The end-Triassic and Early Jurassic mass extinction records in the British Isles

Abstract

The complex crises of the end-Triassic and Early Jurassic (Toarcian) mass extinctions are well recorded in the British Isles where they coincide with major palaeoenvironmental changes. The end-Triassic extinction occurs within the quasi-marine Lilstock Formation, the fauna of which is dominated by eurytopic bivalves. These suffered a major extinction event (77% of species), including all infaunal forms, within the basal beds of the Formation. This crisis slightly predates a series of environmental events that include, in ascending order, an intense phase of seismicity (the onset of Central Atlantic flood basalt volcanism?), sea-level fall and a negative δ 13 C org spike. The extinction is linked most clearly (i.e. closest in timing) with regression and seismicity. A further sea-level fall is recorded at the top of the Formation, followed by transgression and the spread of anoxia. These events are not associated with any further bivalve losses although conodonts disappeared at this time. The link between extinction and oxygen-poor conditions is demonstrated better by the Toarcian crisis, when a bivalve extinction event (85% species loss) coincided with the onset of intense anoxia within the middle of the Semicelatum Subzone of the Yorkshire coast sections. However, these losses are part of several extinction pulses spread over more than a million years. These began earlier, at the Pliensbachian/Toarcian stage boundary when ammonites, brachiopods and ostracods all suffered losses coinciding with regression and cooling. However, the subsequent transgression saw a brief spread of marine anoxia (represented by the Sulphur Band in Yorkshire sections) that may also be implicated in the extinctions. Black shale deposition is extremely widespread in the British Toarcian record, with the remarkable exception of the Somerset and south Dorset areas where condensed, fully-oxygenated, deep-water conditions are recorded by the Leptaena Bed and ‘Junction Bed’. Even more remarkably, severe extinction losses (of brachiopods and ostracods) are recorded in this apparently benign depositional setting.