Introduction
The Aalenian–Bajocian (Middle Jurassic, ca. 170 Ma) is known for many important geological features. The Ligurian-Piemontese Basin began its opening (Bill et al. Reference Bill, O’Dogherty, Guex, Baumgartner and Masson2001), the spreading rate of Central Atlantic Ocean intensified (Labails et al. Reference Labails, Olivet, Aslanian and Roest2010), and the Pacific plate started to evolve from the triple junction between the Izanagi, Farallon, and Phoenix plates (Nakanishi et al. Reference Nakanishi, Tamaki and Kobayashi1992; Bartolini and Larson Reference Bartolini and Larson2001) at the origin of a shift of strontium isotope toward unradiogenic values (data compiled by Jenkyns et al. [Reference Jenkyns, Jones, Gröcke, Hesselbo and Parkinson2002]). This new plate production is related to subduction on the border of the Pacific Superocean inducing volcanism (Yin et al. Reference Yin, Pei, Gou and Jiang1998; Pankhurst et al. Reference Pankhurst, Riley, Fanning and Kelley2000; Bartolini and Larson Reference Bartolini and Larson2001). A bulk carbonate δ13C negative perturbation was recorded in Italy (Bartolini et al. Reference Bartolini, Baumgartner and Hunziker1996), Spain (O’Dogherty et al. Reference O’Dogherty, Sandoval, Bartolini, Bruchez, Bill and Guex2006), and Portugal (Suchéras-Marx et al. Reference Suchéras-Marx, Guihou, Giraud, Lécuyer, Allemand, Pittet and Mattioli2012). This negative excursion is also observed in δ13C coal deposits of Yorkshire indicating input of 12C into the atmospheric reservoir (Hesselbo et al. Reference Hesselbo, Morgans-Bell, McElwain, Rees, Robinson and Ross2003). Meanwhile, leaf stomatal density results suggest an important increase in pCO2 (Hesselbo et al. Reference Hesselbo, Morgans-Bell, McElwain, Rees, Robinson and Ross2003). Together, these observations suggest an increase in volcanic activity. Following the Aalenian–Bajocian negative excursion, a δ13C positive excursion is observed during the early Bajocian in Italy (Bartolini et al. Reference Bartolini, Baumgartner and Hunziker1996), Spain (O’Dogherty et al. Reference O’Dogherty, Sandoval, Bartolini, Bruchez, Bill and Guex2006), Portugal (Suchéras-Marx et al. Reference Suchéras-Marx, Guihou, Giraud, Lécuyer, Allemand, Pittet and Mattioli2012), northern France (Brigaud et al. Reference Brigaud, Durlet, Deconinck, Vincent, Pucéat, Thierry and Trouiller2009), and southern France (Suchéras-Marx et al. Reference Suchéras-Marx, Giraud, Fernandez, Pittet, Lécuyer, Olivero and Mattioli2013). This second event has not yet been observed in terrestrial material. It has been proposed that the δ13C positive excursion of the early Bajocian is linked to an increase in oceanic primary productivity due to eutrophication caused by nutrient input (Bartolini et al. Reference Bartolini, Baumgartner and Hunziker1996; Bartolini and Cecca Reference Bartolini and Cecca1999).
In the same time interval, various paleontological changes are observed. The most striking change is a major turnover in ammonite fauna, in which the latest Aalenian–early Bajocian ammonites ancestors of the Middle and Late Jurassic faunas, replace typical Early Jurassic faunas (O’Dogherty et al. Reference O’Dogherty, Sandoval, Bartolini, Bruchez, Bill and Guex2006). A parallel increase in radiolarian deposits is recorded in the Tethys Ocean, as well as a slight trend toward diversification, related to the increase in primary production during the δ13C positive excursion (Bartolini et al. Reference Bartolini, Baumgartner and Guex1999). Eventually, calcareous nannofossils also diversified (Roth Reference Roth1987; Mattioli and Erba Reference Mattioli and Erba1999; Bown Reference Bown2005). In the Mesozoic history of coccoliths, which are micrometric calcite platelets produced by planktic coccolithophorid algae, one highly successful genus, Watznaueria, appeared during the Toarcian (Early Jurassic, 183-174 Ma; Cobianchi et al. Reference Cobianchi, Erba and Pirini-Radrizzani1992; Mattioli Reference Mattioli1996) and diversified during the Bajocian (170-168 Ma; Cobianchi et al. Reference Cobianchi, Erba and Pirini-Radrizzani1992; Mattioli and Erba Reference Mattioli and Erba1999; Erba Reference Erba2006; Tiraboschi and Erba Reference Tiraboschi and Erba2010). Watznaueria species dominated the pelagic realm for ~70 Myr, from the Middle Jurassic until the end of the Early Cretaceous, and disappeared after the K/Pg boundary (Erba Reference Erba2006; Bernaola and Monechi Reference Bernaola and Monechi2007). Nevertheless, this major evolutionary event within the most important Mesozoic pelagic carbonate producers in the oceans remains poorly documented in term of the dynamics of relative and absolute abundances (most of the works published so far focused on nannofossil taxonomy and biostratigraphy [Cobianchi et al. Reference Cobianchi, Erba and Pirini-Radrizzani1992; Mattioli and Erba Reference Mattioli and Erba1999; Tiraboschi and Erba Reference Tiraboschi and Erba2010]) with some exceptions (Aguado et al. Reference Aguado, O’Dogherty and Sandoval2008; Suchéras-Marx et al. Reference Suchéras-Marx, Guihou, Giraud, Lécuyer, Allemand, Pittet and Mattioli2012).
In this study, we quantify absolute and relative abundance of calcareous nannofossils in the latest Aalenian–early Bajocian time interval in order to estimate the effect of Watznaueria diversification on the calcareous nannofossil assemblages and its relation to the δ13C positive excursion of the early Bajocian. We focus on deposits from Cabo Mondego (Portugal), the Aalenian/Bajocian GSSP (Global Stratotype Section and Point; Pavia and Enay Reference Pavia and Enay1997), and Chaudon-Norante (SE France), the only early Bajocian time-calibrated section based on cyclostratigraphy (Suchéras-Marx et al. Reference Suchéras-Marx, Giraud, Fernandez, Pittet, Lécuyer, Olivero and Mattioli2013).
Geological Settings
Cabo Mondego
The Cabo Mondego section is located in the Lusitanian Basin (Fig. 1), on the western Atlantic coast of Portugal, near Figueira da Foz. The succession is represented by marine deposits of Upper Toarcian to Kimmeridgian age (Ruget-Perrot Reference Ruget-Perrot1961). Cabo Mondego is the Global Stratotype Section and Point (GSSP) for the Aalenian/Bajocian boundary (Pavia and Enay Reference Pavia and Enay1997) as well as the Auxiliary Stratotype Section and Point (ASSP) for the Bajocian/Bathonian boundary (Fernandez-Lopez et al. Reference Fernandez-Lopez, Pavia, Erba, Guiomar, Henriques, Lanza, Mangold, Morton, Olivero and Tiraboschi2009). Numerous ammonites have been collected throughout the succession, allowing the establishment of a detailed biostratigraphical framework (Henriques et al. Reference Henriques, Gardin, Gomes, Soares, Rocha, Marques, Lapa and Montenegro1994).
Paleogeographic distribution of oceans and lands during the Middle Jurassic (after Blakey Reference Blakey2008). On the left, a global view with subduction zones and mid-ocean ridges; on the right, focus on the western Tethys with the locations of Cabo Mondego in the Lusitanian Basin and Chaudon-Norante in the French Subalpine Basin.
The studied part of the Cabo Mondego section extends from the latest Aalenian (Concavum ammonite zone) to the end of the lower Bajocian (base of the Humphriesianum ammonite zone) (Fernandez-Lopez et al. Reference Fernandez-Lopez, Henriques, Mouterde, Rocha and Sadki1988). The early Bajocian is divided into four ammonite zones, namely Discites, Laeviuscula, Propinquans (formerly Sauzei zone), and Humphriesianum. Nannofossil zones (“Nanno. zone” in Figs. 3–6) follow Mattioli and Erba (Reference Mattioli and Erba1999), using the W. britannica occurrence for NTJ9 and W. manivitiae for NTJ10. The Aalenian NTJ8 zone is confirmed thanks to the presence of W. contracta since the base of the studied interval. The nannofossil zones proposed are partly in agreement with Henriques et al. (Reference Henriques, Gardin, Gomes, Soares, Rocha, Marques, Lapa and Montenegro1994), the major difference being in the occurrence of W. britannica in the Aalenian in our observations whereas Henriques et al. (Reference Henriques, Gardin, Gomes, Soares, Rocha, Marques, Lapa and Montenegro1994) observed it in the early Bajocian (Discites ammonite zone). The sedimentary succession consists of alternating marlstone and limestone (Fig. 2); the carbonate fraction is exclusively micritic or microsparitic calcite (Henriques et al. Reference Henriques, Gardin, Gomes, Soares, Rocha, Marques, Lapa and Montenegro1994; Canales and Henriques Reference Canales and Henriques2008). Abundant and diverse benthic foraminiferal assemblages were observed in the middle Aalenian–lower Bajocian limestone-marl alternations, suggesting a distal ramp paleoenvironment (Canales and Henriques Reference Canales and Henriques2013). The sediments corresponding to the Concavum (~5.5 m thick) and Discites (~7.2 m thick) ammonite zones are characterized by irregular nodular bedding but fairly regular alternations of ~20 cm argillaceous limestone and marlstone beds. The interval corresponding to the base of the Laeviuscula (~36 m thick) ammonite zone is limestone-dominated. At the base of the Propinquans (~32 m thick) ammonite zone, the argillaceous limestone beds become more regular and thicker in comparison to the base of the section through the Humphriesianum (~7 m) ammonite zone. From the Propinquans ammonite zone, the succession becomes limestone-dominated (Suchéras-Marx et al. Reference Suchéras-Marx, Guihou, Giraud, Lécuyer, Allemand, Pittet and Mattioli2012).
Calcareous nannofossils photographs from Cabo-Mondego (CM) and Chaudon-Norante (CN) under cross-polarized light. 1. Schizosphaerella sp. CM67; 2. Biscutum dubium CM26; 3. Similiscutum precarium CM17; 4. Biscutum depravatum CM62; 5. Discorhabdus criotus CM62; 6. Discorhabdus ignotus CM26; 7. Discorhabdus striatus CM26; 8. Lotharingius barozii CN420; 9. Lotharingius crucicentralis CN220; 10. Lotharingius velatus CM62; 11. Crepidolithus crassus CN380; 12. Tubirhabdus patulus CM60 (large specimen); 13. Retecapsa incompta CN1680; 14. Carinolithus magharensis CM26; 15. Calyculus sp. CN260; 16. Watznaueria fossacincta CM62; 17. Watznaueria colacicchii CN200; 18. Watznaueria contracta CM26; 19. Watznaueria aff. contracta CM17; 20. Watznaueria aff. contracta CN550; 21. Watznaueria britannica E CM60; 22. Watznaueria britannica F CM62; 23. Watznaueria communis CM62 (large specimen); 24. Watznaueria aff. manivitiae CM60; 25. Watznaueria sp. lateral view CM26.
The Cabo Mondego section showing the δ13C (Suchéras-Marx et al. Reference Suchéras-Marx, Guihou, Giraud, Lécuyer, Allemand, Pittet and Mattioli2012), nannofossil absolute abundance (gray), and nannofossil flux (black) based on time estimation of early Bajocian ammonite zones (Suchéras-Marx et al. Reference Suchéras-Marx, Giraud, Fernandez, Pittet, Lécuyer, Olivero and Mattioli2013); Schizosphaerella spp. absolute (gray) and relative (black) abundances in the nannofossil assemblage; Watznaueria spp., Lotharingius spp., Discorhabdus spp., Biscutum spp. + Similiscutum spp.; and Carinolithus spp. absolute (gray) and relative (black) abundances in the coccolith assemblage. Ammonite biostratigraphy is based on Fernandez-Lopez et al. (Reference Fernandez-Lopez, Henriques, Mouterde, Rocha and Sadki1988) and nannofossil biostratigraphy derived from this study.
The Chaudon-Norante section. Details as in Figure 3. Ammonite biostratigraphy is based on Pavia (Reference Pavia1983) and nannofossil biostratigraphy derived from this study.
The Cabo Mondego section showing the δ13C (Suchéras-Marx et al. Reference Suchéras-Marx, Guihou, Giraud, Lécuyer, Allemand, Pittet and Mattioli2012) and the W. colacicchii, W. contracta, W. aff. contracta, W. britannica and W. aff. manivitiae absolute (gray) and relative (black) abundances in the coccolith assemblage.
Chaudon-Norante
The Chaudon-Norante section is located in the Ravin de Coueste (Digne tectonic nappe, Southern Alps), which was emplaced during the Mio-Pliocene (Lemoine Reference Lemoine1973; Gidon and Pairis Reference Gidon and Pairis1992). It presents a continuous succession from the Toarcian (Early Jurassic) to the base of the Bathonian (Middle Jurassic). This section belongs to the French Subalpine Basin, which was bounded northward by the Jura platform, westward by the Central Massif and the Ardèche platform, and southward by the Provence platform (Fig. 1). The Chaudon-Norante succession is well exposed and the ammonite biostratigraphy for the Bajocian (Pavia Reference Pavia1973, 1983) has high temporal resolution. Nannofossil zones follow Mattioli and Erba (Reference Mattioli and Erba1999), using the W. contracta occurrence for NTJ8, W. britannica for NTJ9, and W. manivitiae for NTJ10. The nannofossil zones proposed here agree with Erba (Reference Erba1990).
The part of the section studied here extends from the end of the Aalenian to the end of the lower Bajocian. It is represented by decimetric hemipelagic marlstone-limestone alternations. The limestones are mainly wackestones to packstones with some bioclastic remains of Bositra (Bivalvia), radiolarians, rare benthic foraminifera, and rare siliceous sponge spicules (Pavia Reference Pavia1983). The top of the Aalenian is dated to the upper part of the Concavum ammonite zone. This 37.9-m-thick interval is composed of fairly regular marlstone-limestone alternations except for the uppermost part, which is marl-dominated. The 131.1-m-thick lower Bajocian succession corresponds to four ammonite zones. From base to top, the Discites ammonite zone is dominated by marlstones, the Laeviuscula ammonite zone displays regular marlstone-limestone alternations, the Propinquans ammonite zone is limestone-dominated, and the Humphriesianum ammonite zone displays regular marlstone-limestone alternations (Suchéras-Marx et al. Reference Suchéras-Marx, Giraud, Fernandez, Pittet, Lécuyer, Olivero and Mattioli2013).
Material and Methods
We quantified nannofossils (both coccoliths and the nannolith Schizosphaerella) in 41 samples from Cabo Mondego (collected in the field in 2004 by F. Giraud and E. Mattioli before the classification of this area as Natural Monument in 2007) and 50 samples from Chaudon-Norante. Sample preparation for absolute abundances per gram of rock used the random-settling method proposed by Beaufort (Reference Beaufort1991) and calibrated by Geisen et al. (Reference Geisen, Bollmann, Herrle, Mutterlose and Young1999), consisting of a suspension of 20 to 30 mg of rock powder mixed with water settled for 24 h on a cover slide. Once cover slides were dry, they were glued to a microscope slide using eukitt®. Usually 300 nannofossils were counted, using a Zeiss optical microscope with a ×1000 magnification. In poor samples, at least 100 coccoliths were counted in a slide in order to reach reliable relative abundance estimations at the genus level. Unfortunately, eight samples from Cabo Mondego and six samples from Chaudon-Norante were too poor and were excluded from the relative abundance data set. The preservation of coccoliths was estimated on a scale from 1 (very poor preservation) to 6 (excellent preservation; modified from Bown and Young Reference Bown and Young1998). Criteria chosen for this scale are mainly based on dissolution and overgrowth patterns, preservation of the crystallographic organization, and preservation of the central area.
We used the counts to calculate absolute abundance of each sample; this represents the number of specimens per gram of rock (Geisen et al. Reference Geisen, Bollmann, Herrle, Mutterlose and Young1999). Relative abundance of the nannolith Schizosphaerella was calculated with respect to the whole nannofossil assemblage, whereas relative abundances of coccolith species (including Watznaueria species) were calculated with respect to the coccolith assemblage only. We derived confidence limits of the sample relative abundances from their underlying binomial distributions by using the algorithm of Beaudoin et al. (Reference Beaudouin, Jouet, Suc, Berné and Escarguel2007a,Reference Beaudouin, Suc, Escarguel, Arnaud and Charmassonb). Given a type I error rate α=0.05, these limits define the 95% confidence intervals associated with each sample’s relative abundance, that is, the range of percentage values within which there is a probability of 1−α=95% of finding the unknown true (parametric) percentage. Eventually, nannofossil fluxes in number of nannofossils per m2 per year were calculated from ammonite zone durations based on Suchéras-Marx et al. (Reference Suchéras-Marx, Giraud, Fernandez, Pittet, Lécuyer, Olivero and Mattioli2013) or Gradstein et al. (Reference Gradstein, Ogg, Schmitz and Ogg2012; only for the Aalenian part of Cabo Mondego). Only the genera of calcareous nannofossils representing >5% of the assemblage in more than ten samples (and >90% of all identified nannofossils when considered together) are presented in the results.
Results
At Cabo Mondego, 50 nannofossil species have been identified (partly presented in Fig. 2). The main genera are, starting from the most abundant: Watznaueria, Schizosphaerella, Discorhabdus, Similiscutum, Biscutum, Carinolithus, and Lotharingius. At Chaudon-Norante, 44 different species of nannofossils have been identified. The main genera are, starting from the most abundant: Watznaueria, Similiscutum, Lotharingius, Discorhabdus, Biscutum, Carinolithus, and Schizosphaerella. Despite the slight difference in total species observed between the two sections, the assemblages are remarkably similar. The preservation of nannofossils in both sections is better in marlstones than in limestones (see counting charts in Supplementary Materials). At Cabo Mondego, preservation in limestones is poor to moderate, whereas it is moderate in marlstones, with some samples showing a good preservation. There is no stratigraphic influence on the preservation; samples with poor to moderate preservation are found all along the section. At Chaudon-Norante, preservation is poor to moderate (but mostly moderate) in limestones; in marlstones it is moderate to moderate-good. Here, also, there is no stratigraphic pattern in preservation, except for marlstone-limestone alternations. Overall, nannofossil preservation is very similar at Cabo Mondego and Chaudon-Norante, thus allowing a direct comparison between the two sections.
Nannofossil Absolute Abundances and Fluxes
In Figure 3, nannofossil absolute abundance and flux are presented, as well as absolute abundance for Schizosphaerella spp., Watznaueria spp., Lotharingius spp., Discorhabdus spp., Biscutum spp. + Similiscutum spp., and Carinolithus spp. for Cabo Mondego. Nannofossil absolute abundance shows the lowest values in the Aalenian part of the section, with values between 106 and 107 nannofossils/g. The nannofossil absolute abundance gradually increases from the Discites ammonite zone up to a maximum of ~109 nannofossils/g in the middle of the Laeviuscula ammonite zone. Then, except for two low values at the end of the Laeviuscula ammonite zone and at the beginning of the Propinquans ammonite zone, the nannofossil absolute abundance remains high and mostly fluctuates between 108 and 109 nannofossils/g. The estimated nannofossil flux shows exactly the same trend as absolute abundance, with the lowest Aalenian values ranging between 108 and 109 nannofossils/m2/year and the highest values exceeding 1011 nannofossils/m2/yr in the middle of the Laeviuscula ammonite zone.
Schizosphaerella spp., Lotharingius spp., Discorhabdus spp., Biscutum spp.+Similiscutum spp., and Carinolithus spp. absolute abundances show the same trend, with a decrease from the base of the section to minimal values at the Aalenian/Bajocian boundary, followed by an increase ending in the middle part of the Discites ammonite zone. Then, the absolute abundances fluctuate between the middle Discites ammonite zone values and values from the beginning of the section. After the main increase, only a few samples show values comparable to the Aalenian/Bajocian minimum. On average, Schizosphaerella spp., Discorhabdus spp., and Biscutum spp.+Similiscutum spp. have comparable absolute abundances whereas Lotharingius spp. and Carinolithus spp. have lower absolute abundances. Watznaueria spp. absolute abundance increases from the base of the section until the middle of the Laeviuscula ammonite zone. Then, absolute abundances remain high and fluctuate between 8×107 and 109 nannofossils/g. Overall, the Watznaueria spp. absolute abundance curve is very similar to the nannofossil absolute abundance curve.
The nannofossil absolute abundance at Chaudon-Norante (Fig. 4) increases from the Aalenian until the upper part of the Discites ammonite zone. Then there is a decrease in the Laeviuscula ammonite zone and a return to high absolute abundance at the beginning of the Propinquans ammonite zone. Within the Propinquans ammonite zone, there is a small decrease and a return to high values at the Propinquans/Humphriesianum ammonite zones boundary. The nannofossil flux has exactly the same trend as the nannofossil absolute abundance.
At Chaudon-Norante, Schizosphaerella spp., Lotharingius spp., Discorhabdus spp., Biscutum spp. + Similiscutum spp., and Carinolithus spp. increase until the upper part of the Discites ammonite zone, except for a high value in the Aalenian for Carinolithus spp. Both Schizosphaerella spp. and the major coccolith genera show absolute abundances lower at Chaudon-Norante than at Cabo Mondego, except for Biscutum spp.+Similiscutum spp. For Watznaueria spp., there is a rising trend from the base of the section until the upper part of the Discites ammonite zone.
Relative Abundance of Most Relevant Nannofossil Genera
Figure 3 shows the relative abundance of Schizosphaerella spp. with respect to the whole nannofossil assemblage at Cabo Mondego, as well as relative abundances of Watznaueria spp., Lotharingius spp., Discorhabdus spp., Biscutum spp.+Similiscutum spp., and Carinolithus spp. with respect to the coccolith assemblage. Schizosphaerella spp. relative abundance decreases from the Aalenian to lower Bajocian except for one sample at the base of the Laeviuscula ammonite zone and three samples at the top of the section. Watznaueria spp. relative abundance gradually increases from the base of the section until the upper part of the Laeviuscula ammonite zone, with maximum values around 80%. Then it decreases and stabilizes between 50% and 60%. The percentages of Lotharingius spp., Discorhabdus spp., Biscutum spp.+Similiscutum spp., and Carinolithus spp. decreases, with minimum values in the middle to the upper part of the Laeviuscula ammonite zone. On average, Biscutum spp.+Similiscutum spp. are more abundant than Lotharingius spp., Discorhabdus spp., and Carinolithus spp.
At Chaudon-Norante (Fig. 4), the relative abundance of Schizosphaerella spp. remains low except for a few samples around 20% in the upper part of the Laeviuscula ammonite zone and in the upper part of the Propinquans ammonite zone. The increasing trend of Watznaueria spp. relative abundance is similar to the rise observed at Cabo Mondego, ending in the middle part of the Laeviuscula ammonite zone. A decreasing trend is observed (except for the last four samples at the top of Chaudon-Norante) for Lotharingius spp., Biscutum spp.+Similiscutum spp., and Carinolithus spp. at Chaudon-Norante as well as at Cabo Mondego. Contrary to Cabo Mondego, the relative abundance of Schizosphaerella spp. in the upper part of the Laeviuscula ammonite zone is higher than that of Discorhabdus spp. in the lower part of Propinquans ammonite zone. On average, Biscutum spp.+Similiscutum spp. are more abundant than Lotharingius spp., Discorhabdus spp., and Carinolithus spp.
Absolute and Relative Abundance of Most Abundant Watznaueria Species
Figure 5 presents the absolute abundances of W. colacicchii, W. contracta, W. aff. contracta, W. britannica, and W. aff. manivitiae and their relative abundances in the coccolith assemblage of Cabo Mondego. Absolute abundances of W. colacicchii and W. contracta increase in the Aalenian and Discites ammonite zone up to a maximum between the end of the Discites ammonite zone and the beginning of the Laeviuscula ammonite zone. Then, a slight decrease is observed. The same pattern is observed for W. aff. contracta, except for the highest values reached in the Laeviuscula ammonite zone. For these three species, there is a slight decrease toward the top of the section but never as low as in the Aalenian. Absolute abundances of W. britannica and W. aff. manivitiae increase until the middle and upper part of the Laeviuscula ammonite zone, respectively, and then remain relatively stable.
W. colacicchii has its highest relative abundance, up to 13%, between the Discites ammonite zone and the beginning of the Laeviuscula ammonite zone; then it gradually decreases to few percent. W. contracta and W. aff. contracta have their maximum percentages in the same interval as W. colacicchii then gradually decrease except for two spikes in W. aff. contracta, to ~15%, in the Propinquans ammonite zone. The relative abundances of W. contracta and W. aff. contracta are in the same range as W. colacicchii in the latest Aalenian and Discites ammonite zone and at the end of the Propinquans ammonite zone, but in the rest of the section their percentages are two to three times higher than those of W. colacicchii. W. britannica has relative abundances of 0–5% in the latest Aalenian and Discites ammonite zone, followed by an increase in the Laeviuscula ammonite zone up to values over 20%. The beginning of the Propinquans ammonite zone is marked by a decrease, with the lowest value down to 8%. The middle part of this ammonite zone is marked by an increase in W. britannica relative abundance up to 20%, followed by another decrease down to 10%. Finally, W. aff. manivitiae remains poorly represented, with percentages under 5% until an increase in the upper part of the Laeviuscula ammonite zone. Then, percentages generally fluctuate between 5% and 20%, with a maximum around 40% in the Humphriesianum ammonite zone.
Figure 6 represents absolute abundances of W. colacicchii, W. contracta, W. aff. contracta, W. britannica, and W. aff. manivitiae and their relative abundances with respect to the coccolith assemblage at Chaudon-Norante. Absolute abundances of W. colacicchii and W. contracta increase in the Aalenian and Discites ammonite zone up to a maximum between the end of the Discites ammonite zone and the beginning of the Laeviuscula ammonite zone. Then, a slight decrease is observed. The same pattern occurs in W. aff. contracta, except that the maximum is reached mainly in the Laeviuscula ammonite zone. Absolute abundances of W. britannica and W. aff. manivitiae increase by steps during the whole interval studied. The first step is reached in the middle part of the Laeviuscula ammonite zone, the second in the lower part of the Propinquans ammonite zone, and the third one around the Propinquans/Humphriesianum ammonite zones boundary.
The Chaudon-Norante section. Details as in Figure 5.
W. colacicchii relative abundances are low at Chaudon-Norante, most of the values ranging between 0% and 4% except for a maximum in the Discites ammonite zone up to 8%. W. aff. contracta is common only in the Laeviuscula ammonite zone where percentages reach 6%. W. contracta is abundant mainly in the Discites and the lower part of the Laeviuscula ammonite zones, with relative abundances over 15%. In the rest of the section, values mainly range between 0% and 5%. W. britannica is almost absent in the Aalenian and the beginning of the Discites ammonite zone. Then its relative abundance gradually increases to a maximum around 40% in the Humphriesianum ammonite zone. Finally, W. aff. manivitiae is almost absent until the Laeviuscula ammonite zone; then, its relative abundance is extremely variable from a few percent to 25% but tends to increase until the upper part of the Propinquans ammonite zone, where it gradually decreases to values between 10% and 15%.
Discussion
Given the calcareous nannofossil assemblage turnover described above, we discuss in the following sections the possible oceanographic, environmental, and biological triggers of such changes.
Origin of the Oceanic Eutrophication
The early Bajocian is marked by a positive excursion of δ13Cbulk carbonate documented in several localities in Europe, e.g., Italy (Bartolini et al. Reference Bartolini, Baumgartner and Hunziker1996, Reference Bartolini, Baumgartner and Guex1999), Spain (O’Dogherty et al. Reference O’Dogherty, Sandoval, Bartolini, Bruchez, Bill and Guex2006), Portugal (Suchéras-Marx et al. Reference Suchéras-Marx, Guihou, Giraud, Lécuyer, Allemand, Pittet and Mattioli2012) (Fig. 2), and France (Brigaud et al. Reference Brigaud, Durlet, Deconinck, Vincent, Pucéat, Thierry and Trouiller2009; Suchéras-Marx et al. Reference Suchéras-Marx, Giraud, Fernandez, Pittet, Lécuyer, Olivero and Mattioli2013) (Fig. 3). This carbon isotope positive excursion, coupled with the radiolarian fossil record, is interpreted as corresponding to a period of eutrophication of the oceans, at least over the western Tethys (Bartolini et al. Reference Bartolini, Baumgartner and Hunziker1996, Reference Bartolini, Baumgartner and Guex1999). The origin of this major environmental change event remains unclear and could have been triggered by paleoceanographic or climatic changes discussed in this section.
Oceanic Eutrophication by Paleoceanographic Changes
The opening of the Ligurian-Piemontese Ocean (Bill et al. Reference Bill, O’Dogherty, Guex, Baumgartner and Masson2001) and enhanced rates of opening in the Atlantic Ocean (Labails et al. Reference Labails, Olivet, Aslanian and Roest2010) are recorded during the latest Aalenian–early Bajocian time interval. Cabo Mondego is located in the Lusitanian Basin adjacent to the Atlantic Ocean, whereas Chaudon-Norante is located in the French Subalpine Basin close to the Liguro-Piemontese Ocean. Hence, the paleoceanographical settings in these sections may have been highly influenced by these tectonically driven changes, leading to a reorganization of the ocean circulation within the Western Tethys, and most particularly to the onset of upwelling zones (O’Dogherty et al. Reference O’Dogherty, Sandoval, Bartolini, Bruchez, Bill and Guex2006; Leonide et al. Reference Léonide, Floquet and Villier2007). Indeed, the neodymium isotope signal suggests that the tectonic reorganization promoted cold-water upwelling in the tropics during the Aalenian–Bathonian interval (Dera et al. Reference Dera, Priunier, Smith, Haggart, Popov, Guzhov, Rogov, Delsate, Thies, Cuny, Pucéat, Charbonnier and Bayon2015). Nevertheless, this hypothesis does not seem to apply to the early Bajocian interval, the Humphriesianum ammonite zone being marked by an increase in seawater temperature at low latitudes (Dera et al. Reference Dera, Brigaud, Monna, Laffont, Puceat, Deconinck, Pellenard, Joachimski and Durlet2011). The neodymium isotope signal may be linked to an increase in radiogenic fluxes through volcanic activity related to major tectonics changes (Dera et al. Reference Dera, Priunier, Smith, Haggart, Popov, Guzhov, Rogov, Delsate, Thies, Cuny, Pucéat, Charbonnier and Bayon2015). Eutrophication due to paleoceanographic changes during the early Bajocian, though not excluded, is still difficult to demonstrate because the early Bajocian geochemical record is rather scarce, with the few available data not having the same time resolution.
Oceanic Eutrophication by Climate Changes
The increase in δ13Cbulk carbonate resulting from eutrophic conditions may have been triggered by climate changes. On the one hand, during the early Bajocian and more precisely in the Humphriesianum ammonite zone, a climate warming has been inferred from δ18O records in belemnite and oyster calcite from the Paris Basin (Brigaud et al. Reference Brigaud, Durlet, Deconinck, Vincent, Pucéat, Thierry and Trouiller2009; Dera et al. Reference Dera, Brigaud, Monna, Laffont, Puceat, Deconinck, Pellenard, Joachimski and Durlet2011) (Fig. 7). On the other hand, a climate cooling has also been proposed, based on glendonite deposits in Siberia (Price Reference Price1999; Rogov and Zakharov Reference Rogov and Zakharov2010) and fossil wood occurrence during the Bajocian. Xenoxylon wood is supposed to have developed under mean annual temperatures between 5° and 15°C. Its occurrence at low latitudes and absence at high latitudes may reflect temperatures <5°C at high latitudes (Philippe and Thevenard Reference Philippe and Thevenard1996). Given the concurrence of the low-latitude temperature increase supported by oyster δ18O and the high-latitude temperature decrease supported by glendonites and Xenoxylon wood, these climatic records point to an increased latitudinal temperature gradient during the early Bajocian. Studies on clay minerals show an assemblage of illite-smectite mixed-layer with moderate illite concentrations, reflecting warm and humid conditions in the Mecsek Mountains in Hungary during the early Bajocian (Raucsik et al. Reference Raucsik, Demény, Borbély Kiss and Szabó2001; Raucsik and Varga Reference Raucsik and Varga2008). In the Humphriesianum ammonite zone of the eastern part of the Paris Basin, the association of illite (50%), illite-smectite mixed layers (30%), and kaolinite (20%) reflects humid conditions (Brigaud et al. Reference Brigaud, Durlet, Deconinck, Vincent, Pucéat, Thierry and Trouiller2009). The abundance of coal over charcoal in Yorkshire (England) in the lower Bajocian also arguably reflects more humid conditions and less fire-prone (presumably seasonal arid) environments (Hesselbo et al. Reference Hesselbo, Morgans-Bell, McElwain, Rees, Robinson and Ross2003). The increase of a latitudinal temperature gradient may have enhanced atmospheric circulation and increased humidity at low to middle latitudes (Price et al. Reference Price, Valdes and Sellwood1998), which in turn may have enhanced the oceanic primary production through continental weathering and bio-limiting nutrient transfer (e.g. phosphorus, iron) during the early Bajocian. This hypothesis of climatically driven fertilization of the oceans remains to be confirmed by further isotopic and mineralogical data with better time calibration.
Summary illustration for the latest Aalenian–early Bajocian interval. A, δ13C global trend. B, Nannofossil flux (Suchéras-Marx et al. Reference Suchéras-Marx, Guihou, Giraud, Lécuyer, Allemand, Pittet and Mattioli2012, Reference Suchéras-Marx, Giraud, Fernandez, Pittet, Lécuyer, Olivero and Mattioli2013). C, Watznaueria species relative abundance and integration steps (this study). D, δ18O of belemnites and oysters (Podlaha et al. Reference Podlaha, Mutterlose and Veizer1998; Jenkyns et al. Reference Jenkyns, Jones, Gröcke, Hesselbo and Parkinson2002; Metodiev and Koleva-Rekalova Reference Metodiev and Koleva-Rekalova2008; Brigaud et al. Reference Brigaud, Durlet, Deconinck, Vincent, Pucéat, Thierry and Trouiller2009; Gomez et al. Reference Gomez, Canales, Ureta and Goy2009; Price Reference Price2010 in Dera et al. Reference Dera, Brigaud, Monna, Laffont, Puceat, Deconinck, Pellenard, Joachimski and Durlet2011). E, 87Sr/86Sr of belemnites from Canada, England, Portugal, and Scotland (Jenkyns et al. Reference Jenkyns, Jones, Gröcke, Hesselbo and Parkinson2002). F, Synthetic temperature and humidity conditions (Brigaud et al. Reference Brigaud, Durlet, Deconinck, Vincent, Pucéat, Thierry and Trouiller2009). G, Ocean fertility based on δ13C (Bartolini et al. Reference Bartolini, Baumgartner and Hunziker1996; Brigaud et al. Reference Brigaud, Durlet, Deconinck, Vincent, Pucéat, Thierry and Trouiller2009; O’Dogherty et al. Reference O’Dogherty, Sandoval, Bartolini, Bruchez, Bill and Guex2006; Suchéras-Marx et al. Reference Suchéras-Marx, Guihou, Giraud, Lécuyer, Allemand, Pittet and Mattioli2012, Reference Suchéras-Marx, Giraud, Fernandez, Pittet, Lécuyer, Olivero and Mattioli2013).
Integration of Watznaueria in the Nannoplankton Community
The increase in nannofossil absolute abundances and fluxes during the early Bajocian observed in this study is coeval with the oceanic fertilization and eutrophication event, and marks the onset of the Watznaueria dominance in the calcareous nannoplankton community throughout the Mesozoic. Indeed, calcareous nannofossil production and sedimentation are dependent on nutrient concentration (Broerse et al. Reference Broerse, Ziveri, van Hinte and Honjo2000; Kinkel et al. Reference Kinkel, Baumann and Cepek2000; Andruleit et al. Reference Andruleit, Stäger, Rogalla and Cepek2003); with the increase in continental flux of bio-limiting nutrients, calcareous nannoplankton flourished during the early Bajocian. While increasing the nannoplankton productivity, such high-fertility environments triggered changes within the calcareous nannoplankton community organization and interactions. At the Aalenian–Bajocian transition, the Watznaueria spp. represented less than 20% of the coccolith assemblage. Then Watznaueria spp. seemingly replaced the other species, which all decreased in relative abundances. By the Laeviuscula ammonite zone, Watznaueria spp. reached >50% of the coccolith assemblage (Figs. 3, 4). Given the relative abundances in both sections, Watznaueria spp. increase is synchronous with a decrease in Biscutum spp.+Similiscutum spp. and, to a lesser extent, all the other coccolith genera, suggesting that Watznaueria spp. replaced these taxa.
At the same time, however, there is also an increase in nannofossil absolute abundance and flux observed during the earliest Bajocian in the two studied sections. This increase in nannofossil absolute abundance is mostly related to the increase in the absolute abundance of Watznaueria spp.; it is not accompanied by any decrease in absolute abundance of other nannofossil species. In fact, there is also in the Discites ammonite zone a slight increase in absolute abundance of Lotharingius spp., Discorhabdus spp., and Biscutum spp.+Similiscutum spp. (observed only at Cabo Mondego).
Therefore, the combined relative and absolute abundance signals indicate that Watznaueria spp. actually integrated into (without ecological displacement/replacement of previous species) rather than invaded (with displacement/replacement of previous species) the early Bajocian nannoplankton community. With this integration, Watznaueria spp. started to dominate the coccolith assemblage without replacing any species, and thus possibly without significantly affecting the ecological organization and functioning of the already present nannoplankton community. At that time, carrying capacity of the nannoplankton community increased with the influx of nutrients, favoring the Watznaueria spp. as new actors playing a new ecological role in the community, and ultimately leading to changes of the relative, but not absolute, abundances of the already existing species.
The Watznaueria integration was achieved through two successive evolutionary steps: first during the late Aalenian up to the end of the Discites ammonite zone, and then during the Laeviuscula ammonite zone up to the Propinquans ammonite zone. During the first step, the striking rise in Watznaueria spp. absolute abundance in the Discites ammonite zone is mainly supported by W. contracta and W. colacicchii (Figs. 5, 6). The increases in absolute abundance of these two species are synchronous and associated with a slight increase in the absolute abundance of other genera, e.g., Discorhabdus spp. and Biscutum spp.+Similiscutum spp. (all belonging to the family Biscutaceae). The second step, corresponding to the end of the increase and the stabilization of Watznaueria spp. absolute abundances, is supported mainly by W. britannica and W. aff. manivitiae (Figs. 5, 6).
No increase in the absolute abundance of other nannofossil genera is observed during this step. The integration of W. contracta and W. colacicchii thus appears as a slightly different event than the integration of W. britannica and W. aff. manivitiae. The former were rare species before the end of the Aalenian, having been present in the nannoplankton community since the Toarcian (Mattioli and Erba Reference Mattioli and Erba1999; Aguado et al. Reference Aguado, O’Dogherty and Sandoval2008), whereas the latter are new species that appeared between the end of the Aalenian and the beginning of the Bajocian (Mattioli and Erba Reference Mattioli and Erba1999). The two-step integration of Watznaueria spp. thus possibly resulted from different species adaptations leading to different paleoecological preferences during each step.
Origin of the Watznaueria Integration: Changes in Exploitation of Environmental and Ecological Resources
The two-step scenario discussed above calls for at least two possible hypotheses explaining the ecological integration of Watznaueria into the early Bajocian nannoplankton community: (1) integration achieved through environmental (physico-chemical parameters) changes, namely increasing ecosystem’s carrying capacity (i.e., opening new, previously unfilled ecological spaces), or (2) integration facilitated by ecological change (interaction between organisms and their environment), namely species’ niche shifting. These two hypotheses are not mutually exclusive and may have co-occurred. Linking them to the two integration steps defined earlier suggests that step 1 was related to the environmental change hypothesis, whereas step 2 was related to the interplay between environmental and ecological changes hypotheses.
Step 1: Integration via Carrying Capacity Increase
The hypothesis of integration rather than invasion of Watznaueria spp. is related to an increase in the carrying capacity of the ecological niche occupied by the calcareous nannoplankton: the oceanic photic zone. The vertical and lateral organization of the nannoplankton community actually involves a complex fragmentation of this niche (e.g., upper and lower photic zone, from coastal to oceanic domains, from upwelling zones to oceanic gyres, in equatorial to polar latitudes[Winter and Siesser Reference Winter and Siesser1994]). Therefore, any modification (by collapse, development, or origination) of the carrying capacity of the calcareous nannoplankton ecological niche is at least potentially related to several more or less independent factors, such as changes in surface-water parameters (e.g., water transparency, temperature or salinity stratification [Ahagon et al. Reference Ahagon, Tanaka and Ujiié1993; Solignac et al. Reference Solignac, de Vernal and Giraudeau2008]), sea-level change (Roth Reference Roth1987), biological competition, or nutrient availability.
Some of those factors are very difficult to detect in the geological record (e.g., water transparency); others are still unknown for the early Bajocian (e.g., water-mass stratification), but for the studied time interval, sea-level and nutrient availability changes are documented. Sea-level rise may have participated in the formation of new niches or an increase in niches’ carrying capacities (Roth Reference Roth1987). Indeed, the early Bajocian is marked by a transgression in the southeastern part of the Paris Basin (Durlet and Thierry Reference Durlet and Thierry2000), the northern French Subalpine Basin (Ferry and Mangold Reference Ferry and Mangold1995), and the Betic cordillera (Vera Reference Vera1988). Also, transgressive deposits are documented in the time interval corresponding to the first two ammonite zones of the Bajocian in Greenland (Surlyk Reference Surlyk2003) as well as in England (Hesselbo Reference Hesselbo2008). Conversely, a regressive pattern is observed in Germany (Deutsche Stratigraphische Kommission 2002). The difference in relative sea-level variation in Western Tethys is related to the regional control exerted by major tectonic changes, which is highlighted by the shift of strontium isotope toward unradiogenic values (Fig. 7) (data compiled by Jenkyns et al. Reference Jenkyns, Jones, Gröcke, Hesselbo and Parkinson2002) and was induced by the opening of the Liguro-Piemontese Ocean and Atlantic Ocean, as well as the North Sea bulging. Sea-level rise can increase the size of the photic zone in a shallow proximal environment. A marked deepening of proximal environment could also remove the continental source of limiting nutrients, thus reducing the nannoplankton productivity, which is more important in proximal environment than in pelagic environment (Baumann et al. Reference Baumann, Böckel and Frenz2004).
Nevertheless, even if the contribution of sea-level changes to ecological niche modification cannot be ruled out, the most significant environmental change recorded during the early Bajocian may have been the eutrophication event discussed earlier. Indeed, nannofossil assemblages are classically more affected by nutrient availability (e.g., Erba Reference Erba2004; Lees et al. Reference Lees, Bown and Mattioli2005; Aguado et al. Reference Aguado, O’Dogherty and Sandoval2008; Giraud et al. Reference Giraud, Courtinat and Atrops2009), than by sea-level variations. Actually, the receptivity of calcareous nannoplankton to sea-level changes seems to be indirectly related to nutrient availability, given that offshore environments are relatively more oligotrophic and nearshore environments more meso- to eutrophic due to the proximity of bio-limiting elements supplied by continental weathering. For instance, the higher absolute and higher relative abundances of Schizosphaerella spp. in the distal ramp setting of Cabo Mondego section, as compared with the hemipelagic setting of Chaudon-Norante, are in agreement with this taxon’s affinities for nearshore carbonate environments (Pittet and Mattioli Reference Pittet and Mattioli2002; Mattioli and Pittet Reference Mattioli and Pittet2004).
Hence, we hypothesize that step 1, the integration of W. contracta and W. colacicchii in the community, was chiefly related to an environmental change, namely the increase in nutrient availability. With such increase, the carrying capacity of the photic zone increased, opening a new ecological space first filled by these two species. Prior to the latest Aalenian, these two species (already present since the Toarcian) were scarce in the nannoplankton assemblages, possibly because they were unable to develop in nutrient-limited environments. Remarkably, these two species show similar morphological characters, being characterized by the presence of a cross spanning the central area of the coccolith. Nevertheless, the biological mechanisms linking a coccolith’s morphology to its ecological preferences remain difficult to assess. Incidentally, this hypothesis is compatible with the slight increase in Biscutaceae coccoliths; these are species often associated with eutrophic environments (e.g., Erba Reference Erba2004; Lees et al. Reference Lees, Bown and Mattioli2005) and their increase testifies to global oceanic eutrophication that mainly favored Watznaueria spp. but also other species production.
Step 2: Integration via Innovation in Ecological Niche Exploitation
The second step, Watznaueria spp.’s stabilization in early Bajocian nannoplankton assemblages, is based on a biological innovation leading to an improvement in ecological niche exploitation. This phase is marked by the increase and then stabilization of the nutrient-rich environment based on the δ13C record. In that context, two new species (W. britannica and W. aff. manivitiae) increased in absolute and relative abundances while W. contracta and W. colacicchii, at first predominant in these nutrient-rich environments, remained stable in absolute abundance. Given the likely absence of competition between W. contracta/W. colacicchii and W. britannica/W. aff. manivitiae (the increase in absolute abundance of the latter is not associated with a decrease in absolute abundance of the former), W. britannica/W. aff. manivitiae likely developed new biological traits allowing exploitation of a still-unoccupied, potentially newly formed ecological niche. This hypothesis is supported by studies that identify W. britannica and W. aff. manivitiae as species newly occurring in the early Bajocian (Mattioli and Erba Reference Mattioli and Erba1999) and developing new ecological capabilities in a nutrient-rich environment. W. britannica across the Middle-Late Jurassic in France has already been identified as an opportunistic species highly competitive in nutrient-rich environment (Giraud Reference Giraud2009).
The available temporal resolution is obviously too low to directly observe the seasonal to yearly nannoplankton community dynamics, but the biological innovation characterizing W. britannica and W. aff. manivitiae very likely resides in their blooming ability. Monospecific laminae of coccolith are described in the fossil record as old as the lower Toarcian (Early Jurassic, ca. 183 Ma) in the Paris Basin Schistes Carton (Goy Reference Goy1981). This is the oldest evidence of coccolithophore bloom in the fossil record—although this phenomenon is well known in living assemblages—but the species observed in those layers are murolith and Calyculus species with no evolutionary relation to Watznaueriaceae (Bown Reference Bown1987). In Upper Jurassic sediments, monospecific laminae containing coccospheres of Watznaueria barnesiae and W. britannica (Gallois and Medd Reference Gallois and Medd1979; Medd Reference Medd1979; Lees et al. Reference Lees, Bown, Young and Riding2004) are interpreted as the evidence for blooms occurring under eutrophic conditions (Lees et al. Reference Lees, Bown, Young and Riding2004, Reference Lees, Bown and Young2006). This is the earliest unequivocal record for blooms of Watznaueria coccolithophores. Similarly, W. barnesiae in the Early Cretaceous is interpreted as a blooming species, because seasonal changes led to the proliferation of these opportunistic taxa first, followed by the development of other calcareous nannofossils (Thomsen Reference Thomsen1989). W. britannica has been interpreted as an opportunistic “r-strategist” blooming in eutrophic conditions, but not W. aff. manivitiae (Lees et al. Reference Lees, Bown, Young and Riding2004, Reference Lees, Bown and Young2006). The latter is actually poorly documented in Jurassic calcareous nannofossil studies. Nevertheless, this species has a morphology very similar to another Watznaueria r-strategist blooming in eutrophic conditions during the Early Cretaceous: W. barnesiae (Lees et al. Reference Lees, Bown, Young and Riding2004, Reference Lees, Bown and Young2006). Likewise, W. manivitiae/britannica, a Watznaueria morphotype with still unclear taxonomical affinities and sharing morphological features from W. britannica and W. aff. manivitiae, is also described as a morphotype inhabiting instable meso-eutrophic environments (Giraud Reference Giraud2009; Giraud et al. Reference Giraud, Courtinat and Atrops2009), environments often dominated by opportunistic species. Even if a direct relationship between a coccolith’s general morphology and its ecological preferences is questionable, we hypothesize that during the early Bajocian, W. aff. manivitiae may have had an ecology close to W. barnesiae. Eventually, W. britannica and W. aff. manivitiae integrated into the nannoplankton community thanks to their adaptation to seasonal blooms in nutrient-rich niches previously unexploited by other species.
Because at this time only W. britannica and W. aff. manivitiae increased in both absolute and relative abundances, other environmental conditions may have favored their growth (or limited growth of other species). The fact that certain species bloomed, but not the others, results from (1) their presence before the blooming conditions became settled, and (2) the coincidence in time between nutrient enrichment and ecological preferences of the blooming species (e.g., temperature, water turbulence, day length; Balch Reference Balch2004). The general increase in nutrient availability during the early Bajocian resulted in seasonal increase in nutrients, which either allowed calcareous nannoplankton blooms to occur or increased the blooming period. Already existing species, with more restrictive ecological preferences (e.g., temperature, water turbulence, day length; Balch 2004) were not adapted to such conditions, but the opportunistic taxa W. britannica and W. aff. manivitiae were adapted to those conditions and could efficiently develop. If both species show common trends in the early Bajocian, however, only W. britannica continued to be abundant afterward. The scarcity of W. aff. manivitiae in the rest of the Mesozoic suggests that both species did not share exactly the same ecological preferences. W. aff. manivitiae may have disappeared because by the Bathonian a new species, namely W. barnesiae, invaded the oceans, replacing W. aff. manivitiae. In order to clarify these points, further quantitative studies on the late Bajocian and Bathonian calcareous nannofossils are needed (e.g., Tiraboschi and Erba Reference Tiraboschi and Erba2010).
Conclusion
The early Bajocian is characterized by major changes in geology, paleoceanography, climate, and biodiversity. This time interval is notably marked by the diversification of the coccolith genus Watznaueria, the most successful Mesozoic coccoliths (Lees et al. Reference Lees, Bown and Mattioli2005; Erba Reference Erba2006). We evaluated the conditions in which this diversification occurred and its effect on the nannofossil assemblages. Our conclusions are as follows:
∙ During the latest Aalenian–early Bajocian (171-169 Ma), relative abundance of Watznaueria increased over other coccoliths; meanwhile, their absolute abundance also increased without significant concomitant decrease in absolute abundances of other coccoliths. Although estimation of calcareous nannofossil absolute abundance has been possible for more than 20 years (e.g., Beaufort Reference Beaufort1991; Geisen et al. Reference Geisen, Bollmann, Herrle, Mutterlose and Young1999), such quantification remains much less common than relative abundance studies. Nevertheless, as our results suggest, it offers interesting outcomes, complementary to relative abundance data, for studying nannoplankton community dynamics;
∙ The increase in nannofossil absolute abundance and flux is mostly due to the emergence of Watznaueria spp.
∙ The diversification and emergence of Watznaueria spp. during the early Bajocian was likely related to a climatically driven eutrophication event.
∙ Watznaueria spp. integrated into rather than invaded the nannoplankton community.
∙ This integration was achieved in two subsequent steps: step 1, involving W. contracta and W. colacicchii, likely relates to an increase in the carrying capacity of the ecological niche due to increased nutrient supply; and step 2, involving W. britannica and W. aff. manivitiae, likely relates to a newly acquired blooming capacity in a previously unoccupied ecological niche.
Our results suggest that Watznaueria started to dominate the calcareous nannofossil assemblages thanks to peculiar ecological preferences during the early Bajocian critical environmental perturbation. Later on, this genus remained dominant in nannoplankton assemblages until the Upper Cretaceous thanks to an opportunistic mode of life. It finally lost the throne with the K/Pg mass extinction, which completely changed the calcareous nannoplankton community structure (Bown Reference Bown2005), giving the way to a new calcareous nannofossil dynasty.
Acknowledgments
This paper is part of B.S.-M.’s Ph.D. project funded by the French Ministry of Research and supported by grants from BQR Université Claude Bernard Lyon1 2006 to F.G., BQR Université Claude Bernard Lyon1 2010, and CNRS-INSU 2011-12 Syster and Interrvie Projects to E.M. This paper is a contribution of the Laboratoire de Géologie de Lyon team “Paléoenvironnements.” B.S.-M. also thanks the new institute OSU-Institut Pythéas. We thank two reviewers and editor Bruce MacFadden for constructive comments that helped us to clarify several points in the manuscript. Cabo Mondego nannofossil smear slides used in this study are curated in the Department of Earth Sciences of Coimbra, Portugal, and in the Collections de Géologie de Lyon, France. Chaudon-Norante smear slides used in this study are curated in the Collections de Géologie de Lyon, France. Lastly, B.S.-M. wish to dedicate this work to the late Martin Chauffrey, who strongly supported him in his early days as a young student in geology.
Supplementary material
Supplementary material deposited at Dryad: doi:10.5061/dryad.jf816
