Lower Jurassic epicontinental carbonates and mudstones from England and Wales: chemostratigraphic signals and the early Toarcian anoxic event

Abstract

Sections through Lower Jurassic epicontinental carbonates from Southern Britain (Junction Bed and equivalent) show a positive carbon‐isotope excursion (δ 13 C carbonate), detectable in bulk rock, in the falciferum Zone of the lower Toarcian. Isotopic data from organic matter in more clay‐rich sections from Wales and north‐east England, together with determinations on belemnite calcite, indicate that highest δ 13 C values are localized in the upper exaratum Subzone of the falciferum Zone. Levels of particular enrichment in organic carbon were developed in the early to mid‐exaratum Subzone and hence pre‐date this δ 13 C maximum. These phenomena reflect the impact of the early Toarcian oceanic anoxic event in the British area. Similar isotopic trends have been recorded in other Toarcian sections from Tethyan Europe and are interpreted as reflecting the chemistry of sea water. On the assumption of isotopic correlation between the English and Tethyan sections, the δ 13 C maximum would be everywhere dated as latest exaratum Subzone in terms of the north European ammonite scheme. Absolute oxygen‐isotope values in carbonates probably reflect both early diagenetic cementation and later temperature‐related burial diagenesis, although a palaeotemperature maximum is tentatively identified as characterizing the early falciferum Zone. Subsequent climatic deterioration may have been triggered by drawdown of CO 2, related to regional excess carbon burial during the oceanic anoxic event. Using the positive δ 13 C excursion as a correlative level in sections from different faunal provinces (Britain, Italy and Spain) implies that lower Toarcian zonal stratigraphy is diachronous between northern and southern Europe. There is evidence for partitioning of water masses between the north European shelf and the Tethyan continental margin during the Early Jurassic.