Fertility changes in surface waters during the Aalenian (mid-Jurassic) of the Western Tethys as revealed by calcareous nannofossils and carbon-cycle perturbations

Marine Micropaleontology 68 (2008) 268–285

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Marine Micropaleontology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a r m i c r o

Fertility changes in surface waters during the Aalenian (mid-Jurassic) of the Western Tethys as revealed by calcareous nannofossils and carbon-cycle perturbations Roque Aguado a,⁎, Luis O'Dogherty b, José Sandoval c a b c

Departamento de Geología, Escuela Politécnica Superior de Linares, Universidad de Jaén, Alfonso X El Sabio 28, E23700 Linares, Spain Departamento de Ciencias de la Tierra, CASEM, E11510 Puerto Real, Spain Departamento de Estratigrafía y Paleontología, Universidad de Granada, E18002 Granada, Spain

a r t i c l e

i n f o

Article history: Received 21 December 2007 Received in revised form 7 May 2008 Accepted 5 June 2008 Keywords: Calcareous nannofossils Radiolarians Carbon–oxygen isotopes Aalenian Palaeoceanography Western Tethys

a b s t r a c t In Aalenian times, the South Iberian Palaeomargin was part of the westernmost Tethys Ocean. The Median Subbetic palaeogeographic domain of the Betic Cordillera was a relatively deep trough in the South Iberian Palaeomargin during the Early Jurassic–Late Cretaceous interval, where mainly pelagic and hemipelagic limestones and marls were deposited. A semiquantitative study of nannofossil assemblages was performed in sediments from the upper Toarcian–lowest Bajocian from two Median Subbetic sections (Agua Larga and Cerro de Mahoma). Nannofossil assemblages are composed mainly of cosmopolitan and Tethyan taxa. The NJ8a, NJ8b and NJ9 Zones as well as other useful biohorizons (FOs of Triscutum tiziense and Carinolithus magharensis and LO of Similiscutum finchii) were identified and directly correlated to ammonite zones. The analysis of the relative abundances of some common to abundant taxa including Biscutum, Carinolithus superbus, Crepidolithus crassus, Lotharingius, Schizosphaerella and Watznaueria display noticeable fluctuations that can be correlated between the two sections. The comparison of these fluctuations with the δ13Ccarb curves and the interpretation of the palaeoecologic significance of some of these taxa provided an outline of the palaeoceanographic trophic regime throughout the interval studied. During the latest Toarcian–Early Aalenian, the high proportions of oligotrophic Schizosphaerella, moderately high proportions of C. crassus and low proportions of eutrophic Biscutum, correlate with low to moderate values in the δ13Ccarb curves. Radiolarians display low abundance throughout this interval. This is interpreted as an interval where mesotrophic to oligotrophic and stable conditions occurred in surface waters. The Middle Aalenian, characterized by high proportions of Schizosphaerella and C. crassus and low proportions of Biscutum, correlates with low values in the δ13Ccarb curves, and was interpreted to correspond to an episode when stable oligotrophic conditions occurred in surface waters. Radiolarians moderately increased throughout this interval. Finally, the Late Aalenian–earliest Bajocian interval, with lower proportions of Schizosphaerella and C. crassus, and higher proportions of Biscutum, also correlates with a significant positive excursion in the δ13Ccarb curve, suggesting a shift from oligo- to eutrophic conditions in surface waters. This change in productivity is also revealed by a conspicuous increase in radiolarian abundance, at the same time as a quasi-complete replacement of Early Jurassic radiolarian fauna took place. The analysis of faunal-flora turnovers reveals a causal link between the global carbon-cycle and the pelagic response. This noticeable faunal-flora change throughout the Late Aalenian–Bajocian can be interpreted as a major biological response to the drastic modification in the western Tethys palaeogeography as consequence of the Atlantic opening, which in turn caused a new pattern in the oceanic circulation. © 2008 Elsevier B.V. All rights reserved.

⁎ Corresponding author. Fax: +34 953 648 622. E-mail addresses: [email protected] (R. Aguado), [email protected] (L. O'Dogherty), [email protected] (J. Sandoval). 0377-8398/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2008.06.001

R. Aguado et al. / Marine Micropaleontology 68 (2008) 268–285

1. Introduction Much progress was made during the last two decades on the Aalenian nannofossil biostratigraphy, (Bown et al., 1988; Cobianchi, 1992; Reale et al., 1992; Kaenel and Bergen, 1993; Mattioli, 1994; Bartolini et al., 1995; Kaenel et al., 1996; Bown and Cooper, 1998; Mattioli and Erba, 1999; Perilli et al., 2002, 2004a), although few data are available on Subbetic sections (Perilli et al., 2004b; Sandoval et al., in press). Also, some recent papers deal with Jurassic calcareous nannofossil palaeoecology and palaeoceanograpy (Cobianchi and Picotti, 2001; Pittet and Mattioli, 2002; Lees et al., 2004; Mattioli and Pittet, 2004; Olivier et al., 2004; Giraud et al., 2006; Tremolada et al., 2006; Bour et al., 2007; Suan et al., 2008), but in them, the study of the Aalenian has received little or no attention. However, this represents a period that recorded major palaeogeographic and palaeoceanographic changes related to the opening of Central Atlantic (Poag, 1991; Boomer and Ballent, 1996; Bill et al., 2001). Aalenian rocks are well exposed in the Median Subbetic domain (Betic Cordillera, S Spain), a relatively deep trough which records the most complete and expanded hemipelagic successions of the Iberian Palaeomargin (Linares and Sandoval, 1993; Aurell et al., 2002). Ammonites are relatively abundant and well preserved, enabling accurate biostratigraphic results at the zone and subzone levels (Linares and Sandoval 1993; Henriques et al., 1996; Sandoval et al., in press). Calcareous nannofossils are also common in marly beds, providing a good correlation between ammonite and

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nannofossil zonations (Sandoval et al., in press). Recently, several pelagic, expanded, and well-dated Aalenian Median Subbetic sections have been analysed for stable isotopes (bulk sample analyses; Sandoval et al., in press) showing distinctive fluctuations in the δ13Ccarb and δ18Ocarb curves. The aim of this work is: (1) to evaluate the fluctuations in relative abundance of calcareous nannofossil assemblages in order to estimate the trophic regime of surface waters throughout the Aalenian by using semiquantitative techniques; (2) to compare the response of the nannoflora with variations in radiolarians abundance, as these latter are the dominant representative of the microzooplankton during the Aalenian, and (3) to test probable links between the stable isotope fluctuations and the evolution of palaeoceanographic trophic regime of surface waters through the interval studied. 2. Aalenian stratigraphy in the Subbetic The sections analysed, Barranco de Agua Larga (Sierra de Alta Coloma, province of Jaén) and Cerro de Mahoma (Sierra de Ricote, province of Murcia) are located in the Betic Cordillera (S Spain) at a distance of 220 km. Palaeogeographically, the two sections belong to the Subbetic (central part of External zones of Betic Cordillera), which was a relatively deep trough in the South Iberian Palaeomargin during the Early Jurassic–Late Cretaceous. In the Aalenian, the South Iberian Palaeomargin was part of the westernmost Tethys Ocean (Fig. 1) at a palaeolatitude of ∼ 20°N (Bassoullet et al., 1993; Golonka, 2004) and where mainly epicontinental

Fig. 1. Palaeogeographic map at 175 Ma (Early Aalenian) showing the location of the two study sections in the South Iberian Palaeomargin (from Bassoullet et al., 1993; Golonka, 2004; modified). 1. Agua Larga section. 2. Cerro de Mahoma section.

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marine sedimentary environments prevailed. The distribution of Aalenian materials in the Subbetic is highly irregular (Linares and Sandoval, 1993). In the Subbetic swells (Internal and External Subbetic domains) Aalenian rocks, generally red or grey nodular limestones, constitute condensed or reduced sections. Discontinuities are common and, locally, stratigraphic gaps can include the whole Aalenian. The Median Subbetic domain (the Subbetic pelagic trough), where the study sections are located, is the only Betic domain with continuous marine pelagic sedimentation during the late Early Jurassic–Late Cretaceous interval. In this palaeogeographical domain, Aalenian rocks are generally made up of marls and marly-limestone rhythmites and can have continuous records throughout the stage, including the lower and upper boundaries (Linares and Sandoval, 1993; Henriques et al., 1996). 2.1. Barranco de Agua Larga section This section (Fig. 2A) is located in the central sector of the Median Subbetic, outcropping along the Agua Larga creek (37°35′10″N, 3°33′1″W), approximately 8 km NW of the village of Campillo de Arenas and 1.5 km N of peak of Alta Coloma, Province of Jaén (S Spain). Upper Aalenian–Lower Bajocian ammonite biostratigraphy of this section was established by Sandoval (1983) and by Linares and Sandoval (1990, 1996) whereas O'Dogherty et al. (2006) and Sandoval et al. (in press) have revised previous biostratigraphic data, analysed the δ13Ccarb isotopic curve, and provided preliminary nannofossil biostratigraphic data throughout the uppermost Toarcian (Aalensis ammonite Zone)–Lower Bajocian (Humphriesianum ammonite Zone) interval. For this study, we analysed the uppermost Toarcian– lowermost Bajocian interval, including the Toarcian–Aalenian and the Aalenian–Bajocian boundaries. This interval is approximately 116 m thick and represented by grey marls, marly limestones, and limestones with radiolarians, thinshelled pelagic bivalves, benthic foraminifers, ostracods, ammonite embryos, etc. Calcareous bed thickness varies from 5 to 100 cm, whereas marly beds can locally encompass 150 cm. The marly sediments are dominant in the Upper Toarcian and some Upper Aalenian levels whereas marly limestones or limestones are the predominant lithofacies in the rest of the section. Small, irregular, and disperse chert nodules occur in some Upper Aalenian beds, coinciding with maximum content in radiolarians, and marls have a higher siliciclastic proportion. Ammonites are abundant and well preserved in the Upper Aalenian–Lower Bajocian interval. In the rest of the section, ammonites, although less common, also provide good biostratigraphic results. The ammonite zones of Aalensis, Opalinum (subzones of Opalinum and Comptum), Murchisonae, Bradfordensis (subzones of Bradfordensis and Gigantea), Concavum (subzones of Concavum and Limitatum) and Discites were recognized (Fig. 2A). 2.2. Cerro de Mahoma section This section (38°7′31″N, 1°28′21″W; Fig. 2B) is located in the Sierra de Ricote (province of Murcia) within the eastern sector of the Median Subbetic and crops out along a forest path, nearly 2500 m SW of the “Caserío de la Bermeja” where Pliensba-

chian–Bathonian rocks are exposed (Seyfried, 1978). Its Upper Toarcian–Aalenian biostratigraphy was previously established by Linares and Sandoval (1993), García-Gómez et al. (1995) and by Sandoval et al. (2001). Recently Sandoval et al. (in press) have revised the previous biostratigraphic ammonite data, analysed δ13Ccarb isotope variations, and provided preliminary nannofossil biostratigraphic data for this interval. The uppermost Toarcian–lowermost Bajocian interval (Fig. 2B) is made up of approximately 60-m-thick set of pelagic to hemipelagic grey or grey-white marls, marly limestones and limestones with radiolarians, sponge spicules, thin-shelled pelagic bivalves, benthic foraminifers and ostracods. Calcareous beds thickness varies from 5 to 70 cm and marly interbeds can locally be thicker than 100 cm. Some limestone beds, especially in the uppermost Middle Aalenian–Upper Aalenian interval, are more compacted than the underlying levels bearing small, irregular, and disperse chert nodules. Ammonites, common throughout the section, present characteristic taxa, which allowed good biostratigraphic results (Linares and Sandoval, 1993; Henriques et al., 1996; Sandoval et al., 2001, in press). The ammonite zones of Aalensis (subzones of Mactra, Aalensis and Buckmani), Opalinum (subzones of Opalinum and Comptum), Murchisonae, Bradfordensis (subzones of Bradfordensis and Gigantea), Concavum (subzones of Concavum and Limitatum) and Discites were recognized and their boundaries well established (Fig. 2B). 3. Materials and methods Smear slides for calcareous nannofossil study were prepared for 134 samples of marls (80 from Agua Larga section and 54 from Cerro de Mahoma section), taken at irregularly spaced points along the interval studied. Sample preparation consisted of a simple mechanical crushing of a small piece of rock, its dilution with water and spreading onto a slide. After drying in a stove, the slides were mounted with coverslips using Canada balsam. Special care was taken to prepare the samples as homogeneously as possible so that particle density in different slides would be comparable. This preparation technique was used to retain the original composition of calcareous nannofossil assemblages. All the smear slides were investigated for nannofossil content using a polarizing light microscope at 1200x magnification. The preservation of nannofossils in each sample was evaluated under the light microscope using a preservation index based on the criteria proposed by Roth and Thierstein (1972), as follows: P = poor, severe dissolution, fragmentation and/or overgrowth; the specific identification is hindered up to 20%; M = moderate, dissolution and/or overgrowth; the specific identification is hindered up to 10%; G = good, little dissolution and/or overgrowth; diagnostic characteristics are preserved, the specimens could be identified to the species level (up to 95%). In general, calcareous nannofossil assemblages from Agua Larga and Cerro de Mahoma display a moderate to good state of preservation. Only 91 samples (51 from Agua Larga and 40 from Cerro de Mahoma), which showed the best preserved and abundant assemblages were retained to perform a semi-quantitative study (see “supplementary material”). For this study, the nannofossils were counted along a longitudinal traverse of the smear slide (1 traverse = 200 fields of view at 1200x

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Fig. 2. Lithologic sketches, ammonite and nannofossil stratigraphy, and δ18Ocarb and δ13Ccarb curves after Sandoval et al. (in press) for the two study sections. Nannofossil Zones and Subzones after Bown et al., 1988. The most relevant biostratigraphical nannofossil events are also marked.

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magnification). In 5 samples, due to the scarcity of nannofossils, two longitudinal traverses were analysed in order to reach a nannofossil number statistically significant for percentage calculation. A minimum of 107 (only in 2 samples) and up to 1180 nannofossil specimens were counted in each sample. For Schizosphaerella, only complete isolated valves or fragments representing more than half a valve were counted. The nannofossil abundance in each sample was calculated by dividing the number of specimens counted by the number of fields of view necessary to count them, and finally expressed as nannofossils/10 fields of view at 1200x. The abundances of individual taxa were expressed as percentages. The percentage of each coccolith species was calculated with respect to the total number of coccoliths. The percentage of Schizosphaerella spp. was calculated with respect to the total nannofossil content. All taxa constituting a significant proportion of the total assemblage (abundance greater than 10% in, at least, one sample and present in more than two samples) were selected. These included: Biscutum dubium, B. intermedium, Carinolithus superbus, Crepidolithus crassus, Discorhabdus striatus, Schizosphaerella spp., and Watznaueria contracta. Also, all the species of Lotharingius (L. barozii, L. crucicentralis, L. hauffii, L. sigillatus, L. umbriensis and L. velatus) were grouped together and also selected (see “supplementary material”). Principal Component Analysis (PCA) on this dataset was performed by using the free statistical software Past v1.46 (Hammer et al., 2001). Also, scattered plots were carried out with the Sigma-Plot v8.0 package. Radiolarian abundances were estimated after a complete analysis of microfacies in Agua Larga and in Cerro de Mahoma sections. Around a hundred of thin sections were examined for sedimentological and palaeontological features. Radiolarian percentages were established after visual estimation by means of Baccelle and Bosellini (1965) relative abundance tables. 4. Results 4.1. Nannofossil stratigraphy Calcareous nannofossils, mainly cosmopolitan and Tethyan taxa, were irregularly distributed, being rare to abundant throughout the two sections studied. Measured abundances in the Agua Larga section vary from 2.7–59.2 (average 12.7) nannofossils/10 fields of view at 1200x, and from 2.2–39.7 (average 14.6) in Cerro de Mahoma. In Agua Larga, higher abundances were more common in the upper part of the section (Concavum ammonite Zone; Fig. 3) whereas in Cerro de Mahoma, abundance peaks were recorded in the lower part (Fig. 4). Species richness varied from 10–25 (average 15.3) in Agua Larga, and from 11–23 (average 16.0) in Cerro de Mahoma (Figs. 3, 4). Consistent record of Triscutum (especially T. tiziense), is noticeable in the sections studied (Sandoval et al., in press). These taxa were also reported from Morocco, Portugal, and Switzerland (Kaenel and Bergen,1993; Kaenel et al.,1996; Perilli et al., 2002), and Basque–Cantabrian area (Perilli et al., 2004a) but not found (Cobianchi,1992; Reale et al.; 1992) or reported as very rare (Mattioli,1994; Bartolini et al.,1995; Mattioli and Erba, 1999) from the majority of Italian sections. Taxa such as Cycla-

gelosphaera margerelii and Diazomatolithus lehmanii, commonly found in SE France and Italian sections (Baldanza et al., 1990; Erba, 1990; Cobianchi, 1992; Reale et al., 1992; Mattioli, 1994; Bartolini et al., 1995; Mattioli and Erba, 1999), were not found in the Aalenian record of the Agua Larga and Cerro de Mahoma (Sandoval et al., in press). Finally, some species of the genus Lotharingius (L. umbriensis, L. barozii and L. hauffii) were last recorded between the end of the Early Aalenian (upper part of the Comptum ammonite Subzone) and the early Middle Aalenian (Murchisonae ammonite Zone) in the study sections. However, these species are known to have last occurrences later in the Jurassic (Cobianchi et al., 1992; Mattioli and Erba, 1999; Mailliot et al., 2006) in other locations. Possibly the last records observed in the study sections could be rather a consequence of its decrease in abundance than true last occurrences. The Tethyan zonation proposed by Mattioli and Erba (1999) could only be partially applied owing to the fact that C. margerelii was not found in the Aalenian record of the Agua Larga and Cerro de Mahoma sections (Sandoval et al., in press). For this reason, we follow the zonation proposed by Bown et al. (1988) and Bown and Cooper (1998). From a biostratigraphic standpoint, nearly the entire interval studied in Agua Larga and Cerro de Mahoma sections (Fig. 2A) should be assigned to the NJ8 nannofossil Zone (Bown et al., 1988; Bown and Cooper, 1998). As R. incompta is an extremely rare species, the lowermost part of the studied interval in both sections was tentatively assigned to the NJ8a nannofossil Subzone. The first occurrence (FO) of Watznaueria contracta was observed near the base of the interval whilst the first W. britannica specimens were found, in both sections, within the Discites ammonite Zone (Fig. 2; Sandoval et al., in press). The last meter of the studied interval in both sections should thus be assigned to the NJ9 nannofossil Zone. The sequence of nannofossil events observed in the Agua Larga and Cerro de Mahoma sections, and its correlation with ammonite zones is summarized in Fig. 2. Also, a list of calcareous nannofossils identified in the study sections with appropriate author citations is provided in the Appendix, and vertical distribution of individual taxa is provided as “supplementary material”. 4.2. Nannofossil preservation and diagenesis Variations in abundance and composition of nannofossil assemblages can be interpreted as the response of phytoplankton to palaeoceanographic and palaeoclimatic changes (Bucefallo Palliani et al., 2002; Erba, 2004; Giraud et al., 2006; Tremolada et al., 2006). However, one of the most common problems is to determine to what extent the fossil assemblages reflect a real ecological signal or are the result of a diagenetic overprint. The genus Watznaueria (especially Watznaueria barnesiae) is considered to be a good index indicating alteration of the Cretaceous assemblages (Thierstein, 1981; Roth and Bowdler, 1981; Roth and Krumbach, 1986; Thierstein and Roth, 1991). Cretaceous nannofossil assemblages containing more than 40% of Watznaueria are considered as heavily altered (Roth and Krumbach, 1986). However, some Middle–Late Jurassic nannofossil assemblages are characterized by a high abundance of Watznaueria, and there is evidence that its dominance is not the result of a diagenetic overprint but a real ecological signal (Busson et al., 1993; Pittet and Mattioli, 2002; Lees et al., 2004;

R. Aguado et al. / Marine Micropaleontology 68 (2008) 268–285 Fig. 3. Ammonite zones and calcareous nannofossil zones, simplified lithology and δ18Ocarb and δ13Ccarb curves for Agua Larga section. Changes in nannofossil abundance expressed as nannofossils/10 fields of view at 1200x magnification (each field of view = 177 μm2). Also, species richness and variations in percentages of some selected taxa including Lotharingius, Watznaueria, Biscutum, Crepidolithus crassus and Schizosphaerella throughout the interval studied. Percentages of coccolith taxa calculated with respect to the total coccolith number. Percentage of Schizosphaerella spp. calculated with respect to the total nannofossil content. Percentage abundance of radiolarians throughout the section is based on estimates from slices using the method by Baccelle and Bosellini (1965).

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274 R. Aguado et al. / Marine Micropaleontology 68 (2008) 268–285 Fig. 4. Ammonite zones and calcareous nannofossil zones, simplified lithology and δ18Ocarb and δ13Ccarb curves for Cerro de Mahoma section. Changes in nannofossil abundance expressed as nannoliths/10 fields of view at 1200x magnification (each field of view = 177 μm2). Also, species richness and variations in percentages of some selected taxa including Lotharingius, Watznaueria, Biscutum, Crepidolithus crassus and Schizosphaerella throughout the interval studied. Percentages of coccolith taxa calculated with respect to the total coccolith number. Percentage of Schizosphaerella spp. calculated with respect to the total nannofossil content. The absence of data throughout most of the Concavum ammonite Zone is due to poorly preserved nannofossil assemblages. Percentage abundance of radiolarians throughout the section is based on estimates from slices using the method by Baccelle and Bosellini (1965).

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Olivier et al., 2004; Tremolada et al., 2006). The lower part of the Middle Jurassic corresponds to the radiation of the genus Watznaueria. Probably its relative abundance was at that time much more subject to the evolutionary history of the genus rather than its preservational factors and should not be used as an index of diagenetic alteration for this time-interval. Throughout the study interval, the genus Watznaueria is represented mainly by W. contracta, with minor proportions of W. colacicchii, W. britannica and W. communis. The average proportion of Watznaueria with respect to the other coccoliths (excluding Schizosphaerella) is relatively small in Agua Larga (19.2%). Only in one sample from the Upper Aalenian (sample -125 at 68.7 m, with a 46.7%), this proportion is higher than 40% (Fig. 3). In the Cerro de Mahoma section, the average proportion of Watznaueria is only 10.3%, and in any sample (Fig. 4) its proportion reaches 40% (the highest value is 32.6%). The observed degree of etching and overgrowth and the fluctuations in species richness (Figs. 3, 4) suggest a slight to moderate diagenetic alteration throughout the study sections. Moreover, the significant proportions of dissolution-susceptible taxa as Biscutum spp. (3.13%–45.5%, average 12.9% in Agua Larga; 1.7%–36.4%, average 10.3% in Cerro de Mahoma) may indicate that fluctuations in nannofossil assemblages are affected by diagenesis to a minor extent (Figs. 3, 4, 5). The degree of correlation between the bulk rock carbonate carbon and oxygen isotopes (δ13Ccarb and δ18Ocarb) is used by several authors (Corfield, 1995; Schmid-Röhl et al., 2002; Godet et al., 2006; Tremolada et al., 2006; Duchamp-Alphonse et al., 2007) as an indicator of the amount of diagenetic alteration in carbonate rocks. Low covariance of δ13Ccarb and δ18Ocarb can be interpreted to be the result of a minor influence of the diagenesis. Simple scatter plots of the δ13Ccarb and δ18Ocarb values prepared for the study sections by Sandoval et al. (in press) were made (Fig. 6). In Agua Larga, the correlation between δ13Ccarb and δ18Ocarb values is weak (r = 0.217; Fig. 6A), suggesting a minor influence of the diagenesis. This correlation is moderate in Cerro de Mahoma (r = 0.469; Fig. 6B), also suggesting a moderate influence of the diagenesis on the isotopic signature. In order to avoid problems related to diagenetic alteration, only the samples containing the best preserved assemblages were retained for palaeoenvironmental interpretations (see “supplementary material”). Also, only the abundant taxa with significant contribution to the total assemblage (greater than 10%, at least in one sample) were used for the PCA statistical analysis. 4.3. Fluctuations in nannofossil assemblages The most outstanding feature within the nannofossil assemblages during the latest Aalenian–earliest Bajocian is the evolutionary transition between Lotharingius and Watznaueria (Cobianchi et al., 1992; Mattioli, 1994; Bown et al., 1996; Bown and Cooper, 1998; Mattioli and Erba, 1999; Perilli et al., 2002; Erba, 2006; Sandoval et al., 2006, among others). In the study interval of the Agua Larga section, the nannofossil assemblages from uppermost Toarcian (Aalensis ammonite Zone) to Lower Aalenian (Opalinum and most the Comptum Subzones) contain a very significant (11.4% average) and diverse proportion of Lotharingius species (Fig. 3, 5) including L. barozii, L. crucicentralis, L. hauffii, L. sigillatus, L.

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umbriensis and L. velatus. However, from the upper part of the Comptum Subzone (sample −189, 34.5 m) to the top of the interval, the proportion of Lotharingius averages 1.4%, implying a decrease of 10% in abundance. In the interval studied, the genus Watznaueria is represented by four species: W. britannica, W. colacicchii, W. communis and W. contracta (Fig. 5). From these, W. britannica and W. communis are present only in the uppermost Aalenian and Bajocian part. W. contracta first occurs in Agua Larga in the latest Toarcian (top of the Aalensis ammonite Zone), and W. colacicchii is present from the base of the section, but never reaches a high proportion (b4%) in the assemblages. Its combined abundance in the assemblages increases progressively towards the top of the section (Fig. 3). Watznaueria is clearly dominant over Lotharingius in the upper part of the section (from the middle of the Bradfordensis ammonite Zone), reaching proportions of 19.2% of the assemblage on average. The results for Cerro de Mahoma section are consistent with those from Agua Larga (Fig. 4). The nannofossil assemblages of the Upper Toarcian and lowermost Aalenian samples contain an average of 6.5% of Lotharingius and 3.7% of Watznaueria. From the base of the Murchisonae ammonite Zone (sample 70a, 22.2 m) to the top of the interval studied, the abundance of Lotharingius decreases to 1.5%, and that of Watznaueria (W.colacicchii + W. contracta) increases to 13.7% on average. Also, as in Agua Larga, Watznaueria is clearly dominant over Lotharingius from the middle part of the Bradfordensis ammonite Zone (sample 77, 25.3 m) onwards, averaging proportions of 14.6%. In both sections, the record of Biscutum dubium and B. intermedium is highly consistent throughout the interval studied, reaching percentages greater than 10% in some Upper Aalenian samples (Concavum and Limitatum ammonite subzones). B. depravatum shows lower abundances and much sparser distribution throughout the sections. Although the records of Similiscutum novum and S. finchii are consistent throughout the study interval in both sections, they are less abundant (average 2.2%), and usually show the greatest proportions in the Lower and Upper Aalenian samples. Finally, S. cruciulus is rare, and found only from uppermost Toarcian samples of Cerro de Mahoma section. All these taxa were considered together as the Biscutum group and their fluctuations in abundance are shown in Figs. 3 and 4. In Agua Larga (Fig. 3), the uppermost Toarcian–Middle Aalenian samples contain an average of 8.1% of Biscutum, but from the lower part of the Concavum ammonite Zone (sample −125, 68.6 m) to the top of the section, the abundance of these taxa reaches 19.4% on average, with peaks exceeding 40%. Despite the lack of data due to the scarcity and poor preservation of nannofossils in an interval covering part of the Concavum ammonite Zone, a similar trend is recorded in Cerro de Mahoma (Fig. 4). In this section, the uppermost Toarcian– Middle Aalenian samples average 7.6% of Biscutum. From the lower part of the Concavum ammonite Zone (sample 100, 38.5 m) to the top of the section, the average abundance of Biscutum is 17.9%, although in some samples the abundance exceeds 30%. Other common taxa displaying consistent fluctuations throughout the interval studied are Schizosphaerella spp. and Crepidolithus crassus (Figs. 3, 4). As Schizosphaerella is considered an incertae sedis taxon (see Appendix), possibly

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Fig. 6. Scatter plots of the δ13Ccarb values with respect to δ18Ocarb values in the Agua Larga (A) and Cerro de Mahoma (B) sections.

a calcareous dinocyst (Kälin and Bernoulli, 1984), its proportion in each sample was calculated with respect to the total nannofossil content (see Section 3). In Agua Larga (Fig. 3), the percentage abundance of Schizosphaerella spp. throughout the uppermost Toarcian–Middle Aalenian is high, reaching 41.6% of the assemblage, on average, with peaks reaching 72.5% and frequently being higher than 45%. In the Upper Aalenian (Concavum ammonite Zone), this proportion decreases to an average of 24.8% with minimum values lower than 1%. A similar pattern is observed in Cerro de Mahoma (Fig. 4), where Schizosphaerella spp. shows high proportions in the uppermost Toarcian–Middle Aalenian (average 36.2%) and smaller proportions throughout the Upper Aalenian (average 26.9%). The decrease in the relative abundance of Schizosphaerella spp. appears as a nearly synchronous event in both sections, located within the upper part of the Gigantea ammonite Subzone (Figs. 3, 4). C. crassus, in Agua Larga (Fig. 3), shows an interval with high average proportions (52.3%) and values frequently exceeding 40% that approximately coincide with the Middle Aalenian. In the uppermost Toarcian–Lower Aalenian interval the average proportion of C. crassus, although with fluctuations, remains

relatively small (24.8%), and the same trend can be seen for the Upper Aalenian–lowermost Bajocian (27.8% average). A similar pattern was observed in Cerro de Mahoma (Fig. 4), where high (38.4%) average proportions of C. crassus were recorded for the Middle Aalenian, whereas the smaller ones correspond to the Upper Toarcian–Lower Aalenian (22.2%) and Upper Aalenian–lowermost Bajocian (29.5%) intervals. 4.4. Radiolarian preservation and abundance Radiolarians and finely shelled pelagic bivalves are the two main components throughout the Aalenian microfacies of the study sections. Concentrations by winnowing due to bottom currents or to turbiditic processes were not recognized. Only some discrete, finely laminated levels were identified in the Lower Aalenian. In general, the Lower Aalenian shows a dominance of finely shelled bivalves over radiolarians. This trend is progressively reversed in the Middle Aalenian, when radiolarians are the dominant skeletal component (Sandoval et al., in press). Radiolarians are abundant in the Middle-Late Aalenian with the exception of few discrete levels where their abundance decreases and fine pelagic bivalve shells occur

Fig. 5. Light micrographs of the most relevant nannofossil taxa found throughout the interval studied in the Agua Larga (AQ) and Cerro de Mahoma (CM) sections. All micrographs under cross-polarized light. 1–2, Biscutum depravatum; 1, sample AQ-65, 0° to crossed nicols; 2, sample AQ-65, 0° to crossed nicols. 3–4, Biscutum dubium; 3, sample AQ-86, 45° to crossed nicols; 4, sample AQ-65, 45° to crossed nicols. 5–6, Biscutum intermedium; 5, sample AQ-240, 40° to crossed nicols; 6, sample AQ-29, 20° to crossed nicols. 7, Similiscutum finchii, sample AQ-72, 50° to crossed nicols. 8, Similiscutum novum, sample AQ-112, 30° to crossed nicols. 9, Bussonius prinsii, sample CM88, 50° to crossed nicols. 10, Calyculus? sp., sample AQ-65, 30° to crossed nicols. 11–12, Crepidolithus crassus; 11, sample AQ-59, 20° to crossed nicols; 12, sample AQ-225, 45° to crossed nicols. 13, 19–20, Carinolithus magharensis; 13, sample CM101, plan view; 19, sample AQ-59, lateral view, 0° to crossed nicols; 20, sample AQ-65, plan view. 14, 21–22, Diductius constans; 14, sample AQ-87, 30° to crossed nicols; 21, sample AQ-59, 0° to crossed nicols; 22, sample AQ-70, 10° to crossed nicols. 15–16, Discorhabdus criotus; 15, sample AQ-87; 16, sample AQ-65. 17–18, Carinolithus superbus; 17, sample AQ-62, 40° to crossed nicols; 18, sample AQ-70, 0° to crossed nicols. 23–24, Discorhabdus striatus; 23, sample AQ-108; 24, sample AQ-120. 25, Lotharingius barozii, sample AQ225, 10° to crossed nicols. 26, Lotharingius crucicentralis, sample CM91, 20° to crossed nicols. 27–28, Lotharingius hauffii; 27, sample CM55, 20° to crossed nicols; 28, sample AQ-243, 60° to crossed nicols. 29–30, Lotharingius sigillatus; 29, sample AQ-225, 20° to crossed nicols; 30, sample AQ-70, 10° to crossed nicols. 31–32, Lotharingius velatus; 31, sample AQ-60, 20° to crossed nicols; 32, sample AQ-65, 15° to crossed nicols. 33–34, Retecapsa incompta; 33, sample AQ-65, 20° to crossed nicols; 34, sample AQ-87, 25° to crossed nicols. 35–36, Tubirhabdus patulus; 35, sample AQ-65, 50° to crossed nicols; 36, sample AQ-59, 0° to crossed nicols. 37–38, Watznaueria colacicchii; 37, sample AQ-245, 20° to crossed nicols; 38, sample AQ-197, 15° to crossed nicols. 39–40, Watznaueria contracta; 39, sample AQ-71, 0° to crossed nicols; 40, sample AQ-65, 10° to crossed nicols. 41, Triscutum tiziense, sample AQ-87, 45° to crossed nicols. 42, Schizosphaerella punctulata, sample CM86. 43, Triscutum sullivanii, sample AQ-55, 0° to crossed nicols. 44–45, Watznaueria britannica; 45, sample CM139, 0° to crossed nicols; 46, sample AQ-38, 20° to crossed nicols. 46–47, Zeugrhabdotus erectus; 47, sample AQ-65, 45° to crossed nicols; 48, sample AQ-60, 0° to crossed nicols.

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again, as at the boundary of the Bradfordensis–Gigantea ammonite Subzones. Despite of the high abundance of radiolarians, their preservation is rather poor and their skeletons are commonly calcified. Preservation is locally better within micro-bioturbated sediments, mainly Chondrites. This is a quite common pattern in the studied sections. Sometimes, as in the Middle Aalenian, the skeletons are heavily calcified and it is difficult to recognise the radiolarian remains from the micritic matrix. Only in the upper part of the Cerro de Mahoma section do radiolarians become quite abundant at the same time as there is an increase in the number of visible chert nodules (Fig. 4). The presence of chert improves radiolarian preservation, and allows acid treatment for taxonomic determination (Sandoval et al. in press). In the Barranco de Agua Larga section no visible chert nodules in layers occurred (Fig. 3), and thus radiolarian extraction was unsuccessful. In thin section, however, a similar trend for radiolarian abundance has been observed (compare Figs. 3, 4). The similar pattern of the abundance curves in the two sections clearly points for a primary control in the abundance fluctuations recorded. The distribution is quite regular throughout the interval studied and abundance values fluctuate between 1 to 30% in the Early and Middle Aalenian, respectively. In the upper part of Middle Aalenian, we notice in both sections an important increase in the abundance (Figs. 3, 4). Abundance during the Lower Aalenian is in general low, recording the lowest values (lower than 5%). The abundance increases slightly (10%) in both sections at the beginning of the Middle Aalenian. Through the Gigantea–Concavum ammonite Subzones the abundances reach values of 20–30%. This important abundance increase of radiolarians predates the positive excursion in δ13Ccarb recorded in the sections investigated (Figs. 3, 4). 5. Discussion 5.1. Nannofossil assemblages and carbon-cycle perturbations Fluctuations in abundance and composition observed in nannofossil assemblages could be interpreted as the response of phytoplankton to palaeoclimatic and palaeoceanographic changes revealed through stable isotope investigations. The isotopic carbon composition of marine surface waters depends on primary productivity and other environmental parameters as temperature, upwelling, and CO2 exchange with the atmosphere (Steinmetz, 1994). Also, calcareous nannoplankton assemblages are affected by such environmental factors as temperature, nutrient availability or salinity of ocean surface waters (Brand, 1994), which may be recorded in stable carbon isotopes of carbonate sediments. According to this, some links should appear between calcareous nannofossil assemblages and δ13Ccarb curves. To analyse a more complete nannofossil community instead of changes in single taxa abundance, the PCA statistical method was applied to percentage values of selected taxa (see above, Section 3) recorded in the interval studied. The PCA method facilitate interpretations of complex data sets, by reducing a large data matrix composed of a great number of variables to a small number of factors representing the main modes of variations. The results are summarized in Fig. 7.

For Agua Larga, two factors show eigenvalues greater than 1, and together represent the 64% of the variance in the data set. Factor 1 (eigenvalue = 2.68; 33.67% of the variance), loading on B. dubium and B. intermedium in opposition with Schizosphaerella spp. and C. crassus and, in minor proportion, with Lotharingius and D. striatus, may be interpreted in terms of nutrient concentration and nutricline depth within the photic zone. (Fig. 7A). Biscutum is generally associated by most authors (Roth and Bowdler, 1981; Roth and Krumbach, 1986; Premoli Silva et al., 1989; Watkins, 1989; Erba, 1992; Mattioli and Pittet, 2004; Tremolada et al., 2006) with eutrophic environments, revealing high fertility of surface waters. Schizosphaerella spp. flourishes under conditions of a deep nutricline in the photic zone and oligotrophic surface waters, and is more abundant in carbonate-rich sediments during the Early–Late Jurassic (Noël et al., 1994; Claps et al., 1995; Mattioli, 1997; Cobianchi and Picotti, 2001; Pittet and Mattioli, 2002; Erba, 2004; Mattioli and Pittet, 2004; Olivier et al., 2004; Tremolada et al., 2006). C. crassus is interpreted as a deep-dweller (Mattioli and Pittet, 2004; Bour et al., 2007) and is also affiliated with oligotrophic conditions in marine surface waters. Factor 2 (eigenvalue = 2.42; 30.35% of the variance), in which Lotharingius and D. striatus are opposed to C. crassus and W. contracta, is more difficult to interpret, but probably reflects in part the replacement of Lotharingius by W. contracta (see Section 4.3, Fig. 3 and 7A). Lotharingius is also considered by some authors (Mattioli and Pittet, 2004) to be more abundant when enhanced trophic conditions prevailed. This remains unclear from our data, as Lotharingius is opposed to B. dubium and B. intermedium in Factor 1, but also is opposed to C. crassus in Factor 2. The scatter plot of the factor scores for Factors 1 and 2 with respect to the corresponding δ13Ccarb and δ18Ocarb values from the Agua Larga section (Fig. 8), shows a low correlation between Factors 1 and 2 and the δ18Ocarb values (Fig. 8C, D). Also, a low correlation is shown between Factor 2 and δ13Ccarb values. However, a better correlation (r = 0.423 to 0.604; see below) is detected between Factor 1 (interpreted in terms of nutricline depth within the photic zone) and δ13Ccarb values (Fig. 8A). Within the Limitatum ammonite Subzone, three samples (−65, 101 m; −62,102.8 m and −55,108 m; Fig. 2A and 3) showed high fertility calcareous nannofossil assemblages characterized by very low proportions of Schizosphaerella spp. and high proportions of Biscutum (B. dubium and B. intermedium), but its values are not so high. In smear slides, significant proportions of small quartz grains and other terrigenous minerals were observed in these samples. Some offset was noted in the δ13Ccarb values of the studied sections that were interpreted to correspond to the variation in carbon isotope of dissolved inorganic carbon of seawater, probably associated with changes in the oxidation rate of organic carbon (Sandoval et al., in press). These δ13Ccarb low values could be interpreted in terms of continental organic matter input in an oxidizing seawater environment (Sandoval et al., in press). In Fig. 8A, the black-dotted points correspond to the scores of Factor 1 in these three samples. The dashed line in Fig. 8A represent the best fit for all the samples studied, revealing a moderately good correlation (r = 0.423) between Factor1 scores and δ13Ccarb values. The solid line in the figure represents the best fit excluding the black-dotted samples, and shows a much higher (r = 0.604) correlation between Factor 1 scores and δ13Ccarb.

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Fig. 7. Results of Principal Component Analysis. The results of the PCA analysis for the study sections are tabulated below the corresponding plots. A) Plot of Factor 1 against Factor 2 loadings for the selected taxa in Agua Larga section (see text for details). B) Plot of Factor 1 against Factor 2 loadings for the selected taxa in Cerro de Mahoma section (see text for details).

For the Cerro de Mahoma section, the results of the PCA analysis allow the extraction of three factors showing eigenvalues greater than 1 and representing together the 74.7% of the variance in the data set. Factor 1 (eigenvalue = 3.00; 37.72% of the variance), in which W. contracta and, in lower proportion, B. intermedium are opposed to D. striatus and Lotharingius and also to Schizosphaerella spp., probably reflects mainly the replacement of Lotharingius by W. contracta (see Section 4.3, Figs. 4 and 7B). However, could also be partially interpreted in terms of nutrient concentration and nutricline depth within the photic zone, as B. intermedium and Schizosphaerella spp. also show opposed loadings values. Factor 2 (eigenvalue = 1.83; 23.07% of the variance; Fig. 7B), loading on B. dubium and B. intermedium in opposition with C. crassus and Schizosphaerella spp., could be interpreted in terms of the nutrient concentration, reflecting nutricline depth within the photic zone, like Factor 1 in Agua Larga section (Fig. 7A). Factor 3 (not shown in scatter plots) with an eigenvalue = 1.11 and representing 14% of the variance, heavily loads on C. superbus and is in opposition to D. striatus, but is actually difficult to interpret. Scatter plots of the factor scores for Factors 1 and 2 with respect to the δ13Ccarb and δ18Ocarb values show a weak

correlation (Fig. 9A,B) or lack of correlation (Fig. 9C,D) for the studied interval of Cerro de Mahoma section. This fact could be the consequence of a more relevant diagenetic overprint in the rocks of this section (see above, Section 4.2). 5.2. Latest Toarcian to earliest Bajocian palaeoceanography and calcareous nannoplankton–radiolarian evolution The main fluctuations in calcareous nannofossil assemblages (Section 4.3) and trophic conditions in the upper photic zone have been already discussed (Section 5.1). Two groups of taxa were separated on the basis of its ecological significance: Biscutum (eutrophic surface-waters indicator) versus Schizosphaerella spp. and C. crassus (associated with a depth nutricline and indicating oligotrophic surface-water conditions). The fluctuation of these fertility indicators of surface water has shown a good correlation with variations in δ13Ccarb values during the Aalenian. These data are discussed here in order to understand the Aalenian palaeoceanographic regime and its possible relationships with calcareous nannoplankton and radiolarian evolution. The latest Toarcian–Early Aalenian interval is characterized by the coexistence of high proportions of Schizosphaerella spp.

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Fig. 8. Scatter plots of Factor 1 and Factor 2 scores against the respective δ13Ccarb and δ18Ocarb values for the Agua Larga section. Lines correspond to the best fit by single polynomial regression. For all plots, the number of data points and the correlation coefficients are indicated. See text for details. A) Plot of Factor 1 scores against δ13Ccarb values. B) Plot of Factor 2 scores against δ13Ccarb values. C) Plot of Factor 1 scores against δ18Ocarb values. D) Plot of Factor 2 scores against δ18Ocarb values.

together with moderate proportions of C. crassus and minor proportions of Biscutum (Fig. 3, 4, 10). During this interval, the δ13Ccarb values remained low to medium. These results point to oligotrophic to mesotrophic conditions in surface photic zone throughout this interval (Fig. 10). This was, in general, an episode of low-nutrient surface waters, with a deep nutricline where high proportions of heavily calcified taxa (Schizosphaerella spp. and C. crassus) proliferated. These taxa dominate the latest Toarcian–Early Aalenian calcareous nannofossil assemblages, and authors credit it with contributing to an important fraction of fine-grained biogenic carbonates (Weissert and Erba, 2004; Erba, 2006). During this interval the radiolarian abundance displays very low values. Coccolithophore species tend to be the most abundant components of the phytoplankton community in warm, stratified, nutrient-poor waters and less dominant in nutrient-rich waters (Brand, 1994; Erba 2006). In modern oceans,

the seasonally stable, oligotrophic environments support the highest diversities, although reproduction rates are low (Brand, 1994; Bown and Young, 1998). Its diversification would be then favoured by low–nutrient availability and stable conditions (Bown et al., 2004; Erba 2006). The importance of nutrient availability for calcareous nannoplankton evolution in the Mesozoic ocean was stressed by Roth (1987, 1989). According to this author, scarcity of limiting nutrients induces competition and speciation whilst nutrient availability is related with extinctions. During the latest Toarcian–Early Aalenian, the Agua Larga and Cerro de Mahoma nannofossil records show an origination phase, with the successive FOs of Watznaueria contracta, Triscutum tiziense and Carinolithus magharensis (Fig. 2). This origination phase could, at least in part, be interpreted as a consequence of this period of stability with the development of meso- to oligotrophic conditions in ocean surface waters.

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Fig. 9. Scatter plots of Factor 1 and Factor 2 scores against the respective δ13Ccarb and δ18Ocarb values for the Cerro de Mahoma section. Lines correspond to the best fit by single polynomial regression. For all plots, the number of data points and the correlation coefficients are indicated. See text for details. A) Plot of Factor 1 scores against δ13Ccarb values. B) Plot of Factor 2 scores against δ13Ccarb values. C) Plot of Factor 1 scores against δ18Ocarb values. D) Plot of Factor 2 scores against δ18Ocarb values.

During the Middle Aalenian, both, Schizosphaerella spp. and C. crassus reach their highest proportions in the assemblages, coinciding with a low abundance of Biscutum and with the lower values in the δ13Ccarb (Figs. 3, 4, 10). The total abundance of radiolarians falls concomitantly with the increase of Schizosphaerella spp. and C. crassus assemblages (Fig. 10). This interval should be interpreted as an episode of highly oligotrophic conditions with very low nutrient concentration in ocean surface waters. The evolutionary transition between Lotharingius and Watznaueria, (Section 4.3 and Figs. 3, 4) could, in part, be linked to this period of enhanced oligotrophic, stable conditions (Fig. 10). According to Roth (1987, 1989) scarcity of nutrients induces competition and probably some Lotharingius species and W. contracta occupied similar ecologic niches. Finally, in the Late Aalenian–earliest Bajocian interval, a higher proportion of Biscutum and a very significant decrease in

the proportions of Schizosphaerella spp. and C. crassus together with the highest values in the δ13Ccarb suggest the onset of meso- to eutrophic conditions in surface waters (Figs. 3, 4, 10). Through the Late Aalenian, the abundance of radiolarians, in general, undergoes a great increase and reaches the highest values of the entire studied interval (i.e. Fig. 4). In the early Late Aalenian a marked palaeoceanographic change occurred in the fertility of surface waters probably related to accelerated hydrological cycle. Similar scenarios have been proposed for the Early Toarcian (Suan et al., 2008), where high rates of continental runoff, increased nutrient input, and reduced salinity, may have induced changes in trophic conditions. However, we cannot dismiss an increase in surface-water productivity due to greater upwelling related to the acceleration of the Tethyan-Atlantic opening (Bill et al., 2001). This episode could also be related to the surge in siliciclastics

282 R. Aguado et al. / Marine Micropaleontology 68 (2008) 268–285 Fig. 10. Fluctuations in the percentage abundance of some selected nannofossil taxa and radiolarians against the δ13Ccarb curves throughout the study interval in Agua Larga and Cerro de Mahoma sections. Also, subintervals differentiated in relation to the trophic regime of surface-water masses. Average percentages of eutrophic (Biscutum) and oligotrophic (Schizosphaerella and Crepidolithus crasssus) surface-waters indicators per ammonite subzone throughout the studied interval in Agua Larga (circles) and Cerro de Mahoma (diamonds). Note that percentage abundances of coccolith taxa were calculated with respect to the total number of coccoliths, and Schizosphaerella percentages were calculated with respect to the total nannofossil content. The average percentage of radiolarians per ammonite subzone in Agua Larga (squares) and Cerro de Mahoma (triangles) is based on estimates from slices using the method by Baccelle and Bosellini (1965). Ammonite zones and subzones and δ13Ccarb curves from the two study sections after Sandoval et al. (in press). Geochronology and Tethyan sequences after Gradstein et al. (2004) and Hardenbol et al. (1998).

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observed in Agua Larga (see Section 2.1) and the presence of chert nodules as well as more abundance of radiolarians in both sections. Consequently, the amount of nutrients in sea surface water increased, leading to more eutrophic conditions. During this interval, nannofossil assemblages contained higher proportions of eutrophic low-calcified taxa (Biscutum) and presumably, its contribution to biogenic fine-grained carbonates remained lower (Weissert and Erba, 2004; Erba, 2006). High fertility coupled with unstable conditions resulted in lowdiversity nannoplankton assemblages and extinctions (Roth, 1987, 1989; Bown et al., 2004; Erba, 2006). During this interval, the Agua Larga and Cerro de Mahoma nannofossil records are characterized by the LO of S. finchii, although the possibility of the LO of other taxa (L. hauffi, L. barozii) cannot be dismissed. The marked palaeoceanographic change that occurred in the Late Aalenian, as revealed by the nannofossil assemblages, is also manifested by a major faunal turnover in the siliceous microplankton. According to Yao (1997) the major radiolarian turnover in the Jurassic occurred in at the Middle–Late Aalenian times; a quasi complete replacement of Early Jurassic fauna is achieved during the Aalenian. Through the Late Aalenian new and important phylogenetic lineages developed which will be the sources for the Middle Jurassic–Early Cretaceous radiolarian genera (Baumgartner et al., 1995, Gorican et al., 2006). This major turnover is located, causally, at time when we recognized an impressive increase in the abundance of radiolarians, as well as a major perturbation in the Carbon cycle (Fig. 10). This drastic faunal change can be interpreted as a major biological response of the siliceous microplankton to the opening of new ecological niches due to major palaeotectonic modifications in the palaeogeography of the western Tethys as consequence of the Atlantic opening (Bill et al., 2000). The increasing values of δ13C in the Late Aalenian–Bajocian may be consequence of a greater oceanic fertilization, as shown by: (a) the higher radiolarian abundance; (b) the qualitative changes occurred in the radiolarian assemblages during the Late Aalenian (Bartolini et al., 1999); and (c) the higher proportions of eutrophic lowcalcified nannofossil taxa (Biscutum). The influence of upwelling, undoubtedly played a major role by enhancing siliceous production (Bartolini et al., 1999, O'Dogherty et al., 2006) in a particular scenario resulted of the opening of the Liguro– Piemontese Ocean which in turn caused a rearrangement, or even a new model, in the oceanic circulation patterns (Bill et al., 2001). 6. Conclusions The cosmopolitan and Tethyan calcareous nannofossil assemblages found in the Aalenian Subbetic show significant fluctuations in total and relative abundances which record palaeoceanographic changes in surface water that affected the most distal part of the western Tethys. The comparison of these fluctuations with the δ13Ccarb curves, abundance of radiolarians, and the interpretation of the palaeoecologic significance of some calcareous nannofossil taxa provide an outline of the palaeoceanographic trophic regime of sea surface waters throughout the time interval studied. The latest Toarcian–Early Aalenian is marked by the coexistence of very low radiolarian content, high proportions of Schizosphaerella spp., moderate proportions of C. crassus, and small proportions of Biscutum. These bioevents together with

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low to medium δ13Ccarb values indicate oligotrophic to mesotrophic palaeoceanographic conditions. This represents an episode of low-nutrient surface waters, with a deep nutricline. The paleoceanographic conditions through the Middle Aalenian record an episode of highly oligotrophic conditions with very low-nutrient surface waters, as shown by high proportions of Schizosphaerella spp. and C. crassus, low abundances of Biscutum and the smaller values in the δ13Ccarb. Throughout the Late Aalenian to earliest Bajocian , oligo- to eutrophic conditions occurred in surface waters as shown by a higher proportion of Biscutum and very significant decreases in the proportions of Schizosphaerella spp. and C. crassus together with the highest values in abundance of radiolarians and the δ13Ccarb. Nannoplankton evolution throughout the interval studied also appears to be linked to these palaeoceanographic changes, in particular, the replacement of Lotharingius by Watznaueria during the Middle Aalenian. Acknowledgments The authors warmly thank Dr. E. Mattioli for constructive discussions and thoroughly review of this manuscript. Comments by one anonymous reviewer and Dr. A. Mackensen greatly improved the quality of this manuscript. Mr. D. Nesbitt corrected the English version of the text. This study forms part of the results of Research Projects: CGL2005-02500, financed by the DGI (Dirección General de Investigación, Spain), and by the RNM-178, 200, 208 Research Groups, Junta de Andalucía, Spain). Appendix A. Index of calcareous nannofossils identified in the study sections with appropriate author citations Biscutum depravatum (Grün and Zweili, 1980) Bown, 1987 Biscutum dubium (Noël, 1965) Grün in Grün et al., 1974 Biscutum intermedium Bown, 1987 Bussonius prinsii (Noël, 1973) Goy, 1979 Calyculus spp. Noël, 1973 Carinolithus magharensis (Moshkovitz and Ehrlich, 1976) Bown, 1987 Carinolithus superbus (Deflandre in Deflandre and Fert,1954) Prins in Grün et al., 1974 Crepidolithus crassus (Deflandre in Deflandre and Fert, 1954) Noël, 1965 Diductius constans Goy, 1979 Discorhabdus criotus Bown, 1987 Discorhabdus ignotus (Górka, 1957) Perch-Nielsen, 1968 Discorhabdus striatus Moshkovitz and Ehrlich, 1976 Lotharingius barozii Noël, 1973 Lotharingius crucicentralis (Medd, 1971) Grün and Zweili, 1980 Lotharingius hauffii Grün and Zweili in Grün et al., 1974 Lotharingius sigillatus (Stradner, 1961) Prins in Grün et al.,1974 Lotharingius umbriensis Mattioli, 1996 Lotharingius velatus Bown and Cooper, 1989 Retecapsa incompta Bown and Cooper, 1989 Schizosphaerella spp. Deflandre and Dangeard, 1938 [incertae sedis] Similiscutum cruciulus de Kaenel and Bergen, 1993

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