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Global Toarcian OAE This Discussion Paper Is/Has Been Under Review for the Journal Solid Earth (SE) Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Solid Earth Discuss., 3, 385–410, 2011 Solid Earth www.solid-earth-discuss.net/3/385/2011/ Discussions SED doi:10.5194/sed-3-385-2011 3, 385–410, 2011 © Author(s) 2011. CC Attribution 3.0 License. Global Toarcian OAE This discussion paper is/has been under review for the journal Solid Earth (SE). Please refer to the corresponding final paper in SE if available. D. R. Grocke¨ et al. An open marine record of the Toarcian Title Page oceanic anoxic event Abstract Introduction Conclusions References 1 2 1 3 D. R. Grocke¨ , R. S. Hori , J. Trabucho-Alexandre , D. B. Kemp , and Tables Figures L. Schwark4 1 Dept. of Earth Sciences, Durham University, Science Laboratories, South Road, Durham, J I DH1 3LE, UK 2Dept. of Earth Sciences, Faculty of Science, Ehime University, Matsuyama 790-8577, Japan J I 3 Dept. of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK Back Close 4Christian-Albrechts-University – Kiel, Institute for Geosciences, Ludewig-Meyn-Str. 10, 24118 Kiel, Germany Full Screen / Esc Received: 4 April 2011 – Accepted: 5 April 2011 – Published: 7 April 2011 Printer-friendly Version Correspondence to: D. R. Grocke¨ ([email protected]) Interactive Discussion Published by Copernicus Publications on behalf of the European Geosciences Union. 385 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract SED Oceanic anoxic events were time intervals in the Mesozoic characterized by widespread distribution of marine organic-rich sediments (black shales) and signifi- 3, 385–410, 2011 cant perturbations in the global carbon cycle. The expression of these perturbations is 5 globally recorded in sediments as excursions in the carbon isotope record irrespective Global Toarcian OAE of lithology or depositional environment. During the Early Toarcian, black shales were deposited on the epi- and peri-continental shelves of Pangaea and these sedimentary D. R. Grocke¨ et al. rocks are associated with a pronounced (ca. 7‰) negative (organic) carbon isotope ex- cursion (CIE) which is thought to be the result of a major perturbation in the global car- Title Page 10 bon cycle. For this reason, the Early Toarcian is thought to represent an oceanic anoxic event (the T-OAE). Associated with this event, there were pronounced perturbations in Abstract Introduction global weathering rates and seawater temperatures. Although it is commonly asserted that the T-OAE is a global event and that the distribution of black shales is likewise Conclusions References global, an isotopic and/or organic-rich expression of this event has as yet only been Tables Figures 15 recognized on epi- and peri-continental Pangaean localities. To address this issue, the 13 carbon isotope composition of organic matter (δ Corg) of Early Toarcian cherts from Japan that were deposited in the open Panthalassa Ocean was analysed. The results J I 13 show the presence of a major (>6‰) negative excursion in δ Corg that, based on J I radiolarian biostratigraphy, is a correlative of the Early Toarcian negative CIE known Back Close 20 from European epicontinental strata. Furthermore, a secondary ca. −2‰ excursion 13 in δ Corg is also recognized lower in the studied succession that, within the current Full Screen / Esc biostratigraphical resolution, is likely to represent the excursion that occurs close to the Pliensbachian/Toarcian boundary and which is also recorded in European epicontinen- Printer-friendly Version tal successions. These results from the open ocean realm suggest that, in conjunction 25 with other previously published datasets, these major Early Jurassic carbon cycle per- Interactive Discussion turbations affected all active global reservoirs of the exchangeable carbon cycle (deep 13 marine, shallow marine, atmospheric). An extremely negative δ Corg value (−57‰) during the peak of the T-OAE is also reported, which suggests that the inferred open 386 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ocean mid-water oxygen minimum layer within which these sediments are thought to have been deposited was highly enriched in methanotrophic bacteria, since these or- SED ganisms are the only plausible producers of such 12C-enriched organic matter. 3, 385–410, 2011 1 Introduction Global Toarcian OAE 5 Over the past quinquennium there has been an increased effort to unravel the cause(s) D. R. Grocke¨ et al. and consequences of the Early Toarcian oceanic anoxic event (T-OAE). Of enormous diagnostic significance over this interval is the presence of a pronounced, transient 13 negative carbon isotope (δ C) excursion that has been recognized in marine inor- Title Page ganic and organic matter (Jenkyns and Clayton, 1988; Schouten et al., 2000; Kemp Abstract Introduction 10 et al., 2005; Suan et al., 2008), and terrestrial plant material (Hesselbo et al., 2000; 12 2007). This feature has been postulated to be the result of an increase in C in the Conclusions References carbon cycle, perhaps following the dissociation of methane hydrates (Hesselbo et al., 2000; Kemp et al., 2005). Associated with the T-OAE is concomitant evidence for: a Tables Figures sudden rise in seawater palaeotemperatures (Rosales et al., 2004; van de Schoot- 15 brugge et al., 2005), a three-fold increase in atmospheric CO2 levels (McElwain et J I al., 2005), a major increase in silicate weathering rates (Cohen et al., 2004; Waltham and Grocke,¨ 2006) and a biotic crisis affecting marine invertebrates and biocalcifying J I micro-organisms (Little and Benton, 1995; Mattioli et al., 2004; Tremolada et al., 2005). Back Close The lithological expression of the T-OAE may be locally very different. The develop- Full Screen / Esc 20 ment of anoxic, organic-rich facies was not generalized in the European Neotethyan realm and, where developed, the petrology and organic richness of the sediments is variable. For instance, certain limestones in the H. falciferum ammonite Zone of north- Printer-friendly Version ern Europe (Hallam, 1967) and of the Tethys (Jenkyns, 1985) are pink/red. These dif- Interactive Discussion ferences illustrate the importance of local environmental conditions that are ultimately 25 more important in determining the local lithological expression of an OAE (Trabucho Alexandre et al., 2010). Indeed, available biostratigraphical data indicate that the de- velopment and demise of organic-rich facies in Tethyan and Boreal provinces of Europe 387 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | are diachronous (Wignall et al., 2005 and references therein): similar to the Cenoma- nian/Turonian boundary OAE (Tsikos et al., 2004). SED Hesselbo et al. (2000) suggested that the cause of the negative CIE was the disso- 3, 385–410, 2011 ciation, release and consequent oxidation of methane derived from continental shelf 13 5 methane hydrates. More recently, a high-resolution δ Corg record across the T-OAE obtained from mudrocks exposed in northeast England was generated by Kemp et Global Toarcian OAE al. (2005). These data show that the negative δ13C excursion was not a single event but three rapid events, the timing of which was paced by astronomically forced climate D. R. Grocke¨ et al. cycles. An alternative hypothesis recently put forward for the cause of the T-OAE is that 12 10 C-enriched thermogenic methane was released into the ocean-atmosphere system Title Page due to an igneous intrusion into coaly organic-rich facies in the Karoo Basin, South Africa. (McElwain et al., 2005; Svensen et al., 2007). This mechanism has subse- Abstract Introduction quently been questioned by Grocke¨ et al. (2009), who showed that both the physical nature of the contacts between intrusions and organic-rich facies and the geochemical Conclusions References 15 pattern within these facies suggest little or no thermogenic methane generation from Tables Figures the Karoo Basin. A key controversy surrounding the T-OAE is that localities where the negative δ13C J I excursion has been recorded have primarily been restricted to epicontinental sections exposed in Europe. This has prompted speculation that the isotopic perturbation is a J I 20 feature affecting only the shallow marine environment (i.e. <200 m water depth) and Back Close driven by localized phenomena (van de Schootbrugge et al., 2005). Recently, how- ever, Al-Suwaidi et al. (2010) documented evidence for the Early Toarcian negative Full Screen / Esc δ13C excursion from strata from the Neuquen´ Basin, Argentina. This record revealed the onset of the excursion, although the main excursion interval and recovery were Printer-friendly Version 25 not documented owing to the presence of an unconformity that truncates the succes- sion. A further commonly cited issue surrounding the event is the lack of evidence for Interactive Discussion the negative δ13C excursion in belemnite calcite (van de Schootbrugge et al., 2005; Wignall et al., 2006; McArthur et al., 2007). However, Suan et al. (2008) have recently reproduced the negative δ13C excursion in brachiopod calcite across the T-OAE from a 388 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | section in Peniche, Portugal. This work thus suggests that belemnites are either: (1) not present during the negative δ13C T-OAE, termed the “belemnite gap” by Hesselbo et SED al. (2007); and/or (2) are shifting habitats and thus sampling different components of 3, 385–410, 2011 the water column thus masking (diluting) the isotopic
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