Black shales and carbon isotopes in pelagic sediments from the Tethyan Lower Jurassic

Abstract

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 δ 13 C 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 –33δ PDB 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.