WAS THE LOWER JURASSIC ‘TOARCIAN OCEANIC ANOXIC EVENT’ AND ASSOCIATED BENTHIC EXTINCTION REALLY A GLOBAL PHENOMENON? A CASE STUDY OF PLIENSBACHIAN–TOARCIAN BENTHIC MACROFAUNAL TURNOVER IN THE LLANBEDR (MOCHRAS FARM) CORE

Hyperthermal conditions and accompanying environmental perturbations linked to Karoo-Ferrar volcanism caused marine anoxia across much of Europe during the early Toarcian. This so-called Toarcian Oceanic Anoxic Event (TOAE) is recognised by the deposition of organic-rich laminated black shales, a global negative carbon isotope excursion (NCIE), and benthic macrofaunal extinction.

Benthic extinction and recovery across the TOAE is well studied in the Cleveland Basin (Yorkshire, England), as well as in Germany, and equivalent strata elsewhere (e.g., Morocco, Canada). Although this pattern is considered universal across northwest Europe, data is limited from localities where laminated black shales are absent. One such example is the Llanbedr (Mochras Farm) core (Cardigan Bay Basin, North Wales), which provides a continuous, expanded section throughout the entire Pliensbachian-Toarcian time interval. Here we identify benthic macrofossils to species level to assess turnover across the Pliensbachian-Toarcian, and we investigate benthic faunal diversity at 5-meter resolution.

Infauna is scarce for most of the upper Pliensbachian in the Mochras core, but a rich epifauna of brachiopods and bivalves dominate the margaritatus and spinatum Zones. The lower spinatum Zone has the highest species-richness across the Pliensbachian-Toarcian at Mochras and correlates to a minor NCIE. A significant drop in overall diversity coincides with a positive carbon isotope excursion in the tenuicostatum Zone, where bivalves completely disappear, and the assemblage comprises intermittent occurrences of gastropods and shallow infaunal brachiopods.

Diversity improves at the onset of the NCIE in the exaratum Subzone (serpentinum Zone), producing an assemblage of epifaunal, shallow infaunal, and deep infaunal bivalves, semi-infaunal gastropods, and small pyritised burrows. Therefore, redox conditions at Mochras during the ‘TOAE’ differed, both in terms of timing and intensity, from coeval sections in the Cleveland Basin, suggesting more oxygenated conditions.

The diversity drop just after the Pliensbachian-Toarcian boundary (much earlier than the TOAE) at Mochras contrasts diversity change recorded in the Cleveland Basin and Canada, but is similar to Morocco and south Germany, questioning the universality of correlation between the timing of extinction and the ‘TOAE’.