Sections through Lower Jurassic epicontinental carbonates from Southern Britain (Junction Bed and equivalent) show a positive carbon‐isotope excursion (δ13Ccarbonate), 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 δ13C 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 δ13C 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 δ13C 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 CO2, related to regional excess carbon burial during the oceanic anoxic event. Using the positive δ13C 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.
Detailed sampling and analysis of Jurassic pelagic limestones and marls from Italy, Hungary and Switzerland have enabled construction of an isotope stratigraphy across the Pliensbachian‐Toarcian boundary with resolution to the zonal level. The oxygen‐isotope record is unremarkable. The carbon isotopes, however, show two positive excursions: one, relatively minor, during the Pliensbachian, margaritatus Zone, subnodosus Subzone, the other, more major, during the Toarcian. early falciferum Zone, where a maximum δ13C value of 4·52%PDB is attained. These intervals are known to be favoured periods of organic‐rich sedimentation in diverse parts of the globe and the isotopic excursions are interpreted as a response to abnormally high rates of storage of organic carbon in the sedimentary record. A comparable phenomenon has been documented from the Cenomanian‐Turonian boundary in the Cretaceous where it has been referred to the influence of an ‘Oceanic Anoxic Event’.
Some Italian sections spanning this Lower Jurassic interval contain organic‐rich shales in the falciferum Zone; the isotopic signatures from their included, locally manganiferous carbonate betray a considerable diagenetic overprint and they cannot therefore be incorporated in a composite isotopic curve. Carbon isotopes from the organic carbon itself are extremely negative, falling to –33δPDB and, in one section examined in detail, correlate with the calcium‐carbonate content of the shales; they may reflect a partial change to a non‐calcified planktonic biota during deposition of this lime‐poor interval, possibly responding to upwelling and increased fertility of near‐surface waters. The onset of upwelling may have been as early as spinatum‐tenuicostatum Zone time, that is, at the Pliensbachian‐Toarcian boundary.
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