A new scenario for the Domerian - Toarcian transition

Abstract

In contrast to the majority of recently published hypotheses, we believe that the main trigger for early Toarcian anoxia is neither increased primary productivity during the Tenuicostatum and Falciferum Zones nor sudden methane hydrate degassing close to the transition between these two zones.In our opinion, this peculiar paleoceanographic episode is linked to a major, though short-lived, regression at the end of Upper Domerian. Sea-level fall resulted from sudden cooling due to increased volcanic activity. This generated global thermal insulation and subsequent glaciation. The regression is responsible for a major hiatus over NW-European epicontinental seas and is later followed by the well-known Lower Toarcian transgression. The interval corresponding to this hiatus allowed vegetation to colonise vast newly emerged surfaces. The leaching and drowning of the accumulated organo-humic matter then triggered the anoxic cycle at the transgressive maximum, concomitant with a global warming. Un nouveau scénario pour le passage Domérien -ToarcienMots clés. -Pliensbachien supérieur, Domérien, Toarcien, Anoxie, Lacune stratigraphiqueRésumé. -Pour tenter de mieux comprendre l'événement d'anoxie océanique globale au Toarcien inférieur, il convient de remonter aux changements environnementaux qui ont précédé cette phase paroxysmale. En effet, la comparaison des séquences sédimentaires et biostratigraphiques du passage Domérien-Toarcien entre la Téthys occidentale (Maroc, Espagne, Portugal) et l'Europe du Nord-Ouest (Causses, Allemagne, Angleterre) fait apparaître une importante lacune dans la région septentrionale.La faune d'Arieticeratinae (Emaciaticeras, Canavaria, Fontanelliceras) et d'Harpoceratinae (Lioceratoides, Neolioceratoides), accompagnée ensuite de Dactylioceras particuliers (groupe mirabile-polymorphum = sous-genre Eodactylites), fait presque totalement défaut en Europe du Nord-Ouest, alors qu'elle abonde dans les dernières alternances marno-calcaires, sans changement lithologique notable avec le Domérien des coupes téthysiennes. Cette faune est intercalée entre les derniers Pleuroceras et les Dactylioceras du groupe tenuicostatum. Ces derniers apparaissent dans les argiles succédant immédiatement au dernier banc calcaire à Dactylioceras mirabile au Portugal notamment. C'est là le diachronisme déjà reconnu entre les limites lithostratigraphique (disparition des bancs calcaires) et biostratigraphique (apparition du genre Dactylioceras) au passage Domérien-Toarcien.Cette observation peut s'intégrer dans un nouveau scénario paléo-océanographique prenant en compte à la fois la tendance régressive majeure du Domérien supérieur (conduisant à une lacune régionale importante), l'abondance de matière charbonneuse dans les premiers dépôts transgressifs du Toarcien et l'événement anoxique global subséquent.Dans la partie supérieure du Domérien, l'existence d'un fort volcanisme peut être déduite des données relatives aux variations des isotopes du strontium [McArthur et al., 2000]. A ce pic de strontium sont associées une f...

Dramatic decrease of pelagic carbonate production by nannoplankton across the Early Toarcian anoxic event (T-OAE)

“…This cooling event is further associated with a sedimentary hiatus well documented in many settings of the NW margin of Tethys, extending into the first ammonite zone of the Toarcian (Guex et al, 2001;Morard et al, 2003). The association of cool temperatures and low sea-level leading to a widespread hiatus can be interpreted as the result of glacioeustatic mechanisms, already postulated by Guex et al (2001).…”

Calcium isotope evidence for dramatic increase of continental weathering during the Toarcian oceanic anoxic event (Early Jurassic)

“…All sample and standard (NIST SRM915b) solutions were loaded in ion-exchange chromatographic columns filled with Eichrom™ Sr-Spec resin to remove the strontium ions (Tacail et al, 2014). Indeed, strontium monatomic bivalent ions can create significant interferences with calcium ions signal due to their extremely close m/z ratio, and therefore compromise calcium isotope measurements for sample with Ca/Sr ratio lower than 10 000 (Morgan et al, 2011). After calcium elution with distilled 3.5N HNO 3 , samples were collected into Teflon beakers and dried down at 70 • C for twelve hours.…”

“…The precision of the measurements, corrected for instrumental drift, was better than 98% based on the reproducibility of standard replicates. The Ca/metal of the solutions being >10 for all these elements, the one-step column chemistry described above was considered sufficient to avoid interference and matrix effects (Morgan et al, 2011) and allow reliable calcium isotope measurements.…”

Black shale deposition during Toarcian super-greenhouse driven by sea level

“…This Maximum Regressive Surface (MRS R5-T6) records an abrupt change in the lithology from sandy limestone (rich in macrofossils) to dark grey calcareous marlstone. In most regions of the NW European realm, this surface has led to the absence of the latest Pliensbachian and earliest Toarcian sediments corresponding to a major sedimentary hiatus at the stage boundary (Morard et al, 2003;Hermoso et al, 2009b). The MFS of T6 occurred around the serpentinum-bifrons Zonal transition (= limit between the Early and Middle Toarcian) in the Paris Basin (Gély and Lorenz, 2006;Guillocheau et al, 2000), and can be placed at Sancerre at approximately 322 m, within the clayed facies of the Marnes à Bifrons Fm (Figs.…”

“…This problem has led to different interpretations pertaining to the stratigraphic relationship between "black shale" occurrence and the carbon isotope curves. In the present study, we define a black shale interval on the basis of sediment lamination rather than a TOC content higher than 5 % (Neuendorf et al, 2005). Indeed, the organic content may be modulated as a result of dilution by clay and carbonate minerals.…”