Early Jurassic extrinsic solar system dynamics versus intrinsic Earth processes – Toarcian sedimentation and benthic life in deep-sea contourite drift facies, Cardigan Bay Basin, UK

Abstract The Cardigan Bay Basin (UK) may have functioned as a narrow, deep strait, and influenced Early Jurassic oceanic circulation through the northern and southern Laurasian Seaway and, in general, between the Boreal and Peri-Tethys domains. Toarcian hemipelagic deposits of the basin in the Mochras borehole show strongly bioturbated contourite facies. Trace fossils are strongly dominated by Phycosiphon incertum (represented by four morphotypes), which was produced by opportunistic colonizers. Thalassinoides, Schaubcylindrichnus and Trichichnus are common (the latter is a deep-tier trace fossil produced by filamentous sulfide-oxidizing bacteria with a high tolerance for dysoxia), accompanied by less common Zoophycos, Planolites, Palaeophycus, Teichichnus, Rhizocorallium, Chondrites and dwelling and resting structures, such as cf. Polykladichnus, Siphonichnus, Skolithos, Arenicolites, Monocraterion and Lockeia. Ichnological and lithological signals suggest repetitive fluctuations in benthic conditions attributed to the a hierarchy of orbital cycles (precession and obliquity [4th order], short eccentricity [3rd order], long eccentricity [2nd order] and Earth–Mars secular resonance [1st order]). The Pliensbachian-Toarcian boundary event appears to be a significant palaeoceanographic turning point, starting a CaCO3 decline with the most severe oxygen crisis (dysoxic, not anoxic), ending at the beginning of the Toarcian negative carbon isotope excursion (To-CIE – linked with the Toarcian Oceanic Anoxic Event). This contrasts with the prevalence of anoxia synchronous with the To-CIE in many other settings. A minor dysoxic event also occurred in the latest thouarsense and dispansum zones. Extreme climate warming during the To-CIE may have enhanced and caused a reversal in the direction of deep marine circulation, improving oxygenation of the sea floor. Spectral analysis of binary data on ichnotaxa appearances gives high confidence in orbital signals and allows refined estimation of ammonite zones and the duration of the Toarcian (minimum ~ 9.7 Myr).