Secular environmental precursors to Early Toarcian (Jurassic) extreme climate changes

Earth and Planetary Science Letters 290 (2010) 448–458

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Earth and Planetary Science Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e p s l

Secular environmental precursors to Early Toarcian (Jurassic) extreme climate changes Guillaume Suan a,⁎, Emanuela Mattioli a, Bernard Pittet a, Christophe Lécuyer a, Baptiste Suchéras-Marx a, Luís Vítor Duarte b, Marc Philippe a, Letizia Reggiani a,c, François Martineau a a b c

Laboratoire CNRS UMR 5125 “PaléoEnvironnements & PaléobioSphère”, Université Claude Bernard Lyon 1, Campus de la Doua, F-69622 Villeurbanne, France Departamento de Ciências da Terra, IMAR-CMA, Faculdade de Ciências e Tecnologia da Universidade de Coimbra, 3000-272 Coimbra, Portugal Dipartimento di Scienze della Terra, Università di Perugia, Piazza Università, 06123 Perugia, Italy

a r t i c l e

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Article history: Received 16 July 2009 Received in revised form 11 December 2009 Accepted 29 December 2009 Available online 18 January 2010 Editor: M.L. Delaney Keywords: Pliensbachian–Toarcian stable isotopes paleotemperature carbon cycle glacio-eustasy carbonate production

a b s t r a c t The Early Toarcian Oceanic Anoxic Event (T-OAE), about 183 myr ago, was a global event of environmental and carbon cycle perturbations, which deeply affected both marine biota and carbonate production. Nevertheless, the long-term environmental conditions prevailing prior to the main phase of marine extinction and carbonate production crisis remain poorly understood. Here we present a ∼ 8 myr-long record of Early Pliensbachian–Middle Toarcian environmental changes from the Lusitanian Basin, Portugal, in order to address the long-term paleoclimatic evolution that ultimately led to carbonate production and biotic crises during the T-OAE. Paleotemperature estimates derived from the oxygen isotope compositions of wellpreserved brachiopod shells from two different sections reveal a pronounced ∼ 5 °C cooling in the Late Pliensbachian (margaritatus–spinatum ammonite Zones boundary). This cooling event is followed by a marked ∼ 7–10 °C seawater warming in the Early Toarcian that, after a second cooling event in the midpolymorphum Zone, culminates during the T-OAE. Calcium carbonate (CaCO3) contents, the amount of nannofossil calcite and the mean size of the major pelagic carbonate producer Schizosphaerella, all largely covary with paleotemperatures, indicating a coupling between climatic conditions and both pelagic and neritic CaCO3 production. Furthermore, the cooling and warming episodes coincided with major marine regressions and transgressions, respectively, suggesting that the growth and decay of ice caps may have exerted a strong control on sea-level fluctuations throughout the studied time interval. This revised chronology of environmental changes shows important similarities with Neogene and Paleozoic episodes of deglacial black shale formation, and thus prompts the reevaluation of ice sheet dynamics as a possible agent of Mesozoic events of extinction and organic-rich sedimentation. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Geochemical, sedimentological and paleontological data indicate that the Late Pliensbachian–Early Toarcian mass extinction event (∼183 myr ago, Early Jurassic) was accompanied by severe environmental changes that included development of anoxic conditions, changes in the hydrological cycle, marked variations in seawater temperatures and changes in marine and terrestrial biota (Jenkyns, 1988; Philippe and Thévenard, 1996; Macchioni and Cecca, 2002; Bailey et al., 2003; Cohen et al., 2004; Wignall et al., 2005; Rosales et al., 2006; Suan et al., 2008a; Gómez et al., 2008). It has been suggested that these major environmental changes could have been triggered by massive releases of greenhouse gases, possibly involving the destabilization of marine gas hydrates or the thermal metamorphism of ⁎ Corresponding author. Present address: Institut de Géologie et de Paléontologie, Université de Lausanne, Anthropole, CH-1015 Lausanne, Switzerland. E-mail addresses: [email protected], [email protected] (G. Suan). 0012-821X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2009.12.047

organic-rich sediments during the intrusive phase of the eruption of the Karoo–Ferrar large igneous province (Hesselbo et al., 2000; Cohen et al., 2007; Svensen et al., 2007). Shallow-water carbonate platforms and calcareous nannofossils, as well as benthic and pelagic invertebrates, were particularly affected by these major environmental changes, notably across the Pliensbachian–Toarcian boundary and during an episode of widespread organic-rich deposition defined as the Toarcian Oceanic Anoxic event (T-OAE) (Bassoullet and Baudin, 1994; Harries and Little, 1999; Cobianchi and Picotti, 2001; Macchioni and Cecca, 2002; Erba, 2004; Mattioli et al., 2004; Tremolada et al., 2005; Wignall et al., 2005; Mattioli et al., 2008; Suan et al., 2008a). Most studies relate these mass extinctions and biocalcification crises to pulses of CO2-induced environmental changes, namely by enhanced nutrient input due to accelerated hydrological cycle, productivitydriven anoxia, rise in seawater temperatures and changes in the saturation state of the ocean with respect to calcite (Erba, 2004; Mattioli et al., 2004; Tremolada et al., 2005; Wignall et al., 2005; Mattioli et al., 2008; Gómez et al., 2008).

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Though many studies focused specifically on the Early Toarcian episode, the long-term environmental conditions that led to severe climatic and biotic perturbations remain poorly understood. In particular, a number of studies have suggested that the Late Pliensbachian was characterized by low seawater temperatures and cool atmospheric conditions, which may have been associated with the transient development of icehouse conditions prior to the T-OAE (Price, 1999; Cobianchi and Picotti, 2001; Guex et al., 2001; Bailey et al., 2003; van de Schootbrugge et al., 2005). Indeed, the temporal coincidence between this cooling episode and a major marine regression has been interpreted as possibly reflecting the growth of continental ice at high latitudes (Price, 1999; Cobianchi and Picotti, 2001; Guex et al., 2001; van de Schootbrugge et al., 2005). It has been proposed that these important environmental perturbations may have also contributed to platform carbonate production decline, and may as well have preconditioned the western epicontinental seas to water mass stratification during the T-OAE (Cobianchi and Picotti, 2001; Bailey et al., 2003). Nevertheless, the relative timing between this cooling episode, the decline of carbonate production and the sealevel fall remain poorly constrained, impeding integration of longand short-term climate variability in our understanding of the Toarcian climatic and biotic events. The aim of this study is to address the long-term evolution of climatic conditions and pelagic and neritic carbonate production during the Pliensbachian–Early Toarcian using a multi-proxy approach based on geochemical data and nannofossil abundance and biometry. Our study is based on data acquired in the Lusitanian Basin, and in particular from the reference section of Peniche, Portugal (Pliensbachian–Toarcian boundary GSSP candidate). The succession provides a continuous exposure of biostratigraphically well-constrained marine hemipelagic sediments of Late Sinemurian to Middle Toarcian age (Duarte and Soares, 2002; Duarte, 2007), and thus represents an ideal stratigraphic framework for such a study. To reconstruct the evolution of both the pelagic and neritic carbonate production, we present CaCO3 contents

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and a quantification of nannofossil abundance and size for the Upper Pliensbachian. We use new carbon and oxygen isotope data of brachiopod shells sampled from the same section in order to access to concomitant changes in carbon cycling and paleotemperatures. We also present an additional carbon and oxygen isotope record of brachiopod shells sampled in a shallow marine environment succession of the Lusitanian Basin (Tomar) to further assess the environmental significance of the stable-isotope signal acquired in Peniche.

2. Geological setting and stratigraphy 2.1. Peniche The Peniche section in Portugal (Fig. 1) presents a complete succession of marine hemipelagic marls and limestones of Late Sinemurian to Middle–Late Toarcian age (Duarte and Soares, 2002) that were deposited in one of deepest part of the Lusitanian Basin (Duarte, 1998, 2007). The ammonite biostratigraphy of the section is well established at the zonal level and refers to the studies of Mouterde (1955), Elmi et al. (1996) and Elmi (2006). The middle part of the Pliensbachian (top of ibex to margaritatus ammonite zones) displays several organic matter-rich levels generally poor in benthic macrofauna that may contain as much as 15% of organic carbon (Oliveira et al., 2006). The uppermost Pliensbachian (spinatum Zone) is characterized by thick carbonate-rich marl limestone alternations rich in belemnite rostra and brachiopod shells. Lithostratigraphy, stable-isotope data, nannofossil abundance and size, and cyclostratigraphic framework for the uppermost Pliensbachian and Lower Toarcian were documented by Hesselbo et al. (2007), Suan et al. (2008a,b) and Mattioli et al. (2008). In this study, we have extended the previously existing record (Suan et al., 2008a) by generating new measurements of the carbon and oxygen isotope compositions of brachiopod shells, CaCO3 contents and size of the main pelagic

Fig. 1. (A) Location of the studied sections; (B), paleogeography of the western margin of the Tethys Ocean during the Early Jurassic and location of the Lusitanian Basin; (C) position of the studied sections relative to the morphology and structure of the Lusitanian Basin during the Early Toarcian (modified after Vanney and Mougenot, 1981). BF: Berlanga– Farilhões horst.

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carbonate producer Schizosphaerella for most of the Lower and Upper Pliensbachian. 2.2. Tomar In order to unravel the paleoenvironmental significance of the brachiopod stable-isotope record of the Peniche section, we have also analyzed the stable isotopic composition of brachiopod shells from another locality of the Lusitanian Basin (Tomar area, Fig. 1). The Pliensbachian and Toarcian marine marls and limestones exposed ∼ 5 km north of the city of Tomar corresponded to a shallower marine setting and to one of the easternmost areas of the westerly dipping homoclinal ramp of the Lusitanian Basin (Duarte, 1997, 1998). Due to limited exposure, the Upper Pliensbachian to Middle Toarcian succession was sampled in two different sections. The Upper Pliensbachian and lowermost Toarcian (ibex to basal levisoni ammonite zones) have been sampled in the Vale Venteiro area, while the Lower and Middle Toarcian (levisoni and bifrons ammonite zones) were sampled in the exposure along the road facing the Prado factory (Casais and Prado). The detailed litho- and biostratigraphy of these sections is given by Mouterde et al. (1971) and Alméras et al. (1997). The Upper Pliensbachian part comprises a succession of marls and limestones, locally rich in bivalve and brachiopod shells. The sequence is characterized by a shallowing upward trend at the end of the Pliensbachian that is capped by a succession of massive grainstones presenting abundant rounded belemnites and bioclasts. The deposition of this unit likely took place under few meters below the sea surface. The top surface of the last massive calcareous bed of this succession contains several Dactylioceras specimens that mark the base of the Toarcian. As observed in the whole Lusitanian Basin with the exception of Peniche, the levisoni Zone begins with a succession of amalgamated and irregular calcarenite beds (Calcários em Plaquetas; Duarte, 1997). A number of these fine- to coarse-sand calcareous beds display characteristic hummocky cross stratifications, amalgamation, graded bedding and erosive-bases, indicating a storm origin for these deposits (tempestites). Macrofauna is largely absent in the most basal part of the levisoni Zone, while it is very abundant in its middle and upper part, comprising numerous and complete specimens of the brachiopod Soaresirhynchia bouchardi and Telothyris jauberti. The presence of relatively high-energy shallow-facies throughout the succession indicates that the Upper Pliensbachian–Lower Toarcian sediments were deposited in a shallower environment (mid-ramp; Duarte, 1998) as compared to that of Peniche. 3. Materials and methods 3.1. Stable-isotope data 3.1.1. Brachiopod shells A total of 82 brachiopod shells were analyzed for their carbon and oxygen isotope compositions and their preservation was assessed. The stable-isotope profiles correspond to a composite of different taxa of rhynchonellids and terebratulids (see supplementary data). The rejection of diagenetically altered brachiopod shell was realized on the basis of the macro- and micro-textural aspects of the shells, following the methodology described by Suan et al. (2008a). The primary layer, which is considered to be secreted out from isotopic equilibrium with oceanic water in modern species (e.g., Carpenter and Lohmann, 1995; Auclair et al., 2003; Parkinson et al., 2005), was carefully removed with a dental tool under a binocular microscope. Sampling for isotopic analyses was realized only on the fibrous calcite of the secondary layer within the anterior part of the shell. Carbon and oxygen isotope compositions of brachiopod shells were determined using an auto sampler Multiprep coupled to a GV Isoprime® mass spectrometer. For each sample, an aliquot of about 300 µg of calcium carbonate was reacted with anhydrous over-

saturated phosphoric acid at 90 °C during 20 min. Isotopic compositions are quoted in the delta notation in permil relative to V-PDB. All sample measurements were duplicated and adjusted to the international reference NIST NBS19. Reproducibility is 0.1‰ (1σ) for δ18O values and 0.05‰ (1σ) for δ13C values. 3.1.2. Fossil wood Fifteen samples of macrofossil wood from the Peniche section were collected for systematical identification and measurements of stable carbon isotope ratios. Each sample was carefully searched for anatomically well-preserved areas. Once located these were gently split away, using a disposable razor blade. Resulting pieces were mounted on aluminum stubs, with double-sided conducting adhesive tape, coated with gold/palladium at 25 kV for 5 min, and then observed under a 10 kV acceleration voltage with a Hitachi S-800 scanning electronic microscope. Determination was performed following principles described in Philippe and Bamford (2008). About 10 mg of macroscopic fossil wood were separated from adhering matrix under a binocular microscope, crushed to fine fraction and decarbonated using 1 N HCl for 24 h at ambient temperature. The samples were then rinsed with deionized water, centrifuged, rinsed again until neutrality was reached and dried in an oven at 60 °C. Samples of ∼ 0.2 mg of fossil wood were weighed in tin capsules and placed in a Eurovector Elemental Analyzer (EuroEA3028-HT) connected to a GV instrument® Isoprime stableisotope-ratio mass spectrometer. The measurements were standardized using IAEA CH7 (NIST-8540) (δ13C = − 31.8 ± 0.18‰) and Tyrosine (δ13C = −23.2 ± 0.1‰) and are quoted in the delta notation relative to V-PDB. All sample measurements were duplicated and adjusted to the international reference NIST NBS19. Reproducibility was close to 0.1‰ (1σ). 3.2. CaCO3 budget and nannofossils Calcium carbonate (CaCO3) contents were determined by measuring the volume of CO2 released by acidification of ∼ 300 mg of rock powder. The measurements were standardized to a 98% carbonate calcium powder. Reproducibility was better than 1% (1σ) for duplicate analyses. Measurements of Schizosphaerella were made using a Zeiss Axioscope 40 optical microscope with a magnification ×1000. Thirty specimens per sample were snapped with a Moticam 2000 camera and biometric measurements were made with Motic Images Plus 2.0 (graphic precision of one pixel of 0.0833 µm long and 0.0847 µm large). An error of measurement has been estimated to ±0.17 µm (i.e., ± 2 pixels), by repetitive (10 times) measurements of 10 schizospheres. Hemi-valves showing evidence of diagenetic overgrowth were excluded. The amount of nannofossil-produced CaCO3 (CaCO3nanno) was calculated using the absolute abundance and volumes of the most abundant nannofossils (see Mattioli and Pittet, 2002; Suan et al., 2008a for details). The absolute abundance (per gram of rock) of the most abundant nannofossil taxa (Schizosphaerella, Crepidolithus and Mitrolithus jansae) was determined using the methodology of Geisen et al. (1999). For each stratigraphic interval, the mean volumes of Schizosphaerella and Crepidolithus were inferred from the mean size of 30 specimens; in contrast, the mean volume of Mitrolithus jansae, which shows a more “patchy” distribution along the studied interval, was estimated from the data of Suan et al. (2008a) for the Lower Toarcian of Peniche and considered constant (v = 44 µm3) throughout the succession. As Mitrolithus jansae represents roughly half the mean volume of Crepidolithus (v = 94 µm3) and about one fifth of the mean volume of Schizosphaerella (v = 230 µm3), we consider that this assumption has no important bearing on the calculated trends of CaCO3nanno contents. These three taxa account for 65% to 85% of total Pliensbachian nannofossil content. Other nannofossil taxa that may

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occur in significant proportions in the studied sediments have volumes much smaller than the three taxa used here for quantifications, and the amounts of carbonate produced by these taxa are negligible (Mattioli and Pittet, 2002). 4. Results 4.1. Stable-isotope ratios 4.1.1. Peniche The overall trends in δ13Cbrachiopod, δ13Cbulk carbonate, and δ13Cwood at Peniche are broadly similar, though the brachiopod and fossil wood data show a relatively higher degree of scatter (Fig. 2). Taken together, these records reveal a long-term shift toward more positive values from the Upper Sinemurian to uppermost Lower Toarcian, which is interrupted by three successive negative excursions at the base of the margaritatus ammonite Zone, at the base of the polymorphum Zone and at the base of the levisoni Zone, respectively. Though δ13Cwood appear somewhat scattered, the values fluctuate between about −26.5 and −22‰ in the Pliensbachian interval, with mean values approximating − 25‰. Significantly, these values contrast markedly with the more negative δ13Cwood values (b−29‰) reported by Hesselbo et al. (2007) at the base of the levisoni Zone (Fig. 2), thus confirming that the δ13Cwood data during this latter interval truly record ‘anomalous’ values (Hesselbo et al., 2007) rather than a return to ‘normal’ values (McArthur, 2007). The bulk of the studied wood specimens belongs to the same and well-characterized wood species (Simplicioxylon hungaricum Andreaszky; see supplementary data), indicating that interspecific variability cannot account for the observed large scatter between individual δ13Cwood values from each stratigraphic level. The maximum variations in δ18Obrachiopod and δ13Cbrachiopod from the same stratigraphic horizon are 1.2 and 0.9‰, respectively. It is interesting to note that δ18Obrachiopod variations are generally much smaller than that recorded for δ13Cbrachiopod. This pattern is fully consistent with stable-isotope variations observed on modern brachiopod species, and may be related to vital and metabolic fractionation effects in the uptake of carbon during mineralization (Brand et al., 2003; Auclair et al., 2003; Parkinson et al., 2005). The δ18Obrachiopod profile records a ∼1‰ negative trend from the base of the jamesoni Zone to the top of the margaritatus Zone, followed by a ∼ 1.5–2‰ positive shift at the base of the spinatum Zone (Fig. 2). The phase of elevated δ18Obrachiopod values during the spinatum Zone is then followed by a 2–2.5‰ shift toward lower values from the upper part of the spinatum Zone to the lower part of the levisoni Zone. A marked negative excursion is recorded at the base of the polymorphum Zone (Fig. 2). 4.1.2. Tomar The trends in δ18Obrachiopod and δ13Cbrachiopod profiles as well as their absolute values are very similar to those evidenced for the upper Pliensbachian and lower Toarcian of Peniche (Figs. 3 and 4). As observed in Peniche, the δ18Obrachiopod shows a marked ∼ 1.5‰ increase across from the margaritatus to spinatum ammonite zones, followed by a marked shift toward lower values occurring across the Pliensbachian–Toarcian boundary (Figs. 3 and 4). Most depleted δ18Obrachiopod values are recorded in the lower part of the levisoni Zone. The δ13Cbrachiopod record reveals a prominent 3.5‰ shift toward positive values in the upper part of the levisoni Zone that correlates remarkably with that recorded in the Peniche section (Fig. 3). This positive shift is followed by a return to lower δ13Cbrachiopod values across the levisoni–bifrons boundary, depicting a large positive excursion. Different brachiopod taxa collected from the same stratigraphic levels record similar values (see supplementary data), suggesting the absence of interspecific isotopic fractionation effects for the studied taxa.

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4.2. CaCO3 budget and nannofossils The CaCO3 contents, CaCO3nanno, and Schizosphaerella size variations show closely parallel trends throughout the whole studied interval. Particularly, these parameters record marked increases across the margaritatus–spinatum boundary, which are coeval of the marked increase in δ18Obrachiopod values at the same level (Fig. 2), and then show a marked decrease across the Pliensbachian–Toarcian boundary. Smallest Schizosphaerella diameters and minimum CaCO3 and CaCO3nanno contents are recorded in the interval corresponding to the marked negative excursion in δ18Obrachiopod, δ13Cbrachiopod, δ13Cbulk carbonate, and δ13Cwood at the base of the levisoni Zone. As Schizosphaerella is the most important carbonate producer in terms of both abundance and volume along the entire studied interval, about 33% of the CaCO3nanno increase estimated across the margaritatus–spinatum ammonite zones is due to the concomitant N2.5 µm increase of the mean diameter of this genus. Nevertheless, the absolute abundance of other dominant taxa, i.e. M. jansae, also increases at this level. The calculated amount of CaCO3nanno represents between 2 and 10 wt.% of the bulk rock, and thus corresponds to a minor proportion of the total CaCO3 throughout the studied interval (between 0.8 to 48%, mean = 10%). It is worth noting, however, that CaCO3nanno contents reach significantly higher values (up to 27% of the bulk rock) in several stratigraphically limited horizons of the latest spinatum and polymorphum ammonite zones (Fig. 2). 5. Discussion 5.1. Evolution of Pliensbachian–Toarcian climatic conditions Assuming on one side that diagenesis had little or no influence on the oxygen isotope composition of the brachiopod shells and on the other side that the world was ice-free over the entire studied interval, isotopic paleotemperatures can be calculated using the equation of Anderson and Arthur (1983) and a mean δ18O composition of seawater (δ18Ow) of −1‰ (V − SMOW). Calculated paleotemperatures are presented in Figs. 2–4, and reveal very large variations, with values ranging from ∼ 14 °C to ∼24 °C. A long-term gradual ∼ 3 °C warming is revealed from the basal part of the jamesoni Zone to the uppermost part of the margaritatus Zone, followed by a pronounced and apparently rapid cooling of about 5 °C across the margaritatus– spinatum boundary. The lowest paleotemperatures are then recorded throughout the spinatum Zone, whilst the highest paleotemperatures occur in the lowermost levisoni Zone (Fig. 4). The ∼10 °C change in the calculated paleotemperature may appear huge given the tropical setting of the studied area. Indeed, temperature changes during recent glacial–interglacial cycles rarely exceeded 3 °C to 5 °C in tropical areas (e.g., Lea, 2004), suggesting that additional factors than temperature may have been responsible for the large δ18Obrachiopod variations. As the δ18Ow of seawater is also strongly influenced by local or regional changes in the evaporation/ precipitation budget and runoff, the δ18Obrachiopod record could plausibly reflect variations in both seawater temperature and salinity. Indeed, it has been suggested on the basis of biomarkers and sedimentological data that the marked positive shift in δ18Obulk carb across the margaritatus–spinatum Zone boundary at Peniche reflects a transition from low- to high saline conditions (Oliveira et al., 2006). Nevertheless, both shallow- and deep-water (a few meters versus a few hundred meters water depth) δ18Obrachiopod record the same positive shift (Fig. 4), making it unlikely that the brachiopod oxygen isotope signal reflects local salinity changes of superficial seawaters. Moreover, a similar large shift towards δ18O positive values has been recorded in belemnite rostra from several localities in Germany (∼ 1.2‰) and Spain (∼ 1.3–1.5‰) (Bailey et al., 2003; van de Schootbrugge et al., 2005; Rosales et al., 2006), indicating that this isotopic feature was not restricted to the Lusitanian Basin.

452 G. Suan et al. / Earth and Planetary Science Letters 290 (2010) 448–458 Fig. 2. Carbon- and oxygen-isotope data, CaCO3 contents and mean diameter of the pelagic nannofossil Schizosphaerella from the reference section of Peniche (Pliensbachian–Toarcian boundary GSSP candidate), Portugal. Note the marked parallelism between the long-term evolution of the oxygen-isotope composition of brachiopod shells, CaCO3 contents and the size of Schizosphaerella, which both reach their highest and minimum values in the spinatum and basal levisoni Zone, respectively. Isotopic paleotemperatures were calculated from the equation of Anderson and Arthur (1983) assuming an isotopic composition of seawater of − 1‰ for an ice-free world. Geochemical (CaCO3, δ18Obrachiopod and δ13Cbrachiopod) and nannofossil data for the latest Pliensbachian–Toarcian (from 100 m to 145 m) are from Suan et al. (2008a). The biostratigraphic zonation is based on Mouterde (1955) for ammonites and on Oliveira et al. (2006) and Mattioli et al. (2008) for nannofossils. Dark shading in biostratigraphy columns indicates lack of age-diagnostic fossils.

G. Suan et al. / Earth and Planetary Science Letters 290 (2010) 448–458 Fig. 3. Geochemical data from the Upper Pliensbachian–Middle Toarcian sedimentary rocks exposed in the Lusitanian Basin, Portugal. Isotopic paleotemperatures were calculated from the equation of Anderson and Arthur (1983) assuming an isotopic composition of seawater of − 1‰ for an ice-free world. The ammonite biostratigraphy based on Mouterde (1955) and Mouterde et al. (1971). Lithology as in Fig. 2. polym.: polymorphum.

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Fig. 4. Composite brachiopod carbon- and oxygen-isotope profiles and inferred seawater paleotemperature trends from the Lusitanian Basin, Portugal (solid circles, Peniche; open squares, Tomar) compared to available relative sea-level curves. Stars refer to episodes of marine invertebrate mass extinction (Harries and Little, 1999; Wignall et al., 2005) and biocalcification crisis in the western Tethyan domain.

The phase of low paleotemperatures at Peniche spans ∼ 25 m, while the marked cooling at the base of the spinatum Zone occurs within less than 5 m (Fig. 2). According to Gradstein et al. (2004), the whole Pliensbachian stage lasted around 6.6 myr; assuming a constant sedimentation rate, this implies durations of ∼1.6 myr and ∼ 300 kyr for the whole phase of low paleotemperatures and the marked cooling, respectively (note that these durations likely represent overestimates, as high concentration of ammonites and encrusted belemnites indicates reduced sedimentation rates close to the Pliensbachian–Toarcian boundary at Peniche). For the Phanerozoic Eon, it is often considered that the primary mechanism that can affect globally the δ18Ow of both deep and superficial water on short timescales (b1 myr) is the growth and decay of continental 18Odepleted ice sheets (Miller et al., 2005a). Because the formation of continental ice would have caused an increase in the ocean δ18Ow at the global scale, it is conceivable that the synchronous increase in δ18Obrachiopod from shallow and deep environments close to the margaritatus–spinatum transition could reflect both a marked cooling event and increased continental ice storage. The Late Pliensbachian is characterized by a marked and widespread sea-level lowstand (Hallam, 1967; de Graciansky et al., 1998a,b; Hesselbo and Jenkyns, 1998) that could be compatible with a glacio-eustatic interpretation of the oxygen isotope signal. In the Tomar area, the 1.5‰ increase in δ18Obrachiopod values across the margaritatus–spinatum boundary coincides with the sudden appearance of coarse carbonate-rich material that represent the shallowest facies observed in the Pliensbachian–Toarcian succession (Fig. 3). Similarly, a major forced regression dated from the spinatum Zone has been documented in several localities of the western Tethys

(Parkinson and Hines, 1995; de Graciansky et al., 1998a,b; Cobianchi and Picotti, 2001; Rosales et al., 2006; Duarte, 2007; Mailliot et al., 2009). This event is remarkably illustrated by the marked progradation of siltstones and sandstones (‘Grès Médioliasiques’) of the spinatum Zone (solare Subzone) over the argillaceous marls of the margaritatus Zone in eastern France (Allouc and Hilly, 1979; de Graciansky et al., 1998b). Importantly, a pronounced sea-level fall has been also documented in several siliciclastic successions of northern and eastern Siberia close to the margaritatus–villigaensis (∼ spinatum) transition (Nikitenko and Mickey, 2004; Zakharov et al., 2006). Corroborating evidence of cool high-latitude climatic conditions compatible with the formation of continental ice at this time comes from the occurrence of glendonites in marine deposits of the villigaensis (∼ spinatum) Zone of northern Siberia (Kaplan, 1978; Nikitenko, 2008). According to Haq et al. (1987), the Late Pliensbachian records a sea-level fall of ∼25 m. Assuming similar relationships between changes in sea level and δ18Ow in the Early Jurassic and Quaternary (∼ 0.1‰/10 m of sea-level fall; Miller et al., 2005a,b), about 0.25‰ of the Late Pliensbachian δ18Obrachiopod shift could be then attributed to high-latitude ice buildup. This sea-level fall would correspond, if principally caused by glacio-eustasy, to the buildup of more than one third of the modern Antarctic ice sheet (Miller et al., 2005b). Conversely, the marked warming recorded across the T-OAE (Fig. 4) corresponds to a marked and well-documented transgressive event (Hallam, 1967, 1981; Haq et al., 1987; Jenkyns, 1988; Wignall, 1991; de Graciansky et al., 1998a,b; Hesselbo and Jenkyns, 1998; Nikitenko and Mickey, 2004), which could hence reflect a relatively sudden and massive decay of high-latitude ice sheets.

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5.2. Variations in CaCO3 accumulation Well-preserved calcareous nannofossil assemblages occur throughout the whole Pliensbachian–Toarcian interval in Peniche, indicating a weak diagenetic control on nannofossils in the studied succession. As the measured Schizosphaerella specimens also displayed moderate to excellent preservation, we exclude the possibility that the Schizosphaerella size records result from differential dissolution/overgrowth. Quantification of nannofossil absolute abundance indicates that the pelagic nannofossil fraction did not produce the bulk of the carbonate mud during the whole studied interval (Fig. 2). Therefore, we assume that the carbonate mud deposited in the Peniche area throughout the studied time interval was, as demonstrated for other areas at the same time (Cobianchi and Picotti, 2001; Mattioli and Pittet, 2002; Mattioli et al., 2004), essentially derived from shallowwater carbonate platforms. Accordingly, the variations in CaCO3 contents should mainly reflect the intensity in carbonate-mud production and export from adjacent shallow-water platforms or variations in its preservation on the seafloor (Suan et al., 2008b). On the other hand, the nannofossil absolute abundances and CaCO3nanno contents were likely greatly affected by the temporal changes in the amount of allochtonous carbonate mud received by the studied basin. For instance, several levels characterized by very high CaCO3nanno contents are associated with exceptionally high concentrations of ammonites and encrusted belemnites (Fig. 2), suggesting that the elevated amounts of nannofossil calcite recorded at these levels can be explained by the limited dilution by allochtonous sediments. Nevertheless, elevated CaCO3nanno contents are not systematically associated with evidence of stratigraphic condensation, and, importantly, are also recorded in intervals characterized by high contents of non-pelagic carbonate (Fig. 2). These observations suggest that the CaCO3nanno record along the studied interval most likely reflects the combined influence of differential inputs of allochtonous sediment and temporal changes in pelagic CaCO3 production. 5.3. Links between temperature changes and variations in CaCO3 accumulation The covariance between the observed trends in platformand nannofossil-derived CaCO3 contents, Schizosphaerella size and δ18Obrachiopod values (Fig. 2) suggests that both the pelagic and neritic CaCO3 production/accumulation were related to seawater temperatures or to the regional or global climatic conditions. Both theoretical and empirical data indicate that variations in pCO2 trough time are intimately coupled to deep-sea carbonate ion concentration, which in turn controls the amount of CaCO3 accumulated on the seafloor through dissolution processes (e.g., Zachos et al., 2005). Nevertheless, the Lusitanian Basin was probably not deeper than a few hundreds of meters (Bjerrum et al., 2001), and was thus probably too shallow to be affected by CO2-induced deep-sea dissolution processes. Moreover, the observed nannofossils do not show any distinctive sign of dissolution, while dissolution-susceptible taxa, such as the genera Similiscutum, Biscutum and Sollasites, are recorded throughout the studied succession, thus excluding differential dissolution processes as the main cause of the observed CaCO3 fluctuations. Sedimentological data from the shallow-water succession of Tomar area indicate that the Late Pliensbachian cooling event was closely associated with the basinward progradation of coarse carbonate-rich material (Fig. 3), likely reflecting a major glacioeustatic fall of the relative sea-level (cf. part 5.1.). This suggest that the simultaneous major increase of platform-derived CaCO3 accumulation at Peniche (Fig. 2) at least partly reflect the regression-forced progradation of carbonate-producing shallow areas. Accordingly, short-term variations of CaCO3 accumulation recorded throughout the studied interval (Fig. 2) could be also interpreted in terms of

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climatically-forced changes in sea level. Because the short-term CaCO3 accumulation for the Lower Toarcian of the same section was orbitally paced (Suan et al., 2008b), this would imply that, as in the Neogene, ice growth and decay throughout the studied interval might have been modulated by orbital dynamics. The size of both coccolithophorids and diatoms has changed in concert with temperature and nutrient availability (Knappertsbusch et al., 1997; Finkel et al., 2005; Bornemann and Mutterlose, 2006; Giraud et al., 2006). Salinity is also found to influence the morphology of the modern coccolithophorid species Emiliania huxleyi (Bollmann and Herrle, 2007), and may have exerted an important control on ancient coccolithophore assemblages as well, by inducing changes in seawater stratification (Mattioli et al., 2008). Recently, several studies based on modern species cultures have suggested that pCO2 may also deeply influence coccolithophorid morphology and calcification, although the response within a single species can be greatly dependant on initial experimental conditions (Riebesell et al., 2000; Langer et al., 2006; Iglesias-Rodriguez et al., 2008). Palaeo-pCO2 in oceanic waters is also inferred to influence coccolith and nannolith size in ancient marine systems (Bornemann et al., 2003; Mattioli et al., 2004). Therefore, the size fluctuations of Schizosphaerella may simply reflect variations in abundance of differently sized species or morphotypes in response to changes in some of these environmental parameters, which may have also triggered changes in neritic CaCO3 production and export. It has been proposed that Schizosphaerella had preferences for oligotrophic environments (Claps et al., 1995; Mattioli and Pittet, 2004; Tremolada et al., 2005) and proliferated in shallow water close to carbonate platform areas (Cobianchi and Picotti, 2001; Mattioli and Pittet, 2002, 2004), although some authors interpret this taxon as a deep-dweller (Erba, 2004; Tremolada et al., 2005). It is then conceivable that the observed size variations reflect climatically induced changes in nutrient availability, which are known to deeply influence carbonate production of shallow-water platforms as well (Hallock and Schlager, 1986; Mattioli and Pittet, 2002, 2004). 5.4. The Late Pliensbachian cooling event and implications for the Early Toarcian environmental crises It has been proposed that the Late Pliensbachian cooling episode resulted from reduced insolation due to enhanced volcanogenic SO2 emissions (Guex et al., 2001). Though the oldest ages of the Karoo– Ferrar large igneous province emplacement might partly coincide with the onset of this cooling (Jourdan et al., 2008), the effectiveness of this mechanism to cool the Earth's climate globally on the 106 year time scale remains uncertain (Wignall, 2001). Significantly, the start of the cooling event at Peniche coincides with a ∼ 1.5‰ positive δ13C excursion (Figs. 2 and 4), which immediately follows a 10 meter-thick interval of organic-rich marls and limestones that contain up to 15% of TOC (Oliveira et al., 2006). A ∼1.5‰ positive δ13C excursion has been also reported in the upper margaritatus Zone in several European localities (Jenkyns and Clayton, 1986; Bailey et al., 2003; Rosales et al., 2006), while age-equivalent black shales are widespread in the Lusitanian Basin, and are also known from other localities in Spain, north-east Siberia, Japan and North America (Hallam, 1981; Borrego et al., 1996; Rosales et al., 2006). These observations suggest that the margaritatus positive δ13C excursion could conceivably reflect a widespread burial of isotopically-light organic carbon, which may have promoted cooler climatic conditions and the growth of terrestrial ice through CO2 drawdown and the inverse greenhouse effect. The marked negative δ13C excursions recorded by wood, brachiopod shells and bulk carbonate immediately above the Pliensbachian– Toarcian boundary and across the T-OAE at Peniche suggest that massive amounts of 13C-depleted carbon were repeatedly injected into the atmospheric and superficial oceanic reservoirs during the Early Toarcian (Hesselbo et al., 2007; Suan et al., 2008a). These

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excursions coincide precisely with marked negative shifts of δ18Obrachiopod values (Figs. 2 and 4), suggesting that successive episodes of massive inputs of greenhouse gases may have been associated with or possibly contributed to the termination of the Late Pliensbachian cooling episode. The possible sources of these carbon inputs are the dissociation of oceanic gas hydrates (Hesselbo et al., 2000; Cohen et al., 2007), the thermogenic methane generated during the intrusive eruption of the Karoo–Ferrar (McElwain et al., 2005; Svensen et al., 2007 but see Gröcke et al., 2009) or the emissions of volcanogenic CO2 (Suan et al., 2008b). Determining the source of this carbon input is beyond the scope of this paper, but it can be further suggested that cool conditions in the Late Pliensbachian and part of the Early Toarcian might have led to an increased storage of 13C-depleted carbon under the form of high-latitude oceanic and continental gas hydrates. Our results indicate that, from a long-term perspective, the Early Toarcian carbonate and biotic crises (close to the Pliensbachian– Toarcian boundary and during the T-OAE; Suan et al., 2008a) corresponded to dramatic shifts from two opposed climatic modes, which likely consisted of rapid returns to greenhouse conditions after transient icehouse episodes (Fig. 4). Interestingly, this revised chronology of Pliensbachian–Toarcian climate changes possesses significant similarities with other events of black shale formation and extinctions. For instance, a large number of marine invertebrate became extinct during an episode of widespread black shale formation at the termination of the transient Late Ordovician glaciation, which likely lasted between 1 to 2 myr (Lüning et al., 2000; Gradstein et al., 2004; Fan et al., 2009). These findings are also compatible with evidence of decreased bottom water ventilation in many areas during Quaternary episodes of deglaciation and sea-level rise (e.g., Cannariato et al., 1999; Negri et al., 2009). Possible causes of deglacial organic-rich deposition include an accelerated hydrological cycle, transgression-related sediment starvation, limited bottom water ventilation due to glacially inherited topography, and global reorganization of oceanic and atmospheric circulation (Cannariato et al., 1999; Lüning et al., 2000; Negri et al., 2009). Obviously, further investigations are needed to determine the respective influence of each of these mechanisms on Early Jurassic bottom water ventilation and biota; future studies should also better constrain the precise timing, volumes and location of putative ice buildup throughout the considered interval. Nevertheless, the inferred chronology and magnitude of Pliensbachian–Toarcian climatic changes may prompt the reexamination ice sheet dynamics as a forcing factor of carbonate and organic matter accumulation during the Mesozoic. 6. Conclusions Our new data from the Lusitanian Basin, Portugal, clearly demonstrate a close coupling between climatic changes and the accumulation of nannofossil- and platform-derived CaCO3 for a ∼ 8 myr-long critical period of the Early Jurassic (Early Pliensbachian to the Middle Toarcian). Our δ18Obrachiopod record reveals a pronounced seawater cooling across the transition between the margaritatus–spinatum ammonite zones, which coincided with a marked progradation of coarse carbonate-rich material in shallow areas. Combined with available sedimentological and geochemical data, these results corroborate the hypothesis that substantial amounts of continental ice may have formed during the Late Pliensbachian– earliest Toarcian, with important implications for the formation of organic-rich deposits during the following T-OAE. Though determining its origin deserves further research, this cooling episode may have resulted from severe organic matter drawdown and CO2 sequestration, as suggested by the widespread occurrence of black shales of margaritatus age and accompanying ∼ 1.5‰ positive δ13C excursion. The termination of this cooling at the base of the T-OAE was accompanied by a rapid marine transgression and by a major decline

of both platform- and nannofossil-derived CaCO3. From a long-term perspective, the T-OAE thus appears to correspond to a period of greenhouse warming and biocalcification crisis, which immediately followed a transient phase of cooling and enhanced biocalcification. The chronological and climatic similarities of the Pliensbachian– Toarcian and other events of organic-rich deposition (e.g., Late Ordovician) indicate that changes in ocean and atmosphere dynamics associated with rapid shifts from icehouse to greenhouse conditions may have repeatedly contributed to the spread of anoxic bottom waters and biotic crisis. Acknowledgments This work was funded by the CNRS program ECLIPSE II. Thanks are due to Yves Alméras for the identification of brachiopod shells from Tomar, to Peggy Vincent and Jean Guex for valuable comments and discussions. We would like to thank Gregory Price and an anonymous reviewer for their constructive reviews that improved the quality of the manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2009.12.047. References Allouc, J., Hilly, J., 1979. Quelques aspects de la sédimentologie du Domérien de la région de Nancy (Est du Bassin de Paris). Sci. Terre 23, 61–91. Alméras, Y., Mouterde, R., Benest, M., Bassoullet, J.P., 1997. Biodiversité et stratégie A: l'exemple des brachiopodes toarciens de la rampe carbonatée de Tomar (Portugal). Geobios, M.S. 21, 113–119. Anderson, T.F., Arthur, M.A., 1983. Stable isotopes of oxygen and carbon and their applications to sedimentologic and palaeontological problems. Econ. Palaeontol. Mineral. Short Course 10 (I. 1–I), 151. Auclair, A.-C., Joachimski, M.M., Lécuyer, C., 2003. Deciphering kinetic, metabolic and environmental controls on stable isotope fractionations between seawater and shell of Tetrabratalia transversa (Brachiopoda). Chem. Geol. 202, 59–78. Bailey, T.R., Rosenthal, Y., McArthur, J.M., van de Schootbrugge, B., Thirlwall, M.F., 2003. Paleoceanographic changes of the Late Pliensbachian–Early Toarcian interval: a possible link to the genesis of an Oceanic Anoxic Event. Earth Planet. Sci. Lett. 212, 307–320. Bassoullet, J.P., Baudin, F., 1994. Le Toarcien inférieur : une période de crise dans les bassins et sur les plates-formes carbonatées de l'Europe du Nord-Ouest et de la Téthys. Geobios, M.S. 17, 645–654. Bjerrum, C.J., Surlyk, F., Callomon, J.H., Slingerland, R.L., 2001. Numerical paleoceanographic study of the Early Jurassic transcontinental Laurasian Seaway. Paleoceanography 16, 390–404. doi:10.1029/2000PA000512. Bollmann, J., Herrle, J.O., 2007. Morphological variation of Emiliania huxleyi and sea surface salinity. Earth Planet. Sci. Lett. 255, 273–288. Bornemann, A., Mutterlose, J., 2006. Size analyses of the coccolith species Biscutum constans and Watznaueria barnesiae from the Late Albian “Niveau Breistroffer” (SE France): taxonomic and palaeoecological implications. Geobios 39, 599–615. Bornemann, A., Aschwer, U., Mutterlose, J., 2003. The impact of calcareous nannofossils on the pelagic carbonate accumulation across the Jurassic–Cretaceous boundary. Palaeogeogr. Palaeoclimatol. Palaeoecol. 199, 187–228. Borrego, A.G., Hagemann, H.W., Blanco, C.G., Valenzuela, M., Suarez de Centi, C., 1996. The Pliensbachian (Early Jurassic) anoxic events in Asturias, northern Spain: Santa Mera Member, Rodiles Formation. Org. Geochem. 25, 295–309. Brand, U., Logan, A., Hiller, N., Richardson, J., 2003. Geochemistry of modern brachiopods: application and implications for oceanography and paleoceanography. Chem. Geol. 198, 305–334. Cannariato, K.G., Kennett, J.P., Richard, J., Behl, R.J., 1999. Biotic response to late Quaternary rapid climate switches in Santa Barbara Basin: ecological and evolutionary implications. Geology 27, 63–66. Carpenter, S.J., Lohmann, K.C., 1995. 18O and 13C values of modern brachiopod shells. Geochim. Cosmochim. Acta 59, 3749–3764. Claps, M., Erba, E., Masetti, D., Melchiorri, F., 1995. Milankovitch-type cycles recorded in Toarcian black shales from the Belluno trough (Southern Alps, Italy). Mem. Sci. Geol. 47, 179–188. Cobianchi, M., Picotti, V., 2001. Sedimentary and biological response to sea-level and paleoceanographic changes of a Lower–Middle Jurassic Tethyan platform margin (Southern Alps, Italy). Palaeogeogr. Palaeoclimatol. Palaeoecol. 169, 219–244. Cohen, A.S., Coe, A.L., Harding, S.M., Schwark, L., 2004. Osmium isotope evidence for the regulation of atmospheric CO2 by continental weathering. Geology 32, 157–160. Cohen, A.S., Coe, A.L., Kemp, D.B., 2007. The Late Palaeocene–Early Eocene and Toarcian (Early Jurassic) carbon isotope excursions: a comparison of their time scales,

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