Abstract
AbstractThe Toarcian Oceanic Anoxic Event (T-OAE, c. 182 Ma) represents a major perturbation of the carbon cycle marked by widespread black shale deposition. Consequently, the onset of the T-OAE has been linked to the combined effects of global warming, high productivity, basin restriction and salinity stratification. However, the processes that led to termination of the event remain elusive. Here, we present palynological data from Arctic Siberia (Russia), the Viking Corridor (offshore Norway) and the Yorkshire Coast (UK), all spanning the upper Pliensbachian – upper Toarcian stages. Rather than a ‘dinoflagellate cyst black-out’, as recorded in T-OAE strata of NW Europe, both the Arctic and Viking Corridor records show high abundance and dinoflagellate diversity throughout the T-OAE interval as calibrated by C-isotope records. Significantly, in the Arctic Sea and Viking Corridor, numerous species of the Parvocysta and Phallocysta suites make their first appearance in the lower Toarcian Falciferum Zone much earlier than in Europe, where these key dinoflagellate species appeared suddenly during the Bifrons Zone. Our results indicate migrations of Arctic dinoflagellate species, driven by relative sea-level rise in the Viking Corridor and the establishment of a S-directed circulation from the Arctic Sea into the Tethys Ocean. The results support oceanographic models, but are at odds with some interpretations based on geochemical proxies. The migration of Arctic dinoflagellate species coincides with the end of the T-OAE and marks the arrival of oxygenated, low-salinity Arctic waters, triggering a regime change from persistent euxinia to more dynamic oxygen conditions.
Highlights
The Toarcian Oceanic Anoxic Event (T-OAE) is generally regarded as a global, short-lived perturbation of the carbon cycle that occurred during the Exaratum Subzone of the Falciferum Zone (Jenkyns, 1988; Hesselbo et al 2000; Suan et al 2008b; Boulila et al 2014) (Fig. 1)
It is marked by a negative carbon isotope excursion (CIE) of around 3–4 per mille in both carbonate and organic carbon substrates (Suan et al 2015), which has been documented in many locations in Europe (Sabatino et al 2009, 2013; Kafousia et al 2011), as well as in NW Africa (Bodin et al 2016), North and South America (Al-Suwaidi et al 2010; Martindale et al 2017), Siberia (Suan et al 2011) and Japan (Izumi et al 2012)
Instead of being a rapid speciation event, palynological records from Siberia indicate that this important group of characteristic Early Jurassic cyst-forming dinoflagellates originated in the high latitudes in relatively cold Arctic waters, and were able to disperse southwards due to deepening of the Viking Corridor at the end of the Toarcian carbon cycle perturbation at the Falciferum–Bifrons Zone transition (Fig. 12)
Summary
The Toarcian Oceanic Anoxic Event (T-OAE) is generally regarded as a global, short-lived perturbation of the carbon cycle that occurred during the Exaratum Subzone of the Falciferum Zone (Jenkyns, 1988; Hesselbo et al 2000; Suan et al 2008b; Boulila et al 2014) (Fig. 1).
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