Ichnology of the Toarcian Oceanic Anoxic Event: An understimated tool to assess palaeoenvironmental interpretations

Abstract The Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma) of the Early Jurassic period is considered a second-order bioevent, of major importance in the Mesozoic. This event determined significant changes in the existing biota, including the extinction of some 15–20% of marine families and genera, while also affecting continental groups. The T-OAE is characterized by dramatic palaeoenvironmental changes with widespread oceanic anoxia being the generally invoked most significant effect. It can be recognized by a pronounced negative carbon isotope excursion (CIE), which occurred between two positive ones. Associated with this negative CIE, the T-OAE is widely documented by the record of organic-rich black shale. The event has been profusely studied, but some aspects are still controversial, including its possible causes and duration, its global character, and the selective incidence on biota. Regarding the latter, trace-fossil analysis is comparatively scarce, and ichnological information remains poorly explored. Yet trace-fossil features of the T-OAE could help to improve its characterization. This work reviews the existing ichnological information on the T-OAE worldwide, evaluating registered variability in light of the general palaeogeography during the early Toarcian, based on T-OAE records from the Tethys, Panthalassa, and Arctic oceans. Moreover, ichnological data from the T-OAE (pre-, syn- and post-TOAE CIE) are interpreted with regard to the temporal evolution of the event. The general trend is of decreasing in ichnodiversity, abundance, and burrow size from pre- to syn-T-OAE CIE, followed by their increase from syn- to post-T-OAE CIE. However, some variations in this general pattern are seen in particular environments, depending on the palaeoenvironmental conditions involved and the asociated facies. This review also revisits the role of anoxia as a global phenomenon during this time —the common presence of bioturbation suggests discarding generalized anoxic conditions. Several possibilities are suggested to interpret the record of bioturbation in black shales, in relation to varying oxygenation; from general and prolonged oxygenated bottom and pore-waters punctuated by short intervals of anoxic to euxinic conditions affecting the seafloor, to very low oxygenation levels, but not anoxia, allowing the presence of meiofauna adapted to low-oxygen, even sulfidic conditions. The influence of palaeoenvironmental conditions other than anoxia, and the variable response of the macrobenthic tracemaker community are considered. This work evidences that ichnological information can be a key to a better understanding of T-OAE.