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- Title
Black shales and carbon isotopes in pelagic sediments from the Tethyan Lower Jurassic.
- Authors
Jankyns, Hugh C.; Clayton, Christopher J.
- 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[SUP13]C value of 4.52%[SUB0PDB] 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 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 -33%[SUB0PDB] 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 water. The onset of upwelling may have been as early as spinatum-tenuicostatum Zone time, that is, at the Pliensbachian-Toarcian boundary.
- Subjects
HUNGARY; SWITZERLAND; ITALY; JURASSIC stratigraphic geology; SAMPLING (Process); LIMESTONE
- Publication
Sedimentology, 1986, Vol 33, Issue 1, p87
- ISSN
0037-0746
- Publication type
Article
- DOI
10.1111/j.1365-3091.1986.tb00746.x