Carbon isotope evidence for sedimentary discontinuities in the lower Toarcian of the Lusitanian Basin (Portugal): Sea level change at the onset of the Oceanic Anoxic Event Bernard Pittet a, ⁎, Guillaume Suan a , Fabien Lenoir a , Luis Vitor Duarte b , Emanuela Mattioli a a UMR CNRS 5276 LGLTPE, Université Lyon 1, Campus de la Doua, Bâtiment Géode, F-69622 Villeurbanne Cedex, France b Departamento de Ciências da Terra and IMAR-CMA, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Largo Marquês de Pombal, 3000-272 Coimbra, Portugal abstract article info Article history: Received 15 October 2013 Received in revised form 9 January 2014 Accepted 10 January 2014 Available online 21 January 2014 Editor: J. Knight Keywords: Carbon isotope Sea-level changes Jurassic Oceanic Anoxic Event The Toarcian Oceanic Anoxic Event (T-OAE), Early Jurassic, was an episode of rapid warming and deep perturba- tion of the carbon cycle, as suggested by the large carbon and oxygen isotope excursions recorded by various carbonate and organic materials of this age. Previous studies have shown that strata deposited immediately below the T-OAE are marked by widespread discontinuities, but their duration and synchronicity remain uncer- tain. In this study, we use the carbon isotope profiles of twelve sections from the Lusitanian Basin, Portugal, to correlate shallower and deeper settings and identify the durations and possible causes of these discontinuities. The obtained correlation suggests the existence of three major discontinuities. The first occurs close to the Pliensbachian–Toarcian boundary and locally corresponds to an interval of condensation due to rapid transgres- sion. The second and most marked discontinuity occurs in the mid-upper part of the Polymorphum ammonite Zone. Depending on the location in the Lusitanian Basin, this discontinuity is interpreted to reflect either marine erosion (in deeper parts of the basin) or subaerial exposure (in the shallow parts of the basin), both induced by a high-amplitude sea-level fall. In intermediate parts of the basin, this discontinuity is interpreted as a transgres- sive ravinement surface due to subsequent sea-level rise. This latter transgression induced the formation of a third discontinuity of sedimentary condensation (uppermost Polymorphum Zone) that corresponds to the inter- val of highest rate of sea-level rise just before the onset of the T-OAE. The comparison of the carbon isotope record of the Lusitanian Basin with that of other European basins indicates that these discontinuities are present on a wide paleogeographical scale within the western Tethys, and correspond to marked changes in seawater temperatures and CO 2 levels, implying a control by high-amplitude, likely glacio-eustatic sea-level changes. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The Early Toarcian Oceanic Anoxic Event (T-OAE), about 182.7 Ma (Early Jurassic, Gradstein et al., 2012), is considered to be one of the most extreme paleoenvironmental perturbations of the Phanerozoic. The T-OAE corresponded to an interval of widespread organic matter burial, 6–7 °C warming in tropical and subtropical seas, biotic turnover and a prominent carbon cycle perturbation of the ocean–atmosphere as record- ed by marked changes in the isotopic composition of fossil wood and ma- rine inorganic and organic carbon (e.g., Jenkyns, 1988; Jenkyns and Clayton, 1997; Hesselbo et al., 2007; Suan et al., 2008a; Caswell et al., 2009; Hermoso et al., 2009). Carbonate-producing systems were also deeply affected by the event, resulting in a dramatic decline in carbonate accumulation on platforms and in the pelagic realm at tropical latitudes in the Early Toarcian (Suan et al., 2008a; Mattioli et al., 2009). Although their causes remain debated, these biotic and abiotic perturbations have often been regarded as the consequences of massive carbon injection from in- trusive volcanism and massive gas hydrate dissociation (Hesselbo et al., 2000; Kemp et al., 2005; Cohen et al., 2007; Svensen et al., 2005). A growing number of studies indicate that the T-OAE was preceded by severe environmental perturbations that started in the Late Pliensbachian. These perturbations are reflected by intense changes in carbonate deposition in neritic environments, multiple carbon- and oxygen-isotope excursions and high rates of turnover in biotic assem- blages (Wignall et al., 2005; Suan et al., 2008a; Caswell et al., 2009; Gómez and Arias, 2010; Littler et al., 2010; Suan et al., 2010; Korte and Hesselbo, 2011). The upper Pliensbachian–lowermost Toarcian succes- sions are also characterized by numerous depositional hiatuses, as testi- fied by frequent hardgrounds, condensed intervals and the widespread absence of some biostratigraphic zones (e.g., Wignall, 1991; Morard et al., 2003; Röhl and Schmid-Röhl, 2005; Léonide et al., 2012). These discontinuities have been variously interpreted as the result of pro- nounced changes in sea level, tectonic activity and hydrological cycling predating the T-OAE, but both their timing and duration, and hence causal mechanisms, remain poorly constrained. Indeed, both the macro- fossil (mainly ammonites) and nannofossil groups used to define Sedimentary Geology 303 (2014) 1–14 ⁎ Corresponding author. E-mail addresses: bernard.pittet@univ-lyon1.fr (B. Pittet), guillaume.suan@univ-lyon1.fr (G. Suan), fabien.lenoir0@gmail.com (F. Lenoir), lduarte@dct.uc.pt (L.V. Duarte), emanuela.mattioli@univ-lyon1.fr (E. Mattioli). 0037-0738/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sedgeo.2014.01.001 Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo