independent of species richness, ab-solute nannofossil abundance, and calcium carbonate content. In fact,well-preserved samples are observed whatever the calcium carbonatecontent of the rock or the richness in nannofossils (number of speciesand absolute abundance; Fig. 3A and B).
A simple plot of the number of species in a given sample versus thesurface of the slide studied (Fig. 3C) allows distinguishing samples witha high richness (where a surface of 0.0025 cm2 is sufficient to achievethemaximumnumber of species), from thosewhere thenumber of spe-cies does not increase with an increasing surface of the slide studied(i.e., low richness).
4.2. Results of PCA
Three factors were extracted by PCA applied to the log-transformedrelative abundance of nannofossil taxa (Fig. 4). Only species thatcontribute for more than 0.5 to each factor are discussed here. Thefirst PCA factor accounts for 47.7% of the total variance and receivesimportant loading of W. barnesiae on negative values, and of most ofthe coccoliths (B. ellipticum/constans, all Staurolithites, D. lehmanii,Cretarhabdus+ Retecapsa, all Rhagodiscus, R. asper, D. ignotus/rotatoriusand all Zugrhabdotus) and of penthaliths (Micrantolithus obtusus andM. hoschultzii) on positive values. The second PCA factor represents28.6% of the variance, and receives loading of W. ovata, W. britannica,W. fossacincta, all Cyclagelosphaera and Haquius on positive values inopposition to N. globulus. The third factor, accounting for 23.7% ofthe variance, shows P. embergeri, C. cuvillieri and all the Conusphaera(positive values) in opposition with N. steinmannii (negative values).
4.3. Stratigraphic changes of factorial scores
The score of PCA factor 1 shows a stratigraphic trend similar to thatof the absolute abundance and of the species richness (Fig. 5). Namely,fluctuating but relatively low values occur in the lower Valanginian.Values increase from the Platycostatus horizon to the Verrucosum ASacross the lower to upper Valanginian boundary. This trend matcheswith the δ13C positive excursion characterizing the base of theWeissertEvent (Fig. 5). Then a gentle decrease is observed up to the PeregrinusAS, followed by a second increase up to the base of the Furcillata AS.PCA factor 1 displays negative values in the uppermost Valanginian–basal Hauterivian.
PCA factor 2 has positive values in two intervals (Fig. 5). The firstinterval exhibits two peaks in the Fuhri horizon, below and above theBarande Layers, respectively. The second interval is in the PeregrinusAS and contains samples coming from the “Faisceau Médian”. A regulardecrease towards negative values is recorded for the rest of theValanginian up to the base of the Hauterivian. The PCA factor 3 showsslight fluctuations and a single interval of very positive values corre-sponding to the negative δ13C excursion in the Barande Layers (Fig. 5).
4.4. Species richness and Shannon Diversity Index
The interval at the base of the upper Valanginian in which absoluteabundances per gramof rock are the highest corresponds to the samplesof highest species richness (Fig. 5). This richness averages 30–35 speciesper sample in most of the studied interval. In some samples from theVerrucosum AS, a maximum of 54 taxa is recorded. The evolution ofthe Shannon Diversity Index is similar to that of PCA factor 1, showingan increase from the basis of the studied interval (with values of 1.4 to2.2) followed by a major drop in diversity (with values down to 0.8)in the Barrande Layers of the Fuhri horizon (Fig. 5). The diversityincreases again from the Platycostatus horizon to the Pronecostatum
Fig. 2. Plate showing some significant nannofossil taxa typically recorded in the studied samples. White bar = 5 μm.
476 E. Mattioli et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 414 (2014) 472–485
040 60 80 100
goodmoderate
poor
goodmoderate
poor
wt% CaCO3
wt% CaCO3
N = 174
N = 174
3x109
2x109
1x109
Num
ber
of n
anno
foss
ilspe
r g
of r
ock
Spe
cies
ric
hnes
sS
peci
es r
ichn
ess
Preservation
Preservation
40 60 80 100
Barrande levels B1-B3
Barrande levels B1-B3
Barrandelevel B4
Barrandelevel B4
15
20
25
30
35
40
45
50
55
A
N = 174
goodmoderate
poor
Preservation
0 0.005 0.01 0.015 0.02 0.025
Surface of the slide studied(cm )2
15
20
25
30
35
40
45
50
55
C
B
Fig. 3. (A) Crossplot between calcium carbonate content and nannofossil absolute abun-dance (specimens per gram of rock). Samples are splitted by preservation state, namelypoor, moderate and good. The Barrande samples are also shown. (B) Crossplot betweenwt.%CaCO3 and species richness, i.e. the number of species recorded in each sample.(C) Crossplot between the surface of the slide studied and species richness. Samples aresplitted by their preservation state.
AS (with values fluctuating around 2), before showing a long-term,slight decrease until the base of the Hauterivian. Table 1 shows thecorrelations between the three PCA factors, species richness andnannofossil flux.
4.5. Comparison with lipid biomarkers
The analysis of TOC content and lipid biomarkers in two samples, thefirst above the Barrande Layer B4 and the second close to the lower–upper Valanginian boundary (Table 2), was in good agreement withdata reported by Kujau et al. (2012) (Fig. 6). Only the value of thesteranes/hopanes ratio from the sample within the Barrande interval
is slightly lower. The ratio of steranes/hopanes provides informationabout the proportion of eukaryote- versus prokaryote-derived organicmatter (Peters andMoldowan, 2005). Dinosterane, is a compound rath-er specific for dinoflagellates although it can occasionally be producedby diatoms (Rampen et al., 2009). Since diatoms did not diversify signif-icantly before the Late Cretaceous, dinosterane can be assigned to dino-flagellate input in this case. An increase in the ratio of dinosterane overregular steranes is observed from the base of the Barrande interval tothe Verrucosum Subzone upsection indicating a proliferation of thisgroup of algae. The ratio pristane over phytane, which informs on thepaleo-redox regime in the water column during organic matter deposi-tion (Didyk et al., 1978; Peters andMoldowan, 2005), displays a peak intheBarrande Layers interval (Fig. 6). The comparison of PCA factorswiththe organic geochemical data shows a striking co-variation betweenthe dinosterane/reg. sterane ratio and PCA factor 1, and between thepristane/phytane ratio and PCA factor 3 (Fig. 6).
5. Discussion
5.1. Primary vs. diagenetic control on nannofossil assemblages
A careful evaluation of nannofossil preservation in the studied sam-ples was performed in order to discriminate between a primary signal(due to an environmental control on assemblage composition) from adiagenetic pattern. The estimated preservation state varies generallyfrom moderate to good. Poorly preserved samples are few and occurmainly in the Barrande Layers (Fig. 3A and B). The other poorlypreserved samples are, however, quite rich in terms of abundance andspecies richness, and all the encountered nannofossil specimens couldbe identified.
In nannofossil studies, it is generally assumed that preservation isthe best in lithologies with a carbonate content between 40 and 55%(Erba, 1986, 1992; Thierstein and Roth, 1991; Mattioli, 1997). In ourstudy, in spite of the fact that absolute abundance of nannofossils andspecies richness decrease with increasing CaCO3 content (Fig. 3A andB), preservation state is moderate to good even in samples with carbon-ate content higher than 55%. Alternatively to diagenesis, a decreasingnannofossil abundance with increasing CaCO3 may result from dilutionby carbonate material derived from platform environments as sug-gested for the Vergol section by Reboulet et al. (2003).We can thus rea-sonably assume that assemblage changes across the Vergol successionwere mainly controlled by environmental parameters and have notbeen induced by diagenesis.
5.2. Interpretation of the three PCA factors and their stratigraphicalvariations
PCA factor 1 strongly correlates with species richness (Fig. 7A) butonly weakly with the Shannon Index (Fig. 7B). It co-varies also stronglywith nannofossil flux (Fig. 7C) and clay accumulation rate (Fig. 7D) cal-culated by Gréselle et al. (2011). These co-variations (Fig. 7; Table 1)allow the use of PCA factor 1 as an indicator of nannoplankton produc-tivity. This is in agreement with characteristic nannoplanktonic speciesloading on PCA factor 1. Some of these species belong to taxa generallyconsidered as indicators of high trophic (meso- to eutrophic) conditionswithin the oceanic photic zone, namely, Biscutum constans +B. ellipticum, Z. erectus and the small Zeugrhabdotus, Discorhabdus, andD. lehmani (Table 3 and references therein). The interpretation of PCAfactor 1 as reflecting nannoplankton productivity is also consistentwith the positive relationship between this factor and clay accumula-tion that is verified for all the samples except for the Barrande LayersB1–B3 (Fig. 7D). Clay accumulation may translate a variable continent-derived nutrient input. When the Barande Layers B1–B3 are not consid-ered in the correlation, a comparable slope of regression is obtained forthe Barrande interval (Fig. 7D) though the correlation index is weaker.This record suggests that peculiar environmental conditions developed
W. barnesiae (W.bar.)
W. britannica (W.brit.)
W. communis (W.com)
W. manivitiae (W.man.)
W. fossacincta (W.fos.)
W. ovata (W.ova.)
C. margerelii+rotaclypeata (Cycl.)
H. circumradiatus+ellipticus (Haqu.)
N. steinmannii (N.ste.)
N. globulus (N.glo.)
N. kamptneri (N.kam.)
C. cuvillieri (C.cuv.)
all Conusphaera (Conu.)
P. embergeri (P.emb.)
R. asper (R.asp.)
all Zeugrhabdotus (Zeug.)
B. ellipticum+constans (Bisc.)
D. lehmanii (D.leh.)
D. subbeticus (D.sub.)
D. rotatorius (D.rot.)
all Rucinolithus (Ruci.)
Rhagodiscus without R. asper (Rhag.)
all Staurolithites (Stau.)
Pentaliths (pent.)
all Tubidiscus (Tubi.)
C. oblongata (C.obl.)
Cretarhabdus+Retecapsa (Creta.+Rete.)
-.748 -.315 .261
.071 .495 .193
-.133 -.051 .429
-.330 -.233 .092
-.065 .494 -.042
-.090 .662 .251
-.308 .462 -.205
-.141 .542 -.109
-.063 .392 -.553
.009 -.542 .108
-.323 .089 .374
.087 .032 .693
-.031 .089 .540
.031 .025 .551
.570 -.003 -.202
.565 -.112 -.031
.760 -.442 -.062
.700 -.286 .065
.454 -.505 -.008
.553 -.541 -.215
.103 .405 -.476
.582 .329 .257
.756 .105 .216
.681 -.259 -.213
.434 -.139 .051
.176 -.073 -.018
.646 -.040 .339
PCA1 PCA2 PCA3Log of relative abundance
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
PCA1(47.7% of the variance)
PCA1(47.7% of the variance)
PC
A2
(28.
6% o
f the
var
ianc
e)P
CA
3(2
3.7%
of t
he v
aria
nce)
W.bar. D.leh.
D.leh.
pent.
pent. Bisc.
Bisc.
R.asp.
R.asp.
Zeug.
Zeug.
Tubi.
Tubi.
D.rot.
D.rot.
D.sub.
D.sub.
Creta.+Rete.
Creta.+Rete.
Stau.
Stau.
Rhag.
Rhag.
C.obl.
C.obl.
N.glo.
N.glo.
P.emb.
P.emb.
C.cuv.
C.cuv.Conu.
Conu.
W.com.
W.com.
W.man.
W.man.
N.kam.
N.kam.
N.ste.
N.ste.
Ruci.
Ruci.
Cycl.
Cycl.
W.fos.
W.bri.
W.bri.
Haqu.
Haqu.
W.ova.
W.ova.W.bar.
W.fos.
Fig. 4. Results of Principal Component Analysis applied to the log value of relative abundance (percentage) of the listed 27 nannofossil taxa. Plots of the three PCA factors obtained. Therelative contribution to the total variance is shown for each PCA factor. Shaded area indicates the species contributing for less than 0.5 to the extracted factors. These species have notbeen used for paleoenvironmental reconstructions.
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during the Barande interval. As PCA factor1 is interpreted as an indicatorof nannoplankton productivity, lower PCA1 scores in the Barrandeinterval than in the other intervals for a same clay accumulation rateindicate lower productivity in this interval.
Other taxa such as the Rhagodiscus and R. asper (that was consideredseparately), the pool Cretarhabdus-Retecapsa, the Micrantholithus(pentaliths) and Staurolitithes also load on positive values of PCA factor1. Although no clear ecological affinities have been established for mostof these taxa, R. asper and Micrantholithus are rather considered asadapted to warm surface waters, while Staurolitithes, and in particularS. stradneri, are supposed to thrive in cold waters (Table 2). It is there-fore intriguing that those taxa with apparently different ecologicalpreferences were flourishing at the same time.
W. barnesiae is the only species to load on the negative values of PCAfactor 1. This pattern can be explained as the result of a relative decreaseof the percentage ofW. barnesiae and, conversely, a relative increase inthe abundance of the species loading on positive values of PCA factor 1that produced such an opposition on PCA factor 1. W. barnesiae wasapparently successfully inhabiting all Mesozoic marine environments(Table 3).
The concomitant increase of PCA factor 1 along with nannofossil ab-solute abundances, species richness, δ13Ccarb and dinosterane (Figs. 5, 6and 7) from the Barrande layers interval up to the Verrucosum AS indi-cates an increase in the production of the two main phytoplanktonicgroups of the Valanginian, namely calcareous nannoplankton and dino-flagellates. PCA factor 1 further indicates that surfacewater fertility wasfluctuating during this period, reaching a peak across the lower–upperValanginian boundary, but slightly decreasing in the “FaisceauMédian”of the Peregrinus AS, and in the Furcillata AZ.
The PCA factor 2 receives significant contribution from differentspecies of Watznaueria, Cyclagelosphaera and Haquius (Fig. 4). The twointervals where PCA2 shows the highest values correspond to majorsea-level falls, one occurring at the base of the Campylotoxus AZ (Haqet al., 1988; Gréselle and Pittet, 2010), and the other at the base of thePeregrinus AZ. These sea-level falls forced carbonate ramp systems toprograde basinwards for more than 100 km (Gréselle and Pittet,2010). The decrease in PCA values in the upper part of the section canthus be explained by a relative sea-level rise, related to a major carbon-ate platform re-flooding in the latest Valanginian (Gréselle and Pittet,2010; Bonin et al., 2012).
PCA factor 3 typically characterizes the interval containing theBarrande Layers and corresponds to maximum values of the pristane/phytane ratio (Fig. 5). This pattern likely indicates enhanced stratifica-tion of the water column in the Barrande interval (Kujau et al., 2012),as previously suggested by Reboulet et al. (2003) on the basis of sedi-mentological and paleontological data. Water stratification likely pro-duced a poor ventilation of the basin and favored the development ofdysoxic/anoxic conditions during the deposition of these organicmatter(OM)-rich levels (Westermann et al., 2010).
5.3. Paleoecology of calcareous nannofossil taxa
This study allows us to refine the paleoecology of some nannofossiltaxa. Table 3 summarizes the main information provided in the litera-ture on the ecological preferences of the species loading on the threePCA factors extracted in this work. Significant discrepancies exist inthe interpretation of the ecological preferences of some taxa, such asR. asper, Nannoconus or W. barnesiae.
Fig. 5. Stratigraphic variations of nannofossil absolute abundance, species richness, ShannonDiversity Index, and the three PCA factors. Comparisonswith δ13C curve. The Barrande intervaland Weissert Event are also shown.
PCA1 gathers many taxa that are often interpreted as typical of highproductivity conditions in surface oceanic waters, and thus adapted toeutrophic conditions. This is especially the case for small coccolithtaxa such as Biscutum, Zeugrhabdotus,Discorhabdus andDiazomatolithus(Table 3) and for other small taxa like Staurolithithes and some
Table 1Correlation indexes between the three PCA factors obtained when analyzing nannofossilassemblages, clay accumulation rates, nannofossil fluxes (specimens per square meter andper year) and species richness. Statistically significant correlation indexes are shown in bold.
Rhagodiscus. Although some larger taxa like Cretarhabdus, Retecapsa orthe pentaliths contribute to this PCA factor, the dominance of small-sized coccoliths characterizes marine environments with high trophiclevels. A similar pattern is observed in contemporary living assemblages(Young, 1994).
These taxa are opposed to W. barnesiae on PCA1. This opposition isinterpreted as the passive consequence of an increase of species posi-tively contributing to PCA1, which induces a relative decrease of theproportion of W. barnesiae in the assemblage. This suggests thatW. barnesiae is less dependent on the trophic level than most of theother coccoliths and that it can thus be considered as a eurytrophic spe-cies. This conciliates the different interpretations given in the literature,associating W. barnesiae to a wide range of trophic conditions, fromoligotrophic to eutrophic (Table 3). In fact, the relative abundance ofW. barnesiae seems essentially resulting from the fluctuations of taxamore sensitive to environmental conditions. Thus, relative changes inabundance of W. barnesiae cannot be confidently used to interpretpaleoenvironmental changes.
Table 2Results of the analysis of organic matter and of lipidic compounds in two samples taken, respectively, above the 4th Barrande Layer and close to the lower/upper Valanginian boundary.
Samples Hopanes Steranes S Steranes S Hopanes S/H Pr/Ph %TOC δ15Nave δ13Cave
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Remarkably, the PCA2 factor seems to be related to variations in sealevel. Species loading on positive values are more abundant during sea-level falls, while species contributing to negative values are more com-mon during sea-level high stands. This suggests that species contribut-ing to positive values on PCA2 were inhabiting proximal areas. Byextension, Haquius can also be associated to proximal environments.Accordingly, C. margereii and W. britannica are regularly considered tothrive and dominate neritic or even lagoon environments (Table 2).Opposed to these taxa, N. globulus likely indicates more distal condi-tions, or a deeper habitat within the photic zone. All nannoconids areoften interpreted as deep-dwellers (Table 3). If this seems indeed plau-sible for N. globulus, according to the data presented here, the variationsin abundance of N. steinmannii are different from those of N. globulus.Also, N. steinmanni contributes mostly to another PCA factor (PCA3).
PCA3 shows in opposition N. steinmanni and species typically abun-dant in the interval containing the OM-rich Barrande Layers that result-ed from a stratified water column favoring dysoxic/anoxic conditionsnear the seafloor (Reboulet et al., 2003; Westermann et al., 2010;Kujau et al., 2012). The paucity of N. steinmanni under such conditionsis not consistent with a deep-dweller mode of life. Indeed, deep-dwellers likely develop when nutricline is deep in the water column,like under stratified conditions. Different paleoenvironmental prefer-ences were already proposed for various species of Nannoconus(Barbarin et al., 2012; Table 3).
The coccoliths of the C. cuvillieri, P. embergeri and Conusphaera taxa,which contribute positively to PCA3, are here interpreted as deep-dwellers because they proliferate preferentially during periods ofenhanced stratification of the water column. Interestingly, these aretypically robust coccoliths. Thick coccoliths may increase the mass ofthe coccosphere, thus playing a role in the regulation of the depth inwhich the organism is living (Young, 1994). Also, thick calcite elementsmay enhance light availability within the cell, because calcite has a re-fraction index higher than water (Gartner and Bukry, 1969). This is animportant ecological mechanism, which would enable a species livingin the deep photic zone to accumulate enough light for photosynthesis.It is noticeable that C. rothi possesses a morphology very close to that ofthe Jurassic species Mitrolithus jansae, which is interpreted by variousauthors as a deep-dweller (Erba, 2004; Mattioli and Pittet, 2004).
5.4. Paleoenvironmental changes during the Weissert Event
Based on the above interpretations, stratigraphical changes in thescores of the three extracted PCA factors (Fig. 5) can be used to discussthe paleoenvironmental evolution from the late Early to the latestValanginan. PCA1 retraces changes through time of the trophic condi-tions. Intermediate trophic conditions characterize the lowermostCampylotoxus AZ with two intervals of oligotrophic conditions, notablyduring the deposition of the OM-rich Barrande Layers, as already sug-gested by Reboulet et al. (2003). This interpretation is consistent withthe record of a relatively low dinosterane/reg. sterane ratio in theBarrande Layers (Fig. 6). As the steranes/hopanes ratio is relativelyhigh in the Barrande Layers (Fig. 6), prokaryote producers (presumablycyanobacteria; Farrimond et al., 2004; Dumitrescu and Brassel, 2005)were likely even rarer than eukaryote algae mainly represented by nan-noplankton and dinoflagellates in the studied interval. Furthermore, lowclay accumulation rates may suggest low clay and nutrient inputs fromemerged lands (Gréselle et al., 2011). In the uppermost CampylotoxusAZ and Verrucosum AS, PCA1 scores increase simultaneously with bulk
δ13C values of carbonates (Fig. 5) and with clay accumulation rate(Gréselle et al., 2011). This suggests relatively high trophic conditionsduring the positive shift in δ13C, and a possible link between this shiftand an increase in productivity of both calcareous nannoplankton anddinoflagellates in marine surface waters. The concomitant increasein terrigenous input and primary productivity was also observed inother localities of the Vocontian Basin (Duchamp-Alphonse et al.,2007). This is also in agreement with the increase in the Detrital Index(DI) and Weathering Index (WI) established by Duchamp-Alphonseet al. (2011). The decrease of PCA1 from the mid-Verrucosum AS to thebase of the Peregrinus AS (“Faisceau Médian”), where it has generallynegative values (Fig. 5), suggests relatively oligotrophic conditions insurface waters. This trend is also contemporaneous with the decreasein clay accumulation rate calculated by Gréselle et al. (2011) and of theDI andWI of Duchamp-Alphonse et al. (2011). Upsection, PCA1 changesfrom moderately positive values likely corresponding to mesotrophicconditions, to moderately negative values that probably indicate oligo-trophic conditions. Duchamp-Alphonse et al. (2007) inferred a similarevolution pattern of trophic levels.
PCA2 represents the relative abundance of taxa inhabiting proximal(positive values) vs. distal (negative values) environments. Changes inPCA2 values thus possibly translate the relative displacement of thecoastline, and then transgressive–regressive cycles in the VocontianBasin. These cycles (Fig. 5) can be compared to the recently publishedsequence-stratigraphic interpretations of the Valanginian in theVocontian Basin (Gréselle and Pittet, 2010). This is especially true forthe two major sea-level falls identified by Gréselle and Pittet (2010)(their 3rd order sequence boundaries F and G; Fig. 5) that correspondto the highest values of PCA2 observed in the Campylotoxus AZ and inthe Peregrinus AZ (Faisceau Médian).
The first positive peak of PCA2 is interrupted by an interval of lowvalues that corresponds to a peak of PCA3 values (Fig. 5). This last factoris typically high in the interval containing the Barrande Layers. TheseOM-rich levels are interpreted as being deposited during short-lastingepisodes of reduced accumulation rate and stratification of the watercolumn (Reboulet et al., 2003) that promoted dysoxic/anoxic conditionsclose to the sea floor (Westermann et al., 2010), as attested by anincrease of the pristane/phytane ratio (Fig. 6; see also Kujau et al.,2012). This interval also corresponds to an episode of low nutrient con-tent in surface waters as suggested by low values of PCA1. In the Vergolsection, this is the unique interval where PCA3 shows high values(Fig. 5) thus indicating a stratified water column. Water column strati-fication did not occur in the interval of theWeissert Event. This patternis consistent with the lack of anoxic/dysoxic conditions in the VocontianBasin during the positive carbon isotope excursion (Kujau et al., 2012).
Higher trophic conditions during theWeissert Event are also support-ed by nannofossil record from other Tethyan settings like Southern Alps(Bersezio et al., 2002; Erba and Tremolada, 2004), as well as the AtlanticBornemann and Mutterlose (2008) and Pacific Oceans (Erba et al.,2004). Concomitant to the development of more humid climate condi-tions at low to intermediate paleolatitudes (Price et al., 1998), a wide-spread ocean eutrophication, occurring at least at low latitudes seems tohave constituted a major feature characterizing the Weissert Event.
6. Summary and conclusions
The integration of calcareous nannofossil data with already pub-lished and new organic geochemical and sedimentary data allows a
Fig. 6. Comparison between carbon isotope data, organic matter and lipidic compound analyses, and PCA analysis of calcareous nannofossil assemblages. The new data of wt.%TOC, s nes/hopane and pristane/phytane ratios measured in this workare compared to the previous data of Kujau et al. (2012) obtained for the same section.
Barrande interval without Barrande Layers; R2 = 0.433; p<0.0001
C
A B
D
Barrandelayers B1-B3
B4
-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
-3
-2.5
-2
-1.5
-1
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0
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1
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2
2.5
-3
-2.5
-2
-1.5
-1
-0.5
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Fig. 7. Crossplot between PCA factor 1, nannofossil species richness (A), Shannon Diversity Index (B), fluxes (billions of specimens per square meters and per year; C), and clay accumu-lation rates (μmper year; D). Nannofossilfluxes and clay accumulation rates are fromGréselle et al. (2011). (D)Different R2were calculated including or excluding theBarrande samples orinterval.
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better comprehension of paleoenvironmental changes occurred duringthe mid-Valanginian Weissert Event. The most relevant pattern we ob-served is related to fertility conditions in surface waters. All of the ana-lyzed parameters indicate that a short-lived but major crash in primaryproduction occurred during the deposition of the OM-rich BarrandeLayers. Indeed, nannofossil abundance and richness are especially lowin this interval, aswell as dinosterane/reg. sterane attesting for low pro-ductivity of the major phytoplanktonic groups at that time. Organicmatter accumulation in the Barrande Layers was mainly due to preser-vation related to water mass stratification, as attested by highpristane/phytane ratios and peaks in the abundance of robust coccoliths(P. embergeri, C. cuvillieri and Conusphaera) likely thriving in the lowerphotic zone when nutricline was deep.
Eventually, a long-lasting event of ocean fertilization occurred, notonly within the Vocontian Basin, but also in Atlantic and Pacific Oceansat low paleolatitudes. This oceanic fertilization occurred in times of highdetrital input from continents related to a humid climate (Fesneau et al.,2009; Gréselle et al., 2011). The increase in trophic levels is supportedby increasing nannofossil abundances and by assemblages dominatedby small coccoliths (like Biscutum, Zeugrhabdotus, Discorhabdus,Diazomatolithus, Staurolithithes and some Rhagodiscus). High levelsof dinosterane are concomitantly recorded. The positive excursion in
carbon stable isotope characterizing the Weissert Event is thereforerelated to this enhanced phytoplanktonic production.
Nannofossil assemblages also fluctuate in response to sea-levelchanges as inferred by previous authors (Gréselle and Pittet, 2010;Gréselle et al., 2011; Bonin et al., 2012). Some taxa like Haquius,C. margereii andW. britannicaweremore abundantly recorded in the as-semblages in times of sea-level falls, and converselyN. globulus seems toindicate relativelymore distal environments in times of higher sea level.
This study investigates the entire nannofossil community by meansof PCAmethods, and not only species-specific abundance as it is usuallydone in paleoenvironmental studies. We show that some taxa likeBiscutum, Zeugrhabdotus, Discorhabdus can be considered as indicativeof high trophic levels in surface waters. Interpretation of the ecologyof other taxa is, however, much more difficult. In particular,W. barnesiae that is one of the most debated species in the literaturefor its ecological preferences, should be probably used with parsimonybecause of its high plasticity with respect to environmental conditions.Also, nannoconids that are commonly described as a deep-dweller oroligotrophic group (see Table 3), should be analyzed separately speciesby species, because each one shows peculiar paleoenvironmental pref-erence. The use of some nannofossil taxa in paleoceanographic recon-structions thus needs to be critically revised.
Table 3Main ecological preferences of lower Cretaceous nannofossil taxa according to the literature concerned. The reported ecological preferences follow the definitions given in the referredpaper. Numbers refer to the papers listed below, while in bold are reported the interpretations used in the present paper. 1. Bersezio et al. (2002); 2. Bornemann and Mutterlose(2008); 3. Bornemann et al. (2003); 4. Bown et al. (1998); 5. Bralower et al. (1993); 6. Busson and Noël (1991); 7. Busson et al. (1992); 8. Coccioni et al. (1992); 9. Crux (1989); 10. Du-champ-Alphonse et al. (2007); 11. Erba (1992); 12. Erba (1994); 13. Erba and Tremolada (2004); 14. Erba et al. (1992); 15. Erba (2004); 16. Giraud et al. (2003); 17. Heimhofer et al.(2006); 18. Herrle et al. (2003); 19. Herrle (2003); 20. Lees et al. (2004); 21. Lees et al. (2005); 22. Melinte and Mutterlose (2001); 23. Mutterlose and Kessels (2000); 24. Mutterloseet al. (2003); 25. Mutterlose et al. (2005); 26. Mutterlose (1987, 1989); 27. Pauly et al. (2012); 28. Pittet and Mattioli (2002); 29. Premoli Silva et al. (1989); 30. Roth and Bowdler(1981); 31. Roth and Krumbach (1986); 32. Roth (1981); 33. Scarparo Cunha and Shimabukuro (1997); 34. Street and Bown (2000); 35. Tribovillard et al. (1992); 36. Watkins (1989);37. Williams and Bralower (1995).
Nannofossil taxa Trophic level/productivity Temperature Distribution Water depthhabitat
Other ecological preference
Biscutum spp. High productivity10,31,32 Cold waters30,31
Biscutum constans# High productivity2,12,14,15,23,26,28,30,31,36 Cold waters18,22 Boreal Realm22
We wish to thank the Editor, Finn Surlyk, and two anonymousreviewers for valuable comments to our manuscript. We are gratefulto Stéphane Reboulet (Université Lyon 1) for discussions and enjoyable
fieldtrip. François Baudin (Université Paris 6) is warmly thanked forthe analysis of wt.%TOC in two samples. Slides for nannofossil analysisare housed in the paleontological collections of the University ClaudeBernard Lyon 1, Villeurbanne, France (FSL). This study was funded bythe INSU-CNRS French programs ‘ECLIPSE II’, Syster and Interrvie.
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