Evidence for major environmental perturbation prior to and during the Toarcian (Early Jurassic) oceanic anoxic event from the Lusitanian Basin, Portugal

Abstract

[1] The timing and causal relationships between the pronounced negative C isotope excursion and paleoenvironmental perturbations associated with the Toarcian oceanic anoxic event (Early Jurassic) remain unclear, particularly because biotic crises and carbonate production decrease appear to have been initiated earlier than the main C isotope anomaly. Here we present a new quantification of Late Pliensbachian-Early Toarcian calcareous nannofossils abundance and size from the Peniche reference section (Portugal) together with O and C isotope records of well-preserved brachiopod shells from the same section. The brachiopod shell d 13C curve parallels that of bulk carbonate and records two pronounced negative isotopic excursions, close to the Pliensbachian-Toarcian boundary ($À2%) and during the Toarcian oceanic anoxic event ($À3.5%). Our results indicate that both C isotope negative excursions were characteristic of benthic and shallow-water environments, suggesting that these two carbon cycle perturbations affected all epioceanic reservoirs. Coeval shifts toward lower values of brachiopod oxygen isotope compositions and closely correlated northward migrations of Mediterranean ammonite fauna suggest that both events coincided with major rises in seawater temperatures, probably as a result of increased CO 2 levels and enhanced greenhouse conditions. CO 2 -induced changes in seawater chemistry likely affected the calcification potential of both neritic and pelagic systems, as evidenced by synchronous drops of platform-derived carbonate accumulation and drastic reductions in size (>3 mm) of the main pelagic carbonate producer Schizosphaerella. We suggest that the Early Toarcian paleoenvironmental crisis occurred in two distinct episodes that were most likely related to two successive phases of intense volcanic degassing in the Karoo-Ferrar province.Citation: Suan, G., E. Mattioli, B. Pittet, S. Mailliot, and C. Lécuyer (2008), Evidence for major environmental perturbation prior to and during the Toarcian (Early Jurassic) oceanic anoxic event from the Lusitanian Basin,

An open ocean record of the Toarcian oceanic anoxic event

“…The latter value is somewhat similar to the recent estimate by Suan et al (2008), who assigned a value of 600 kyr for the duration of the T-OAE negative excursion in the Lusitanian Basin. Suan et al (2008) further suggested that the entire excursion (negative and positive excursion) lasted 900 kyr. If the end of the T-OAE excursion at Katsuyama is taken to occur between ca.…”

“…5) represents the excursion found in European sections at the Pliensbachian/Toarcian boundary. Evidence for a Pliensbachian/Toarcian negative CIE from Europe has been documented from sections in Yorkshire (Littler et al, 2009) and Peniche (Hesselbo et al, 2007;Suan et al, 2008) and is also apparent in the Mochras Farm record of Wales (Jenkyns et al, 2001) (Fig. 6).…”

“…Moreover, the negative δ 13 C excursion is absent in data generated using belemnite calcite from NW Europe Wignall et al, 2006;McArthur et al, 2008). The excursion is, however, present in bulk calcite; additionally, Suan et al (2008) recently reproduced the negative δ 13 C excursion in brachiopod calcite across the T-OAE from a section in Peniche, Portugal. Nevertheless, the absence of a negative excursion in belemnite calcite prompted speculation that the isotope perturbation affected only the shallow marine environment (i.e.…”

“…The presence of a pronounced, transient negative carbon isotope (δ 13 C) excursion is of diagnostic significance. The excursion has been recognized in marine inorganic and organic matter (Jenkyns and Clayton, 1986;Schouten et al, 2000;Kemp et al, 2005;Suan et al, 2008), and terrestrial plant material (Hesselbo et al, 2000(Hesselbo et al, , 2007. Associated with the T-OAE is concomitant evidence for: a threefold increase in atmospheric CO 2 levels (McElwain et al, 2005), a rise in seawater palaeotemperatures (Rosales et al, 2004;, an increase in silicate weathering rates (Cohen et al, 2004;Waltham and Gröcke, 2006) and a biotic crisis affecting marine invertebrates and biocalcifying microorganisms (Little and Benton, 1995;Mattioli et al, 2004;Tremolada et al, 2005).…”

Palaeoclimatic oscillations in the Pliensbachian (Early Jurassic) of the Asturian Basin (Northern Spain)

“…Especially relevant are the latest Pliensbachian−Early Toarcian climate changes, which have been documented in many sections from Western Europe (i. e. Saelen et al, 1996;McArthur et al, 2000;Röhl et al, 2001;Schmidt-Röhl et al, 2002;Bailey et al, 2003;Jenkyns, 2003;Rosales et al, 2004;Gómez et al, 2008;Metodiev and Koleva-Rekalova, 2008;Suan et al, 2008Suan et al, , 2010Dera et al, 2009Dera et al, , 2010Dera et al, , 2011Gómez and Arias, 2010;García Joral et al, 2011;Gómez and Goy, 2011;Fraguas et al, 2012), as well as in Northern Siberia and in the Artic Region (Zakharov et al, 2006;Nikitenko, 2008;Suan et al, 2011). The close correlation between the severe Late Pliensbachian Cooling and the Early Toarcian Warming events, and the major Early Toarcian mass extinction indi- cates that warming was one of the main causes of this faunal turnover (Kemp et al, 2005;Gómez et al, 2008;Gómez and Arias, 2010;García Joral et al, 2011;Gómez and Goy, 2011;Fraguas et al, 2012;Clémence, 2014;Clémence et al, 2015;Baeza-Carratalá et al, 2015).…”